Self-comparative holography and shearography

Abstract

Double-exposure holography and double-exposure shearography are two interferometric techniques that are particularly useful for precision measurement, and for non-destructive inspection and evaluation of the structural integrity of test objects. Quite often, objects with symmetrical deformation are tested and the challenge to the engineer is to check for symmetry in the deformation. While the holographic and shearographic fringe patterns are able to achieve this, it is felt that the inspection process can be simplified and expedite considerably if the engineer needs only to look for either the absence or the presence of fringes. In other words, a "go, no-go" gauging method is desired so that the absence of interference fringes depicts symmetrical deformation and the presence of interference fringes depicts asymmetrical deformation. This paper demonstrates that this gauging method can be realized with the use of a commercial beam-splitting cube. By carefully orientating the beam-splitting cube, the image of the test object is brought to overlap, either fully or partially, with the mirror-image of the test object. With fully overlapping images, the relative displacement between two symmetrically located points on the test surface is obtained, resulting in the technique of self-comparative holography. This method is found useful in checking the symmetrically of deformation. With partially overlapping images, the relative displacement-derivative between two symmetrically located points on the test surface is obtained, resulting in the technique of self-comparative shearography.