Improvements in or relating to the examination of sheet material

880,135. Photo-electric inspection apparatus. GENERAL ELECTRIC CO. Ltd. May 16, 1960 [Feb. 24, 1959], No. 6339/59. Drawings to Specification. Class 40(3). An apparatus for the inspection of sheet material which has a smooth and highly reflecting surface includes a light source, a photosensitive detector to receive light from the source which has been reflected from said surface, and means responsive to the output of the detector to mark the material if a flaw is detected. The sheet material and the detector are moved relative to each other, and in the embodiment described with reference to the Figures the light source comprises a lamp which extends across the sheet and is enclosed in an enclosure having a slit in it. The image of this slit is focused towards a further slit in front of an elongated photosensitive unit which also extends across the sheet. It is arranged that the image does not fall on the second slit, and hence actuate the photosensitive unit, unless there is a flaw in the sheet material. The output of the photosensitive unit controls, via a delay if necessary, a device for marking the material, e.g. with paint, near the edge of the sheet and in line with the flaw. The output may also control a means for sorting faulty pieces of sheet from flawless ones when the material is in plate form. In further embodiments the detector comprises a number of photosensitive units, each associated with a separate marking means so that the sheet is marked near to any detected flaw. Alternatively the detector may scan the illuminated strip of the sheet, the marking means being moved in synchronism with it, or there being a number of marking means which are switched synchronously to the detector output. A cutting device may be included which, in response to the detector output, cuts out any flaw-containing portion of the sheet. To overcome the effect of stray light the light source output is preferably chopped and the detector output passed to an A.C. amplifier. A number of pairs of light sources and detectors may be used, e.g. in echelon formation, to cover the width of the sheet, In yet further embodiments the light source comprises one or more cathode ray tubes, scanned repeatedly to illuminate the surface of the sheet.