Patent GB-1315654-A (GB)
Title: Detection of faults in transparent material using lasers
IPC: G01N21/896
Filed: 1969-05-21
Abstract: 1315654 Fault finding PILKINGTON BROS Ltd P H GASKELL and D LOWE 14 May 1970 [21 May 1969] 25996/69 Heading G1A A method or apparatus for inspecting transparent ribbon or sheet material for internal defects includes directing a laser beam into the inside of the sheet material, effecting relative movement between the laser beam and the ribbon or sheet material so that the beam scans the ribbon or sheet material, locating a detector at a position removed from the normal light entry and exit paths of the sheet and detecting light scattered by a defect when the scanning beam impinges on the defect. Two kinds of embodiment are disclosed one in which light enters the sheet by its edge and the detector receives scattered light from the major surface of the sheet, the other in which the light is directed onto the major surface of the sheet and a detector inclined to the major surface receives any light scattered from any defect present. In both embodiments facilities are provided by means of logic circuits for determining the position and exact nature of any defects present. In the first embodiment, Figs.1 and 2, to one side of a continuously moving glass sheet 1 a laser 3 is positioned to direct a beam through a suitable lens 4 and onto a rotating member 5. The member 5 has eight identical vertical mirrored faces so the laser beam reflected from the member 5 makes successive scans about an arc in a horizontal plane. Before entering the edge face of the glass sheet the beam passes through a cylindrical lens 6 and a narrow slit 7. The beam entering the glass is contained between the upper and lower surfaces by internal reflection until it reaches any defects 13 whereupon it is scattered out through a major surface and is received and detected by means of a combination of a slit aperture 9, a lens system 12, a filter 11 and a photo-sensitive device 10. A further photosensitive device 8 is placed at such a position that the laser beam impinges on the device at the beginning of the scan. The time elapsed between emission of a reference pulse from the photosensitive device 8 and any pulse from photosensitive device 10 is a measure of the distance between the reference device 8 and the defect 13. This time interval is ascertained by means of electronic logic circuitry 14 and an amplifier 15. The magnitude of the pulse may also be processed to distinguish between large and small defects. A number of modifications are disclosed: three detectors viewing the same portion of glass thereby improving signal to noise ratio and helping to distinguish different types of defects; an integrating enclosure surrounding the portion of glass being inspected; and lastly, employing a bunch of glass fibres shaped so that one end has an elongated rectangular cross section whilst the other end has a circular cross section. Another embodiment Figs. 3 and 4 (not shown), has similar components and operation to the above except that light is directed onto the major surface and the detector is inclined to the surface to receive the scattered light. The state of polarization of the scattered light can be monitored by including a suitable filter so that the different types of defects can be distinguished.