Research on key factors in the digital shearing speckle pattern interferometry

Digital shearing speckle pattern interferometry (DSSPI) is a powerful tool in the measurement of strain, residual stress and the non-destructive testing (NDT). It is widely used in the fields of wood testing, tire inspection and aerospace, etc. The DSSPI system based on the Wollaston prism is attractive because of its compact arrangement, high immunity to disturbance, and, most of all, the ability to measure strain directly. It could show the strain distribution of the measured object by producing fringe patterns. In conventional DSSPI systems, phase-shifting devices are commonly adopted to extract phase information from the fringe patterns, which means dynamical measurement cannot be realized. In this paper, the principle of a novel DSSPI system based on the temporal analysis is presented. Phase information is extracted with the Fourier Transform method instead of the phase-shifting devices in this system. In this way, dynamical measurement is realized. The measuring accuracy is mostly determined by the quality of the fringe patterns (the fringe width and the contrast ratio). There are several factors that influence the quality of the fringe patterns. Here we mainly discuss the influence of the shearing distance and the polarization state. The preliminary experiments with different shearing distances and polarization states are conducted. The ideal shearing distance and polarization state are suggested.