Patent GB-944247-A (GB)
Title: Improvements in or relating to light valve systems and electrical signal transducer apparatus therefor
IPC: H04N5/82
Filed: 1959-12-22
Abstract: 944,247. Projection systems: gratings. GENERAL ELECTRIC CO. Dec. 22, 1959 [Dec. 24, 1958; March 13, 1959], No. 43476/59. Heading G2J. [Also in Divisions H1 and H4] Production, of diffraction gratings. In a colour television system (see Division H4) the image information is recorded by forming two or more superposed diffraction gratings in a medium, the lines of one grating extending in a direction normal to the direction of the lines of another grating. The gratings occupy an area corresponding to the television image and at each elementary picture point serve during reproduction to cause the correct proportions of light in two or more component colours to be diffracted through an optical system to create a coloured image in accordance with the original information. The gratings are produced by scanning a medium with electron beams in a discharge tube, reference being made to the production of both intensity and phase gratings. The invention is described in detail however only as applied to the production of phase gratings on a deformable medium where the electric charge laid down by the scanning beam varies at each point in accordance with the intensity of the desired light so as to vary correspondingly the depths of the grating lines. In one application the gratings are created in a deformable transparent film (oil, beeswax or a methyl silicon fluid) carried by a mirror incorporated in light modulating system so as to permit the simultaneous production of an image on a screen. In another application, the gratings are created as a permanent recording, so as to permit reproduction subsequently, by the use of a thermoplastic medium in the form of a record tape comprised of a base bearing cuprous iodide to which a thin film of polystyrene is adhered. The medium is first softened by heat to allow the electric charges to deform the surface. One grating may be created with a line spacing appropriate to the production of green light whilst two further gratings are created with their lines normal to those of the green grating and having appropriate line spacings for red and blue light. Alternatively one grating may be for red light whilst a further single grating normal to the first may have its grating spacing varied throughout in accordance with both blue and green image information so that at each point it diffracts the appropriate wavelengths along the line joining the blue and green points in the standard chromaticity diagram. Optical apparatus, Fig. 4. Where the diffraction grating 21<SP>1</SP>, 22<SP>1</SP>, are in the form of a permanent recording on a thermoplastic tape 12 the image is reproduced with aid of an optical system in which light from a source 72, 73 projects on to a screen 85 through masking members 74 and 80 between which is located the record tape. The masking members have orthoganally arranged sets of apertures 75, 76 and 81, 82 and the light from apertures 75, 76 is focused on mask 80 by a lens 77, the arrangement being such that, in the absence of diffraction gratings in tape 12, the light falls on the opaque regions between apertures 80, 81 so that no light reaches screen 83. When gratings are present, the light is diffracted and passes through the apertures to reach the screen. Light through aperture 80 is controlled by the grating formed with horizontal lines, whilst the vertical grating controls the light through aperture 81. The aperture sizes and spacings are chosen in relation to diffraction grating spacings to select light of the appropriate colour (green for grating 21<SP>1</SP>, and red and blue for grating 22<SP>1</SP> in the example illustrated). The Specification described the method of determining the sizes and spaces of the apertures to achieve a desired spectral distribution of light, the arrangement being based on the use of the first order diffraction pattern. In a first modification, Fig. 9 (not shown), the masks 74 and 80 each have only a single set of parallel apertures arranged at an angle chosen to provide appropriate components in the horizontal and vertical directions. In a second modification, Fig. 12 (not shown),the elongated apertures are replaced by appropriately sized and spaced circular apertures. Where the diffraction gratings are produced in a medium forming part of a light modulating system (see Division H4) the optical apparatus co-operates with the mirror bearing the medium and is formed in due embodiment Fig. 10 (not shown), of two sets of curved reflecting bars in another embodiment Fig. 13 (not shown) of an opaque member bearing an assembly of circular reflectors interspersed with circular apertures in the member.