Patent GB-1362340-A (GB)
Title: Method of measuring small objects
IPC: G01B11/00
Filed: 1972-07-14
Abstract: 1362340 Holograms; spacial filtering BBC BROWN BOVERI & CO Ltd 14 July 1972 [17 July 1971] 33020/72 Heading G2J [Also in Division G1] The dimensions of a test object and a reference object are matched by producing Fraunhofer diffraction patterns from each object and adjusting the real or apparent size of the reference object until the diffraction patterns are congruent. As shown, Fig.1 light from a laser 1 is split into two parallel beams 4 and 6 which respectively illuminate the test object 5, which is a wire, and the reference object 7, which is a slit of adjustable width. A lens 12 produces the Fraunhofer diffraction patterns of the two objects and a photo-multiplier or photo-diode 13 measures the intensity of the radiation at a point displaced from the centre of the pattern. The beams incident on the detector 13 are alternately switched to produce an alternating signal whose amplitude is proportional to the difference in intensity of the incident parts of the diffraction patterns. The signal is applied to a phase sensitive detector 19 connected to a servo unit 20 which provides a control voltage to adjust the width of the slit 7 by means of a Piezo-electric transducer 9 until the intensities of the detected portions of the diffraction patterns are equal, i.e. the patterns are congruent. With circular test objects the reference object may comprise a fixed circular aperture and a lens may be used to produce a real image the size of which can be varied by adjusting the longitudinal position of the lens by Piezo-electric transducer Fig.4 (not shown). The intensities of the diffraction patterns can be measured at a second off centre point to permit adjustments of the relative intensities to be made. The reference object may be complementary to the test object as described above or may be similar, the diffraction patterns being the same in both cases. The beam switching can be achieved (a) by using a beam splitter 3 which orthogonally polarises the emergent rays and a rotating polariser 16 (or 15), a linear polarizer 14 being inserted between the laser 1 and the splitter 3 to adjust the relative intensities of the beams; (b) by using a rotating shutter at position 15 or 16; (c) by using an orthogonally polarizing beam splitter 3 and inserting a Pockels cell followed by a polarizer in each of the beams; (d) by using a gas laser subject to Zeeman splitting, so that the plane of polarization rotates with the magnetic field, and an orthogonally polarizing beam splitter 3. In an arrangement to reduce noise, rotating polarizers directed orthogonally are inserted into the two beams and an orthogonally polarizing beam splitter is placed in front of two photo-detectors which therefore receive anti-phase signals which can be subtracted to eliminate common mode noise. Electro-optical modulators may replace the rotating polarizers. Other light sources such as laser diodes, luminescent diodes and lamps of limited spectral distribution may be used together with narrow slits to provide spacial coherency and filters to provide a narrow spectral width as necessary.