A new phase shift method that requires only a single image to create the phase map is needed to expand the application of the phase shift digital shearography technique to include dynamic loading conditions. In this case, the acquisition rate will only be limited by the camera instead of by the multiplephase-stepping procedure. As only one image is needed, there is no need to make assumptions about the phase distribution between each phase-stepping image. This new phase shift method is called the spatial phase shift method.
The spatial phase shift technique can be divided into two major categories: the multiple-channel spatial phase shift approach and the frequency carrier spatial phase shift method. The earlier approach to solve dynamic loading problems was to use the multichannel spatial phase shift method. Later, the frequency carrier spatial phase shift method, a more advanced technique, was introduced. This method records multiple speckle interferograms on a single image. A Fourier transform method provides a way to extract and separate the phase information from this single image. In the remainder of this chapter, the multiple-channel spatial phase shift approach will be introduced, and the frequency carrier spatial phase shift method will be discussed in detail.
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