GB1145353A - Method of testing transparent materials - Google Patents

Abstract

1,145,353. Photo-electric measurement. INSTITUT NATIONAL DU VERRE. March 15, 1966[March 31, 1965], No.11346/66. Heading G1A. Angularity (non-parallelism of opposed surfaces) and local distortions (e. g. lens effects due to chemical or physical heterogeneities) in sheet material 12 (e. g. a lens or windscreen) are detected and measured using a detector 24 located to receive light transmitted through the sheet and distorted thereby, but in such a position as to be on the same axis as the light beam 10 incident on the sheet. In the described embodiment, the basic optical system comprises lenses 6 and 15, focusing the image of an opaque disc-shaped mask illuminated by source 2, on to the opaque centre portion 22 of an annular aperture 21 on a screen 20 located in front of photo-cell 24. When no sheet is present in the light path, the image is superimposed exactly on the centre portion 22. The presence of a sheet having a degree of angularity causes and angular displacement (refraction) of the light and the image overlaps on to the open part of the annular aperture, cutting down the amount of light reaching detector 24. The output signal of the detector is thus representative of the presence and the degree of angularity of the sheet. In order to avoid measurement inaccuracies due to changes in light source intensity and transparency or clearness &c. of the sheet, a beam splitter 17 is provided to direct light to a second (reference) photo-cell 28, which may be connected in a Wheatstone bridge circuit with cell 24, or some other suitable circuit. A visual (e. g. meter) or audio indication may be taken, or a record (photographic, magnetic, &c. made. The record can be a continuous one taken as a particular line across the sheet is moved through the light path or as the sheet is rotated. From this record, thickness variations may be determinable. The effect of a change in light flux on the reference cell 28 due to angularity changes, is reduced by having a diffusing screen and large adjustable aperture in front of the cell. Instead of the opaque mask 8, an attenuator, or colour or polaroid filter may be used, an appropriate attenuator or filter being provided instead of the opaque portion 22 in front of cell 24. The mask may be of semicircular or quadrant form and the screen aperture 21 circular or partly obturated by a segment, Figs. 2, 3 (not shown). Such masks may be rotated instead of moving the sheet, when angularity variations in all directions are required.