Optical Characteristics of Concave and Convex Defects on Steel Sheets for Low-angle Incident Light and Its Application to Defect Inspection

Concave and convex defects on steel sheets sometimes become problem in steel production line. These defects are usually invisible because their height or depth is as small as surface roughness of steel sheets. Therefore they could be detected by human inspection only after grinding steel surface using grindstone. This paper describes optical characteristics of such kinds of defects on steel sheets with certain roughness, especially on low-angle light incident condition, and proposes optical inspection system utilized “Magic mirror” phenomenon with low angle laser incidence to detect these defects without grinding. The on-line inspection system has been built and proved to have the capability to detect such kinds of invisible defects.