Patent GB-1052841-A (GB)
IPC: G01N21/23
Abstract: 1,052,841. Polarizing apparatus. COMPAGNIE DE SAINT-GOBAIN. Aug. 2, 1963 [Aug. 2, 1962], No. 30762/63. Heading G2J. [Also in Division G1] A method of measuring phase differences between the vibrational components of a beam of polarized light with reference to interference fringes produced as a result of the phase differences, involves the use of a continuously and periodically varying additional path difference between the components so as to continuously and periodically to displace the fringes, conversion of the light intensity at least one point in the zone of formation of the fringes into an electrical signal which varies with the intensity, and measurement of the phase of this signal. The method may be applied to the examination of the stress variations in or at the surface of a transparent doubly-refracting medium. The apparatus for measuring phase differences within the sample is shown in Fig. 11. Light from a tube 7, comprising a mercury discharge source 9, condenser 10 Fig. 12 (not shown), lens system 11 and filter f, is brought to a focus at some point within the sample 8. The sample is masked at 19, 20, 21 to prevent unwanted diffusion at the surface and the incident ray is stopped down by diaphragms 20, 21. The light receiving tube 12 contains a lens system which produces an enlarged image of a point within the sample. The variable compensator comprises a quarter-wave plate 13, whose axes are perpendicular and parallel to the planes of polarization of the light, attached to the end of the tube 12, and a continuously rotating analyser disc 14 driven by motor 15, the phase difference introduced varying periodically between -#and +#. The image, masked from stray light at 16, is investigated by movable photon guide plates 18, feeding to photomultipliers 17 which produce electrical signals corresponding to the intensities of the light at the points investigated. The law governing the variation of the additional path difference is assumed to the linear with time, but if it is not perfectly linear, an adjustment to the compensator can be made by altering the position of the analyser on its driving spindle. To obtain the path difference between any two points A, B along a light ray within the specimen, Fig. 4 (not shown), two corresponding points A, B, Fig. 1 (not shown), in the image are chosen to be investigated. The time for a fringe to be displaced (due to rotation of the analyser) from A to B, is proportional to the path difference between those points. The variation of intensity at A and B is detected by the two photomultipliers 17 and the electrical signals fed into the circuit of Fig. 17 (not shown), where they are amplified by resonant circuits and the final signals can be displayed on an oscilloscope or fed to a reading instrument. Measurement of path differences between different points along a plane parallel to the face of entry of the sample can also be made and the variation indicated by an oscilloscope or by a demonstration device. The latter receives rectified signals from the circuit of Fig. 17 (not shown) which amplifies signals from the photomultipliers. The demonstration device comprises a lamp 5 Fig. 7 (not shown) fitted on the end of an arm 6, which is rotated at the same speed as the rotating analyser. Since the signal received by the lamp is rectified, the lamp is energized during every alternate half-cycle. If the specimen is moved in a plane parallel to its surface and there is a stress variation between the points (e. g. A, A', Fig. 4 (not shown)) in the specimen plane being examined, the arc of energization of the lamp will swing through an angle α Fig. 9 (not shown) representative of this phase difference, and therefore stress variation, between the points. A second lamp added to the equipment will indicate the phase differences between points (e. g. B, B') in another specimen plane. Apparatus especially suitable for examining stress variations between points on the surface of the specimen is also described Fig. 19 (not shown). This comprises a monochromatic source S, an optical system L 1 , a polarizer P, the specimen E, a Babinet compensator B, a diaphragm D, at which the fringes are formed, a “ wave plate, the rotatable analyser A, a lens system L 2  and a detector M.