An Instrument for Recording the Optical–Path Variation of Transmitting Objects

Abstract

An instrument is described for recording the variations in optical path length, particularly for small periodic line structures, in thin transparent objects. It 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. 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 in much 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. The fringe contour accurately describes the variation of optical path despite variations of density. Samples having spatial frequencies as high as several hundred lines per millimeter or more can be measured, depending upon the microscope optics and the nature of the sample. The lower spatial frequency limit depends upon the number of lines of the sample that are contained within the illuminated field of view of the microscope. Although focusing of the microscope was found to be quite critical, for most cases of interest sufficient focusing accuracy can be attained by using an auxiliary white light source. The instrument is convenient to operate and provides an accuracy that is considerably better than can be obtained with visual interference microscopes. With minor changes it can be adapted for operation as a conventional microdensitometer.