---
title: "DU spectrophotometer"
chunk: 3/5
source: "https://en.wikipedia.org/wiki/DU_spectrophotometer"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T03:41:22.285535+00:00"
instance: "kb-cron"
---

When RCA could not meet Beckman's demand for experimental phototubes, National Technical Laboratories again had to design its own components in-house. They developed a pair of phototubes, sensitive to the red and blue areas of the spectrum, capable of amplifying the signals they received. With the incorporation of Beckman's UV-sensitive phototubes, the Model D became the Model DU UV–Vis spectrophotometer. Its designation as a "UV–Vis" spectrophotometer indicates its ability to measure light in both the visible and ultraviolet spectra.
The DU was the first commercially viable scientific instrument for measuring the amount of ultraviolet light absorbed by a substance.  As he had done with the pH meter, Beckman had replaced an array of complicated equipment with a single, easy-to-use instrument. One of the first fully integrated instruments or "black boxes" used in modern chemical laboratories, it sold for $723 in 1941.
It is generally assumed that the "DU" in the name was a combination of "D" for the Model D on which it was based, and "U" for the ultraviolet spectrum. However, it has been suggested that "DU" may also reference Beckman's fraternity at the University of Illinois, Delta Upsilon, whose members were called "DU"s.
A publication in the scholarly literature compared the optical quality of the DU to the Cary 14 Spectrophotometer, another leading UV–Vis spectrophotometer of the time.

== Design ==

From 1941 until 1976, when it was discontinued, the Model DU spectrophotometer was built upon what was essentially the same design. It was a single beam instrument.
The DU spectrophotometers used a quartz prism to separate light from a lamp into its absorption spectrum and a phototube to electrically measure the light energy across the spectrum. This allowed the user to plot the light absorption spectrum of a substance to obtain a standardized "fingerprint" characteristic of a compound.  All modern UV–Vis spectrophotometer are built on the same basic principles as the DU spectrophotometer.

"Light from the tungsten lamp is focused by the condensing mirror and directed in a beam to the diagonal slit entrance mirror. The entrance mirror deflects the light through the entrance slit and into the monochromator to the collimating mirror. Light falling on the collimating mirror is rendered parallel and reflected to the quartz prism where it undergoes refraction. The back surface of the prism is aluminized so that light refracted at the first surface is reflected back through the prism, undergoing further refraction as it emerges from the prism. The desired wavelength of light is selected by rotating the Wavelength Selector which adjusts the position of the prism. The spectrum is directed back to the collimating mirror which centers the chosen wavelength on the exit slit and sample. Light passing through the sample strikes the phototube, causing a current gain. The current gain is amplified and registered on the null meter." Model DU Optical System 
Although the default light source for the instrument was tungsten, a hydrogen or mercury lamp could be substituted depending on the optimal range of measurement for which the instrument was to be used.  The tungsten lamp was suitable for transmittance of wavelengths between 320 and 1000 millimicrons; the hydrogen lamp for 220 to 320 millimicrons, and the mercury lamp for checking the calibration of the spectrophotometer.

As advertised in the 1941 News Edition of the American Chemical Society, the Beckman Spectrophotometer used an autocollimating quartz crystal prism for a monochromator, capable of covering a range from the ultraviolet (200 millimicrons) to the infrared (2000 millimicrons), with a nominal bandwidth of 2 millimicrons or less for most of its spectral range. The slit mechanism was continuously adjustable from .01 to 2.0 mm and claimed to have less than 1/10% of stray light over most of the spectral range. It featured an easy-to-read wavelength scale, simultaneously reporting % Transmission and Density information.
The sample holder held up to 4 cells. Cells could be moved into the light path via an external control, allowing the user to take multiple readings without opening the cell compartment. As described in the DU's manual, absorbance measurements of a sample were made in comparison to a blank, or standard, "a solution identical in composition with the sample except that the absorbing material being measured is absent." The standard could be a cell filled with a solvent such as distilled water or a prepared solvent of a known concentration. At each wavelength two measurements are made: with the sample and with the standard in the light beam. This enables the ratio, transmittance, to be obtained. For quantitative measurements transmittance is converted to absorbance which is proportional to the solute concentration according to Beer's law. This makes possible the quantitative determination of the amount of a substance in solution.
The user could also switch between phototubes without removing the sample holder. A 1941 advertisement indicates that three types of phototubes were available, with maximum sensitivity to red, blue and ultraviolet light ranges.
The 1954 DU spectrophotometer differs in that it claims to be useful from 200 to 1000 millimicrons, and does not mention the ultraviolet phototube. The wavelength selector, however, still ranged from 200 to 2000 millimicrons. and an "Ultraviolet accessory set" was available. This shift away from using the DU for infrared measurement is understandable, since by 1954 Beckman Instruments was marketing a separate infrared spectrophotometer. Beckman developed the IR-1 infrared spectrophotometer during World War II, and redesigned it as the IR-4 between 1953 and 1956.

== Use ==