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Webvision: Color Perception <br />Figure 6. Munsell colours ystem, illustrating 1„ xuc, value and ctvoma..(27 K j�L, ima ?c <br />1931 Commision Internationale d'Elairage (CIE) chromaticity system <br />Trichromatic colour matches using three colours can be illustrated on Newton's colour <br />circle (figure 7). Newton's colour circle consists of the following components: <br />Page 5 of 18 <br />4 <br />a circle representing the spectral colours (although not shown here, mixtures of blue and red <br />(purple locus are not spectral colours and hence a straight line between R and B is more <br />appropriate) <br />a triangle whose vertices represent the three primary colours used to make colour matches (R, G <br />and B) <br />the centre of the circle representing white (W) <br />Figure 7. Newton's colour circ1e.(j.7 Kipvg ima e <br />Newton's colour circle provides a qualitative description of colour matches and can be used to explain <br />why two colours may not be sufficient to make colour matches and also the use of 'negative' colours. For <br />example, if 500 nm is required to be matched (spectral colour located on the circle), blue and green will <br />be required. However, blue and green primaries alone will produce a desaturated 500 nm. Therefore, red <br />must be added to the spectral colour to desaturate it and make the match (figure 8) When the third <br />primary is added to desaturate the colour mixture, negative tristimulus values result (figure 5). This can <br />be demonstrated by the following equation: <br />500 nm (S) + R = G+ B <br />or <br />http://www.webvision.med.utah.edu/KallColor.html 6/18/01 <br />