Profile measurement of objects with discontinuous surfaces by modified temporal phase unwrapping

Abstract

A profile measurement technique for objects with spatially isolated surfaces is presented. The technique is based on the Fourier fringe analysis combined with temporal phase unwrapping. A sequence of sinusoidal fringe patterns with a varying pitch is projected onto objects, and the temporal variation of the fringe signal is recorded with a CCD camera. The phase of the temporal fringe signal is detected at each pixel by the Fourier transform method, and is temporally phase unwrapped, independently from other pixels. The temporal frequency of the fringe signal estimated from the time slope of the unwrapped phase provides the information about the absolute surface heights of the objects. It will be shown that, because of its filtering function that can exclude higher order harmonics, the Fourier transform method is more robust to the nonlinear characteristics of a LCD projector and a CCD image sensor, than the phase shift technique that assumes a pure sinusoidal fringe signal. The measurement of objects with large discontinuities and/or spatially isolated surfaces is demonstrated by experiment.