A direction/orientation-based method for shape measurement by shadow moiré

Abstract

In this paper, a methodology to process fringe patterns is presented. The core of the signal processing technique is the use of the direction information, which is a modulo 2 pi quantity that locally indicates the direction along which fringes grow at a maximum rate. By using this information, it was possible to perform adaptive and direction/orientation-based operations on fringe images to remove unwanted effects (mainly distortions at the image borders), to enhance the contrast, and to extract the phase information encoded. The method has been applied on shadow-moire interferograms with carrier fringes in order to measure the surface of small objects. The developed algorithm allows processing of fringes whose phase information is not monotonically increasing everywhere and to more accurately estimate the phase itself. The results from two experimental fringe patterns are presented. These results are compared with the measurements that are performed by means of a conoscopic system that is used as a reference. Hence, it was possible to calculate the uncertainty of the performed measurements. Aside from the possibility of decoding fringe patterns that present closed fringes, the proposed method provides measurements that are characterized by an uncertainty that is more than halved compared with Fourier-based methods.