The problem considered is one in which a thin, nonscattering photosensitive film, either free standing or coated on a substrate, is placed in contact with a document and exposed through the photosensitive film. Energy-balance equations are written at each boundary and an expression for the energy absorbed in the film is obtained. The expression is dependent on the reflectance and absorptance of the film and the reflectance of the document. The energy-absorption equation is interpreted with the use of the characteristic curve of the photosensitive material and a figure of merit called the reflex index is defined. A means for evaluating the optical parameters and measurement of the reflex index is discussed.
GILBERT ZWEIG, "Influence of the Optical Properties of Photosensitive Films in Reflex Copying," J. Opt. Soc. Am. 59, 1659-1665 (1969)