Patent GB-1494182-A (GB)
Title: Apparatus for large scale screen display of images
IPC: G03G15/26
Filed: 1976-01-20
Abstract: 1494182 Electro-photographic projection apparatus COULTER INFORMATION SYSTEMS  Inc 20 Jan 1976 02201/76 Headings G2A G2E and G2J Apparatus for projecting an image on a transparent electro-photographic plate, including a photo-conductive coating, on a screen 36 comprises a translatable carrier 20 for at least one plate 16, means to move the carrier 20 stepwise to position the plate 16 successively at a number of stations along the path of carrier 20, the stations comprising (1) a record station comprising a projector for forming an image on the plate 16 and stationary means for charging, exposing and toning the plate 16 to form an image-carrying transparency, (2) a projection station wherein an image of the transparency is projected on screen 36 and (3) an erasing station having means for removing all the toner from plate 16, the recording station having a housing 44 including a wall with a window 46, housing 44 enclosing the means for charging, exposing and toning so that the transparency may be prepared therein. The toner is not fixed. As shown in Fig. 3 an image is formed of the screen of a CRT 12 by an optical system 14 on the plate 16 mounted in a window 18 formed in the disc-like carrier 20. Carrier 20 is driven by a motor 26 in steps, the first step carrying member 16 from the exposure station 1 to a projection station 2, where an image formed on plate 16 is projected by a light source 32 and an optical system 34 on to a screen 36. The next step carries member 16 to station 3, where toner particles are removed, e.g. by a revolving brush 96. Optionally, the member 16 is exposed to a bright light 92 before returning to station 1 to repeat the cycle. There may be more than one (e.g. three) members 16 mounted in windows in carrier 20, and the members may be presented for projection at station 2 at a rate which produces an illusion of movement by persistence of vision at the screen 36. Plate 16, Fig. 2, consists of a substrate 42 carrying a thin film 39 of ohmic material and a thin film 40 of photo-conductive material, e.g. r.t. sputtered CdS. The optical system 14 and the means for charging, exposing and toning are all enclosed in the housing 44, which has a window 46 aligned with the window 18 in carrier 20, in which the plate 16 is supported. A back plate 58 on the housing 44 covers the back of plate 16 so that it may be prepared for exposure in the dark. The plate 16 is charged by corona discharge from a wire 58. After opening a shutter 68 Fig. 1 to make an exposure, plate 16 is flooded with toner (liquid or powder) by a toner device 66. An adjustable iris diaphragm 70 is provided e.g. in the optical system 14, for adjusting the depth of focus. A photo-cell 72, Fig. 3, intercepts at least some of the imageforming beam from optical system 14; it may be formed as a transparent coating on one of the lenses. The corona wire is connected to a supply 56, and the charge produced on plate 16 is measured by a measuring instrument with a probe 76. The charge on plate 16 and the output from photo-cell 72 are compared in a control circuit 78, and when a given relationship is reached, a signal from the control circuit 78 switches off the supply 56. At the same time shutter 68 is opened for a given time; on closure the toner device 66 is operated and a bias is applied to plate 16 to assist toning.