A novel optical method without phase unwrapping for subsurface flaw detection

This paper presents a novel optical method without phase unwrapping for subsurface flaw detection, based on the computer-vision reflection grating technique. In the setup, a computer-generated fringe pattern displayed on a computer-monitor is placed in front of the test object surface and forms a mirror image behind the reflective surface. The fringe pattern is perturbed according to the surface slope distribution. Unlike the current fringe pattern analysis techniques, which require one/several images, the analysis algorithm of the present method involves a sequence of fringe patterns having multiple frequencies to deduce the 'pixel-slope' information which is directly related to the surface slope of the test object. The novel method offers independent analysis for each pixel of the data array, and thus the problem of phase discontinuity in phase unwrapping can be eliminated. In this paper, experimental demonstration of the novel optical method to subsurface flaw detection is presented. © 2008 Elsevier Ltd. All rights reserved.