Can Be Used For The Digital Evaluation System Of Large Diameter Precision Surface Defect Detection

Along with developing modern technology, the demands for precise surface quality are also higher and higher. Light will be scattered if there're scratches and other macroscopic defects on the surfaces. Scattering will lead to light energy dissipation, and even damage optical elements seriously, that will affect the performance of optical systems. The automatic inspection of surface imperfections of precise elements is a problem expected to be solved.Based on the defects image are bright in black background in scattering imaging system, a complete digital evaluation system of surface imperfections, is presented in this paper.Then, the principle and method of the instrument design has been described. The equipment is composed of optical system, mechanical structure, photo-translating module and computer software system.For defects are in random distribution and random shape on the surface of optical element, implementing the inspection of large-aperture surface, it is necessary to stitch image by sub-aperture scanning technique. Using the XY- scanning system to detect surface (e.g. the large aperture is 320×320mm2, sub-aperture of XY-scanning is 4×4mm2), corresponding coordinates are built, eliminating redundant information. Based on isometry serial images, digital image could be stitched speedily and precisely. Mathematical morphology is introduced to defect image recognition. How to implement binary image eliminating noise, segmentation and marginal test, feature extraction by using erosion and dilation algorithm was studied. The different defects size feature (e.g. length, width, area) are analyzed and evaluated by criterion. The experimental setting has 1μm resolving power and obtained credible defect grades evaluated result.