GB1399474A - Measuring methods and apparatus - Google Patents

Abstract

1399474 Photo-electric measurements using diffraction and interference of light T R PRYOR 22 May 1972 23924/72 Heading G1A [Also in Division G2] General. The spacing W between objects 4 and 5, Fig. 1, or a change in the spacing, is determined by diffracting light (visible or infrared) at edges 2 and 3 to form an interference pattern 6 having fringe spacing Xn related to the object spacing. The pattern may be examined and measured: (1) visually with the aid of vernier calipers; (2) photoelectrically by traversing a detector 9 over the pattern with a micrometer; (3) photo-electrically by counting the fringes passing a fixed detector; or (4) by comparing the pattern with a reference pattern. The pattern may be recorded photographically before examination. Spacing between the objects 20, 21 in orthogonal directions x and z, Fig. 2, results in an asymmetrical pattern 25 in which the fringe spacing is a function of both the spacing We and the displacement Z. Examination (fringe counting) by two detectors 27 and 28 symmetrically positioned with respect to the object axis permits both components of the changes to be determined: A laser 1, 24 is used as the light source; where a pulsing laser is used stroboscopic slowing down or stopping of a change of spacing may be effected. Interpolation between fringes is achieved by: (1) a voltage controlled mechanism to displace a photo-electric detector until the next count is obtained, the voltage being a function of fringe fraction; (2) analysis of the amplitude of the photo-electric signal; (3) varying the phase of the light at one edge with respect to that at the other (Michelson interferometa) until the next count is obtained, the phase change required being a function of fringe fraction; (4) using a multiple edge arrangement with separate detectors, Fig. 4 (not shown), to increase the total of counts for a given spacing change. A second pair of edges with an associated detector operating at 90 degrees phase relationship with the first is used to determine the direction of a spacing change. Extension of the range of measurement. Fig. 8 (not shown), is effected by forming a regular series of edges on one object the members of which are brought into measuring relationship in succession with an edge on the other object. Errors in measurement when the spacing tends towards zero are avoided by bringing into operation a second measuring system. Modifications. As an alternative to the use of object edges, slits may be provided, Fig. 6 (not shown) or diffraction gratings attached to the objects, Fig. 7 (not shown). Applications. (1) Sheet thickness or surface roughness is measured, Fig. 3 (not shown) by engaging one object with the sheet or surface so as to be displaced by it. The arrangement employs fringe counting with a split photo-detection system arranged to respond to pattern slope reversals. Provision is made to adjust the detector entry aperture width. A mark in the light source path is provided to permit measurement to be made at a given time by withdrawing the mark. (2) Strains are measured, Fig. 9 (not shown), by attaching the objects with diffracting edges to the stressed body and backing the edges with a reflector. A detachable assembly of a light source and apertured reflector is brought into use to illuminate the edges and form the interference pattern by reflected light. The strain pattern may be compared with a reference pattern which is produced in the absence of strain and used as a filter to produce moire fringes for visual examination. A variable reference pattern may be generated with a medium subjected to vibrations from an adjustable ultrasonic source. (3) Changes in the profile of a body (in both the x and z directions - see Fig. 2 above) as the body is traversed through a measuring station, Fig. 10 (not shown), are determined by using the profile as a diffracting edge in conjunction with a reference edge. A cylindrical lens concentrates light to a spot to avoid errors due to lack of parallelism between the profile and reference edge. Two measuring arrangements, one at each side of the body permit determination of diameter. (4) Angle of rotation is measured with the aid of a series of edges disposed radially on a body.