The light source is uniform and diffuse, and falls on a little angle of the reflecting surface of the object to inspect. 

The camera is placed in the same angle so that we can have a reflected image of the light source in the surface of the inspected object. A dark spot will appear in the image taken by the camera if there is any imperfection in the object. The dark spot is due to the variation of the angle of reflexion that produces the imperfection on regard to the angle of the incident light, losing the reflect of the lighting source.

Ideal to detect lines, fissures and distortions in objects with flat and bright surfaces.