Suppose you want a machine vision system to automatically detect scratches on a plastic part such as a cell phone case. You know that the scratches will be nearly straight lines and that they can be low contrast and discontinuous. You can’t predict their orientations or lengths, but only scratches longer than some length must be reported.
You set up a machine vision system with part positioning, lighting, lens, camera, processor, and vision software, and do some experiments. As always, proper lighting is critical for showing the defects. For this task a low-angle ring light “catches” the raised edges of each scratch and so shows the scratch as brighter lines. This lighting also “catches” dust particles on the part.