# US 4,395,088 A · Color selective filters > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 4,395,088 A - Document ID: US4395088A - Status: Granted - Kind: A - Application number: 06/242,610 - Filed: 1981-03-11 - Granted: 1983-07-26 - Earliest claimed: 1980-03-13 - Assignees: The Marconi Company Limited - Inventors: Kenneth Firth; Richard K. Howard - Web page: https://patentgrep.com/patent/US4395088A ## CPC - G02B 5/203: {having holographic or diffractive elements (diffraction gratings per se G02B5/18; holographic elements per se G02B5/32; generating the spectrum using diffraction elements G01J3/18)} - G02B 5/32: Holograms used as optical elements - G03H 2001/0417: {for recording single beam Lippmann hologram wherein the object is illuminated by reference beam passing through the recording material} - G03H 2001/0439: {for recording Holographic Optical Element [HOE]} - G03H 2222/36: Scanning light beam - Y10S 359/90: Methods - G02B: OPTICAL ELEMENTS, SYSTEMS OR APPARATUS - G03H: HOLOGRAPHIC PROCESSES OR APPARATUS (holograms, e.g. point holograms, used as ordinary optical elements G02B5/32; producing stereoscopic or other three-dimensional effects G02B30/00; diffraction-grating systems G02B27/44; systems using moiré fringes G02B27/60; optical logic elements G02F3/00; stereo-photography G03B35/00; photosensitive materials or processes for photographic purposes G03C; {stereo-photographic or similar processes G03C9/00}; apparatus for processing exposed photographic materials G03D; analogue computers performing mathematical operations with the aid of optical elements G06E3/00; authentication by radiation, of concealed information carried by holograms or diffraction gratings G06K19/16; holographic storage G11B7/0065, G11C13/04; {stereoscopic or other three dimensional effects in television systems H04N13/00}) - Y10S: TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS ## Abstract A color selective filter in the form of a holographic reflector is produced by a method which includes the step of exposing a photosensitive film of gelatine to a monochromatic laser beam. In order to obtain acceptably short exposure times and to enable substantially the whole of the available laser light to be utilized, a relatively narrow laser beam is sequentially scanned across the surface area of the gelatine film in a scanning pattern which consists of a large number of parallel straight lines. The diameter of the laser beam spot is very large as compared with the spacing between adjacent line scans. In this way each point on the gelatine film is scanned a very large number of times. The laser beam 8 is generated by a laser 7 and deflected by two movable mirrors 10 and 11, which are controlled so that the beam scans the whole of the gelatine film 1. ## Claims ### Claim 1 A method of making a colour selective filter in the form of a holographic reflector including the steps of: illuminating a photosensitive film with a beam of light from a coherent monochromatic laser in which the area of the beam spot on the film is small as compared with the total area of the film; moving the laser beam relative to the film so that each point on the area of the film is illuminated a large number of times and the total illumination received by each point is substantially constant; causing a wave derived from the illuminating beam to interfere with the illuminating beam; and developing the film so as to change the latent image thereon into a colour selective filter. ### Claim 2 (depends on 1) A method as claimed in claim 1 and wherein the beam is scanned systematically over the surface of the film in a sequence of line scans with adjacent line scans being spaced apart by a distance which is small compared with any dimension of the beam spot on the film. ### Claim 3 (depends on 2) A method as claimed in claim 2 and wherein the beam is arranged to scan sequentially across the surface of the film in a series of straight lines which are parallel to each other, with movement of the beam from one straight line to the next being accomplished in abrupt steps. ### Claim 4 (depends on 2) A method as claimed in claim 2 and wherein the surface of the film is flat. ### Claim 5 (depends on 2) A method as claimed in claim 2 and wherein the surface of the film is curved to present a concave surface to the illuminating beam. ### Claim 6 (depends on 1) A method as claimed in claim 1 and wherein the photosensitive film comprises a layer of gelatine which is located at the surface of a rigid transparent plate, which surface is opposite the surface of said plate on which the illuminating beam is first incident, so that the gelatine is exposed by the laser beam after the beam has traversed the plate. ### Claim 7 (depends on 6) A method as claimed in claim 6 and wherein the surface of the film which is remote from said glass plate is closely adjacent to a smooth highly reflecting surface. ### Claim 8 (depends on 7) A method as claimed in claim 7 and wherein a thin film of liquid is interposed between said photosensitive film and said reflecting surface, with the refractive index of the liquid being at least approximately the same as that of the photosensitive film. ### Claim 9 (depends on 1, 2, 3, 4, 5, 6, 7, 8) A colour selective filter when made by the method claimed in any of claims 1, 2, 3, 4, 5, 6, 7 or 8.