Improved phase-shifted digital speckle shearography for time-dependent deformation measurement

Some enhancement measures are studied for improving phase-shifted digital speckle shearography toward time-dependent application, such as the evaluation of materials' time-dependent mechanical behavior. To ensure quick and accurate phase shifting, a liquid-crystal variable retarder and a Wollaston prism are integrated into a compact unit, and an in situ phase-shift calibration procedure is developed. Algorithms are studied, and the most appropriate one is implemented, in which four phase-shifted reference shearograms and one shearogram per deformation state are processed to obtain deformation-related phase changes. Sine-cosine-averaging phase filtering is combined with a multigrid unweighted least-squares phase-unwrapping algorithm to ensure correct real phase extraction. Theories backing the enhancements are illustrated in detail, and verification tests are presented to demonstrate the effectiveness of the suggested improvement measures. The study shows good potential for phase-shifted digital speckle shearography to be applied to time-dependent measurement of surface slope and out-of-plane displacement. (C) 2008 Society of Photo-Optical Instrumentation Engineers.