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. Large ring lights, spot lights and line lights can all be used in the dark field (image 1).
The sequence of images (image 2-4) shows the principle of dark field illumination when detecting surface scratches or defects on flat objects.
On this flat, unmarked surface there is a low angle of incidence, and virtually all of the light is reflected out of the field of view of the camera. The resultant image would be very dark with no distinctive features implying a defective object (image 2).
Here one can see that when surface features are present, the light hits them and is scattered in all directions. Some of this scattered light will enter the camera to reveal the scratch or feature (image 3).
As this detail shows, light hitting a sharp edge will disperse widely (image 4).