Patent GB-738118-A (GB)
Title: Improvements relating to colour printing
Inventor: CROSFIELD JOHN FOTHERGILL
IPC: H04N1/60
Filed: 1953-08-25
Abstract: 738,118. Copying in colour by scanning. CROSFIELD, Ltd., J. F. Aug. 19, 1954 [Aug. 25, 1953], No. 23459/53. Class 40 (3). In a method of producing corrected colour separation prints from a colour transparency of an original or from uncorrected separation prints of the original, a print to be exposed is scanned directly by a moving spot of light the brightness of which is controlled by information derived from photo-electric cells receiving light of a number of colours from the transparency or uncorrected prints, the latter being scanned by a spot of light originating from the same source as that of the first mentioned spot. Fig. 1 illustrates the invention as applied to the production of prints from a transparency. The scanning light spots are produced by a cathode-ray tube 2 the beam of which is scanned to produce a raster 3 and the light from which is split into two beams by partially silvered mirror 13. The reflected light is focused by a lens 14 on to a plate 15 on which the corrected print is to be formed whilst the transmitted light is focused by lens 11 on to the original transparency 12. Light passing through the transparency is divided into three paths by partially silvered mirrors 17 and 18, and is arranged to fall on three photo-electric cells 19, 20, 21 associated with filters 22 so as to be responsive respectively to red, green and blue light. The cells provide outputs through cathode-followers 24. In an alternative arrangement dichroic mirrors may be employed and the filters 22 omitted. A further photo-electric cell 25 receives light directly from the scanning raster and provides an output through cathode follower 26 proportional to the instantaneous brightness. The outputs from cells 19-21 are proportional to the transmission factors of the transparency for the particular colours selected multiplied by the instantaneous brightness of the scanning spot and in order to render the signals dependent solely on the transmission factors they are divided by the brightness signal from cell 25. This is effected by first deriving the logarithms of the signals in circuits 27M, 27Y, 27C and 29 and then subtracting the brightness signal from each of the colour signals in mixing circuits 28M, 28Y and 28C. The resulting outputs are proportional to the logarithms of the transmission factors for the particular colours, but since the inverse density at any point in the transparency for a particular colour is itself proportional to the logarithm of the transmission factor, the output from the mixing circuits are proportional directly to the densities of the complementary colours, i.e. magenta, yellow and cyan, of the corrected prints to be produced. " Masking " of each of the signals with respect to the other two is performed by means of circuits 29M, 29Y and 29C, and the resulting signals are then applied to a computer circuit 31 the function of which it to produce a black printer signal proportional to the least of the colour signals and to reduce each of the colour signals by the amount of the black signal. The final signals are then modified in non-linear circuits 32 to emphasise high-lights and shadows by reducing amplitude in the middle ranges and further modified in non-linear circuits 33 to correct for the characteristics of the cathode-ray tube. By means of switch 34 the four signals are connected in turn to the cathode-ray tube grid 8, each for the duration of a complete scanning to produce the corrected prints on plates placed at 15. By the use of phasereversing stages in the signal channels either positive or negative prints may be produced. The signal for cancelling the brightness component, instead of being derived from a photoelectric cell 25, may be taken directly from the signal applied to grid 3. Furthermore, instead of being divided logarithmically into the colour signals, it may be employed to control reciprocally the gains of amplifiers through which the signals are fed. A high-frequency oscillator 10 modulates grid 8 in order to produce alternating currents in the photo-electric cell outputs with the object of simplifying the construction of the circuits. In a modified method of using the above system, the corrected print may be partially exposed, either subsequently or beforehand, to light of constant intensity passed through the original and a colour filter. In a system for use with uncorrected separation prints of the original, the optical system is modified slightly, Fig. 2 (not shown), in order to direct the light passing through lens 11 on to three separation prints behind which are situated the photo-electric cells 19-21.