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Webvision: Color Perception <br />.[overview] [Colour Specification] Colour Discrimination Functions] <br />[Theories of Colour 'vision Colour Vision Deficiencies] [Colour Vision <br />Vests] [Referenc A <br />Overview. <br />Page 1 of 18' <br />M <br />Colour vision processing in the primate visual system is initiated by absorption of light by <br />three different spectral classes of cones. Consequently, colour vision is described as being <br />trivariant or trichromatic, and initial psychophysical studies demonstrated that colours could be matched <br />by the use of three different primaries. In 1802, Thomas Young proposed a model that perception of <br />colour can be coded by three principal colour receptors rather than thousands of colour receptors coding <br />for individual colours. <br />Spectral sensitivity of cones can be determined through several methods. Two of these methods include <br />isolating receptoral responses (Baylor et al, 1984) using calculation from colour matching function of <br />normals and dichromats (Smith and Pokorny, 1975; a dichromat is a subject whose retina has one cone <br />photopigment missing), microspectrometry (Bowmaker and Dartnall, 1980) or reflection densitometry <br />(Rushton, 1963, 1966). The microspectrometer technique involves isolating a single cone and passing <br />light through it. The change in transmission of different wavelengths can be used to calculated the <br />spectral absorption of the cone or determine the change in electrical response. Reflection densitometry <br />involves directing light in the retina and determining the change in absorption as a function of <br />wavelength. These results are subsequently used to calculate spectral absorption. <br />Three classes of cones in the human retina have been isolated from the above techniques. These three <br />classes of cones are the short -wavelength sensitive (S-cones), middle -wavelength sensitive (M-cones) <br />and long -wavelength sensitive (L-cones), and all have different but overlapping spectral sensitivities. <br />The spectral sensitivity of S-cones peak at approximately 440 nm, M-cones peak at 545 nm and L-cones <br />peak at 565 nm after corrected for pre -retinal light loss, although the various measuring techniques result <br />in slightly different maximum sensitivity values (figure 1). <br />wave, ength (nw) <br />F_i yre 1. Snecftal sensitivity of the S -a nQ, IM -cone and L-cone. Combined results from various authors u -inn different, methods includine retinal densitometrj, irc m <br />^`�J Rushton(t and ) microsnectromet_� ! orn B own and Wald nand r ),and increment threshoId �roducin� artifcia! monochromasy fromI3rinlet f1� and <br />mcrecnent. thresholei muasurcments from L alci (3 t _(F_ronx Adages. <br />Itfob sA. v ComT�aand a . 994. F.d Adler's Pht siolo�'v of the Etc Clinical 1Pplication. SL Louis: Tho C. N'. <br />Wnn1ve nvT7.Q2 K pj g image <br />°�http://www.webvision.med.utah.edu/KallColor.html 6/18/01 <br />