Digital phase-stepping shearography is a speckle interferometric technique that uses laser speckles to generate the phase map of the displacement derivatives of a stressed object, and hence can map the stresses of a deformed object directly. Conventional digital phase-stepping shearography relies on the use of video cameras of relatively lower resolution, in the order of 5 megapixels or lower, operating at a video rate. In the present work, we propose a novel method of performing high spatial resolution phase stepping shearography. This method uses a 24 megapixel still digital imaging device (DSLR camera) and a Michelson-type shearing arrangement with an edge-clamped, center-loaded plate. Different phase-stepping algorithms were used, and all successfully generated shearograms. The system enabled extremely high-resolution phase maps to be generated from relatively large deformations applied to the test plate. Quantitative comparison of the maximum achieved spatial resolution is made with the video-rate cameras used in conventional shearography. By switching from conventional (video) imaging methods to still imaging methods, significantly higher spatial resolution (by about 5 times) can be achieved in actual phase-stepping shearography, which is of great usefulness in industrial non-destructive testing (NDT).

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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 edgeclamped, 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.

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Phase-stepping shearography is a useful speckle interferometric technique having wide applicability in various fields. Nyquist criterion has long been considered to be the gold standard in signal processing and imaging. This criterion states that the spatial frequency of image sampling should be more than twice the maximum spatial frequency in the image. As applied to speckle imaging in shearography, this implies that, if a two-dimensional charge-coupled device (CCD) is used for imaging, the pixel separation of the CCD device should be half of the minimum feature size in the image, or half of the speckle size. We examined the applicability of the Nyquist criterion in image sampling in phase-stepping shearography. We used a shearographic system employing a 24-megapixel digital still camera and a Michelson interferometer based shearing arrangement. We generated phase maps from a deformed object, which was a center-loaded thin aluminum plate fixed at the periphery. For a given amount of deformation and image shear, detailed phase maps where generated using the 4+4 phase stepping protocol for a different F-numbers, representing different sampling situations, ranging from the sub-Nyquist domain (undersampling) through the Nyquist domain, to the above-Nyquist (oversampling) domain. From the generated phase maps and quantitative measurements, we show that adherence to the Nyquist criterion is not a strict requirement in digital shearography. Operation in the sub-Nyquist domain is acceptable, even desirable, in many cases. Phase maps of good contrast and quality, with or without filtering, can be generated in the sub-Nyquist domain, which can be useful in strain analysis and non-destructive testing applications.