Differential holographic interferometry

Abstract

To compare the change in surface contour of a test object subjected to a stress with the changes that occur in a similar master object, subjected to the same stress, first and second holograms are formed of the surface of the master object before and after stressing employing convergent reference waves. A third double-exposure hologram is then formed wherein the first exposure is made of the test object illuminated using the first hologram reconstructed using a wave conjugate to the reference wave employed in its formation. Similarly, the second exposure is made of the test object stressed in the same manner as the master and illuminated using the second hologram reconstructed with a conjugate wave. The image of the test object as reconstructed from the third hologram will include fringes representative of the difference in strain between the model and test objects as a result of the stressing. Alternatively, the difference between surface contours of the model and test object may be analyzed by successively forming first and second holograms of the model while it is illuminated with light of first and second different wavelengths, respectively, and then using the two holograms successively, as sources of object illuminating waves during the formation of a double exposure hologram of the test object while illuminated using the two different wavelengths, successively. The image of the second object reconstructed from the double exposure hologram will contain fringes arrayed as a function of the difference in shape of the master and test objects.