---
title: "Spectronic 20"
chunk: 1/2
source: "https://en.wikipedia.org/wiki/Spectronic_20"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T03:42:09.986392+00:00"
instance: "kb-cron"
---

The Spectronic 20 is a brand of single-beam spectrophotometer, designed to operate in the visible spectrum across a wavelength range of 340 nm to 950 nm, with a spectral bandpass of 20 nm. It is designed for quantitative absorption measurement at single wavelengths. Because it measures the transmittance or absorption of visible light through a solution, it is sometimes referred to as a colorimeter. The name of the instrument is a trademark of the manufacturer.
Developed by Bausch & Lomb and launched in 1953, the  Spectronic 20 was the first low-cost spectrophotometer. It rapidly became an industry standard due to its low cost, durability and ease of use, and has been referred to as an "iconic lab spectrophotometer". Approximately 600,000 units were sold over its nearly 60 year production run. It has been the most widely used spectrophotometer worldwide.  Production was discontinued in 2011 when it was replaced by the Spectronic 200, but the Spectronic 20 is still in common use.  It is sometimes referred to as the "Spec 20".

== Design ==

The Bausch & Lomb Spectronic 20 colorimeter uses a diffraction grating monochromator combined with a system for the detection, amplification, and measurement of light wavelengths in the 340 nm to 950 nm range.

As shown in the schematic optical diagram (see left), polychromatic light from a source in the system passes through lenses which are reflected and dispersed by the diffraction grating to restrict the range of light wavelengths. This restricted range of wavelengths is then passed through the sample to be measured. The intensity of the transmitted light is determined by a phototube detector. Mechanical movement of the diffraction grating by means of the cam attached to the wavelength control enables the user to select for various wavelengths. This is the "λ knob", wherein λ refers to wavelength of light used for the measurement.

== Quantitative measurements ==

Many substances absorb light in the ultraviolet - visible light range. Absorption at any particular wavelength in the ultraviolet visible range is proportional to the concentration of the substances in the solution or other medium, in accord with the Beer–Lambert relationship. In a practical sense, the Beer–Lambert relationship can be stated as:

       A = ε x l x c

in which A is the absorbance measured by the instrument, ε is the molar absorption coefficient of the sample, l is the pathlength of the light beam through the sample, and c is the concentration of the substance in the solution or medium. The Spectronic 20 is thereby commonly used for quantitative determination of the concentration of a substance of interest. The Spectronic 20 measures the absorbance of light at a pre-determined concentration, and the concentration is calculated from the Beer–Lambert relationship.
The absorbance of the light is the base 10 logarithm of the ratio of the Transmittance of the pure solvent to the transmittance of the sample, and so the two absorbance and transmittance can be interconverted. Either transmittance or absorbance can therefore be plotted versus concentration using measurements from the Spectronic 20.  Plotting a curve using percent transmittance of light yields an exponential curve. However, absorbance is linearly related to concentration, and so absorbance is often preferred for plotting a standard curve. This type of standard curve relates the concentration of the solution (on the x-axis) to measures of its absorbance (y-axis).
To obtain such a curve, a series of dilutions of known concentration of a solution are prepared and readings are obtained for each of the dilutions (see plot at left).  In this plot, the slope of the line is the product ε x l. By measuring a series of standards and creating the standard curve, it is possible to quantify the amount or concentration of a substance within a sample by determining the absorbance on the Spec 20 and finding the corresponding concentration on the calibration curve. Alternatively, the logarithm of percent transmittance can be plotted versus concentration to create a standard curve using the same procedure.
The absorbance measured by the Spectronic 20 is the sum of the absorbance of each of the constituents of the solution. Therefore, the Spectronic 20 can be used to analyze more complex solutions. For example, if a sample solution has two light-absorbing compounds in it, then the user performs measurements at two different wavelengths and constructs standard curves for each compound. Then the concentration of each compound can be calculated algebraically.
The Spectronic 20 can be used for turbidimetric measurements. In microbiological work, the turbidity of a liquid culture of bacterial cells relates to the cell count, and OD600 measurements can be conducted for this purpose using the Spectronic 20. Likewise the turbidity of water suspensions of clays and other particles of size suitable for light scattering can be quantitatively determined by means of a Spectronic 20. In the past, the Spectronic 20 was used for clinical diagnostic purposes.