The optics of photometric measurements

The influence of the apertures which limit the light transmitted from the source to the photometer, and of the reflection and transmission coefficients of the specimen and apparatus used, including the source and the photometer themselves, on the quantities determinable from a photometric balance are considered. The way in which the apparatus is set up must take into account the precise information to be obtained. A general theory is given, which leads to the conclusion that in precise photometry the measurement of the light reflected in both directions, in addition to that transmitted, is necessary when the properties of materials are being determined. A suitable experimental arrangement for making the measurements is described. The method of calculating these quantities from the observations is given, and numerical examples are included.