Explanation
The illustrations on this page show an enlarged section of a typical magazine illustration.
The picture in the magazine was printed using inks in four colours—cyan, magenta, yellow and black.
This process is called 4-colour printing or
CMYK printing.
(Here the K stands for blacK, because B usually stands for Blue.)
In 4-colour printing, each ink is applied as an array of small dots.
(This array is sometimes called a half-tone screen, for historic reasons.)
Each array is aligned in a different direction to reduce the chance of producing Moiré patterns,
which would be distracting.
The dots are spaced further apart on poorer quality paper such as newsprint
because the ink spreads further in this kind of paper before it dries.
(This spreading is called dot gain.)
The varying colours in the image are produced by varying the density of each ink.
The final colours are produced by a fairly complex combination of processes
but a simplified explanation
will work well enough for most practical purposes.
Technical notes
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CMYK printing is commonly used in
newspapers, books and magazines;
it’s used for both analogue and digital photographic prints;
it’s used by most computer printers, both inkjet printers and colour laser printers.
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It’s impossible to produce coloured inks with perfectly matched absorbtion spectra.
It’s difficult to produce a good match if the colours are to be reasonably
light fast.
In practice, cyan, magenta and yellow inks combine to produce a more or less dark brown,
not a dense black.
In CMYK printing, the black ink is used to
compensate for this problem and allow for a greater range of dark colours.
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Some high-quality images involve one or more extra ink colours, called spot colours.
These colours may be chosen specifically to suit a particular image.
They allow an image to include colours outside the colour gamut
of a normal CMYK print,
but they increase the cost of the print.
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Some inkjet printers achieve an extended colour gamut
by using two distinct cyan inks, two distinct magenta inks and two distinct yellow inks.
Not surprisingly, this may be called 6-colour printing.
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Even in the 21st century, the spacing of half-tone dots, the screen ruling,
is normally quoted in 19th-century units!
A newspaper, for example, may describe its screen ruling as 85#lpi,
meaning 85 LPI
(lines per inch); an inkjet printer may claim to have a
resolution of 600 DPI
(dots per inch).
