COLOUR VISION AND PERCEPTIONThe sub

COLOUR VISION AND PERCEPTION

The subject of colour perception is an area of science which is still a long way from being completely understood. Quite apart from the lack of complete understanding of the mechanics of the visual process, there are also the problems of adaptation and contrast effect. A great deal has been written concerning these aspects, and in this chapter we can



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do little more than 'scratch the surface'. However, they probably remain the most significant technical stumbling block to the complete automation of colour matching procedures or quantitative assessment of colour reproduction.
For many purposes the mechanism of the visual process or the influence of adaptation effects may be ignored and then physical measurement will offer a perfect definition of colour but, as we shall see, this is all too frequently impossible.
For very many years the theory of colour vision, most widely accepted by colour scientists, was as follows. In the retina of the eye there exist three reception: one principally responsive to short wavelengths of the visible electromagnetic spectrum; one to middle wavelengths; and one to long wavelengths. These receptors, known as cone, are connected to the visual cortex of the brain by a series of neural networks and together these produce the perception of colour.
This theory has been supported by experiments which show that all colours may be matched with mixtures of three, widely separated, monochromatic radiation sources. There is little doubt today that it is the accepted hypothesis of the mechanism of the response of the eye to colour. Nevertheless, it does have certain shortcomings as a complete theory of colour vision, partly in the classification of those with defective colour vision, but perhaps of more interest is the problem of yellow.
While there is no doubt that yellow light may be produced by suitable mixtures of red and green light, at the same time it is a colour which has no apparent redness or greenness within it. This property is shared by only three other colour: red, green and blue. These four colours together are known as the psychological primaries and have unitary hues; that is, each of them is perceived as being totally independent of the other three primaries. An alternative theory of colour vision is based on this principle and is referred to as the opponent response theory. It is postulated that are three pairs of response processes, white-black, yellow-blue and red-green, which take place in the visual mechanism.
It is likely that any hypothesis of colour vision, which is finally proved correct, will contain both of these concepts and defind some complex neural connections bridging the gap between them. One such theory, is postulated in Measuring Colour1.