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

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V'(l) V(l)

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wavelength (nm)

Con

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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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wavelength (nm)

RedGreen L-M+S BlueYellow L+M-S

Achromatic L+M+.05*S

Schematic color opponent response

+- +

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V(l)

wavelength (nm)

RedGreen L-M+S BlueYellow L+M-S

Achromatic L+M+.05*S

Schematic color opponent response

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Log Spatial Frequency (cpd)

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Luminance

Red-Green

Blue-Yellow

Color matching

• Grassman laws of linearity:()(l(l(l

ll• Hence for any stimulus s(l) and response

r(l), total response is integral of s(l) r(l), taken over all l or approximately s(l)r(l)

Primarylights

Test light

Bipartitewhitescreen

Surround field

Test light Primary lights

Subject

Surround light

Color Matching

• Spectra of primary lights s1(l), s2(l), s3(l)

• Subject’s task: find c1, c2, c3, such thatc1s1(l)+c2s2(l)+c3s3(l)

matches test light.

• Problems (depending on si(l))

– [c1,c2,c3] is not unique (“metamer”)

– may require some ci<0 (“negative power”)

Color matching

• What about three monochromatic lights?• M(l) = R*R(l) + G*G(l) + B*B(l)• 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

Primarylights

Test light

Bipartitewhitescreen

Surround field

Test light Primary lights

Subject

Surround light

Color matching

• Solution: CIE XYZ basis functions

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CIE 1931 standard colorimetric observer color matching functions

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wavelength (nm)

Color matching

• Note Y is V(l)

• None of these are lights

• Euclidean distance in RGB and in XYZ is not perceptually useful.

• Nothing about color appearance

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3.5Stiles and Burch 1959 10-degree bipartite field color matching functions

primary lights at 645.2 nm 525.3 nmand 444.4 nm

b10(l) g10(l)

r10(l)