page_title: DU spectrophotometer
section_title: Design
hierarchical_section_title: DU spectrophotometer / Design
caption_attribution_description: Figure 2. Diagram of Model DU Optical System from Beckman Instruments Instruction Manual / The Beckman Model DU Spectrophotometer and Accessories Beckman Division, Beckman Instruments, Inc. Fullerton, CA, [1954]. Back cover illustration states March, 1954 No copyright statement. No copyright renewals. Full caption to Figure 2, page 3, reads as follows: “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. Figure 2. Diagram of Model DU Optical System”
context_page_description: The DU spectrophotometer or Beckman DU, introduced in 1941, was the first commercially viable scientific instrument for measuring the amount of ultraviolet light absorbed by a substance. This model of spectrophotometer enabled scientists to easily examine and identify a given substance based on its absorption spectrum, the pattern of light absorbed at different wavelengths. Arnold O. Beckman's National Technical Laboratories developed three in-house prototype models and one limited distribution model before moving to full commercial production with the DU. Approximately 30,000 DU spectrophotometers were manufactured and sold between 1941 and 1976.
  Sometimes referred to as a UV–Vis spectrophotometer because it measured both the ultraviolet and visible spectra, the DU spectrophotometer is credited as being a truly revolutionary technology. It yielded more accurate results than previous methods for determining the chemical composition of a complex substance, and substantially reduced the time needed for an accurate analysis from weeks or hours to minutes.
context_section_description: 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 infra
caption: Diagram of Model DU optical system, Beckman manual, 1954
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