Described is a technique for optically subtracting the phase terms in holographic interferometry to yield fringe patterns showing the differences between a master object and an object under test. The technique may be applied to the strain fringes produced in holographic nondestructive testing or to contour fringes used to measure shapes of parts. In either case, the fringe spatial frequency is greatly reduced by eliminating unwanted phase terms. The test stress may then be greatly increased, yielding increased detection sensitivity, or the contour interval may be greatly reduced, providing more accurate measurement.
Don B. Neumann,
"Comparative Holography: A Technique For Eliminating Background Fringes In Holographic Interferometry," Optical Engineering 24(4), 244625 (1 August 1985). https://doi.org/10.1117/12.7973540