Many brilliant colours – the Colour separation module
The mechanical printing of coloured images is normally performed
by printing four colours (cyan, magenta, yellow and black) in various
intensities superimposed on each other (see Colour printing
in
the Fundamentals
part of this manual). Variations in intensity
for each colour are achieved by the use of rasters. But as the four
colours have to be applied in four separate printing processes, this
also requires four printing masters, the so-called colour extracts.
The division of a coloured document into these four colour masters is
called Colour separation. If the document also uses spot colours
(solid colour patches used for adornment rather than images) then even
more colour extracts may be required. These too have to be separated
in that case.
The basics of colour printing were already described briefly in
Fundamentals / Colour printing
. But the subject with all its
physical and print-technical phenomena is so complex that it is not
possible to provide a comprehensive discussion about it here.
Reference should be made to the relevant technical literature for
further details.
The basic idea is as follows: Using a total of four printing
colours, varying their intensities can produce any required colour
tint or shade. Unlike a television screen, the printing process cannot
vary the strength or shade of the inks to print individual points
lighter
or darker
. For this reason the rasterizing
process already described several times in this manual is used.
Rasters vary the spacing and/or size of the ink dots to mimic the
appearance of different colour intensities.
In theory, colour printing is quite simple. Since all colours in the document are saved in the RGB (Red Green Blue) system, we first convert them to the CMY (Cyan Magenta Yellow) system. From the resulting colour we calculate a fourth element, black, which will then be subtracted from all three basic colours. Finally, the resulting intensity values are converted to raster points of different size, appropriate to their relative intensities, which are then printed:
So much for theory. In practice, additional problems to those described for rasterization arise. The greatest problem is due to the printing colours themselves. Unfortunately, even today's chemical and colour industries cannot produce a colour which, for example, absorbs only the red part of the light spectrum and allows green and blue to be reflected unhindered. This optimum cyan is just as nonexistent as an ideal yellow or an optimum magenta. Therefore, to print an accurate colour picture, the theoretical colours in the computer have to be modified to allow for shortcomings in the printer's process colours.
To counter these colour imperfections, a total of eight control curves are built into the process illustrated above. These curves define the relationship between the theoretical values in the computer image and the actual output values. You may already be familiar with control curves from the Frame editing module. In the Colour separation module, you can use the control curves to adjust and correct information sent to the printer. The effect of these settings, however, will only be visible in the printed result, as these are purely control curves for printing.
