GB835111A - Improvements in or relating to colour reproduction - Google Patents

Abstract

835,111. Photo-electric control of photographic exposure. CROSFIELD Ltd., J. F. April 24, 1957 [April 24, 1956], No. 12545/56. Class 40 (3). A method of obtaining toned or coloured reproductions employs a single light source modulated in accordance with the output of an electrical computing device and serving both to scan uncorrected transparencies to provide information from which the required correction can be computed and also to expose an emulsion in accordance with the corrected information, and separate scanning beams derived from the light source are transmitted through a number of uncorrected transparencies and impinge on a number of light-sensitive devices, the modulated electrical signals from which are applied to the electrical computer which determines the required correction from the signal variations, and the emulsion to be exposed is placed behind the uncorrected transparency for which the correction has been computed so that light passing through the latter to the associated light-sensitive device also passes through the emulsion to be exposed. In the arrangement shown, a rectangular raster formed on the screen of the cathode-ray tube 6 is focused by a lens 22 on a blue transparency or separation negative 25B and is also reflected by mirrors 23, 24 on red and green transparencies or separation negatives 25R, 25G. To obtain the corrected blue photographic plate the element 25B is backed by a photographic plate 26-28 comprising an emulsion 26 sensitive solely to the blue end of the spectrum, and the filter backing 28 is arranged to absorb blue light and to transmit light in the remainder of the spectrum. The light passed by the transparency is collected by the unit 32B and controls a photomultiplier 34B having its output connected to a cathode follower 36B and a logarithmic circuit 38B. Signals from the transparencies 25G, 25R associated with diffusion plates 30G, 30R are fed via cathode followers 36G, 36R to logarithmic circuits 38G, 38R. The output of the circuits correspond to the added logarithms of the spot brightness and of the transmission factor of the element of the uncorrected negative which is being scanned, and to derive a correcting signal from which the intensity component has been removed, a brightness signal derived from a photo-cell 40 is passed to a logarithmic circuit 44 whose output is fed to operate subtractively in circuits 46R, 46B, 46G so that appropriate signals representing the transmission factor are passed to masking circuits 50R, 50B, 50G and also to attenuating and inverting circuits 48R, 48B, 48G which feed signals to the masking circuits of the other two channels-e.g. outputs of 48R pass to 50B, 50G. The masked signals from 50R, 50B, 50G are fed to antilogarithmic circuits 51R, 51B and 51G, the outputs from which are passed to limiters 52R, 52B, 52G and thence to circuits 53R, 53B, 53G which provide brightness control signals applied through switch 16 in position B and gate circuit 18 to the grid of the C.R.T. 6. The corrected photographic plate for red or green is obtained by moving the switch 54 to the appropriate position R or G, and replacing the diffusion plate 30R, 30G by a plate corresponding to 26-28 and provided with the appropriate emulsion and filter. In a modified and simplified arrangement, Fig. 2 (not shown), the signal from one channel, e.g. blue, is corrected by the signal from another channel, e.g. green, passed through an inverter circuit and then to a subtractor circuit operated by the two signals and passing its output via a limiter and switch, similar to 54, to the control grid of the C.R.T. The system may also be used for successive two-stage masking, e.g. by obtaining a corrected yellow positive from uncorrected yellow and magenta negatives, registering the first corrected yellow positive with an uncorrected magenta negative to produce a further mask which is then registered with the uncorrected yellow negative to make a second corrected yellow positive. Screened positives and negatives may be formed by using a contact screen having a dotted density pattern in two mutually perpendicular directions. Alternatively, a ruled screen may be interposed between the lens system and the uncorrected transparency or separation negative. The signal impulses applied to the C.R.T. may be time modulated instead of amplitude modulated with circuit modifications to deal with the output of the photomultiplier which will be time modulated in respect of the light-spot intensity, and amplitude modulated in respect of the transmission factor of the corresponding separation negative. In lieu of the mirror beamsplitting system shown in Fig. 1, the light spot from the C.R.T. 6 may be focused on the three separation negatives by three separate lens systems. Specifications 737,768 and 738,118 are referred to.