Unified approach for holography and shearography in surface deformation measurement and nondestructive testing

Holography and shearography are two useful whole-field non-contacting optical tools for nondestructive flaw detection and precision measurements. Holography serves as a displacement transducer since it gives direct measurements on displacements whereas shearography serves as a strain gage since it gives direct measurements on displacement gradients. This paper views holography and shearography and their variations as a single optical technique having the same basic mathematical formulation and instrumentation. A key optical component used in both techniques is a doubly-refractive prism that combines two angularly separated laser rays to interfere at near collinearity, thereby permitting the use of a low-resolution CCD camera for recording the interference pattern. Shearography uses a doubly-refractive prism with small image shearing so that two neighboring points on the test surface are brought to interfere at the image plane of the camera, whereas holography, on the other hand, uses a doubly-refractive prism with large image shearing so that light scattered from two different objects-a test object and a reference surface (serving as a reference beam)-are brought to interfere at the image plane of the camera. Hence, testing and measurements made using holography may also be made using shearography, and vice versa. (C) 2003 Society of Photo-Optical Instrumentation Engineers.