Optical scanning apparatus for detecting faults in transparent material wherein the plane of incident light is arranged at the breuster angle to the normal to the surface

An optical scanning apparatus for transparent, plate-like material having a surface, a normal thereto and an upper side and a lower side, said apparatus comprising:

• a light source;
• means for producing linearly polarized light;
• a light deflecting device for generating a scanning beam for executing a periodic scanning movement in a scanning plane extending obliquely to the surface of the material at the brewster angle relative to the normal, the scanning beam generating a scanning light bead on the surface of the material;
• photoelectric light receiving means disposed at an angle of reflection for differentially detecting light reflected from the upper side and from the lower side of the material including a position sensitive photo receiver for determining angular deviations between the beams reflected from the upper side and from the lower side of the material;
• and
• wherein said means for producing linearly polarized light polarizes the light alternately parallel to and perpendicular to the scanning plane.