L.R. Watkins, S.M. Tan and T.H. Barnes
An interferometer measures the optical path difference between a reference flat and a test surface, producing a pattern of fringes whose spatial frequency is proportional to the local gradient of the test surface. To obtain the two-dimensional phase distribution (and hence the surface shape), it is necessary to resolve the sign ambiguity of the local slope. In phase-stepping interferometry, this is done by acquiring three or more accurately phase-stepped images by moving either the test surface or the reference flat with piezo actuators and grabbing images with a CCD camera. Inaccuracies in the phase-stepping, however, generate errors in the extracted phase distribution.
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