Detection and Discrimination of Surface Defects by Analyzing Diffraction Pattern of Laser Beam

CONVENTIONAL SURFACE INSPECTION We have developed a new apparatus to cope W I T H LASER BEAM. with the surface inspection of products having even undulating or stepped surfaces. To realize such surface inspection, we combined the three dimensi~nal shape measurement through laser beam scanning and the flaw and defwct identification t h m u g h diffraction light pattern analysis. This combination makes it possible to detect locations, sizes, kind and degrees of flews or detects if any. A couple of parabolic cylindrical mirrors are used to scan the laser beam perpendicularly to the specimen transfer direction and ta converge the ditfraction light on a apecifledIy designed photo-detecting sensor through a m e c h a n i s m to maintain the inspecting surface at a constant level. A couple of cylindrical lenses a r e also used For a TV camera to improve the height measurement eccuracy. INTRODUCTlON The developments of image processing, pattern-recognition and knowledge enginmrings have stimulated the automation of inspection processes for manufactured products. However, for the inspection of product surfaces, the present stete of the art is not mature and the visual check by i n ~ p e c t o r s is still a common practice at producuon Iines becausv nT t l t t * v m i e t y o l surfme flaws and defects of p d u c t s and their complexity. The development of autornabd surface inspection system I s therefore expected not only to replace the visual check b u t also to realize t h e production of flaw-and-defect free products through the identification of flaw-and-defect causes with the system. This paper deals with such a surface inspection a p p a r a t u s and a method developed on a n experimental basis. The apparatus can cope with the inspection of products having even undulating o r stepped surfaces on them and can detect locations, sizes and kinds of flows and defects if any. The inspection method applied here combines t h e three dimensional shape measurement of a product through a laser beam scanning and the analysis of diffracted light pattern of the projected lases beam light en the surfaces af the product.