butterfly/kb
1
0
Fork 0
Code Issues Pull Requests Actions Packages Projects Releases Wiki Activity

Scrape wikipedia-science: 5771 new, 3166 updated, 9202 total (kb-cron)

Browse Source
This commit is contained in:
turtle89431 2026-05-05 02:44:17 -07:00
parent 8d33c32756
commit 1467547cab
101 changed files with 5127 additions and 3 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

BIN
_index.db
View File

Binary file not shown.

41
data/en.wikipedia.org/wiki/ADRON-RM-0.md Normal file
View File

@ -0,0 +1,41 @@
---
title: "ADRON-RM"
chunk: 1/1
source: "https://en.wikipedia.org/wiki/ADRON-RM"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:44:02.707682+00:00"
instance: "kb-cron"
---
ADRON-RM (Autonomous Detector of Radiation of Neutrons Onboard Rover at Mars) is a neutron spectrometer to search for subsurface water ice and hydrated minerals. This analyser is part of the science payload on board the European Space Agency's Rosalind Franklin rover, tasked to search for biosignatures and biomarkers on Mars. The rover is planned to be launched not earlier than 2028 and land on Mars in 2029.
ADRON-RM is a near copy of ADRO-EM on the stationary ExoMars 2020 surface platform and the Dynamic Albedo of Neutrons (DAN) neutron detector on board NASA's Curiosity rover, all designed by Igor Mitrofanov from the Russian Space Research Institute (IKI).
== Overview ==
ADRON-RM is a neutron spectrometer that will search for hydrogen in the form of bound water or water ice, and hydrogen-bearing compounds. It will be used in combination with WISDOM instrument (a ground-penetrating radar) to study the subsurface beneath the rover and to search for optimal sites for drilling and sample collection. It can also detect trace elements such as Gd and major elements that constitute soil, such as Cl, Fe, Ti. It will also monitor the neutron component of the radiation background on Mars' surface.
== Development ==
The Principal Investigator is Igor Mitrofanov from the Russian Space Research Institute (IKI). The instrument is almost a reproduction of the Dynamic Albedo of Neutrons (DAN) neutron detector on board NASA's Curiosity rover also developed in Russia. Mitrofanov is also developing the active gamma and neutron spectrometer, ADRON-EM (Active Detection of Radiation of Nuclei-ExoMars) for the stationary Kazachok lander—the primary goal of which will be to measure water distribution in the Martian subsurface. Measurements by ADRON-RM and ADRON-EM will work in synergy with other ExoMars instruments.
ADRON-RM uses two 3He proportional counters with a cylindrical shape of about 25 mm in diameter and 55 mm in total length. Each counter is filled with 3He gas under 4 atmospheres of pressure. Each neutron detector will measure two 32-channel spectra. The data will be obtained as routine and passive measurements, which will not be saved but will be immediately transmitted from the instrument to the rover computer. This means that all ADRON-RM measurements will be performed only when the 'Rover Compute Element' is active (daytime).
ADRON-RM is installed inside the ExoMars rover body at the rear balcony. The height above the surface is 0.8 m (2.6 ft).
== Objectives ==
The stated objectives of the ADRON-RM scientific investigation include:
Measurement of the distribution of bulk hydrogen content in the form of free or bound water.
Evaluation of the bulk composition of major soil neutron absorption elements (Cl, Fe, Ti S, etc.)
Monitoring of the neutron component of the natural radiation background and estimation of neutron radiation dose at the Martian surface from Galactic cosmic rays and solar particle events.
The potential to monitor seasonal changes of the neutron environment due to variations of atmospheric and subsurface properties.
== See also ==
Astrobiology
Life on Mars
Water on Mars
== References ==

20
data/en.wikipedia.org/wiki/Accelerograph-0.md Normal file
View File

@ -0,0 +1,20 @@
---
title: "Accelerograph"
chunk: 1/1
source: "https://en.wikipedia.org/wiki/Accelerograph"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:48.291339+00:00"
instance: "kb-cron"
---
An accelerograph can be referred to as a strong-motion instrument or seismograph, or simply an earthquake accelerometer. They are usually constructed as a self-contained box, which previously included a paper or film recorder (an analogue instrument) but now they often record directly on digital media and then the data is transmitted via the Internet.
Accelerographs are useful for when the earthquake ground motion is so strong that it causes the more sensitive seismometers to go off-scale. There is an entire science of strong ground motion, that is dedicated to studying the shaking in the vicinity of earthquakes (roughly within about 100 km of the fault rupture).
Accelerographs record the acceleration of the ground with respect to time. This recording is often called an accelerograms, strong-motion record or acceleration time-history. From this record strong-motion intensity measures (IMs, also called parameters) can be computed. The simplest of which is peak ground acceleration (PGA). Other IMs include Arias intensity, peak ground velocity (PGV), for which the accelerogram needs to be integrated once, peak ground displacement (PGD), for which double integration is required. Often a response spectrum is computed to show how the earthquake would affect structures of different natural frequencies or periods. These observations are useful to assess the seismic hazard of an area.
As well as their engineering applications, accelerograms are also useful for the study earthquakes from a scientific viewpoint. For example, accelerograms can be used to reconstruct the detailed history of rupture along a fault during an earthquake, which would not be possible with seismograms from standard instruments because they would be too far away to resolve the details. An example of an accelerograph array that was established to improve knowledge of near-source earthquake shaking as well as earthquake rupture propagation is the Parkfield Experiment, which involved a massive set of strong motion instrumentation.
Within the accelerograph, there is an arrangement of three accelerometer sensing heads. In recent low-cost instruments these are usually micro-machined (MEMS) chips that are sensitive to one direction. Thus constructed, the accelerometer can measure full motion of the device in three dimensions.
Unlike the continually recording seismometer, accelerometers nearly always work in a triggered mode. That means a level of acceleration must be set which starts the recording process. For analogue and older digital instruments this makes maintenance much more difficult without a direct Internet connection (or some other means of communication). Many trips have been made to accelerometers after a large earthquake, only to find that the memory was filled with extraneous noise, or the instrument was malfunctioning.
Accelerometers are used to monitor the response of structures to earthquakes. Analysis of these records along with the shaking recorded at base of the structure can improve building design, through earthquake engineering.
== References ==

37
data/en.wikipedia.org/wiki/Advanced_Technology_Microwave_Sounder-0.md Normal file
View File

@ -0,0 +1,37 @@
---
title: "Advanced Technology Microwave Sounder"
chunk: 1/1
source: "https://en.wikipedia.org/wiki/Advanced_Technology_Microwave_Sounder"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:44:04.496319+00:00"
instance: "kb-cron"
---
The Advanced Technology Microwave Sounder (ATMS) is a 22-channel scanning microwave radiometer for observation of the Earth's atmosphere and surface. It is the successor to the Advanced Microwave Sounding Unit (AMSU) on NOAA weather satellites. ATMS units have been flown on the Suomi NPP and on the Joint Polar Satellite System.
== Applications ==
ATMS measurements are assimilated into numerical weather prediction models
and atmospheric profiles retrieved by the combination of ATMS and the Cross-track Infrared Sounder on the same satellites are useful for synoptic scale meteorology.
Also, ATMS continues the record from its predecessor instruments MSU and AMSU of measurements in the 5-mm band of oxygen for monitoring of atmospheric temperature trends.
== Instrument characteristics ==
All of the channels are contained within one unit, unlike the AMSU which comprises two instruments (AMSU-A and AMSU-B).
The radiometer's antenna scans underneath the satellite through nadir, and its polarization vector rotates with the scan angle.
The sampling rate satisfies the Nyquist criterion for channels 1-16; thus, images produced from the data are not aliased. However, ATMS images exhibit striping, attributed to receiver gain fluctuations (1/f noise), which can be removed by filtering the data.
Table 1 lists some characteristics of the ATMS channels.
Table 1 ATMS Radiometric characteristics
Notes
"Vertical polarization near nadir" (also known as quasi-vertical) means that for this cross-track scanning arrangement, the E-vector is parallel to the scan direction when the antenna views nadir; "horizontal polarization" means the orthogonal direction.
The NEDT values were measured on the Suomi-NPP unit. Two subsequent units showed similar or slightly better noise performance.
== References ==
== External links ==
https://www.jpss.noaa.gov/atms.html Archived 2022-01-20 at the Wayback Machine

34
data/en.wikipedia.org/wiki/Advanced_Very-High-Resolution_Radiometer-0.md Normal file
View File

@ -0,0 +1,34 @@
---
title: "Advanced Very-High-Resolution Radiometer"
chunk: 1/4
source: "https://en.wikipedia.org/wiki/Advanced_Very-High-Resolution_Radiometer"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:44:05.717614+00:00"
instance: "kb-cron"
---
The Advanced Very-High-Resolution Radiometer (AVHRR) instrument is a space-borne sensor that measures the reflectance of the Earth in five spectral bands that are relatively wide by today's standards. AVHRR instruments are or have been carried by the National Oceanic and Atmospheric Administration (NOAA) family of polar orbiting platforms (POES) and European MetOp satellites. The instrument scans several channels; two are centered on the red (0.6 micrometres) and near-infrared (0.9 micrometres) regions, a third one is located around 3.5 micrometres, and another two the thermal radiation emitted by the planet, around 11 and 12 micrometres.
The first AVHRR instrument was a four-channel radiometer. The final version, AVHRR/3, first carried on NOAA-15 launched in May 1998, acquires data in six channels. The AVHRR has been succeeded by the Visible Infrared Imaging Radiometer Suite, carried on the Joint Polar Satellite System spacecraft.
== Operation ==
Prior to 2025, NOAA had at least two polar-orbiting meteorological satellites in orbit at all times, with one satellite crossing the equator in the early morning and early evening and the other crossing the equator in the afternoon and late evening. The primary sensor on board both satellites was the AVHRR instrument. Morning-satellite data were most commonly used for land studies, while data from both satellites were used for atmosphere and ocean studies. Together they provided twice-daily global coverage, and ensured that data for any region of the earth are no more than six hours old. The swath width, the width of the area on the Earth's surface that the satellite can "see", is approximately 2,500 kilometers (~1,540 mi). The satellites orbit between 833 or 870 kilometers (+/− 19 kilometers, 516–541 miles) above the surface of the Earth.
The highest ground resolution that can be obtained from the current AVHRR instruments is 1.1-kilometer (0.68 mi) per pixel at the nadir.
Data from AVHRR (in its three evolutions) has been collected continuously since 1981.
The primary purpose of these instruments is to monitor clouds and to measure the thermal emission of the Earth. These sensors have proven useful for a number of other applications, however, including the surveillance of land surfaces, ocean state, aerosols, etc. AVHRR data are particularly relevant to study climate change and environmental degradation because of the comparatively long records of data already accumulated (over 20 years). The main difficulty associated with these investigations is to properly deal with the many limitations of these instruments, especially in the early period (sensor calibration, orbital drift, limited spectral and directional sampling, etc.). Whereas, the follow-on VIIRS instruments on JPSS and METimage in MetOp-SG satellites have on-board calibration mechanisms.
The AVHRR instrument also flies on the MetOp series of satellites and as of 2025 these are the only remaining AVHRR instruments. The Metop-B&C MetOp satellites are part of the EUMETSAT Polar System (EPS) run by EUMETSAT, which will be succeeded by MetOp-SG.
== Calibration and validation ==
Remote sensing applications of the AVHRR sensor are based on validation (matchup) techniques of co-located ground observations and satellite observations. Alternatively, radiative transfer calculations are performed. There are specialized codes which allow simulation of the AVHRR observable brightness temperatures and radiances in near infrared and infrared channels.
=== Pre-launch calibration of visible channels (Ch. 1 and 2) ===
Prior to launch, the visible channels (Ch. 1 and 2) of AVHRR sensors are calibrated by the instrument manufacturer, ITT, Aerospace/Communications Division, and are traceable to NIST standards. The calibration relationship between electronic digital count response (C) of the sensor and the albedo (A) of the calibration target are linearly regressed:
A = S * C + I
where S and I are the slope and intercept (respectively) of the calibration regression [NOAA KLM]. However, the highly accurate prelaunch calibration will degrade during launch and transit to orbit as well as during the operational life of the instrument [Molling et al., 2010]. Halthore et al. [2008] note that sensor degradation is mainly caused by thermal cycling, outgassing in the filters, damage from higher energy radiation (such as ultraviolet (UV)), and condensation of outgassed gases onto sensitive surfaces.
One major design constraint of AVHRR instruments is that they lack the capability to perform accurate, onboard calibrations once on orbit [NOAA KLM]. Thus, post-launch on-orbit calibration activities (known as vicarious calibration methods) must be performed to update and ensure the accuracy of retrieved radiances and the subsequent products derived from these values [Xiong et al., 2010]. Numerous studies have been performed to update the calibration coefficients and provide more accurate retrievals versus using the pre-launch calibration.
=== On-orbit individual/few sensor absolute calibration ===
==== Rao and Chen ====
Rao and Chen [1995] use the Libyan Desert as a radiometrically stable calibration target to derive relative annual degradation rates for Channels 1 and 2 for AVHRR sensors on board the NOAA -7, -9, and -11 satellites. Additionally, with an aircraft field campaign over the White Sands desert site in New Mexico, USA [See Smith et al., 1988], an absolute calibration for NOAA-9 was transferred from a well calibrated spectrometer on board a U-2 aircraft flying at an altitude of ~18 km in a congruent path with the NOAA-9 satellite above. After being corrected for the relative degradation, the absolute calibration of NOAA-9 is then passed onto NOAA −7 and −11 via a linear relationship using Libyan Desert observations that are restricted to similar viewing geometries as well as dates in the same calendar month [Rao and Chen, 1995], and any sensor degradation is corrected for by adjusting the slope (as a function of days after launch) between the albedo and digital count signal recorded [Rao and Chen, 1999].

33
data/en.wikipedia.org/wiki/Advanced_Very-High-Resolution_Radiometer-1.md Normal file
View File

@ -0,0 +1,33 @@
---
title: "Advanced Very-High-Resolution Radiometer"
chunk: 2/4
source: "https://en.wikipedia.org/wiki/Advanced_Very-High-Resolution_Radiometer"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:44:05.717614+00:00"
instance: "kb-cron"
---
==== Loeb ====
In another similar method using surface targets, Loeb [1997] uses spatiotemporal uniform ice surfaces in Greenland and Antarctica to produce second-order polynomial reflectance calibration curves as a function of solar zenith angle; calibrated NOAA-9 near-nadir reflectances are used to generate the curves that can then derive the calibrations for other AHVRRs in orbit (e.g. NOAA-11, -12, and -14).
It was found that the ratio of calibration coefficients derived by Loeb [1997] and Rao and Chen [1995] are independent of solar zenith angle, thus implying that the NOAA-9-derived calibration curves provide an accurate relation between the solar zenith angle and observed reflectance over Greenland and Antarctica.
==== Iwabuchi ====
Iwabuchi [2003] employed a method to calibrate NOAA-11 and -14 that uses clear-sky ocean and stratus cloud reflectance observations in a region of the NW Pacific Ocean and radiative transfer calculations of a theoretical molecular atmosphere to calibrate AVHRR Ch. 1. Using a month of clear-sky observations over the ocean, an initial minimum guess to the calibration slope is made. An iterative method is then used to achieve the optimal slope values for Ch. 1 with slope corrections adjusting for uncertainties in ocean reflectance, water vapor, ozone, and noise. Ch. 2 is then subsequently calibrated under the condition that the stratus cloud optical thickness in both channels must be the same (spectrally uniform in the visible) if their calibrations are correct [Iwabuchi, 2003].
==== Vermote and Saleous ====
A more contemporary calibration method for AVHRR uses the on-orbit calibration capabilities of the VIS/IR channels of MODIS. Vermote and Saleous [2006] present a methodology that uses MODIS to characterize the BRDF of an invariant desert site. Due to differences in the spectral bands used for the instruments' channels, spectral translation equations were derived to accurately transfer the calibration accounting for these differences. Finally, the ratio of AVHRR observed to that modeled from the MODIS observation is used to determine the sensor degradation and adjust the calibration accordingly.
==== Others ====
Methods for extending the calibration and record continuity also make use of similar calibration activities [Heidinger et al., 2010].
=== Long-term calibration and record continuity ===
In the discussion thus far, methods have been posed that can calibrate individual or are limited to a few AVHRR sensors. However, one major challenge from a climate point of view is the need for record continuity spanning 30+ years of three generations of AVHRR instruments as well as more contemporary sensors such as MODIS and VIIRS. Several artifacts may exist in the nominal AVHRR calibration, and even in updated calibrations, that cause a discontinuity in the long-term radiance record constructed from multiple satellites [Cao et al., 2008].
==== International Satellite Cloud Climatology Project (ISCCP) method ====
Brest and Rossow [1992], and the updated methodology [Brest et al., 1997], put forth a robust method for calibration monitoring of individual sensors and normalization of all sensors to a common standard. The International Satellite Cloud Climatology Project (ISCCP) method begins with the detection of clouds and corrections for ozone, Rayleigh scatter, and seasonal variations in irradiance to produce surface reflectances. Monthly histograms of surface reflectance are then produced for various surface types, and various histogram limits are then applied as a filter to the original sensor observations and ultimately aggregated to produce a global, cloud free surface reflectance.
After filtering, the global maps are segregated into monthly mean SURFACE, two bi-weekly SURFACE, and a mean TOTAL reflectance maps. The monthly mean SURFACE reflectance maps are used to detect long-term trends in calibration. The bi-weekly SURFACE maps are compared to each other and are used to detect short-term changes in calibration.
Finally, the TOTAL maps are used to detect and assess bias in the processing methodology. The target histograms are also examined, as changes in mode reflectances and in population are likely the result of changes in calibration.
==== Long-term record continuity ====
Long-term record continuity is achieved by the normalization between two sensors. First, observations from the operational time period overlap of two sensors are processed. Next, the two global SURFACE maps are compared via a scatter plot. Additionally, observations are corrected for changes in solar zenith angle caused by orbital drift. Ultimately, a line is fit to determine the overall long-term drift in calibration, and, after a sensor is corrected for drift, normalization is performed on observations that occur during the same operational period [Brest et al., 1997].

27
data/en.wikipedia.org/wiki/Advanced_Very-High-Resolution_Radiometer-2.md Normal file
View File

@ -0,0 +1,27 @@
---
title: "Advanced Very-High-Resolution Radiometer"
chunk: 3/4
source: "https://en.wikipedia.org/wiki/Advanced_Very-High-Resolution_Radiometer"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:44:05.717614+00:00"
instance: "kb-cron"
---
==== Calibration using the moderate-resolution imaging spectroradiometer ====
Another recent method for the absolute calibration of the AHVRR record makes use of the contemporary MODIS sensor onboard NASA's TERRA and AQUA satellites. The MODIS instrument has high calibration accuracy and can track its own radiometric changes due to the inclusion of an onboard calibration system for the VIS/NIR spectral region [MCST]. The following method utilizes the high accuracy of MODIS to absolutely calibrate AVHRRs via simultaneous nadir overpasses (SNOs) of both MODIS/AVHRR and AVHRR/AVHRR satellite pairs as well as MODIS-characterized surface reflectances for a Libyan Desert target and Dome-C in Antarctica [Heidinger et al., 2010]. Ultimately, each individual calibration event available (MODIS/AVHRR SNO, Dome C, Libyan Desert, or AVHRR/AVHRR SNO) is used to provide a calibration slope time series for a given AVHRR sensor. Heidinger et al. [2010] use a second-order polynomial from a least-squares fit to determine the time series.
The first step involves using a radiative transfer model that will convert observed MODIS scenes into those that a perfectly calibrated AVHRR would see. For MODIS/AVHRR SNO occurrences, it was determined that the ratio of AVHRR to MODIS radiances in both Ch1 and Ch2 are modeled well by a second-order polynomial of the radio of MODIS reflectances in channels 17 and 18. Channels 17 and 18 are located in a spectral region (0.94mm) sensitive to atmospheric water vapor, a quantity that affects the accurate calibration of AVHRR Ch. 2. Using the Ch17 to Ch 18 ratio, an accurate guess at the total precipitable water (TPW) is obtained to further increase the accuracy of MODIS to AVHRR SNO calibrations. The Libyan Desert and Dome-C calibration sites are used when MODIS/AVHRR SNOs do not occur. Here, the AVHRR to MODIS ratio of reflectances is modeled as a third-order polynomial using the natural logarithm of TWP from the NCEP reanalysis. Using these two methods, monthly calibration slopes are generated with a linear fit forced through the origin of the adjusted MODIS reflectances versus AVHRR counts.
To extend the MODIS reference back for AVHRRs prior to the MODIS era (pre-2000), Heidinger et al. [2010] use the stable Earth targets of Dome C in Antarctica and the Libyan Desert. MODIS mean nadir reflectances over the target are determined and are plotted versus the solar zenith angle. The counts for AVHRR observations at a given solar zenith angle and corresponding MODIS reflectance, corrected for TWP, are then used to determine what AVHRR value would be provided it had the MODIS calibration. The calibration slope is now calculated.
==== Calibration using direct AVHRR/AVHRR SNOs ====
One final method used by Heidinger et al. [2010] for extending the MODIS calibration back to AVHRRs that operated outside of the MODIS era is through direct AVHRR/AVHRR SNOs. Here, the counts from AVHRRs are plotted and a regression forced through the origin calculated. This regression is used to transfer the accurate calibration of one AVHRRs reflectances to the counts of an un-calibrated AVHRR and produce appropriate calibration slopes. These AVHRR/AVHRR SNOs do not provide an absolute calibration point themselves; rather they act as anchors for the relative calibration between AVHRRs that can be used to transfer the ultimate MODIS calibration.
== Next-generation system ==
Operational experience with the MODIS sensor onboard NASA's Terra and Aqua led to the development of AVHRR's follow-on, VIIRS. VIIRS is currently operating on board the Suomi NPP and NOAA-20 satellites. Whereas EUMETSAT MetOp satellites with AVHRR instruments will be succeeded by MetOp-SG satellites with a European MetImage instrument.
== Launch and service dates ==
== See also ==
Ocean temperature
== References ==

35
data/en.wikipedia.org/wiki/Advanced_Very-High-Resolution_Radiometer-3.md Normal file
View File

@ -0,0 +1,35 @@
---
title: "Advanced Very-High-Resolution Radiometer"
chunk: 4/4
source: "https://en.wikipedia.org/wiki/Advanced_Very-High-Resolution_Radiometer"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:44:05.717614+00:00"
instance: "kb-cron"
---
== Further reading ==
Frey, C.; Kuenzer, C.; Dech, S. (2012). "Quantitative comparison of the operational NOAA AVHRR LST product of DLR and the MODIS LST product V005". International Journal of Remote Sensing. 33 (22): 7165–7183. Bibcode:2012IJRS...33.7165F. doi:10.1080/01431161.2012.699693. S2CID 128981116.
Brest, C.L. and W.B. Rossow. 1992. Radiometric calibration and monitoring of NOAA AVHRR data for ISCCP. International Journal of Remote Sensing. Vol. 13. pp. 235–273.
Brest, C.L. et al. 1997. Update of Radiance Calibrations for ISCCP. Journal of Atmospheric and Oceanic Technology. Vol 14. pp. 1091–1109.
Cao, C. et al. 2008. Assessing the consistency of AVHRR and MODIS L1B reflectance for generating Fundamental Climate Data Records. Journal of Geophysical Research. Vol. 113. D09114. doi:10.1029/2007JD009363.
Halthore, R. et al. 2008. Role of Aerosol Absorption in Satellite Sensor Calibration. IEEE Geoscience and Remote Sensing Letters. Vol. 5. pp. 157–161.
Heidinger, A. K. et al. 2002. Using Moderate Resolution Imaging Spectrometer (MODIS) to calibrate Advanced Very High Resolution Radiometer reflectance channels. Journal of Geophysical Research. Vol. 107. doi:10.1029/2001JD002035.
Heidinger, A.K. et al. 2010. Deriving an inter-sensor consistent calibration for the AVHRR solar reflectance data record. International Journal of Remote Sensing. Vol. 31. pp. 6493–6517.
Iwabuchi, H. 2003. Calibration of the visible and near-infrared channels of NOAA-11 and NOAA-14 AVHRRs by using reflections from molecular atmosphere and stratus cloud. International Journal of Remote Sensing. Vol. 24. pp. 5367–5378.
Loeb, N.G. 1997. In-flight calibration of NOAA AVHRR visible and near-IR bands over Greenland and Antarctica. International Journal of Remote Sensing. Vol. 18. pp. 477–490.
MCST. MODIS Level 1B Algorithm Theoretical Basis Document, Version 3. Goddard Space Flight Center. Greenbelt, MD. December 2005.
Molling, C.C. et al. 2010. Calibrations for AVHRR channels 1 and 2: review and path towards consensus. International Journal of Remote Sensing. Vol. 31. pp. 6519–6540.
NOAA KLM User's Guide with NOAA-N, -N' Supplement. NOAA NESDIS NCDC. Asheville, NC. February 2009.
Rao, C.R.N. and J. Chen. 1995. Inter-satellite calibration linkages for the visible and near-infrared channels of the Advanced Very High Resolution Radiometer on the NOAA-7, −9, and −11 spacecraft. International Journal of Remote Sensing. Vol. 16. pp. 1931–1942.
Rao, C.R.N. and J. Chen. 1999. Revised post-launch calibration of the visible and near-infrared channels of the Advanced Very High Resolution Radiometer on the NOAA-14 spacecraft. International Journal of Remote Sensing. Vol. 20. pp. 3485–3491.
Smith, G.R. et al. 1988. Calibration of the Solar Channels of the NOAA-9 AVHRR Using High Altitude Aircraft Measurements. Journal of Atmospheric and Oceanic Technology. Vol. 5. pp. 631–639.
Vermote, E.F. and N.Z. Saleous. 2006. Calibration of NOAA16 AVHRR over a desert site using MODIS data. Remote Sensing of Environment. Vol. 105. pp. 214–220.
Xiong, X. et al. 2010. On-Orbit Calibration and Performance of Aqua MODIS Reflective Solar Bands. IEEE Transactions on Geoscience and Remote Sensing. Vol 48. pp. 535–546.
== External links ==
What is AVHRR? at National Atlas
Advanced Very High Resolution Radiometer at NOAA
Advanced Very High Resolution Radiometer at USGS
[1] at NASA
[2] at NASA

43
data/en.wikipedia.org/wiki/Alice_(spacecraft_instrument)-0.md Normal file
View File

@ -0,0 +1,43 @@
---
title: "Alice (spacecraft instrument)"
chunk: 1/1
source: "https://en.wikipedia.org/wiki/Alice_(spacecraft_instrument)"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:44:06.912554+00:00"
instance: "kb-cron"
---
Alice is any one of two ultraviolet imaging spectrometers; one used on the New Horizons spacecraft and the other used on the Rosetta spacecraft. Alice is a small telescope with a spectrograph and a special detector with 32 pixels each with 1024 spectral channels detecting ultraviolet light. Its primary role is to determine the relative concentrations of various elements and isotopes in Pluto's atmosphere.
Alice has an off-axis telescope which sends light to a Rowland-circle spectrograph, and the instrument has a field of view of 6 degrees. It is designed to capture airglow and solar occultation at the same time, and has two inputs to allow this.
== Overview ==
Alice uses an array of potassium bromide and caesium iodide type of photocathodes. It detects in the extreme and far ultraviolet spectrum, from 700–2,050 Å (70–205 nm) wavelengths of light, with a spectral resolution of 8–12 Å (0.80–1.20 nm) and a spatial resolution of 500 metres (1,600 feet) per 50 km (31 miles) of altitude.
Alice is intended, among its capabilities, to detect ultraviolet signatures of noble (aka inert) gases including helium, neon, argon, and krypton. Alice should also be able to detect water, carbon monoxide, and carbon dioxide in the ultraviolet. Although the instrument was designed to study Pluto's atmosphere, ALICE will also be tasked with studying Pluto's moon Charon, in addition to various Kuiper-belt objects.
ALICE was built and operated by the Southwest Research Institute for NASA's Jet Propulsion Laboratory. The instrument is powered using a radiation hardened version of an Intel 8052 micro-processor. The instrument uses 32KB of programmable read only memory (PROM), 128 KB of EEPROM, and 32KB of SRAM. The command and data handling electronics are contained across four circuit boards which sit behind the detectors.
ALICE operates in two separate data modes; Pixel List mode (PLM) and Histogram mode (HM). In Pixel List mode, the number of photons/second are recorded. In Histogram mode, the sensor array collects data/photons for a defined period of time. This data is then read as a 2D image. Furthermore, whilst the image is being read from the first memory bank, a second exposure can be started using the secondary memory bank. An advantage of utilising two different data modes is that the method of data collection can be tailored to the scientific goals. PLM provides time resolution, where as HM consistently requires same amount of memory, regardless of exposure length.
== Naming ==
Alice is not an acronym. The name was chosen by principal investigator Alan Stern for personal reasons.
== Alice on New Horizons ==
In August 2018, NASA confirmed, based on results by Alice on the New Horizons spacecraft, the detection of a "hydrogen wall" at the outer edges of the Solar System that was first detected in 1992 by the two Voyager spacecraft which have detected a surplus of ultraviolet light determined to be coming from hydrogen.
The New Horizons version of Alice uses an average power of 4.4 watts and weighs 4.5 kg (9.9 pounds).
== Alice on Rosetta ==
On Rosetta, a mission to a comet, Alice performed ultraviolet spectroscopy to search and quantify the noble gas content in the comet nucleus.
On Rosetta it is a 3.1 kg (6.8 lb) instrument which uses 2.9 watts.
== See also ==
UVS (Juno) (ultraviolet imaging spectrometer on Juno Jupiter orbiter)
Ultraviolet–visible spectroscopy
List of New Horizons topics
== References ==

55
data/en.wikipedia.org/wiki/Alpha_particle_X-ray_spectrometer-0.md Normal file
View File

@ -0,0 +1,55 @@
---
title: "Alpha particle X-ray spectrometer"
chunk: 1/1
source: "https://en.wikipedia.org/wiki/Alpha_particle_X-ray_spectrometer"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:44:08.100387+00:00"
instance: "kb-cron"
---
APXS is also an abbreviation for APache eXtenSion tool, an extension for Apache web servers.
An alpha particle X-ray spectrometer (APXS) is a spectrometer that analyses the chemical element composition of a sample from scattered alpha particles and fluorescent X-rays after a sample is irradiated with alpha particles and X-rays from radioactive sources. This method of analysing the elemental composition of a sample is most often used on space missions, which require low weight, small size, and minimal power consumption. Other methods (e.g. mass spectrometry) are faster, and do not require the use of radioactive materials, but require larger equipment with greater power requirements. A variation is the alpha proton X-ray spectrometer, such as on the Pathfinder mission, which also detects protons.
Over the years several modified versions of this type of instrument such as APS (without X-ray spectrometer) or APXS have been flown: Surveyor 5-7, Mars Pathfinder, Mars 96, Mars Exploration Rover, Phobos, Mars Science Laboratory, the Philae comet lander and the Chandrayaan-3 lunar rover.
APS/APXS devices will be included on several upcoming missions.
== Sources ==
Several forms of radiation are used in APXS. They include alpha particles, protons, and X-rays. Alpha particles, protons, and X-rays are emitted during the radioactive decay of unstable atoms. A common source of alpha particles is curium-244. It emits particles with an energy of 5.8 MeV. X-rays of 14 and 18 keV are emitted in the decay of plutonium-240. The Mars Exploration Rovers' Athena payload uses curium-244 with a source strength of approximately 30 millicuries (1.1 GBq).
== Alpha particles ==
Some of the alpha particles of a defined energy are backscattered to the detector if they collide with an atomic nucleus. The physical laws for Rutherford backscattering in an angle close to 180° are conservation of energy and conservation of linear momentum. This makes it possible to calculate the mass of the nucleus hit by the alpha particle.
Light elements absorb more energy of the alpha particle, while alpha particles are reflected by heavy nuclei nearly with the same energy. The energy spectrum of the scattered alpha particle shows peaks from 25% up to nearly 100% of the initial alpha particles. This spectrum makes it possible to determine the composition of the sample, especially for the lighter elements. The low backscattering rate makes prolonged irradiation necessary, approximately 10 hours.
== Protons ==
Some of the alpha particles are absorbed by the atomic nuclei. The [alpha,proton] process produces protons of a defined energy which are detected. Sodium, magnesium, silicon, aluminium and sulfur can be detected by this method. This method was only used in the Mars Pathfinder APXS. For the Mars Exploration Rovers the proton detector was replaced by a second alpha particle sensor. So it is also called alpha particle X-ray spectrometer.
== X-ray ==
The alpha particles are also able to eject electrons from the inner shell (K- and L-shell) of an atom. These vacancies are filled by electrons from outer shells, which results in the emission of a characteristic X-ray. This process is termed particle-induced X-ray emission and is relatively easy to detect and has its best sensitivity and resolution for the heavier elements.
== Specific instruments ==
Alpha-X, for DAS lander on Phobos 1 and Phobos 2.
ALPHA, for Mars 96 landers. Collaboration between Germany, Russia, and USA.
Alpha Proton X-Ray Spectrometer, for Mars Pathfinder by the Max Planck Institute and the University of Chicago.
Alpha Particle X-ray Spectrometer, for Spirit (MER-A) and Opportunity (MER-B) Mars Exploration Rovers.
Alpha Particle X-ray Spectrometer, for Curiosity (MSL). The principal investigator for Curiosity's APXS is Ralf Gellert, a physicist at the University of Guelph in Ontario, Canada. It was developed and funded by the Canadian Space Agency, with operations also supported by Guelph and United States' space administration.
Alpha Particle X-ray Spectrometer, for Philae, the European Space Agency's lander attached to Rosetta, to study the comet 67P/Churyumov–Gerasimenko.
== Gallery ==
== References ==
== External links ==
Media related to Alpha particle X-ray spectrometer (APXS) at Wikimedia Commons
H. Wänke; J. Brückner; G. Dreibus; R. Rieder; I. Ryabchikov (2001). "Chemical Composition of Rocks and Soils at the Pathfinder Site". Space Science Reviews. 96 (1/4): 317–330. Bibcode:2001SSRv...96..317W. doi:10.1023/A:1011961725645.

37
data/en.wikipedia.org/wiki/Atmosphere-Space_Interactions_Monitor-0.md Normal file
View File

@ -0,0 +1,37 @@
---
title: "Atmosphere-Space Interactions Monitor"
chunk: 1/1
source: "https://en.wikipedia.org/wiki/Atmosphere-Space_Interactions_Monitor"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:44:09.250846+00:00"
instance: "kb-cron"
---
Atmosphere-Space Interactions Monitor (ASIM) is a project led by the European Space Agency to place cameras and X-ray/γ-ray detectors on the International Space Station to observe the upper atmosphere in order to study sprites, jets and elves and terrestrial gamma-ray flashes in connection with thunderstorms. It is hoped that measurements of these phenomena from space will contribute to the understanding of Earth's upper atmosphere.
The ASIM components, originally planned to be completed in 2014, were launched on 2 April 2018 and mounted on the Columbus External Payload Facility on 13 April 2018. Danish tech company Terma A/S is running the technical part of the project for ESA and DTU Space (National Space Institute) from the Technical University of Denmark provides the scientific leadership of the project. Mission operations will be performed by the Belgian User Support and Operations Centre (B.USOC) in Uccle, Belgium.
First results from the measurements revealed that gamma ray bursts form when powerful electric fields course through the atmosphere, just before a lightning bolt travels along the same path. These results were published in July 2019.
== Instruments ==
The ASIM payload has a mass of 314 kg (692 lb) and consists of sub-systems CEPA and DHPU, and two scientific instruments called MXGS and MMIA:
The Columbus External Payload Adapter (CEPA) and Data Handling and Power Unit (DHPU) form the structural and electrical connections, respectively, to the Columbus module.
The Modular X and Gamma Ray Instrument (MXGS) is a pair of terrestrial gamma-ray flash (TGF) detectors. The low-energy detector is sensitive from 15 keV to 400 keV, and the high-energy detector is sensitive from 200 keV to 40 MeV.
The Modular Multi-Imaging Assembly (MMIA) is an optical imaging system capable of observing 12 frames per second continuously in the 777.4 nm and 337 nm bands at 5 nm wide intervals.
== See also ==
European contribution to the International Space Station
Scientific research on the International Space Station
Spacecraft Atmosphere Monitor
== References ==
== External links ==
ASIM.dk
ASIM webpage at the European Space Agency's Human Spaceflight Research portal
ASIM webpage by Terma A/S
ASIM webpage at NASA's Space Station Research & Technology portal

2
data/en.wikipedia.org/wiki/Atmospheric_Chemistry_Suite-0.md
View File

@ -4,7 +4,7 @@ chunk: 1/1
source: "https://en.wikipedia.org/wiki/Atmospheric_Chemistry_Suite" source: "https://en.wikipedia.org/wiki/Atmospheric_Chemistry_Suite"
category: "reference" category: "reference"
tags: "science, encyclopedia" tags: "science, encyclopedia"
date_saved: "2026-05-05T06:25:07.865114+00:00" date_saved: "2026-05-05T09:44:10.564246+00:00"
instance: "kb-cron" instance: "kb-cron"
--- ---

51
data/en.wikipedia.org/wiki/Atmospheric_infrared_sounder-0.md Normal file
View File

@ -0,0 +1,51 @@
---
title: "Atmospheric infrared sounder"
chunk: 1/1
source: "https://en.wikipedia.org/wiki/Atmospheric_infrared_sounder"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:44:11.758008+00:00"
instance: "kb-cron"
---
The atmospheric infrared sounder (AIRS) is one of six instruments flying on board NASA's Aqua satellite, launched on May 4, 2002. The instrument is designed to support climate research and improve weather forecasting.
Working in combination with its partner microwave instrument, the Advanced Microwave Sounding Unit (AMSU-A), AIRS observes the global water and energy cycles, climate variation and trends, and the response of the climate system to increased greenhouse gases. AIRS uses infrared technology to create three-dimensional maps of air and surface temperature, water vapor, and cloud properties. AIRS can also measure trace greenhouse gases such as ozone, carbon monoxide, carbon dioxide, and methane.
AIRS and AMSU-A share the Aqua satellite with the Moderate Resolution Imaging Spectroradiometer (MODIS), Clouds and the Earth's Radiant Energy System (CERES), and the Advanced Microwave Scanning Radiometer-EOS (AMSR-E). Aqua is part of NASA's "A-train," a series of high-inclination, Sun-synchronous satellites in low Earth orbit designed to make long-term global observations of the land surface, biosphere, solid Earth, atmosphere, and ocean.
AIRS data are free and available to the public through the Goddard Earth Sciences Data Information and Services Center.
NASA's Jet Propulsion Laboratory in Pasadena, California, manages AIRS for NASA's Science Mission Directorate in Washington, D.C.
== Technology ==
The term "sounder" in AIRS's name refers to the fact that the instrument measures temperature and water vapor as a function of height (atmospheric sounding).
AIRS measures the infrared brightness coming up from Earth's surface and from the atmosphere. Its scan mirror rotates around an axis along the line of flight and directs infrared energy from the Earth into the instrument. As the spacecraft moves along, this mirror sweeps the ground creating a scan swath that extends roughly 800 kilometers on either side of the ground track. Within the instrument, an advanced, high-resolution spectrometer separates the infrared energy into wavelengths.
Each infrared wavelength is sensitive to temperature and water vapor over a range of heights in the atmosphere, from the surface up into the stratosphere. By having multiple infrared detectors, each sensing a particular wavelength, a temperature profile, or sounding of the atmosphere, can be made. While prior space instruments had only 15 detectors, AIRS has 2378. This greatly improves the accuracy, making it comparable to measurements made by weather balloons.
Thick clouds act like a wall to the infrared energy measured by AIRS. However, microwave instruments on board Aqua can see through the clouds with limited accuracy. Using a special computer algorithm, data from AIRS and the microwave instruments are combined to provide highly accurate measurements in all cloud conditions resulting in a daily global snapshot of the state of the atmosphere.
== AIRS Science and Applications ==
AIRS and its companion microwave sounder AMSU observe the entire atmospheric column from Earth's surface to the top of the atmosphere. The primary data they return is the infrared spectrum in 2378 individual frequencies. The infrared spectrum is rich in information on numerous gases in the atmosphere.
AIRS primary scientific achievement has been to improve weather prediction and provide new information on the water and energy cycle. The instrument also yields information on several important greenhouse gases.
Weather and climate forecasting
AIRS data are used by weather forecasting centers around the world. By incorporating AIRS measurements into their models, forecasters have been able to extend reliable mid-range weather forecasts by more than six hours. AIRS data have also improved forecasts of the location and magnitude of predicted storms.
AIRS temperature and water vapor profiles are available in real time to regional weather forecasters, providing twice-daily weather measurements for the entire Pacific Ocean, once in the morning and once in the evening.
AIRS measurements form a "fingerprint" of the state of the atmosphere for a given time and place that can be used as a climate data record for future generations. They have become important tools for understanding current climate and increasing the ability to predict the future.
Atmospheric Composition, Greenhouse Gases, and Air Quality
AIRS maps the concentration of carbon dioxide and methane globally. Its ability to provide simultaneous observations of the Earth's atmospheric temperature, water vapor, ocean surface temperature, and land surface temperature and infrared spectral emissivity, as well as humidity, clouds and the distribution of greenhouse gases, makes AIRS/AMSU a very useful space instrument to observe and study the response of the atmosphere to increased greenhouse gases.
The instrument can detect carbon monoxide emissions from the burning of plant materials and animal waste by humans in rainforests and large cities. It can follow giant plumes of this gas moving across the planet from these large burns, allowing scientists to better monitor pollution transport patterns.
AIRS provides a global daily 3-D view of Earth's ozone layer, showing how ozone is transported. The instrument also gives scientists their best view of atmospheric ozone in the Antarctic region during the polar winter.
AIRS is also able to identify concentrations of sulphur dioxide and dust.
== References ==
This article incorporates public domain material from How Airs Works. National Aeronautics and Space Administration. (and other articles).
== External links ==
AIRS homepage at JPL
AIRS/AMSU/HSB on the Aqua mission
NASA's Earth Observing System
Aqua Project Science Page
Global Climate Change: NASA's Eyes on the Earth Archived December 14, 2012, at the Wayback Machine
NASA Earth Observatory

43
data/en.wikipedia.org/wiki/Automatic_picture_transmission-0.md Normal file
View File

@ -0,0 +1,43 @@
---
title: "Automatic picture transmission"
chunk: 1/2
source: "https://en.wikipedia.org/wiki/Automatic_picture_transmission"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:44:12.925851+00:00"
instance: "kb-cron"
---
The Automatic Picture Transmission (APT) system is an analog image transmission system developed for use on weather satellites. It was introduced in the 1960s and over four decades has provided image data to relatively low-cost user stations at locations in most countries of the world. A user station anywhere in the world can receive local data at least twice a day from each satellite as it passes nearly overhead.
== Transmission ==
=== Structure ===
The broadcast transmission is composed of two image channels, telemetry information, and synchronization data, with the image channels typically referred to as Video A and Video B. All this data is transmitted as a horizontal scan line. A complete line is 2080 pixels long, with each image using 909 pixels and the remainder going to the telemetry and synchronization. Lines are transmitted at 2 per second, which equates to a 4160 words per second, or 4160 baud.
=== Images ===
On NOAA POES system satellites, the two images are 4 km / pixel smoothed 8-bit images derived from two channels of the advanced very-high-resolution radiometer (AVHRR) sensor. The images are corrected for nearly constant geometric resolution prior to being broadcast; as such, the images are free of distortion caused by the curvature of the Earth.
Of the two images, one is typically long-wave infrared (10.8 micrometers) with the second switching between near-visible (0.86 micrometers) and mid-wave infrared (3.75 micrometers) depending on whether the ground is illuminated by sunlight. However, NOAA can configure the satellite to transmit any two of the AVHRR's image channels.
=== Synchronization and telemetry ===
Included in the transmission are a series of synchronization pulses, minute markers, and telemetry information.
The synchronization information, transmitted at the start of each video channel, allows the receiving software to align its sampling with the baud rate of the signal, which can vary slightly over time. The minute markers are four lines of alternating black then white lines which repeat every 60 seconds (120 lines).
The telemetry section is composed of sixteen blocks, each 8 lines long, which are used as reference values to decode the image channels. The first eight blocks, called "wedges," begin at 1/8 max intensity and successively increase by 1/8 to full intensity in the eighth wedge, with the ninth being zero intensity. Blocks ten through fifteen each encode a calibration value for the sensor. The sixteenth block identifies which sensor channel was used for the preceding image channel by matching the intensity of one of the wedges one through six. Video channel A typically matches either wedge two or three, channel B matches wedge four.
The first fourteen blocks should be identical for both channels. The sixteen telemetry blocks repeat every 128 lines, and these 128 lines are referred to as a frame.
=== Broadcast signal ===
The signal itself is a 256-level amplitude modulated 2400Hz subcarrier, which is then frequency modulated onto the 137 MHz-band RF carrier. Maximum subcarrier modulation is 87% (±5%), and overall RF bandwidth is 34 kHz. On NOAA POES vehicles, the signal is broadcast at approximately 37dBm (5 watts) effective radiated power.
== Receiving images ==
An APT signal is continuously broadcast, with reception beginning at the start of the next line when the receiver is within radio range. Images can be received in real-time by relatively unsophisticated, inexpensive receivers during the time the satellite is within radio range, which typically lasts 8 to 15 minutes.
As of 2004 there were almost 5,000 APT receiving stations registered with the World Meteorological Organization (WMO). It is unclear what percent of the total user-base this represents, since registration is not a requirement, and was only available after 1996.
=== Software-defined radio ===
A popular device used to receive the APT signals is a Software-defined radio (SDR). Many of these are cheap, and allow reception of frequencies from 500 kHz up to- in some cases- 7 GHz. Open-source software is available to set up the receiver, and since many have Plug and play it makes reception of APT accessible to the non-specialist. A cheap and reliable example is the RTL-SDR v3/v4, as it can reach up to 2.56 MHz in bandwidth which more than adequate for the reception of the APT signals (34 kHz). The SDR can decode on all the modulations, in this case it would be the NFM.
=== Radio receiver ===
The bandwidth required to receive APT transmissions is approximately 34 kHz. Most older scanners (police and fire type receivers) are the standard 15 kHz bandwidth which were designed to support voice transmissions. Newer VHF general coverage receivers are equipped with multiple IF bandpasses; some are, but not limited to: 6 kHz, 15 kHz 50 kHz & 230 kHz(broadcast FM). Use of a receiver with too narrow a bandwidth will produce pictures that are saturated in the blacks and whites, as well as possible inversion. Too wide, and the noise floor of the receiver will be too high to acquire a good picture. For the amateur enthusiast, a computer controller receiver is the best option to allow the software to automatically tune and set the required modes for proper reception. There are also dedicated APT receivers made specifically for computer control and APT reception. Specifically, ICOM PCR1000, PCR1500 & PCR2500 will produce excellent results. Searching on the web for "NOAA APT (RECEPTION or RECEIVER)" will produce a wealth of information on receivers, software, and antennas.
=== Antenna ===
APT images from weather satellites can be received with a right-hand circular polarized, 137 MHz antenna. Normally, there is no need to have the antenna follow the satellite and a fixed position antenna will provide good results.
The three most frequently recommended antennas are the crossed dipole, V dipole antenna or the quadrifilar helix antenna (QHA or QFH).

43
data/en.wikipedia.org/wiki/Automatic_picture_transmission-1.md Normal file
View File

@ -0,0 +1,43 @@
---
title: "Automatic picture transmission"
chunk: 2/2
source: "https://en.wikipedia.org/wiki/Automatic_picture_transmission"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:44:12.925851+00:00"
instance: "kb-cron"
---
=== Displaying the images ===
Years ago, to receive APT images, a specialized decoder was required in addition to the receiver to display or print images, much like HF WEFAX (serving the maritime community). Often both receiver and decoder were combined into one unit.
Nowadays, with the advent of personal computers, all that is required is dedicated software such as WXtoIMG (many of which offer "free" versions [1]) and a sound card. The sound card acquires and digitizes the slow scan video (in the audible range) coming from the speaker, phones, or line-out of the receiver, and then the software will process the various visible and infrared channels of the AVHRR sensor.
==== Enhanced images ====
Since each channel of the AVHRR sensor is sensitive to only one wavelength of light, each of the two images is luminance only, also known as grayscale. However, different materials tend to emit or reflect with a consistent relative intensity. This has enabled the development of software that can apply a color palette to the images which simulates visible light coloring. If the decoding software knows exactly where the satellite was, it can also overlay outlines and boundaries to help in utilizing the resulting images.
== History ==
Developed by the National Earth Satellite Service
Tested on TIROS-8, launched December 21, 1963
Nimbus 1, launched August 28, 1964, was the first application satellite
First NOAA polar-orbiting vehicle to use it was TIROS-N, launched on October 13, 1978, and it has flown on all NOAA polar-orbiting vehicles since then.
Also flown on the Soviet METEOR, Sich, Resurs and Okean weather satellites.
== Current status ==
There are currently no satellites that transmit APT. NOAA-15, the last satellite to transmit APT, was decommissioned on August 19, 2025. Previous satellites such as Sich-1 and the METEOR series of satellites operated on APT, but have since reached the end of their operational lifespans.
== Future ==
With improvements in electronics, analog transmission systems have given way to digital transmissions systems. NOAA-19, called NOAA-N' prior to its launch on 6 February 2009, is the last satellite to carry an APT system. The MetOp program, a collaboration between NOAA and EUMETSAT, as well as the Russian METEOR-M program have switched to Low Rate Picture Transmission (LRPT) for their new polar-orbit satellites.
Since the NOAA-20 satellite, most of the NOAA satellites have their lowest transmission frequency at 7812 MHz in the C Band, making reception by the general public impractical since unlike older schemes (APT, LRPT) they require a dish antenna with a specialised antenna feed etc.
== See also ==
EUMETSAT
Radiofax
Weather satellites
== References ==
== External links ==
POES spacecraft status NOAA
NEODAAS Dundee Satellite Receiving Station
APT images received at Thirteen Island Lake Ontario Canada
Decoding Software for APT Satellite Reception

2
data/en.wikipedia.org/wiki/CCOR-2-0.md
View File

@ -4,7 +4,7 @@ chunk: 1/1
source: "https://en.wikipedia.org/wiki/CCOR-2" source: "https://en.wikipedia.org/wiki/CCOR-2"
category: "reference" category: "reference"
tags: "science, encyclopedia" tags: "science, encyclopedia"
date_saved: "2026-05-05T09:40:15.017695+00:00" date_saved: "2026-05-05T09:43:00.904773+00:00"
instance: "kb-cron" instance: "kb-cron"
--- ---

103
data/en.wikipedia.org/wiki/COCONet-0.md Normal file
View File

@ -0,0 +1,103 @@
---
title: "COCONet"
chunk: 1/1
source: "https://en.wikipedia.org/wiki/COCONet"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:49.481321+00:00"
instance: "kb-cron"
---
Continuously Operating Caribbean GPS Observational Network (COCONet) was a global positioning system (GPS) observation network that spanned across the Caribbean and the neighboring area It was part of UNAVCO (University Navstar Corporation). UNAVCO and IRIS (Incorporated Research Institutions for Seismology) Consortium later merged to create EarthScope Consortium in 2023.
The project was initiated after the devastation of the 2010 Haiti Earthquake, which was a 7.0 Mw earthquake. Starting from 2011, UNAVCO built and operated COCONet for the National Science Foundation (NSF). It was a network of continuous GPS meteorology or cGPS/Met sites. Along with the NSF-funded TLALOCNet GPS network in Mexico, the two networks of cGPS-Met instrumentation were available to support research in Mexico, Central America, and the Caribbean.
== Function ==
The purpose of COCOnet was to:
Provide details on the tectonics of the entire Caribbean region.
Enhance atmospheric observations that can be used for testing and validation of climate and weather models.
Improve the analysis of local geodetic (the science of planetary measurement) measurements by providing access to an integrated backbone electronic network of reference stations.
Increase our ability to model and predict the natural hazards (earthquakes, hurricanes and so on) in the region.
To accomplish the function of climate modeling and other objectives., the network also had tide gauges in Mexico, Jamaica, the Dominican Republic, and Panama. In terms of atmospheric measurements and related goals, the COCOnet stations were able to assist the Constellation Observing System for Meteorology, Ionosphere, and Climate of the University Corporation for Atmospheric Research (UCAR/COSMIC) as it created continuous estimates of precipitable water vapor.
== Stations ==
A notable station is the station on the isolated Isla del Coco (Cocos Island), Costa Rica, in that it is the only GPS station continuously tracking the Cocos Plate, as it passes underneath the Caribbean Plate, at a rate of 78 millimeters (mm) per year. Because the island is the only land mass of the Cocos Plate that is above sea level, this was the only place where Cocos Plate motion observations could be measured in this GPS network. A continuous GPS station was built and instrumented on the island in May 2011. Data from the station show a steady motion of the island at a speed of 90.9±1.5mm/yr. or approximately 90 millimeters a year.
== Partnerships ==
The following organizations were members of the partnership for the network when it had existed:
Universidad Politécnica de Ingeniería (UPI) (Honduras)
Puerto Rico Seismic Network
Instituto Sismológico Universitario (ISU) - Autonomous University of Santo Domingo (UASD)
Oficina Nacional de Meteorología (ONAMET) (Dominican Republic)
University of Zulia, Venezuela
National Meteorological Service (Mexico)
Nicaraguan Institute of Territorial Studies Intra-Americas Studies of Climate Processes
Jamaica Climate Service
Pennsylvania State University
State University Haiti
Colombian Institute for Hydrology, Meteorology and Environmental Studies
Geographic Institute of Venezuela Simón Bolívar
Venezuelan Foundation for Seismological Research
National Geographic Institute of El Salvador
University of Texas, Arlington
Camagüey Meteorological Center (Camagüey, Cuba)
National Geographic Institute of Honduras
Panama Canal Authority
Caribbean Community Climate Change Centre, Belize
Volcanological and Seismological Observatory of Costa Rica
University of the West Indies - Seismic Research Center
Institut de Physique du Globe de Paris
Montserrat Volcano Observatory (Caribbean island of Montserrat)
Purdue University
University of Puerto Rico, Mayagüez
Meteorological Service of the Netherlands, Antilles, and Aruba
University of Arizona
University of Technology, Jamaica
Colombian Institute of Geology and Mining
National Geographic Institute, Guatemala City
National Autonomous University of Honduras
University Corporation for Atmospheric Research (UCAR)
National Autonomous University of Mexico (UNAM)
National Oceanic and Atmospheric Administration (NOAA)
U.S. Geological Survey (USGS)
National Aeronautics and Space Administration (NASA)
National Science Foundation (NSF)
Bahamas Department of Meteorology
Meteorological Department Curaçao
Earthquake Unit, Jamaica
Real Estate Jurisdiction of the Dominican Republic
Universidad Nacional Pedro Henriquez Ureña (UNPHU) (Dominican Republic)
== Meetings and workshops ==
COCOnet had held for workshops. The meetings can help to understand the history of the project:
The original COCONet project proposal was covered in three workshops:
The first one was planned to be held in San Juan, Puerto Rico (February 3–4, 2011)
The second meeting was to be held in Port-of-Spain, Trinidad, Republic of Trinidad & Tobago (June 28–29, 2011) with one of several goals being for Caribbean network operators to address the specifics of choosing existing and new stations.
The third COCONet workshop focused primarily on longer-term operations and maintenance for GPS stations installed in the Caribbean, and related issues.
== Data centers ==
The following were the data centers:
Servicio Geólogico Colombiano (SGC) (Columbian Geological Survey) - Regional Data Center
Caribbean Institute for Meteorology & Hydrology (CIMH), Barbados - Regional Data Center
Instituto Nicaragüense de Estudios Territoriales (INETER) - Regional Data Center
== Last publications and workshops ==
The last publications and workshops of COCOnet were the following:
Geirsson, Halldor; Lafemina, Peter C.; Demets, Charles; Hernandez, Douglas Antonio; Mattioli, Glen S.; Rogers, Robert; Rodriguez, Manuel; Marroquin, Griselda; Tenorio, Virginia (2015). "The 2012 August 27 M w7.3 El Salvador earthquake: Expression of weak coupling on the Middle America subduction zone". Geophysical Journal International. 202 (3): 1677–1689. doi:10.1093/gji/ggv244.
Douilly, R.; Aochi, H.; Calais, E.; Freed, A. M. (2015). "Three-dimensional dynamic rupture simulations across interacting faults: The Mw7.0, 2010, Haiti earthquake" (PDF). Journal of Geophysical Research: Solid Earth. 120 (2): 1108–1128. doi:10.1002/2014JB011595.
The last workshop was COCONet - Results, Sustainability, and Capacity Building, which had been held May 3–5, 2016 in Punta Cana, Dominican Republic.
== References ==

31
data/en.wikipedia.org/wiki/Cachecam-0.md Normal file
View File

@ -0,0 +1,31 @@
---
title: "Cachecam"
chunk: 1/1
source: "https://en.wikipedia.org/wiki/Cachecam"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:44:14.125067+00:00"
instance: "kb-cron"
---
The Cachecam, a photographic camera, is mounted inside the rover underbelly, at the top of the sample cache of NASA's Perseverance rover mission to Mars.
== Overview ==
The Cachecam's main job is to photograph the top of a sample tube after the sample is gathered in order to verify acquisition before the tube is sealed. It may also take images at other points in the sample processing.
The Cachecam will acquire color images of the samples using its 20 megapixel CMOS detector.
It is considered an engineering camera. The other sets of engineering cameras on the Mars 2020 Rover are the Navcams and hazcams.
== See also ==
Astrionics
List of NASA cameras on spacecraft
Mars rover
== References ==
== External links ==
The Cameras on the Mars 2020 Rover

26
data/en.wikipedia.org/wiki/Cheiroscope-0.md Normal file
View File

@ -0,0 +1,26 @@
---
title: "Cheiroscope"
chunk: 1/1
source: "https://en.wikipedia.org/wiki/Cheiroscope"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:02.140168+00:00"
instance: "kb-cron"
---
A cheiroscope (also: chiroscope) is an optical device consisting of a viewing instrument equipped with a drawing pad, with the viewing instrument set up as a haploscope that blends a left and/or right image into view over the drawing.
The cheiroscope was presented in an article published in 1929. The author E. E. Maddox writes that compared to the earlier amblyoscope,
"[t]he cheiroscope approaches the problem from a different and complementary angle, on the simple principle of pressing the hand into service to educate the eye."
A cheiroscope can be operated in different manners. For example, using a cheiroscope, a line image can be presented to one eye and the image of a blank sheet to the other eye, and the subject is intended to make a drawing that reproduces the line image.
The cheiroscope is used for diagnostic purposes to test binocular vision, to assess certain conditions of strabism in particular related to binocular stability and alignment, cyclotropia, and the presence and extent of suppression. It can also be used in vision therapy to train amblyopic subjects in desuppression and eye–hand coordination.
A stereoscope can be modified to function as a cheiroscope.
== See also ==
Diplopia
Amblyopia
Orthoptist
== References ==

53
data/en.wikipedia.org/wiki/Chemistry_and_Camera_complex-0.md Normal file
View File

@ -0,0 +1,53 @@
---
title: "Chemistry and Camera complex"
chunk: 1/1
source: "https://en.wikipedia.org/wiki/Chemistry_and_Camera_complex"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:44:15.324222+00:00"
instance: "kb-cron"
---
Chemistry and Camera complex (ChemCam) is a suite of remote sensing instruments on Mars for the Curiosity rover. As the name implies, ChemCam is actually two different instruments combined as one: a laser-induced breakdown spectroscopy (LIBS) and a Remote Micro Imager (RMI) telescope. The purpose of the LIBS instrument is to provide elemental compositions of rock and soil, while the RMI will give ChemCam scientists high-resolution images of the sampling areas of the rocks and soil that LIBS targets. The LIBS instrument can target a rock or soil sample from up to 7 m (23 ft) away, vaporizing a small amount of it with about 30 5-nanosecond pulses from a 1067 nm infrared laser and then observing the spectrum of the light emitted by the vaporized rock.
== Overview ==
ChemCam has the ability to record up to 6,144 different wavelengths of ultraviolet, visible, and infrared light. Detection of the ball of luminous plasma is done in the visible, near-UV and near-infrared ranges, between 240 nm and 800 nm. The first initial laser testing of the ChemCam by Curiosity on Mars was performed on a rock, N165 ("Coronation" rock), near Bradbury Landing on August 19, 2012.
Using the same collection optics, the RMI provides context images of the LIBS analysis spots. The RMI resolves 1 mm (0.039 in) objects at 10 m (33 ft) distance, and has a field of view covering 20 cm (7.9 in) at that distance. The RMI has also been used to take images of distant geologic features and landscapes.
The ChemCam instrument suite was developed by the Los Alamos National Laboratory and the French CESR laboratory. The flight model of the mast unit was delivered from the French CNES to Los Alamos National Laboratory.
== Instrumentation ==
=== Laser Induced Breakdown Spectroscopy ===
ChemCam marks the first use of Laser Induced Breakdown Spectroscopy (LIBS) as part of a planetary science mission. The laser is positioned on the mast of the Curiosity rover and focused by the telescope that also resides on the mast, while the spectrometer is housed in the rover's body. Typically, the laser fires 30 shots at a single point, gathering spectroscopic readings from the vaporized rock for each laser shot, and samples multiple points on a chosen target. For bedrock observations, the first 5 shots of a point are discarded as they are considered to be contaminated by Martian dust. The remaining shots of one point are averaged together for chemical composition calculations. It is common for there to be 9 or 10 points of analysis on any given target, but this is not always the case. Some targets have as few as 4 points while some targets have 20 points.
=== Remote Micro-Imager ===
The Remote Micro-Imager is primarily used to capture high-resolution, black and white images of ChemCam targets for context and documentation. Usually, an image of the target of interest is captured before and after the laser is fired. Often, the laser makes "LIBS pits" that can be visible in the RMI to show where the laser sampled specifically on a particular target. The resolution of the RMI is higher than the black and white navigational camera (navcam) and the color mast cameras (mastcam).
==== Long Distance Imaging ====
The RMI is primarily used to obtain close-up images of targets sampled by ChemCam, but it can also be used to gather high-resolution images of distant outcrops and landscapes. The RMI has a higher spatial resolution than the mastcam M100 camera, which is a color camera also capable of imaging nearby objects or distant geologic features. The RMI has been used by the mission for reconnaissance of up-coming terrain as well as imaging distant features such as the rim of Gale Crater.
== Scientific contributions ==
ChemCam has been used, in conjunction with other instruments of the Curiosity rover, to make advancements in understanding the chemical composition of rocks and soils on Mars. LIBS makes it possible to detect and quantify the major oxides: SiO2, Al2O3, FeOT, MgO, TiO2, CaO, Na2O, and K2O of bedrock targets. There are distinguishable geologic units determined from orbital analyses that have been confirmed by averaged bedrock compositions determined from ChemCam and other instruments aboard Curiosity. The identification is based on multivariate PLS and PCA models classified using SIMCA with calibration models made using "The Unscrambler" software. ChemCam has also quantified soil chemistry. ChemCam has seen two distinct soil types at Gale crater: a fine-grained mafic material that is more representative of global Martian soils or dust and a coarse-grained felsic material that originates from local Gale crater bedrock. ChemCam has the capability to measure minor or trace elements such as lithium, manganese, strontium, and rubidium. ChemCam has measured MnO up to 25 wt% in fracture fills that suggests Mars was once a more oxygenating environment.
== Images ==
== See also ==
== References ==
== External links ==
Media related to Chemistry and Camera complex (ChemCam) at Wikimedia Commons
Curiosity home page at NASA.gov
How Does ChemCam Work? at MSL-ChemCam.com

17
data/en.wikipedia.org/wiki/Coherence_scanning_interferometry-0.md Normal file
View File

@ -0,0 +1,17 @@
---
title: "Coherence scanning interferometry"
chunk: 1/1
source: "https://en.wikipedia.org/wiki/Coherence_scanning_interferometry"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:03.323582+00:00"
instance: "kb-cron"
---
Coherence scanning interferometry (CSI) is any of a class of optical surface measurement methods wherein the localization of interference fringes during a scan of optical path length provides a means to determine surface characteristics such as topography, transparent film structure, and optical properties. CSI is currently the most common interference microscopy technique for areal surface topography measurement. The term "CSI" was adopted by the International Organization for Standardization (ISO).
The technique encompasses but is not limited to instruments that use spectrally broadband, visible sources (white light) to achieve interference fringe localization. CSI uses either fringe localization alone or in combination with interference fringe phase, depending on the surface type, desired surface topography repeatability and software capabilities. The table below compiles alternative terms that conform at least in part to the above definition.
== References ==

2
data/en.wikipedia.org/wiki/Colorimeter_(chemistry)-0.md
View File

@ -4,7 +4,7 @@ chunk: 1/1
source: "https://en.wikipedia.org/wiki/Colorimeter_(chemistry)" source: "https://en.wikipedia.org/wiki/Colorimeter_(chemistry)"
category: "reference" category: "reference"
tags: "science, encyclopedia" tags: "science, encyclopedia"
date_saved: "2026-05-05T06:25:16.738183+00:00" date_saved: "2026-05-05T09:43:04.500380+00:00"
instance: "kb-cron" instance: "kb-cron"
--- ---

21
data/en.wikipedia.org/wiki/Dipleidoscope-0.md Normal file
View File

@ -0,0 +1,21 @@
---
title: "Dipleidoscope"
chunk: 1/1
source: "https://en.wikipedia.org/wiki/Dipleidoscope"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:05.701644+00:00"
instance: "kb-cron"
---
A dipleidoscope is an instrument used to determine true noon; its name comes from the Greek for double image viewer. It consists of a small telescope and a prism that creates a double image of the sun. When the two images overlap, it is local true noon. The instrument is capable of determining true noon to within ten seconds.
The dipleidoscope was invented by Giovanni Battista Amici in the first half of the 19th century.
Edward John Dent, a chronometer and clockmaker in London, was working in the 1830s on a simple contrivance that would allow the public to set clocks correctly based on the transit of the sun (more complex and expensive transit telescopes had been developed by Ole Rømer in 1690). By 1840, he felt he had come to a suitable design using shadows, however when he communicated his ideas to J.M. Bloxam (a barrister), he found he had also been working on his own design using reflections, which Dent felt was superior. The two formed a partnership and worked together on the device, and after a further 2 years of work, they finalised the design and patented it (GB Patent 9793 of 1843), with Dent manufacturing and selling it as Dent's Dipleidoscope. The instrument could use the moon as well as the sun and when correctly calibrated and aligned the accuracy was said to be less than a second. Dent exhibited the device at the Great Exhibition of 1851. After Edward Dent died in 1853, his son Frederick William Dent took over manufacture.
The significance of this device relates in part to the development of the railways, when an absolute knowledge of the time became more important, whereas previously it was often sufficient that an entire rural community would use the parish clock, and this would periodically be set by 'the announcement of the guard of the mail coach' or similar. The instrument came with a detailed instruction booklet, which had a substantial section on correcting local time to Greenwich Mean Time (as used by the railways).
== References ==
== External links ==
A dipleidoscope of the National Observatory of Athens

24
data/en.wikipedia.org/wiki/Disappearing-filament_pyrometer-0.md Normal file
View File

@ -0,0 +1,24 @@
---
title: "Disappearing-filament pyrometer"
chunk: 1/1
source: "https://en.wikipedia.org/wiki/Disappearing-filament_pyrometer"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:06.929588+00:00"
instance: "kb-cron"
---
The disappearing-filament pyrometer is an optical pyrometer, in which the temperature of a glowing incandescent object is measured by comparing it to the light of a heated filament. Invented independently in 1901 by Ludwig Holborn and Ferdinand Kurlbaum in Germany and Everett Fleet Morse in the United States, it was the first device which could measure temperatures above 1000 °C. Disappearing filament pyrometers have been used to measure temperatures between about 600 °C and 3000 °C. Like other optical pyrometers they are used to measure the temperature of objects too hot for contact thermometers, such as molten metals. Widely used in the steel and ceramics industries as well as for research, they have been almost totally superseded by electronic spectral-band pyrometers.
The simplest design has optics like a Keplerian telescope. A thin wire (filament), placed at the focal plane of the objective lens, is heated by electric current. When seen through the eyepiece, the wire appears silhouetted in front of the hot luminous object under investigation. The user compares the brightness of the glowing filament with the object behind, and adjusts the current through the filament until it seems to "disappear" in front of the glowing object. At that point the filament and object are at the same temperature. The user then reads the temperature off the filament current control dial, which is calibrated by the filament's current-vs-temperature curve. or in some instruments from a current-vs-temperature table.
The filament seems to "disappear" against the background of the object because two objects at the same temperature have the same black-body spectrum.
In other designs the current through the filament is kept constant, and the radiation allowed through from the target object is varied with calibrated attenuating wedges in the optical path, or a prism is used to place the images of the target object and a calibrated glowing surface next to each other, e.g. as a disk inside a ring.
Many disappearing-filament pyrometers use a red filter. The combination of the filter and the human eye's response only allows through a narrow band of red wavelengths, so the luminosity comparison is made over only a narrow band of wavelengths. This reduces errors due to the target and filament not having identical emission spectra. For very hot objects, additional filters can be used to protect the eye from excessive light. The resolution of the instrument depends somewhat on the operator, but with a skilled operator a resolution of 10 °C for temperatures up to 2000 °C can be achieved.
Disappearing-filament pyrometers can be used only if the object under study emits visible light similar to a hot black body; this means that its temperature must be high enough (around 600 °C and up) and the object must not be fully transparent or highly reflective. For good accuracy, the object should appear dark gray or black when cold.
== See also ==
Mirror galvanometer
Glossmeter
== References ==

16
data/en.wikipedia.org/wiki/Dynameter-0.md Normal file
View File

@ -0,0 +1,16 @@
---
title: "Dynameter"
chunk: 1/1
source: "https://en.wikipedia.org/wiki/Dynameter"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:08.099108+00:00"
instance: "kb-cron"
---
A dynameter is an instrument that measures the magnification of a telescope. It is usually a double-image micrometer used to measure the diameter of the image of the object glass. The magnifying power is found by comparing the actual diameter of the glass with the measured diameter of the image of the glass.
== References ==
Dictionary entry for dynameter.
The DYNAMETER, archived from the original

35
data/en.wikipedia.org/wiki/Erygmascope-0.md Normal file
View File

@ -0,0 +1,35 @@
---
title: "Erygmascope"
chunk: 1/1
source: "https://en.wikipedia.org/wiki/Erygmascope"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:09.276943+00:00"
instance: "kb-cron"
---
An erygmascope is a late 19th-century electric lighting apparatus designed for the examination of the strata of earth traversed by boring apparatus.
It consisted of a very powerful incandescent lamp enclosed in a metallic cylinder. One of the two semi-cylindrical sides constitutes the reflector, and the other, which is of thick glass, allows the passage of light, which illuminates the strata of earth traversed by the instrument. The base, which is inclined at an angle of 45°, is an elliptical mirror, and the top, of straight section, is open in order to permit the observer standing at the mouth of the well, and provided with a powerful spyglass, to see in the mirror the image of the geological layers or of the structure of crystalline rocks. It somewhat resembles to a kind of inverted semi-periscope aimed to look at earth downwards. The lamp is so mounted that its upwardly emitted rays are intercepted.
The whole apparatus was suspended from a long cable, formed of two conducting wires, which winds around a windlass with metallic journals which are electrically insulated. These journals communicate, through the intermedium of two friction springs, with the conductors on the one hand and, on the other, with the poles of a portable battery. This permits of lowering and raising the apparatus at will, without derangement, and without its being necessary to interrupt the light and the observation.
The erygmascope was described in an 1891 edition of the Scientific American Supplement; To what extent it was put to practical use is unknown.
== Etymology ==
The etymology of the term remains enigmatic, but could originate from a Greek word ήρυγμα meaning "trench". However, a shaft should be translated in Greek as πηγάδι (pegadi), and a water well as φρέαρ (phrear). The origin of the suffix scope is classical for an optical instrument: from σκοπεῖν (skopein), meaning to look at.
== In art and popular culture ==
Sonic and visual artistic collaboration between Tracy Hill, artist, Ralph Hoyte, poet and writer, and Phill Phelps, musician, working in partnership with Lancashire Wildlife Trust and City of Trees. It symbolizes the curiosity of the three artists for looking beyond the surface whilst working on their opus Stratum.
== See also ==
Borehole image logs
Optical borehole imager used for geological mapping and breakouts localization
Well logging
== References ==
== External links ==
"Electric Erygmascope. Scientific American Supplement, No. 787, January 31, 1891. From the Project Gutenberg. EBook #14009. Release Date: November 10, 2004". Gutenberg.org. November 10, 2004. Retrieved 3 April 2021.

148
data/en.wikipedia.org/wiki/Galperin_configuration-0.md Normal file
View File

@ -0,0 +1,148 @@
---
title: "Galperin configuration"
chunk: 1/1
source: "https://en.wikipedia.org/wiki/Galperin_configuration"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:50.630864+00:00"
instance: "kb-cron"
---
Galperin configuration are a particular configuration of sensing elements found in a class of seismic instruments measuring ground motion and are named after Soviet seisomologist Evsey Iosifovich Galperin, who introduced it in 1955 for petroleum exploration.
== Description ==
Common triaxial seismometers provide signal outputs in three orthogonal axes oriented towards east–west (E), north–south (N) and up-down (Z), i.e. in the Cartesian coordinate system. In contrast, the Galperin configuration consists of three orthogonal axes (U, V, W) that are oriented at precisely the same angle with respect to the horizontal plane (α=35.26°). The projection of all three axes onto the horizontal plane are all separated by 120°, which results in the "symmetric triaxial" design. The recordings acquired with the Galperin configuration are brought to the Cartesian coordinate system by the following coordinate transformation, where β=30°:
[
E
N
Z
]
=
[
−
cos
⁡
α
cos
⁡
α
sin
⁡
β
cos
⁡
α
sin
⁡
β
0
cos
⁡
α
cos
⁡
β
−
cos
⁡
α
cos
⁡
β
sin
⁡
α
sin
⁡
α
sin
⁡
α
]
[
U
V
W
]
{\displaystyle {\begin{bmatrix}E\\N\\Z\end{bmatrix}}={\begin{bmatrix}-\cos \alpha &\cos \alpha \sin \beta &\cos \alpha \sin \beta \\0&\cos \alpha \cos \beta &-\cos \alpha \cos \beta \\\sin \alpha &\sin \alpha &\sin \alpha \end{bmatrix}}{\begin{bmatrix}U\\V\\W\end{bmatrix}}}
A main advantage of the Galperin configuration is that all three receivers have identical orientation with respect to the vertical axis and, thus have identical instrument responses. Another advantage is the ability to build smaller packages (i.e., instruments) compared to the Cartesian orientation, which makes the Galperin configuration especially applicable for borehole installations. Other benefits of the Galperin configuration include easier distinction between external and internal noise sources and the fact that the configuration is not sensitive to rotation around the vertical axis. However, the main drawback of the configuration is that all input vectors are linked by the rotational matrix, which causes failure of the entire system when one of the three sensor is malfunctioning. In the Cartesian configuration, for example, both horizontal components still provide useful data in case the vertical (Z) component fails.
The Galperin configuration found wide application in seismometer design, including models for borehole, ocean bottom, and vault installations. The Galperin configuration can also be applied at the source side to simulate three-component seismic sources
== References ==

57
data/en.wikipedia.org/wiki/Geophone-0.md Normal file
View File

@ -0,0 +1,57 @@
---
title: "Geophone"
chunk: 1/1
source: "https://en.wikipedia.org/wiki/Geophone"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:51.799682+00:00"
instance: "kb-cron"
---
A geophone is a device that converts ground movement (velocity) into voltage, which may be recorded at a recording station. The deviation of this measured voltage from the base line is called the seismic response and is analyzed for structure of the Earth.
== Etymology ==
The term geophone derives from the Greek word "γῆ (ge)
" meaning "earth" and "phone" meaning "sound".
== Construction ==
Geophones have historically been passive analog devices and typically comprise a spring-mounted wire coil moving within the field of a case-mounted permanent magnet to generate an electrical signal. Recent designs have been based on microelectromechanical systems (MEMS) technology which generates an electrical response to ground motion through an active feedback circuit to maintain the position of a small piece of silicon.
The response of a coil/magnet geophone is proportional to ground velocity, while MEMS devices usually respond proportional to
acceleration. MEMS have a much higher noise level (50 dB velocity higher) than geophones and can only be used in strong motion or active seismic applications.
== Frequency response ==
The frequency response of a geophone is that of a harmonic oscillator, fully determined by corner frequency (typically around 10 Hz) and damping (typically 0.707). Since the corner frequency is proportional to the inverse square root of the moving mass, geophones with low corner frequencies (< 1 Hz) become impractical. It is possible to lower the corner frequency electronically, at the price of higher noise and cost.
Although waves passing through the Earth have a three-dimensional nature, geophones are normally constrained to respond to single dimension - usually the vertical. However, some applications require the full wave to be used and three-component or 3-C geophones are used. In analog devices, three moving coil elements are mounted in an orthogonal arrangement within a single case.
== Distinction from seismometers ==
Geophones are similar to seismometers in their design and are also used to register seismic waves. In the past, there were clear differences between geophones and seismometers. Compared to conventional geophones, seismometers are more suitable for detecting extremely small ground movements as they cover a wider frequency band, including the frequency range below their natural frequency, usually from 0.01 to 50 Hz. In conventional geophones, the frequency band is in the range of 1-15 Hz. They are cheaper than seismometers and are therefore more commonly used in arrays for large area detection with better specialised resolution. However, with the development of new technologies, the frequency coverage in compact devices has also increased significantly, so that geophones can cover frequency bands from 0 to 500 Hz and the boundaries between geophones and seismometers are becoming blurred.
== Uses ==
The majority of geophones are used in reflection seismology to record the energy waves reflected by the subsurface geology. In this case the primary interest is in the vertical motion of the Earth's surface. However, not all the waves are upwards traveling. A strong, horizontally transmitted wave known as ground-roll also generates vertical motion that can obliterate the weaker vertical signals. By using large areal arrays tuned to the wavelength of the ground-roll the dominant noise signals can be attenuated and the weaker data signals reinforced.
Analog geophones are very sensitive devices which can respond to very distant tremors. These small signals can be drowned by larger signals from local sources. It is possible though to recover the small signals caused by large but distant events by correlating signals from several geophones deployed in an array. Signals which are registered only at one or few geophones can be attributed to unwanted, local events and thus discarded. It can be assumed that small signals that register uniformly at all geophones in an array can be attributed to a distant and therefore significant event.
The sensitivity of passive geophones is typically 30 volts per (meter per second), so they are in general not a replacement for
broadband seismometers.
Conversely, some applications of geophones are interested only in very local events. A notable example is in the application of remote ground sensors (RGS) incorporated in unattended ground sensor (UGS) systems. In such an application there is an area of interest which when penetrated a system operator is to be informed, perhaps by an alert which could be accompanied by supporting photographic data.
Geophones were used on the Moon for a number of active and passive experiments as part of the Apollo Lunar Surface Experiments Package.
== See also ==
Accelerometer
Hydrophone
Michelson interferometer
Seismometer
== References ==
== External links ==
PSR-1 Seismic Intrusion Detector (Vietnam era military device)

36
data/en.wikipedia.org/wiki/Glossmeter-0.md Normal file
View File

@ -0,0 +1,36 @@
---
title: "Glossmeter"
chunk: 1/2
source: "https://en.wikipedia.org/wiki/Glossmeter"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:10.484367+00:00"
instance: "kb-cron"
---
A glossmeter (also gloss meter) is an instrument which is used to measure specular reflection gloss of a surface. Gloss is determined by projecting a beam of light at a fixed intensity and angle onto a surface and measuring the amount of reflected light at an equal but opposite angle.
There are a number of different geometries available for gloss measurement, each being dependent on the type of surface to be measured. For non-metals such as coatings and plastics the amount of reflected light increases with a greater angle of illumination, as some of the light penetrates the surface material and is absorbed into it or diffusely scattered from it depending on its colour. Metals have a much higher reflection and are therefore less angularly dependent.
Many international technical standards are available that define the method of use and specifications for different types of glossmeter used on various types of materials including paint, ceramics, paper, metals and plastics. Many industries use glossmeters in their quality control to measure the gloss of products to ensure consistency in their manufacturing processes. The automotive industry is a major user of the glossmeter, with applications extending from the factory floor to the repair shop.
== History ==
Of the many internationally recorded publications relating to gloss measurement, the earliest recorded studies (perceived and instrumental) are attributed to Leonard R. Ingersoll,
who in 1914 developed a means to measure the glare of paper. The Ingersoll "Glarimeter", the earliest known instrument developed for gloss measurement, was based on the principle that light is polarised in specular reflection. The instrument employed incident and viewing angles of 57.5° and used a contrast method to subtract the specular component from the total reflection using a polarising element. Ingersoll successfully applied for and patented this instrument a few years later in 1917.
In 1922 L. A. Jones, during his study of gloss of photographic papers using goniophotometry, developed a glossmeter based on his research, which provided closer correlation to gloss ratings assigned by visual evaluation. Jones's glossmeter used a geometric configuration of 45°/0°/45° whereby the surface was illuminated at 45° and two incident reflective angles measured and compared at 0° (diffuse reflectance) and 45° (diffuse plus specular reflectance). Jones was the first to emphasize the importance of using goniophotometric measurements in studies of gloss.
Early work in 1925 by A. H. Pfund led to the development of a variable angle "glossimeter" to measure specular gloss which was later patented in 1932. Pfund's instrument, allowed the angle of measurement to be varied, but maintained the angle of view to the angle of illumination. Reflected light was measured using a pyrometer lamp as a photometer. The 'glossimeter' was the first to use black glass standards as a basis for reflectance setting. As the angle was variable this instrument could also be used for the measurement of sheen or specular gloss at near grazing angles.
During this time, growing interest in this field resulted in a number of similar studies by other individuals each having their own method for gloss measurement, most of which published as technical articles in scientific journals of that time. A few of these also resulted in patents.
In 1937 Hunter, as part of a research project for the U.S. National Bureau of Standards, produced a paper on the methods of determining gloss. In this paper he discussed instruments that were available at the time (including the ones mentioned previously) in relation to the classification of six different types of gloss. In this paper Hunter also detailed the general requirements for a standardised glossmeter.
Standardisation in gloss measurement was led by Hunter and ASTM (American Society for Testing and Materials) who produced ASTM D523 Standard test method for specular gloss in 1939. This incorporated a method for measuring gloss at a specular angle of 60°. Later editions of the Standard (1951) included methods for measuring at 20° (high gloss) and 85° (matt, or low, gloss). ASTM has a number of other gloss-related standards designed for application in specific industries.
In the paint industry, measurements of specular gloss are made according to International Standard ISO 2813. This standard is equivalent to national standards ASTM D523 (United States), BS 3900, Part 5 (United Kingdom); DIN 67530 (Germany), NFT 30-064 (France), AS 1580 (Australia), JIS Z8741 (Japan).
== Construction ==
A typical glossmeter consists of a fixed mechanical assembly comprising a standardised light source that projects a parallel beam of light onto the test surface to be measured and a filtered detector located to receive the rays reflected from the surface. The ASTM Method states that the illumination should be defined such that the source-detector combination is spectrally corrected to give the CIE luminous efficiency, V(?), with CIE illuminant SC.
A number of instruments are commercially available that conform to the above standards in terms of their measurement geometry. The instruments are calibrated using reference standards that are usually made from highly polished, plane, black glass with a refractive index of 1.567 for the Sodium D line, and these are assigned a gloss value of 100 for each geometry.
== Measurement and angle selection ==
The glossmeter provides a quantifiable way of measuring gloss intensity ensuring consistency of measurement by defining the precise illumination and viewing conditions. The configuration of both illumination source and observation reception angles allows measurement over a small range of the overall reflection angle.
The measurement results of a glossmeter are related to the amount of reflected light from a black glass standard with a defined refractive index. The ratio of reflected to incident light for the specimen, compared to the ratio for the gloss standard, is recorded as gloss units (GU).
Measurement angle refers to the angle between the incident light and the perpendicular. Three measurement angles (20°, 60°, and 85°) are specified to cover the majority of industrial coatings applications. The angle is selected based on the anticipated gloss range, as shown in the following table.

49
data/en.wikipedia.org/wiki/Glossmeter-1.md Normal file
View File

@ -0,0 +1,49 @@
---
title: "Glossmeter"
chunk: 2/2
source: "https://en.wikipedia.org/wiki/Glossmeter"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:10.484367+00:00"
instance: "kb-cron"
---
For example, if the measurement made at 60° is greater than 70 GU, the measurement angle should be changed to 20° to optimise measurement accuracy.
Three types of instruments are available on the market: 60° single angle instruments, a combination of 20° and 60° and one type that combines 20°, 60° and 85°.
Two additional angles are used for other materials. An angle of 45° is specified for the measurement of ceramics, films, textiles and anodised aluminium, whilst 75° is specified for paper and printed materials.
== Gloss units ==
The measurement scale, gloss units (GU), of a glossmeter is a scaling based on a highly polished reference black glass standard with a defined refractive index having a specular reflectance of 100GU at the specified angle.
This standard is used to establish an upper point calibration of 100 with the lower end point established at 0 on a perfectly matte surface. This scaling is suitable for most non-metallic coatings and materials (paints and plastics) as they generally fall within this range. For other materials, highly reflective in appearance (mirrors, plated / raw metal components), higher values can be achieved reaching 2000 Gloss Units. For transparent materials, these values can also be increased due to multiple reflections within the material. For these applications it is common to use percent reflection of incident light rather than gloss units
== Standards ==
== Calibration ==
Each glossmeter is set up by the manufacturer to be linear throughout its measuring range by calibrating to a set of master calibration tiles traceable to a national or international standard like ISO 2813 or Germany's Federal Institute for Materials Research and Testing (BAM).
In order to maintain the performance and linearity of the glossmeter it is recommended to use a checking standard tile. This standard tile has assigned gloss unit values for each angle of measurement which are also traceable to a national standard such as German's BAM. The instrument is calibrated to this checking standard which is commonly referred to as a 'calibration tile' or 'calibration standard'. The interval of checking this calibration is dependent on the frequency of use and the operating conditions of the glossmeter.
It has been seen that standard calibration tiles kept in optimum conditions can become contaminated and change by a few gloss units over a period of years. Standard tiles which are used in working conditions will require regular calibration or checking by the instrument manufacturer or glossmeter calibration specialist.
A period of one year between standard tile recalibration should be regarded as a minimum period. If a calibration standard becomes permanently scratched or damaged at any time it will require immediate recalibration or replacement as the glossmeter may give incorrect readings.
International standards state that it is the tile that is the calibrated and a traceable artefact not the glossmeter. However it is often recommended by manufacturers that the instrument also be checked to verify its operation on a frequency dependent on the operating conditions.
== Development ==
The glossmeter is a useful instrument for measuring the gloss of a surface. However, it is not sensitive to other common effects which reduce appearance quality such as haze and orange peel.
Haze is caused by microscopic surface structure which slightly changes the direction of a reflected light causing a bloom adjacent to the specular (gloss) angle. The surface has less reflective contrast and a shallow milky effect.
Orange peel is caused by an uneven surface formation of large surface structures distorting the reflected light.
Two high gloss surfaces can measure identically with a standard glossmeter but can be visually very different. Instruments are available to quantify orange peel by measuring distinctness of image (DOI) or reflected image quality (RIQ) and haze.
== Applications ==
The glossmeter is adopted by many industries, from paper mills to automotive and is used at each stage of the manufacturing process from goods receipt through to final inspection. Examples include: paints; powder and wood coatings; additives; inks; plastics; automative, glass, and yacht manufacturing; aerospace, polished stone and metal; consumer electronics; and anodised metals.
== See also ==
Drawdown card – Tool for assessing opacity and contrast ratio of coatings
Paint sheen – Glossiness of a paint finish
Visual appearance – How matter appears by virtue of interaction with light
== References ==
== External links ==
PCI Magazin article: What is the Level of Confidence in Measuring Gloss?
Gloss Meter 60° - Caltech India
Tri Angle Gloss Meter (20°/60°/85°) - Caltech India

23
data/en.wikipedia.org/wiki/Haze_meter-0.md Normal file
View File

@ -0,0 +1,23 @@
---
title: "Haze meter"
chunk: 1/1
source: "https://en.wikipedia.org/wiki/Haze_meter"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:11.625842+00:00"
instance: "kb-cron"
---
A haze meter measures the amount of light that is diffused or scattered when passing through a transparent material. Transparency is important because a material needs to be more or less see-through depending on its practical usage, e.g. a grocery bag needs the light to be more diffused so that less can be seen while food packaging film needs the light to be less diffused so that the contents can be seen clearly. For reasons such as these haze meters are necessary to determine which material is needed for which practical purpose.
Haze is measured with a wide angle scattering test in which light is diffused in all directions which results in a loss of contrast. That percentage of light that when passing through that deviates from the incident beam by greater than 2.5 degrees on average is defined as haze
See through quality is measured with a narrow angle scattering test in which light is diffused in a small range with high concentration. This test measures the clarity with which finer details can be seen through the object being tested.
The haze meter also measures total transmittance. Total transmittance is the measure of the total incident light compared to the light that is actually transmitted (e.g. total transmittance). So the incident light may be 100%, but because of absorption and reflection the total transmittance may only be 94%.
The data gained from the haze meter can be transferred to a PC for further data processing to ensure a consistent product.
== References ==
== External links ==
Caltech India - Portable Haze Meter
Devices to measure haze

44
data/en.wikipedia.org/wiki/Hilger_&_Watts-0.md Normal file
View File

@ -0,0 +1,44 @@
---
title: "Hilger & Watts"
chunk: 1/1
source: "https://en.wikipedia.org/wiki/Hilger_&_Watts"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:12.767255+00:00"
instance: "kb-cron"
---
Hilger & Watts was a well-known British manufacturing company that made theodolites and scientific instruments.
== History ==
It was founded on 20 February 1948 when Adam Hilger, Ltd, founded in 1874, merged with Messrs E. R. Watts and Son by Edwin Richard Watts (1833–1901) and George William Watts (c. 1871–1954), founded in 1865. The company was taken over by the Rank Organisation in 1969 and later sold on.
== Structure ==
It employed around 1,300 people in six factories in the late 1940s. It was situated on Camberwell Road (A215 road) in Camberwell, near the junction with the B214, between Walworth (to the north) and Camberwell (to the south) on the western edge of Burgess Park, now part of the London Borough of Southwark. There was a factory in Highbury, together with the head office in Camden Town and a small factory situated between Margate and Ramsgate in Kent. These locations were primarily "Hilger" products, whereas Camberwell was primarily Watts products. The company also had a factory at Loughton (Debden) in Essex
== Products ==
Optical instruments
Photometers
Theodolite and surveying equipment
Tripods
Computer controlled X-ray diffractometers
Amongst other devices, the Camden location produced PDP-8 computer-driven X-ray diffractometers in the mid-late 1960s, one of which is believed to be still functional at Oxford University Chemistry Dept. in 2017. Development began with a linear diffractometer in the late 1950s - early 1960s but this was superseded around 1965 by the Y290, a four-circle diffractometer, the electronics for which were developed at the University of Manchester by Prof David B.G. Edwards (Computer Science) along with Owen S. Mills (Chemistry). The early models (actually referenced Y230) used a Ferranti computer for controlling the diffractometer but, due to reliability issues, the cheaper and more compact PDP-8 became the computer of choice for the Y290 model. One of the production engineers claimed that the electronic research department in Camden (headed by Arthur Long) had PDP-8 No 4 from Digital Equipment Corporation (DEC), so Hilger and Watts was likely to have been one of the first PDP-8 customers. The book ‘Single Crystal Diffractometry’ by U.W. Arndt and B.T.M. Willis (C.U.P., 1966) has photographs and other excellent diagrams showing the construction of these machines; both authors were involved in their development. The four-circle diffractometer was originally designed for neutron work at the Atomic Energy Research Establishment Harwell Laboratory but it became better known in field of X-ray crystallography. Photographs showing prototypes of both instruments may be seen in a review article by U.W. Arndt. For commercial production of the four-circle diffractometer, Hilger and Watts won two Queen's Awards to Industry, the first for Services to Export in 1966 and the second for Technological Achievement in 1968. In the late 1960s the cost of the four-circle system was around $130,000.
The Y290 diffractometer had an optical motor-positioning system based on moiré fringes which were recorded by photocells - sometimes the user would find that after centring their crystal under a bright light and forgetting to turn it off, the diffractometer motors would completely lose their settings. The correct positioning could be restored by a command which returned the motors to built-in datum points. In fact, the motors were regularly driven back to datum during data collection as a check on their positioning. The X-ray data were written to paper tape and, by the mid-1980s, users were struggling to find computer facilities which could still read their tapes for downstream analysis. At the Laboratory of Molecular Biophysics, University of Oxford, a 5-circle version of the machine was developed for protein crystallography which could measure up to five X-ray reflections in each scan due to the addition of a tiltable linear array of 5 counters to the detector arm.
In Galway a BBC micro hard wired to the PDP8 front panel and a reflection indexing program were used to automate the Y290 (getting rid of the paper tape) and another group has described complete modernisation of the Y290 motors and control electronics.
== See also ==
Watts reflected decimal code
== References ==
The Times, page 9, 25 February 1948
== External links ==
https://hilgerwatts.blogspot.com - unofficial Hilger & Watts diffractometer history

19
data/en.wikipedia.org/wiki/Hydroscope-0.md Normal file
View File

@ -0,0 +1,19 @@
---
title: "Hydroscope"
chunk: 1/1
source: "https://en.wikipedia.org/wiki/Hydroscope"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:13.924162+00:00"
instance: "kb-cron"
---
A hydroscope is any of several instruments related to water:
One kind is an instrument for making observations below the surface of water, such as a long tube fitted with various lenses arranged so that objects lying at the bottom can be reflected upon a screen on the deck of the ship that carries it. These are built with a large tire tube that supports the screen and covered by an acrylic dome for protection.
Another kind detects subsurface water through nuclear magnetic resonance using the surface nuclear magnetic resonance technique.
An instrument (likely a hydrometer) described by Synesius in his Letter 15 to Hypatia, written in 402 AD. There are references to such instruments as early as the fourth century.
Another ancient Greek instrument: a water clock or clepsydra.
== Sources and notes ==

19
data/en.wikipedia.org/wiki/Interferometry-0.md Normal file
View File

@ -0,0 +1,19 @@
---
title: "Interferometry"
chunk: 1/10
source: "https://en.wikipedia.org/wiki/Interferometry"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:15.174742+00:00"
instance: "kb-cron"
---
Interferometry is a technique which uses the interference of superimposed waves to extract information. Interferometry typically uses electromagnetic waves and is an important investigative technique in the fields of astronomy, fiber optics, engineering metrology, optical metrology, oceanography, seismology, spectroscopy (and its applications to chemistry), quantum mechanics, nuclear and particle physics, plasma physics, biomolecular interactions, surface profiling, microfluidics, mechanical stress/strain measurement, velocimetry, optometry, and making holograms.
Interferometers are devices that extract information from interference. They are widely used in science and industry for the measurement of microscopic displacements, refractive index changes and surface irregularities. In the case with most interferometers, light from a single source is split into two beams that travel in different optical paths, which are then combined again to produce interference; two incoherent sources can also be made to interfere under some circumstances. The resulting interference fringes give information about the difference in optical path lengths. In analytical science, interferometers are used to measure lengths and the shape of optical components with nanometer precision; they are the highest-precision length measuring instruments in existence. In Fourier transform spectroscopy they are used to analyze light containing features of absorption or emission associated with a substance or mixture. An astronomical interferometer consists of two or more separate telescopes that combine their signals, offering a resolution equivalent to that of a telescope of diameter equal to the largest separation between its individual elements.
== Basic principles ==
Interferometry makes use of the principle of superposition to combine waves in a way that will cause the result of their combination to have some meaningful property that is diagnostic of the original state of the waves. This works because when two waves with the same frequency combine, the resulting intensity pattern is determined by the phase difference between the two waves—waves that are in phase will undergo constructive interference while waves that are out of phase will undergo destructive interference. Waves which are not completely in phase nor completely out of phase will have an intermediate intensity pattern, which can be used to determine their relative phase difference. Most interferometers use light or some other form of electromagnetic wave.
Typically (see Fig. 1, the well-known Michelson configuration) a single incoming beam of coherent light will be split into two identical beams by a beam splitter (a partially reflecting mirror). Each of these beams travels a different route, called a path, and they are recombined before arriving at a detector. The path difference, the difference in the distance traveled by each beam, creates a phase difference between them. It is this introduced phase difference that creates the interference pattern between the initially identical waves. If a single beam has been split along two paths, then the phase difference is diagnostic of anything that changes the phase along the paths. This could be a physical change in the path length itself or a change in the refractive index along the path.
As seen in Fig. 2a and 2b, the observer has a direct view of mirror M1 seen through the beam splitter, and sees a reflected image M′2 of mirror M2. The fringes can be interpreted as the result of interference between light coming from the two virtual images S′1 and S′2 of the original source S. The characteristics of the interference pattern depend on the nature of the light source and the precise orientation of the mirrors and beam splitter. In Fig. 2a, the optical elements are oriented so that S′1 and S′2 are in line with the observer, and the resulting interference pattern consists of circles centered on the normal to M1 and M'2. If, as in Fig. 2b, M1 and M′2 are tilted with respect to each other, the interference fringes will generally take the shape of conic sections (hyperbolas), but if M′1 and M′2 overlap, the fringes near the axis will be straight, parallel, and equally spaced. If S is an extended source rather than a point source as illustrated, the fringes of Fig. 2a must be observed with a telescope set at infinity, while the fringes of Fig. 2b will be localized on the mirrors.
Use of white light will result in a pattern of colored fringes (see Fig. 3). The central fringe representing equal path length may be light or dark depending on the number of phase inversions experienced by the two beams as they traverse the optical system. (See Michelson interferometer for a discussion of this.)

33
data/en.wikipedia.org/wiki/Interferometry-1.md Normal file
View File

@ -0,0 +1,33 @@
---
title: "Interferometry"
chunk: 2/10
source: "https://en.wikipedia.org/wiki/Interferometry"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:15.174742+00:00"
instance: "kb-cron"
---
== History ==
The law of interference of light was described by Thomas Young in his 1803 Bakerian Lecture to the Royal Society of London. In preparation for the lecture, Young performed a double-aperture experiment in a water ripple tank. His interpretation in terms of the interference of waves was rejected by most scientists at the time because of the dominance of Isaac Newton's corpuscular theory of light proposed a century before.
The French engineer Augustin-Jean Fresnel, unaware of Young's results, began working on a wave theory of light and interference and was introduced to François Arago. Between 1816 and 1818, Fresnel and Arago performed interference experiments at the Paris Observatory. During this time, Arago designed and built the first interferometer, using it to measure the refractive index of moist air relative to dry air, which posed a potential problem for astronomical observations of star positions. The success of Fresnel's wave theory of light was established in his prize-winning memoire of 1819 that predicted and measured diffraction patterns. The Arago interferometer was later employed in 1850 by Leon Foucault to measure the speed of light in air relative to water, and it was used again in 1851 by Hippolyte Fizeau to measure the effect of Fresnel drag on the speed of light in moving water.
Jules Jamin developed the first single-beam interferometer (not requiring a splitting aperture as the Arago interferometer did) in 1856. In 1881, the American physicist Albert A. Michelson, while visiting Hermann von Helmholtz in Berlin, invented the interferometer that is named after him, the Michelson Interferometer, to search for effects of the motion of the Earth on the speed of light. Michelson's null results performed in the basement of the Potsdam Observatory outside of Berlin (the horse traffic in the center of Berlin created too many vibrations), and his later more-accurate null results observed with Edward W. Morley at Case College in Cleveland, Ohio, contributed to the growing crisis of the luminiferous ether. Einstein stated that it was Fizeau's measurement of the speed of light in moving water using the Arago interferometer that inspired his theory of the relativistic addition of velocities.
== Categories ==
Interferometers and interferometric techniques may be categorized by a variety of criteria:
=== Homodyne versus heterodyne detection ===
In homodyne detection, the interference occurs between two beams at the same wavelength (or carrier frequency). The phase difference between the two beams results in a change in the intensity of the light on the detector. The resulting intensity of the light after mixing of these two beams is measured, or the pattern of interference fringes is viewed or recorded. Most of the interferometers discussed in this article fall into this category.
The heterodyne technique is used for (1) shifting an input signal into a new frequency range as well as (2) amplifying a weak input signal (assuming use of an active mixer). A weak input signal of frequency f1 is mixed with a strong reference frequency f2 from a local oscillator (LO). The nonlinear combination of the input signals creates two new signals, one at the sum f1 + f2 of the two frequencies, and the other at the difference f1 − f2. These new frequencies are called heterodynes. Typically only one of the new frequencies is desired, and the other signal is filtered out of the output of the mixer. The output signal will have an intensity proportional to the product of the amplitudes of the input signals.
The most important and widely used application of the heterodyne technique is in the superheterodyne receiver (superhet), invented in 1917-18 by U.S. engineer Edwin Howard Armstrong and French engineer Lucien Lévy. In this circuit, the incoming radio frequency signal from the antenna is mixed with a signal from a local oscillator (LO) and converted by the heterodyne technique to a lower fixed frequency signal called the intermediate frequency (IF). This IF is amplified and filtered, before being applied to a detector which extracts the audio signal, which is sent to the loudspeaker.
Optical heterodyne detection is an extension of the heterodyne technique to higher (visible) frequencies. While optical heterodyne interferometry is usually done at a single point it is also possible to perform this widefield.
=== Double path versus common path ===
A double-path interferometer is one in which the reference beam and sample beam travel along divergent paths. Examples include the Michelson interferometer, the Twyman–Green interferometer, and the Mach–Zehnder interferometer. After being perturbed by interaction with the sample under test, the sample beam is recombined with the reference beam to create an interference pattern which can then be interpreted.
A common-path interferometer is a class of interferometer in which the reference beam and sample beam travel along the same path. Fig. 4 illustrates the Sagnac interferometer, the fibre optic gyroscope, the point diffraction interferometer, and the lateral shearing interferometer. Other examples of common path interferometer include the Zernike phase-contrast microscope, Fresnel's biprism, the zero-area Sagnac, and the scatterplate interferometer.
=== Wavefront splitting versus amplitude splitting ===
==== Wavefront splitting inferometers ====
A wavefront splitting interferometer divides a light wavefront emerging from a point or a narrow slit (i.e. spatially coherent light) and, after allowing the two parts of the wavefront to travel through different paths, allows them to recombine. Fig. 5 illustrates Young's interference experiment and Lloyd's mirror. Other examples of wavefront splitting interferometer include the Fresnel biprism, the Billet Bi-Lens, diffraction-grating Michelson interferometer, and the Rayleigh interferometer.

27
data/en.wikipedia.org/wiki/Interferometry-2.md Normal file
View File

@ -0,0 +1,27 @@
---
title: "Interferometry"
chunk: 3/10
source: "https://en.wikipedia.org/wiki/Interferometry"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:15.174742+00:00"
instance: "kb-cron"
---
In 1803, Young's interference experiment played a major role in the general acceptance of the wave theory of light. If white light is used in Young's experiment, the result is a white central band of constructive interference corresponding to equal path length from the two slits, surrounded by a symmetrical pattern of colored fringes of diminishing intensity. In addition to continuous electromagnetic radiation, Young's experiment has been performed with individual photons, with electrons, and with buckyball molecules large enough to be seen under an electron microscope.
Lloyd's mirror generates interference fringes by combining direct light from a source (blue lines) and light from the source's reflected image (red lines) from a mirror held at grazing incidence. The result is an asymmetrical pattern of fringes. The band of equal path length, nearest the mirror, is dark rather than bright. In 1834, Humphrey Lloyd interpreted this effect as proof that the phase of a front-surface reflected beam is inverted.
==== Amplitude-splitting inferometers ====
An amplitude splitting interferometer uses a partial reflector to divide the amplitude of the incident wave into separate beams which are separated and recombined.
The Fizeau interferometer is shown as it might be set up to test an optical flat. A precisely figured reference flat is placed on top of the flat being tested, separated by narrow spacers. The reference flat is slightly beveled (only a fraction of a degree of beveling is necessary) to prevent the rear surface of the flat from producing interference fringes. Separating the test and reference flats allows the two flats to be tilted with respect to each other. By adjusting the tilt, which adds a controlled phase gradient to the fringe pattern, one can control the spacing and direction of the fringes, so that one may obtain an easily interpreted series of nearly parallel fringes rather than a complex swirl of contour lines. Separating the plates, however, necessitates that the illuminating light be collimated. Fig 6 shows a collimated beam of monochromatic light illuminating the two flats and a beam splitter allowing the fringes to be viewed on-axis.
The Mach–Zehnder interferometer is a more versatile instrument than the Michelson interferometer. Each of the well separated light paths is traversed only once, and the fringes can be adjusted so that they are localized in any desired plane. Typically, the fringes would be adjusted to lie in the same plane as the test object, so that fringes and test object can be photographed together. If it is decided to produce fringes in white light, then, since white light has a limited coherence length, on the order of micrometers, great care must be taken to equalize the optical paths or no fringes will be visible. As illustrated in Fig. 6, a compensating cell would be placed in the path of the reference beam to match the test cell. Note also the precise orientation of the beam splitters. The reflecting surfaces of the beam splitters would be oriented so that the test and reference beams pass through an equal amount of glass. In this orientation, the test and reference beams each experience two front-surface reflections, resulting in the same number of phase inversions. The result is that light traveling an equal optical path length in the test and reference beams produces a white light fringe of constructive interference.
The heart of the Fabry–Pérot interferometer is a pair of partially silvered glass optical flats spaced several millimeters to centimeters apart with the silvered surfaces facing each other. (Alternatively, a Fabry–Pérot etalon uses a transparent plate with two parallel reflecting surfaces.) As with the Fizeau interferometer, the flats are slightly beveled. In a typical system, illumination is provided by a diffuse source set at the focal plane of a collimating lens. A focusing lens produces what would be an inverted image of the source if the paired flats were not present, i.e., in the absence of the paired flats, all light emitted from point A passing through the optical system would be focused at point A'. In Fig. 6, only one ray emitted from point A on the source is traced. As the ray passes through the paired flats, it is multiply reflected to produce multiple transmitted rays which are collected by the focusing lens and brought to point A' on the screen. The complete interference pattern takes the appearance of a set of concentric rings. The sharpness of the rings depends on the reflectivity of the flats. If the reflectivity is high, resulting in a high Q factor (i.e., high finesse), monochromatic light produces a set of narrow bright rings against a dark background. In Fig. 6, the low-finesse image corresponds to a reflectivity of 0.04 (i.e., unsilvered surfaces) versus a reflectivity of 0.95 for the high-finesse image.
Fig. 6 illustrates the Fizeau, Mach–Zehnder, and Fabry–Pérot interferometers. Other examples of amplitude splitting interferometer include the Michelson, Twyman–Green, Laser Unequal Path, and Linnik interferometer.
==== Michelson-Morley ====
Michelson and Morley (1887) and other early experimentalists using interferometric techniques in an attempt to measure the properties of the luminiferous aether, used monochromatic light only for initially setting up their equipment, always switching to white light for the actual measurements. The reason is that measurements were recorded visually. Monochromatic light would result in a uniform fringe pattern. Lacking modern means of environmental temperature control, experimentalists struggled with continual fringe drift even though the interferometer might be set up in a basement. Since the fringes would occasionally disappear due to vibrations by passing horse traffic, distant thunderstorms and the like, it would be easy for an observer to "get lost" when the fringes returned to visibility. The advantages of white light, which produced a distinctive colored fringe pattern, far outweighed the difficulties of aligning the apparatus due to its low coherence length. This was an early example of the use of white light to resolve the "2 pi ambiguity".
== Applications ==
=== Physics and astronomy ===

23
data/en.wikipedia.org/wiki/Interferometry-3.md Normal file
View File

@ -0,0 +1,23 @@
---
title: "Interferometry"
chunk: 4/10
source: "https://en.wikipedia.org/wiki/Interferometry"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:15.174742+00:00"
instance: "kb-cron"
---
In physics, one of the most important experiments of the late 19th century was the famous "failed experiment" of Michelson and Morley which provided evidence for special relativity. Recent repetitions of the Michelson–Morley experiment perform heterodyne measurements of beat frequencies of crossed cryogenic optical resonators. Fig 7 illustrates a resonator experiment performed by Müller et al. in 2003. Two optical resonators constructed from crystalline sapphire, controlling the frequencies of two lasers, were set at right angles within a helium cryostat. A frequency comparator measured the beat frequency of the combined outputs of the two resonators. As of 2009, the precision by which anisotropy of the speed of light can be excluded in resonator experiments is at the 10−17 level.
Michelson interferometers are used in tunable narrow band optical filters and as the core hardware component of Fourier transform spectrometers.
When used as a tunable narrow band filter, Michelson interferometers exhibit a number of advantages and disadvantages when compared with competing technologies such as Fabry–Pérot interferometers or Lyot filters. Michelson interferometers have the largest field of view for a specified wavelength, and are relatively simple in operation, since tuning is via mechanical rotation of waveplates rather than via high voltage control of piezoelectric crystals or lithium niobate optical modulators as used in a Fabry–Pérot system. Compared with Lyot filters, which use birefringent elements, Michelson interferometers have a relatively low temperature sensitivity. On the negative side, Michelson interferometers have a relatively restricted wavelength range and require use of prefilters which restrict transmittance.
Fig. 8 illustrates the operation of a Fourier transform spectrometer, which is essentially a Michelson interferometer with one mirror movable. (A practical Fourier transform spectrometer would substitute corner cube reflectors for the flat mirrors of the conventional Michelson interferometer, but for simplicity, the illustration does not show this.) An interferogram is generated by making measurements of the signal at many discrete positions of the moving mirror. A Fourier transform converts the interferogram into an actual spectrum.
Fig. 9 shows a doppler image of the solar corona made using a tunable Fabry-Pérot interferometer to recover scans of the solar corona at a number of wavelengths near the FeXIV green line. The picture is a color-coded image of the doppler shift of the line, which may be associated with the coronal plasma velocity towards or away from the satellite camera.
Fabry–Pérot thin-film etalons are used in narrow bandpass filters capable of selecting a single spectral line for imaging; for example, the H-alpha line or the Ca-K line of the Sun or stars. Fig. 10 shows an Extreme ultraviolet Imaging Telescope (EIT) image of the Sun at 195 Ångströms (19.5 nm), corresponding to a spectral line of multiply-ionized iron atoms. EIT used multilayer coated reflective mirrors that were coated with alternate layers of a light "spacer" element (such as silicon), and a heavy "scatterer" element (such as molybdenum). Approximately 100 layers of each type were placed on each mirror, with a thickness of around 10 nm each. The layer thicknesses were tightly controlled so that at the desired wavelength, reflected photons from each layer interfered constructively.
The Laser Interferometer Gravitational-Wave Observatory (LIGO) uses two 4-km Michelson–Fabry–Pérot interferometers for the detection of gravitational waves. In this application, the Fabry–Pérot cavity is used to store photons for almost a millisecond while they bounce up and down between the mirrors. This increases the time a gravitational wave can interact with the light, which results in a better sensitivity at low frequencies. Smaller cavities, usually called mode cleaners, are used for spatial filtering and frequency stabilization of the main laser. The first observation of gravitational waves occurred on September 14, 2015.
The Mach–Zehnder interferometer's relatively large and freely accessible working space, and its flexibility in locating the fringes has made it the interferometer of choice for visualizing flow in wind tunnels, and for flow visualization studies in general. It is frequently used in the fields of aerodynamics, plasma physics and heat transfer to measure pressure, density, and temperature changes in gases.
Mach–Zehnder interferometers are also used to study one of the most counterintuitive predictions of quantum mechanics, the phenomenon known as quantum entanglement.
An astronomical interferometer achieves high-resolution observations using the technique of aperture synthesis, mixing signals from a cluster of comparatively small telescopes rather than a single very expensive monolithic telescope.
Early radio telescope interferometers used a single baseline for measurement. Later astronomical interferometers, such as the Very Large Array illustrated in Fig 11, used arrays of telescopes arranged in a pattern on the ground. A limited number of baselines will result in insufficient coverage. This was alleviated by using the rotation of the Earth to rotate the array relative to the sky. Thus, a single baseline could measure information in multiple orientations by taking repeated measurements, a technique called Earth-rotation synthesis. Baselines thousands of kilometers long were achieved using very long baseline interferometry.

21
data/en.wikipedia.org/wiki/Interferometry-4.md Normal file
View File

@ -0,0 +1,21 @@
---
title: "Interferometry"
chunk: 5/10
source: "https://en.wikipedia.org/wiki/Interferometry"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:15.174742+00:00"
instance: "kb-cron"
---
Astronomical optical interferometry has had to overcome a number of technical issues not shared by radio telescope interferometry. The short wavelengths of light necessitate extreme precision and stability of construction. For example, spatial resolution of 1 milliarcsecond requires 0.5 μm stability in a 100 m baseline. Optical interferometric measurements require high sensitivity, low noise detectors that did not become available until the late 1990s. Astronomical "seeing", the turbulence that causes stars to twinkle, introduces rapid, random phase changes in the incoming light, requiring data collection rates to be faster than the rate of turbulence. Despite these technical difficulties, three major facilities are now in operation offering resolutions down to the fractional milliarcsecond range.
The wave character of matter can be exploited to build interferometers. The first examples of matter interferometers were electron interferometers, later followed by neutron interferometers. Around 1990 the first atom interferometers were demonstrated, later followed by interferometers employing molecules.
Electron holography is an imaging technique that photographically records the electron interference pattern of an object, which is then reconstructed to yield a greatly magnified image of the original object. This technique was developed to enable greater resolution in electron microscopy than is possible using conventional imaging techniques. The resolution of conventional electron microscopy is not limited by electron wavelength, but by the large aberrations of electron lenses.
Neutron interferometry has been used to investigate the Aharonov–Bohm effect, to examine the effects of gravity acting on an elementary particle, and to demonstrate a strange behavior of fermions that is at the basis of the Pauli exclusion principle: Unlike macroscopic objects, when fermions are rotated by 360° about any axis, they do not return to their original state, but develop a minus sign in their wave function. In other words, a fermion needs to be rotated 720° before returning to its original state.
Atom interferometry techniques are reaching sufficient precision to allow laboratory-scale tests of general relativity.
Interferometers are used in atmospheric physics for high-precision measurements of trace gases via remote sounding of the atmosphere. There are several examples of interferometers that utilize either absorption or emission features of trace gases. A typical use would be in continual monitoring of the column concentration of trace gases such as ozone and carbon monoxide above the instrument.
=== Engineering and applied science ===
Newton (test plate) interferometry is frequently used in the optical industry for testing the quality of surfaces as they are being shaped and figured. Fig. 13 shows photos of reference flats being used to check two test flats at different stages of completion, showing the different patterns of interference fringes. The reference flats are resting with their bottom surfaces in contact with the test flats, and they are illuminated by a monochromatic light source. The light waves reflected from both surfaces interfere, resulting in a pattern of bright and dark bands. The surface in the left photo is nearly flat, indicated by a pattern of straight parallel interference fringes at equal intervals. The surface in the right photo is uneven, resulting in a pattern of curved fringes. Each pair of adjacent fringes represents a difference in surface elevation of half a wavelength of the light used, so differences in elevation can be measured by counting the fringes. The flatness of the surfaces can be measured to millionths of an inch by this method. To determine whether the surface being tested is concave or convex with respect to the reference optical flat, any of several procedures may be adopted. One can observe how the fringes are displaced when one presses gently on the top flat. If one observes the fringes in white light, the sequence of colors becomes familiar with experience and aids in interpretation. Finally one may compare the appearance of the fringes as one moves ones head from a normal to an oblique viewing position. These sorts of maneuvers, while common in the optical shop, are not suitable in a formal testing environment. When the flats are ready for sale, they will typically be mounted in a Fizeau interferometer for formal testing and certification.
Fabry-Pérot etalons are widely used in telecommunications, lasers and spectroscopy to control and measure the wavelengths of light. Dichroic filters are multiple layer thin-film etalons. In telecommunications, wavelength-division multiplexing, the technology that enables the use of multiple wavelengths of light through a single optical fiber, depends on filtering devices that are thin-film etalons. Single-mode lasers employ etalons to suppress all optical cavity modes except the single one of interest.

17
data/en.wikipedia.org/wiki/Interferometry-5.md Normal file
View File

@ -0,0 +1,17 @@
---
title: "Interferometry"
chunk: 6/10
source: "https://en.wikipedia.org/wiki/Interferometry"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:15.174742+00:00"
instance: "kb-cron"
---
The Twyman–Green interferometer, invented by Twyman and Green in 1916, is a variant of the Michelson interferometer widely used to test optical components. The basic characteristics distinguishing it from the Michelson configuration are the use of a monochromatic point light source and a collimator. Michelson (1918) criticized the Twyman–Green configuration as being unsuitable for the testing of large optical components, since the light sources available at the time had limited coherence length. Michelson pointed out that constraints on geometry forced by limited coherence length required the use of a reference mirror of equal size to the test mirror, making the Twyman–Green impractical for many purposes. Decades later, the advent of laser light sources answered Michelson's objections. (A Twyman–Green interferometer using a laser light source and unequal path length is known as a Laser Unequal Path Interferometer, or LUPI.) Fig. 14 illustrates a Twyman–Green interferometer set up to test a lens. Light from a monochromatic point source is expanded by a diverging lens (not shown), then is collimated into a parallel beam. A convex spherical mirror is positioned so that its center of curvature coincides with the focus of the lens being tested. The emergent beam is recorded by an imaging system for analysis.
Mach–Zehnder interferometers are being used in integrated optical circuits, in which light interferes between two branches of a waveguide that are externally modulated to vary their relative phase. A slight tilt of one of the beam splitters will result in a path difference and a change in the interference pattern. Mach–Zehnder interferometers are the basis of a wide variety of devices, from RF modulators to sensors to optical switches.
Some proposed and future extremely large astronomical telescopes, such as the Thirty Meter Telescope and the Extremely Large Telescope, will be of segmented design. Their primary mirrors will comprise hundreds of hexagonal mirror-segments. Polishing and figuring these highly aspheric and non-rotationally symmetric mirror segments presents a major challenge. Traditional means of optical testing compare a surface against a spherical reference with the aid of a null corrector. Computer-generated holograms supplement null correctors in test setups for complex aspheric surfaces. Fig. 15 illustrates how this is done. (Unlike the figure, actual CGHs have line spacing on the order of 1 to 10 μm.) When laser light is passed through the hologram, the zero-order diffracted beam experiences no wavefront modification. The wavefront of the first-order diffracted beam, however, is modified to match the desired shape of the test surface. In the illustrated Fizeau interferometer test setup, the zero-order diffracted beam is directed towards the spherical reference surface, and the first-order diffracted beam is directed towards the test surface in such a way that the two reflected beams combine to form interference fringes.
Ring laser gyroscopes (RLGs) and fibre optic gyroscopes (FOGs) are interferometers used in navigation systems. They operate on the principle of the Sagnac effect. The distinction between RLGs and FOGs is that in a RLG, the entire ring is part of the laser while in a FOG, an external laser injects counter-propagating beams into an optical fiber ring, and rotation of the system then causes a relative phase shift between those beams. In a RLG, the observed phase shift is proportional to the accumulated rotation, while in a FOG, the observed phase shift is proportional to the angular velocity.
In telecommunication networks, heterodyning is used to move frequencies of individual signals to different channels which may share a single physical transmission line. This is called frequency division multiplexing (FDM). For example, a coaxial cable used by a cable television system can carry 500 television channels at the same time because each one is given a different frequency, so they don't interfere with one another. Continuous wave (CW) doppler radar detectors are basically heterodyne detection devices that compare transmitted and reflected beams.
Optical heterodyne detection is used for coherent Doppler lidar measurements capable of detecting very weak light scattered in the atmosphere and monitoring wind speeds with high accuracy. It has application in optical fiber communications, in various high resolution spectroscopic techniques, and the self-heterodyne method can be used to measure the linewidth of a laser.

15
data/en.wikipedia.org/wiki/Interferometry-6.md Normal file
View File

@ -0,0 +1,15 @@
---
title: "Interferometry"
chunk: 7/10
source: "https://en.wikipedia.org/wiki/Interferometry"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:15.174742+00:00"
instance: "kb-cron"
---
Optical heterodyne detection is an essential technique used in high-accuracy measurements of the frequencies of optical sources, as well as in the stabilization of their frequencies. Until relatively few years ago, lengthy frequency chains were needed to connect the microwave frequency of a cesium or other atomic time source to optical frequencies. At each step of the chain, a frequency multiplier would be used to produce a harmonic of the frequency of that step, which would be compared by heterodyne detection with the next step (the output of a microwave source, far infrared laser, infrared laser, or visible laser). Each measurement of a single spectral line required several years of effort in the construction of a custom frequency chain. Currently, optical frequency combs have provided a much simpler method of measuring optical frequencies. If a mode-locked laser is modulated to form a train of pulses, its spectrum is seen to consist of the carrier frequency surrounded by a closely spaced comb of optical sideband frequencies with a spacing equal to the pulse repetition frequency (Fig. 16). The pulse repetition frequency is locked to that of the frequency standard, and the frequencies of the comb elements at the red end of the spectrum are doubled and heterodyned with the frequencies of the comb elements at the blue end of the spectrum, thus allowing the comb to serve as its own reference. In this manner, locking of the frequency comb output to an atomic standard can be performed in a single step. To measure an unknown frequency, the frequency comb output is dispersed into a spectrum. The unknown frequency is overlapped with the appropriate spectral segment of the comb and the frequency of the resultant heterodyne beats is measured.
One of the most common industrial applications of optical interferometry is as a versatile measurement tool for the high precision examination of surface topography. Popular interferometric measurement techniques include phase shifting interferometry (PSI), and vertical scanning interferometry (VSI), also known as scanning white light interferometry (SWLI) or by the ISO term coherence scanning interferometry (CSI), CSI exploits coherence to extend the range of capabilities for interference microscopy. These techniques are widely used in micro-electronic and micro-optic fabrication. PSI uses monochromatic light and provides very precise measurements; however it is only usable for surfaces that are very smooth. CSI often uses white light and high numerical apertures, and rather than looking at the phase of the fringes, as does PSI, looks for best position of maximum fringe contrast or some other feature of the overall fringe pattern. In its simplest form, CSI provides less precise measurements than PSI but can be used on rough surfaces. Some configurations of CSI, variously known as Enhanced VSI (EVSI), high-resolution SWLI or Frequency Domain Analysis (FDA), use coherence effects in combination with interference phase to enhance precision.
Phase Shifting Interferometry addresses several issues associated with the classical analysis of static interferograms. Classically, one measures the positions of the fringe centers. As seen in Fig. 13, fringe deviations from straightness and equal spacing provide a measure of the aberration. Errors in determining the location of the fringe centers provide the inherent limit to precision of the classical analysis, and any intensity variations across the interferogram will also introduce error. There is a trade-off between precision and number of data points: closely spaced fringes provide many data points of low precision, while widely spaced fringes provide a low number of high precision data points. Since fringe center data is all that one uses in the classical analysis, all of the other information that might theoretically be obtained by detailed analysis of the intensity variations in an interferogram is thrown away. Finally, with static interferograms, additional information is needed to determine the polarity of the wavefront: In Fig. 13, one can see that the tested surface on the right deviates from flatness, but one cannot tell from this single image whether this deviation from flatness is concave or convex. Traditionally, this information would be obtained using non-automated means, such as by observing the direction that the fringes move when the reference surface is pushed.
Phase shifting interferometry overcomes these limitations by not relying on finding fringe centers, but rather by collecting intensity data from every point of the CCD image sensor. As seen in Fig. 17, multiple interferograms (at least three) are analyzed with the reference optical surface shifted by a precise fraction of a wavelength between each exposure using a piezoelectric transducer (PZT). Alternatively, precise phase shifts can be introduced by modulating the laser frequency. The captured images are processed by a computer to calculate the optical wavefront errors. The precision and reproducibility of PSI is far greater than possible in static interferogram analysis, with measurement repeatabilities of a hundredth of a wavelength being routine. Phase shifting technology has been adapted to a variety of interferometer types such as Twyman–Green, Mach–Zehnder, laser Fizeau, and even common path configurations such as point diffraction and lateral shearing interferometers. More generally, phase shifting techniques can be adapted to almost any system that uses fringes for measurement, such as holographic and speckle interferometry.

18
data/en.wikipedia.org/wiki/Interferometry-7.md Normal file
View File

@ -0,0 +1,18 @@
---
title: "Interferometry"
chunk: 8/10
source: "https://en.wikipedia.org/wiki/Interferometry"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:15.174742+00:00"
instance: "kb-cron"
---
In coherence scanning interferometry, interference is only achieved when the path length delays of the interferometer are matched within the coherence time of the light source. CSI monitors the fringe contrast rather than the phase of the fringes. Fig. 17 illustrates a CSI microscope using a Mirau interferometer in the objective; other forms of interferometer used with white light include the Michelson interferometer (for low magnification objectives, where the reference mirror in a Mirau objective would interrupt too much of the aperture) and the Linnik interferometer (for high magnification objectives with limited working distance). The sample (or alternatively, the objective) is moved vertically over the full height range of the sample, and the position of maximum fringe contrast is found for each pixel. The chief benefit of coherence scanning interferometry is that systems can be designed that do not suffer from the 2 pi ambiguity of coherent interferometry, and as seen in Fig. 18, which scans a 180μm x 140μm x 10μm volume, it is well suited to profiling steps and rough surfaces. The axial resolution of the system is determined in part by the coherence length of the light source. Industrial applications include in-process surface metrology, roughness measurement, 3D surface metrology in hard-to-reach spaces and in hostile environments, profilometry of surfaces with high aspect ratio features (grooves, channels, holes), and film thickness measurement (semi-conductor and optical industries, etc.).
Fig. 19 illustrates a Twyman–Green interferometer set up for white light scanning of a macroscopic object.
Holographic interferometry is a technique which uses holography to monitor small deformations in single wavelength implementations. In multi-wavelength implementations, it is used to perform dimensional metrology of large parts and assemblies and to detect larger surface defects.
Holographic interferometry was discovered by accident as a result of mistakes committed during the making of holograms. Early lasers were relatively weak and photographic plates were insensitive, necessitating long exposures during which vibrations or minute shifts might occur in the optical system. The resultant holograms, which showed the holographic subject covered with fringes, were considered ruined.
Eventually, several independent groups of experimenters in the mid-60s realized that the fringes encoded important information about dimensional changes occurring in the subject, and began intentionally producing holographic double exposures. The main Holographic interferometry article covers the disputes over priority of discovery that occurred during the issuance of the patent for this method.
Double- and multi- exposure holography is one of three methods used to create holographic interferograms. A first exposure records the object in an unstressed state. Subsequent exposures on the same photographic plate are made while the object is subjected to some stress. The composite image depicts the difference between the stressed and unstressed states.
Real-time holography is a second method of creating holographic interferograms. A holograph of the unstressed object is created. This holograph is illuminated with a reference beam to generate a hologram image of the object directly superimposed over the original object itself while the object is being subjected to some stress. The object waves from this hologram image will interfere with new waves coming from the object. This technique allows real time monitoring of shape changes.
The third method, time-average holography, involves creating a holograph while the object is subjected to a periodic stress or vibration. This yields a visual image of the vibration pattern.

21
data/en.wikipedia.org/wiki/Interferometry-8.md Normal file
View File

@ -0,0 +1,21 @@
---
title: "Interferometry"
chunk: 9/10
source: "https://en.wikipedia.org/wiki/Interferometry"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:15.174742+00:00"
instance: "kb-cron"
---
Interferometric synthetic aperture radar (InSAR) is a radar technique used in geodesy and remote sensing. Satellite synthetic aperture radar images of a geographic feature are taken on separate days, and changes that have taken place between radar images taken on the separate days are recorded as fringes similar to those obtained in holographic interferometry. The technique can monitor centimeter- to millimeter-scale deformation resulting from earthquakes, volcanoes and landslides, and also has uses in structural engineering, in particular for the monitoring of subsidence and structural stability. Fig 20 shows Kilauea, an active volcano in Hawaii. Data acquired using the space shuttle Endeavour's X-band Synthetic Aperture Radar on April 13, 1994 and October 4, 1994 were used to generate interferometric fringes, which were overlaid on the X-SAR image of Kilauea.
Electronic speckle pattern interferometry (ESPI), also known as TV holography, uses video detection and recording to produce an image of the object upon which is superimposed a fringe pattern which represents the displacement of the object between recordings. (see Fig. 21) The fringes are similar to those obtained in holographic interferometry.
When lasers were first invented, laser speckle was considered to be a severe drawback in using lasers to illuminate objects, particularly in holographic imaging because of the grainy image produced. It was later realized that speckle patterns could carry information about the object's surface deformations. Butters and Leendertz developed the technique of speckle pattern interferometry in 1970, and since then, speckle has been exploited in a variety of other applications. A photograph is made of the speckle pattern before deformation, and a second photograph is made of the speckle pattern after deformation. Digital subtraction of the two images results in a correlation fringe pattern, where the fringes represent lines of equal deformation. Short laser pulses in the nanosecond range can be used to capture very fast transient events. A phase problem exists: In the absence of other information, one cannot tell the difference between contour lines indicating a peak versus contour lines indicating a trough. To resolve the issue of phase ambiguity, ESPI may be combined with phase shifting methods.
A method of establishing precise geodetic baselines, invented by Yrjö Väisälä, exploited the low coherence length of white light. Initially, white light was split in two, with the reference beam "folded", bouncing back-and-forth six times between a mirror pair spaced precisely 1 m apart. Only if the test path was precisely 6 times the reference path would fringes be seen. Repeated applications of this procedure allowed precise measurement of distances up to 864 meters. Baselines thus established were used to calibrate geodetic distance measurement equipment, leading to a metrologically traceable scale for geodetic networks measured by these instruments. (This method has been superseded by GPS.)
Other uses of interferometers have been to study dispersion of materials, measurement of complex indices of refraction, and thermal properties. They are also used for three-dimensional motion mapping including mapping vibrational patterns of structures.
Potential military applications of laser interferometry, documented as of 1964, included "precision positioning in research, manufacture, and in field use of equipment, such as missile alignment or turret aiming equipment; in geodesy and mapping, where one does not need a map accurate to microinches, but where the accurate establishment of base lines is essential; and in the remote measuring of variations of temperature in the atmosphere [...] which affect optical seeing ability."
=== Biology and medicine ===
Optical interferometry, applied to biology and medicine, provides sensitive metrology capabilities for the measurement of biomolecules, subcellular components, cells and tissues. Many forms of label-free biosensors rely on interferometry because the direct interaction of electromagnetic fields with local molecular polarizability eliminates the need for fluorescent tags or nanoparticle markers. At a larger scale, cellular interferometry shares aspects with phase-contrast microscopy, but comprises a much larger class of phase-sensitive optical configurations that rely on optical interference among cellular constituents through refraction and diffraction. At the tissue scale, partially-coherent forward-scattered light propagation through the micro aberrations and heterogeneity of tissue structure provides opportunities to use phase-sensitive gating (optical coherence tomography) as well as phase-sensitive fluctuation spectroscopy to image subtle structural and dynamical properties.
Optical coherence tomography (OCT) is a medical imaging technique using low-coherence interferometry to provide tomographic visualization of internal tissue microstructures. As seen in Fig. 22, the core of a typical OCT system is a Michelson interferometer. One interferometer arm is focused onto the tissue sample and scans the sample in an X-Y longitudinal raster pattern. The other interferometer arm is bounced off a reference mirror. Reflected light from the tissue sample is combined with reflected light from the reference. Because of the low coherence of the light source, interferometric signal is observed only over a limited depth of sample. X-Y scanning therefore records one thin optical slice of the sample at a time. By performing multiple scans, moving the reference mirror between each scan, an entire three-dimensional image of the tissue can be reconstructed. Recent advances have striven to combine the nanometer phase retrieval of coherent interferometry with the ranging capability of low-coherence interferometry.

31
data/en.wikipedia.org/wiki/Interferometry-9.md Normal file
View File

@ -0,0 +1,31 @@
---
title: "Interferometry"
chunk: 10/10
source: "https://en.wikipedia.org/wiki/Interferometry"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:15.174742+00:00"
instance: "kb-cron"
---
Phase contrast and differential interference contrast (DIC) microscopy are important tools in biology and medicine. Most animal cells and single-celled organisms have very little color, and their intracellular organelles are almost totally invisible under simple bright field illumination. These structures can be made visible by staining the specimens, but staining procedures are time-consuming and kill the cells. As seen in Figs. 24 and 25, phase contrast and DIC microscopes allow unstained, living cells to be studied. DIC also has non-biological applications, for example in the analysis of planar silicon semiconductor processing.
Angle-resolved low-coherence interferometry (a/LCI) uses scattered light to measure the sizes of subcellular objects, including cell nuclei. This allows interferometry depth measurements to be combined with density measurements. Various correlations have been found between the state of tissue health and the measurements of subcellular objects. For example, it has been found that as tissue changes from normal to cancerous, the average cell nuclei size increases.
Phase-contrast X-ray imaging (Fig. 26) refers to a variety of techniques that use phase information of a coherent x-ray beam to image soft tissues. (For an elementary discussion, see Phase-contrast x-ray imaging (introduction). For a more in-depth review, see Phase-contrast X-ray imaging.) It has become an important method for visualizing cellular and histological structures in a wide range of biological and medical studies. There are several technologies being used for x-ray phase-contrast imaging, all utilizing different principles to convert phase variations in the x-rays emerging from an object into intensity variations. These include propagation-based phase contrast, Talbot interferometry, Moiré-based far-field interferometry, refraction-enhanced imaging, and x-ray interferometry. These methods provide higher contrast compared to normal absorption-contrast x-ray imaging, making it possible to see smaller details. A disadvantage is that these methods require more sophisticated equipment, such as synchrotron or microfocus x-ray sources, x-ray optics, or high resolution x-ray detectors.
== See also ==
Coherence
Coherence scanning interferometry
Fine Guidance Sensor (HST) (HST FGS are interferometers)
Holography
Interferometric visibility
Interference lithography
List of types of interferometers
Ramsey interferometry
Seismic interferometry
Superposition principle
Very-long-baseline interferometry
Zero spacing flux
== References ==
Media related to Interferometry at Wikimedia Commons

41
data/en.wikipedia.org/wiki/Level_(optical_instrument)-0.md Normal file
View File

@ -0,0 +1,41 @@
---
title: "Level (optical instrument)"
chunk: 1/2
source: "https://en.wikipedia.org/wiki/Level_(optical_instrument)"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:16.386089+00:00"
instance: "kb-cron"
---
A level is an optical instrument used to establish or verify points in the same horizontal plane in a process known as levelling. It is used in conjunction with a levelling staff to establish the relative height or levels (the vertical separation) of objects or marks. It is widely used in surveying and construction to measure height differences and to transfer, measure, and set heights of known objects or marks.
It is also known as a surveyor's level, builder's level, dumpy level or the historic "Y level". It operates on the principle of establishing a visual level relationship between two or more points, for which an inbuilt optical telescope and a highly accurate bubble level are used to achieve the necessary accuracy. Traditionally the instrument was completely adjusted manually to ensure a level line of sight, but modern automatic versions self-compensate for slight errors in the coarse levelling of the instrument, and are thereby quicker to use.
The optical level should not be confused with a theodolite, which can also measure angles in the vertical plane.
== Description ==
The complete unit is normally mounted on a tripod, and the telescope can freely rotate 360° in a horizontal plane. The surveyor adjusts the instrument's level by coarse adjustment of the tripod legs and fine adjustment using three precision levelling screws on the instrument to make the rotational plane horizontal. The surveyor does this with the use of a bull's eye level built into the instrument mount.
The surveyor looks through the eyepiece of the telescope while an assistant holds a vertical level staff which is graduated in inches or centimeters. The level staff is placed with its foot on the point for which the level measurement is required. The telescope is rotated and focused until the level staff is plainly visible in the crosshairs. In the case of a tilting level, the fine level adjustment is made by an altitude screw, using a high accuracy bubble level fixed to the telescope. This can be viewed by a mirror whilst adjusting, or the ends of the bubble in a "split bubble" display can be viewed within the telescope. This also allows assurance of the accurate level of the telescope whilst the sight is being taken. However, in the case of an automatic level, altitude adjustment is done automatically by a suspended prism due to gravity, as long as the coarse levelling of the instrument base is accurate within certain limits.
When level, the staff graduation readings at the crosshairs and stadia marks are recorded, and an identifying mark or marker placed where the level staff rested on the object or position being surveyed.
== Invention ==
In 1832, English civil engineer and inventor William Gravatt, who was commissioned to examine a scheme for the South Eastern Railway's route from London to Dover, became frustrated with the slow and cumbersome operation of the "Y" level during the survey work, and devised the more transportable, easier-to-use "dumpy" level, so called because of its shorter appearance.
The telescope of the historic "Y" level is held in two brass arms, which are part of the mount and the telescope could be easily removed to allow sighting reversal though 180 degrees or an axial rotation of the telescope; both to compensate for optical collimation errors. Because the telescope is not fixed to the level adjusting mechanism, the "Y" instrument is assembled and disassembled for each sighting station. However, the dumpy level is permanently secured to its two support arms and the levelling mechanism, thereby reducing measurement uncertainty and considerably reducing the time taken to set up the instrument. The dumpy uses the same basic principle of level sighting.
== Survey operation ==
After careful setup of the level, the height of the crosshairs is determined by either sighting from a known benchmark with known height determined by a previous survey or an arbitrary point with an assumed height is used.
Sighting is done with an assistant surveyor who holds a graduated staff vertical at the point under measurement. The surveyor rotates the telescope until the graduated staff is in the crosshairs and records the reading. This is repeated for all sightings from that datum. Should the instrument be moved to another position within sighting distance, it is re-levelled, and a sighting taken of a known level in the previous survey. This relates any new levels to the previous levels.
== Variants ==
=== Y level ===
The Y level or wye level is the oldest and bulkiest of the older style optical instruments. A low-powered telescope is placed in a pair of clamp mounts, and the instrument then leveled using a spirit level, which is mounted parallel to the main telescope.
=== Dumpy level ===
The term dumpy level (also builder's level) endures despite the evolution in design. They can be manual or automatic, the latter being much quicker to set up.
=== Tilting level ===
A tilting level is a variant which has a precision vertical adjustment screw which tilts both the telescope and the high accuracy bubble level attached to it to make them level. This reduces the complete reliance on the levelling accuracy of the instruments' bottom mount, and the "split bubble" display gives additional assurance that the telescope is level whilst taking the sight. This allows faster operation as the bottom mount need not be truly level, though it will introduce a slight error as the vertical axis of the mount is not completely coincident with the telescope centre. The split bubble works by displaying half of both ends of the bubble side by side in the telescope, and when the curved ends are aligned it is level.
=== Automatic level ===

36
data/en.wikipedia.org/wiki/Level_(optical_instrument)-1.md Normal file
View File

@ -0,0 +1,36 @@
---
title: "Level (optical instrument)"
chunk: 2/2
source: "https://en.wikipedia.org/wiki/Level_(optical_instrument)"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:16.386089+00:00"
instance: "kb-cron"
---
An automatic level, self-levelling level, or builder's auto level includes an internal compensator mechanism (a swinging prism) that, when set close to level, automatically removes any remaining variation. This reduces the need to set the instrument base truly level, as with a dumpy level. Self-levelling instruments are the preferred instrument on building sites, construction, and during surveying due to ease of use and rapid setup time. The world's first automatic level was introduced on 8 March 1950. It was produced by the German company Zeiss-Opton in Oberkochen. Main components of an automatic level include a tribrach – a lower, fixed, triangular or circular element containing three sockets for adjusting screws (or wedge rings), attached to the tripod with a heart screw – and an alidade – a movable (rotatable) part. In automatic levels, the spirit level is most often replaced by a pendulum-based opto-mechanical device called a compensator.
=== Digital electronic level ===
A digital electronic level is also set level on a tripod and reads a bar-coded staff using electronic laser methods. The height of the staff where the level beam crosses the staff is shown on a digital display. This type of level removes interpolation of graduation by a person, thus removing a source of error and increasing accuracy. During night time, the dumpy level is used in conjunction with an auto cross laser for accurate scale readings.
=== Transit level ===
A transit level also has the ability to measure both the altitude and azimuth of a target object with respect to a reference in the horizontal plane. The instrument is rotated to sight the target, and the vertical and horizontal angles are read off calibrated scales
== In popular culture ==
In the first chapter of Thomas Hardy's 1887 novel The Woodlanders, the narrator states, "He knew every subtle incline of the ten miles of ground between Abbot's Cernel and Sherton—the market town to which he journeyed—as accurately as any surveyor could have learnt it by a Dumpy level."
In the online game World of Warcraft, there is a quest in Wetlands given by Surveyor Thurdan to retrieve his lost dumpy level. He even comments on the name, saying, "I didn't name the bloody thing, alright? Go look it up!"
== See also ==
Glossary of levelling terms
Rotary laser level, laser levels used in surveying and construction
Line laser level, laser levels used in carpentry and home improvement
Theodolite
Total station
Philadelphia rod
Water level (device)
== References ==
== External links ==
Checking a level
Wye level (Y level)

17
data/en.wikipedia.org/wiki/List_of_viewing_instruments-0.md Normal file
View File

@ -0,0 +1,17 @@
---
title: "List of viewing instruments"
chunk: 1/1
source: "https://en.wikipedia.org/wiki/List_of_viewing_instruments"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:17.548748+00:00"
instance: "kb-cron"
---
A viewing instrument is a type of optical instrument that is used to assist viewing or visually examining an object or scenery.
== Types ==
== References ==

22
data/en.wikipedia.org/wiki/Lovibond_comparator-0.md Normal file
View File

@ -0,0 +1,22 @@
---
title: "Lovibond comparator"
chunk: 1/1
source: "https://en.wikipedia.org/wiki/Lovibond_comparator"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:18.709605+00:00"
instance: "kb-cron"
---
The Lovibond comparator is an example of a colorimeter made in Britain by The Tintometer Ltd. It was invented in the 19th century by Joseph Williams Lovibond, a British brewer and chemist, as a tool to standardize the production of beer. Updated versions are still available.
== Description ==
The device is used to determine the color of liquids. A sample is put in a glass tube. The tube is inserted in the comparator and compared with a series of colored glass discs until the nearest possible match is found.
Among other things, the device is used to determine the concentration of certain chemicals in solution. In this use, some assumptions are made about what is in the sample. Given those assumptions, the concentration will be indicated by the disc which best matches the color of the solution.
There are a number of standard tests in which a sample to be tested is mixed with a color reagent. In such tests, the resulting color indicates the concentration of the sample under test (see Beer–Lambert law).
Results can be approximate, compared to other testing techniques, but the comparator is useful for field work because it is portable, rugged and easy to use. If a more exact measurement is required other tests can be conducted in a laboratory.
It may be used in chemistry laboratories to approximately measure the pH of a sample.
== References ==

64
data/en.wikipedia.org/wiki/MERMAID-0.md Normal file
View File

@ -0,0 +1,64 @@
---
title: "MERMAID"
chunk: 1/1
source: "https://en.wikipedia.org/wiki/MERMAID"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:53.042340+00:00"
instance: "kb-cron"
---
MERMAID is a marine scientific instrument platform, short for Mobile Earthquake Recorder for Marine Areas by Independent Divers.
MERMAID evolved from a first prototype, developed and built by Scripps Institution of Oceanography in partnership with Princeton University, to a second, built by Teledyne Webb Research in collaboration with the University of Nice Sophia Antipolis, and now third-generation model, operational today, commercialized by OSEAN SAS in Le Pradet, France. Fourth-generation models add hydrographic monitoring capability to complement the acoustic sensor suite, and are designed to carry out measurement profiles to depths exceeding 4,000 m.
MERMAID is a freely-drifting float equipped with a hydrophone to collect hydroacoustic data for the study of earthquakes worldwide. Typically floating at a parking depth of 1500 m, the instrument uses a buoyancy engine (a hydraulic oil bladder system) to return to the surface for triggered data transmission (on average every 6–7 days) via the Iridium satellite constellation, to respond to on-demand data requests, and to receive mission parameter updates. MERMAID carries lithium-ion batteries, sufficient to power about 250 descend/ascend cycles, which translates to an instrument autonomy of about 5 years. A pressure sensor monitors descent depth, and a GPS receiver provides location and time corrections during the brief intervals that MERMAID surfaces (on average less than one hour).
Fourth-generation models are multidisciplinary and carry a conductivity-temperature-depth sensor to collect hydrographic profiles of ocean temperature and salinity (similar to those from the Argo program) during their voyages. They can be additionally equipped with high-frequency hydrophones for the study of, e.g. cetacean (whale) vocalizations, and other sensors.
== Scientific objectives and capabilities ==
Imaging Earth's interior via the technique of seismic tomography is reliant on dense source-receiver distribution, or data coverage, but two thirds of the Earth's surface are covered by water. Increasing station density in the oceanic domain is an objective widely shared in the global seismological research community. After the first detections of teleseismic events by first-generation MERMAID, relatively small-scale deployments of second-generation MERMAID instruments in the Mediterranean, the Indian Ocean, and in the Pacific around the Galápagos, demonstrated MERMAID's potential for closing the oceanic seismic coverage gap, both for global and regional seismic events, and for seismic tomography of the Earth's mantle.
The ongoing multinational experiment SPPIM (South Pacific Plume Imaging and Modeling), coordinated by Ifremer with JAMSTEC, deployed an array of fifty-one third-generation instruments to image, in detail, the massive mantle plume in the lower mantle beneath the South Pacific. The instruments are owned by Southern University of Science and Technology, Princeton University, JAMSTEC and Géoazur, and the data management and distribution is handled by EarthScope-Oceans.
The standard third-generation model reports pressure time-series, waveforms triggered by earthquakes, whereas the third-generation model deployed in the Mediterranean was configured to report time-resolved hydroacoustic spectral densities.
== Deployments and network configuration ==
MERMAIDs first-generation model (2003-2005) retired after gathering about 120 hours of acoustic pressure data from a depth of around 700 m offshore from La Jolla, California.
Of the second-generation MERMAIDs (2012-2016), the first were deployed in the Mediterranean and recovered after 10, respectively 18 months of autonomous operations. Other deployments followed in the Indian Ocean, and in the Pacific around Galápagos, where an array of nine MERMAIDs remained operational for about two years.
Sixty-seven third-generation MERMAIDs (2018-now) were launched in the Pacific Ocean, the South China Sea, and the Mediterranean, from a variety of international (French, Japanese, Chinese) research vessels.
== Data collection and distribution ==
Every MERMAID instrumental sensor has a unique identifier. In contrast to conventional (land-based) seismometers or ocean-bottom seismometers (OBS), MERMAID instruments are passively adrift with the ocean currents: they do not remain at any fixed geographic location. Data from particular units are location-tagged hydroacoustic time-series as recorded at depth in the oceans. Data segments triggering transmission mostly contain pressure-wave signals from particular earthquakes worldwide, but also noise generated by a variety of sources (e.g. microseisms or volcanic eruptions). Since GPS signals do not penetrate under water, the actual location of recording specific events is derived from interpolation in post-processing.
Seismic data from the US and French MERMAIDs are being deposited with the IRIS Consortium. Primary seismoacoustic arrivals from distant teleseismic earthquakes are prioritized for automatic reporting, although the complete records (and the year-long buffer, which can be queried remotely) contain multiple other types of seismic arrivals. Seismic waveforms are released to the public through the IRIS Data Management Center, after a rolling embargo period of typically two years.
Trajectory metadata are released by EarthScope-Oceans in near real-time. Float trajectories allow for the reconstruction of ocean currents, and are used in educational and outreach programs, e.g. using the free iOS app Adopt-A-Float.
== The EarthScope-Oceans Consortium ==
EarthScope-Oceans is an international academic consortium that collects seismic data using robotic mobile—drifting—diving platforms (profiling floats) in the world's oceans, and distributes them to scientific user communities, with the objective to plug the oceanic data coverage gap in earthquake detection. MERMAID is EarthScope-Oceans' chief instrument platform.
Funded in part by the US National Science Foundation (NSF), EarthScope-Oceans is not affiliated with NSF's EarthScope program. EarthScope is a trademark of the IRIS Consortium.
EarthScope-Oceans is one of 361 Decade Actions endorsed by the Intergovernmental Oceanographic Commission of UNESCO, part of the Ocean Observing Co-Design program, falling under the umbrella of the United Nations Decade of Ocean Science for Sustainable Development (2021-2030).
The expansion of the EarthScope-Oceans fleet to include new multidisciplinary MERMAID models adds oceanography, meteorology, climate science, and bioacoustics to the seismological domain of interest of the EarthScope-Oceans Consortium.
EarthScope-Oceans is a member organization of the International Federation of Digital Seismograph Networks. Its network code is MH, and its doi 10.7914/SN/MH. EarthScope-Oceans is a member of the Marine Technology Society.
== References ==
== External links ==
EarthScope-Oceans Data & Metadata
EarthScope-Oceans: 300 MERMAIDS
UN Decade of Ocean Science for Sustainable Development
EarthScope-Oceans Code Repository
Adopt-A-Float iOS App
Adopt-A-Float iOS App Code Repository
The Argo Program
South Pacific Plume Imaging and Modeling (SPPIM)
The GLOBALSEIS Project
IRIS Data Management Center
The Global Seismographic Network
Hi Tech hydrophones
OSEAN SAS
RBR profiling CTD sensors
Sea-Bird profiling CTD sensors
Teledyne Webb Research

25
data/en.wikipedia.org/wiki/Mirror_galvanometer-0.md Normal file
View File

@ -0,0 +1,25 @@
---
title: "Mirror galvanometer"
chunk: 1/2
source: "https://en.wikipedia.org/wiki/Mirror_galvanometer"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:19.898895+00:00"
instance: "kb-cron"
---
A mirror galvanometer is an ammeter that indicates it has sensed an electric current by deflecting a light beam with a mirror. The beam of light projected on a scale acts as a long massless pointer. In 1826, Johann Christian Poggendorff developed the mirror galvanometer for detecting electric currents. The apparatus is also known as a spot galvanometer after the spot of light produced in some models.
Mirror galvanometers were used extensively in scientific instruments before reliable, stable electronic amplifiers were available. The most common uses were as recording equipment for seismometers and submarine cables used for telegraphy.
In modern times, the term mirror galvanometer is also used for devices that move laser beams by rotating a mirror through a galvanometer set-up, often with a servo-like control loop. The name is often abbreviated as galvo.
== Kelvin's galvanometer ==
The mirror galvanometer was improved significantly by William Thomson, later to become Lord Kelvin. He coined the term mirror galvanometer and patented the device in 1858. Thomson intended the instrument to read weak signal currents on very long submarine telegraph cables. This instrument was far more sensitive than any which preceded it, enabling the detection of the slightest defect in the core of a cable during its manufacture and submersion.
Thomson decided that he needed an extremely sensitive instrument after he took part in the failed attempt to lay a transatlantic telegraph cable in 1857. He worked on the device while waiting for a new expedition the following year. He first looked at improving a galvanometer used by Hermann von Helmholtz to measure the speed of nerve signals in 1849. Helmholtz's galvanometer had a mirror fixed to the moving needle, which was used to project a beam of light onto the opposite wall, thus greatly amplifying the signal. Thomson intended to make this more sensitive by reducing the mass of the moving parts, but in a flash of inspiration while watching the light reflected from his monocle suspended around his neck, he realised that he could dispense with the needle and its mounting altogether. He instead used a small piece of mirrored glass with a small piece of magnetised steel glued on the back. This was suspended by a thread in the magnetic field of the fixed sensing coil. In a hurry to try the idea, Thomson first used a hair from his dog, but later used a silk thread from the dress of his niece Agnes.
The following is adapted from a contemporary account of Thomson's instrument:
The mirror galvanometer consists of a long fine coil of silk-covered copper wire. In the heart of that coil, within a little air-chamber, a small round mirror is hung by a single fibre of floss silk, with four tiny magnets cemented to its back. A beam of light is thrown from a lamp upon the mirror, and reflected by it upon a white screen or scale a few feet distant, where it forms a bright spot of light. When there is no current on the instrument, the spot of light remains stationary at the zero position on the screen; but the instant a current traverses the long wire of the coil, the suspended magnets twist themselves horizontally out of their former position, the mirror is inclined with them, and the beam of light is deflected along the screen to one side or the other, according to the nature of the current. If a positive electric current gives a deflection to the right of zero, a negative current will give a deflection to the left of zero, and vice versa.
The air in the little chamber surrounding the mirror is compressed at will, so as to act like a cushion, and deaden the movements of the mirror. The needle is thus prevented from idly swinging about at each deflection, and the separate signals are rendered abrupt. At a receiving station the current coming in from the cable has simply to be passed through the coil before it is sent into the ground, and the wandering light spot on the screen faithfully represents all its variations to the clerk, who, looking on, interprets these, and cries out the message word by word. The small weight of the mirror and magnets which form the moving part of this instrument, and the range to which the minute motions of the mirror can be magnified on the screen by the reflected beam of light, which acts as a long impalpable hand or pointer, render the mirror galvanometer marvellously sensitive to the current, especially when compared with other forms of receiving instruments. Messages could be sent from the United Kingdom to the United States through one Atlantic cable and back again through another, and there received on the mirror galvanometer, the electric current used being that from a toy battery made out of a lady's silver thimble, a grain of zinc, and a drop of acidulated water.
The practical advantage of this extreme delicacy is that the signal waves of the current may follow each other so closely as almost entirely to coalesce, leaving only a very slight rise and fall of their crests, like ripples on the surface of a flowing stream, and yet the light spot will respond to each. The main flow of the current will of course shift the zero of the spot, but over and above this change of place the spot will follow the momentary fluctuations of the current which form the individual signals of the message. What with this shifting of the zero and the very slight rise and fall in the current produced by rapid signalling, the ordinary land line instruments are quite unserviceable for work upon long cables.
== Moving coil galvanometer ==

29
data/en.wikipedia.org/wiki/Mirror_galvanometer-1.md Normal file
View File

@ -0,0 +1,29 @@
---
title: "Mirror galvanometer"
chunk: 2/2
source: "https://en.wikipedia.org/wiki/Mirror_galvanometer"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:19.898895+00:00"
instance: "kb-cron"
---
Moving coil galvanometer was developed independently by Marcel Deprez and Jacques-Arsène d'Arsonval about 1880. Deprez's galvanometer was developed for high currents, while D'Arsonval designed his to measure weak currents. Unlike in the Kelvin's galvanometer, in this type of galvanometer the magnet is stationary and the coil is suspended in the magnet gap. The mirror attached to the coil frame rotates together with it. This form of instrument can be more sensitive and accurate and it replaced the Kelvin's galvanometer in most applications. The moving coil galvanometer is practically immune to ambient magnetic fields. Another important feature is self-damping generated by the electro-magnetic forces due to the currents induced in the coil by its movements the magnetic field. These are proportional to the angular velocity of the coil.
== Modern uses ==
In modern times, high-speed mirror galvanometers are employed in laser light shows to move the laser beams and produce colorful geometric patterns in fog around the audience. Such high speed mirror galvanometers have proved to be indispensable in industry for laser marking systems for everything from laser etching hand tools, containers, and parts to batch-coding semiconductor wafers in semiconductor device fabrication. They typically control X and Y directions on Nd:YAG and CO2 laser markers to control the position of the infrared power laser spot. Laser ablation, laser beam machining and wafer dicing are all industrial areas where high-speed mirror galvanometers can be found.
This moving coil galvanometer is mainly used to measure very feeble or low currents of order 10−9 A.
To linearise the magnetic field across the coil throughout the galvanometer's range of movement, the d'Arsonval design of a soft iron cylinder is placed inside the coil without touching it. This gives a consistent radial field, rather than a parallel linear field.
== See also ==
String galvanometer
== References ==
== Further reading ==
"Marcel Deprez's Galvanometer for Strong Currents". Nature. 22 (559): 246–7. 15 July 1880. Bibcode:1880Natur..22R.246.. doi:10.1038/022246b0.
== External links ==
Mirror Galvanometer - Interactive Java Tutorial National High Magnetic Field Laboratory

58
data/en.wikipedia.org/wiki/N-slit_interferometer-0.md Normal file
View File

@ -0,0 +1,58 @@
---
title: "N-slit interferometer"
chunk: 1/1
source: "https://en.wikipedia.org/wiki/N-slit_interferometer"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:21.075248+00:00"
instance: "kb-cron"
---
The N-slit interferometer is an extension of the double-slit interferometer also known as Young's double-slit interferometer. One of the first known uses of N-slit arrays in optics was illustrated by Newton. In the first part of the twentieth century, Michelson described various cases of N-slit diffraction.
Feynman described thought experiments the explored two-slit quantum interference of electrons, using Dirac's notation. This approach was extended to N-slit interferometers, by Duarte and colleagues in 1989, using narrow-linewidth laser illumination, that is, illumination by indistinguishable photons. The first application of the N-slit interferometer was the generation and measurement of complex interference patterns. These interferograms are accurately reproduced, or predicted, by the N-slit interferometric equation for either even (N = 2, 4, 6,...), or odd (N = 3, 5, 7,...), numbers of slits.
== N-slit laser interferometer ==
The N-slit laser interferometer, introduced by Duarte, uses prismatic beam expansion to illuminate a transmission grating, or N-slit array, and a photoelectric detector array (such as a CCD or CMOS) at the interference plane to register the interferometric signal. The expanded laser beam illuminating the N-slit array is single-transverse-mode and narrow-linewidth. This beam can also take the shape, via the introduction of a convex lens prior to the prismatic expander, of a beam extremely elongated in the propagation plane and extremely thin in the orthogonal plane. This use of one-dimensional (or line) illumination eliminates the need of point-by-point scanning in microscopy and microdensitometry. Thus, these instruments can be used as straight forward N-slit interferometers or as interferometric microscopes.
The disclosure of this interferometric configuration introduced the use of digital detectors to N-slit interferometry.
== Applications ==
=== Secure optical communications ===
These interferometers, originally introduced for applications in imaging, are also useful in optical metrology and have been proposed for secure optical communications in free space, between spacecraft. This is due to the fact that propagating N-slit interferograms suffer catastrophic collapse from interception attempts using macroscopic optical methods such as beam splitting. Recent experimental developments include terrestrial intra-interferometric path lengths of 35 meters and 527 meters.
These large, and very large, N-slit interferometers are used to study various propagation effects including microscopic disturbances on propagating interferometric signals. This work has yielded the first observation of diffraction patterns superimposed over propagating interferograms.
These diffraction patterns (as shown in the first photograph) are generated by inserting a spider web fiber (or spider silk thread) into the propagation path of the interferogram. The position of the spider web fiber is perpendicular to the propagation plane.
=== Clear air turbulence ===
N-slit interferometers, using large intra interferometric distances, are detectors of clear air turbulence. The distortions induced by clear air turbulence upon the interferometric signal are different, in both character and magnitude, from the catastrophic collapse resulting from attempted interception of optical signals using macroscopic optical elements.
=== Expanded beam interferometric microscopy ===
The original application of the N-slit laser interferometer was interferometric imaging. In particular, the one dimensionally expanded laser beam (with a cross section 25-50 mm wide by 10-25 μm high) was used to illuminate imaging surfaces (such as silver-halide films) to measure the microscopic density of the illuminated surface. Hence the term interferometric microdensitometer. Resolution down to the nano regime can be provided via the use of interinterferometric calculations. When used as a microdensitometer the N-slit interferometer is also known as a laser microdensitometer.
The multiple-prism expanded laser beam is also described as an extremely elongated laser beam. The elongated dimension of the beam (25-50 mm) is in the propagation plane while the very thin dimension (in the μm regime) of the beam is in the orthogonal plane. This was demonstrated, for imaging and microscopy applications, in 1993. Alternative descriptions of this type of extremely elongated illumination include the terms line illumination, linear illumination, thin light sheet illumination (in light sheet microscopy), and plane illumination (in selective plane illumination microscopy).
=== Other applications ===
N-slit interferometers are of interest to researchers working in atom optics, Fourier imaging, optical computing, and quantum computing.
== See also ==
Beam expander
Clear air turbulence
Diffraction from slits
Double-slit experiment
Free-space optical communication
Laser communication in space
Microscopy
Microdensitometer
N-slit interferometric equation
List of laser articles
== References ==

35
data/en.wikipedia.org/wiki/Ocean-bottom_seismometer-0.md Normal file
View File

@ -0,0 +1,35 @@
---
title: "Ocean-bottom seismometer"
chunk: 1/2
source: "https://en.wikipedia.org/wiki/Ocean-bottom_seismometer"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:54.235687+00:00"
instance: "kb-cron"
---
An ocean-bottom seismometer (OBS) is a seismometer that is designed to record the earth's motion under oceans and lakes from man-made sources and natural sources.
Sensors at the sea floor are used to observe acoustic and seismic events. Seismic and acoustic signals may be caused by different sources, by earthquakes and tremors as well as by artificial sources. Computing and analyzing the data yields information about the kind of source and, in case of natural seismic events, the geophysics and geology of the sea floor and the deeper crust. The deployment of OBS along a profile will give information about the deep structure of the Earth's crust and upper mantle in offshore areas. OBS may be equipped with a maximum of a three-component geophone in addition to a hydrophone, and thus it needs a capacity of more than 144 Mbytes, which would be the minimum for adequate MCS profiling. In a typical survey, the instruments should be operational for several days (deployments can exceed 12 months), which requires a data storage capacity of more than 500 Mbyte. Other experiments, such as tomographic investigations within a 3D-survey or seismological monitoring, demand even larger capacities.
== Instrument package ==
The OBS consists of an aluminum sphere which contains sensors, electronics, enough alkaline batteries to last 10 days on the ocean bottom, and an acoustic release. The two sphere halves are put together with an O-ring and a metal clamp to hold the halves together. A slight vacuum is placed on the sphere to better ensure a seal. The sphere by itself floats, so an anchor is needed to sink the instrument to the bottom. In this case, the anchor is a flat metal plate 40 inches (1.02 meters) in diameter. The instrument has been designed to be able to deploy and recover off almost any vessel. All that is needed (for deployment and recovery) is enough deck space to hold the instruments and their anchors and a boom capable of lifting an OBS off the deck and swinging it over to lower it into the water. The OBS is bolted to the anchor and then dropped (gently) over the side.
== Working ==
Seismometers work using the principle of inertia. The seismometer body rests securely on the sea floor. Inside, a heavy mass hangs on a spring between two magnets. When the earth moves, so do the seismometer and its magnets, but the mass briefly stays where it is. As the mass oscillates through the magnetic field it produces an electric current which the instrument measures. The seismometer itself is a small metal cylinder; the rest of the footlocker-sized OBS consists of equipment to run the seismometer (a data logger and batteries), weight to sink it to the sea floor, a remote-controlled acoustic release and flotation to bring the instrument back to the surface.
== Types of OBS ==
The ground motion caused by earthquakes can be extremely small (less than a millimeter) or large (several meters). Small motions have high frequencies, so monitoring them requires measuring movement many times per second and produces huge amounts of data. Large motions are much rarer, so instruments need to record data less frequently, to save memory space and battery power for longer deployments. Because of this variability, engineers have designed two basic kinds of seismometers:
=== Short-period OBSs ===
They record high-frequency motions (up to hundreds of times per second). They can record small, short-period earthquakes and are also useful for studying the outer tens of kilometers of the seafloor. Technical details for two models: WHOI D2 and Scripps L-CHEAPO.
=== Long-period OBSs ===
They record a much broader range of motions, with frequencies of about 10 per second to once or twice a minute. They are used for recording mid-sized earthquakes and seismic activity far from the instrument. Technical details for two models: WHOI long-deployment OBS and Scripps long-deployment OBS.
=== Custom OBSs ===
Custom OBS are beginning to be developed, as the need for expansion on coverage in the field of seismology increases and permanent deployments are necessary. One customizations to improve the data quality of the seismometers is to borehole the seismometer in an aluminium casing into the surface (~1 m) to create stability in the soft sediment of the ocean floor. Another customization that is possible is to add a differential pressure gauge (DPG) and/or current meter, to understand how the pressure is changing around the seismometer. It can also be practical to store the datalogger and battery in a glass Benthos sphere to be able to connect to the ship through the use of a remotely operated vehicle (ROV), which is a necessary advancement to have and maintain permanent OBS deployments.
== Advantages ==
Very stable clocks make comparable the readings from many far-flung seismometers. (Without reliable time-stamps, data from different machines would be unusable.) Development of these clocks was a crucial advance for seismologists studying the Earth's interior. After recovering an ocean-bottom seismometer, scientists can offload the instrument's data by plugging in a data cable. This feature saves the task of gingerly disassembling the instrument's protective casing while aboard a rolling ship. The ability to connect a seismometer to a mooring or observatory makes the instrument's data instantly available. This is a huge advantage for geologists scrambling to respond to a major earthquake.

23
data/en.wikipedia.org/wiki/Ocean-bottom_seismometer-1.md Normal file
View File

@ -0,0 +1,23 @@
---
title: "Ocean-bottom seismometer"
chunk: 2/2
source: "https://en.wikipedia.org/wiki/Ocean-bottom_seismometer"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:54.235687+00:00"
instance: "kb-cron"
---
== Disadvantages ==
The environment of these deployments complicates standard methods that are used in analyzing the data because of the ocean on top of the seismometer, as opposed to free-air above a typical land station. These seismometers also have a decreased signal-to-noise ratio because of noise created by the movement of the oceans due to wind driven tides, particularly at periods of 7 and 14 seconds. This long period motion and current flowing around the seismometer can create problems of long period noise on the horizontal components because the soft (saturated) sediment that the seismometer is resting on is more susceptible to allow the seismometer to tilt and ideally, the horizontal component will not move and be perpendicular to gravity to get the best results out of the seismometer. The saturated sediment also reduces signal-to-noise ratio significantly because the velocity of the P and S waves decreases and the seismic waves get trapped in the sediment layer creating a large amplitude ringing due to the conservation of energy.
== Notable deployments ==
One of the largest OBS deployments ever was The Big Mantle Electromagnetic and Tomography (Big MELT) Experiment, involving almost 100 OBS in the East Pacific Rise with the goal of understanding magma generation and mid-ocean ridge development. The Cascadia Initiative is an offshore/onshore deployment to observe the deformation of the Juan de Fuca and Gorda plates, as well as topics ranging from megathrust earthquakes to volcanic arc structure in the Pacific Northwest. The Hawaiian PLUME (Plume-Lithosphere Undersea Melt Experiment) was an onshore/offshore (predominantly offshore) deployment to better understand what type of mantle plume is beneath Hawaii and to better understand the mantle upwelling in this region and its relationship to the lithosphere. The Asthenospheric and Lithospheric Broadband Architecture from the California Offshore Region Experiment (ALBACORE) deployment from 2010 to 2011 of 34 OBS to help better understand the tectonic interaction at the Pacific-North America plate boundary and deformation styles of the Pacific plate and the nearby microplates.
== References ==
== External links ==
"French OBS" Archived 2014-01-16 at the Wayback Machine
"https://woodshole.er.usgs.gov/operations/obs/whatobs.html"
"K.U.M. OBS"
"GEOMAR Technologie"

252
data/en.wikipedia.org/wiki/Optical_correlator-0.md Normal file
View File

@ -0,0 +1,252 @@
---
title: "Optical correlator"
chunk: 1/1
source: "https://en.wikipedia.org/wiki/Optical_correlator"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:22.231203+00:00"
instance: "kb-cron"
---
An optical correlator is an optical computer for comparing two signals by utilising the Fourier transforming properties of a lens. It is commonly used in optics for target tracking and identification.
== Introduction ==
The correlator has an input signal which is multiplied by some filter in the Fourier domain. An example filter is the matched filter which uses the cross correlation of the two signals.
The cross correlation or correlation plane,
c
(
x
,
y
)
{\displaystyle c(x,y)}
of a 2D signal
i
(
x
,
y
)
{\displaystyle i(x,y)}
with
h
(
x
,
y
)
{\displaystyle h(x,y)}
is
c
(
x
,
y
)
=
i
(
x
,
y
)
⊗
h
∗
(
−
x
,
−
y
)
{\displaystyle c(x,y)=i(x,y)\otimes h^{*}(-x,-y)}
This can be re-expressed in Fourier space as
C
(
ξ
,
η
)
=
I
(
ξ
,
η
)
H
∗
(
−
ξ
,
−
η
)
{\displaystyle C(\xi ,\eta )=I(\xi ,\eta )H^{*}(-\xi ,-\eta )}
where the capital letters denote the Fourier transform of what the lower case letter denotes. So the correlation can then be calculated by inverse Fourier transforming the result.
== Implementation ==
According to Fresnel Diffraction theory a convex lens of focal length
f
{\displaystyle f}
will produce the exact Fourier transform at a distance
f
{\displaystyle f}
behind the lens of an object placed
f
{\displaystyle f}
distance in front of the lens. So that complex amplitudes are multiplied, the light source must be coherent and is typically from a laser. The input signal and filter are commonly written onto a spatial light modulator (SLM).
A typical arrangement is the 4f correlator. The input signal is written to an SLM which is illuminated with a laser. This is Fourier transformed with a lens and this is then modulated with a second SLM containing the filter. The resultant is again Fourier transformed with a second lens and the correlation result is captured on a camera.
== Filter design ==
Many filters have been designed to be used with an optical correlator. Some have been proposed to address hardware limitations, others were developed to optimize a merit function or to be invariant under a certain transformation.
=== Matched filter ===
The matched filter maximizes the signal-to-noise ratio and is simply obtained by using as a filter the Fourier transform of the reference signal
r
(
x
,
y
)
{\displaystyle r(x,y)}
.
H
(
ξ
,
η
)
=
R
(
ξ
,
η
)
{\displaystyle H(\xi ,\eta )=R(\xi ,\eta )}
=== Phase-only filter ===
The phase-only filter is easier to implement due to limitation of many SLMs and has been shown to be more discriminant than the matched filter.
H
(
ξ
,
η
)
=
R
(
ξ
,
η
)
|
R
(
ξ
,
η
)
|
{\displaystyle H(\xi ,\eta )={\frac {R(\xi ,\eta )}{\left\vert R(\xi ,\eta )\right\vert }}}
== References ==

34
data/en.wikipedia.org/wiki/Optical_power_meter-0.md Normal file
View File

@ -0,0 +1,34 @@
---
title: "Optical power meter"
chunk: 1/2
source: "https://en.wikipedia.org/wiki/Optical_power_meter"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:23.426777+00:00"
instance: "kb-cron"
---
An optical power meter (OPM) is a device used to measure the power in an optical signal. The term usually refers to a device for testing average power in fiber optic systems. Other general purpose light power measuring devices are usually called radiometers, photometers, laser power meters (can be photodiode sensors or thermopile laser sensors), light meters or lux meters.
A typical optical power meter consists of a calibrated sensor, measuring amplifier and display.
The sensor primarily consists of a photodiode selected for the appropriate range of wavelengths and power levels.
On the display unit, the measured optical power and set wavelength is displayed. Power meters are calibrated using a traceable calibration standard.
A traditional optical power meter responds to a broad spectrum of light, however, the calibration is wavelength dependent. This is not normally an issue, since the test wavelength is usually known, however, it has a couple of drawbacks. Firstly, the user must set the meter to the correct test wavelength, and secondly, if there are other spurious wavelengths present, then wrong readings will result.
Optical power meters are available as stand-alone bench or handheld instruments or combined with other test functions such as an Optical Light Source (OLS), Visual Fault Locator (VFL), or as sub-system in a larger or modular instrument. Commonly, a power meter on its own is used to measure absolute optical power, or used with a matched light source to measure loss.
When combined with a light source, the instrument is called an Optical Loss Test Set, or OLTS, typically used to measure optical power and end-to-end optical loss. More advanced OLTS may incorporate two or more power meters, and so can measure Optical Return Loss. GR-198, Generic Requirements for Hand-Held Stabilized Light Sources, Optical Power Meters, Reflectance Meters, and Optical Loss Test Sets, discusses OLTS equipment in depth.
Alternatively, an Optical Time Domain Reflectometer (OTDR) can measure optical link loss if its markers are set at the terminus points for which the fiber loss is desired. However, this is an indirect measurement. A single-direction measurement may quite inaccurate if there are multiple fibers in a link, since the back-scatter coefficient is variable between fibers. Accuracy can be increased if a bidirectional average is made. GR-196 Archived 2012-03-07 at the Wayback Machine, Generic Requirements for Optical Time Domain Reflectometer (OTDR) Type Equipment, discusses OTDR equipment in depth.
== Sensors ==
The major semiconductor sensor types are Silicon (Si), Germanium (Ge) and Indium Gallium Arsenide (InGaAs). Additionally, these may be used with attenuating elements for high optical power testing, or wavelength selective elements so they only respond to particular wavelengths. These all operate in a similar type of circuit, however, in addition to their basic wavelength response characteristics, each one has some other particular characteristics:
Si detectors tend to saturate at relatively low power levels, and they are only useful in the visible and 850 nm bands, where they offer generally good performance.
Ge detectors saturate at the highest power levels, but have poor low power performance, poor general linearity over the entire power range, and are generally temperature sensitive. They are only marginally accurate for "1550 nm" testing, due to a combination of temperature and wavelength affecting responsivity at e.g. 1580 nm, however they provide useful performance over the commonly used 850 / 1300 / 1550 nm wavelength bands, so they are extensively deployed where lower accuracy is acceptable. Other limitations include: non-linearity at low power levels, and poor responsivity uniformity across the detector area.
InGaAs detectors saturate at intermediate levels. They offer generally good performance, but are often very wavelength sensitive around 850 nm. So they are largely used for single-mode fiber testing at 1270 - 1650 nm.
An important part of an optical power meter sensor is the fiber optic connector interface. Careful optical design is required to avoid significant accuracy problems when used with the wide variety of fiber types and connectors typically encountered.
Another important component is the sensor input amplifier. This needs very careful design to avoid significant performance degradation over a wide range of conditions.
== Power measuring range ==
A typical OPM is linear from about 0 dBm (1 milli Watt) to about -50 dBm (10 nano Watt), although the display range may be larger. Above 0 dBm is considered "high power", and specially adapted units may measure up to nearly + 30 dBm ( 1 Watt). Below -50 dBm is "low power", and specially adapted units may measure as low as -110 dBm. Irrespective of power meter specifications, testing below about -50 dBm tends to be sensitive to stray ambient light leaking into fibers or connectors. So when testing at "low power", some sort of test range / linearity verification (easily done with attenuators) is advisable. At low power levels, optical signal measurements tend to become noisy, so meters may become very slow due to use of a significant amount of signal averaging.
To calculate dBm from power meter output :
The linear-to-dBm calculation method is:
dB = 10 log ( P1 / P2 )
where P1 = measured power level ( e.g. in mWatts ), P2 = reference power level, which is 1 mW

49
data/en.wikipedia.org/wiki/Optical_power_meter-1.md Normal file
View File

@ -0,0 +1,49 @@
---
title: "Optical power meter"
chunk: 2/2
source: "https://en.wikipedia.org/wiki/Optical_power_meter"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:23.426777+00:00"
instance: "kb-cron"
---
== Calibration and accuracy ==
Optical Power Meter calibration and accuracy is a contentious issue. The accuracy of most primary reference standards (e.g. Weight, Time, Length, Volt, etc.) is known to a high accuracy, typically of the order of 1 part in a billion. However the optical power standards maintained by various National Standards Laboratories, are only defined to about one part in a thousand. By the time this accuracy has been further degraded through successive links, instrument calibration accuracy is usually only a few %. The most accurate field optical power meters claim 1% calibration accuracy. This is orders of magnitude less accurate than a comparable electrical meter.
Calibration processes for optical power meters are given in IEC 61315 Ed. 3.0 b:2019 - Calibration of fibre-optic power meters.
Furthermore, the in-use accuracy achieved is usually significantly lower than the claimed calibration accuracy, by the time additional factors are taken into account. In typical field applications, factors may include: ambient temperature, optical connector type, wavelength variations, linearity variations, beam geometry variations, detector saturation.
Therefore, achieving a good level of practical instrument accuracy and linearity is something that requires design skill, and high manufacturing proficiency.
With the increasing global importance in the reliability of data transmission and optical fiber, and also the sharply reducing optical loss margin of these systems in data centres, there is increased emphasis on the accuracy of optical power meters, and also proper traceability compliance via International Laboratory Accreditation Cooperation (ILAC) accredited calibration, which includes metrological traceability to national standards and external laboratory accreditation to ISO/IEC 17025 to improve confidence in overall accuracy claims.
== Extended sensitivity meters ==
A class of laboratory power meters has an extended sensitivity, of the order of -110 dBm. This is achieved by using a very small detector and lens combination, and also a mechanical light chopper at typically 270 Hz, so the meter actually measures AC light. This eliminates unavoidable dc electrical drift effects. If the light chopping is synchronized with an appropriate synchronous (or "lock-in") amplifier, further sensitivity gains are achieved. In practice, such instruments usually achieve lower absolute accuracy due to the small detector diode, and for the same reason, may only be accurate when coupled with single-mode fiber. Occasionally such an instrument may have a cooled detector, though with the modern abandonment of Germanium sensors, and the introduction of InGaAs sensors, this is now increasingly uncommon.
== Pulse power measurement ==
Optical power meters usually display time-averaged power. So for pulse measurements, the signal duty cycle must be known to calculate the peak power value. However, the instantaneous peak power must be less than the maximum meter reading, or the detector may saturate, resulting in wrong average readings. Also, at low pulse repetition rates, some meters with data or tone detection may produce improper or no readings.
A class of "high power" meters has some type of optical attenuating element in front of the detector, typically allowing about a 20 dB increase in maximum power reading. Above this level, an entirely different class of "laser power meter" instrument is used, usually based on thermal detection.
== Common fiber optic test applications ==
Measuring the absolute power in a fiber optic signal. For this application, the power meter needs to be properly calibrated at the wavelength being tested, and set to this wavelength.
Measuring the optical loss in a fiber, in combination with a suitable stable light source. Since this is a relative test, accurate calibration is not a particular requirement, unless two or more meters are being used due to distance issues. If a more complex two-way loss test is performed, then power meter calibration can be ignored, even when two meters are used.
Some instruments are equipped for optical test tone detection, to assist in quick cable continuity testing. Standard test tones are usually 270 Hz, 1 kHz, 2 kHz. Some units can also determine one of 12 tones, for ribbon fiber continuity testing.
== Test automation ==
Typical test automation features usually apply to loss testing applications, and include:
The ability to set the unit to read 0 dB at a reference power level, typically the test source.
The ability to store readings into internal memory, for subsequent recall and download to a computer.
The ability to synchronize the wavelength with a test source, so that the meter sets to the source wavelength. This requires a specifically matched source. The simplest way of achieving this, is by recognizing a test tone, but the best way is by transfer of data. The data method has benefits that the source can send additional useful data such as nominal source power level, serial number etc.
== Wavelength-selective meters ==
An increasingly common special-purpose OPM, commonly called a "PON Power Meter" is designed to hook into a live PON (Passive Optical Network) circuit, and simultaneously test the optical power in different directions and wavelengths. This unit is essentially a triple power meter, with a collection of wavelength filters and optical couplers. Proper calibration is complicated by the varying duty cycle of the measured optical signals. It may have a simple pass/ fail display, to facilitate easy use by operators with little expertise.
Wavelength sensitivity of fiber optic power meter is a problem when using a photodiode for voltage current measurement. If the temperature-sensitive measurement replaces voltage-current measurement by photodiode the wavelength sensitivity of an OPM can be reduced. Thus if the photodiode is reverse biased by a constant voltage source and supplied with constant current, when triggered by light the junction dissipates power. The temperature of the junction rises and the temperature rise measured by thermistor is directly proportional to the Optical power. Due to constant current supply, the reflection of power to photodiode is nearly zero and the transition to and fro of electrons between valence band and conduction band is stable.
== See also ==
Optical attenuator
== References ==
== External links ==
OPM Application Notes
greenTEG Application Note Laser Power Measurement
Guidelines for specifying OPMs

26
data/en.wikipedia.org/wiki/Perspective_machine-0.md Normal file
View File

@ -0,0 +1,26 @@
---
title: "Perspective machine"
chunk: 1/1
source: "https://en.wikipedia.org/wiki/Perspective_machine"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:24.616799+00:00"
instance: "kb-cron"
---
A perspective machine is an optical instrument designed to help artists create perspective drawings. The earliest machines were built centuries ago when the theory of perspective was being worked out, and modern versions are still in use.
== Timeline ==
1510: Leonardo da Vinci's Draftsman drawing an armillary sphere shows an early form of perspective machine in use.
1525: Albrecht Dürer, in his illustration Man drawing a lute, shows an artist using a perspective machine to create a drawing. The machine consists of a wooden frame with a taut string passing through it to represent the viewer's line of sight. Dürer built his second model of such a machine in the same year.
c.1765: Scottish engineer James Watt designs a machine based on an easel, with a pantograph mechanism allowing the artist to trace an object using a sight arm and transfer the movement of the sight to a pen drawing on paper. Watt stated that his machine was based on an invention by a Mr Hurst, who lived in India.
1763: Philosopher Thomas Reid uses a machine to investigate his theory of perception.
1825: English inventor Francis Ronalds patents two perspective tracing machines. One generated an accurate drawing of an object or scene in nature and the other created a perspective view of an object from drawings of the plan and elevations. Ronalds manufactured the machines and sold several hundreds of them.
== References ==
== External links ==
DrawingMachines.org - pictorial archive of mechanical drawing aids

31
data/en.wikipedia.org/wiki/Phase_Shift_Interferometry-0.md Normal file
View File

@ -0,0 +1,31 @@
---
title: "Phase Shift Interferometry"
chunk: 1/1
source: "https://en.wikipedia.org/wiki/Phase_Shift_Interferometry"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:25.774011+00:00"
instance: "kb-cron"
---
Phase Shift Interferometry (PSI) is a technique for areal surface characterisation that focuses on digitising the interference data acquired during a controlled phase shift, usually introduced by the mechanical oscillation of an interference objective.
The technique provides full 3D images with typical height measurement repeatability of less than 1 nm, independent of field size. It is a powerful method that allows analysing interferograms to recover phase information. Traditionally, interferograms were measured by locating the centre of a fringe and then tracing along it. Phase shift interferometry avoids the need to track the location of fringes and enables a point-by-point reconstruction of the wavefront.
PSI offers significantly lower measurement noise, potentially as low as 0.01 nm. It is the preferred technique for measuring the surface roughness of super-smooth surfaces.
== Principle of Operation ==
Interferometers utilize the wave properties of light to analyse surface characteristics, particularly surface height variations. For evaluating areal surface topography, interferometers separate the source light so that it follows two independent paths: one includes a reference surface and the other the object surface. The separated light beams then recombine and are directed to a digital camera that measures the resultant light intensity over multiple image points simultaneously.
PSI acquires a sequence of images with a precisely controlled phase change between them. When a few fringes are visible on the surface, this manifests as a shift in fringe position between the images captured by a camera. The phase shifting is almost always generated by mechanical motion of the interference objective, which allows for fast, non-contact metrology.
In traditional interferometry, the phase information is encoded in the interference pattern. However, extracting this information can be challenging due to the ambiguity in determining the sign of the phase. PSI overcomes this limitation by introducing a known phase shift between the interfering waves.
By acquiring multiple interferograms with different phase shifts, PSI enables the accurate determination of phase information.
== Applications ==
PSI is widely used in high-precision manufacturing for surface metrology and quality control. Common applications include:
Optical component testing
Precision engineering
Semiconductor manufacturing
== References ==

42
data/en.wikipedia.org/wiki/Photometer-0.md Normal file
View File

@ -0,0 +1,42 @@
---
title: "Photometer"
chunk: 1/2
source: "https://en.wikipedia.org/wiki/Photometer"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:26.920738+00:00"
instance: "kb-cron"
---
A photometer is an instrument for measuring photometric quantities such as luminous flux, illuminance, or luminance.
Historically, photometry was done by estimation, comparing the luminous flux of a source with a standard source. By the 19th century, common photometers included Rumford's photometer, which compared the depths of shadows cast by different light sources, and Ritchie's photometer, which relied on equal illumination of surfaces. Another type was based on the extinction of shadows.
Most modern photometers detect light by converting it into an electric current using a photoresistor, photodiode, or photomultiplier. Some models employ photon counting, measuring light by counting individual photons. They are especially useful in areas where the irradiance is low. Photometers have wide-ranging applications including photography, where they determine the correct exposure, and science, where they are used in absorption spectroscopy to calculate the concentration of substances in a solution, infrared spectroscopy to study the structure of substances, and atomic absorption spectroscopy to determine the concentration of metals in a solution.
== History ==
Before electronic light sensitive elements were developed, photometry was done by estimation by the eye. The relative luminous flux of a source was compared with a standard source. The photometer is placed such that the illuminance from the source being investigated is equal to the standard source, as the human eye can judge equal illuminance. The relative luminous fluxes can then be calculated as the illuminance decreases proportionally to the inverse square of distance. A standard example of such a photometer consists of a piece of paper with an oil spot on it that makes the paper slightly more transparent. When the spot is not visible from either side, the illuminance from the two sides is equal.
By 1861, three types were in common use. These were Rumford's photometer, Ritchie's photometer, and photometers that used the extinction of shadows, which was considered to be the most precise.
=== Rumford's photometer ===
Rumford's photometer (also called a shadow photometer) depended on the principle that a brighter light would cast a deeper shadow. The two lights to be compared were used to cast a shadow onto paper. If the shadows were of the same depth, the difference in distance of the lights would indicate the difference in intensity (e.g. a light twice as far would be four times the intensity).
=== Ritchie's photometer ===
Ritchie's photometer depends upon equal illumination of surfaces. It consists of a box (a,b) six or eight inches long, and one in width and depth. In the middle, a wedge of wood (f,e,g) was angled upwards and covered with white paper. The user's eye looked through a tube (d) at the top of a box. The height of the apparatus was also adjustable via the stand (c). The lights to compare were placed at the side of the box (m, n)—which illuminated the paper surfaces so that the eye saw both surfaces at once. By changing the position of the lights, they were made to illuminate both surfaces equally, with the difference in intensity corresponding to the square of the difference in distance.
=== Method of extinction of shadows ===
This type of photometer depended on the fact that if a light throws the shadow of an opaque object onto a white screen, there is a certain distance that, if a second light is brought there, obliterates all traces of the shadow.
== Operating principles ==
Most photometers detect the light with photoresistors, photodiodes or photomultipliers. To analyze the light, the photometer may measure the light after it has passed through a filter or through a monochromator for determination at defined wavelengths or for analysis of the spectral distribution of the light.
== Photon counting ==
Some photometers measure light by counting individual photons rather than incoming flux. The operating principles are the same but the results are given in units such as photons/cm2 or photons·cm−2·sr−1 rather than W/cm2 or W·cm−2·sr−1.
Due to their individual photon counting nature, these instruments are limited to observations where the irradiance is low. The irradiance is limited by the time resolution of its associated detector readout electronics. With current technology this is in the megahertz range. The maximum irradiance is also limited by the throughput and gain parameters of the detector itself.
The light sensing element in photon counting devices in NIR, visible and ultraviolet wavelengths is a photomultiplier to achieve sufficient sensitivity.
In airborne and space-based remote sensing such photon counters are used at the upper reaches of the electromagnetic spectrum such as the X-ray to far ultraviolet. This is usually due to the lower radiant intensity of the objects being measured as well as the difficulty of measuring light at higher energies using its particle-like nature as compared to the wavelike nature of light at lower frequencies. Conversely, radiometers are typically used for remote sensing from the visible, infrared though radio frequency range.
== Photography ==
Photometers are used to determine the correct exposure in photography. In modern cameras, the photometer is usually built in. As the illumination of different parts of the picture varies, advanced photometers measure the light intensity in different parts of the potential picture and use an algorithm to determine the most suitable exposure for the final picture, adapting the algorithm to the type of picture intended (see Metering mode). Historically, a photometer was separate from the camera and known as an exposure meter. The advanced photometers then could be used either to measure the light from the potential picture as a whole, to measure from elements of the picture to ascertain that the most important parts of the picture are optimally exposed, or to measure the incident light to the scene with an integrating adapter.

38
data/en.wikipedia.org/wiki/Photometer-1.md Normal file
View File

@ -0,0 +1,38 @@
---
title: "Photometer"
chunk: 2/2
source: "https://en.wikipedia.org/wiki/Photometer"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:26.920738+00:00"
instance: "kb-cron"
---
== Visible light reflectance photometry ==
A reflectance photometer measures the reflectance of a surface as a function of wavelength. The surface is illuminated with white light, and the reflected light is measured after passing through a monochromator. This type of measurement has mainly practical applications, for instance in the paint industry to characterize the colour of a surface objectively.
== UV and visible light transmission photometry ==
These are optical instruments for measurement of the absorption of light of a given wavelength (or a given range of wavelengths) of coloured substances in solution. From the light absorption, Beer's law makes it possible to calculate the concentration of the coloured substance in the solution. Due to its wide range of application and its reliability and robustness, the photometer has become one of the principal instruments in biochemistry and analytical chemistry. Absorption photometers for work in aqueous solution work in the ultraviolet and visible ranges, from wavelength around 240 nm up to 750 nm.
The principle of spectrophotometers and filter photometers is that (as far as possible) monochromatic light is allowed to pass through a container (cell) with optically flat windows containing the solution. It then reaches a light detector, that measures the intensity of the light compared to the intensity after passing through an identical cell with the same solvent but without the coloured substance. From the ratio between the light intensities, knowing the capacity of the coloured substance to absorb light (the absorbency of the coloured substance, or the photon cross section area of the molecules of the coloured substance at a given wavelength), it is possible to calculate the concentration of the substance using Beer's law.
Two types of photometers are used: spectrophotometer and filter photometer. In spectrophotometers a monochromator (with prism or with grating) is used to obtain monochromatic light of one defined wavelength. In filter photometers, optical filters are used to give the monochromatic light. Spectrophotometers can thus easily be set to measure the absorbance at different wavelengths, and they can also be used to scan the spectrum of the absorbing substance. They are in this way more flexible than filter photometers, also give a higher optical purity of the analyzing light, and therefore they are preferably used for research purposes. Filter photometers are cheaper, robuster and easier to use and therefore they are used for routine analysis. Photometers for microtiter plates are filter photometers.
== Infrared light transmission photometry ==
Spectrophotometry in infrared light is mainly used to study structure of substances, as given groups give absorption at defined wavelengths. Measurement in aqueous solution is generally not possible, as water absorbs infrared light strongly in some wavelength ranges. Therefore, infrared spectroscopy is either performed in the gaseous phase (for volatile substances) or with the substances pressed into tablets together with salts that are transparent in the infrared range. Potassium bromide (KBr) is commonly used for this purpose. The substance being tested is thoroughly mixed with specially purified KBr and pressed into a transparent tablet, that is placed in the beam of light. The analysis of the wavelength dependence is generally not done using a monochromator as it is in UV-Vis, but with the use of an interferometer. The interference pattern can be analyzed using a Fourier transform algorithm. In this way, the whole wavelength range can be analyzed simultaneously, saving time, and an interferometer is also less expensive than a monochromator. The light absorbed in the infrared region does not correspond to electronic excitation of the substance studied, but rather to different kinds of vibrational excitation. The vibrational excitations are characteristic of different groups in a molecule, that can in this way be identified. The infrared spectrum typically has very narrow absorption lines, which makes them unsuited for quantitative analysis but gives very detailed information about the molecules. The frequencies of the different modes of vibration varies with isotope, and therefore different isotopes give different peaks. This makes it possible also to study the isotopic composition of a sample with infrared spectrophotometry.
== Atomic absorption photometry ==
Atomic absorption photometers are photometers that measure the light from a very hot flame. The solution to be analyzed is injected into the flame at a constant, known rate. Metals in the solution are present in atomic form in the flame. The monochromatic light in this type of photometer is generated by a discharge lamp where the discharge takes place in a gas with the metal to be determined. The discharge then emits light with wavelengths corresponding to the spectral lines of the metal. A filter may be used to isolate one of the main spectral lines of the metal to be analyzed. The light is absorbed by the metal in the flame, and the absorption is used to determine the concentration of the metal in the original solution.
== Luminometers ==
Luminometers are sensitive photometers that are used to measure luminescence and fluorescence in biology and chemistry.
== See also ==
Radiometry
Raman spectroscopy
Photodetector – A transducer capable of accepting an optical signal and producing an electrical signal containing the same information as in the optical signal.
== References ==
Article partly based on the corresponding article in Swedish Wikipedia

84
data/en.wikipedia.org/wiki/Polarimeter-0.md Normal file
View File

@ -0,0 +1,84 @@
---
title: "Polarimeter"
chunk: 1/4
source: "https://en.wikipedia.org/wiki/Polarimeter"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:28.106070+00:00"
instance: "kb-cron"
---
A polarimeter is a scientific instrument used to measure optical rotation: the angle of rotation caused by passing linearly polarized light through an optically active substance.
Some chemical substances are optically active, and linearly polarized (uni-directional) light will rotate either to the left (counter-clockwise) or right (clockwise) when passed through these substances. The amount by which the light is rotated is known as the angle of rotation. The direction (clockwise or counterclockwise) and magnitude of the rotation reveals information about the sample's chiral properties such as the relative concentration of enantiomers present in the sample.
== History ==
Polarization by reflection was discovered in 1808 by Étienne-Louis Malus (1775–1812).
== Measuring principle ==
The ratio, the purity, and the concentration of two enantiomers can be measured via polarimetry. Enantiomers are characterized by their property to rotate the plane of linear polarized light. Therefore, those compounds are called optically active and their property is referred to as optical rotation. Light sources such as a light bulb, Tungsten Halogen, or the sun emit electromagnetic waves at the frequency of visible light. Their electric field oscillates in all possible planes relative to their direction of propagation. In contrast to that, the waves of linear-polarized light oscillate in parallel planes.
If light encounters a polarizer, only the part of the light that oscillates in the defined plane of the polarizer may pass through. That plane is called the plane of polarization. The plane of polarization is turned by optically active compounds. According to the direction in which the light is rotated, the enantiomer is referred to as dextro-rotatory or levo-rotatory.
The optical activity of enantiomers is additive. If different enantiomers exist together in one solution, their optical activity adds up. That is why racemates are optically inactive, as they nullify their clockwise and counter clockwise optical activities. The optical rotation is proportional to the concentration of the optically active substances in solution. Polarimeters may therefore be applied for concentration measurements of enantiomer-pure samples. With a known concentration of a sample, polarimeters may also be applied to determine the specific rotation when characterizing a new substance. The specific rotation
[
α
]
λ
T
{\displaystyle [\alpha ]_{\lambda }^{T}}
is a physical property and defined as the optical rotation α at a path length l of 1 dm, a concentration c of 10 g/L, a temperature T (usually 20 °C) and a light wavelength λ (usually sodium D line at 589.3 nm):
[
α
]
λ
T
=
100
×
α
l
×
c
{\displaystyle [\alpha ]_{\lambda }^{T}={\frac {100\times \alpha }{l\times c}}}
This tells us how much the plane of polarization is rotated when the ray of light passes through a specific amount of optically active molecules of a sample. Therefore, the optical rotation depends on temperature, concentration, wavelength, path length, and the substance being analyzed.
== Construction ==
The polarimeter is made up of two Nicol prisms (the polarizer and analyzer). The polarizer is fixed and the analyzer can be rotated. The prisms may be thought of as slits S1 and S2. The light waves may be considered to correspond to waves in the string. The polarizer S1 allows only those light waves which move in a single plane. This causes the light to become plane polarized. When the analyzer is also placed in a similar position it allows the light waves coming from the polarizer to pass through it. When it is rotated through the right angle no waves can pass through the right angle and the field appears to be dark. If now a glass tube containing an optically active solution is placed between the polarizer and analyzer the light now rotates through the plane of polarization through a certain angle, the analyzer will have to be rotated in same angle.
== Operation ==
Polarimeters measure this by passing monochromatic light through the first of two polarising plates, creating a polarized beam. This first plate is known as the polarizer. This beam is then rotated as it passes through the sample. After passing through the sample, a second polarizer, known as the analyzer, rotates either via manual rotation or automatic detection of the angle. When the analyzer is rotated such that all the light or no light can pass through, then one can find the angle of rotation which is equal to the angle θ by which the analyser was rotated in the former case, or 90-θ in the latter case.
== Types of polarimeter ==

25
data/en.wikipedia.org/wiki/Polarimeter-1.md Normal file
View File

@ -0,0 +1,25 @@
---
title: "Polarimeter"
chunk: 2/4
source: "https://en.wikipedia.org/wiki/Polarimeter"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:28.106070+00:00"
instance: "kb-cron"
---
=== Laurent's half-shade polarimeter ===
When plane-polarised light passes through some crystals, the velocity of left-polarized light is different from that of the right-polarized light, thus the crystals are said to have two refractive indices, i.e. double refracting.
Construction: The polarimeter consists of a monochromatic source S which is placed at focal point of a convex lens L. Just after the convex lens there is a Nicol Prism P which acts as a polariser. H is a half shade device which divides the field of polarized light emerging out of the Nicol P into two halves, generally of unequal brightness. T is a glass tube in which an optically active solution is filled. The light, after passing through T, is allowed to fall on the analyzing Nicol A which can be rotated about the axis of the tube. The rotation of the analyzer can be measured with the help of a scale C.
Working principle: To understand the need of a half-shade device, let us suppose that it is not present. The position of the analyzer is adjusted so that the field of view is dark when the tube is empty. The position of the analyzer is noted on the circular scale. Now the tube is filled with the optically active solution and it is set in its proper position. The optically active solution rotates the plane of polarization of the light emerging out of the polarizer P by some angle, so the light is transmitted by analyzer A and the field of view of the telescope becomes bright. Now the analyzer is rotated by a finite angle so that the field of view of the telescope again becomes dark. This will happen only when the analyzer is rotated by the same angle by which the plane of polarization of light is rotated by the optically active solution.
The position of the analyzer is again noted. The difference of the two readings will give the angle of rotation of the plane of polarization.
A difficulty faced in the above procedure is that when analyzer is rotated for the total darkness, then it is attained gradually and hence it is difficult to find the exact position correctly for which complete darkness is obtained. To overcome the above difficulty, the half-shade device is introduced between polarizer P and the glass tube T.
Half shade device: It consist of two semicircular plates ACB and ADB. One half ACB is made of glass while other half is made of quartz. Both halves are cemented together. The quartz is cut parallel to the optic axis. Thickness of the quartz is selected in such a way that it introduces a path difference of ’A/2 between ordinary and extraordinary ray. The thickness of the glass is selected in such a way that it absorbs the same amount of light as is absorbed by the quartz half.
Consider that the vibration of polarization is along OP. On passing through the glass half the vibrations remain along OP. But on passing through the quartz half these vibrations will split into 0- and £-components. The £-components are parallel to the optic axis while O- component is perpendicular to optic axis. The O-component travels faster in quartz and hence an emergence 0-component will be along OD instead of along OC. Thus components OA and OD will combine to form a resultant vibration along OQ which makes the same angle with optic axis as OP. Now if the Principal plane of the analyzing Nicol is parallel to OP then the light will pass through the glass half unobstructed. Hence the glass half will be brighter than the quartz half or we can say that the glass half will be bright and the quartz half will be dark. Similarly if the principal plane of the analyzing Nicol is parallel to OQ then the quartz half will be bright and the glass half will be dark.
When the principal plane of the analyzer is along AOB then both halves will be equally bright. On the other hand, if the principal plane of the analyzer is along DOC then both the halves will be equally dark.
Thus it is clear that if the analyzing Nicol is slightly disturbed from DOC then one half becomes brighter than the other. Hence by using the half shade device, one can measure the angle of rotation more accurately.
Determination of specific rotation: In order to determine a specific rotation of an optically active substance (say, sugar), the polarimeter tube is first filled with pure water and the analyzer is adjusted for equal darkness (both the halves should be equally dark) point. The position of the analyzer is noted with the help of the scale. Now the polarimeter tube is filled with a sugar solution of known concentration and again the analyzer is adjusted in such a way that again the equally dark point is achieved. The position of the analyzer is again noted. The difference of the two readings will give the angle of rotation θ. Hence, a specific rotation S is determined as S = θ/LC, where L is the optical path length and C is concentration of the substance.
=== Biquartz polarimeter ===
A biquartz polarimeter uses a biquartz plate, consisting of two semicircular plates of quartz, each of thickness 3.75mm. One half consists of right-handed optically active quartz, while the other is left-handed optically active quartz.

39
data/en.wikipedia.org/wiki/Polarimeter-2.md Normal file
View File

@ -0,0 +1,39 @@
---
title: "Polarimeter"
chunk: 3/4
source: "https://en.wikipedia.org/wiki/Polarimeter"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:28.106070+00:00"
instance: "kb-cron"
---
=== Manual ===
The earliest polarimeters, which date back to the 1830s, required the user to physically rotate one polarizing element (the analyzer) whilst viewing through another static element (the detector). The detector was positioned at the opposite end of a tube containing the optically active sample, and the user used his/her eye to judge the "alignment" when least light was observed. The angle of rotation was then read from a simple fixed to the moving polariser to within a degree or so.
Although most manual polarimeters produced today still adopt this basic principle, the many developments applied to the original opto-mechanical design over the years have significantly improved measurement performance. The introduction of a half-wave plate increased "distinction sensitivity", whilst a precision glass scale with vernier drum facilitated the final reading to within ca. ±0.05º. Most modern manual polarimeters also incorporate a long-life yellow LED in place of the more costly sodium arc lamp as a light source.
=== Semi-automatic ===
Today, semi-automatic polarimeters are available. The operator views the image via a digital display adjusts the analyzer angle with electronic controls.
=== Fully automatic ===
Fully automatic polarimeters are now widely used and simply require the user to press a button and wait for a digital readout. Fast automatic digital polarimeters yield an accurate result within a few seconds, regardless of the rotation angle of the sample. In addition, they provide continuous measurement, facilitating high-performance liquid chromatography and other kinetic investigations.
Another feature of modern polarimeters is the Faraday modulator. The Faraday modulator creates an alternating current magnetic field. It oscillates the plane of polarization to enhance the detection accuracy by allowing the point of maximal darkness to be passed through again and again and thus be determined with even more accuracy.
As the temperature of the sample has a significant influence on the optical rotation of the sample, modern polarimeters have already included Peltier elements to actively control the temperature. Special techniques as temperature controlled sample tubes reduce measuring errors and ease operation. Results can directly be transferred to computers or networks for automatic processing. Historically, accurate filling of the sample cell had to be checked outside the instrument, as an appropriate control from within the device was not possible. Nowadays a camera system can help to monitor the sample and accurate filling conditions in the sample cell. Furthermore, features for automatic filling introduced by few companies are available on the market. When working with caustic chemicals, acids, and bases it can be beneficial to not load the polarimeter cell by hand. Both of these options help to avoid potential errors caused by bubbles or particles.
== Sources of error ==
The angle of rotation of an optically active substance can be affected by:
Concentration of the sample
Wavelength of light passing through the sample (generally, angle of rotation and wavelength tend to be inversely proportional)
Temperature of the sample (generally the two are directly proportional)
Length of the sample cell (input by the user into most automatic polarimeters to ensure better accuracy)
Filling conditions (bubbles, temperature and concentration gradients)
Most modern polarimeters have methods for compensating or/and controlling these errors.
== Calibration ==
Historically, a sucrose solution with a defined concentration was used to calibrate polarimeters relating the amount of sugar molecules to the light polarization rotation. The International Commission for Uniform Methods of Sugar Analysis (ICUMSA) played a key role in unifying analytical methods for the sugar industry, set standards for the International Sugar Scale (ISS) and the specifications for polarimeters in sugar industry. However, sugar solutions are prone to contamination and evaporation. Moreover, the optical rotation of a substance is very sensitive to temperature. A more reliable and stable standard was found: crystalline quartz which is oriented and cut in a way that it matches the optical rotation of a normal sugar solution, but without showing the disadvantages mentioned above. Quartz (silicon dioxide, SiO2) is a common mineral, a trigonal chemical compound of silicon and oxygen. Nowadays, quartz plates or quartz control plates of different thickness serve as standards to calibrate polarimeters and saccharimeters. In order to ensure reliable and comparable results, quartz plates can be calibrated and certified by metrology institutes. Alternatively, calibration may be checked using a Polarization Reference Standard, which consists of a plate of quartz mounted in a holder perpendicular to the light path. These standards are available, traceable to NIST, by contacting Rudolph Research Analytical, located at 55 Newburgh Road, Hackettstown, NJ 07840, USA. A calibration first consists of a preliminary test in which the fundamental calibration capability is checked. The quartz control plates must meet the required minimum requirements with respect to their dimensions, optical pureness, flatness, parallelism of the faces and optical axis errors. After that, the actual measurement value - the optical rotation - is measured with the precision polarimeter. The measurement uncertainty of the polarimeter amounts to 0.001° (k=2).
== Applications ==
Because many optically active chemicals such as tartaric acid, are stereoisomers, a polarimeter can be used to identify which isomer is present in a sample – if it rotates polarized light to the left, it is a levo-isomer, and to the right, a dextro-isomer. It can also be used to measure the ratio of enantiomers in solutions.
The optical rotation is proportional to the concentration of the optically active substances in solution. Polarimetry may therefore be applied for concentration measurements of enantiomer-pure samples. With a known concentration of a sample, polarimetry may also be applied to determine the specific rotation (a physical property) when characterizing a new substance.

29
data/en.wikipedia.org/wiki/Polarimeter-3.md Normal file
View File

@ -0,0 +1,29 @@
---
title: "Polarimeter"
chunk: 4/4
source: "https://en.wikipedia.org/wiki/Polarimeter"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:28.106070+00:00"
instance: "kb-cron"
---
=== Chemical industry ===
Many chemicals exhibit a specific rotation as a unique property (an intensive property like refractive index or specific gravity) which can be used to distinguish it. Polarimeters can identify unknown samples based on this if other variables such as concentration and length of sample cell length are controlled or at least known. This is used in the chemical industry.
By the same token, if the specific rotation of a sample is already known, then the concentration and/or purity of a solution containing it can be calculated.
Most automatic polarimeters make this calculation automatically, given input on variables from the user.
=== Food, beverage and pharmaceutical industries ===
Concentration and purity measurements are especially important to determine product or ingredient quality in the food & beverage and pharmaceutical industries. Samples that display specific rotations that can be calculated for purity with a polarimeter include:
Polarimeters are used in the sugar industry for determining quality of both juice from sugar cane and the refined sucrose. Often, the sugar refineries use a modified polarimeter with a flow cell (and used in conjunction with a refractometer) called a saccharimeter. These instruments use the International Sugar Scale, as defined by the International Commission for Uniform Methods of Sugar Analysis (ICUMSA).
== See also ==
Optical rotation
Polarimetry
Polarization
Chirality
Enantiomers
== References ==

29
data/en.wikipedia.org/wiki/Post_Instrument-0.md Normal file
View File

@ -0,0 +1,29 @@
---
title: "Post Instrument"
chunk: 1/3
source: "https://en.wikipedia.org/wiki/Post_Instrument"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:29.277739+00:00"
instance: "kb-cron"
---
The Post Plotting Instrument, or simply Post Instrument and sometimes the Observer Instrument, was the standard optical sighting system used by the UK's Royal Observer Corps (ROC) to determine the location of aircraft. It was used during the period from the mid-1930s into the early 1950s, and was one of the main sources of daytime tracking information during World War II.
There were two versions of the Post Instrument, a pre-war model using a pantograph, and a wartime version of somewhat more sophistication. Both required the operator to estimate the altitude of the aircraft and enter that into the device, then point a mechanical indicator, or sight, at the aircraft. The motion of the sight moved an indicator on a small Ordnance Survey National Grid map. The grid location indicated by the pointer was then telephoned to central control rooms, where several such reports were combined to produce a more accurate location estimate.
Later models added the Micklethwait Height Corrector, which allowed the posts to measure altitude with some accuracy and thus improve the quality of the measurements. The ROC also developed a methodology that allowed the Post Instrument to be used to produce measurements purely by sound, but it is unclear how often this was used.
== Background ==
Prior to the introduction of radar, optical tracking systems of widely varying complexity were commonly used to spot and report aircraft positions. The Post Instrument was intended to be at the simple end of the scale, an inexpensive and easy to use instrument to make rough but rapid measurements of the locations of aircraft.
Post Instruments were installed at hundreds of observation posts across the UK, typically in small groups of three or four posts about 3 to 5 miles (4.8 to 8.0 km) apart. This spacing allowed the operators to cross check each other's altitude measurements. Each post was normally manned by two or three operators, one operating the Post Instrument, another using the telephone to report the locations to a plotting center, and the third, if present, operating as a lookout and helper.
== Pre-war model ==
The original Post Instrument was mounted on a metal rod extending vertically from the centre of a circular table. A small section of a map showing the surrounding area was attached to the tabletop.
The instrument itself consisted of an open rectangle of metal bars, with the long axis horizontal. Hinges at the connection points between the bars allowed the bars to be rotated to form various parallelograms. Similar hinges were located at the midpoints of the long horizontal bars of the rectangle. These midpoint pivots connected to the vertical bar on the table. The result was a pantograph that allowed the long horizontal bars to be rotated into the vertical to point upward at an aircraft, sighting along the upper bar.
A final piece was a separate vertical bar connected to the two horizontals and pivoted in the same way so that it remained pointing vertically as the horizontal bars were rotated. This bar was able to be moved along the horizontal bars, fore and aft, which was used to adjust the estimated altitude.
To use the system, the operator would first estimate the altitude of the target aircraft and then move the smaller pointer to that altitude as measured against a scale on the upper horizontal arm. They would then rotate the apparatus around the vertical shaft so the target aircraft lay along the line of the upper bar and then rotate the bar vertically until it pointed at the aircraft. The vertical pointer now pointed to a grid location on the map, which could be read off to the reporting centre.
== Wartime model ==
The original model worked but was somewhat difficult and time consuming to use. Just prior to the war a new version was introduced that was easier to use. Officially known as the Observer Instrument, Mark 2, the first examples were built by R.B. Pullin & Co., starting in 1934.
The vertical rod of the original version was replaced by a horizontal framework, roughly T-shaped, that was suspended above the table on three wheels running on a metal track around the rim of the map. This provided a much more robust framework for holding the sighting system, and rotated much more smoothly. A pointer behind the front wheel made it easy to read off the bearing, when required. Travelling along the framework horizontally fore and aft was a sliding mechanism that held the sights. This formed the altitude adjustment that would be set prior to sighting. The map pointer was connected to the bottom of the slider.
The sights, in the form of an open-framework tube containing a crosshairs, was mounted to the horizontal slider on a vertical square tube. Sightings could be taken either through the crosshairs or along open sights on the top of the tube. A geared rack running down the back of the tube held the sights at a selected angle, and the angle was adjusted by rotating a geared knob on the right side of the sights. As the sights were rotated upwards, they forced the horizontal slider to the rear, moving the pointer over the map.

23
data/en.wikipedia.org/wiki/Post_Instrument-1.md Normal file
View File

@ -0,0 +1,23 @@
---
title: "Post Instrument"
chunk: 2/3
source: "https://en.wikipedia.org/wiki/Post_Instrument"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:29.277739+00:00"
instance: "kb-cron"
---
== Micklethwait Height Corrector ==
Wartime models were modified in 1940 with the Micklethwait Height Corrector, named for its inventor, Eric Walter Eustace Micklethwait. Micklethwait was an observer at the Gower Street Post on the roof of a building at University of London, near Euston Square tube station. Formerly a patent clerk, he devised the Corrector and quickly patented it.
The Corrector consisted of a second map pointer on a second horizontal slider, with a crank that moved the horizontal slider fore and aft. A second arm suspended from the main sight tube was pushed up and down as the horizontal portion slid. This arm was measured against a short vertical bar marked with altitude corrections. The system indicated only corrections, not the actual altitude. Two or more posts had to work in concert to use the system, using two measured angles and simple trigonometry to solve the altitude.
The simplest measurement took place when an aircraft flew directly over one of the posts. Other posts that could see the same aircraft would continue to track the target as normal, set to whatever altitude they had initially estimated. When the first post called that the aircraft was directly overhead, the other posts would crank the Corrector until its pointer lay over a marking for the other post printed on the map.
For instance, if the original estimate was 10,000 feet (3,000 m) and the aircraft was actually at 11,000 feet (3,400 m), the operator at a second post would set his instrument to 10,000 and continue to measure as normal until they heard the first post call "aircraft overhead". At this point they would stop measuring the aircraft and instead crank the Corrector until its pointer lay over the marking on the map indicating the location of the first post. By comparing the position of the bar suspended from the sighting tube to the Corrector's vertical scale, they would see it indicate +1000. This correction was called to the ROC center who then forwarded it to all the posts in the area to update their altitude settings to 11,000 feet.
When the target did not pass directly over a post the calculation was somewhat more complex. In this case two posts would measure the location of the aircraft at the same time, and then one would call the aircraft's measured grid reference to the other. The second would then place a ruler on the map lying along the line from their measured location on the map to the one called in from the other post. They would then crank the Corrector until its pointer lay directly above the nearest point on the ruler, and the correction could then be read off as normal.
An alternate procedure involved the use of an operator at the observer center. Gathering indicated grid locations from several sites set to the same arbitrary altitude, they would triangulate the grid location of the target and pass this information back to the posts. The operators at the posts would then crank the Corrector until its pointer lay over the calculated grid location, at which point the correction could be read.
== Sound measures ==
The Post Instrument was introduced in an era when sound locating was still common, and some techniques for measuring the angle by sound were developed. This basically consisted of moving the horizontal slider until the indicator pointer was over the "sound line", a circle on the map representing a 5 miles (8.0 km) distance around the post. The operator would then rotate the sights horizontally and vertically to try to point the sights in the direction they estimated the sound to be coming from. Instead of using the map, the operator instead called the horizontal and vertical angles to their operations room. The horizontal angle could be read off a scale around the outer edge of the map, but the vertical angle was instead measured by dropping the last three zeros of the altitude measurement, so if the sights were over the 14,000 foot marker, they would call in "angle 14".
In the operations room, a plotter trained in sound measurements would take the angle measurements from multiple stations and determine the location by plotting the angles on a map and looking for the intersections. They would then calculate the distance from one or more of the posts and calculate the altitude using the formula altitude = angle x calculated distance ÷ 5. For instance, if they found that the aircraft was 4 miles (6.4 km) from a particular post that indicated the angle was 14, then the altitude would be 14 x 4 / 5 = 56/5 = about 11,000 feet. Plotters were equipped with pre-calculated tables to make these calculations quickly.
== Reporting system ==

30
data/en.wikipedia.org/wiki/Post_Instrument-2.md Normal file
View File

@ -0,0 +1,30 @@
---
title: "Post Instrument"
chunk: 3/3
source: "https://en.wikipedia.org/wiki/Post_Instrument"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:29.277739+00:00"
instance: "kb-cron"
---
The Observer Corp was an expansion of a system originally set up in World War I to coordinate the reports from observers in the London area, part of the London Air Defence Area (LADA). In this system, originally set up by Edward Ashmore, observers telephoned reports of aircraft to a plotting center in the Horse Guards building in London. Information from the map would then be forwarded to the searchlights and anti-aircraft guns in the LADA area.
In the post-World War I era the system was taken over by the Air Defence of Great Britain organization, formally part of the Royal Air Force but containing British Army and Royal Engineer units as well. It was re-organized and expanded, covering not only the London area but adding similar reporting organizations in The Midlands. They also introduced new techniques to deal with faster aircraft.
In the new systems, plotters would take the reports from the observers and place a colored marker on a large map inside the indicated grid location. The marker held information about the number and altitude of the aircraft. The marker colors changed every five minutes, based on a sector clock, and when the marker was moved to a new location, a smaller marker was left behind in its former location. This produced a trail of colored markers on the map that allowed observers to easily see the track of the aircraft, as well as estimate how quickly it was moving.
The Dowding system was built on top of this reporting system. It added a central filter room that acted as a plotting station for all of the Chain Home radar stations. Reports from the filter room were then forwarded to Group plotting rooms where they were combined with information from the Observer Corps. The same basic system using colored markers indicating the time, altitude and number of aircraft was used throughout the system. Just prior to the war, two additional Groups were added to cover Scotland and the north, and the southwest.
Starting in 1942, additional charts were installed at the Group plotting centers that allowed information from neighbouring Groups to be recorded. This was useful for handing over tracks that were moving across Group boundaries.
== Notes ==
== References ==
=== Citations ===
=== Bibliography ===
Craine, Simon; Ryan, Noel (2011). "Protection from the Cold": Cold War Protection in Preparedness for Nuclear War. Lulu.com. ISBN 9781904098195.
Instructions for Observer Posts (PDF) (Technical report). Air Ministry. April 1941.
Observers' Tale: The Story of Group 17 of the R.O.C. Roland Brothers. 1950.
ROC Training Manual (PDF) (Technical report). Air Ministry. 1951.
Routledge, N.W. (1994). A History of the Royal Regiment of Artillery Anti-Aircraft Artillery, 1914-55. Brassey's. ISBN 1-85753-099-3.
Holmes, Lawrence (24 December 2009). "What's a Micklethwait?". Royal Observer Corps Association.
Zimmerman, David (2013). "Information and the Air Defence Revolution, 1917–40". In Goldman, Emily (ed.). Information & Revolutions in Military Affairs. Routledge. ISBN 978-1-136-82779-2. First published in: Zimmerman, David (2004). "Information and the Air Defence Revolution, 1917–40". Journal of Strategic Studies. 27 (2): 370–394. doi:10.1080/0140239042000255968. S2CID 153613073.

36
data/en.wikipedia.org/wiki/Pseudoscope-0.md Normal file
View File

@ -0,0 +1,36 @@
---
title: "Pseudoscope"
chunk: 1/1
source: "https://en.wikipedia.org/wiki/Pseudoscope"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:30.433063+00:00"
instance: "kb-cron"
---
A pseudoscope is a binocular optical instrument that reverses depth perception. It is used to study human stereoscopic perception. Objects viewed through it appear inside out, for example: a box on a floor would appear as a box-shaped hole in the floor.
It typically uses sets of optical prisms, or periscopically arranged mirrors to swap the view of the left eye with that of the right eye.
== Purpose ==
In the 1800s Charles Wheatstone coined the name from the Greek ψευδίς σκοπειν – 'false view'. The device was used to explore his theory of stereo vision.
Basically, pseudoscopic vision is three-dimensional vision in reverse. For example, in aerial photography, swimming pools appear to look like buildings and buildings appear to look like swimming pools. In red and green plotters like the Kelsh and Multiplex this is achieved by reversing the lenses on the 3D glasses. The images will be reverse order. The right image will be viewed through the left eye, and the left image will be through the right eye.
== Effect ==
Switching the two pictures in a standard stereoscope changes all the elevated parts into depressions, and vice versa. The pseudoscope also changes convex into concave, and high-relief into low-relief.
== History ==
Before the pseudoscope itself was created intentionally, it existed in binocular instruments as an imperfection. The first binocular microscope was invented by the Capuchin friar Cherubin d'Orleans. Because his instrument consisted of two inverting systems, it produced a pseudoscopic impression of depth by accident, although not recognized by microscopists of the time.
The instrument subsequently fell into complete neglect for nearly two centuries. It was revived in 1852 by Charles Wheatstone, who published his ideas in his paper "On Binocular Vision", in the Philosophical Transactions for 1852. Wheatstone's paper stimulated the investigation of binocular vision and many variations of pseudoscopes were created, chief types being the mirror or the prismatic.
In 1853 the American scientist John Leonard Riddell (1807–1865) devised his binocular microscope, which contained the essentials of Wheatstone's pseudoscope.
== References ==
== External links ==
Further reading & commercial pseudoscope
Make your own Pseudoscope for 10 dollars
History of design prismatic pseudoscope

19
data/en.wikipedia.org/wiki/Pyramid_wavefront_sensor-0.md Normal file
View File

@ -0,0 +1,19 @@
---
title: "Pyramid wavefront sensor"
chunk: 1/1
source: "https://en.wikipedia.org/wiki/Pyramid_wavefront_sensor"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:31.620901+00:00"
instance: "kb-cron"
---
A pyramid wavefront sensor is a type of a wavefront sensor. It measures the optical aberrations of an optical wavefront. This wavefront sensor uses a pyramidal prism with a large apex angle to split the beam into multiple parts at the geometric focus of a lens. A four-faceted prism, with its tip centered at the peak of the point spread function, will generate four identical pupil images in the absence of optical aberrations. In the presence of optical aberrations, the intensity distribution among the pupils will change. The local wavefront gradients can be obtained by recording the distribution of intensity in the pupil images. The wavefront aberrations can be evaluated from the estimated wavefront gradients.
It has potential applications in astronomy and ophthalmology.
== Modulation ==
The prism is often modulated (mechanically moved in a circle/square) for averaging purposes and to make sure that the ray spends an equal fraction of the total time on every face of the pyramidal prism. This makes the wavefront sensor slightly inconvenient to use due to the need for mechanically moving parts – either the prism or the beam is modulated. Using a light diffusing plate, mechanically moving parts can be eliminated. Alternatively, it has been shown that the need for mechanically moving parts can be overcome in a digital pyramid wavefront sensor with the spatial light modulator.
== References ==

21
data/en.wikipedia.org/wiki/Reynolds_and_Branson-0.md Normal file
View File

@ -0,0 +1,21 @@
---
title: "Reynolds and Branson"
chunk: 1/3
source: "https://en.wikipedia.org/wiki/Reynolds_and_Branson"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:32.838993+00:00"
instance: "kb-cron"
---
Reynolds & Branson Leeds was a business based at 13 Briggate and 14 Commercial Street in Leeds, England. The business lasted from 1816 (1816) to 1972 (1972). Edward Matterson managed the company in 1822, and William West F.R.S. took over in 1833. The National Archives Records about the company include a day book, sales ledger, and prescription books. The records were created by Reynolds & Branson Ltd. Reynolds & Branson was registered in July 1898 as a limited corporation with a capital of £34,000 in shares of £10 each by Messrs. R. Reynolds, F. W. Branson. No remuneration was given to Mr. R. Reynolds, but a £700 per annum was given to each of the others. In 1890, Richard Reynold's son, Richard Freshfield (Fred) Reynolds was made a partner.
The firm was in the business of wholesale and retail for chemists and surgical instrument makers.
== Origins ==
The original company can be traced back to 1816 (see Grace's Guide which is the leading source of historical information on industry and manufacturing in Britain). Edward Matterson was a druggist who ran the firm after being employed by Allen and Hanburys. He was educated at Leeds Grammar School. In 1822 the company moved to 13 Briggate, Leeds. In 1833 William West F.R.S. took over the company after Matterson went bankrupt (see The bankrupt directory: being a complete register of all the bankruptcies, with their residences, trades, and dates when they appeared in the London gazette, from December 1820 to April 1843). In 1839 Thomas Harvey joined the business, when William West left the company to pursue analytical chemistry. The firm was then renamed Thomas Harvey (Quaker). Harvey was born in 1811 at Barnsley into a Quaker family. His father was a linen manufacturer. The second of five children, he was educated at Barnsley Grammar in Yorkshire. From 1822 to 1825 Harvey studied at Ackworth and afterwards became a chemist apprentice for David Doncaster of Sheffield. Upon Doncaster's death he trained at Thomas Southalls in Birmingham for eight years. In 1837 Harvey settled in Leeds as a chemist. He became an anti-slavery campaigner and philanthropist.
== Richard Reynolds ==
In 1844 Richard Reynolds joined the company as an apprentice. He was born in 1829 and was the eldest son of an apothecary who died when the boy was only four years old. From 1850 to 1851 he attended the School of Pharmacy in London where he took first prizes in chemistry, materia medica and botany in a contest held by the pharmaceutical society. He then went to Mr. Henry Deane at Chapman for two years and returned to the Leeds business. In 1854 Richard Reynolds joined Thomas Harvey as a partner and the company then became Harvey & Reynolds. In 1861 the firm was joined by a Mr. Fowler and became Harvey, Reynolds & Fowler. By 1864 Thomas Harvey had retired (Noted in 1884). At the age of 72, he undertook an arduous journey to Canada on a Quaker mission but it exhausted him. He died on 25 December at his home at Ashwood, Headingley Lane. Mr. Haw then joined the business and the company became Haw & Reynolds. In 1867 the business was listed as Haw, Reynolds, & Co. In 1883 Fredrick W Branson joined the business. An 1884 advertisement listed the partnership between Reynolds & Branson (late Harvey, Reynolds & co).
== 1898–1914 ==
The firm of Reynolds & Branson was registered in July 1898 as a limited corporation with a capital of £34,000 in shares of £10 each by Messrs. R. Reynolds, F. W. Branson. No remuneration was given to Mr. R. Reynolds, but a £700 each per annum to the others. In 1890 Richard Reynold's son, Richard Freshfield (Fred) Reynolds was made a partner. The firm was in the business of wholesale and retail chemists and surgical instrument makers.[5] Fredrick W Branson now focused on the development of scientific apparatus and chemical glassware for the business. The company was flourishing under his management. Frederick Hartridge attended the University of Leeds in 1905, and then attending the School of Pharmaceutical Society in London in 1909. In 1901, during the outbreak of lead poisoning at Morley, the company was called in. Frederick W. Branson made recommendations which freed Morley from this scourge. In collaboration with A. F. Dimmock, M.D., he contributed to the British Medical Association meeting in 1903 a paper " A new method for the determination of uric acid in urine" (Br. Med. J., 1903, 2, 585). For this process he devised a correction scale which was contributed to the British Pharmaceutical Conference in 1904. At the 1905 meeting of the British Medical Association a further paper by these two authors was read, " A rapid and simple process for the estimation of uric acid " (ibid., 1905, 2, 1104), in which uric acid was precipitated and the precipitate measured in a specially graduated tube. In 1914, in collaboration with Dr. Gordon Sharp, he contributed a paper to the British Pharmaceutical Conference on the activity of digitalis leaves and the stability and standardisation of tinctures of digitalis.

37
data/en.wikipedia.org/wiki/Reynolds_and_Branson-1.md Normal file
View File

@ -0,0 +1,37 @@
---
title: "Reynolds and Branson"
chunk: 2/3
source: "https://en.wikipedia.org/wiki/Reynolds_and_Branson"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:32.838993+00:00"
instance: "kb-cron"
---
== The war years ==
During the First World War, he actively pursued efforts to standardize the size and shape of chemical glassware. In 1916, he was elected as an inaugural member of the Society of Glass Technology. He organized research and published works on these topics. Branson sought to secure in Great Britain the manufacturing process for the glass required for the equipment of munition factories.[8][9] Branson contributed a paper on the composition of some types of chemical glassware to the Society of Chemical Industry (J. SOC. Chem. Ind., 1915, 34, 471) in collaboration with his son Frederick Hartridge, a paper to the Transactions of the Society of Glass Technology (1919, 3, 249) "A proposed standard formula for a glass for lamp workers". Branson was chairman until retirement in 1932. His son, Frederick Hartridge, Associate of the Royal Institute of Chemistry AIC, became chairman and managing director of Reynolds & Branson.[5][7] he run the company for 20 years, until his untimely death on 10 February 1952, Frederick Hartridge had appointed his 3 Sons and Daughter as directors of Reynolds & Branson, Frederick Norman the eldest son who attended Ilkley Grammar an all-boys school, Eileen his only daughter, Peter Orchard who as Director of Phospherade, which was the mineral water company, He attended Roundhay all-boys grammar school.
In the Second World War he was in the RAF with 54 Spitfire squadron, in 1942 married Rita Blackburn, he went to Australia with 54 Spitfire squadron at the end of 1942, he met Patricia A Grant his second wife. He married Patricia in Leeds 1948, Peter emigrated to Australia 1953, when his father died leaving the bulk of the business to his eldest brother Frederick Norman, He set up his own pharmacy in Blackburn South in 1955, He later become a Podiatrist retiring at the age of 90, Richard Orchard who attended Roundhay all-boys grammar school, Second World War Richard was also in RAF as a pilot he died on active service 1945. His eldest son Frederick Norman Branson became Chairman & Managing Director of Reynolds & Branson in 1953, he would run Reynolds & Branson for almost 20 years, at this point the company had a workforce of 150 people, In 1972 Frederick Norman Branson sold the business to Barclay, later selling to the asset strippers Slater & Walker.
== Reynolds & Branson Chronological Time Frame ==
1822, Edward Matterson druggist, dealer in paint and colours, Location 12, Briggate, Leeds Baines's Directory and Gazette.
1829, Edward Matterson, druggist located 13 Briggate & 4 Blundel place. Pigot's Directory
1833, William West F.R.S. took over the company after Matterson went bankrupt,
1839, Thomas Harvey, chemist and druggist, 5 Commercial Street. Leeds.
1841, Thomas Harvey, chemist and druggist, 13 Briggate. Leeds.
1854, Reynolds returned to Leeds as partner with Harvey in the chemist business and the firm became Harvey & Reynolds.
1856, Harvey & Reynolds, chemist and druggist, 13 Briggate. Leeds.
1861, Harvey, Reynolds & Fowler. Chemist and druggist, 10 Briggate. Leeds.
1864, Haw & Reynolds. Chemist and druggist. Briggate. Leeds. as Thomas Harvey had retired.
1867, Haw, Reynolds, & Co. Chemist and druggist. Briggate. Leeds.
1872, Haw, Reynolds & Co. Chemist. 14 Commercial Street, and 13 Briggate. Leeds.
1886, Reynolds & Branson. Makers of the first short clinical thermometer.
1891, Reynolds & Reynolds, chemist and druggist, 13 Briggate. Leeds
1901, Reynolds & Reynolds, chemist and druggist, 13 Briggate. Leeds
1911, Reynolds & Reynolds, chemist and druggist, 13 Briggate. Leeds
== X-ray pioneers ==
On 24 July 1896, Reynolds and Branson attended the Photographic Convention of the United Kingdom at Leeds. The firm was represented in various sessions. During the session on Orthochromatic Photography, Branson gave a presentation on X-ray apparatus that included a well received demonstration and repeated as follows:"... Mr. Branson, of Messrs. Reynolds and Branson, who had made a special study of X-ray work, gave a demonstration which for lucidity and completeness has rarely been equalled. In the course of his remarks he fully explained the construction and exhaustion of the tubes, and showed various forms and explained his method of making calcium tungstate, which was to mix solutions of sodium tungstate and calcium chloride, collect, wash, and dry the precipitate of calcium tungstate which was formed, and then to fuse this in a small muffle furnace at the temperature of the melting point of cast-iron, and reduce to small crystals in a mortar, mix with varnish, and coat a screen. With such a screen in contact with the plate he had been able to show osseous structure of the hand, measuring only one-hundredth of an inch, with an exposure of one minute. A comparison of the fluorescent appearance of the three salts, calcium tungstate, platinocyanide of barium, and platinocyanide of potassium, was shown, the first and last being the best for photographic work, as the fluorescence was blue, and the barium salt was most satisfactory for visual work, as the fluorescence was yellow."
At the same convention, during the session on Photography at the Seaside the firm displayed some of their product line that included X-ray apparatus, as follows:"Reynolds & Branson, of Commercial Street, Leeds, had a very high-class show, special prominence being given to apparatus for X-ray work. A case of lenses of all the leading makers, together with a very well-made photo-micrographic outfit, a cabinet of chemicals, another of cameras, and all the little odds and ends of apparatus, made up a very fine show."
== Reynolds & Branson Trade Catalogues ==
Reynolds and Branson trade catalogues listed:

36
data/en.wikipedia.org/wiki/Reynolds_and_Branson-2.md Normal file
View File

@ -0,0 +1,36 @@
---
title: "Reynolds and Branson"
chunk: 3/3
source: "https://en.wikipedia.org/wiki/Reynolds_and_Branson"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:32.838993+00:00"
instance: "kb-cron"
---
Reynolds and Branson, 1887. Handy Guide to Surgical Instruments and Appliances etc. Reynolds and Branson, 14 Commercial Street, Leeds. Gloucester: John Bellows. 1887. 246p.
Reynolds and Branson, 1890. Illustrated Catalogue of Chemical and Physical Apparatus, Pure Chemicals and Reagents. Reynolds and Branson, 14 Commercial Street, Leeds. 1890. 200p.
Reynolds and Branson, 1903. Catalogue of Special Preparations. Reynolds and Branson, 14 Commercial Street, Leeds. 1903. 64p.
Reynolds and Branson, 1907. Illustrated Catalogue of Optical Lanterns, Slides, Compressed Gases and Accessory Apparatus. Reynolds and Branson, 14 Commercial Street, Leeds. Leeds: McCorquodale & Co. 1907. 204p.
Reynolds and Branson, 1908. Illustrated Catalogue of Surgical Instruments and Appliances. Reynolds and Branson, 14 Commercial Street, Leeds. Leeds: Chorley & Pickersgill. 1908. 119p.
Reynolds and Branson, 1912. Catalogue of Special Preparations, Surgical Instruments, Trusses etc. Reynolds and Branson, 14 Commercial Street, Leeds. 1912. 84p.
Reynolds and Branson, 1912–1920. Catalogue of Laboratory Fittings and Furniture. Reynolds and Branson. 1912–1920. 29p.
== Reynolds & Branson Patents ==
Patents include: #1120 in 1885, #16373 in 1893, #14102 in 1899.
Improvements in photographic ‘shutters’ for instantaneous photography. #1650. 1883.
Means or apparatus for measuring quantities of highly volatile liquids. No. 3490. 1904.
== References ==
== External links ==
Reynolds and Branson @Grace's Guide
Science Museum Group List of items manufactured by R&B
Oak Ridge museum, Electroscope manufactured by R&B
Camera manufactured by R&B
http://historiccamera.com/cgi-bin/librarium2/pm.cgi?action=app_display&app=datasheet&app_id=2517
http://www.thoresby.org.uk/content/people/harvey.php
http://www.huntsearch.gla.ac.uk/cgi-bin/foxweb/huntsearch/DetailedResults.fwx?collection=instruments&searchTerm=105190
http://collections.wakefield.gov.uk/photographs/index.asp?page=hitlist&mwsquery=(%7BCategory%7D%3D%7Bphotographs%7D)&mwsQueryTemplate=%5B%7Bcontrol%3DthemeList%7D%7Bindex%3DClassification%7D%7Brelation%3D%3D*%7D%5D&themeList=Community+Life+/+Health+and+Welfare+/+Workhouses+and+children%27s+homes
[1]

36
data/en.wikipedia.org/wiki/Rhinoscope-0.md Normal file
View File

@ -0,0 +1,36 @@
---
title: "Rhinoscope"
chunk: 1/1
source: "https://en.wikipedia.org/wiki/Rhinoscope"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:33.971738+00:00"
instance: "kb-cron"
---
A rhinoscope (or nasoscope) is a thin, tube-like instrument used to examine the inside of the nose. A rhinoscope has a light and a lens for viewing and may have a tool to remove tissue.
== Types ==
Rhinoscopy is performed by two procedures.
Anterior rhinoscopy using a nasal speculum
Posterior rhinoscopy using an endoscopic rhinoscope
=== Anterior rhinoscopy ===
In anterior rhinoscopy, the rhinoscope is advanced through the nose to examine the nasal cavity.
=== Posterior rhinoscopy ===
In posterior rhinoscopy, the endoscope is advanced through the mouth to examine the back of the nasal cavity above the soft palate, and can be used to visualise the oropharynx below that.
Structures seen in posterior rhinoscopy: posterior border of nasal septum, fossa of Roosenmuller, eustachian tube opening and upper surface of soft palate.
== References ==
== External links ==
Rhinoscope entry in the public domain NCI Dictionary of Cancer Terms
This article incorporates public domain material from Dictionary of Cancer Terms. U.S. National Cancer Institute.

454
data/en.wikipedia.org/wiki/Seismic_array-0.md Normal file
View File

@ -0,0 +1,454 @@
---
title: "Seismic array"
chunk: 1/3
source: "https://en.wikipedia.org/wiki/Seismic_array"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:55.448067+00:00"
instance: "kb-cron"
---
A seismic array is a system of linked seismometers arranged in a regular geometric pattern (cross, circle, rectangular etc.) to increase sensitivity to earthquake and explosion detection. A seismic array differs from a local network of seismic stations mainly by the techniques used for data analysis. The data from a seismic array is obtained using special digital signal processing techniques such as beamforming, which suppress noises and thus enhance the signal-to-noise ratio (SNR).
The earliest seismic arrays were built in the 1950s in order to improve the detection of nuclear tests worldwide. Many of these deployed arrays were classified until the 1990s. Today they have become part of the International Monitoring System (IMS) as primary or auxiliary stations. Seismic arrays are not only used to monitor earthquakes and nuclear tests but also used as a tool for investigating nature and source regions of microseisms as well as locating and tracking volcanic tremor and analyzing complex seismic wave-field properties in volcanic areas.
== Layout ==
Seismic arrays can be classified by size, which is defined by the array's aperture given by the largest distance between the single seismometers.
The sensors in a seismic array are arranged in different geometric patterns horizontally. The arrays built in the early 1960s were either cross (orthogonal linear) or L-shaped. The aperture of these arrays ranged from 10 to 25 km. Modern seismic arrays such as NORES and ARCES are located on concentric rings spaced at log-periodic intervals. Each ring consists of an odd number of seismometer sites. The number of rings and aperture differ from array to array, determined by economy and purpose.
Using the NORES design as an example, seismometers are placed on 4 concentric rings. The radii of the 4 rings are given by:
R
n
=
R
m
i
n
⋅
2.15
n
(
n
=
0
,
1
,
2
,
3
)
,
{\displaystyle R_{n}=R_{min}\cdot 2.15^{n}(n=0,1,2,3),}
R
m
i
n
=
150
m
{\displaystyle R_{min}=150m}
If the three sites in the inner ring are placed at 36, 156 and 276 degrees from due North, the five sites in the outer ring might be placed at 0, 72, 144, 216 and 288 degrees. This class of design is considered to provide the best overall array gain.
== Data processing ==
=== Array beamforming ===
With a seismic array, the signal-to-noise ratio (SNR) of a seismic signal can be improved by summing the coherent signals from the individual array sites. The most important point during the beamforming process is to find the best delay times by which the single traces must be shifted before summation in order to get the largest amplitudes due to coherent interference of the signals.
For distances from the source much larger than about 10 wavelengths, a seismic wave approaches an array as a wavefront that is close to planar. The directions of approach and propagation of the wavefront projected onto the horizontal plane are defined by the angles Φ and Θ.
Φ Backazimuth (BAZ) = angle of wavefront approach, measured clockwise from the North to the direction towards the epicenter in degree.
Θ Direction in which the wavefront propagates, measured in degree from the North, with Θ = Φ ±180°.
dj Horizontal distances between array site j and center site in [km].
s Slowness vector with absolute value s = 1/ vapp
vapp Apparent velocity vector with the absolute value vapp = 1/s . vapp = (vapp,x ,vapp,y ,vapp,z), where vapp,x ,vapp,y ,vapp,z are the single apparent velocity components in [km/s] of the wavefront crossing an array.
vapp,h Absolute value of the horizontal component of the apparent velocity.
v
a
p
p
,
h
=
v
a
p
p
,
x
2
+
v
a
p
p
,
y
2
{\displaystyle v_{app,h}={\sqrt {v_{app,x}^{2}+v_{app,y}^{2}}}}
In most cases, the elevation differences between single array sites are so small that travel-time differences due to elevation differences are negligible. In this case, we cannot measure the vertical component of the wavefront propagation. The time delay τj between the center site 0 and site j with the relative coordinates (xj, yj) is
τ
j
=
d
j
v
a
p
p
,
h
=
−
x
j
s
i
n
Φ
−
y
j
c
o
s
Φ
v
a
p
p
,
h
{\displaystyle \tau _{j}={\frac {d_{j}}{v_{app,h}}}={\frac {-x_{j}sin\Phi -y_{j}cos\Phi }{v_{app,h}}}}
In some cases, not all array sites are located on one horizontal plane. The time delays τj also depends on the local crustal velocities (vc) below the given site j. The calculation of τj with coordinates (xj, yj, zj) is
τ
j
=
−
x
j
s
i
n
Φ
−
y
j
c
o
s
Φ
v
a
p
p
,
h
+
z
j
c
o
s
Φ
v
c
{\displaystyle \tau _{j}={\frac {-x_{j}sin\Phi -y_{j}cos\Phi }{v_{app,h}}}+{\frac {z_{j}cos\Phi }{v_{c}}}}
In both the calculation can be written in vector syntax with position vector
r
j
{\displaystyle r_{j}}
and slowness vector
s
j
{\displaystyle s_{j}}
:
τ
j
=
r
j
⋅
s
j
{\displaystyle \tau _{j}=r_{j}\cdot s_{j}}
Let wj(t) be the digital sample of the seismometer from site j at time t, then the beam of the whole array is defined as
b
(
t
)
=
1
M
∑
j
=
1
M
w
j
(
t
+
τ
j
)
{\displaystyle b(t)={\frac {1}{M}}\sum _{j=1}^{M}w_{j}(t+\tau _{j})}
If seismic waves are harmonic waves S(t) without noise, with identical site responses, and without attenuation, then the above operation would reproduce the signal S(t) accurately.
Real data w(t) are the sum of background noise n(t) plus the signal of interest S(t), i.e. w(t) = S(t) + n(t). Assuming that the signal is coherent and not attenuated, calculating the sum of M observations and including noise we get
B
(
t
)
=
M
⋅
S
(
t
)
+
∑
j
=
1
M
n
j
(
t
+
τ
j
)
{\displaystyle B(t)=M\cdot S(t)+\sum _{j=1}^{M}n_{j}(t+\tau _{j})}

568
data/en.wikipedia.org/wiki/Seismic_array-1.md Normal file
View File

@ -0,0 +1,568 @@
---
title: "Seismic array"
chunk: 2/3
source: "https://en.wikipedia.org/wiki/Seismic_array"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:55.448067+00:00"
instance: "kb-cron"
---
Assuming that the noise nj(t) has a normal amplitude distribution with zero mean and variance σ2 at all sites, then the variance of the noise after summation is
σ
s
2
=
M
σ
2
{\displaystyle \sigma _{s}^{2}=M\sigma ^{2}}
and the standard deviation is
M
σ
2
{\displaystyle {\sqrt {M}}\sigma ^{2}}
. That means the standard deviation of the noise is multiplied by
M
{\displaystyle {\sqrt {M}}}
while the coherent signal is multiplied by
M
{\displaystyle M}
. The theoretical improvement of the SNR by beamforming (aka array gain) will be
G
=
M
{\displaystyle G={\sqrt {M}}}
for an array containing M sites.
==== The N-th root process ====
N-th root process is a non-linear method to enhance the SNR during beamforming. Before summing up the single seismic traces, the N-th root is calculated for each trace retaining the sign information. signum{wj(t)} is a function defined as -1 or +1, depending on the sign of the actual sample wj(t). N is an integer that has to be chosen by the analyst
B
N
(
t
)
=
∑
j
=
1
M
n
j
(
t
+
τ
j
)
N
⋅
s
i
g
n
u
m
{
w
j
(
t
)
}
{\displaystyle B_{N}(t)=\sum _{j=1}^{M}{\sqrt[{N}]{n_{j}(t+\tau _{j})}}\cdot signum\{w_{j}(t)\}}
Here the value of the function
s
i
g
n
u
m
{
w
j
(
t
)
}
{\displaystyle signum\{w_{j}(t)\}}
is defined as ±1 depending on the sign of the actual sample wj(t). After this summation, the beam has to be raised to the power of N
b
(
t
)
=
|
B
N
(
t
)
|
N
⋅
s
i
g
n
u
m
{
w
j
(
t
)
}
{\displaystyle b(t)=|B_{N}(t)|^{N}\cdot signum\{w_{j}(t)\}}
The N-th root process was first proposed by K. J. Muirhead and Ram Dattin in 1976. With the N-th root process, the suppression of uncorrelated noise is better than with linear beamforming. However, it weighs the coherency of a signal higher than the amplitudes, which results in a distortion of the waveforms.
==== Weighted stack methods ====
Schimmel and Paulssen introduced another non-linear stacking technique in 1997 to enhance signals through the reduction of incoherent noise, which shows a smaller waveform distortion than the N-th root process. Kennett proposed the use of the semblance of the signal as a weighting function in 2000 and achieved a similar resolution.
An easily implementable weighted stack method would be to weight the amplitudes of the single sites of an array with the SNR of the signal at this site before beamforming, but this
does not directly exploit the coherency of the signals across the array. All weighted stack methods can increase the slowness resolution of velocity spectrum analysis.
==== Double beam technique ====
A cluster of earthquakes can be used as a source array to analyze coherent signals in the seismic coda. This idea was consequently expanded by Krüger et al. in 1993 by analyzing seismic array data from well-known source locations with the so-called "double beam method". The principle of reciprocity is used for source and receiver arrays to further
increase the resolution and the SNR for small amplitude signals by combining both arrays in a single analysis.
=== Array transfer function ===
The array transfer function describes sensitivity and resolution of an array for seismic signals with different frequency contents and slownesses. With an array, we are able to observe the wavenumber
k
=
2
π
/
λ
=
2
π
⋅
f
⋅
s
{\displaystyle k=2\pi /\lambda =2\pi \cdot f\cdot s}
of this wave defined by its frequency f and its slowness s. While time-domain analog-to-digital conversion may give aliasing effects in the time domain, the spatial sampling may give aliasing effects in the wavenumber domain. Thus the wavelength range of seismic signals and the sensitivity at different wavelengths must be estimated.
The difference between a signal w at the reference site A and the signal wn at any other sensor An is the travel time between the arrivals at the sensors. A plane wave is defined by its slowness vector so
w
n
(
t
)
=
w
(
t
−
r
n
⋅
s
0
)
{\displaystyle w_{n}(t)=w(t-r_{n}\cdot s_{0})}
, where
r
n
{\displaystyle r_{n}}
is the position vector of site n
The best beam of an array with M sensors for a seismic signal for the slowness so is defined as
b
(
t
)
=
1
M
∑
j
=
1
M
w
j
(
t
+
r
j
⋅
s
0
)
{\displaystyle b(t)={\frac {1}{M}}\sum _{j=1}^{M}w_{j}(t+r_{j}\cdot s_{0})}
If we calculate all time shifts for a signal with the slowness so with respect to any other slowness s, the calculated beam becomes
b
(
t
)
=
1
M
∑
j
=
1
M
w
j
(
t
+
r
j
⋅
(
s
0
−
s
)
)
{\displaystyle b(t)={\frac {1}{M}}\sum _{j=1}^{M}w_{j}(t+r_{j}\cdot (s_{0}-s))}
The seismic energy of this beam can be calculated by integrating over the squared amplitudes
E
(
t
)
=
∫
−
∞
∞
b
2
(
t
)
d
t
=
∫
−
∞
∞
[
1
M
∑
j
=
1
M
w
j
(
t
+
r
j
⋅
(
s
0
−
s
)
)
]
2
d
t
{\displaystyle E(t)=\int _{-\infty }^{\infty }b^{2}(t)dt=\int _{-\infty }^{\infty }[{\frac {1}{M}}\sum _{j=1}^{M}w_{j}(t+r_{j}\cdot (s_{0}-s))]^{2}dt}
This equation can be written in the frequency domain with
w
¯
(
ω
)
{\displaystyle {\bar {w}}(\omega )}
being the Fourier transform of the seismogram w(t), using the definition of the wavenumber vector k = ω⋅ s

393
data/en.wikipedia.org/wiki/Seismic_array-2.md Normal file
View File

@ -0,0 +1,393 @@
---
title: "Seismic array"
chunk: 3/3
source: "https://en.wikipedia.org/wiki/Seismic_array"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:55.448067+00:00"
instance: "kb-cron"
---
E
(
ω
,
k
0
−
k
)
=
1
2
π
∫
−
∞
∞
|
w
¯
(
ω
)
|
2
⋅
|
C
(
k
0
−
k
)
|
2
d
ω
{\displaystyle E(\omega ,k_{0}-k)={\frac {1}{2\pi }}\int _{-\infty }^{\infty }|{\bar {w}}(\omega )|^{2}\cdot |C(k_{0}-k)|^{2}d\omega }
, where
C
(
k
0
−
k
)
=
1
M
∑
j
=
1
M
e
i
w
r
j
(
k
0
−
k
)
{\displaystyle C(k_{0}-k)={\frac {1}{M}}\sum _{j=1}^{M}e^{iwr_{j}(k_{0}-k)}}
This equation is called the transfer function of an array. If the slowness difference is zero, the factor
|
C
(
k
0
−
k
)
|
2
{\displaystyle |C(k_{0}-k)|^{2}}
becomes 1.0 and the array is optimally tuned for this slowness. All other energy propagating with a different slowness will be suppressed.
=== Slowness estimation ===
Slowness estimation is a matter of forming beams with different slowness vectors and comparing the amplitudes or the power of the beams, and finding out the best beam by looking for the vapp and backazimuth combination with the highest energy on the beam.
==== f-k analysis ====
Frequency-wavenumber analysis is used as a reference tool in array processing for estimating slowness. This method was proposed by Capon in 1969 and further developed to include wide-band analysis, maximum-likelihood estimation techniques, and three-component data in the 1980s.
The methodology exploits the deterministic, non-periodic character of seismic wave propagation to calculate the frequency-wavenumber spectrum of the signals by applying the multidimensional Fourier transform. A monochromatic plane wave w(x,t) will propagate along the x direction according to equation
w
(
x
,
t
)
=
A
e
i
2
π
(
f
0
t
−
k
0
x
)
{\displaystyle w(x,t)=Ae^{i2\pi (f_{0}t-k_{0}x)}}
It can be rewritten in frequency domain as
W
(
k
x
,
f
)
=
A
δ
(
f
−
f
0
)
δ
(
k
x
−
k
0
)
{\displaystyle W(k_{x},f)=A\delta (f-f_{0})\delta (k_{x}-k_{0})}
which suggests the possibility to map a monochromatic plane wave in the frequency-wavenumber domain to a point with coordinates (f, kx) = (f0, k0).
Practically, f-k analysis is performed in the frequency domain and represents in principle beamforming in the frequency domain for a number of different slowness values. At NORSAR slowness values between -0.4 and 0.4 s/km are used equally spaced over 51 by 51 points. For every one of these points the beam power is evaluated, giving an equally spaced
grid of 2601 points with power information.
==== Beampacking ====
A beampacking scheme was developed at NORSAR to apply f-k analysis of regional phases to data of large array. This algorithm performs time-domain beamforming over a predefined grid of slowness points and measures the power of the beam.
In practice the beampacking process gives the same slowness estimate as for the f-k analysis in the frequency domain. Compared to the f-k process, the beampacking process results in
a slightly (about 10%) narrower peak for the maximum power.
==== Plane wave fitting ====
Another way of estimating slowness is to pick carefully times of the first onset or any other common distinguishable part of the same phase (same cycle) for all instruments in an
array. Let ti be the arrival time picked at site i, and tref be the arrival time at the reference site, then τi = ti − tref is the observed time delay at site i. We observe the plane wave at M sites. With M ≥ 3. The horizontal components (sx, sy) of the slowness vector s can be estimated by
s
^
=
m
i
n
s
∑
j
=
1
M
(
τ
j
−
r
j
⋅
s
)
2
{\displaystyle {\hat {s}}={\underset {s}{min}}\sum _{j=1}^{M}(\tau _{j}-r_{j}\cdot s)^{2}}
Plane wave fitting requires interactive analyst's work. However, to obtain automatic time picks and thereby provide a slowness estimate automatically, techniques like cross-correlation or just picking of peak amplitude within a time window may be used. Because of the amount of required computations, plane wave fitting is most effective for arrays with a smaller number of sites or for subarray configurations.
== Applications ==
Current seismic arrays worldwide:
=== Gräfenberg ===
The Gräfenberg array is the first digital broadband array that has a continuous data history from 1976 until today. This array consists of 13 broadband stations in the Fränkische Alb. It extends approximately 100 kilometers north-south and approximately 40 kilometers east-west.
=== YKA ===
YKA or Yellowknife Seismological Array is a medium size seismic array established near Yellowknife in the Northwest Territories, Canada, in 1962, in cooperative agreement between the Department of Mines and Technical Surveys (now Natural Resources Canada) and the United Kingdom Atomic Energy Authority (UKAEA), to investigate the feasibility of teleseismic detection and identification of nuclear explosions. YKA currently consists of 19 short period seismic sensors in the form of a cross with an aperture of 2.5 km, plus 4 broadband seismograph sites with instruments able to detect a wide range of seismic wave frequencies.
=== LASA ===
LASA or Large Aperture Seismic Array is the first large seismic array. It was built in Montana, USA, in 1965.
=== NORSAR ===
NORSAR or Norwegian Seismic Array was established at Kjeller, Norway in 1968 as part of the Norwegian-US agreement for the detection of earthquakes and nuclear explosions. It has been an independent, not-for-profit, research foundation within the field of geo-science since 1999. NORSAR was constructed as a large aperture array with a diameter of 100 km. It is the largest stand-alone array in the world.
=== NORES and ARCES ===
NORES was the first regional seismic array constructed in southern Norway in 1984. A sister array ARCES was established in northern Norway in 1987. NORES and ARCES are small aperture arrays with a diameter of only 3 km.
=== GERES ===
GERES is a small aperture array built in the Bavarian Forest near the border triangle of Germany, Austria and Czechia, in 1988. It consists of 25 individual seismic stations arranged in 4 concentric rings with radius of 200m, 430m, 925m and 1988m.
=== SPITS ===
SPITS is a very small aperture array at Spitsbergen, Norway. It was originally installed in 1992 and upgraded to IMS standard in 2007 by NORSAR.
== See also ==
Seismometer
Array processing
== References ==

32
data/en.wikipedia.org/wiki/Seismometer-0.md Normal file
View File

@ -0,0 +1,32 @@
---
title: "Seismometer"
chunk: 1/6
source: "https://en.wikipedia.org/wiki/Seismometer"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:56.624148+00:00"
instance: "kb-cron"
---
A seismometer is an instrument that responds to ground displacement and shaking caused by quakes, volcanic eruptions, and explosions. They are usually combined with a timing device and a recording device to form a seismograph. The output of such a device—formerly recorded on paper (see picture) or film, now recorded and processed digitally—is a seismogram. Such data is used to locate and characterize earthquakes, and to study the internal structure of Earth.
== Basic principles ==
A simple seismometer, sensitive to up-down motions of the Earth, is like a weight hanging from a spring, both suspended from a frame that moves along with any motion detected. The relative motion between the weight (called the mass) and the frame provides a measurement of the vertical ground motion. A rotating drum is attached to the frame and a pen is attached to the weight, thus recording any ground motion in a seismogram.
Any movement from the ground moves the frame. The mass tends not to move because of its inertia, and by measuring the movement between the frame and the mass, the motion of the ground can be determined.
Early seismometers used optical levers or mechanical linkages to amplify the small motions involved, recording on soot-covered paper or photographic paper. Modern instruments use electronics. In some systems, the mass is held nearly motionless relative to the frame by an electronic negative feedback loop. The motion of the mass relative to the frame is measured, and the feedback loop applies a magnetic or electrostatic force to keep the mass nearly motionless. The voltage needed to produce this force is the output of the seismometer, which is recorded digitally.
In other systems the weight is allowed to move, and its motion produces an electrical charge in a coil attached to the mass which voltage moves through the magnetic field of a magnet attached to the frame. This design is often used in a geophone, which is used in exploration for oil and gas.
Seismic observatories usually have instruments measuring three axes: north-south (y-axis), east–west (x-axis), and vertical (z-axis). If only one axis is measured, it is usually the vertical because it is less noisy and gives better records of some seismic waves.
The foundation of a seismic station is critical. A professional station is sometimes mounted on bedrock. The best mountings may be in deep boreholes, which avoid thermal effects, ground noise and tilting from weather and tides. Other instruments are often mounted in insulated enclosures on small buried piers of unreinforced concrete. Reinforcing rods and aggregates would distort the pier as the temperature changes. A site is always surveyed for ground noise with a temporary installation before pouring the pier and laying conduit. Originally, European seismographs were placed in a particular area after a destructive earthquake. Today, they are spread to provide appropriate coverage (in the case of weak-motion seismology) or concentrated in high-risk regions (strong-motion seismology).
== Nomenclature ==
The word derives from the Greek σεισμός, seismós, a shaking or quake, from the verb σείω, seíō, to shake; and μέτρον, métron, to measure, and was coined by David Milne-Home in 1841, to describe an instrument designed by Scottish physicist James David Forbes.
Seismograph is another Greek term from seismós and γράφω, gráphō, to draw. It is often used to mean seismometer, though it is more applicable to the older instruments in which the measuring and recording of ground motion were combined, than to modern systems, in which these functions are separated. Both types provide a continuous record of ground motion; this record distinguishes them from seismoscopes, which merely indicate that motion has occurred, perhaps with some simple measure of how large it was.
The technical discipline concerning such devices is called seismometry, a branch of seismology.
The concept of measuring the "shaking" of something means that the word "seismograph" might be used in a more general sense. For example, a monitoring station that tracks changes in electromagnetic noise affecting amateur radio waves presents an rf seismograph. And helioseismology studies the "quakes" on the Sun.
== History ==
The first seismometer was made in China during the 2nd century. It was invented by Zhang Heng, a Chinese mathematician and astronomer. The first Western description of the device comes from the French physicist and priest Jean de Hautefeuille in 1703. The modern seismometer was developed in the 19th century.
Seismometers were placed on the Moon starting in 1969 as part of the Apollo Lunar Surface Experiments Package. In December 2018, a seismometer was deployed on the planet Mars by the InSight lander, the first time a seismometer was placed onto the surface of another planet.
=== Ancient era ===

18
data/en.wikipedia.org/wiki/Seismometer-1.md Normal file
View File

@ -0,0 +1,18 @@
---
title: "Seismometer"
chunk: 2/6
source: "https://en.wikipedia.org/wiki/Seismometer"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:56.624148+00:00"
instance: "kb-cron"
---
Some secondary sources mention unverified claims that a device resembling an earthquake detector may have existed in Ancient Egypt, but as of January 2026, no reliable historical sources confirm this, and the earliest well-documented seismoscope was invented in China by Zhang Heng in AD 132.
In AD 132, Zhang Heng of China's Han dynasty is said to have invented the first seismoscope (by the definition above), which was called Houfeng Didong Yi (translated as, "instrument for measuring the seasonal winds and the movements of the Earth"). The description we have, from the History of the Later Han Dynasty, says that it was a large bronze vessel, about 2 meters in diameter; at eight points around the top were dragon's heads holding bronze balls. When there was an earthquake, one of the dragons' mouths would open and drop its ball into a bronze toad at the base, making a sound and supposedly showing the direction of the earthquake. On at least one occasion, probably at the time of a large earthquake in Gansu in AD 143, the seismoscope indicated an earthquake even though one was not felt. The available text says that inside the vessel was a central column that could move along eight tracks; this is thought to refer to a pendulum, though it is not known exactly how this was linked to a mechanism that would open only one dragon's mouth. The first earthquake recorded by this seismoscope was supposedly "somewhere in the east". Days later, a rider from the east reported this earthquake.
=== Early designs (1259–1839) ===
By the 13th century, seismographic devices existed in the Maragheh observatory (founded 1259) in Persia, though it is unclear whether these were constructed independently or based on the first seismoscope. French physicist and priest Jean de Hautefeuille described a seismoscope in 1703, which used a bowl filled with mercury which would spill into one of eight receivers equally spaced around the bowl, though there is no evidence that he actually constructed the device. A mercury seismoscope was constructed in 1784 or 1785 by Atanasio Cavalli, a copy of which can be found at the University Library in Bologna, and a further mercury seismoscope was constructed by Niccolò Cacciatore in 1818. James Lind also built a seismological tool of unknown design or efficacy (known as an earthquake machine) in the late 1790s.
Pendulum devices were developing at the same time. Neapolitan naturalist Nicola Cirillo set up a network of pendulum earthquake detectors following the 1731 Puglia Earthquake, where the amplitude was detected using a protractor to measure the swinging motion. Benedictine monk Andrea Bina further developed this concept in 1751, having the pendulum create trace marks in sand under the mechanism, providing both magnitude and direction of motion. Neapolitan clockmaker Domenico Salsano produced a similar pendulum which recorded using a paintbrush in 1783, labelling it a geo-sismometro, possibly the first use of a similar word to seismometer. Naturalist Nicolo Zupo devised an instrument to detect electrical disturbances and earthquakes at the same time (1784).
The first moderately successful device for detecting the time of an earthquake was devised by Ascanio Filomarino in 1796, who improved upon Salsano's pendulum instrument, using a pencil to mark, and using a hair attached to the mechanism to inhibit the motion of a clock's balance wheel. This meant that the clock would only start once an earthquake took place, allowing determination of the time of incidence.
After an earthquake taking place on October 4, 1834, Luigi Pagani observed that the mercury seismoscope held at Bologna University had completely spilled over, and did not provide useful information. He therefore devised a portable device that used lead shot to detect the direction of an earthquake, where the lead fell into four bins arranged in a circle, to determine the quadrant of earthquake incidence. He completed the instrument in 1841.

18
data/en.wikipedia.org/wiki/Seismometer-2.md Normal file
View File

@ -0,0 +1,18 @@
---
title: "Seismometer"
chunk: 3/6
source: "https://en.wikipedia.org/wiki/Seismometer"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:56.624148+00:00"
instance: "kb-cron"
---
=== Early Modern designs (1839–1880) ===
In response to a series of earthquakes near Comrie in Scotland in 1839, a committee was formed in the United Kingdom in order to produce better detection devices for earthquakes. The outcome of this was an inverted pendulum seismometer constructed by James David Forbes, first presented in a report by David Milne-Home in 1842, which recorded the measurements of seismic activity through the use of a pencil placed on paper above the pendulum. The designs provided did not prove effective, according to Milne's reports. It was Milne who coined the word seismometer in 1841, to describe this instrument. In 1843, the first horizontal pendulum was used in a seismometer, reported by Milne (though it is unclear if he was the original inventor). After these inventions, Robert Mallet published an 1848 paper where he suggested ideas for seismometer design, suggesting that such a device would need to register time, record amplitudes horizontally and vertically, and ascertain direction. His suggested design was funded, and construction was attempted, but his final design did not fulfill his expectations and suffered from the same problems as the Forbes design, being inaccurate and not self-recording.
Karl Kreil constructed a seismometer in Prague between 1848 and 1850, which used a point-suspended rigid cylindrical pendulum covered in paper, drawn upon by a fixed pencil. The cylinder was rotated every 24 hours, providing an approximate time for a given quake.
Luigi Palmieri, influenced by Mallet's 1848 paper, invented a seismometer in 1856 that could record the time of an earthquake. This device used metallic pendulums which closed an electric circuit with vibration, which then powered an electromagnet to stop a clock. Palmieri seismometers were widely distributed and used for a long time.
By 1872, a committee in the United Kingdom led by James Bryce expressed their dissatisfaction with the current available seismometers, still using the large 1842 Forbes device located in Comrie Parish Church, and requested a seismometer which was compact, easy to install and easy to read. In 1875 they settled on a large example of the Mallet device, consisting of an array of cylindrical pins of various sizes installed at right angles to each other on a sand bed, where larger earthquakes would knock down larger pins. This device was constructed in 'Earthquake House' near Comrie, which can be considered the world's first purpose-built seismological observatory. As of 2013, no earthquake has been large enough to cause any of the cylinders to fall in either the original device or replicas.
=== The first seismographs (1880–present) ===
The first seismographs were invented in the 1870s and 1880s. The first seismograph was produced by Filippo Cecchi in around 1875. A seismoscope would trigger the device to begin recording, and then a recording surface would produce a graphical illustration of the tremors automatically (a seismogram). However, the instrument was not sensitive enough, and the first seismogram produced by the instrument was in 1887, by which time John Milne had already demonstrated his design in Japan.

21
data/en.wikipedia.org/wiki/Seismometer-3.md Normal file
View File

@ -0,0 +1,21 @@
---
title: "Seismometer"
chunk: 4/6
source: "https://en.wikipedia.org/wiki/Seismometer"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:56.624148+00:00"
instance: "kb-cron"
---
In 1880, the first horizontal pendulum seismometer was developed by the team of John Milne, James Alfred Ewing and Thomas Gray, who worked as foreign-government advisors in Japan, from 1880 to 1895. Milne, Ewing and Gray, all having been hired by the Meiji Government in the previous five years to assist Japan's modernization efforts, founded the Seismological Society of Japan in response to an Earthquake that took place on February 22, 1880, at Yokohama (Yokohama earthquake). Two instruments were constructed by Ewing over the next year, one being a common-pendulum seismometer and the other being the first seismometer using a damped horizontal pendulum. The innovative recording system allowed for a continuous record, the first to do so. The first seismogram was recorded on 3 November 1880 on both of Ewing's instruments. Modern seismometers would eventually descend from these designs. Milne has been referred to as the 'Father of modern seismology' and his seismograph design has been called the first modern seismometer.
This produced the first effective measurement of horizontal motion. Gray would produce the first reliable method for recording vertical motion, which produced the first effective 3-axis recordings.
An early special-purpose seismometer consisted of a large, stationary pendulum, with a stylus on the bottom. As the earth started to move, the heavy mass of the pendulum had the inertia to stay still within the frame. The result is that the stylus scratched a pattern corresponding with the Earth's movement. This type of strong-motion seismometer recorded upon a smoked glass (glass with carbon soot). While not sensitive enough to detect distant earthquakes, this instrument could indicate the direction of the pressure waves and thus help find the epicenter of a local quake. Such instruments were useful in the analysis of the 1906 San Francisco earthquake. Further analysis was performed in the 1980s, using these early recordings, enabling a more precise determination of the initial fault break location in Marin county and its subsequent progression, mostly to the south.
Later, professional suites of instruments for the worldwide standard seismographic network had one set of instruments tuned to oscillate at fifteen seconds, and the other at ninety seconds, each set measuring in three directions. Amateurs or observatories with limited means tuned their smaller, less sensitive instruments to ten seconds.
The basic damped horizontal pendulum seismometer swings like the gate of a fence. A heavy weight is mounted on the point of a long (from 10 cm to several meters) triangle, hinged at its vertical edge. As the ground moves, the weight stays unmoving, swinging the "gate" on the hinge.
The advantage of a horizontal pendulum is that it achieves very low frequencies of oscillation in a compact instrument. The "gate" is slightly tilted, so the weight tends to slowly return to a central position. The pendulum is adjusted (before the damping is installed) to oscillate once per three seconds, or once per thirty seconds. The general-purpose instruments of small stations or amateurs usually oscillate once per ten seconds. A pan of oil is placed under the arm, and a small sheet of metal mounted on the underside of the arm drags in the oil to damp oscillations. The level of oil, position on the arm, and angle and size of sheet is adjusted until the damping is "critical", that is, almost having oscillation. The hinge is very low friction, often torsion wires, so the only friction is the internal friction of the wire. Small seismographs with low proof masses are placed in a vacuum to reduce disturbances from air currents.
Zollner described torsionally suspended horizontal pendulums as early as 1869, but developed them for gravimetry rather than seismometry.
Early seismometers had an arrangement of levers on jeweled bearings, to scratch smoked glass or paper. Later, mirrors reflected a light beam to a direct-recording plate or roll of photographic paper. Briefly, some designs returned to mechanical movements to save money. In mid-twentieth-century systems, the light was reflected to a pair of differential electronic photosensors called a photomultiplier. The voltage generated in the photomultiplier was used to drive galvanometers which had a small mirror mounted on the axis. The moving reflected light beam would strike the surface of the turning drum, which was covered with photo-sensitive paper. The expense of developing photo-sensitive paper caused many seismic observatories to switch to ink or thermal-sensitive paper.
After World War II, the seismometers developed by Milne, Ewing and Gray were adapted into the widely used Press-Ewing seismometer.
== Modern instruments ==

36
data/en.wikipedia.org/wiki/Seismometer-4.md Normal file
View File

@ -0,0 +1,36 @@
---
title: "Seismometer"
chunk: 5/6
source: "https://en.wikipedia.org/wiki/Seismometer"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:56.624148+00:00"
instance: "kb-cron"
---
Modern instruments use electronic sensors, amplifiers, and recording devices. Most are broadband covering a wide range of frequencies. Some seismometers can measure motions with frequencies from 500 Hz to 0.00118 Hz (1/500 = 0.002 seconds per cycle, to 1/0.00118 = 850 seconds per cycle). The mechanical suspension for horizontal instruments remains the garden-gate described above. Vertical instruments use some kind of constant-force suspension, such as the LaCoste suspension. The LaCoste suspension uses a zero-length spring to provide a long period (high sensitivity). Some modern instruments use a "triaxial" or "Galperin" design, in which three identical motion sensors are set at the same angle to the vertical but 120 degrees apart on the horizontal. Vertical and horizontal motions can be computed from the outputs of the three sensors.
Seismometers unavoidably introduce some distortion into the signals they measure, but professionally designed systems have carefully characterized frequency transforms.
Modern sensitivities come in three broad ranges: geophones, 50 to 750 V/m; local geologic seismographs, about 1,500 V/m; and teleseismographs, used for world survey, about 20,000 V/m. Instruments come in three main varieties: short-period, long-period and broadband. The short- and long-period instruments measure velocity and are very sensitive; however they 'clip' the signal or go off-scale for ground motion that is strong enough to be felt by people. A 24-bit analog-to-digital conversion channel is commonplace. Practical devices are linear to roughly one part per million.
Delivered seismometers come with two styles of output: analog and digital. Analog seismographs require analog recording equipment, possibly including an analog-to-digital converter. The output of a digital seismograph can be simply input to a computer. It presents the data in a standard digital format (often "SE2" over Ethernet).
=== Teleseismometers ===
The modern broadband seismograph can record a very broad range of frequencies. It consists of a small "proof mass", confined by electrical forces, driven by sophisticated electronics. As the earth moves, the electronics attempt to hold the mass steady through a feedback circuit. The amount of force necessary to achieve this is then recorded.
In most designs the electronics holds a mass motionless relative to the frame. This device is called a "force balance accelerometer". It measures acceleration instead of velocity of ground movement. Basically, the distance between the mass and some part of the frame is measured very precisely, by a linear variable differential transformer. Some instruments use a linear variable differential capacitor.
That measurement is then amplified by electronic amplifiers attached to parts of an electronic negative feedback loop. One of the amplified currents from the negative feedback loop drives a coil very like a loudspeaker. The result is that the mass stays nearly motionless.
Most instruments measure directly the ground motion using the distance sensor. The voltage generated in a sense coil on the mass by the magnet directly measures the instantaneous velocity of the ground. The current to the drive coil provides a sensitive, accurate measurement of the force between the mass and frame, thus measuring directly the ground's acceleration (using f=ma where f=force, m=mass, a=acceleration).
One of the continuing problems with sensitive vertical seismographs is the buoyancy of their masses. The uneven changes in pressure caused by wind blowing on an open window can easily change the density of the air in a room enough to cause a vertical seismograph to show spurious signals. Therefore, most professional seismographs are sealed in rigid gas-tight enclosures. For example, this is why a common Streckeisen model has a thick glass base that must be glued to its pier without bubbles in the glue.
It might seem logical to make the heavy magnet serve as a mass, but that subjects the seismograph to errors when the Earth's magnetic field moves. This is also why seismograph's moving parts are constructed from a material that interacts minimally with magnetic fields. A seismograph is also sensitive to changes in temperature so many instruments are constructed from low expansion materials such as nonmagnetic invar.
The hinges on a seismograph are usually patented, and by the time the patent has expired, the design has been improved. The most successful public domain designs use thin foil hinges in a clamp.
Another issue is that the transfer function of a seismograph must be accurately characterized, so that its frequency response is known. This is often the crucial difference between professional and amateur instruments. Most are characterized on a variable frequency shaking table.
=== Strong-motion seismometers ===
Another type of seismometer is a digital strong-motion seismometer, or accelerograph. The data from such an instrument is essential to understand how an earthquake affects man-made structures, through earthquake engineering. The recordings of such instruments are crucial for the assessment of seismic hazard, through engineering seismology.
A strong-motion seismometer measures acceleration. This can be mathematically integrated later to give velocity and position. Strong-motion seismometers are not as sensitive to ground motions as teleseismic instruments but they stay on scale during the strongest seismic shaking.
Strong motion sensors are used for intensity meter applications.
=== Other forms ===
Accelerographs and geophones are often heavy cylindrical magnets with a spring-mounted coil inside. As the case moves, the coil tends to stay stationary, so the magnetic field cuts the wires, inducing current in the output wires. They receive frequencies from several hundred hertz down to 1 Hz. Some have electronic damping, a low-budget way to get some of the performance of the closed-loop wide-band geologic seismographs.
Strain-beam accelerometers constructed as integrated circuits are too insensitive for geologic seismographs (2002), but are widely used in geophones.
Some other sensitive designs measure the current generated by the flow of a non-corrosive ionic fluid through an electret sponge or a conductive fluid through a magnetic field.

59
data/en.wikipedia.org/wiki/Seismometer-5.md Normal file
View File

@ -0,0 +1,59 @@
---
title: "Seismometer"
chunk: 6/6
source: "https://en.wikipedia.org/wiki/Seismometer"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:56.624148+00:00"
instance: "kb-cron"
---
=== Interconnected seismometers ===
Seismometers spaced in a seismic array can also be used to precisely locate, in three dimensions, the source of an earthquake, using the time it takes for seismic waves to propagate away from the hypocenter, the initiating point of fault rupture (See also Earthquake location). Interconnected seismometers are also used, as part of the International Monitoring System to detect underground nuclear test explosions, as well as for Earthquake early warning systems. These seismometers are often used as part of a large-scale governmental or scientific project, but some organizations such as the Quake-Catcher Network, can use residential size detectors built into computers to detect earthquakes as well.
In reflection seismology, an array of seismometers image sub-surface features. The data are reduced to images using algorithms similar to tomography. The data reduction methods resemble those of computer-aided tomographic medical imaging X-ray machines (CAT-scans), or imaging sonars.
A worldwide array of seismometers can actually image the interior of the Earth in wave-speed and transmissivity. This type of system uses events such as earthquakes, impact events or nuclear explosions as wave sources. The first efforts at this method used manual data reduction from paper seismograph charts. Modern digital seismograph records are better adapted to direct computer use. With inexpensive seismometer designs and internet access, amateurs and small institutions have even formed a "public seismograph network".
Seismographic systems used for petroleum or other mineral exploration historically used an explosive and a wireline of geophones unrolled behind a truck. Now most short-range systems use "thumpers" that hit the ground, and some small commercial systems have such good digital signal processing that a few sledgehammer strikes provide enough signal for short-distance refractive surveys. Exotic cross or two-dimensional arrays of geophones are sometimes used to perform three-dimensional reflective imaging of subsurface features. Basic linear refractive geomapping software (once a black art) is available off-the-shelf, running on laptop computers, using strings as small as three geophones. Some systems now come in an 18" (0.5 m) plastic field case with a computer, display and printer in the cover.
Small seismic imaging systems are now sufficiently inexpensive to be used by civil engineers to survey foundation sites, locate bedrock, and find subsurface water.
=== Fiber optic cables as seismometers ===
A new technique for detecting earthquakes has been found, using fiber optic cables.
In 2016 a team of metrologists running frequency metrology experiments in England observed noise with a wave-form resembling the seismic waves generated by earthquakes. This was found to match seismological observations of an Mw6.0 earthquake in Italy, ~1400 km away. Further experiments in England, Italy, and with a submarine fiber optic cable to Malta detected additional earthquakes, including one 4,100 km away, and an ML3.4 earthquake 89 km away from the cable.
Seismic waves are detectable because they cause micrometer-scale changes in the length of the cable. As the length changes so does the time it takes a packet of light to traverse to the far end of the cable and back (using a second fiber). Using ultra-stable metrology-grade lasers, these extremely minute shifts of timing (on the order of femtoseconds) appear as phase-changes.
The point of the cable first disturbed by an earthquake's p wave (essentially a sound wave in rock) can be determined by sending packets in both directions in the looped pair of optical fibers; the difference in the arrival times of the first pair of perturbed packets indicates the distance along the cable. This point is also the point closest to the earthquake's epicenter, which should be on a plane perpendicular to the cable. The difference between the P wave/S wave arrival times provides a distance (under ideal conditions), constraining the epicenter to a circle. A second detection on a non-parallel cable is needed to resolve the ambiguity of the resulting solution. Additional observations constrain the location of the earthquake's epicenter, and may resolve the depth.
This technique is expected to be a boon in observing earthquakes, especially the smaller ones, in vast portions of the global ocean where there are no seismometers, and at much lower cost than ocean-bottom seismometers.
=== Deep-Learning ===
Researchers at Stanford University created a deep-learning algorithm called UrbanDenoiser which can detect earthquakes, particularly in urban cities. The algorithm filters out the background noise from the seismic noise gathered from busy cities in urban areas to detect earthquakes.
== Recording ==
Today, the most common recorder is a computer with an analog-to-digital converter, a disk drive and an internet connection; for amateurs, a PC with a sound card and associated software is adequate. Most systems record continuously, but some record only when a signal is detected,
as shown by a short-term increase in the variation of the signal, compared to its long-term
average (which can vary slowly because of changes in seismic noise), also known as a STA/LTA trigger.
Prior to the availability of digital processing of seismic data in the late 1970s, the records were done in a few different forms on different types of media. A "Helicorder" drum was a device used to record data into photographic paper or in the form of paper and ink. A "Develocorder" was a machine that record data from up to 20 channels into a 16-mm film. The recorded film can be viewed by a machine. The reading and measuring from these types of media can be done by hand. After the digital processing has been used, the archives of the seismic data were recorded in magnetic tapes. Due to the deterioration of older magnetic tape medias, large number of waveforms from the archives are not recoverable.
== See also ==
Accelerometer
Galitzine, Boris Borisovich
Geophone
Inge Lehmann
IRIS Consortium
John Milne
Pacific Northwest Seismic Network
Plate tectonics
Quake-Catcher Network
Wood-Anderson seismometer
== References ==
== External links ==
The history of early seismometers
The Lehman amateur seismograph, from Scientific American Archived 2009-02-04 at the Wayback Machine- not designed for calibrated measurement.
Sean Morrisey's professional design of an amateur teleseismograph Also see Keith Payea's version Both accessed 2010-9-29 Morrissey was a professional seismographic instrument engineer. This superior design uses a zero-length spring to achieve a 60-second period, active feedback and a uniquely convenient variable reluctance differential transducer, with parts scavenged from a hardware store. The frequency transform is carefully designed, unlike most amateur instruments. Morrisey is deceased, but the site remains up as a public service.
SeisMac Archived 2010-03-06 at the Wayback Machine is a free tool for recent Macintosh laptop computers that implements a real-time three-axis seismograph.
The Development Of Very-Broad-Band Seismography: Quanterra And The Iris Collaboration Archived 2016-08-10 at the Wayback Machine discusses the history of development of the primary technology in global earthquake research.
Video of seismograph at Hawaiian Volcano Observatory – on Flickr – retrieved on 2009-06-15.
Seismoscope – Research References 2012
Iris EDU – How Does A Seismometer Work?
Seismometers, seismographs, seismograms – what's the difference? How do they work? – USGS

20
data/en.wikipedia.org/wiki/Slit_lamp-0.md Normal file
View File

@ -0,0 +1,20 @@
---
title: "Slit lamp"
chunk: 1/3
source: "https://en.wikipedia.org/wiki/Slit_lamp"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:35.131086+00:00"
instance: "kb-cron"
---
In ophthalmology and optometry, a slit lamp is an instrument consisting of a high-intensity light source that can be focused to shine a thin sheet of light into the eye. It is used in conjunction with a biomicroscope. The lamp facilitates an examination of the anterior segment and posterior segment of the human eye, which includes the eyelid, sclera, conjunctiva, iris, natural crystalline lens, and cornea. The binocular slit-lamp examination provides a stereoscopic magnified view of the eye structures in detail, enabling anatomical diagnoses to be made for a variety of eye conditions. A second, hand-held lens is used to examine the retina.
== History ==
Two conflicting trends emerged in the development of the slit lamp. One trend originated from clinical research and aimed to apply the increasingly complex and advanced technology of the time. The second trend originated from ophthalmologic practice and aimed at technical perfection and a restriction to useful methods. The first man credited with developments in this field was Hermann von Helmholtz (1850) when he invented the ophthalmoscope.
In ophthalmology and optometry, the instrument is called a "slit lamp", although it is more correctly called a "slit lamp instrument". Today's instrument is a combination of two separate developments, the corneal microscope and the slit lamp itself. The first concept of a slit lamp dates back to 1911 credited to Allvar Gullstrand and his "large reflection-free ophthalmoscope." The instrument was manufactured by Zeiss and consisted of a special illuminator connected to a small stand base through a vertical adjustable column. The base was able to move freely on a glass plate. The illuminator employed a Nernst glower which was later converted into a slit through a simple optical system. However, the instrument never received much attention and the term "slit lamp" did not appear in any literature again until 1914.
It was not until 1919 that several improvements were made to the Gullstrand slit lamp made by Vogt Henker. First, a mechanical connection was made between lamp and ophthalmoscopic lens. This illumination unit was mounted to the table column with a double articulated arm. The binocular microscope was supported on a small stand and could be moved freely across the tabletop. Later, a cross slide stage was used for this purpose. Vogt introduced Koehler illumination, and the reddish Nernst glower was replaced with the brighter and whiter incandescent lamp. Special mention should be paid to the experiments that followed Henker's improvements in 1919. On his improvements the Nitra lamp was replaced with a carbon arc lamp with a liquid filter. At this time the great importance of color temperature and the luminance of the light source for slit lamp examinations were recognized and the basis created for examinations in red-free light.
In the year 1926, the slit lamp instrument was redesigned. The vertical arrangement of the projector made it easy to handle. For the first time, the axis through the patient's eye was fixed along a common swiveling axis, although the instrument still lacked a coordinate cross-slide stage for instrument adjustment. The importance of focal illumination had not yet been fully recognized.
In 1927, stereo cameras were developed and added to the slit lamp to further its use and application. In 1930, Rudolf Theil further developed the slit lamp, encouraged by Hans Goldmann. Horizontal and vertical co-ordinate adjustments were performed with three control elements on the cross-slide stage. The common swivel axis for microscope and illumination system was connected to the cross-slide stage, which allowed it to be brought to any part of the eye to be examined. A further improvement was made in 1938. A control lever or joystick was used for the first time to allow for horizontal movement.
Following World War II the slit lamp was improved again. On this particular improvement the slit projector could be swiveled continuously across the front of the microscope. This was improved again in 1950, when a company named Littmann redesigned the slit lamp. They adopted the joystick control from the Goldmann instrument and the illumination path present in the Comberg instrument. Additionally, Littmann added the stereo telescope system with a common objective magnification changer.
In 1965, the Model 100/16 Slit Lamp was produced based on the slit lamp by Littmann. This was soon followed by the Model 125/16 Slit Lamp in 1972. The only difference between the two models was their operating distances of 100 mm to 125 mm. With the introduction of the photo slit lamp further advancements were possible. In 1976, the development of the Model 110 Slit Lamp and the 210/211 Photo Slit Lamps were an innovation by which each were constructed from standard modules allowing for a wide range of different configurations. At the same time, halogen lamps replaced the older illumination systems to make them brighter and essentially daylight quality. From 1994 onwards, new slit lamps were introduced which took advantage of new technologies. The last major development was in 1996 in which included new slit lamp optics.

54
data/en.wikipedia.org/wiki/Slit_lamp-1.md Normal file
View File

@ -0,0 +1,54 @@
---
title: "Slit lamp"
chunk: 2/3
source: "https://en.wikipedia.org/wiki/Slit_lamp"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:35.131086+00:00"
instance: "kb-cron"
---
== General procedure ==
While a patient is seated in the examination chair, they rest their chin and forehead on a support area to steady the head. Using the biomicroscope, the ophthalmologist or optometrist then proceeds to examine the patient's eye. A fine strip of paper, stained with fluorescein, a fluorescent dye, may be touched to the side of the eye; this stains the tear film on the surface of the eye to aid examination. The dye is naturally rinsed out of the eye by tears.
A subsequent test may involve placing drops in the eye in order to dilate the pupils. The drops take about 15 to 20 minutes to work, after which the examination is repeated, allowing the back of the eye to be examined. Patients will experience some light sensitivity for a few hours after this exam, and the dilating drops may also cause increased pressure in the eye, leading to nausea and pain. Patients who experience serious symptoms are advised to seek medical attention immediately.
Adults need no special preparation for the test; however children may need some preparation, depending on age, previous experiences, and level of trust.
== Illumination ==
Various methods of slitlamp illumination are required to obtain full advantage of slit-lamp biomicroscope. There are mainly six type of illuminating options:
Diffuse illumination,
Direct focal illumination,
Specular reflection,
Transillumination or retroillumination,
Indirect lateral illumination or Indirect proximal illumination and
Sclerotic scatter.
Oscillatory Illumination is sometimes considered an illumination technique.
Observation with an optical section or direct focal illumination is the most frequently applied method of examination with the slit lamp. With this method, the axes of illuminating and viewing path intersect in the area of the anterior eye media to be examined, for example, the individual corneal layers.
=== Diffuse illumination ===
If media, especially that of the cornea, are opaque, optical section images are often impossible depending on severity. In these cases, diffuse illumination may be used to advantage. For this, the slit is opened very wide and a diffuse, attenuated survey illumination is produced by inserting a ground glass screen or diffuser in the illuminating path. "Wide beam" illumination is the only type that has the light source set wide open. Its main purpose is to illuminate as much of the eye and its adnexa at once for general observation.
=== Direct focal illumination ===
Observation with an optical section or direct focal illumination is the most frequently applied method. It is achieved by directing a full-height, hairline to medium width, medium-bright beam obliquely into the eye and focusing it on the cornea so that a quadrilateral block of light illuminates the transparent medias of eye. Viewing arm and illuminating arm are kept parfocal. This type of illumination is useful for depth localization. Direct focal illumination is used for grading cells and flare in anterior chamber by shortening height of beam to 2–1 mm.
=== Specular reflection ===
Specular reflection, or reflected illumination is just like patches of reflection seen on sunlit lake water surface. To achieve specular reflection, the examiner directs a medium to narrow beam of light (it must be thicker than an optical section) toward the eye from the temporal side. The angle of illumination should be wide (50°-60°) relative to the examiners axis of observation
(which should be slightly nasal to the patients visual axis). A bright zone of specular reflection will be evident on the temporal, midperipheral corneal epithelium. It is used to see endothelial outline of cornea.
=== Transillumination or retroillumination ===
In certain cases, illumination by optical section does not yield sufficient information or is impossible. This is the case, for example, when larger, extensive zones or spaces of the ocular media are opaque. Then the scattered light that is not very bright normally is absorbed. A similar situation arises when areas behind the crystalline lens are to be observed. In this case the observation beam must pass a number of interfaces that may reflect and attenuate the light.
=== Indirect illumination ===
With this method, light enters the eye through a narrow to medium slit (2 to 4 mm) to one side of the area to be examined. The axes of illuminating and viewing path do not intersect at the point of image focus, to achieve this; the illuminating prism is decentered by rotating it about its vertical axis off the normal position. In this way, reflected, indirect light illuminates the area of the anterior chamber or cornea to be examined. The observed corneal area then lies between the incident light section through the cornea and the irradiated area of the iris. Observation is thus against a comparatively dark background.
=== Sclerotic scatter or scattering sclero-corneal illumination ===
With this type of illumination, a wide light beam is directed onto the limbal region of the cornea at an extremely low angle of incidence and with a laterally de-centered illuminating prism. Adjustment must allow the light beam to transmit through the corneal parenchymal layers according to the principle of total reflection allowing the interface with the cornea to be brightly illuminated. The magnification should be selected so that the entire cornea can be seen at a glance.
== Special techniques ==
=== Fundus observation and gonioscopy with the slit lamp ===

67
data/en.wikipedia.org/wiki/Slit_lamp-2.md Normal file
View File

@ -0,0 +1,67 @@
---
title: "Slit lamp"
chunk: 3/3
source: "https://en.wikipedia.org/wiki/Slit_lamp"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:35.131086+00:00"
instance: "kb-cron"
---
Fundus observation is generally performed via ophthalmoscopy, where the observer (fundus camera or observing eye) is focused to infinity, which brings the subject's fundus into focus due to the refractive power of the subject's optical media. In contrast, the microscope in slit lamp biomicroscopy is focused to the anterior segments of the eye, such that direct observation of the fundus is impossible due to the subject's refractive power. However, with the use of auxiliary optics, the fundus can be brought within the focusing range of the microscope. These optics usually take the form of a lens placed on or near the subject's cornea, which range in optical properties and practical application.
Watzke–Allen test is a test used in diagnosis of a full thickness macular hole and also to assess retinal function after surgical closure of the hole, with the help of slit lamp.
== Light filters ==
Most slit-lamps have five light filters options:
Unfiltered,
Heat absorption- for increased patient comfort
Grey filter,
Red free- for better visualisation of nerve fibre layer and haemorrhages and blood vessels.
Cobalt blue- after staining with fluorescein dye, for seeing corneal ulcers, contact lens fitting, Seidel's test
=== Cobalt blue light ===
Slit lamps produce light of the wavelength 450 to 500 nm, known as "cobalt blue". This light is specifically useful for looking for problems in the eye once it has been stained with fluorescein.
== Types ==
There are two distinct slit lamp types based on the location of their illumination system:
=== Zeiss type ===
In the Zeiss-type slit lamp, the illumination is located below the microscope. This type of slit lamp is named after the manufacturing company Carl Zeiss.
=== Haag Streit type ===
In the Haag Streit-type slit lamp, the illumination is located above the microscope. This type of slit lamp is named after the manufacturing company Haag Streit.
== Interpretation ==
The slit lamp exam may detect many diseases of the eye, including:
Cataract
Conjunctivitis
Corneal injury such as corneal ulcer or corneal swelling
Diabetic retinopathy
Fuchs' dystrophy
Keratoconus (Fleischer ring)
Macular degeneration
Retinal detachment
Retinal vessel occlusion
Retinitis pigmentosa
Sjögren's syndrome
Toxoplasmosis
Uveitis
Wilson's disease (Kayser–Fleischer ring)
A sign that may be seen in slit lamp examination is a "flare", which is when the slit-lamp beam is visible in the anterior chamber. This occurs when there is breakdown of the blood-aqueous barrier with resultant exudation of protein.
== References ==
== Further reading ==
Vivino MA, Chintalagiri S, Trus B, Dati.les M., "Development of a Scheimpflug slit lamp camera system for quantitative densitometric analysis", Computer Systems Laboratory, National Eye Institute, National Institutes of Health, Bethesda, MD. Eye (Lond). 1993;7 (Pt 6):791–798.
"Slit-Lamp Gonioscopy." Postgraduate Medical Journal 39.451 (1963): 310.
Jobe, Frederick W. Slit Lamp. United States BAUSCH & LOMB, assignee. Patent "2235319" March 1941.
Nikon, Slit Lamp CS-1 Microscope, accessed February 6, 2011.
Ledford, Janice K. and Sanders, Valerie N. "The slit lamp primer", 2nd ed., SLACK Incorporated, ISBN 978-1-55642-747-3, 2006.
Schwartz, Gary S., "The eye exam: a complete guide", pp. 109–128 Slit Lamp Biomicroscopy, SLACK Incorporated, ISBN 978-1-55642-755-8, 2006.
Koppenhöfer, Eilhard, "From Lateral Illumination to Slit Lamp – An Outline of Medical History", online published 2012

111
data/en.wikipedia.org/wiki/Spherometer-0.md Normal file
View File

@ -0,0 +1,111 @@
---
title: "Spherometer"
chunk: 1/1
source: "https://en.wikipedia.org/wiki/Spherometer"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:36.305316+00:00"
instance: "kb-cron"
---
A spherometer is an instrument used for the precise measurement of the radius of curvature of a curved surface. Originally, these instruments were primarily used by opticians to measure the curvature of the surface of a lens.
== Design ==
A spherometer usually consists of:
A frame with three legs, arranged in an equilateral triangle of known radius. The outer legs of some spherometers can be moved to a set of inner holes in order to accommodate a smaller surface. The lower ends of the legs are finely tapered and terminate in hemispheres.
A central leg, which can be raised or lowered via a screw.
A reading device for measuring the distance the central leg is moved. Often this consists of a marked dial attached to the top of the screw and a vertical scale attached to the frame. This both indicates the number of turns of the screw and serves as an index for reading the divisions on the dial. A lens may be fitted in order to magnify the scale divisions.
On new spherometers, the vertical scale is marked off in units of 0.5 mm. One complete turn of the dial also corresponds to 0.5 mm and each small graduation on the dial represents 0.005 mm. The graduations on old spherometers are 0.001 mm.
== Principles of operation ==
To measure the radius of a sphere—e.g. the curvature of a lens—the spherometer is leveled and read, then placed on the sphere, adjusted until the four points exert equal pressure, and read again. The difference gives the thickness of that portion of the sphere cut off by a plane passing through the three feet. A contact-lever, delicate level or electric contact may be attached to the spherometer in order to indicate the moment at which the four points exert equal pressure.
The spherometer directly measures a sagitta, h. If the mean length between two outer legs is a, the spherical radius R is given by the formula
R
=
h
2
+
a
2
6
h
{\displaystyle R={\frac {h}{2}}+{\frac {a^{2}}{6h}}}
Using a spherometer with a circle cup of diameter D, the spherical radius R is instead given by the formula
R
=
h
2
+
D
2
8
h
{\displaystyle R={\frac {h}{2}}+{\frac {D^{2}}{8h}}}
== Alternative uses ==
Since the spherometer is essentially a type of micrometer, it can be employed for purposes other than measuring the curvature of a spherical surface. For example, it can be used to measure the thickness of a thin plate.
To do so, the instrument is placed on a perfectly level plane surface and the screw turned until the point just touches; the exact instant when it does so is defined by a sudden diminution of resistance followed by a considerable increase. The dial and scale are read, the screw is raised, the thin plate slipped under it, and the process is repeated. The difference between the two readings gives the required thickness.
Similarly, the instrument can measure the depression in an otherwise flat plate. The method is as for measuring the thickness of a plate, except that the micrometer portion is placed over the depression and the measurement is taken below the surface instead of above.
This type of instrument is commonly used in inspecting oil field tool pipe for metal surface pits, fractures, and roundness before being shipped to drilling sites for exploratory wells, so that weakened drill pipe will not fracture during drilling. Tool pipe with thicker than 1" walls for a 4" diameter tube of hardened steel, fitted with tapered thread collars, are re-used after drilling is complete and thinner-walled tubular oil-well casing is in place. Electronic instruments similar to the spherometer in design are used at inspection plants for casing, tubing, and drill pipe. The equivalent measurements in optics would be for a cylinder, or lens with a cylindrical component having an optical axis, where a plane through the lens would produce an oval circumference.
An alternate approach using coordinate geometry was developed recently. This approach reproduces the well-known result for the spherometer and also leads to a scheme to study aspherical surfaces.
A related device is the cylindrometer (also known as cylindro-spherometer or sphero-cylindrometer), which can additionally measure the radius of curvature of a right-circular cylinder.
== See also ==
Lapidary
Lens clock
Lens (optics)
Lensometer
== References ==
== External links ==

14
data/en.wikipedia.org/wiki/Stauroscope-0.md Normal file
View File

@ -0,0 +1,14 @@
---
title: "Stauroscope"
chunk: 1/1
source: "https://en.wikipedia.org/wiki/Stauroscope"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:37.484613+00:00"
instance: "kb-cron"
---
A stauroscope is an optical instrument used in determining the position of the planes of light-vibration in sections of crystals. The word comes from Greek for cross + scope. It was invented by Wolfgang Franz von Kobell in 1855.
== References ==

20
data/en.wikipedia.org/wiki/Stereoautograph-0.md Normal file
View File

@ -0,0 +1,20 @@
---
title: "Stereoautograph"
chunk: 1/1
source: "https://en.wikipedia.org/wiki/Stereoautograph"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:38.646837+00:00"
instance: "kb-cron"
---
The stereoautograph is a complex opto-mechanical measurement instrument for the evaluation of analog or digital photograms. It is based on the stereoscopy effect by using two aero photos or two photograms of the topography or of buildings from different standpoints.
It was invented by Eduard von Orel in 1907.
The photograms or photographic plates are oriented by measured passpoints in the field or on the building. This procedure can be carried out digitally (by methods of triangulation and projective geometry or iteratively (repeated angle corrections by congruent rays). The accuracy of modern autographs is about 0.001 mm.
Well known are the instruments of the companies Wild Heerbrugg (Leica), e.g. analog A7, B8 of the 1980s and the digital autographs beginning in the 1990s, or special instruments of Zeiss and Contraves.
== References ==
Gilbert Willy U.S. patent 1,477,082
Military Topography and Photography by Floyd D. Carlock, U.S. Army, 1916, p.104 ff, with photos (Available online at Google Books)

32
data/en.wikipedia.org/wiki/Strainmeter-0.md Normal file
View File

@ -0,0 +1,32 @@
---
title: "Strainmeter"
chunk: 1/1
source: "https://en.wikipedia.org/wiki/Strainmeter"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:57.834911+00:00"
instance: "kb-cron"
---
A strainmeter is an instrument used by geophysicists to measure the deformation of the Earth. Linear strainmeters measure the changes in the distance between two points, using either a solid piece of material (over a short distance) or a laser interferometer (over a long distance, up to several hundred meters).
The type using a solid length standard was invented by Benioff in 1932, using an iron pipe; later instruments used rods made of fused quartz. Modern instruments of this type can make measurements of length changes over very small distances, and are commonly placed in boreholes to measure small changes in the diameter of the borehole. Another type of borehole instrument detects changes in a volume filled with fluid (such as silicone oil). The most common type is the dilatometer invented by Sacks and Evertson in the USA (patent 3,635,076); a design that uses specially shaped volumes to measure the strain tensor has been developed by Sakata in Japan.
All these types of strainmeters can measure deformation over frequencies from a few Hz to periods of days, months, and years. This allows them to measure signals at lower frequencies than can be detected with seismometers. Most strainmeter records show signals from the earth tides, and seismic waves from earthquakes. At longer periods, they can also record the gradual accumulation of stress (physics) caused by plate tectonics, the release of this stress in earthquakes, and rapid changes of stress following earthquakes.
The most extensive network of strainmeters is installed in Japan; it includes mostly quartz-bar instruments in tunnels and borehole strainmeters, with a few laser instruments. Starting in 2003 there has been a major effort (the Plate Boundary Observatory) to install many more strainmeters along the Pacific/North-America plate boundary in the United States. The aim is to install about 100 borehole strainmeters, primarily in Washington, Oregon and California, and five laser strainmeters, all in California.
== See also ==
Deformation monitoring
Deformation (physics)
Extensometer
Infinitesimal strain theory
== References ==
Agnew DC (1986). "Strainmeters and tiltmeters". Reviews of Geophysics. 24 (3): 579–624. doi:10.1029/RG024i003p00579.
== External links ==
Piñon Flat Observatory, CA: laser strainmeters
GTSM Technologies, AUS: borehole strainmeters
Plate Boundary Observatory Archived 2009-11-24 at the Wayback Machine
US Geological Survey, see under Fault Monitoring Archived 2010-04-12 at the Wayback Machine

41
data/en.wikipedia.org/wiki/Theodolite-0.md Normal file
View File

@ -0,0 +1,41 @@
---
title: "Theodolite"
chunk: 1/4
source: "https://en.wikipedia.org/wiki/Theodolite"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:39.815016+00:00"
instance: "kb-cron"
---
A theodolite () is a precision optical instrument for measuring angles between designated visible points in the horizontal and vertical planes. The traditional use has been for land surveying, but it is also used extensively for building and infrastructure construction, and some specialized applications such as meteorology and rocket launching.
It consists of a moveable telescope mounted so it can rotate around horizontal and vertical axes and provide angular readouts. These indicate the orientation of the telescope, and are used to relate the first point sighted through the telescope to subsequent sightings of other points from the same theodolite position. Depending on the instrument, these angles can be measured with accuracies down to microradians or seconds of arc. From these readings a plan can be drawn, or objects can be positioned in accordance with an existing plan. The modern theodolite has evolved into what is known as a total station where angles and distances are measured electronically, and are read directly to computer memory.
A transit theodolite has a telescope short enough to rotate about the instrument's horizontal trunnion axis, turning the scope through the vertical plane and its zenith; vertical rotation in non-transit instruments is restricted to a limited arc.
The optical level is sometimes mistaken for a theodolite, but it does not measure vertical angles, and is used only for leveling on a horizontal plane (though often combined with medium accuracy horizontal range and direction measurements).
== Principles of operation ==
=== Preparation for making sightings ===
Temporary adjustments are a set of operations necessary in order to make a theodolite ready for taking observations at a station. These include its setting up, centering, leveling up and elimination of parallax, and are achieved in four steps:
Setting up: fixing the theodolite onto a tripod along with approximate leveling and centering over the station mark.
Centering: bringing the vertical axis of theodolite immediately over station mark using a centering plate also known as a tribrach.
Leveling: leveling of the base of the instrument to make the vertical axis vertical usually with an in-built bubble-level.
Focusing: removing parallax error by proper focusing of objective and eye-piece. The eye-piece only requires adjustment once at a station. The objective will be re-focused for each subsequent sighting from this station because of the different distances to the target.
=== Sightings ===
Sightings are taken by the surveyor, who adjusts the telescope's vertical and horizontal angular orientation so the cross-hairs align with the desired sighting point. Both angles are read either from exposed or internal scales and recorded. The next object is then sighted and recorded without moving the position of the instrument and tripod.
The earliest angular readouts were from open vernier scales directly visible to the eye. Gradually these scales were enclosed for physical protection. Angular micrometer scales were also used (not to be confused with the micrometer (device), a device used for length measurements). Finally, angle readings became an indirect optical readout, with convoluted light paths to bring them to a convenient place on the instrument for viewing. The modern digital theodolites have electronic displays.
Micrometers are also used in telescopes and microscopes to measure the apparent diameter of celestial bodies or microscopic objects or the angular distances between two such objects. The micrometer used with a telescope was invented about 1638 by William Gascoigne, an English astronomer.
=== Errors in measurement ===
Index error
The angles in the vertical axis should read 90° (100 grad) when the sight axis is horizontal, or 270° (300 grad) when the instrument is transited. Half of the difference between the two positions is called the index error. This can only be checked on transit instruments.
Horizontal axis error
The horizontal and vertical axes of a theodolite must be perpendicular; if not then a horizontal axis error exists. This can be tested by aligning the tubular spirit bubble parallel to a line between two footscrews and setting the bubble central. A horizontal axis error is present if the bubble runs off central when the tubular spirit bubble is reversed (turned through 180°). To adjust, the operator removes half the amount the bubble has run off using the adjusting screw, then re-level, test and refine the adjustment.
Collimation error
The optical axis of the telescope must also be perpendicular to the horizontal axis; if not, then a collimation error exists.
Index error, horizontal-axis error (trunnion-axis error) and collimation error are regularly determined by calibration and are removed by mechanical adjustment. Their existence is taken into account in the choice of measurement procedure in order to eliminate their effect on the measurement results of the theodolite.
== History ==

25
data/en.wikipedia.org/wiki/Theodolite-1.md Normal file
View File

@ -0,0 +1,25 @@
---
title: "Theodolite"
chunk: 2/4
source: "https://en.wikipedia.org/wiki/Theodolite"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:39.815016+00:00"
instance: "kb-cron"
---
=== Historical background ===
Prior to the theodolite, instruments such as the groma, geometric square and the dioptra, and various other graduated circles (see circumferentor) and semicircles (see graphometer) were used to obtain either vertical or horizontal angle measurements. Over time their functions were combined into a single instrument that could measure both angles simultaneously.
The first occurrence of the word theodolite is found in the surveying textbook A geometric practice named Pantometria (1571) by Leonard Digges. The origin of the word is unknown. The first part of the Neo-Latin theo-delitus might stem from the Greek θεᾶσθαι 'to behold or look attentively upon' The second part is often attributed to an unscholarly variation of the Greek word: δῆλος 'evident' or 'clear'. Other Neo-Latin or Greek derivations have been suggested as well as an English origin from alidade.
The early forerunners of the theodolite were sometimes azimuth instruments for measuring horizontal angles, while others had an altazimuth mount for measuring horizontal and vertical angles. Gregorius Reisch illustrated an altazimuth instrument in the appendix of his 1512 book Margarita Philosophica. Martin Waldseemüller, a topographer and cartographer made the device in that year calling it the polimetrum. In Digges's book of 1571, the term theodolite was applied to an instrument for measuring horizontal angles only, but he also described an instrument that measured both altitude and azimuth which he called a topographicall instrument [sic]. Possibly the first instrument approximating to a true theodolite was the built by Josua Habemel in 1576, complete with compass and tripod. The 1728 Cyclopaedia compares graphometer to half-theodolite. As late as the 19th century, the instrument for measuring horizontal angles only was called a simple theodolite and the altazimuth instrument, the plain theodolite.
The first instrument to combine the essential features of the modern theodolite was built in 1725 by Jonathan Sisson. This instrument had an altazimuth mount with a sighting telescope. The base plate had spirit levels, compass and adjusting screws. The circles were read with a vernier scale.
=== Development of the theodolite ===
The theodolite became a modern, accurate instrument in 1787, with the introduction of Jesse Ramsden's famous great theodolite, which he created using a very accurate dividing engine of his own design. Ramsden's instruments were used for the Principal Triangulation of Great Britain. At this time the highest precision instruments were made in England by such makers as Edward Troughton. Later the first practical German theodolites were made by Breithaupt together with Utzschneider, Reichenbach and Fraunhofer.
As technology progressed the vertical partial circle was replaced with a full circle, creating the transit theodolite. Developed from 18th-century astronomical transit instruments used to measure accurate star positions, these had finely graduated vertical and horizontal circles. The transition into a transit was pioneered by early 19th century by instrument makers such as Edward Troughton and William Simms, and became the standard theodolite design. Development of the theodolite was spurred on by specific needs, such as the British Ordnance Survey, which produced a 1820s requirement for theodolites capable of providing sufficient accuracy for large scale triangulation and mapping. The Survey of India at this time produced a requirement for more rugged and stable instruments, such as the lower center of gravity Everest pattern theodolite.
Railway engineers working in the 1830s in Britain commonly referred to a theodolite as a "Transit". The 1840s was the start of a period of rapid railway building in many parts of the world which resulted in a high demand for theodolites wherever railways were being constructed. It was also popular with American railroad engineers pushing west, and it replaced the railroad compass, sextant and octant. Theodolites were later adapted to a wider variety of mountings and uses. In the 1870s, an interesting waterborne version of the theodolite (using a pendulum device to counteract wave movement) was invented by Edward Samuel Ritchie. It was used by the U.S. Navy to take the first precision surveys of American harbors on the Atlantic and Gulf coasts.
In the early 1920s a step change in theodolite design occurred with the introduction of the Wild T2 made by the Swiss Wild Heerbrugg company. Heinrich Wild designed a theodolite with divided glass circles with readings from both sides presented at a single eyepiece close to the telescope so the observer did not have to move to read them. The Wild instruments were not only smaller, easier to use and more accurate than contemporary rivals but also sealed from rain and dust. Canadian surveyors reported that while the Wild T2 with 3.75 inch circles was not able to provide the accuracy for primary triangulation it was the equal in accuracy to a 12 inch traditional design. The Wild T2, T3, and A1 instruments were made for many years.
In 1926 a conference was held at Tavistock in Devon, UK where Wild theodolites were compared with British ones. The Wild product outclassed the British theodolites so manufacturers such as Cooke, Troughton & Simms and Hilger & Watts set about improving the accuracy of their products to match their competition. Cooke, Troughton and Simms developed the Tavistock pattern theodolite and later the Vickers V. 22.
Wild went on to develop the DK1, DKM1, DM2, DKM2, and DKM3 for Kern Aarau company. With continuing refinements, instruments steadily evolved into the modern theodolite used by surveyors today. By 1977 Wild, Kern and Hewlett-Packard were all offering "Total stations" which combined angular measurements, electronic distance measurement and microchip functions in a single unit.
=== Operation in surveying ===

29
data/en.wikipedia.org/wiki/Theodolite-2.md Normal file
View File

@ -0,0 +1,29 @@
---
title: "Theodolite"
chunk: 3/4
source: "https://en.wikipedia.org/wiki/Theodolite"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:39.815016+00:00"
instance: "kb-cron"
---
Triangulation, as invented by Gemma Frisius around 1533, consists of making direction plots of the surrounding landscape from two separate standpoints. The two graphing papers are superimposed, providing a scale model of the landscape, or rather the targets in it. The true scale can be obtained by measuring one distance both in the real terrain and in the graphical representation.
Modern triangulation as, e.g., practiced by Willebrord Snellius, is the same procedure executed by numerical means. Photogrammetric block adjustment of stereo pairs of aerial photographs is a modern, three-dimensional variant.
In the late 1780s, Jesse Ramsden, a Yorkshireman from Halifax, England, who had developed the dividing engine for dividing angular scales accurately to within a second of arc (≈ 0.0048 mrad or 4.8 μrad), was commissioned to build a new instrument for the British Ordnance Survey. The Ramsden theodolite was used over the next few years to map the whole of southern Britain by triangulation.
In network measurement, the use of forced centering speeds up operations while maintaining the highest precision. The theodolite or the target can be rapidly removed from, or socketed into, the forced centering plate with sub-millimeter precision. Nowadays GPS antennas used for geodetic positioning use a similar mounting system. The height of the reference point of the theodolite—or the target—above the ground benchmark must be measured precisely.
=== Transit theodolite ===
The term transit theodolite, or transit for short, refers to a type of theodolite where the telescope is short enough to rotate in a full circle on its horizontal axis as well as around its vertical axis. It features a vertical circle which is graduated through the full 360 degrees and a telescope that can rotate 360-degrees (and thus "transit the scope"). By reversing the telescope and at the same time rotating the instrument through 180 degrees about the vertical axis, the instrument can be used in 'plate-left' or 'plate-right' modes ('plate' refers to the vertical protractor circle). By measuring the same horizontal and vertical angles in these two modes and then averaging the results, centering and collimating errors in the instrument can be eliminated. Some transit instruments are capable of reading angles directly to thirty arc-seconds (≈ 0.15 mrad). Modern theodolites are usually of the transit-theodolite design, but engraved plates have been replaced with glass plates designed to be read with light-emitting diodes and digital circuitry, greatly improving accuracy up to arc-second (≈ 0.005 mrad) levels.
== Use with weather balloons ==
There is a long history of theodolite use in measuring winds aloft, by using specially-manufactured theodolites to track the horizontal and vertical angles of special weather balloons called ceiling balloons or pilot balloons (pibal). Early attempts at this were made in the opening years of the nineteenth century, but the instruments and procedures weren't fully developed until a hundred years later. This method was extensively used in World War II and thereafter, and was gradually replaced by radio and GPS measuring systems from the 1980s onward.
The pibal theodolite uses a prism to bend the optical path by 90 degrees so the operator's eye position does not change as the elevation is changed through a complete 180 degrees. The theodolite is typically mounted on a rugged steel stand, set up so it is level and pointed north, with the altitude and azimuth scales reading zero degrees. A balloon is released in front of the theodolite, and its position is precisely tracked, usually once a minute. The balloons are carefully constructed and filled, so their rate of ascent can be known fairly accurately in advance. Mathematical calculations on time, rate of ascent, azimuth and angular altitude can produce good estimates of wind speed and direction at various altitudes.
== Modern electronic theodolites ==
In modern electronic theodolites, the readout of the horizontal and vertical circles is usually done with a rotary encoder. These produce signals indicating the altitude and azimuth of the telescope which are fed to a microprocessor. CCD sensors have been added to the focal plane of the telescope allowing both auto-targeting and the automated measurement of residual target offset. All this is implemented in embedded software of the processor.
Many modern theodolites are equipped with integrated electro-optical distance measuring devices, generally infrared based, allowing the measurement in one step of complete three-dimensional vectors—albeit in instrument-defined polar coordinates, which can then be transformed to a preexisting coordinate system in the area by means of a sufficient number of control points. This technique is called a resection solution or free station position surveying and is widely used in mapping surveying.
Such instruments are "intelligent" theodolites called self-registering tacheometers or colloquially "total stations", and perform all the necessary angular and distance calculations, and the results or raw data can be downloaded to external processors, such as ruggedized laptops, PDAs or programmable calculators.
== Gyrotheodolites ==

22
data/en.wikipedia.org/wiki/Theodolite-3.md Normal file
View File

@ -0,0 +1,22 @@
---
title: "Theodolite"
chunk: 4/4
source: "https://en.wikipedia.org/wiki/Theodolite"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:39.815016+00:00"
instance: "kb-cron"
---
A gyrotheodolite is used when the north-south reference bearing of the meridian is required in the absence of astronomical star sights. This occurs mainly in the underground mining industry and in tunnel engineering. For example, where a conduit must pass under a river, a vertical shaft on each side of the river might be connected by a horizontal tunnel. A gyrotheodolite can be operated at the surface and then again at the foot of the shafts to identify the directions needed to tunnel between the base of the two shafts. Unlike an artificial horizon or inertial navigation system, a gyrotheodolite cannot be relocated while it is operating. It must be restarted again at each site.
The gyrotheodolite comprises a normal theodolite with an attachment that contains a gyrocompass, a device which senses the rotation of the Earth in order to find true north and thus, in conjunction with the direction of gravity, the plane of the meridian. The meridian is the plane that contains both the axis of the Earth's rotation and the observer. The intersection of the meridian plane with the horizontal defines the true north-south direction found in this way. Unlike magnetic compasses, gyrocompasses are able to find true north, the surface direction toward the north pole.
A gyrotheodolite will function at the equator and in both the northern and southern hemispheres. The meridian is undefined at the geographic poles. A gyrotheodolite cannot be used at the poles where the Earth's axis is precisely perpendicular to the horizontal axis of the spinner, indeed it is not normally used within about 15 degrees of the pole where the angle between the earth's rotation and the direction of gravity is too small for it to work reliably. When available, astronomical star sights are able to give the meridian bearing to better than one hundred times the accuracy of the gyrotheodolite. Where this extra precision is not required, the gyrotheodolite is able to produce a result quickly without the need for night observations.
== See also ==
Manufacturers
== References ==
== External links ==
Media related to Theodolites at Wikimedia Commons

32
data/en.wikipedia.org/wiki/Tiltmeter-0.md Normal file
View File

@ -0,0 +1,32 @@
---
title: "Tiltmeter"
chunk: 1/1
source: "https://en.wikipedia.org/wiki/Tiltmeter"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:59.000611+00:00"
instance: "kb-cron"
---
A tiltmeter is a sensitive inclinometer designed to measure very small changes from the vertical level, either on the ground or in structures. Tiltmeters are used extensively for monitoring volcanoes, the response of dams to filling, the small movements of potential landslides, the orientation and volume of hydraulic fractures, and the response of structures to various influences such as loading and foundation settlement. Tiltmeters may be purely mechanical or incorporate vibrating-wire or electrolytic sensors for electronic measurement. A sensitive instrument can detect changes of as little as one arc second.
Tiltmeters have a long, diverse history, somewhat parallel to the history of the seismometer. The very first tiltmeter was a long-length stationary pendulum. These were used in the very first large concrete dams, and are still in use today, augmented with newer technology such as laser reflectors. Although they had been used for other applications such as volcano monitoring, they have distinct disadvantages, such as their huge length and sensitivity to air currents. Even in dams, they are slowly being replaced by the modern electronic tiltmeter.
Volcano and Earth movement monitoring then used the water-tube, long baseline tiltmeter. In 1919, the physicist, Albert A. Michelson, noted that the most favorable arrangement to obtain high sensitivity and immunity from temperature perturbations is to use the equipotential surface defined by water in a buried half-filled water pipe. This was a simple arrangement of two water pots, connected by a long water-filled tube. Any change in tilt would be registered by a difference in fill-mark of one pot compared to the other. Although extensively used throughout the world for Earth-science research, they have proven to be quite difficult to operate. For example, due to their high sensitivity to temperature differentials, these always have to be read in the middle of the night.
The modern electronic tiltmeter, which is slowly replacing all other forms of tiltmeter, uses a simple bubble level principle, as used in the common carpenter level. As shown in the figure, an arrangement of electrodes senses the exact position of the bubble in the electrolytic solution, to a high degree of precision. Any small changes in the level are recorded using a standard datalogger. This arrangement is quite insensitive to temperature, and can be fully compensated, using built-in thermal electronics.
A newer technology using microelectromechanical systems (MEMS) sensors enables tilt angle measuring tasks to be performed conveniently in both single and dual axis mode. Ultra-high precision 2-axis MEMS driven digital inclinometer/ tiltmeter instruments are available for speedy angle measurement applications and surface profiling requiring very high resolution and accuracy of one arc second. The 2-axis MEMS driven inclinometers/ tiltmeters can be digitally compensated and precisely calibrated for non-linearity and operating temperature variation, resulting in higher angular accuracy and stability performance over wider angular measurement range and broader operating temperature range. Further, digital display of readings can effectively prevent parallax error as experienced when viewing traditional ‘bubble’ vials located at a distance.
The most dramatic application of tiltmeters is in the area of volcanic eruption prediction. As shown in this figure from the USGS, the main volcano in Hawaii (Kilauea) has a pattern of filling the main chamber with magma, and then discharging to a side vent. The graph shows this pattern of swelling of the main chamber (recorded by the tiltmeter), draining of that chamber, and then an eruption of the adjoining vent. Each number at the peak of tilt, on the graph, is a recorded eruption.
== Gallery ==
== See also ==
Dam safety system
Differential GPS
Geomechanics
Inclinometer
Remote sensing methods
Rock mechanics
Tilt test (geotechnical engineering)
== References ==

27
data/en.wikipedia.org/wiki/Tiny_Ionospheric_Photometer-0.md Normal file
View File

@ -0,0 +1,27 @@
---
title: "Tiny Ionospheric Photometer"
chunk: 1/1
source: "https://en.wikipedia.org/wiki/Tiny_Ionospheric_Photometer"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:40.992547+00:00"
instance: "kb-cron"
---
The tiny ionospheric photometer (TIP) is a small space-based photometer that observes the Earth's ionosphere at 135.6 nm. The TIP instruments were designed and built by the US Naval Research Laboratory (NRL) and are a part of the COSMIC program.
== Operation ==
Although each TIP instrument is fairly simple in design and operation, the value of this instrument is that six of them were launched at once, and they observe the Earth simultaneously from three orbital planes spaced equally apart around the Earth. The data of this instrument when combined with the data from the other COSMIC payloads allows a 3D tomographic analysis of the Earth's ionosphere to be performed.
== See also ==
Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC)
== References ==
== External links ==
Official Website

35
data/en.wikipedia.org/wiki/Total_station-0.md Normal file
View File

@ -0,0 +1,35 @@
---
title: "Total station"
chunk: 1/2
source: "https://en.wikipedia.org/wiki/Total_station"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:42.149349+00:00"
instance: "kb-cron"
---
A total station or total station theodolite is an electronic/optical instrument used for surveying and building construction. It is an electronic transit theodolite integrated with electronic distance measurement (EDM) to measure both vertical and horizontal angles and the slope distance from the instrument to a particular point, and an on-board computer to collect data and perform triangulation and position resection and intersection calculations.
Robotic or motorized total stations allow the operator to control the instrument from a distance via remote control. In theory, this eliminates the need for an assistant staff member, as the operator holds the retroreflector and controls the total station from the observed point. In practice, however, an assistant surveyor is often needed when the surveying is being conducted in busy areas such as on a public carriageway or construction site. This is to prevent people from disrupting the total station as they walk past, which would necessitate resetting the tripod and re-establishing a baseline. Additionally, an assistant surveyor discourages opportunistic theft, which is not uncommon due to the value of the instrument. If all else fails, most total stations have serial numbers. In the United States the National Society of Professional Surveyors hosts a registry of stolen equipment which can be checked by institutions that service surveying equipment to prevent stolen instruments from circulating. These motorized total stations can also be used in automated setups known as "automated motorized total station".
== Function ==
=== Angle measurement ===
Most total station instruments measure angles by means of electro-optical scanning of extremely precise digital bar-codes etched on rotating glass cylinders or discs within the instrument. The best quality total stations are capable of measuring angles within a standard deviation of 0.5 arc-seconds. Inexpensive "construction grade" total stations can generally measure angles within standard deviations of 5 or 10 arc-seconds.
Angle measurement is typically performed by the operator first occupying a known point, aiming the head of the instrument at a target or prism which exists at either another known point or along an azimuth, which is to be held as a backsight — sighting with the reticle inside the eyepiece — then holding that line as an angle of 00°00‘̣00“̣. The operator then will turn the head of the instrument at a target or feature that is to be observed as a foresight and record the AR (Angle Right) from the backsight measured by the instrument in which a horizontal angle is produced. Angular error in the instrument as well as collimation error can be mitigated in many total stations by performing a set collection. This entails witnessing any angles recorded an equal number of times in both "direct" and "reverse" modes by sighting the observed backsight and foresights with the instrument facing the targets normally as well as with the scope flipped or "plunged" 180°. The recorded sets of angles taken from each target will be averaged together and a mean angle will be generated.
=== Distance measurement ===
Measurement of distance is accomplished with a modulated infrared carrier signal, generated by a small solid-state emitter within the instrument's optical path, and reflected by a prism reflector or the object under survey. The modulation pattern in the returning signal is read and interpreted by the computer in the total station. The distance is determined by emitting and receiving multiple frequencies, and determining the integer number of wavelengths to the target for each frequency. Most total stations use purpose-built glass prism (surveying) reflectors for the EDM signal. A typical total station can measure distances up to 1,500 meters (4,900 ft) with an accuracy of about 1.5 millimeters (0.059 in) ± 2 parts per million.
Reflectorless total stations can measure distances to any object that is reasonably light in color, up to a few hundred meters.
=== Coordinate measurement ===
The coordinates of an unknown point relative to a point with known coordinates can be determined using the total station as long as a direct line of sight can be established between the two points. Angles and distances are measured from the total station to points under survey, and the coordinates (X, Y, and Z; or easting, northing, and elevation) of surveyed points relative to the total station position are calculated using trigonometry and triangulation.
To determine an absolute location, a total station requires line of sight observations and can be set up over a known point or with line of sight to 2 or more points with known location, called free stationing.
For this reason, some total stations also have a global navigation satellite system (GNSS) receiver and do not require a direct line of sight to determine coordinates. However, GNSS measurements may require longer occupation periods and offer relatively poor accuracy in the vertical axis.
=== Data processing ===
Some models include internal electronic data storage to record distance, horizontal angle, and vertical angle measured, while other models are equipped to write these measurements to an external data collector, such as a hand-held computer.
When data is downloaded from a total station onto a computer, application software can be used to compute results and generate a map of the surveyed area. The newest generation of total stations can also show the map on the touch-screen of the instrument immediately after measuring the points.
== Applications ==
Most large-scale excavation or mapping projects benefit greatly from the proficient use of total stations. They are mainly used by land surveyors and civil engineers, either to record features as in topographic surveying or to set out features (such as roads, houses or boundaries). They are used by police, crime scene investigators, private accident reconstructionists and insurance companies to take measurements of scenes. Total stations are also employed by archaeologists, offering millimeter accuracy difficult to achieve using other tools as well as flexibility in setup location. They prove crucial in recording artifact locations, architectural dimensions, and site topography.

58
data/en.wikipedia.org/wiki/Total_station-1.md Normal file
View File

@ -0,0 +1,58 @@
---
title: "Total station"
chunk: 2/2
source: "https://en.wikipedia.org/wiki/Total_station"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:42.149349+00:00"
instance: "kb-cron"
---
=== Mining ===
Total stations are the primary survey instrument used in mining surveying.
A total station is used to record the absolute location of the tunnel walls, ceilings (backs), and floors, as the drifts of an underground mine are driven. The recorded data are then downloaded into a CAD program and compared to the designed layout of the tunnel.
The survey party installs control stations at regular intervals. These are small steel plugs installed in pairs in holes drilled into walls or the back. For wall stations, two plugs are installed in opposite walls, forming a line perpendicular to the drift. For back stations, two plugs are installed in the back, forming a line parallel to the drift.
A set of plugs can be used to locate the total station set up in a drift or tunnel by processing measurements to the plugs by intersection and resection.
=== Mechanical and electrical construction ===
Total stations have become the highest standard for most forms of construction layout.
They are most often used in the X and Y axes to lay out the locations of penetrations out of the underground utilities into the foundation, between floors of a structure, as well as roofing penetrations.
Because more commercial and industrial construction jobs have become centered around building information modeling (BIM), the coordinates for almost every pipe, conduit, duct and hanger support are available with digital precision. The application of communicating a virtual model to a tangible construction potentially eliminates labor costs related to moving poorly measured systems, as well as time spent laying out these systems in the midst of a full-blown construction job in progress.
=== Meteorology ===
Meteorologists also use total stations to track weather balloons for determining upper-level winds. With the average ascent rate of the weather balloon known or assumed, the change in azimuth and elevation readings provided by the total station as it tracks the weather balloon over time are used to compute the wind speed and direction at different altitudes. Additionally, the total station is used to track ceiling balloons to determine the height of cloud layers. Such upper-level wind data is often used for aviation weather forecasting and rocket launches.
== Instrument manufacturers ==
Carl Zeiss (historical)
GeoMax, part of Hexagon AB
Hewlett-Packard (historical)
Hilti Corporation
Leica Geosystems, part of Hexagon AB
Nikon, part of Trimble
North Group LTD (historical)
Sokkia, part of Topcon
South Group
Spectra Geospatial, part of Trimble Navigation Ltd.
Stonex (company)
TI Asahi Co. Ltd, sold under the Pentax brand
Topcon
Trimble Navigation Ltd.
Wild Heerbrugg AG (historical), part of Leica Geosystems
== See also ==
Compass
LIDAR
Local attraction
Resection (free stationing)
Surveying
Tacheometry
Theodolite
Trigonometric leveling
== References ==
== External links ==
Using a Total Station
Total Station Manual with Laser Scanner Function
Online computation of total station measurements

38
data/en.wikipedia.org/wiki/Upside_down_goggles-0.md Normal file
View File

@ -0,0 +1,38 @@
---
title: "Upside down goggles"
chunk: 1/1
source: "https://en.wikipedia.org/wiki/Upside_down_goggles"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:43.314607+00:00"
instance: "kb-cron"
---
Upside down goggles, also known as "invertoscopes" by Russian researchers, are optical instruments that invert the image received by the retinas upside down. They are used to study human visual perception, particularly psychological process of building a visual image in the brain. Objects viewed through such a device appear upside down and mirrored. They are constructed using sets of optical right-angle prisms, concave mirrors, or a mirror plus right-angle prisms with unequal cathethus.
== Purpose ==
Upside down goggles can be used to demonstrate human adaptation to inverted vision, and as a method of preventing motion sickness. Hubert Dolezal recommended using upside down goggles for "nausea adaptation" for space travel.
They can also be used to train spatial abilities and possibly cognitive functions.
== Effect ==
Under normal circumstances, an inverted image is formed on the retina of the eye. With the help of upside down goggles, the image on the retina of the observer's eyes is turned back (straightened) and thus the space around the observer looks upside down.
== History ==
George M. Stratton designed the first upside down goggles for a psychological experiment. His device used short-focus lenses. Stratton used a one-tube, monocular device because this also reverses left and right and he wished to set up an experiment without distortion of depth perception.
In 1931 Theodor Erismann and Ivo Kohler conducted a series of experiments using mirror-prismatic upside down goggles employing only one mirror.
After experimenting since 1984, in 1991 Hubert Dolezal procured a US patent for comfortable light weight upside down goggles.
Modern upside down goggles consist of two prisms fixed onto a comfortable ski mask-like base.
== Notes ==
== References ==
== External links ==
How it looks for childhood perception

74
data/en.wikipedia.org/wiki/W._Watson_and_Son-0.md Normal file
View File

@ -0,0 +1,74 @@
---
title: "W. Watson and Son"
chunk: 1/1
source: "https://en.wikipedia.org/wiki/W._Watson_and_Son"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:44.517589+00:00"
instance: "kb-cron"
---
W. Watson and Son was an optical instrument maker. In 1837, the William Watson business was established in London for the manufacture of optical instruments. By the 1840s, the company moved into lanterns, slides and associated equipment. In 1868, the name was changed to W. Watson & Son and by this time were located at 313 High Holborn, London. In the 1870s, the company added photographic equipment and became known as a leading manufacturer of the Highest Class Photographic Instruments and Apparatus in England. Into the 1940s, the company remained at 313 High Holborn.
== W. Watson and Sons ==
On 9 January 1881 William Watson died. In 1883, the name of the company was changed to W. Watson & Sons as the son, Charles Henry Watson joined the business. The 1883 Kelly's Business Directory listed both Charles Henry Watson and Henry Watson as associated with the Watson & Sons company located at 23 Walton Street, Aylesbury, England. In 1889, the company participated in the formation of a Photographic Trades Section of the London Chamber of Commerce. In 1894, the company exhibited scientific instruments at the Antwerp Exhibition.
In the 1890s, the business continued to grow and advertised in catalogues their factories for instruments, optical glass and cabinet work located at Fullwood Rents W.C. The warehouse and show room remained at 313 High Holborn. In 1900, W. Watson & Sons purchased the John Browning & Co. In 1903, a section in the journal Knowledge lists an assortment of equipment available from the company: microscopes, astronomical telescopes (educational model for £5 1s), Crookes' spinthariscope (pocket model for £1 1s), and Electro-Therapeutics apparatus that included complete outfits for radiography from £30. The company also offered Finsen-type lamps.
== W. Watson and Sons Ltd. ==
In 1908, the firm became W. Watson & Sons Ltd. On 10 August 1938, Charles Henry Watson died. In 1912, the company employed their equipment and demonstrated the utilization of alternating current electricity to enhance the growth of plants in a nursery near London. In 1929, an advertisement in the British Industries Fair Catalogue announced an Optical, Scientific and Photographic Exhibit. The exhibition featured manufacturers of microscopes for medical, industrial, and educational purposes and for the amateur, prism binoculars, astronomical and portable telescopes, photographic lenses and cameras, surveying and measuring instruments, photometers, and scientific apparatus of every description. The W. Watson & Son company exhibited in the Scientific Section at Stand No. N.24. In 1947, the firm was a Listed Exhibitor at the British Industries Fair. The Fair featured manufacturers of microscopes for all purposes and auxiliary optical and mechanical accessories. The company offered photometers, telescopes, prism binoculars, photographic lenses of all types, and optical elements in every form. W. Watson & Son exhibited in the Olympia Room, Ground Floor at Stand No. A.1020.
The company was also engaged c1930 to produce three prototype cipher typewriters designed and patented by Morgan O'Brien for evaluation by the military.
== Medical electrology and radiology ==
In January 1905, the display of apparatus at the Annual Meeting and Exhibition of the British Electrotherapeutic Society was quite remarkable. Those in attendance were treated to items and displays of much interest, several items on display for the first time. High frequency apparatus was featured to a lesser extent than shown in previous years, but appliances for X-ray work were plentiful. Excerpt from the Exhibit handbook follows:
W. Watson & Sons (313, High Holborn, W.C.).
"showed Protective X-ray Gloves of a useful type, made of opaque flexible material, and very convenient for practical use; also an X-ray tube with a special arrangement of electrodes for improving the quality of the X-rays when operated with a coil by diverting the inverse impulses of the field of action. It is claimed that, with one of tubes, a high potential transformer can be employed direct in X-ray work without the need for any valve tube or other " rectifying " device. The anode, or second terminal, is at a distance from the antikathode, and is so placed in the tube that when the current is in the reverse direction its bombardment is directed into a depression at the back of the antikathode where it is innocuous."
== Exhibition of 1905 ==
Other companies that attended the Annual Meeting and Exhibition of the British Electrotherapeutic Society in January 1905:
Harry W. Cox, Limited at 1A, Rosebery Avenue, W.C.
A.E. Dean at 82, Hatton Garden, E.C.
The Medical Supply Association at 228, Gray's Inn Road, W.C.
Leslie Miller at 93, Hatton Garden, E.C.
Messrs. Newton & Co. at 3, Fleet Street, London
Sanitas Electrical Co. at 33 and 7A., Soho Square
Mr. Karl Friedrich Schall at 75, Cavendish Street, W.
== People linked to W. Watson and Son ==
William Watson (c.1815–1881) – partner, 1868–1881
George Watson (1857-after 1881) – partner, 1881
Thomas Watson (1855–1897) – partner, 1890s–1897
Charles Watson (1866–1938) – partner, 1930s–1938
Harold Armytage Sanders (1867–1940) – employee, c.1881–1900
Alexander Eugen Conrady (1866–1944) – scientific adviser and lens designer, c.1901–1917
Harry Arthur Crowhurst (1868–1943) – employee, 1900s–1900
G.P. Norman (fl.1890s–1900s) – employee, 1890s–1897
Jasper Redfern (1871–1928) – photographer's apprentice, and later, agent, 1885–1895
== Known locations/addresses ==
1888–1958, business address at: 313 High Holborn, London WC1, England
1890, factory at: 9, 10, 11, 16, 17 Fulwood's Rents, London, England
1890–1891, branch office at: 251 Swanston Street, Melbourne, Victoria, Australia
1897–1901, business address at: 78 Swanston Street, Melbourne, Victoria, Australia
1901, business address at: 16 Forrest Road, Edinburgh, Midlothian, Scotland
1914–1939, factory at: Bell's Hill, High Barnet, Hertfordshire, England
1914, business address at: 184 Great Portland Street, London W, England
1939–1950, factory at: 25 West End Lane, Barnet, Hertfordshire, England
1959–1969, business address at: Barnet, Hertfordshire, England
== References ==
== External links ==
W. Watson and Son camera wiki
A microscope by Watson & Son

21
data/en.wikipedia.org/wiki/Wavefront_curvature_sensor-0.md Normal file
View File

@ -0,0 +1,21 @@
---
title: "Wavefront curvature sensor"
chunk: 1/1
source: "https://en.wikipedia.org/wiki/Wavefront_curvature_sensor"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T09:43:45.713577+00:00"
instance: "kb-cron"
---
A wavefront curvature sensor is a device for measuring the aberrations of an optical wavefront. Like a Shack–Hartmann wavefront sensor it uses an array of small lenses (or lenslets) to focus the wavefront into an array of spots. Unlike the Shack-Hartmann, which measures the position of the spots, the curvature sensor measures the intensity on either side of the focal plane. If a wavefront has a phase curvature, it will alter the position of the focal spot along the axis of the beam, thus by measuring the relative intensities in two places the curvature can be deduced.
== See also ==
Adaptive optics
Wavefront sensor
== Sources ==
Roddier, François (1988). "Curvature sensing and compensation: a new concept in adaptive optics". Applied Optics. 27 (7): 1223–1225. Bibcode:1988ApOpt..27.1223R. doi:10.1364/AO.27.001223. PMID 20531543.
Roddier, François; Roddier, Claude (March 1988). Ulrich, M. H. (ed.). "Curvature Sensing and Compensation: A New Concept in Adaptive Optics". Very Large Telescopes and Their Instrumentation. European Southern Observatory Conference and Workshop Proceedings. 30 (7): 1223–5. Bibcode:1988ESOC...30..667R. doi:10.1364/AO.27.001223. PMID 20531543.

Some files were not shown because too many files have changed in this diff Show More