Patent GB-534812-A (GB)
Title: Improvements in or relating to light polarization
IPC: F21S8/10
Filed: 1939-06-13
Abstract: 534,812. Polarizing apparatus. INTERNATIONAL POLAROID CORPORATION. June 13, 1939, No. 17271. Convention date, June 29, 1938. [Class 97 (i)] A light-polarizing sheet is constructed so as to resolve a beam incident on one surface into differently polarized undiffused components transmitted in directions which are different but may be closely alike ; it may comprise a number of layers each having the same index of refraction for light vibrating in one plane, and different indices for light vibrating in a perpendicular plane, total reflection being caused at contact faces of the layers, and it may be combined with means, preferably in the form of a sheet, for producing the same state of polarization in both components. Fig. 2 shows a sheet comprising parallel layers 10 of isotropic material assembled with thinner layers 12 of birefringent material having its higher or lower refractive index equal to that of the isotropic material. The thicknesses and inclination of the layers to the surface of the sheet may be such that each ray normal to the surface impinges once, and once only, and from the high index side, upon a contact face between the layers, and is reflected thereby. Various resins, cellulose compounds, crystals &c. are set out in the Specification as suitable for the layers ; all the layers may be birefringent. The alternate layers may be formed by assembling birefringent layers having applied to their faces plasticizers which produce an isotropic state in the surface portions thereof, and the change in index of refraction so produced may be gradual; if the layers have minute oriented elements therein, the orientation may be destroyed in the surface portions by heat, or by a solvent &c. Modifications are described wherein the broader parallel layers have convergent lenticules on the incident surface of the sheet, wherein two sheets having layers as in Fig. 2 are superposed, the layers in the two sheets being inclined in opposite directions, and wherein the sheet is formed otherwise than by parallel layers. Fig. 7a shows layers 72 of one material assembled in other material 70 ; the beam reflected at the face 76 passes through the face 78 without reflection. The layers 72 may be omitted and replaced by similarly placed surfaces of different materials having the shapes of the portions 70. Fig 9a shows sodium nitrate crystals 90 in a medium 92 having a refractive index equal to the higher refractive index of the sodium nitrate, the crystals being cut at d so as to reduce interference by one crystal with the light deflected by an adjacent crystal. Another construction wherein, as in Fig. 9a, the separation of the components is effected by refraction is described. The polarizing sheet may be combined with a quarter-wave or other fractional-wave plate, in the form of a cellulosic sheet to render the components circularly polarised in opposite senses. Alternatively, it may be combined with means for producing the same state of polarization in the two components Such means may comprise a half-wave uniadial birefringent sheet having its principal axis normal to its surface, a fractional-wave plate being positioned between the polarizing and half-wave sheets, or it may comprise a sheet, Fig. 3, built up of layers of birefringent material 54 and of isotropic material 52, the layers lying in planes substantially parallel to the direction of transmission of one of the components so as to alter its polarization. A built up sheet comprising reflecting layers is also described. The sheets may be used in conjunction with car headlights or other lights for reducing glare. Specification 514,860 and U.S.A. Specification 2,031,045 are referred to.