Colour Perception

Function of Color Perception

Evolved primarily to help humans search for things, like foraging for berries, to make judgements about food and safety (poisonous animals often coloured), attract mates

Visible Light

Electromagnetic radiation with wavelengths varying from about 400 nm to about 700 nm.

• white light is mixture of all wavelengths

Opaque Object

An object that light cannot pass through = colour is determined by the light that it reflects

Transparent Object Color

The color of a transparent object is determined by the color it transmits.

Munsell Color System Categories

Value (lightness), Hue (color), Chroma (saturation).

Rods

Photoreceptors in the retina that cannot distinguish between colors and are active only at low light levels = not active in normal light light conditions

Three Types of Cones

S cones (419 nm - blue), M cones (531 nm - green), L cones (558 nm - red) = active in normal light conditions. by comparing relative activites of 3 cones humans are able to distinguish between colours

• e.g. if L cones are most active light is primarily red

Monochromatism

no functioning cones and see the world in shades of gray. Only functioning rods

Dichromatism

A condition where individuals are lacking one of the three types of cones. 3 types = protanopes, deuteranopes, tritanopes

Protanopes

Lacking L cones; red = green; see thru shades of blue and yellowish-green.

Deuteranopes

Lacking M cones; red = green; sees thru shades of blue and yellowish green

Tritanopes

Lacking S cones; green = blue; see thru shades of blue and red

Three Color Opponent Channels

Red-green, blue-yellow, and white-black.

Afterimage

image continues to appear after exposure to the original stimulus has ceased. evidence for opponent-process theory of colour vision

impossible colour combinations

bluish-yellow and reddish-green due to opponent processing.

Color Constancy Factors

The color of light an object reflects is determined by its reflectance and the color of light shining on it.

• reflectance x illumination = reflected light

Habituation

Becoming less sensitive to a colour

2 ways visual system achieves colour constancy

Discounting the illuminant and habituation

discounting the illuminant

is a process where the visual system ignores the color of the light source to maintain stable color perception under varying lighting conditions.

opponent-process theory of colour vision

color perception is controlled by opposing processes between colors, specifically red-green, black-white and blue-yellow channels.