Dual level out-of-focus light source for amplification of defects on a surface

摘要

A web defect inspection system includes line scan cameras positioned across the width of a web and a dual-level out-of-focus light source for illuminating the web. An illumination surface of the dual-level light source has a bright portion adjacent a dark portion. One such dual-level light source is provided by covering a portion of a diffused light surface with an opaque material having a sharp edge. The pixels of each line scan camera are aligned with the dark to light transition of the light source so that each pixel has a no-defect brightness level equal to half of a relative brightness level of the bright portion of the dual-level light source. An image of a defect-free portion of the web consists of pixels having relative brightness levels within a pre-determined range of the no-defect brightness level. Images of defects on the web surface appear three-dimensional as bright and dark areas. The nature of the defect, that is, whether the defect is concave or convex, is determined based upon the orientation of the dual-level light source and the direction of movement of the web.