Automatic on-line measurements of three-dimensional shape by shadow casting moiré topography

Abstract When the shadow of a grating falls on an object, and the object is viewed through the same grating, the shadow and the grating interfere to generate a moire fringe pattern. Provided that a number of geometric conditions are met, the moire fringes describe contours of equal height on the surface of the object. Perhaps because of its simplicity (the technique requires only a grating and a source of light for its implementation) the shadow moire method has become popular in the visualization and semiquantitative measurement of three-dimensional shape. Early work in this field relied on photography and subsequent manual fringe analysis to provide quantitative interpretation of the contour pattern. More recently, the advent of digital image processing has opened up the possibility of on-line data capture and analysis. In this paper we will describe a version of shadow casting moire topography in which the contours are automatically interpreted using digital image processing. By analysing the contour pattern in terms of the phase rather than the intensity of the fringes, we facilitate fine subdivision of the contour interval and thereby achieve a higher accuracy than is normally associated with moire contouring techniques.