The 1940 Applied Research Laboratories’ projection comparator-densitometer, using spectrum plates either 2″ × 10″ or 4″ × 10″, provides dual projection of a sample spectrogram and a master onto a translucent screen mounted directly before the operator. Using a slit system that moves into contact with the plate; a vacuum photo-cell, a two-stage compensated amplifier, and sturdy ammeter, the comparator-densitometer provides transmission readings on any spectral line, accurate to a few tenths of a percent. The unit is fast in operation allowing the operator to read easily as many as six lines per minute. Operating from 115-v, 60-cycle power supply, it achieves this accuracy with the minimum amount of care and upkeep.
HARRY W. DIETERT and JOHN SCHUCH, "A New Projection Comparator-Densitometer," J. Opt. Soc. Am. 31, 54-57 (1941)