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Computer Architecture D75 P34

The Primary, Secondary and Tertiary Colours of Light.

A standard CRT monitor works by firing electrons at a glass screen, coated with phosphor. As the phosphor becomes charged, it begins to glow, which creates the picture we see on the screen. This is exactly the same mechanism as a domestic TV set, and this technology has changed little in the last 60 years.

The discovery that different colours of light are composed of three base, or "primary" colours, is a very old one. One of the first scientists to experiment with the splitting of white light into the visible spectrum was Sir Isaac Newton, over 400 years ago. (As a young Professor at Cambridge, his teachings of this phenomenon were thought by the Church to be heretic and he only narrowly escaped being tried for witchcraft!)

The three primary colours of light, out of which all other colours in the visible spectrum are composed, are red, green and blue.

The three secondary colours, created by combining two of the pure primaries, are yellow (from red and green), magenta (from red and blue), and cyan (from blue and green).

The two tertiary colours of light are white (additive - all the colours of light) and black (subtractive - no light at all, of any colour).

You can remember all the colours by using the following mnemonic...

Note that these are not the same as the primary colours you will have learned in Art class - red, yellow and blue - these are known as the primary colours of pigment, and absorb light in a different way. This can lead to problems when trying to print an image dispayed on a screen (light primaries) onto paper, using ink or other printing medium (pigment primaries). We shall see in a moment how this problem can be overcome.

The three colour guns at the neck end of the CRT monitor would create individual images as above. It is the combination of the three beams that create on-screen images in millions of colours (below).

Traditional methods of printing colour images use separate plates, based on the primary colours of pigment - red, yellow and blue. (Normally a black plate is also added, for definition.) Our photograph would now look like this ...
The problem with traditional colour printing methods is that extra software would be required to translate digital imagery, based on RGB, to the pigment primaries RYB. This would be prohibitively expensive for colour printers for everyday use. So the photograph is split into what are known as CYMK channels - the secondary light colours, plus black - before sending the appropriate binary signals to the printer. The four "layers" would appear like this -
Note that these colours correspond exactly to the ink cartridges on a standard colour inkjet printer!