N-point spatial phase-measurement techniques for non-destructive testing

Spatial phase-measurement interferometry techniques commonly used in non-destructive testing are affected by a number of fundamental error sources. This paper focuses on the major limitations for phase calculations using standard N-point algorithms. These limitations include: the wrong carrier frequency, unequally spaced fringes, detector nonlinearities and variations in the de fringe intensity. The character and magnitude of these errors are quantified by using computer simulations. A displacement measurement of a bent plate using different algorithms on the same data shows that practical limitations are unequally-spaced fringes and variations in dc intensity and fringe visibility. These errors limit the resolution of this type of measurement to about a tenth of a wave r.m.s.