Patent GB-913050-A (GB)
Title: Improvements in or relating to colour reproduction systems
Inventor: MAWBY DAVID HARRY
IPC: H04N1/60
Filed: 1960-06-07
Abstract: 913,050. Copying by scanning. CROSFIELD ELECTRONICS Ltd. May 31, 1961 [June 7, 1960], No. 19960/60. Class 40 (3). In a method of producing colour-corrected colour reproductions of originals employing a black printer, the black printer or a colour printer from which undercolour has been removed is obtained by using an electro-optical scanner for scanning the coloured original or colour separation images thereof to obtain colour components of the elements of the original, selecting for each scanned element the minimum colour channel signal representing the neutral colour component (i.e. the signal which departs least from the corresponding colour signal for white), and exposing an element of the black printer photographic emulsion, or modifying the exposure of an element of a colour printer photographic emulsion to remove as a function of the minimum colour channel signal only when the remaining colour channel signals or the larger of the remaining colour channel signals is not in excess of the minimum colour channel signal to more than a predetermined extent. Signal generation.-In a first embodiment, Fig. 1, the colour channel signals a, b, c, Fig. 2, which as an example represent the result of scanning across an area which shades progressively from pure green to black, are applied to minimum and maximum signal selectors 12 and 14 and the selected signals d and e are combined in a mixer 18 after first reversing the polarity (# ) of the maximum signal in a stage 16. The signal g obtained from the mixer, which is of negative polarity in the example considered, represents the excess of the maximum signal over the minimum. This signal is then added to the original selected minimum signal e is a mixer 20 to produce a signal b which is positive and negative respectively for areas where the neutral component is greater and less than the amount by which the maximum colour component exceeds the minimum or neutral colour component. The signal h is added to signal g in a mixer 22 to produce a signal i which is positive only for areas in which the neutral component is greatly in excess of the amount by which the maximum colour component exceeds the minimum. This effect is then intensified by the use of a further mixer 24, resulting in signal j. If desired less or even more mixer stages may be used than the three illustrated. The final mixer signal is applied to a blocking circuit 26 which passes only the positive-going signals to produce the signal k. This final signal comprises the signal which is employed to produce the black printer and effect undercolour removal in a colour printer. In a second embodiment, Fig. 3 (not shown), a similar result is obtained by amplifying the output of minimum signal selector 12 before it is combined with the output of the maximum signal selector 14 in mixer 18. The output of mixer 18 is then applied directly to blocking circuit 26. A typical gain factor for the amplifier is 3. An alternative arrangement (not disclosed in detail) involves adding the minimum and maximum signals in suitable proportions in an attenuating network, amplifying the result and applying it to the blocking circuit. Reference is also made, without giving details, to an arrangement making use of the sum of the differences between the minimum signal and each of the remaining two colour signals. Undercolour removal.-The signal from blocking circuit 26 may be employed to effect undercolour removal electronically by subtracting it in a conventional summing circuit from the colour signal employed to produce the colour printer. Alternatively the signal may be subtracted optically by employing it to modulate the light from a cathode-ray tube which is employed to expose the emulsion of a colour printer through a selected separation transparency. In this respect, reference is made to the apparatus described in Specification 835,111. This signal from the apparatus of the present invention is added to that produced by the colour correction computer and the combined signal is applied to modulate the cathode-ray tube, Fig. 4 (not shown). Black printer production.-The signal from blocking circuit 26 is applied to modulate a light source, e.g. a cathode-ray tube which exposes a black printer emulsion directly. Alternatively, in apparatus as described in Specification 835,111, the black printer is exposed through a selected colour separation transparency and the signal from the apparatus of the present invention together with a signal from the photo-cell associated with the selected separation transparency are employed to modulate the cathode-ray tube, Fig. 5 (not shown). In a modification, Fig. 6 (not shown), the final mixer in the apparatus of the present invention is applied to two blocking circuits, one corresponding to circuit 26 to select the positivegoing signals and the other to select the negative-going signals. The outputs from both blocking circuits are combined and applied to modulate the cathode-ray tube, that from the other blocking circuit being reversed in phase in a variable-gain amplifier.