Patent GB-723534-A (GB)
IPC: G03G15/22
Abstract: 723,534. Optical image projection system employing electrophotographic " intermediate film " technique. FERRANTI, Ltd. Aug. 18, 1952 [Aug. 21, 1951], No. 19640/51. Classes 97 (1) and 98 (2). [Also in Group XL (c)] An image projection system comprises one or more conductive plates each bearing a layer of photo-conductive insulating material, a charging chamber in which the surface of said layer is uniformly charged whilst shielded from light, an exposure chamber in which the charged surface is exposed to light from the object to be optically projected, a development chamber in which finely-divided powder is deposited on the exposed surface adhering thereto to form a visible image in conformation with the electrostatic image formed by exposure, a projector for projecting each said visible image on a viewing screen, an erasing position in which the visible image is destroyed by removal of the powder, and an endless conveyer by which each of the plates is conveyed repetitively through the charging, exposure, and development chambers and the projector and erasing position in that order. If more than one plate is provided two or more projectors may be used to project simultaneously on to a common viewing screen successive images whereby movements in parts of said object are indicated by non-registration of the images of that part. As shown, an endless conveyer belt 13 trained about vertical rollers 14 intermittently driven by a motor 16 through a number of treatment stations with a dwell at each, during which the device there is operated under the control of belt-drive actuated means, and guided by a pair of vertical rollers 15 engaging only the belt margins to follow a re-entrant course, carries a series of electrophotographic plates 17, comprising a metal support, a conductive oxide coating, and a covering coating of amorphous selenium, and each secured to the belt by a clip at top and bottom, successively through (1) a plate charging station 20 having corona discharge needles 24 at a positive potential of 10 kv.; (2) a camera 21 having a shutter 25 which photographs the screen 11 of a P.P.I. tube 12; (3) a powder developing chamber 22 as described and claimed in Specification 723,539 in which a fine homogeneous light-colour powder is forced from a conical container 26 to form a cloud and the positively charged particles of which are caused by the field between the grid 28 at +220 v. and the plate to adhere to the dark selenium-coated face of the plate 17 in direct ratio to the charge dissipated by the exposure to yield a positive picture; (4) an optical projection unit comprising three episcopic projectors 30 which project on to a screen 10 three registered images of the trace at successive times on the screen of the P.P.I, tube; and (5) a cleaning station 31 at which the loosely adherent powder image is removed by an air blast or a stream of coarse granular particles from a jet 32 or by wiping; the charging, exposing, and developing stations 20, 21, 22 being located in a light-tight housing 23. In modifications, instead of a shutter for the camera 21, exposure may be terminated by blanking out the scanning beam of the tube 12; the grid of the developing apparatus may be raised to a negative potential or dispensed with so that the powder is deposited in inverse relation to the loss of charge to form a negative picture; the " present time " state of the P.P.I. trace may be shown on the screen 10 by projecting in superposition with the other images an image of the screen of a slave P.P.I. tube. Instead of a belt conveyer, a series of connected trolleys running along a closed track may be used to carry the plates 17. If the radar trace comprises echo signals on a linear time base trace, the conveyer may be caused to dwell in each station for several scan periods so that the signal pulses are emphasized relatively to the noise pulses. In an alternative arrangement in which electrophotographic plates having a transparent base are used, the camera and developing chamber may face each other on opposite sides of the conveyer belt which may be transparent or apertures with the selenium coating towards the developing chamber, exposure being effected either slightly before or simultaneously with the powder application. Instead of a number of plates 17, a single plate may be used when a single projector 30 only is used.