colour perception Flashcards
function of colour perception
It has been suggested that colour perception can help humans search for things, eg. foraging for berries and help us make judgements, eg is a banana ripe? to see if an animal is venomous (they often have colour on them), and to attract mates (peacock).
Physics of colour
Visable light is on an electromagnetic radiation with wavelengths varying from 400 nm to 700 nm. white light= mixture of all these wavelengths.
opaque and transparent objects
opaque object= light cannot pass through, the colour of an opaque object is determined by the light that it reflects.
transparent= determined by the colour it transmits, if an object absorbs blue but transmits red, then it will appear red.
Mixing paint
Blue= absorbs red light but reflects blue and green light
yellow= absorbs blue light but reflects red and green light
Mixing light
Yellow= Red L and green L
All colours= white light
Munsell colour system
colour categorized according to Value (lightness), Hue (colour) and Chroma (sensation)
Trichromatic theory of vision
Retina= photorecptors, such as rods and cones
rods= cannot distinguish between colours and active in low levels of light
Cones= normal light conditions and can distinguish colour.
Trichromatic theory of vision- cones
S cones- blue
M cones- green
L cones- red
By comparing the relative activities of these three cone tupes, humans are able to distinguish between colours. Eg, if the L cones are most accurate the colour is red.
colour deficiency- Monochromatism
M usually do not have functioning cones, only have functioning cones and they are truly color blind and see the wolrd only in shades of grey. They are very sensitive to light
Dichromatism
Dichromats are lacking one of the three types of cones (Protanopes, Deuranopes, Tritanopes.) One way to know what dichromats can or can’t see is to measure what colors they can and cannot distinguish.
unilateral dichromats
these people have normal trichromatic vision in one eye, but dichromatic vision in the other eye, which lets us know what dichromatic vision is.
opponent- process theory of color vision
what colour we perceived is determined not only by the cones in our retina. The signals from these cones are processed by the cortex where they are combined into three colour opponent channels red-green, blue-yellow, White-black.
evidence for the opponent process theory- Afterimages
when viewing the red stimulus, the L cones are more stimulated than the M cones. Consequently, the ‘red’ percept is more activated than the ‘green; percept, which is subsequently oppressed. When a white canvas is shown, then the red will show rather than the green.
opponent process theory- impossible colour combinations
It is possible to have a bluish red (purple), a yellowish red (orange, a yellowish green and a bluish green (aqua). However, we can’t perceive a bluish yellow or a reddish green- because these are the ‘opponent process colours’
colour constancy
The light reflected by an object is determined by the product of its reflectance and the illumination. The colour of an object is not just based on the light it absorbs, but also what colour is shining on it. If white light is shone on a green jumper, it will predominantly be green. However, if red light is shone on a green sweater it will reflect red light.
