UNION MINIERE (UM) is an international firm that has developed a special process, called preweathering, which gives rolled zinc sheets a natural patina look or a slate gray color. The strip of preweathered zinc can be affected by surface defects caused by the roll mill or by the preweathering process. We have equipped a production line of preweathered zinc in the plant with a computer vision based system in order to automatically inspect one side of the strip. The system is composed of a personal computer system, a light source, a line scan camera and an acquisition board. The basic purpose of this system is to provide effective on-line inspection. The main problems to be solved are, (1) that the imperfections show a considerable variation in length, from a few centimeters in the case of local defects, to several decimeters in the case of periodic spaced defects due to rollers, (2) that the contrast between the local defect and the strip is poor, (3) the grayness varies across the width of the strip, (4) that the grayness varies across the length of the roll. The real time inspection system has now been implemented and is currently undergoing evaluation in the plant.