Linear Densitometer Circuit
P. Hariharan, M S Bhalla · Review of Scientific Instruments · 1956
A densitometer circuit is described which uses two photomultiplier tubes operated at constant anode current as a balanced range compressor, followed by a logarithmic difference amplifier. A density scale linear to ±0.01 density unit over a density range of 2.5 is obtained. The circuit is stable, requires no critical adjustments, and is unaffected by changes in the luminous output of the source.