Systems and method are presented for identifying process variations during manufacture of products such as semiconductor wafers. At a predetermined stage during manufacture of a first products, images of an area of the first product are obtained using different values of at least one imaging parameter. The images are then analyzed to generate a first contrast signature for said first product indicating variations of contrast with said at least one imaging parameter. At the same predetermined stage during manufacture of a second product, images of an area of said second product are obtained corresponding to said area of said first product using different values of said at least one imaging parameter. The images are analyzed to generate a second contrast signature for said second product indicating variations of contrast with said at least one imaging parameter. The first and second contrast signatures are compared to identify whether a variation in process occurred between manufacture of said first and second products.