GB790683A - Improvements in or relating to photographic colour reproduction systems - Google Patents

Abstract

790,683. Photo-electric control systems. HUNTER-PENROSE, Ltd. Oct. 28, 1955 [Nov. 18, 1954], No. 33382/54. Class 40 (3). In an arrangement for producing colour and tone corrected colour-separation photographic negatives (e.g. for use in the production of printing plates) by exposing in camera 2 a colour separation plate to light from a coloured original 1 the latter is illuminated by scanning in line sequential fashion by means of a spot of light from a lamp 4, or from a cathode-ray tube 130, and some of the light from the original is directed via blue, green and red colour filters on to photo-electric cells 14, 15, 16 respectively, the signals produced by which are combined in predetermined proportions in a control circuit 22 to produce a signal which is employed to modulate the intensity of the scanning spot via a light modulator 6 or via the control grid of tube 130. In this manner the light response of the original is modified by e.g. increasing the illumination of colours which do not transmit (or reflect) sufficient light to permit the production of adequately dense negatives when the latter are employed in the production of printing plates for use with yellow, cyan, magenta and black inks the colours of which do not conform to the theoretical ideal. For the production of each negative a separate plate is exposed to a complete scanning of the original, blue, red and green colour filters being employed in the case of the plates from which the yellow, cyan and magenta printing plates are to be prepared, and the modulation of the scanning light spot is effected by a (switch) selected output of the control circuit 22 appropriate to the colourseparation negative to be produced. The circuit 22, details of which are given with reference to Figs. 3A, 3B and 12 (not shown), comprises amplifiers for each of the photo-electric cells 14, 15, 16 (the signals generated by the latter being rendered suitable for A.C. amplification by means of a light " chopper " 11, not shown in Figs. 9 and 10), the amplifier outputs being combined to produce " process " signals by eliminating from the primary colour signals components representative of grey tones in the original and the " process " signals then being corrected by combining them with the primary colour signals to produce the signals for operation of modulator 6. In the embodiments of Figs. 2 and 9 the original 1, which is opaque in the former and translucent in the latter embodiment, is clamped between transparent sheets 3, 126 and scanning is effected by movement of the housing 8 (Fig. 2) or the housings 8 and 125 (Fig. 9) along the rails shown (to effect line scanning) and in a direction at right angles thereto by means not shown, at the end of each line scan, to effect frame scanning, constancy of focus being assured by means of rollers 9 rolling on the sheets 3, 126. As shown in Fig. 9, the light source 4, modulator 6 and associated optical devices are contained in a separate housing 125 integral with the housing 8, which contains the remainder of the apparatus of the housing 8 (Fig. 2), the light source being arranged to provide two beams via mirrors 128, 129 the former of which is directed on to the photographic plate (not shown) and the latter, which is arranged to be slightly in advance of the former beam, directed into the housing 8 (containing the photo-electric cells) to control the production of the modulating signal for application to the modulator 6. If desired an. unmodulated light beam from a separate source may be employed for the latter purpose. In the embodiment of Fig. 10 the original 1 is a " pure dye image transparency " and since this is non-light scattering it may be positioned, as shown, close to the scanning raster on the screen of tube 130. In a modification of this embodiment, for use when the original is " light-scattering," the latter is backed by a diffusing screen on to which the luminous raster is projected by means of a lens system (Fig. 11, not shown).