While looking in the mirror, have you ever wondered whether you are actually watching the mirror itself, or what is reflected by it, that is yourselves? How can a vision system analyze a reflecting surface if the incoming image is that of the recording camera itself? To solve this problem a special illumination technique called Dark-Field can be used. Usually realized by a ring illumination system with a much higher diameter than the framed object and a low angle, this technique is effective to highlight anomalies like scratches or surface flaws, not depending on the color of the object. The surface remains dark, while edges or concavities appear bright in the image. Changing the angle of the light source so the rays of light hit the reflecting surface with a high inclination, light bounces off from the surface so it cannot arrive to the camera. This way all the vision system remains in the shadow and thus not visible "in the mirror". Only surface flaws like scratches or roughness of the reflecting surface can deflect the light upward to the camera, thus appearing illuminated in the image.
Bright Field and Dark Field Illumination
Edges emphasized through Darkfield