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Colour Management - Introduction
Many graphics software programs give you the choice to work
in either RGB or CMYK. These are called "color spaces".
Scanners and digital cameras (also known as input devices)
work in the RGB colour space using combinations of just
three colours: Red, Green and Blue (called "RGB"). These are
the primary colors of light, which computers use to display
images on your screen.
Printers (also known as output devices) work in the colour
space known as CMYK.At Elam we advise that you work in the
RGB colour space; here you do all your colour, effects and
contrast manipulation then convert your document to CMYK
before the document is sent to the printer.
RGB - Additive
A large percentage of the visible spectrum can be
represented by mixing red, green, and blue (RGB) colored
light in various proportions and intensities. Where the
colors overlap, they create cyan, magenta, yellow, and
white.
Because the RGB colors combine to create white, they are
also called additive colors. Adding all colors together
creates white--that is, all visible wavelengths are
transmitted back to the eye. Additive colors are used for
lighting, video, and monitors. Your monitor, for example,
creates color by emitting light through red, green, and blue
phosphors.
CMYK - Subtractive
The CMYK model is based on the light-absorbing quality of
ink printed on paper. As white light strikes translucent
inks, certain visible wavelengths are absorbed while others
are reflected back to your eyes.
In theory, pure cyan (C), magenta (M), and yellow (Y)
pigments should combine to absorb all light and produce
black. For this reason these colors are called subtractive
colors. Because all printing inks contain some impurities,
these three inks actually produce a muddy brown and must be
combined with black (K) ink to produce a true black. (K is
used instead of B to avoid confusion with blue.) Combining
these inks to reproduce color is called four-color process
printing.
The subtractive (CMY) and additive (RGB) colors are
complementary colors. Each pair of subtractive colors
creates an additive color, and vice versa. CMYK
Gamut
Is the range of colours and tones a device or colorspace is
capable of recording or reproducing. The human eye can sense
many more colours than can be reproduced on a computer
monitor in RGB colorspace. The human eye has a larger Gamut
than the computer monitor.
Both RGB and CMYK can only represent at best a relatively
small part of the total colour space, and there are some
colours that can be produced in one but not in the other and
vice-versa. In general, there are often some yellows and
possibly blue-greens that can be printed but not shown on a
monitor, while monitors can usually display more intense
greens, reds, blues and magentas than can be produced by
normal four colour printing. Printing also generally allows
you to get the impression of a deeper black than a monitor
can provide.
RGB to CMYK - History
Traditionally Pre-Press has been just that...prior to going
to press. The work was done by printers or bureaus servicing
the print industry. The emphasis was exclusively on printing
with ink on paper. Consequently the conversion from RGB to
CMYK was done on-the-fly right in the scanner. All of the
retouching, image manipulation, special effects, and color
correcting were done on the CMYK file.
In addition scanning and manipulating the image in RGB is a
far superior because the colour gamut is larger than that of
CMYK. By scanning in CMYK you are limiting the colour
information that can be used in your final print.
Note:
Different colour spaces have different gamuts, and when
changing from one to another, some colour information will
be lost because it cannot be represented in the new space.
So normally you should only change from one space to another
when preparing for print.
When changing between the two spaces colours will shift, but
if the machine is set up correctly the change shouldn’t be
drastic.
As colour spaces have different gamuts so do inkjet papers,
for example the colours and tones displayed in a matte
paper, semi-gloss and high gloss will display the same
colours and tones differently because the paper may limit
the CMYK gamut even further or use as much of the CMYK gamut
as possible.
Soft Proofs
Photoshop in particular allows you to preview the effect of
printing by making use of its 'soft proof' feature. In the
View menu, you can use the 'Proof Setup' option to load a
profile for the printer colour space. You can then toggle
between your RGB working space and this other space using
Ctrl and Y (or Command and Y.)
If you are printing to an inkjet printer, you will send data
to the printer as an RGB file, and the printer driver will
carry out the conversion needed for your inkset - whether
the conventional CMYK of 4 colour printers, (the RIP colour
has six and the large format has four) or more complex six
or seven ink solutions. Your printer profile file allows you
to soft proof before printing as above.
Eventually, as printers begin to understand that they are
only one of many possible output destinations in the world
of digital imaging they, too, will begin to do most of their
image processing in RGB colorspace before converting to CMYK.
Even though monitors always use RGB to display colors, the
colors you see on your monitor will more closely match the
final printed piece if you are viewing them in the CMYK
color space.
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