Illumination: Dark field illumination
This technique is mainly used to highlight surface defects, scratches or engraving. Dark field illumination usually uses a low angle ring light that is mounted very close to the object. Dark field is the opposite of bright field because most of the light reflected from the surface of the target will fall outside the FOV of the camera with the camera only "seeing" scattered light that is reflected by a defect on the surface.
The sequence of images shows the principle of dark field illumination when detecting surface scratches or defects on flat objects.