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Webvision: Color Perception <br />Figure 3. Ndditive colour mixtures of blue, Breen and rcd to produce clan, maucnta_y_eliay+ attd wlutc, l( 2 IC,j eg_ ima <br />Page 3 of 18 <br />Subtractive colour mixtures involve the selectively absorption of wavelengths. Cyan, magenta and <br />yellow are subtractive primaries. If white light was shone through a yellow filter, the yellow filter will <br />absorb blue and transmit red and green (which makes yellow). Therefore, yellow can be considered as a <br />-B filter. A magenta filter subtracts or absorbs green (-G filter) from white light and cyan subtracts or <br />absorbs red (-R filter) from white light (figure 4). <br />Fi?ttre 4. Subtractive colour mixture-, of evart :, nagenta andyeJlow to Lroducc blue,�reen and r L 13 Iij cn ima c _ <br />Grassmann's Laws provide a quantitative description of colour matching data (Grassman 1855). They <br />hold up well within a prescribed set of conditions (with respect to brightness, adaptation of the observer, <br />size of the field, etc). Grassmann's Laws are useful in quantifying colour matching data, but as a rule, <br />colour matching data are affected by the following: <br />• macular pigment (xanthophyll) variations in subject's central 4 - 5 degrees of vision <br />• chromatic aberrations <br />• rod intrusion, especially when large fields and low photopic light levels are involved <br />• failure of Abney's Law of luminance additivity including the Helmholtz -Kohlrausch effect <br />• Bezold -Brucke hue shift at bright intensities <br />Color Specifleation. <br />l <br />The Munsell Colour System <br />All colours can be fully specified in terms of their hue, lightness and saturation. The Munsell system has <br />three dimensions: hue, value and chroma. These three dimensions correspond to the three perceptual <br />attributes of human colour vision. <br />http://www.webvision.med.utah.edu/KallColor.html 6/18/01 <br />