Patent GB-1096376-A (GB)
Title: Improvements in or relating to photographic reproduction
Inventor: KYTE DEREK JOHN
IPC: H04N1/58
Filed: 1965-09-20
Abstract: 1,096,376. Copying in colour by scanning. D. J. KYTE. Sept. 6, 1966 [Sept. 20, 1965], No. 40081/65. Heading H4F. Apparatus for improving the sharpness i.e. contract, of colour separation images produced from coloured originals comprises photo-electric sharp scanning equipment and associated colour separation equipment for producing electrical colour separation signals e.g. yellow (Y), magenta (M) and cyan (C) signals, photoelectric unsharp scanning equipment for producing electrical " white " (W) signals, means for determining a first and a second difference (if any) between the maximum individual signal amplitude occurring among the Y, M, C and W signals and the Y, M and C signals and between the minimum individual signal amplitude occurring among the Y, M, C and W signals and the Y, M and C signals respectively, and means for adding the sum of the first and second differences, referred to as a detail contrast signal, to each of the colour separation signals to form the signals to be used in the production of the colour separation images. A black printer separation signal (B) is also produced to which is added the sum of the first and second differences. The Specification discusses with reference to Figs. 3A-3P (not shown) the manner in which the detail contrast signal improves the contrast of neutral details (greys) and of boundaries between neutral and coloured areas without substantially affecting the coloured details themselves. When the detail contrast signal is used to produce the black printer separation (the visual effect on the printed result being more noticeable for this separation) (a) where a grey detail stands against a light neutral background, the grey detail is made darker whilst the background adjacent to the detail is made lighter; (b) where a grey detail stands against a dark neutral background, the grey detail is made lighter whilst the background adjacent to the detail is made darker; (c) where coloured details stand against a lighter background, the background adjacent to the details is made lighter; and (d) where coloured details stand against a dark background, the background adjacent to the details is made darker. In the embodiment described, Fig. 1, the unsharp scanning equipment which produces the W signal comprises partially reflecting surface 25, filter 26, photo-cell 27, amplifier 28 and circuit 29, Figs. 2 and 4, not shown, which includes maximum and minimum selectors, subtracters and an adder for producing the detail contrast signal. Photo-cell 27 receives light from a larger element of the transparency than photo-cells 14, 15, 16 by arranging for the image of the illuminated transparency 1 on aperture plate 6 to be larger than the aperture. Filter 26 ensures that the sensitivity of photocell 27 is equal in the three regions of the visible spectrum to that defined by the blue, green and red colour filters 11, 12 and 13, in combination with the photocathodes of photo-cells 14, 15 and 16. Circuit 29 includes two potentiometers which are used to adjust the amplitudes of the two differences and thus affect independently dark details on light backgrounds and light details on dark backgrounds. When scanning a broad area of colour, one of the Y, M and C signals will be greater than the W signal and one or other will be less than the W signal and thus any noise will cancel in the subtraction circuits. When scanning large neutral areas, however, the Y, M, C and W signals are all equal and thus any noise signals present will not necessarily be cancelled. These noise signals may be eliminated by introducing threshold values in the maximum and minimum selector circuits for the signals Y, M, C and W so that the W signal is only selected if it is greater than either the Y, M, or C signals by a certain amount in the maximum selector and less than the Y, M or C signals by a certain amount in the minimum selector. The value of the threshold is preferably chosen to be greater than the peak noise voltage expected on the W signal plus the peak noise voltage expected on the Y, M or C signals.