Sub-Nyquist interferometry.

The primary limitation of conventional phase-shifting interferometry (PSI) is its inability to measure surfaces with large aspheric departures. A new method of data analysis, sub-Nyquist interferometry (SNI), is described and demonstrated to overcome this problem. SNI is an extension of PSI, and it preserves the measurement precision that is inherent to PSI. For some types of wavefronts, measurement range improvements of more than 2 orders of magnitude are shown, and these improvements result from the utilization of a priori knowledge about the wavefront. Simple and reasonable assumptions are found to be very powerful for improving the aspheric measurement capability of the interferometer system.