Theories of Color Vision: Young Helmholtz Trichromatric Theory, Slides of Physiology

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Colour Vision 2

Metamers and Theories of

Colour Perception

PSY305 Lecture 10 JV Stone

Structure

• Metamers: colour mixing (of light).

• Three theories of colour vision

– Trichromatic Theory

– Opponent process Theory

– Dual process Theory

• Colour aftereffects as evidence for these

theories.

Metamerism

This yellow can be made from a mixture of red and green lights ...

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If two different

luminance

spectra are

perceived as

the same

colour then

these two

spectra are

metamers of

each other.

Metamers

Different spectra, Same perceived colour

Why are there metamers?

‘Metamers’ for one cone

If you had only one cone type then the output produced by a given wavelength (eg 520nm) can be reproduced exactly by a different wavelength (600nm). The same type of behaviour ensures that many different spectra can induce the same outputs in the three cone types. This is the principle of univariance : a cone’s output indicates how much light it absorbed, not the wavelength of that light.

Wavelength λ L-cone response To test light Test light M-cone response To test light Metameric matching for two cone types

• If we had 2 cone types then we could match any single test wavelength with a combination of two coloured torches.

10 Wavelength λ L-cone response To test light Test light M-cone response To test light Green torch L-cone response To green torch M-cone response To green torch ‘Difference in M-cone output Difference in L-cone output Metameric matching for two cone types

• The difference in cone outputs can be made up by adjusting the brightness of a red torch …

11 Wavelength λ L-cone response To test light Test light M-cone response To test light Red torch Green torch L-cone response To green torch M-cone response To green torch ‘Missing’ M-cone output ‘Missing’ L-cone output Metameric matching for two cone types

• M-cone response to test light = M-cone response to green torch + M-cone response to red torch.
• L-cone response to test light = L-cone response to green torch + L-cone response to red torch. In practice, we get to adjust Brightness of torches too.

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Different illuminance spectrum

S

M

L

cones -10mV -20mV -30mV Perceived^ colour2= perceived colour Luminance spectrum2=/=spectrum reflected from white surface

Different illuminance spectrum gives different luminance spectrum,

BUT same cone outputs, and therefore same perceived colour

(when viewed in isolation).

Torch wavelengths Cone triplet outputs

Metamer summary

• Metamers exist because any single cone triplet output can be produced by many different luminance spectra.
• In terms of metameric matching, this means that any cone triplet can be produced by many settings for three torches used to create the luminance spectrum.
• Most importantly, because a given cone triplet produces a single perceived colour (when viewed in isloation), each perceived colour can be produced by many different spectra.

Theories of colour perception

The correspondence problem

LIGHT INPUT OUTPUT

PROCESS EXPERIENCE OF COLOUR PERCEPTION In order to understand colour perception, we must establish what takes place here. A first stage in doing this is to consider the facts about the psychological experience of colour perception. This was what the original theories of colour perception were based on.

Strengths of Trichromatic Theory

• Accounts for the 3 (red, green, blue) dimensions of colour space.
• Accounts for metameric matching (mixing of 3 colours (red, green, blue) is sufficient to match any other visible colour).
• Partially accounts for the three basic varieties of colour blindness: Protanopes lack the long-wavelength receptors Deutranopes lack the mid-wavelength receptors Tritanopes lack the short-wavelength receptors.

Doesn’t fully explain colour blindness: In reality, colour loss in colour blindness occurs in pairs: inability to discriminate pairs red-green or yellow-blue. Colours are never lost in isolation and pairings other than those above are never found. Weaknesses of Trichromatic Theory