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The theory and use of the integrating sphere in three methods of reflectometry, as proposed by Sharp and Little, Karrer, and Taylor, respectively, are discussed in connection with a new absolute method in reflectometry involving no direct use of an integrating device. The new method is based upon a general law of reciprocity, first stated by Helmholtz, by means of which certain reciprocal relations between the reflective properties for unidirectional and diffused illumination are derived and applied in the method. Under completely diffused illumination the brightness of a sample, in general, increases with increasing angle of observation from the normal to its surface. The illumination on the sample being known, these brightness data are shown to yield (with the aid of the reciprocity law) the reflectance of the sample (ratio of total reflected to total incident light) for any manner of illumination ranging from unidirectional to completely diffused. Equipment is described for the measurement of reflective properties of materials under either completely diffused or unidirectional illumination for various directions of observation. The samples used include various kinds of materials chosen to cover a wide range of reflectance and to represent various degrees of departure from the perfect diffusor. Comparative measurements by all the methods studied are made on these samples.
H. J. McNicholas (Sun,) studied this question.