Patent GB-1135629-A (GB)
Title: Method and apparatus for inspecting transparent members
IPC: G01B11/06
Filed: 1966-03-31
Abstract: 1,135,629. Optical apparatus. LIBBEYOWENS-FORD GLASS CO. March 31, 1966 [May 26, 1965], No. 14308/66. Heading G2J. Also in Division G1]  The thickness or refractive index of a transparent member e. g. a cooling glass strip moving in an annealing lehr, is measured by directing a beam of light at the member to form spaced reflected beams due to reflection from the spaced surfaces of the strip and investigating these beams using a photo-electric detection system. The optical system is mounted within a transversely movable casing situated on the top of the lehr and various cooling arrangements are described. Light from the source assembly 61 provided with a slit is focused by lens system 62 on to the surface of the moving glass strip 5, after reflection by an adjustable mirror arrangement 64 and passage through a heat-resistant transparent member 70. Two beams are formed due to reflection at the top and bottom surfaces of the strip, these beams being spaced by an amount representative of the thickness and the refractive index of the strip. They pass through another heat-resistant member 70, are reflected by a further mirror arrangement 65 and finally focused by a lens system 63 on to the entrance to a detector system 66. The mirror arrangement 64 includes a bracket carrying a plane mirror (102) Figs. 8, 9 (not shown) whereby adjustment is possible in two dimensions so that the desired angle of incidence of the beam on to the glass strip can be obtained. The arrangement 65 is a lateral inversion of that 64 and directs the beam towards the detector. The constructions of the lens assemblies 62, 63, Fig. 4 and of the mountings for the transparent members 70 (also in Fig. 7, not shown) are also described. The detector is a photo-multiplier contained together with a lens system within a rotating tubular chopper member, provided with a slit the same size as that in the source assembly. The rotation of the slit allows the spaced beams to reach the photo-multiplier successively, producing a pair of time-spaced pulses in each cycle. These pulses are differentiated by a capacitor-resistor arrangement, so that the time interval is made independent of the amplitude of the pulses, Figs. 11, 12 (not shown), the time interval being converted into a digital form, a digital to analog converter being finally used to enable the signals to be recorded on a strip chart recorder. If the refractive index of the glass is known, the recorder can be calibrated in thickness, or alternatively the refractive index of a constant thickness strip may be recorded. In these cases, the angle of incidence, and details of the detector chopper arrangement are known. In order to decrease errors due to distortion (wedge) effects, the lens systems 62, 63 are located halfway between source and strip, strip and chopper, respectively. Further, the length of the slit image is arranged to be transverse to the direction of movement of the strip. Means are also provided for recording the width of the strip and its position with respect to the width of the lehr. A potentiometer is driven in association with the movement of the optical system across the lehr, and produces an output voltage to control a pen recorder. The pen is arranged to mark the chart only when a pulse signal received at the beginning of each scan of the optical system across the lehr, and the two marks obtained thus give an indication of the width of the strip. If the edges of the lehr are represented by the edges of the chart, the position of the strip in the lehr is also indicated.