The Colour of Light

If you are like me, then you were probably taught in school that there are three primary colours: red, yellow and blue. So why do TVs and computer screens use red, green and blue? And why does the office printer use CYMK (cyan, yellow, magenta and key)? And isn't light a spectrum anyway?

To answer the last question first; yes, light is a spectrum. Light is part of the electro-magnetic spectrum like microwaves, radio waves and x-rays that all have different wave lengths. Visible light (to the human eye) has wave lengths between about 400 and 700 nanometres (one nanometre is 0.000 000 001 metres). The longer the wave length, the more red the light seems (shorter wave length look more blue).

The three colours concept is partly to do with our eyes. We see colour through 'cone cells' on our retinas. Humans have three types of cone cells that respond to three different wave lengths, and our brain combines the responses from all three types to determine the colours in between. We can use this knowledge of how the eye works to trick our brains into seeing the whole spectrum by using just three colours.

For additive colours, like TVs, the three best colours for allowing us to see the whole spectrum are red green and blue. TVs are called additive colours because the produce the specific wave lengths that our eyes respond to, and by adding our response to each wave length together, we see the colour in the middle. For example, if we are seeing yellow on a TV, the TV is actually emitting red light and green light. This is picked up by our red and green cones, and our brain interprets the two signals by perceiving yellow (which is in between red and green on the spectrum). If the TV emits all three at once, then all the cones are reacting, and we perceive white.

Printers, on the other hand, use subtractive colours: the pigments absorb light of different wave lengths to produce different colours. Normal daylight contains the whole spectrum of colours, and the pigments absorb specific bits of the spectrum. For example, if a pigment absorbed more blue light than any other wave length, then when we look at that pigment, the cones that respond to blue won't react, but the ones that respond to red and green will, and we will see yellow. If you then add in a pigment that absorbs red light (which by itself would look green-blue or cyan), then only the cones that respond to green will react. So if you mix a yellow pigment and a cyan pigment, you get green. Magenta is a pigment that absorbs green. If you mix all three pigments, all the light should be absorb, producing black. In reality, it is almost impossible to produce black through mixing three pigments like that, so K (just a black pigment) used by printers for better efficiency.

So what of red yellow and blue? According to the wikipedia page (http://en.wikipedia.org/wiki/RYB_color_model) , they where historically used from the 18th century. Although because bright greens are hard to produce with red, yellow and blue pigments, green was also often used as primary colour. Similarly, black and white were (are) also often used as primary colours too.