Two Kinds of Contrast of Surface Imaging

Application of Scanning Acoustic Microscopy for nondestructive evaluation of mechanical properties of objects having non-planar surface reveal two kinds of contrast. Images of ceramic bearing balls (Chow et al., 1991) and the images of steel spheres (Kolosov et al., 1991) exhibit the shape of a light spot which diameter is proportional to the diameter of the spheres. In contrary, acoustic micrographs of steel bearing ball obtained by (Weglein, 1981) and cavities in alumina (Poirier et al., 1984) exhibited a fringe pattern associated with a V(z) curve (Poirier et al., 1984). In this paper we present the theoretical explanation of this phenomena using the approximation of the uneven surface by an inclined plane (Weise et al., 1994). The dependence of the contrast of surface images on the wave length, radius of surface curvature and the aperture angle of the lens is investigated. The conditions for different kinds of contrast are obtained.