Extracting the shape and roughness of specular lobe objects using four light photometric stereo

Abstract

A noncontact method of measuring surface shape and surface roughness for part inspection is proposed. The method, called four-light photometric stereo, uses four lights that sequentially illuminate the object under inspection, and a video camera that takes images of the object. Conceptually, the problem has three parts: shape extraction, pixel segmentation, and roughness extraction. The shape information is produced directly by three-light and four-light photometric stereo methods. After shape information is obtained, statistical segmentation techniques can be applied to determine which pixels are specular and which are nonspecular. Then the specular pixels and shape information can be used, in conjunction with a simplified Torrance-Sparrow reflectance model, to determine the surface roughness. The method has successfully been applied to a number of synthetic and real objects.