Holographic interferograms are three-dimensional representations of a four-dimensional space. The fourth dimension is visualized by interference surfaces in space that inter-sect the three-dimensional image of the object and represents, e.g., loci of equal displacement. In sandwich hologram interferometry these interference surfaces can be titled by an analogous tilt of the sandwich hologram. Using this method of fringe manipulation makes possible the elimination of the influence of unwanted motions, the evaluation of the sign of object tilt, the direct study of the derivative of the fringe frequency, and the sutdy of both larger and smaller motions than possible using e.g., double-exposure holography. The tilt of the sandwich hologram causes a moire effect between the two sets of interference fringes that are recorded on the surfaces of the two hologram plates. The result is another moire effect in the image plane producing changes in the interference fringe pattern seen on the object. Thus similar results can be reached by producing a moire effect either in the Fourier plane or directly in the image plane.
Nils Abramson, Nils Abramson,
"Holography As Measuring Tool In Science And Industry", Proc. SPIE 0349, Industrial Applications of Holographic Nondestructive Testing, (25 October 1982); doi: 10.1117/12.933874; https://doi.org/10.1117/12.933874