A defect inspection technique using polarized images for steel strip surface

An in-line defect inspection technique using polarized images for steel strip surface is developed. In inspection for low contrast defects, excessive-detection will be caused by harmless patterns such as slight oil patterns, chemical liquid patterns, and other patterns. We have adopted quasi-ellipsometric method using polarized images of the target samples to obtain their ellipsometric parameters, and found that the ellipsometric characteristics of the defects and the harmless patterns differ from each other. Based on this finding, we have developed an inspection system utilizing three polarized images with different azimuth angles to discriminate defects from harmless patterns at a high-speed production line.