color vision

hue

perceptual quality we refer to as color

saturation

purity or vividness of color

brightness

intensity of light

spectral power distribution (SPD)

plot showing intensity of a light source as a function of wavelength

selective spectral reflectance

objects reflect some wavelengths and absorb others

subtractive mixing

mixing pigments removes wavelengths

additive mixing

mixing light adds wavelengths

principle of univariance

single photoreceptor can’t distinguish wavelength from intensity

s cones

short, blue

m cones

medium, green

l cones

long, red

opponent neurons

RG or BY channels built from cone interactions

negative afterimage

cone bleaching leads to opponent rebound

color constancy

perceived color remains stable despite lighting changes

achromatopsia

loss of color vision from V4 damage

brightness

overall intensity of light

heterochromatic light

light composed of many wavelengths

monochromatic light

light composed of single wavelength

white light

heterochromatic light with roughly equal power at many visible wavelengths

complementary colors

hues that, when mixed in equal proportion, yield an achromatic result

why the sky is blue (rayleigh scattering)

atmospheric particles scatter short wavelength light more, making scattered light appear blue

why sunsets are red/orange

sunlight passes through more atmosphere, short wavelengths scattered away, leaving long (red) wavelengths

why a single receptor type can not support color vision

changes in wavelength and intensity can produce identical responses

why trichromacy requires 3 cones

three distinct spectral sensitivities allow unambiguous decoding of wavelength mixtures

trichromatic theory

color vision arises from 3 cone types and their relative activation

opponent process theory of color vision

color is encoded in pairs of opposing channels (red-green, blue-yellow)

chromatic adaptation

cone-specific adaptation due to prolonged exposure to a particular color

how negative afterimages relate to cone bleaching

after heavy stimulation, adapted cones respond less, making opponent colors dominate

how opponent coding reduces redundancy

M and L cones overlap heavily, subtracting their signals emphasizes differences rather than shared info

why color constancy is needed

illumination changes often, while surface reflectance usually does not

monochromacy

only one class of photoreceptor for bright light seeing, no color discrimination

rod monochromacy

only rods, no cones, extremely poor day vision and no color

cone monochromacy

only one cone type, rods handle dim light, single cone types handles bright light, but no color