Abstract: The inspection for surface deformation of flat glass is carried out by the flying-spot method wherein the beam of light is thrown by a rotating mirror 13 onto a folding mirror 6 which reflects the light onto the continuous sheet of glass 2 to be inspected. The reflected light beam 4 strikes a light-diffusing plate 1 disposed ahead of the receiver 11. The area of the light-diffusing plate 1 on which the beam of light impinges when the sheet of glass 2 is free of defects is opaque. On this opaque area there are disposed in a spaced relationship two photodiodes 7 between which a light bar formed by the scanning light beam extends. When the thickness of the flat glass 2 varies gradually, light will strike one of the photodiodes 7. The pulse so triggered controls a tracking mechanism 50 which adjusts the folding mirror 6 so that the light bar again extends between the photodiodes 7.