These instructions for phototube replacement describe some of the inner workings of the first model D/DU spectrophotometer made by National Technical Laboratories in 1945. The phototube being replaced in these instructions was produced by NTL after RCA couldn't keep up with the demand.

The first Beckman Instruments DU Spectrophotometer was developed in 1940, when the company was still called National Technical Laboratories. Spurred by employee Howard Cary (who had previously been involved in pH meter development), NTL produced its first DU Spectrophotometer in 1940. This first Model A contained a glass prism, while subsequent models used quartz crystal. The DU was initially developed under a U.S. government contract, but was ultimately used in many important scientific discoveries and developments of the 20th century, including penicillin and synthetic rubber research, petroleum refining, and the creation of explosives.