Patent GB-1049508-A (GB)
Title: Improvements relating to colour correction in image reproduction systems
Inventor: MAWBY DAVID HARRY
IPC: H04N1/60
Filed: 1964-03-10
Abstract: 1,049,508. Copying in colour by scanning. CROSFIELD ELECTRONICS Ltd. March 10, 1965 [March 10, 1964], No. 10066/64. Heading H4F. The invention relates to a colour reproduction process, in which separation photographs employed for the production of printing plates are obtained with the aid of signals representative of yellow, cyan and magenta which are derived by the photo-electric scanning of an original, and is concerned with the development of a correction signal for the cyan signal (i.e. the signal obtained from the original via a red filter). In a first embodiment, Fig. 1, the signal is obtained by inverting the yellow and magenta signals in circuits 13 and 16 and then adding them to the cyan signal in adders 14 and 17 to obtain difference signals. Blocking circuits 15 and 18 eliminate the portions of the signals which are negative with respect to the zero axis and the remaining portions are combined in adder 19 to form the correcting signal. This may either be added to the cyan signal 20 or may be employed to modulate a scanning light source by means of which a photograph is produced via a cyan colour separation transparency obtained directly from the original. The operation of the system is illustrated by waveforms of signals derived by scanning a colour pattern formed of black (B), white (W), cyan (C), violet (V), magenta (M), red (R), yellow (Y) and green (G) patches. The original signals have a polarity such that black is positive. The effect of the circuit operations is to produce a correction signal from which there are eliminated signal elements representative of all colours (including black) other than cyan and the neighbouring hues of green and violet. The invention is used in a process where the yellow ink density is reduced in accordance with the magenta ink density and the magenta ink density is reduced in accordance with the cyan ink density to allow for the overlapping spectral characteristics of the inks. The effect of these modifications is to compress the grey scale representation in yellow and magenta, and it is the object of the invention to permit a corresponding compression in cyan. The correction signal has the effect of increasing the amplitude of the signal elements representative of cyan and neighbouring hues to the same as that for black. The overall signal may then be adjusted in amplitude until the contrast of cyan is the same as that for yellow and magenta. Fig. 1 may be modified by the use of a single three-stage adder in place of adders 19 and 20. In a second embodiment, Fig. 2 (not shown), the larger (i.e. closest in amplitude to black) of the yellow and magenta signals at each instant is selected by circuit 30 and then inverted at 32 and combined with the cyan signal in adder 33. The negative components are blocked at 35 and the resulting signal comprises the correction signal. In this embodiment correction is achieved only for cyan. A third embodiment, Fig. 3 (not shown), is similar to Fig. 1, but differs in that the outputs of blocking circuits 15 and 18 are combined by a maximum signal selector 31. A fourth embodiment, Fig. 4 (not shown), is based on Fig. 1 and includes also a minimum signal selector 41 which responds to inverted yellow, magenta and cyan signal and supplies an output which is combined in adder 19 with the outputs from blocking circuits 15 and 28. The effect of this arrangement is to introduce a negative black signal to the correction signal, whereby correction is effected by reducing black as well as increasing cyan and neighbouring hues.