PART 3. SOME BASICS OF COLOUR VISION

All students know about the colour wheel, the spectrum, and the primary colours. They can tell you that the primary colours can be mixed to make all other colours, and can not themselves be mixed at full intensity from other colours. They may know just enough to get into endless debates as to whether the "real" primaries are the red, yellow and blue of the conventional artists colour wheel, or the yellow, magenta and cyan printer's primaries, or perhaps the three optimal primaries for mixing coloured lights - red, green and blue. But few have reflected on the inconsistencies that seem to lurk within this universally held knowledge. For example, few have wondered why there are three primary colours. If the visible spectrum consists of a continuous range of wavelengths, why should just three colours be special? Why not five? And why for that matter do colours form a circle, when the spectrum does not?

These questions do not arise in most treatments of "colour theory" for artists. Though fundamental to the subject, they do not occur to us, perhaps because we internalize our knowledge about colour at an early and uncritical age. To answer them we need to understand some basics of colour vision. In broad outline, the processing of colour information by our visual system seems to operate in three successive stages:

1. Trichromatic image capture: colour information is recorded by the responses of the L,M and S cone cells in the retina.

2. Opponent processing: responses from the L,M and S cones are converted into three signals, for brightness, yellowness vs blueness, and redness vs. greenness respectively, the latter two providing our perception of hue and saturation.

3. Processing for colour constancy: information on hue, brightness and saturation from throughout the visual field is analysed and resolved into an interpretation of the hue, brightness and saturation of the illumination, and the hue, value and chroma of the visible surfaces.

We will review these topics briefly in the succeeding pages, not only to answer the questions raised in the first paragrah, but also to gain the background we need in order to discuss the dimensions of colour in detail. For a much more comprehensive online account of current understanding of all aspects of colour vision, the Webvision site of the University of Utah is strongly recommended.


 

 

 

 

 


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