GB521378A - Improvements relating to optical polarizing devices - Google Patents

Abstract

521,378. Optical polarizers. LAND, E. H. Nov. 17, 1938, No. 33449. Convention date, Nov. 20, 1937, [Class 97 (i)] In a polarizing device of the kind in which the polarizing medium divides a beam of non- diffused incident light into two differentlypolarized components, one of which is diffused and the other transmitted without diffusion, a lens is associated with the polarizer to form an image of the source of light the area of which is a small fraction of the cross-sectional area of the beam and a bi-refringent element is placed in the image plane, either in the area occupied by the image or in the area not so occupied, said element being such as to produce the retardation required to bring the diffused and non-diffused components to the same state of polarization. Preferably, instead of using a single lens, the area of the beam is covered by a number of co-planar lenses, and a bi-refringent element is associated with each lens. The Fig. shows a polarizer forming a closure for the front of an automobile head-lamp 11. The parallel rays from the lamp reflector are incident on a polarizing sheet 30, which is of the above-mentioned kind and may comprise a set suspension of birefringent crystals in a transparent isotropic matrix the refractive Index of which is equal to one of the refractive indices of the crystals, so that one of the polarized components passes directly through the sheet and the other is diffused by multiple internal reflections. Adjacent the sheet 30 is a glass &c. sheet 32 having its surface formed as a number of spherical or cylindrical lenses 36, and behind the sheet 32 in the focal plane of the lenses is a sheet 38 provided ;at the foci of the lenses with areas 40 of bi-refringent material of such thickness as to produce a half-wave retardation of the focused rays. The areas 42 traversed by the diffused polarized component, which is not focused by the lenses 36, is isotropic, so that on emerging from the sheet 38 the two components have the same state of polarization. A sheet 34, similar to the sheet 32 and having similar lens elements 36, may be provided to collimate the non-diffused component. A quarter-wave plate may be associated with the sheet 30 to produce two circulary-polarized components. A polarizing sheet may be provided adjacent the lens surfaces of the sheet 34 and so oriented as to block the small part of the diffused component which has traversed the areas 40 and therefore has characteristics different from those of the rest of the beam. Both the areas 40 and the areas 42 may comprise bi-refringent material, provided the difference between their retardations is such as to produce the desired result. In a modification, the sheet 30 is arranged between the lenses and their focal plane.