A photometer for measuring small changes in optical absorption

Abstract
A simple and inexpensive double beam photometer is described. The photometer can detect changes in optical absorption as small as one part in 105. For ease of mechanical and optical construction, a DC system is used, and the unusual features of the design are aimed at reducing noise to a satisfactory level. These features include not using the electron multiplication stages of the photodetectors, and locking the light intensity via a feedback loop from the reference detector.

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