---
title: "DU spectrophotometer"
chunk: 2/5
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"
---

=== Model A (prototype) ===
The first prototype Beckman spectrophotometer, the Model A, was created at National Technologies Laboratories in 1940. It used a tungsten light source with a glass Fery prism as a monochromator. Tungsten was used for incandescent light filaments because it was strong, withstood heat, and emitted a steady light. Types of light sources differed in the range of wavelengths of light that they emitted. Tungsten lamps were useful in the visible light range but gave poor coverage in the ultraviolet range. However, they had the advantage of being readily available because they were used as automobile headlamps. An external amplifier from the Beckman pH meter and a vacuum tube photocell were used to detect wavelengths.

=== Model B (prototype) ===

It was quickly realized that a glass dispersive prism was not suitable for use in the ultraviolet spectrum. Glass absorbed electromagnetic radiation below 400 millimicrons rather than dispersing it. In the Model B, a quartz prism was substituted for the earlier glass.
A tangent bar mechanism was used to adjust the monochromator. The mechanism was highly sensitive and required a skilled operator. Only two Model B prototypes were made. One was sold: in February 1941, to the University of California Chemistry department in Los Angeles.
The Model B prototype should be distinguished from a later production model of spectrophotometer that was also referred to as the Model "B". The production Model "B" was introduced in 1949 as a less-expensive, simple-to-use alternative to the Beckman DU. It used a glass Fery prism as a chromator and operated in a narrower range, roughly from 320 millimicrons to 950 millimicrons, and 5 to 20 Å.

=== Model C (prototype) ===
Three Model C instruments were then built, improving the instrument's wavelength resolution. The Model B's rotary cell compartment was replaced with a linear sample chamber. The tangent bar mechanism was replaced by a scroll drive mechanism, which could be more precisely controlled to reset the quartz prism and select the desired wavelength. With this new mechanism, results could be more easily and reliably obtained, without requiring a highly skilled operator. This set the pattern for all of Beckman's later quartz prism instruments. Although only three Model B prototypes were built, all were sold, one to Caltech and the other two to companies in the food industry.

=== Model D (limited production) ===

The A, B, and C prototype models all coupled an external Beckman pH meter to the optical component to obtain readouts. In developing the Model D, Beckman took the direct-coupled amplifier circuit from the pH meter and combined the optical and electronic components in a single housing, making it more economical.
Moving from a prototype to production of the Model D involved challenges. 
Beckman originally approached Bausch and Lomb about making quartz prisms for the spectrophotometer. When they turned down the opportunity, National Technical Laboratories designed its own optical system, including both a control mechanism and a quartz prism. Large, high optical quality quartz suitable for creating prisms was difficult to obtain. It came from Brazil, and was in demand for wartime radio oscillators. Beckman had to obtain a wartime priority listing for the spectrophotometer to get access to suitable quartz supplies.
Beckman had previously attempted to find a source of reliable hydrogen lamps, seeking better sensitivity to wavelengths in the ultraviolet range than was possible with tungsten. As described in July 1941, the Beckman spectrophotometer could use a "hot cathode hydrogen discharge tube" or a tungsten light source interchangeably. However, Beckman was still unsatisfied with the available hydrogen lamps. National Technical Laboratories designed its own hydrogen lamp, an anode enclosed in a thin blown-glass window. By December 1941, the in-house design was being used in production of the Model D.
The instrument's design also required a more sensitive phototube than was commercially available at that time. Beckman was able to obtain small batches of an experimental phototube from RCA for the first Model D instruments.
The Model D spectrophotometer, using the experimental RCA phototube, was shown at MIT's Summer Conference on Spectroscopy in July 1941. The paper that Cary and Beckman presented there was published in the Journal of the Optical Society of America. In it, Cary and Beckman compared designs for a modified self-collimating quartz Fery prism, a mirror-collimated quartz Littrow prism, and various gratings. The Littrow prism was a half-prism, which had a mirrored back face, so that the light went through the front face twice. Use of a tungsten light source with the quartz Littrow prism as a monochromator was reported to minimize light scattering within the instrument.
The Model D was the first model to enter actual production. A small number of Model D instruments were sold, beginning in July 1941, before it was superseded by the DU.

=== Model DU ===