Leading into World War II, American scientists scrambled for a way to determine what vitamins, particularly vitamin A, were in food in order to keep US soldiers well-nourished. But state-of-the-art ultraviolet and visible (UV-vis) spectrophotometry, which measures electronic transitions of a wide range of molecules as they absorb light, was cumbersome and expensive.
In July 1941, Arnold Beckman, founder of his eponymous company, introduced his DU UV-vis spectrophotometer. It was the production version of the Model D prototype that he and Howard Cary had first built. It featured a molecular hydrogen lamp, a monochromator made of a Brazilian quartz prism, and a UV-sensitive phototube. Light from the lamp passed through a series of slits and mirrors and separated into the complete visible and UV spectrum at the prism. Once through the sample, the light collected in a phototube for measurement.
The instrument allowed food analysis, but also, of course, much more. In 1947, Edwin Chargaff used his Beckman DU to determine that the ratio of nucleotide monomers in DNA was the same in any given organism, the basis for the discovery of complementary base pairing in DNA. Current Uv-vis spectrophotometers are far more advanced in function and design than their precursors, yet in some form they retain most of the features found in the first advertised model.