Patent GB-1428372-A (GB)
Title: Optical apparatus for determining deviations from a predeter mined form of a surface
IPC: G01N21/88
Filed: 1973-04-18
Abstract: 1428372 Detecting surface deviations photoelectrically R N SCOTT 8 June 1973 [9 June 1972 18 April 1973] 26942/72 and 18718/73 Heading G1A Deviations of a surface are converted into changes in the plane of polarization of a light beam generated by a laser, the beam being analyzed by dividing it into two components having orthogonal planes of polarization which are applied to two photo-detectors the signals from which are combined to give an output representing the deviations. In a first embodiment, Fig. 1, the laser 1 produces a beam 2 which defines a reference axis. A sensing member 3 carries a linear polarizer 31 and the beam is analyzed by a polarizing beam-splitter 5 dividing the beam into two orthogonally polarized components which are detected by photocells 61, 62. The outputs of the photo-cells are amplified, and applied to difference and sum circuits (73, 74) and the resulting signals are applied to a ratio circuit (75). The polarizer 31 is arranged so that when the surface sensed by member 3 is in the desired plane the plane of polarization 12 is at 45 degrees to the planes of the analyzed components thus providing equal signals from the detectors and a zero output from the ratio circuit. Deviations of the surface are followed by sensor 3 and cause rotation of the plane of the polarizer 31 and produce a positive or negative output from the system. In a second embodiment, Fig. 2 (not shown), the laser beam is passed through a fixed linear polarizer (11) and the sensor carries a polarization rotator comprising a quartz wedge cemented to a glass wedge. The amount of rotation produced depends on the thickness of the quartz in the optical path and therefore varies with the position along the length of the wedge at which the laser beam is incident. Transverse displacement of the wedge relative to the optical axis by the sensor therefore results in a rotation of the plane of polarization of the beam. The beam-splitter used in the analyzer is a Woolaston prism. .In an arrangement for measuring twist Fig. 5 (not shown), the laser beam passes through a linear polarizer (31) mounted on the sensor and then through a Faraday modulator (9) fed from the output of the processing circuitry (7) to null the effects of rotations of the polarizer. The drive current to the Faraday modulator is used to provide the output indication. An arrangement similar to the embodiment of (Fig. 2) but with the analyzer of Fig. 1 may be used to measure deviations in the ways of machine tool Fig. 6 (not shown). A second compensated quartz wedge of opposite hand may be used to adjust the plane of polarization to produce a zero reading when the rays are at the desired distance from the optical axis. In a further arrangement Fig. 7 (not shown) the laser may be arranged to be laterally displaced, the beam then being polarized and passed through two oppositely handed compensated quartz wedges to the analyzer. The eccentricity of a bore in a hollow cylinder may be measured by means of the arrangement of Fig. 8 in which a laser 1 produces a polarized beam 2 which defines the optical axis. A compensated quartz wedge 31b is mounted on a pair of leafsprings 36 secured to a bearing 35 carried in a tube 34 readily slidable along the bore of the test object 1. The wedge 31 carries a probe 37 which bears on the inner surface S of the bore. If the bore is concentric with the optical axis rotation of the wedge produces no radial displacement and thus no variation in the plane of polarization. Lateral displacement of the polarized laser beam incident on a fixed compensated quartz wedge followed by an analyzer may be produced by a movable lens which may be mounted within a hostile environment Fig. 9 (not shown). In a further arrangement the beam passes through a first compensated quartz wedge to a retroreflector which returns it through a second, oppositely handed quartz wedge to the analyzer. The wedges are displaced together to produce an additive effect. A lens 16 and a mica quarter wave plate are placed in front of the retroreflector. This arrangement may be modified by directing a diverging beam of radiation from the laser to the retroreflector system which is arranged to be laterally displaced by the sensor. In another arrangement a divergent beam from a laser is passed through an optical element to a retroreflector which returns it through the same element to the analyzer. The optical element comprises two oppositely handed quartz wedges cemented together and is laterally displaced by the sensor. Errors due to the varying sensitivity of the photo-cells over their surface area can be eliminated by enclosing them in integrating spheres so that the same response is achieved irrespective of the direction of incidence of the radiation. The output of the system may be indicated by a meter or a chart recorder or may be used for control purposes.