Colour printing systems are based on the subtractive colour model, mixing the subtractive primaries, cyan, yellow and magenta, to produce other colours. Unfortunately, the reflective properties of printing inks are affected by impurities and experience showed that printing black, which should theoretically be possible by combining cyan, yellow and magenta, produced instead a muddy brown. To overcome this, most colour printers include black ink as a fourth ‘colour’ in the print process.
As well as allowing correct printing of black, this results in improved shadow density and overall contrast. The nineteenth-century discovery of the halftone process showed how the juxtaposition of small enough dots of cyan, yellow, magenta and black inks could produce an image which, to the naked eye, would appear to produce continuous tones, colours being produced not by the physical mixing of the inks, but in the optical mixing of primary colours by the viewer’s eye.
The majority of modern low resolution desktop printers use this principle, laying down dots in various ‘dither’ patterns to produce colour output ranging from crude – with the dot pattern clearly visible – to a quality verging on photorealistic. In the offset process, the dot pattern is created by photographing the original artwork through a halftone screen.
To separate a full colour image into yellow, magenta and cyan, it is necessary to photograph the copy three times, through filters which are the same colour as the additive primaries – red, green and blue. When the copy is photographed through the red filter, green and blue are absorbed and the red passes P * through, producing a negative with a record of the red.
By making a positive of this negative we will obtain a record of everything that is not red, or more specifically, a record of the green and blue. The green and blue, as we have seen ear- \ Halftone dotpattem lier,iombine to producecyan; therefore, we have a record of cyan. The process is repeated, using a green filter to produce a record of magenta and using a blue filter to produce a record of yellow.
As each filter covers one-third of the spectrum a record of all the colours in the original copy has been created. Finally, to improve shadow density and overall contrast, a black separation is made by using a yellow filter. When printed with the subtractive colours – cyan, yellow and magenta – plus black, all the colours and tones of the original are reproduced