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 yourself? How can a vision system analyse 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 realised 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, regardless of the colour 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 reach 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.