Patent GB-840052-A (GB)
Title: Improvements in or relating to the reproduction of visual images
Inventor: ALLEN GORDON STANLEY JAMES
IPC: G03B27/73
Filed: 1957-10-04
Abstract: 840,052. Photo-electric control of photographic exposures. CROSFIELD Ltd., J. F. Sept. 29, 1958 [Oct. 4, 1957], No. 31183/57. Class 40 (3). The invention of Specification 835,111 is modified by scanning simultaneously four transparencies (three colours and black) each with associated light-sensitive devices and each with its own source of light, at least one source being modulated in accordance with the output of a computer fed with signals from the light-sensitive devices. In Fig. 1 light from lamps 10R, 10G, 10B and 10BL falls on transparencies 12R, 12G, 12B, 12BL, the last named being obtained by exposure of the multi-colour original to white light. Each transparency has an associated photographic layer to be exposed 14R, 14G, 14B, 14BL (unlike Specification 835,111 where only one is exposed at a time), and each layer is backed with a filter to absorb light coming from the back to which the layer is sensitive (usually blue light). Photo-multipliers 20R, 20G, 20B, 20BL thus give outputs which are proportional to the instantaneous brightness of the corresponding lamps and to the transmission factor of the corresponding transparency at the point being scanned at that instant. These outputs are passed through cathode followers and logarithmic circuits to subtractors 30R, 30G, 30B, 30BL where the logarithms of the brightness variations of the lamps are subtracted in the form of signals from photo-cells 26R, 26G, 26B, 26BL. The outputs from the first three subtractors are combined in the same way as in Specification 835,111, with the addition of a signal from unit 36 representing the minimum of 30R, 30G, 30B at any instant. The combined outputs are passed to antilogarithm and limiting circuits as in Specification 835,111 and thence to control the brightness of the lamps 10R, 10G, 10B. The " black " output from 30BL is combined with the above " minimum " signal in the antilogarithm circuit 44 and applied via a limiter to control the " black " lamp 10BL. The E.H.T. supply to the photo-multipliers 20 is preferably pulse modulated for ease in dealing with the signals. The simultaneous scanning is carried out by mounting the four transparencies on a table 46 (Fig. 4) which runs on balls 48 in grooves on a base 52 and is caused to reciprocate by an hydraulic piston 54. The lamps are mounted (Fig. 2, not shown) on arms 59 which are moved at a slower speed at right angles to the table by a further hydraulic piston 64. Modifications.-The photo-cells 26 can be omitted and the appropriate signals obtained directly from the outputs of the limiter circuits or from the other two channels as in Fig. 2 of Specification 835,111. The system can be used for successive two-stage masking (Fig. 5, not shown) and for making screened positives and negatives as in Specification 835,111 or by using photographic plates already having a screen image exposed on them. A mask can be inserted between the transparency and the emulsion to compensate for a variation in the density of the picture or to remove part of it. The above scanning arrangement can be replaced by a series of cathode-ray tubes or by a helically moving hollow transparent cylinder with the transparencies outside and photomultipliers inside or vice versa.