Patent GB-707060-A (GB)
Title: Apparatus for the production of polarized light, particularly for searchlights and like illuminants
IPC: F21S8/10
Filed: 1951-02-26
Abstract: 707,060. Polarizing apparatus. GEFFCKEN, W., and SCHRODER, H. Feb. 26, 1951, No. 4561/51. Class 97(i) Apparatus for use with a large light source such as a searchlight, to produce polarized light comprises a plurality of beam-splitting interference polarizers, each consisting of a plurality of alternate layers of high or low refractive index located between transparent prismatic bodies arranged to form a single compact body presenting parallel faces to all light rays travelling therethrough, the polarizers dividing the beam into two perpendicularly polarized components, means being provided to rotate the plane of polarization of one beam into that of the other and to deflect the separated component into the original direction of the beam. Fig. 4 shows a system in which the light reflected by the polarizing layers is returned to the source. As shown the interference polarizer comprises a plurality of superposed layers 21 of alternately high and low refractive index cemeted to the joining surface of two prismatic plates 20 to form a continuous layer 22. Light from the source 1 rendered parallel by a parabolic reflector 3 falls on the faces 24'of the first plate which are perpendicular to the beam. The light falls at the appropriate angle of incidence on the polarizing layer 22, and that transmitted by the layer 22 is polarized in the plane of incidence while that reflected thereby is polarized perpendicularly to said plane and emerges from the faces 23 of the first plate. A mirror 8 reflects this beam back into the system whence it is replaced by the layer 22 and mirror 3 to form image adjacent the source, a phase plate 10 being provided to rotate the plane of polarization of the beam through 90 degrees on a double passage. The light from the image can therefore be transmitted by the layers 22. Modifications are described in which the mirror 8 is omitted, the layer 22 being of V or zig-zag shape so that part of the layer reflects the reflected beam back to the source. Modifications are described in which the mirror 8 is cylindrically concave and a small convex cylindrical mirror is placed before the source to render the reflected beam parallel and direct it back through the polarizer. Other modifications are described in which the reflected beam is passed through a phaseretarding plate or plates carried by the polarizing assembly and is directed by suitable reflectors into the same direction as the transmitted beam. As shown in Fig. 8 the layers 21 are cemented between two prismatic plates the incident surfaces 24 of which are perpendicular to the incident light. Such light as is transmitted by the layers 21 emerges from the surfaces 24<SP>1</SP> while the perpendicularly polarized reflected light is reflected by a surface 25 of the prism to pass through the layer 21, the plane of polarization being rotated through a rightangle by a phase plate 30 placed before the reflecting surface 25. As shown a cylindrical telescopic lens system 31, 32 is provided to concentrate the light on to the surfaces 24. If this system be omitted the surface 25 may be made reflecting on the outside to direct the beam on to an inclined reflector whence it passes to a further polarizer. Alternatively a reflecting prism may be provided on the surface 25 to return the light to the source. Fig. 10 shows a modification in which the beam reflected by the polarizing layers 36 is reflected forwardly by the adjacent layer 36<SP>1</SP> to pass through a phase plate 38 to emerge parallel with the transmitted beam. Reflectors 39 return the non-effective parts of the beam to the source, or the telescopic system of Fig. 8 may be employed. To adapt this system for divergent light the prisms may be shaped to be located on a circular arc about the light source so that each. handle of incident beams strikes the polarizing layer at the correct angle. Alternatively two prism arrays as shown in Fig. 10 may be placed one behind the other. The layers of low refractive index may be of silicic acid, alkaline earth fluorides or cryolite, whilst the layers of high refractive index may be of metal chlorides such as lead chloride or thallium chloride, of the sulphides of zinc or cadmium, or of metallic oxides such as those of titanium, antimony or tin.