Shape measurement by source displacement in three-dimensional shearography

ABSTRACTShearography is a full field non-contact optical technique generally used for measurement of the derivative of thedisplacement of the surface on an object subjected to mechanical or thermal loading. This paper describes the use of shearography for surface slope and shape measurement. Correlation of interferometric speckle patterns obtained before andafter displacement of the optical source generates correlation fringes which are, in general, a mixture of slope and carrierrelated fringes. Carrier fringes are generated when the source is displaced along the source-object optical axis and slopefringes are generated by movement orthogonal to the source-object optical axis. The sensitivity of the slope fringes to theobject slope is determined by the illumination and imaging geometry, the optical wavelength, the applied shear and themagnitude of the source displacement. The slope fringes are distorted by the necessary off-axis illumination, so a correctionis made by subtracting the slope fringes generated on a flat plate. Mathematically modelled and experimentally generatedphase-stepped slope fringes are unwrapped and integrated to recover the object shape.Keywords: slope/shape measurement, source movement, shearography, three-dimensional