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�Webvision: Color Perception Page 8 of 18 Fi u� re 73. -tears ttial-elenictdiscrimination Curve.From Dnson_ 11 The Frg. ti'oi Z. I..c radon_ >cademil ress. 19fi2 .() 1 k^jn g image) Saturation discrimination describes the degree of paleness of the colour. Saturation is related to colorimetric purity (P) which is also defined as: P= L/(Lev+L) where L is the luminance of the spectral colour and Lw is the luminance of the white that is mixed with the spectral colour. The colorimetric purity of a colour quantifies the amount of white mixed with the spectral colour. If the spectral colour is pure (no white added), then the colorimetric purity is 1. In saturation discrimination experiments, the luminance is kept constant. A bipartate field -is used with white (Lw) on one side and white mixed with the spectral colour on the other side (Lw + A L). In can be seen in figure 14, that more 570 nm colour is required to make the white patch appear coloured. Therefore, yellow has low saturating power, whereas blue and red have high saturation power. i 7 cr t o 3 FSk�4ML -1-4 - - �Calc� 4 �it3 lks 7rG'�LrS'4'L��Sjd1�P�P7'J�.t,7 Fi_r,Lrre 14. Saturatim dis; "_T tination_o'f F ries2 audlBk_icllvidi19.4 £rain Ciraha.n <.:. II.. (_d .Vision d Vis?xaI I'crccntirn�. r cpm Y o±3_:.tohn l� ile_l'_ d ionss inc.., 11654,. if 7 K peg irnar The V(X ) function closely matches the sensation of brightness and hence is commonly considered to reflect brightness discrimination. The wavelength 555 nm is. perceived as the brightest in the colour spectrum (see earlier section on the photopic and scotopic luminosity function). Theories of Colour Vision. Any theory of colour vision must predict all the perceptual attributes noted earlier. We present here a simplistic view of the trichromatic and colour opponent theories. See the Colour Vision section by Peter Gouras for a discussion on the physiological correlate of colour opponency. The trichromatic theory was first proposed by Thomas Young in 1802 and was explored further by Helmholtz in 1866. This theory is primarily based on colour mixing experiment and suggests that a combination of three channels explain colour discrimination functions. Evidence for the trichromatic theory includes: 1. Identification of the spectral sensitivities of two cone pigments by Rushton's retinal densitometry (Rushton, 1963). 2. Identification of three cone pigments by microspectrometry (Marks, Dobelle and MacNichol, http://www.webvision.med.utah.edu/KallColor.html 6/18/01