Measurement of complex amplitude transmittance

Thesis (Ph.D.)--University of Rochester. College of Engineering and Applied Science. Institute of Optics, 1969.The complex transmittance of a transparent object can be determined through measurements of both the light flux transmittance and of the optical path through the object. For small detail structure, the transmittance can be determined through the use of a microdensitometer, but some type of interference microscope is required to measure the optical path. In this thesis an instrument is described which serves to record the variations of optical path, particularly for small periodic line structures. This is especially useful for studying relief images on photographic films and plates. The instrument contains an interference microscope in which the interference takes place between a beam transmitted by the sample, and a reference beam, which is generated by a Ronchi grating, passed through the sample, and subsequently spatially filtered. Except for the spatial filter, the microscope uses "off-the'shelf" components throughout. A helium-neon laser is used as a light source. The instrument also includes a system for scanning the interference fringes and recording the fringe shape, much in the same manner that a microdensitometer records density. Because the system scans as many as forty fringes simultaneously, the averaging that is necessary for measuring the mean fringe contours of relatively coarse-grained photographic films is automatically provided. An analysis of the system shows that the fringe contour accurately describes the variation of optical path even when variations in density are present on the film. The limit of spatial frequencies that can be measured is at least several hundred lines per mm, depending upon the microscope optics and the nature of the sample itself. The lower spatial frequency limit was found to depend upon the number of lines of the sample that are contained within the illuminated field of view of the microscope. Some problems were encountered in that focusing of the microscope on the sample was found to be quite critical. However, an analysis showed that for most cases that would be of interest, sufficient focusing accuracy could be attained by using an auxillary white light source. In general, the instrument has been found to be conveneint to operate and to provide an accuracy that is considerably better than can be obtained with visual interference microscopes. With relatively minor changes the instrument can be adapted to operate as a conventional microdensitometer. A series of measurements was made of the relief images obtained when Panatomic-X Film was exposed with sinusoidal line grating patterns. The developed images were scanned to determine the peak-to-peak variations in density and optical path. For the conditions studied, the path variation was found to depend linearly upon the density difference in the pattern, but for a constant density difference it depended upon the average density. Through the use of a curve relating density to the mass of silver per unit area of the developed film, it was possible to examine the relationship between the variation of optical path and the variation of the mass of silver. This appeared to provide a fairly linear relationship in that the ratio of the two showed little, if any, dependency upon the average density of the pattern. Because of this linear relationship, an attempt was made to apply linear systems theory. This proved to be successful in that the path variation associated with an edge exposure could be calculated 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 of refractive index within the photographic emulsion and the thickness variation constituting the relief image. Data are presented for Panatomic-X Film and for Kodak Spectroscopic Plates, Type 649F that have been bleached by various processes