A Coparator Microphotometer of new design is described. An optical system has been developed in which all of the available light flux is used for both viewing the spectrum and measuring the transmission of selected spectral lines. Besides high viewing screen intensity and great sensitivity, a unique electronic system is employed. The advantages of this null-ratio system include: reduced drift and noise, independence of individual vacuum tube characteristics, and freedom from the logarithmic damping responce characteristic of galvanometers and meters.
HAROLD B. BEAL, RICHARD K. BREHM, and RICHARD F. JARRELL, "Console Comparator Microphotometer Featuring High Screen Intensity and a Null-Ratio Measurement System," J. Opt. Soc. Am. 47, 306-306 (1957)