I show how phase shifting interferometry can be extended to account for multiple interference effects using a Fourier based analysis technique combined with wavelength tuning and a particular four-surface interferometer geometry. The technique is demonstrated by simultaneously measuring both surface profiles, the optical thickness variation and index homogeneity of a parallel plate. In addition, unlike traditional phase shifting techniques, the linear component of the homogeneity can be measured with high precision. It is shown that a significant linear component exists in a commercially supplied flat.
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