Patent GB-966387-A (GB)
Title: Improvements in or relating to apparatus for examining the optical properties of a surface layer of a transparent material or medium
IPC: G01N21/43
Filed: 1960-09-01
Abstract: 966,387. Polarizing apparatus. COMPAGNIE DE SAINT-GOBAIN. Sept.1, 1960 [Sept.1, 1959], No.30220/60. Heading G2J. An apparatus for examining the surface layers of a transparent material comprises means for producing a beam of light, means for directing the beam on to the surface of the material at a selected angle of incidence such that at least some rays of the light enter the material through that surface and travel through a very thin superficial layer of the material in a path localised along said surface before they emerge through the latter, means for obscuring parasitic light rays and means for receiving and observing the light which has traversed the surface layers of the material being examined. As shown, the apparatus comprises a lamp 1 with a rectilinear filament 6, the lamp being axially displaceable and being located above a slot 5 which extends parallel to the length of the filament 6. The width of the slot 5 is adjustable by screw operation of one of the slit jaws 7. The flat beam of light issuing through the slot 5 passes through a polarizer 9 having its plane of polarization at 45 degrees to the medial plane of the beam. The polarized beam is then incident on an optical element 11 which totally reflects the beam at 12 on to the stressed sample 14 at an angle of incidence such that the refracted rays within the specimen have an angle of refraction of substantially 90 degrees. Between the surfaces of the optical element 11 and the sample 14 there is disposed a liquid having a refractive index between those of the sample and of the element 11, A slot 16 in the element 11 is filled with a light absorbent material, such as pitch, to screen off parasitic light rays which may be produced by reflection of part of the incident light beam from the surface of the sample, or by diffusion in the sample of some of the refracted rays. Some of the refracted rays traverse the surface layers of the sample 14 and penetrate again into the optical element 11 and are totally reflected at 17 into a tube 18. This tube 18 contains a compensator 19 which, as shown in Fig. 2, comprises a parallel faced plate 20 having its optic axis 21 at right angles to the optic axis 23 of a prismatic plate 22, the optic axis 23 being parallel to the refractive edge of the plate 22. The compensator is located with the refractive edge of the prismatic plate 22 in the plane of emergence of the light rays and the plate is so arranged that fringes due to the stresses in sample 14 are produced in a plane spaced from and parallel to the face 24 of the plate 22. In this plane is disposed a graticule 29 carried by a tube 30, which is rotatable by moving a graduated ring 31. Within this tube 30 is disposed an eyepiece 27 which can be displaced for focusing on the graticule 29. By rotating the tube 30 from a zero position until the graticule coincides with a fringe, the value of the stresses within the sample 14 may be read off directly from the ring 31. A slidably mounted bar 33 is also provided in the tube 18, this bar being formed with two apertures, one of which constitutes a mounting for an objective lens 32 which, when it is located in the axis of the tube 18, projects an image of a point marked on the surface of the sample into the observation plane of the eyepiece. Alternatively, the stresses in the transparent sample may be evaluated by using a compensator comprising relatively movable quartz wedges which are moved to restore the fringes to their original position.