The difference between the phase shift occurring at the object surface owing to displacement and the phase shift occurring at the observation plane of the imaging system of the interferometer is studied. Analytical expressions for the phase shift for a number of surface displacements are found. From these expressions it is found that the difference between the phase shifts at the object and the observation planes depends on the number of speckle-correlation modes in the observation plane and the product between the relative aperture and the relative defocus of the imaging system. For general displacement the results indicate that the accuracy of a phase-shift measurement with a small-aperture interferometer is limited only by the number of speckle-correlation modes at the observation plane for the case of a focused system. For a large-aperture interferometer the phase shift at the observation plane becomes sensitive to defocusing of the imaging system. Agreement between theory and experiments is observed.
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