A Photoelectric Comparator for Precise and Rapid Measurement of Reflection and Transmission

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Abstract
An apparatus is described, designed especially for the determination of reflection and transmission factors either in white light or in the various regions of the spectrum. A beam of light emerging from a lamp or from the slit of a monochromator falls at 45 degrees incidence upon a rotating glass disk, one-half of the surface of which is silvered. This splits the beam into two beams passing in alternation over two separate paths and through variable sectors by which their intensity may be decreased in a measurable manner. The sample is placed in one beam. The beams are reunited on the surface of a photoelectric cell, the current of which is amplified. In the output circuit is a vibration galvanometer tuned to the frequency of the two light beams, which is used as a null instrument. A strict substitution method is used in making the measurements. The spectral range which can be covered is determined by the character of the source of radiation by properties of the optical train and by the response characteristics of the particular photoelectric cell employed. Different cells can be used for different regions. In use, the apparatus has shown itself to be satisfactory from the point of view of high accuracy, rapidity, and simplicity of operation.

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