The photoelectric photodensitometer described was developed for determinging fine structure from “match-point” spectrum photographs. It consists of a push-pull photo-cell circuit, transfomercoupled to an amplifier and cathode-ray oscillograph, and two optical systems with a.c. operated light sources for projecting the spectra onto the photo-cell slit and for forming an image of the wave-length scale which indicates the region of the spectrum being method. As the projected spectra are moved horizontally across the slit, a change in phase occurs in the a.c. output of the photo-cell circuit on passing through a match point. The corresponding change in oscullograph pattern indicates whether the absorption is increasing or decreasing, and permits true match points to be distinguished from decreases in intensity due to dense spectrum lines in matching line spectra.
JOHN R. LOOFBOUROW, "A Push-Pull Photoelectric Photodensitometer for Determining Fine Structure in Ultraviolet Absorption Spectra," J. Opt. Soc. Am. 29, 535-537 (1939)