Extended unambiguous range interferometry

Phase measuring interferometers generally measure phase modulo 2pi. We present a system which uses fringe contrast to help determine the absolute phase in the interference image within the limits of the coherence length of the illumination. This approach obviates the need for phase unwrapping and is unaffected by surface discontinuities or by data dropout. Since the phase is determined on a point-by-point basis, the processing could be pipelined. The system is set up on a microscope interferometer and produces surface profiles over an array of 512 x 512 points. The measurement range is related to the coherence length of the source and can easily be varied from 0.5 to 2.5 microm. The resolution is limited by the 8-bit quantization of the output.