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.
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Hariharan et al. (1956) studied this question.
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