Patent GB-1293348-A (GB)
Title: A method of and apparatus for analysing objects with two or three dimensional shapes and/or patterns
Inventor: BAKER LIONEL RICHARD
IPC: G01B11/24
Filed: 1968-12-10
Abstract: 1293348 Photo-electric investigation of geometrical characteristics of objects SIRA INSTITUTE 10 Dec 1969 [10 Dec 1968] 58649/68 Heading G1A [Also in Division G2] The geometrical characteristics of an object e.g. shape or the pattern within its external shape, are analysed by a Fourier transform technique involving irradiating the object with a beam of electromagnetic radiation to form a diffraction pattern, particular areas of which are investigated to derive electrical signals representative of the object characteristics. The object may be a diffraction grating, target, scale, graticule, photographic transparency, or mechanical component, and a typical application is to compare the object with a standard object to detect differences therebetween (e.g. faults). Fixed objects may be analysed or a series of photographs of an article or articles on a production line may be taken on a moving, progressively developed, film which is passed through the apparatus for analysis. Optical apparatus. In one embodiment, Fig. 1, light from a laser 10, is chopped at 9, split into two beams by a prism or half-silvered mirror 12, and one beam used to illuminate object 4 and the other a similar (e.g. reference) object B. Each beam comprises a microscope objective 15 (or 23), a pinhole 16 (or 24), a collimator 17 (or 25) and a lens 18 (or 26) focusing the light diffracted by the object in the plane of a spatial filter member 19 (or 27), a detector e.g. photo-multiplier or vidicon tube 20 (or 28), picking up light transmitted thereby. A comparator 29 detects any difference in the detector output signals as indicative of differences between the spatial frequencies introduced into the respective beams by the two objects A, B. Thus a zero difference indicates identity of the objects. In a second embodiment, Fig. 7 (not shown), only one light beam and detector is used, with the two objects placed side by side in the collimated beam. A chopper member in the path cuts off the light reaching each object alternately. In a third embodiment, Fig. 8 (not shown) - another single beam arrangement - a single test object A in the collimated beam can give an absolute measurement value or a comparison measurement if a spatial filter 19 in the form of a photographic mask prepared by photographing the diffraction pattern formed by a standard object is also introduced. Devices for scanning different areas of the diffraction pattern are described and can be used, if required, in combination with a spatial filter 19. In one arrangement, Fig. 4 (not shown), a cone-shaped lens (30) is placed in the plane of the or each diffraction pattern, the spherical aberration of the lens causing spreading along the optical axis of the beam the foci of the different annular bands in diffraction pattern. An aperture (31), moving along the axis, thus allows different areas of the diffraction pattern to be passed to the detector (32) which moves with it. In Fig. 5 (not shown) an arrangement of annular photo-cells of increasing diameter is used, the outputs thereof being switched so as to scan continuously or incrementally annular bands in the diffraction pattern. The arrangement of Fig. 6 (not shown) as the same, but uses an annular aperture of continuously increasing diameter formed by a variable iris and a moving cone. An integrating sphere collects light passing through for detection by photo-cell(s).