LECTURE 5: the perception of colour
Colour: not a physical property, it is made up by the brain.
Green filter: nose and stars appear equally bright but are actually a very different colour.
Problem of univariance: an infinite set of different wavelength-intensity combinations can elicit
exactly the same response from a single type of photoreceptor.
It’s better to talk about S M and L cones rather than red, green and yellow cones
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 type of photopigment molecule: Rhodopsin
Newton: prisms break up sunlight into spectral components
Young-Helmholtz (-Maxwell) theory: trichromatic colour vision. Colour vision is based on 3
photoreceptors sensitive to particular ranges of wavelengths.
*Maxwell’s colour matching techniques
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.
Additive colour mixture: a mixture of lights. If light A and light B are both reflected from a surface
to the eye, in the perception of colour, the effects of those two lights add together.
This is what happens when mixing light with different colours.
Subtractive colour mixture: a mixture of pigments A and B mix, some of the light shining on the
surface will be subtracted by A and some by B. only the remainder contributes to the perception of
Non-spectral hues: hues that don’t exist as pure forms of light but only as mixtures of different
Opponent colour theory: the theory that perception of colour is based on the output of three
mechanisms, each of them on an opponency between two colours: red-green, blue-yellow, and
Afterimage: a visual image seen after the stimulus as been removed. LGN (lateral geniculate nucleus) has colour-opponent cells: neurons whose output is based on a
difference between sets of cones.
Achromatopsia: an inability to perceive colours that is due to damage to the central nervous
Two types of colour blindness:
- cone monochromat- have only one cone in their retina
- rod monochromat- no cones of any type; truly colorblind