Thickness contours of highly aberrant phase objects. II. Double-exposure grating interferometer for direct phase measurement at reduced sensitivity

Abstract

An analysis of a double-exposure grating interferometer for the study of transparent phase samples is presented. This shows that, if the test sample is placed behind the grating in contact with it, the resulting interferogram gives directly its absolute thickness contours as they would be obtained by a desensitized two beam interferometer. The presence of a finite distance between the grating and the test sample introduces an additional path difference that corresponds to shearing the object wave in the direction perpendicular to grating lines. The conditions under which the two effects could be separated have been examined. The low sensitivity inherent in the double-exposure grating interferometer makes it useful for measuring the absolute phase variations of highly aberrant samples. However, restoration of high sensitivity in order to visualize low phase variations is also possible. The limitations imposed on the nature of the test samples have been pointed out.