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Why
is that Screen Colour Different to the Printer Colour ?
You know
the problem - you have a perfect screen image with all the right
colours on it and you go to print it and then it comes out looking
completely different. Why does this happen ? There are several reasons
behind this, all of which are caused by the differences in the way
that colour is produced on a computer screen and a computer colour
printer.
How
Does the Computer Hold Colour Information ?
The computer
works entirely with numbers, so colour information must be held
as numbers. There are many different "colour systems"
for representing a colour as a series of numbers but the most common
ones used by computers are the "Hue, Value, Saturation"
(HVS) and "Red, Green, Blue" (RGB). With both systems
a colour is stored as three numbers each with a value between 0
and 255. The computer then uses electronics to convert these numbers
to an electrical signal that produces an image on the screen.
How
Does the Screen Display Colour ?
Firstly,
the screen generates and emits light whereas a printed page reflects
the ambient light. This difference is easily demonstrated if you
put both in a completely dark room. You will still see the computer
screen but you can't see the paper printout without any background
light.
A computer screen
comprises thousands of tiny phosphor dots that can be made to emit
coloured light. Each dot is RED, GREEN, or BLUE and can only emit
light of that colour. A single pixel on the screen is made up of
groups of three dots, one of each colour. The intensity of light
emitted by each dot can be varied. The dots are so small that the
eye tends to merge them together so that complex colours are actually
made up of varying amounts of RED, GREEN, and BLUE dots of varying
intensities. Try looking at a white computer screen through a magnifying
glass - close up you will see the red, green, and blue of the phosphors
and no actual white colour at all.
How
Does the Printer Display Colour ?
A printed
image uses tiny dots of ink on white paper. The primary colours
used here are simply the colours of ink or toner provided to the
printer. These are generally CYAN, MAGENTA, YELLOW, and these are
often supplemented with BLACK. Depending upon the actual printer,
dots of colour are laid side by side, overlaid, or varied in size
when printing. The small size of each dot (typically between 1/300th
and 1/1440th of an inch) means that the eye tends to merge adjacent
dots of colour together and they resulting "blend" of
different colour dots produces an apparent wide range of shades.
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