Typical applications include reflective flat surface defect or edge detection, where the majority of the light may reflect away from the camera on the flat, featureless surface but defects may scatter the light to the camera, creating feature appropriate contrast. 

Dark field illuminators can be dedicated ring lights, or any general bar or spot light oriented at low angles of incidence, depending on defect orientation.  Very low angle dark field ring lights must be deployed at close working distances.