GB1375021A - - Google Patents

Abstract

1375021 Measuring transparent material with fibre optics guides COMPAGNIE DES COMPTEURS 5 Nov 1971 [6 Nov 1970 15 Jan 1971] 51519/71 Heading G1A The thickness or distance away of transparent material 64, Fig.5, having front 61 and rear faces 61' both capable of reflecting radiation is measured by a fibre optics arrangement consisting of a central guide 60 directing radiation on to the surfaces and two receiving guides 62 1 , 62 2 directing the reflected light on to corresponding detectors 63 1 , 632 (photo-diodes or photo-transistors). One radiation receiving guide 62 1 receives radiation from the front face 61 (plus possibly some radiation from the back face, depending upon the material's thickness and the reflectivity of the back face); the other light receiving guide 62 2 is spaced transversely from the central guide 60 and receives radiation from the back face 71 only. The areas of the light receiving faces and/or the gain of amplifiers 66 1 , 66 2 of the detectors 63 1 , 63 2 are adjusted so that the linear portion Figs.5A (not shown) of the characteristic curve of intensity of light received versus distance away from the emitting face has the same slope (sensitivity) in order that subtraction in a differential amplifier 65 of the two detector output signals (which both provide a distance measurement) may provide an output signal directly proportional to the material's thickness. In a modification, Fig.6, an extra light receiving guide 122 2 adjacent the central guide 110 is used to help eliminate the effect of the inner most receiving guide 122 1 receiving light reflected from the back face 111'. The detector output signals of the two adjacent inner most receiving guides are subtracted from one another and the "compensated" resulting signal is fed to a further differential amplifier 132 which also receives the outer most guide's detector's 113 n+1 output. A series of light receiving guides Figs.4, 4B (not shown) for receiving radiation reflected from the back face may be provided to cope with a series of possible thickness ranges of the material since the position of the guide 122 n+ 1 used depends upon the expected thickness of the material, and the guide used should preferably be one where this thickness in the central portion of the linear part of its characteristic curve. Since the detector's outputs are linear over only a portion of their characteristic curves, two adjacent series of guides Fig.4B (not shown), (whose light receiving ends are formed in two lines individual members of which in one line are staggered relative to corresponding members in the other line), may be provided so that the instrument can be used continuously within a range without any periodic portions having to be omitted due to non-linearity of the output signals. A detailed mathematical analysis of the physics of the operation of the apparatus is disclosed.