Phase extraction by spiral phase transform in digital shearography

Digital shearography [1] is a practical inspection tool for industrial applications. Traditionally, the output of shearography is in the form of shearogram depicting the phase change caused by deformation. To determine the phase change quantitatively, it is necessary to capture at least three shearograms with different values of phase-shift at each deformed state. This phase-shifting techniqueworks well for static deformation measurement, but it is unsuitable for measuring dynamic deformation. Therefore, phase demodulation from a single shearogram is of great interest in practical applications. Previous work by other researchers has demonstrated that the Fourier transform (FT) method [2]and the spiral phase transform (SPT) method [3-4] can be used to extract the phase map from a single fringe pattern. However, the FT method requires that a large spatial carrier frequency should present in the fringe pattern. The reported SPT method estimates the fringe orientation from intensity derivatives, which will cause errors when there are more than two extremes in the phase map. However, the phase of a shearogram usually has two or more extremes in digital shearography. To deal with this problem, in this paper we describe a new procedure to extract phase from shearogram. We calculate the fringe orientation by optical flow from two successive shearograms and then determine the phase of the shearogram by spiral phase transform. We describe the principle of the method and then demonstrate it through both simulation and experimental results.