Jerry,
The hardest part is probably defining precisely what you are testing.
You need to get good lighting to evenly illuminate the entire surface with no glare or dark areas. You probably need a good color camera.
Once you have a good picture, you will use different processing to detect different features. For example, scratches and debris could be located by using edge detection and checking for edges in areas that shouldn't have any edges (center of button, etc.). Perhaps examining the color with certain limits would locate discolored areas (too dark, too light, wrong color, etc.)
Bruce
Bruce Ammons
Ammons Engineering
