PART 3. SOME BASICS OF COLOUR VISION

All students know about the colour wheel. They all know that there are three primary colours, and can tell you that these primaries can be mixed to make all other hues, but can not themselves be mixed at full intensity from other colours. Some know just enough to get into interminable debates as to whether the "real" primaries are the red, yellow and blue of the conventional artists colour wheel, or the printer's primaries of yellow, magenta and cyan, or perhaps the three primaries that we use for mixing coloured lights - red, green and blue. But few can answer why there are three primaries. If the visible spectrum is made up of a continuous range of wavelengths, why should just three colours be special? Why not five? And why for that matter do hues form a continuous circle, when the spectrum does not?

This section concerns some essential topics of colour vision that we need to review in order to discuss the dimensions of colour in detail. For a detailed account of current understanding of all aspects of the physiology of colour vision the Webvision site of the University of Utah is strongly recommended.

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