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Lecture 11

PSYB51H3 Lecture Notes - Lecture 11: Scotopic Vision, Color Vision, Trichromacy

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Matthias Niemeier

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Newton’s Discovery
o Newton: prisms break up (refract) white light into spectral components (rainbow)
o Any single component could not be refracted into a different color (monochromatic
o 2nd prism that combines all the colours would re-create white light
o We perceive the continuum of wavelengths as qualitatively different phenomena
o Colour is created in our mind we have three colour mechanisms in our eyes
Perception of Colour
o Without colour cues, it is hard to detect objects
o The problem of univariance an infinite set of different wavelength-intensity
combinations that can elicit exactly the same response from a single type of
o One type of photoreceptor cannot make colour discriminations based on
Photoreceptors respond differently depending on wavelength
They also respond differently depending on light energy n-cone (sees
green light) might respond to other colours if they are brighter
o Trichromacy
o Scotopic: referring to dim light levels at or below the level of bright moonlight
Rods are sensitive to scotopic light levels
All rods contain same the type of photopigment molecule called
All rods have the same sensitivity to wavelength, making is impossible to
discriminate light of different wavelengths
o Young-Helmholtz (-Maxwell) theory: theory of trichromatic colour vision. Colour
vision is based on 3 photoreceptors sensitive to particular ranges of wavelengths
Maxwell’s colour-matching technique
o Cone photoreceptors: three kinds
o S-cones: short wavelengths, 420 nm (‘blue’ cones)
o M-cones: middle wavelengths, 534 nm (‘greencones)
o L-cones: wavelengths, 565 nm (‘red cones)
o With three cone types, we can tell the difference between light of different
o Metamers: any pair of stimuli that are perceived as identical in spite of physical
differences. In terms of light: different mixtures of wavelengths that look identical
o E.g. Maxwell’s colour matching
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