A Modulated-Beam Microdensitometer

The employment of barrier layer cells in microdensitometers designed for use in spectrochemical analysis has found considerable favor, as indicated by the fact that this type of photosensitive element has been incorporated into three commercially available instruments: the Vincent and Sawyer (1) microphotometer manufactured by the Leeds and Northrup Company, and the Jarrel-Ash and Hilger instruments. Sensitive galvanometers are employed in all three of these densitometers. There seems to be no case in which such an instrument has been equipped with an amplifier in order to improve the precision of the higher density readings, presumably because the low internal resistance of the barrier layer cell is not conducive to amplification. However, development in recent years of the General Motors breaker-type amplifier (2), designed to match low input impedances, has simplified the problem of amplifying such photocurrents. The present paper is concerned with the use of such an amplifier on a microdensitometer. In the course of this investigation the principle of modulated-beam operation was ultimately adopted