High Resolution Phase-stepping Shearography by a Using 24 Megapixel Digital Still Imaging Device

Digital phase-stepping shearography is a modern precision measurement technique for quantifying microscopic displacement gradients and strains of an object surface by interferometric speckle techniques. The phase map of the displacement derivatives of a stressed object is generated using laser speckles in this technique. As a result, the strains of a deformed object can be directly mapped. Phase-stepping shearography is also very useful in industrial non-destructive testing (NDT). In conventional digital phase stepping shearography, a video camera of limited resolution is used for imaging the laser speckles. The maximum resolution of the video camera is only of the order of 5 Megapixels. This limits the spatial resolution for the generated shearograms and phase maps, and consequently, limits the maximum value of the deformations that can be successfully observed in a given situation. We improved the shearography technique and, in particular, performed advanced shearographic experiments with substantially higher spatial resolution than is now achievable. A 24 megapixel still digital image device (DSLR camera) and a Michelson-type shearing setup with an edge-clamped, center-loaded plate are used in this novel technique. Different phase-stepping algorithms were tested, and all of them produced shearograms satisfactory quality. This effectively increases the useful spatial resolution of phase-stepping shearography by roughly 5 times compared to the conventional method using video-rate cameras, and will also improve spatial resolution in many possible applications.