Shape measurement by source displacement in three-dimensional shearography

摘要

Shearography is a full fidld non-contact optical technique generally used for measurement of the derivative of the displacement 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 and after displacement of the optical source generates correlation fringes which are, in general, a mixture of slope and carrier related fringes. Carrier fringes are generated when the source is displaced along the source-object optical axis and slope fringes are generated by movement orthogonal to the source-object optical axis. The senstitivity of the slope fringes to the object slope is determined by the illumination and imaging geometry, the optical wavelength, the applied shear and the magnitude of the source displacement. The slope frinmges are distorted by the necessary off-axis illumination, so a correction is made by subtracting the slope fringes generated on a flat plate. Mathematically modelled and experimentally generated phase-stepped slope fringes are unwrapped and integrated to recover the object shape.