A photoelectric densitometer

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Abstract
An a.c. operated self-balancing photoelectric densitometer is descr. It measures photographic density over a range of 0-3, and has reproducibility and sensitivity of better than 0.01 over the entire range. A pair of emissive photoelectric cells are employed, and the measurement is made by a wedge substitution method. Special features are the employment of a high-speed telegraphic relay for the conversion of the photoelectric currents into 50 c/s a.c, and the use of a two-phase servomotor for self-balancing. The system gives high sensitivity, freedom from zero drift, while the simplicity of operation and rigid construction made this densitometer suitable for factory use.

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