Patent GB-874392-A (GB)
Title: Improvements in or relating to optical measuring apparatus
IPC: G01B11/16
Filed: 1957-09-05
Abstract: 874,392. Polarising apparatus. COMPAGNIE DE SAINT-GOBAIN. Sept. 5, 1957 [Sept. 6, 1956], No. 28120/57. Addition to 827,288. Class 97(1) The apparatus of the parent Specification having beam directing means for directing a plane polarised light beam obliquely against one face of a specimen and means for measuring the bi-refringence of the specimen is improved by including in the beam directing means an element whereby a beam of polarised light initially directed perpendicular to said face is redirected at the required oblique angle of incidence and by including in the measuring means an element whereby the emergent light beam can be directed perpendicular to said one face. In the form shown in Fig. 1, the apparatus comprises relatively movable coaxial tubes 3 and 9 of which tube 3 supports a lamp 7 and collimating lens 8 illuminating a plane polariser 5 carried by a transparent plate 4. The underside of plate 4 carries totally reflecting prisms 41 one of which directs plane polarised light through prism 1 on to the test plate P, while the other receives the light after reflection at A from the lower surface of plate P and directs it as at 20 parallel to the common axis NN 1  of tubes 3, 9. At 10 is a prism which changes the direction of beam 20 so that the latter passes through Babinet compensator 13 and analyser 14 into the eyepiece 15. By making observations in planes of incidence coinciding with the orthogonal principal stress planes, the main stresses in plate P may be determined the apparatus being turned 90 degrees about its axis for this purpose. Simultaneous measurement of the bi-refringence of plate P in two orthogonal directions can be obtained with the modified form of Fig. 2, in which is provided two illuminating systems 7, 8 with their axes parallel and in planes intersecting the axis NN 1  at right angles, two pairs of reflecting prisms, one pair co-operating with each illuminator for directing plane polarised light on to plate 9 and directing the reflected light towards the eyepiece 15, a doubly reflecting prism 10 for directing the light from one prism pair along the eyepiece axis and a doubly reflecting prism 101 for directing into the eyepiece the light from the other prism pair after it has been deflected by the prism 26 shown in Fig. 9. This prism is disposed in a plane normal to the axis NN 1  and comprises a transparent sheet having a bevelled edge 27 which deflects horizontally the light beam 201 from the co-operating prism pair so that it is reflected at 29 and at 28 into the doubly reflecting prism 101. A further modification is described and shown in Fig. 6, whereby the main stresses in plate P can be measured. A single illuminating system directs plane polarised light via reflecting prism 41 and prism 1 on to plate P and the reflected light is received by totally reflecting prism 411 and passes into a prism 32 so shaped that it directs the beam downwardly parallel to axis NN 1  and in a plane of incidence normal to that of the beam from the illuminator. The light is then directed by totally reflecting prism 4111 so as to fall for the second time on to the prism 1 and the plate P. After reflection from surface 19 the light strikes a reflecting face of prism 1 so that it returns along a path substantially parallel to that of the incident light and is directed by prism 4<SP>111 </SP>on to doubly reflecting prism 10 and thence into eyepiece 15.