Automatic adaption of projected fringe patterns using a programmable LCD-projector

Triangulation is a method often used to register absolute coordinates of an object. The basic idea of this method if to measure the three-dimensional coordinates by structuring the illumination using spectral or spatial light encoding. This technology has been established in the field of quantitative measuring free-styled surfaces, including the generating of CAD-description of the test-object (reverse engineering). For fringe-projection a periodic grating with constant frequency is mainly used. Advantage of this this kind of grating is that it is easy to produce. But sometimes the measurement fails since shadows, high density of the fringes, edges, overmodulation, reflection and blurred regions of the object refuse exact evaluation. This paper describes a method using fringe-projection-technology which is more robust. This aint is accomplished by monitoring a programmable. high-resolution, high-contrast spatial light-modulator using information from the result of projection. Consequently the main component of this process is a programmable light modulator based on LCD. First step is the detection of problematical regions on the object caused by shadows, overmodulation, reflection, edges of the object and high density of fringes. This analysis is based on skeletonizing the projected fringes in the image, After detecting the faulty areas on the object the pattern, frequency and intensity of the projector is modified for the detected area only until the best result is guaranteed. With these modifications to the conventional fringe-projection-technology a fast, precise and complete shape measurement is achieved.