Optical EngineeringProfilometry using temporal phase unwrapping and a spatial light modulator based fringe projector
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A fringe projector based on a low-cost spatial light modulator has been used to measure the shapes of discontinuous objects. By changing the fringe phase and the fringe pitch, a sequence of wrapped phase maps can be acquired at different sensitivities. This sequence can then be converted to a surface profile by the recently proposed method of temporal unwrapping rather than by a conventional spatial unwrapping approach. The main advantages are that the method is simple and robust, and that objects with surface discontinuities are profiled as easily and accurately as smooth ones. The absolute distance from the camera to an object is measured at each pixel independently of the other pixels in the image. In addition to accurate measurement of surface shape, one possible application may therefore be as a 3-D robot vision system. A simple calibration procedure is described which avoids the need for accurate positioning of the camera and projector and which takes account of instrumental artifacts. A measurement accuracy better than 1/1000 of the field of view has been achieved. A number of practical applications of the technique are illustrated.