Trichromacy Helmholtz thought three separate images went forward, R, G, B. Wrong because retinal processing combines them in opponent channels. Hering proposed opponent models, close to right.

Opponent Models Three channels leave the retina: –Red-Green (L-M+S = L-(M-S)) –Yellow-Blue(L+M-S) –Achromatic (L+M+S) Note that chromatic channels can have negative response (inhibition). This is difficult to model with light.

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Log Spatial Frequency (cpd) Contrast Sensitivity Luminance Red-Green Blue-Yellow

Color matching Grassman laws of linearity: (     )(   (   (   Hence for any stimulus s( ) and response r( ), total response is integral of s( ) r( ), taken over all or approximately  s( )r( )

Primary lights Test light Bipartite white screen Surround field Test lightPrimary lights Subject Surround light

Color Matching Spectra of primary lights s 1 ( ), s 2 ( ), s 3 ( ) Subject’s task: find c 1, c 2, c 3, such that c 1 s 1 ( )+c 2 s 2 ( )+c 3 s 3 ( ) matches test light. Problems (depending on s i ( )) –[c 1,c 2,c 3 ] is not unique (“metamer”) –may require some c i <0 (“negative power”)

Color matching What about three monochromatic lights? M( ) = R* R ( ) + G* G ( ) + B* B ( ) Metamers possible good: RGB functions are like cone response bad: Can’t match all visible lights with any triple of monochromatic lights. Need to add some of primaries to the matched light

Primary lights Test light Bipartite white screen Surround field Test lightPrimary lights Subject Surround light

Color matching Solution: CIE XYZ basis functions

Color matching Note Y is V( ) None of these are lights Euclidean distance in RGB and in XYZ is not perceptually useful. Nothing about color appearance