Patent GB-831620-A (GB)
Title: Improvements in or relating to photographic colour reproduction apparatus
IPC: G03B27/73
Filed: 1957-09-24
Abstract: 831,620. Production of photographic colour separation negatives. KILMINSTER, A. R. Sept. 17, 1958 [Sept. 24, 1957], No. 30012/57. Class 40 (3). In apparatus for the preparation of colour separation photographic negatives from a coloured opaque original or a colour transparency, from which three coloured and one black negatives are required, the amount of light incident on the photographic plate, exposed for a particular colour, is determined by a modulating signal derived from signals representing the three primary colours of the original. The light source 4, Fig. 1, is arranged to scan sequentially the original 1 via the condenser lens 5 and electrically operated modulating means 6. Light is reflected from the original to expose a colour separation negative, e.g. a yellow printer, in the camera 2 via a blue filter; in addition light is reflected to the three photomultipliers 7, 8'and 9 via associated filters blue 10, green 11 and red 12 respectively. The photomultipliers produce signals which are a function of the intensity of the light source and the density of their respective colours in that area of the original from which the light is reflected. These primary colour signals are amplified, Fig. 2, and applied to the deflection plates of the C.R.T. 42, the red signals being applied to the plates 60 and 58, Fig. 7, and the blue and green signals to plates 59 and 57 respectively. As can be seen the tube spot takes up a position which is a function of the colour of the area being scanned. The density of the screen 61 interposed between screen face and photo-multiplier 62 varies from area to area, and the signal produced in the multiplier is a function of the spot position, being a maximum for grey tones and a minimum for the colour for which the plate is being exposed (yellow). The tube screen, and photomultiplier act as a colour analysing device producing a colour signal which is applied after amplification to the deflecting plate 77, Fig. 10, of a second C.R.T. 72, the plates 78 and 80 are earthed and plate 79 is connected via the colour selection switch 41 to the photomultiplier 7 supplying the yellow signal. Colour and tone correction is provided for this signal by the mask 75 interposed between the tube 72 and the photomultiplier 76. The position of the spot on the face of tube 72 is determined by the density and colour components of the area being scanned, line D, Fig. 10, indicates the grey scale of densities from white to black, F indicates the scale from white to full colour of the colour being printed and E the density scale of the remaining colours. The optical density of the mask 75 is varied from area to area to provide the required tone and colour correction. The output of the photomultiplier 76 modulates the intensity of the light source 6 in accordance with this corrected signal. The screen 61 is changed for each colour being exposed, the screen 75 is fixed for each colour but changed during the production of the black printer when a black signal is derived from a combination of the three primary colours. Two further circuit arrangements are described in which the three colour photomultiplier signals are changed to logarithmic signals before application to the correcting circuits. In one of the arrangements the correcting C.R.T. mask and photomultiplier are dispensed with and the correction is applied to the valve driving the light modulating means.