PCT No. PCT/GB89/01030 Sec. 371 Date Apr. 11, 1990 Sec. 102(e) Date Apr. 11, 1990 PCT Filed Sep. 4, 1989 PCT Pub. No. WO90/02930 PCT Pub. Date Mar. 22, 1990.An interferometric procedure, such as electronic speckle pattern interferometry, involves generating two signals representing the point-by-point variations in intensity of respective patterns of electromagnetic radiation resulting from the interference of first and second beams of such radiation derived from a coherent source, with at least the first beam from each pattern being scattered, before interference with its respective second beam, from a common object surface, and with a corresponding pair of the beams, one for each pattern, having a predetermined relative phase difference of other than a multiple of pi ; and determining from the two signals values for a datum phase of the radiation at the object surface. Preferably, as a preliminary to this last determination. DC components are removed from the two signals. Conveniently, to simplify the determination, the phase difference is an odd multiple of pi /4 or pi /2.
Phase-stepping interferometry