Patent GB-637310-A (GB)
Title: Improvements in or relating to color filters
IPC: G02B27/28
Filed: 1947-05-16
Abstract: 637,310. Colour filters and polarizers. INTERNATIONAL POLAROID CORPORATION. May 16, 1947, No. 13223. Convention date, May 31, 1946. [Class 97 (i)] [Also in Group XL (b)] A filter the colour of which can be varied electrically comprises at least one dichroic filter juxtaposed with a Kerr or like cell. For three-colour television (see Group XL (b)) or kinematography a laminate containing three such filters is used. As shown in Fig. 4, the filter comprises three differently coloured dichroic filters 131, 132, 133, which may consist of molecularly oriented polyvinyl material dyed with suitable dyes. Filter 131 is a minus red filter, i.e. it removes the red component of light vibrating in one direction and passes the other components of that light and all components of the light vibrating perpendicular thereto. Similarly the filters 132, 133 are minus green and minus blue, all the filters having their axes parallel. Interposed between the filters are solid Kerr cells 121, 122, 123, consisting of plates of potassium or ammonium dihydrogen phosphate, rubidium dihydrogen phosphate, zinc sulphide or quartz coated on both surfaces with a transparent electrically conducting layer of gold, silver or stannous oxide connected respectively to leads 141, 142, 143. The axes of the layers are so arranged, for example, that vertically polarized light is passed by all the colour filters, light is passed unaffected by the cells when not energized, and the plane of polarization of light is rotated through 90 degrees by the cells when they are energized. Light is differentially absorbed by the coloured layers when it is horizontally polarized. A polarizer 112, for example of the type described in Specifications 548,759 and 525,956, passing only vertically polarized light completes the filter. Voltages of the order of 9000 volts may be applied to the cells 121, 122, 123 and when a white light beam 100 falls on the cell the emergent beam is vertically polarized and of a colour depending on which of the cells is energized. As an alternative the beam 100 may be incident first on the polarizer 112. In the modification shown in Fig. 5, which will give only the colours red, green and blue on energization the dichroic filter 231 comprises two dyed oriented layers so arranged that the first is minus red for vertically polarized light and the second is minus green for horizontally polarized light. A cell 221 is interposed between this filter and a second filter 232 which is minus red for vertically polarized light and minus blue for horizontally polarized light. A cell 222 and a polarizer 212 passing horizontally polarized light completes the assembly. When applied to colour kinematography successive frames of the film are exposed to red, green and blue light passing through a filter of the type described above and are projected through a similar filter, control of the voltages to produce the appropriate colour being effected, for example, by photoelectric cells controlled by light beams passing through the film. The colour filter may also be employed for illumination purposes, and in all the systems described above circularly or elliptically polarized light may be used instead of plane polarized. The invention may also be applied to two-colour or fourcolour systems. Many examples of suitable dichroic dyes are given and those described in Specification 549,668 may be employed. Polarizers of the type described in Specification 605,489 may be used. The Specification as open to inspection under Sect. 91 describes also a stereoscopic colour television system in which two cathode-ray tubes 250, 260, Fig. 8 (Cancelled), at the receiver may each receive colour record images respectively of the right and left eyes. Colour filters 251, 261 with their polarizing axes 253, 263 at right-angles are placed before the tubes and electrically controlled according to the particular colour record being received. The images are projected in superposition on a non-depolarizing screen 270 by optcal systems 280, 290 and are viewed through suitably arranged analyzers 301, 311. Alternatively a single tube and filter may be used together with an additional Kerr cell which rotates the plane of polarization of light from successive pictures. Alternatively the colour filter may be omitted and the system employed for black and white stereoscopic television. This subject-matter does not appear in the Specification as accepted.