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Printing and Colours
Article: Printing and Colours
If you're wondering how printing
presses work, or what the difference is between RGB and CYMK,
these notes will help you get a better understanding of colour
usage in the print industry.
Offset
Printing
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Let's say we want to reproduce
a full colour photographic image. Firstly, the original image
must be output on film separated into 4 colours. These 4 films
are transferred to printing plates in much the same way as photos
are developed - ultraviolet light is allowed to pass through
the film to 'expose' the plate, causing a chemical reaction that
activates an ink-receptive coating.
On the press, the plate is
dampened by water, then ink. The ink adheres to the image area,
and water to the non-image area. The result is transferred to
a rubber blanket, and from there to the paper. That's why it's
called "offset", as the image does not go directly
to the paper from the plates.
Each of the 4 plates will
be printed in turn, over the top of the previous colour. As each
layer of ink is added to the paper, the colours merge and fool
the eye into seeing the visual appearance of the original image.
The images to the right show how a printed picture
will appear at different stages of the printing process using
this four colour ink system, known as CMYK.
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CMYK
System
The four colours used are
Cyan, Magenta, Yellow and Black, therefore named CMYK. The letter
K is used instead of B, so as not to be confused with Blue. CMYK
is the standard colour model used in offset printing for full-colour
documents, and because such printing uses inks of these four
basic colours, it is often called four-colour, full-colour or
Process printing.
CMYK is a colour model called
'subtractive'. This colour process is based on light reflected
from an object and passed through inks that absorb certain light,
allowing others to be reflected. To reproduce colour on the paper,
transparent CMYK pigments are used to filter out light in various
combinations. In theory (and scientifically), the combination
of these three colours should produce black - but due to natural
imperfections of ink, the fourth colour (black) is needed to
produce 'true' black.
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CMYK - Subtractive |
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RGB System
In contrast to CMYK, display
devices (such monitors and TV's) generally use a different colour
model called RGB, which stands for Red Green Blue. RGB is an
'additive' colour system, which means that colour is added to
a black background. One of the most difficult aspects of colour
publishing is colour matching, ie. properly converting the RGB
colours into CMYK colours, so the printed document matches what
appears on the monitor.
The conversion of RGB colours
into CMYK are usually not noticeable, but sometimes with the
case of a fluroescent (or extremely bright) colour, the printed
product will be slightly duller. In most cases when creating
an image or colour scheme, using the CMYK model instead of RGB
will save any problems in reproduction.
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RGB - Additive |
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Pantone
System
On printed jobs that don't need to be reproduced
in full-colour, there is a separate plate made for each colour
needed. For example, our H.W.Inprint logo uses only 2 colours:
red and black. In contrast to Process colours, these are known
as Spot Colours.
Many logos don't have colour-specific needs,
but some companies require that the colour(s) in their logo must
exactly match in all reproductions. In these cases, they usually
specify a particular Pantone
Matching System colour. The Pantone
Matching System is an international reference for selecting,
specifying, matching and controlling ink colors.
The Pantone formula guide (a three-guide set
consisting of 1114 solid Pantone Colors on coated, uncoated and
matte stock), shows corresponding printing ink formulas for each
color, and the three-book set of solid chips provides coated,
uncoated and matte perforated tear-out chips that can be used
for quality control.
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Pantone book |
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