A description is given of the application of the electron-multiplier photo-tube and the light-chopping method to the design of a densitometer. The techniques discussed are generally applicable to the measurement of very low light intensities.
R. O’B. CARPENTER, "A New Light Balancing Circuit for the Non-Recording Densitometer," J. Opt. Soc. Am. 36, 676-678 (1946)