The Colour Triangle
The final aim of this book is to obtain a comprehensive survey of all colour perceptions. If we recall the considerations of § 5–7 it will be clear that it must be possible to represent the complete three-dimensional collection of colour sensations by points in a three-dimensional space. In § 5 we considered the case of a source of a given spectral energy distribution, the luminance of which was varied, and we saw that as long as this variation in luminance applied to all the lights in the field of view, the character of the colours was not affected. Therefore the members of a set of colours, differing only in luminance, are so closely related that we say they have the same chromaticity. While for representing all possible colours we must use a three-dimensional system of co-ordinates, a plane will suffice to represent the various chromaticities. In § 5 we also indicated to what extent information is lost when we restrict ourselves to chromaticities. When the luminance of one object in the field of vision is reduced while the surroundings remain unaltered, the character of the colour perception evoked by this object will be changed; yellow will turn into brown, greenish yellow into olive green and white into grey. Neither brown, olive green or grey are represented in the chromaticity diagram as they are variations of colour sensations arising from variations in luminance. A further point is that because the quantity of light is not taken into account, the relations to be discussed in this chapter are qualitative rather than quantitative.
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