Coating density analyzer and method using non-synchronous tdi camera

Abstract

An image processor based system and method for recognizing predefined-types of coating density imperfections in a web, specifically continuous type or streak imperfections. Continuous type imperfections are recognized in a continuous web moved at a certain rate through an imaging region illuminated by a stripe of substantially constant illumination. A time-delay integrating CCD camera is focused on the illuminated imaging region. The TDI CCD camera comprises an array of N rows of M light sensitive CCD elements each imaged on a fixed discrete pixel-related image area of the illuminated imaging region. The charge levels accumulated in the CCD elements of each row are shifted to the succeeding row or CCD elements and summed with the charge levels therein at a line shift clock frequency that ensures that an asynchronous relationship exists with respect to the incremental movement of the web. During the clock cycle of the N rows, the corresponding pixel areas of the illuminated web shift asynchronously or creep through the discrete pixel-related image areas. The accumulated pixel charge values derived from the pixel-related image areas of the illuminated region of said moving web emphasize imaging of longitudinal streak imperfections in the web due to the asynchronous movement the web.