Extracting the Shape and Roughness of Specular Lobe Objects Using Four Light Photometric Stereo

Abstract

Two important aspects of part inspection are the measurement or surface shape and surface roughness. We propose a noncontact method of measuring surface shape and surface roughness. The method, which we call "four light photometric stereo," uses four lights which sequentially illuminate the object under inspection, and a video camera for taking images of the object. Conceptually, the problem we are solving 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 we have shape information, we can apply statistical segmentation techniques to determine which pixels are specular and which are nonspecular. Then, we can use the specular pixels and shape information, in conjugation with the simplified Torrance-Sparrow reflectance model to determine the surface roughness. The method has successfully been applied to a number of synthetic and real objects.