Optical-Path Variation in a Photographic Emulsion

Abstract

The variation of optical path length through the developed photographic image has been determined for Kodak Panatomic-X film by exposing with a spatial-frequency series of sinusoidal line patterns and scanning the developed images to determine the peak-to-peak variation of density and of optical path for each pattern. For the conditions studied, the ratio of the optical-path variation to the density difference in each pattern depended upon the average density of the pattern. A similar study was made to examine the relationship between the variation of optical path and the variation of silver per unit area on the film. Because the ratio of these showed little, if any, dependence upon the average density, it was concluded that a fairly linear relationship existed between optical path and the mass of silver in an area of the emulsion. Application of linear-systems theory proved to be successful in that the path variation associated with an isolated line exposure could be calculated from the sine-wave data by means of convolution and Fourier-transform techniques. By immersing the film sample in a liquid of known refractive index, it was possible to determine both the variation in optical path resulting from a variation in refractive index and that from the variations in thickness that constitute the relief image.