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Color Vision

The stimulus

The color of objects

•  “white” light (all wavelengths) reaches the object

•  “green” light (medium wavelengths) bounces off

•  other wavelengths are absorbed by swimsuit

Color mixture

Lights Pigments

Example: mixing blue and yellow paint Photoreceptors

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Photoreceptors

•  Few S cones, many L and M cones •  No S cones in fovea

Trichromacy theory

•  Palmer, Young, Maxwell, Helmholtz •  Human color vision is based on 3 primary

color sensations

Opponent-process theory

•  Hering •  Color vision based on 4 primaries (not 3) – What are the primary colors? –  Observers pick out red, green, blue, and yellow

as “pure” –  Some color combinations are never reported •  blueish-yellow •  reddish-green

Opponent-process theory

•  4 primaries, arranged in opposing pairs – red-green pair – blue-yellow pair – (also “black-white” or luminance pair)

Opponent-process theory

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Color interactions

•  Simultaneous color contrast - color of background or other stimuli can impact an object’s perceived color

Simultaneous color contrast

https://www.facebook.com/akiyoshi.kitaoka/videos/10205703441520067/?pnref=story

Simultaneous color contrast

Lots of “blue” in cone response to disk

Even more “blue” in cone response to blue background

Much less “blue” in cone response to purple background

Color interactions

•  Color adaptation – Adapt to one color (stare at it for a while) – Afterimage appears to be the opponent

color

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https://www.youtube.com/watch?v=qA2brNUo7WA

Color adaptation

•  Before: equal response to white in all 3 cone classes

Color adaptation

•  After adaptation to yellow (R+G), the highest response is from the S cones

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Color constancy

•  Retinal color = object color & lighting color

Color constancy

•  Constraints on light sources, surface reflectance, objects

Color constancy Color constancy

xkcd

Color deficiency

•  Anomalous trichromacy –  about 6% of males, <1% of females –  All 3 cone classes are present –  L and M have more similar sensitivities,

making color discrimination a little harder –  Red, orange, yellow, yellow-green, and green,

appear either more greenish or more reddish, and paler

–  Hard to tell red from orange traffic light

Color deficiency

•  Dichromacy – Missing one class of cones •  Protanope - missing L cone (red) •  Deuteranope - missing M cone (green) •  Tritanope - missing S cone (blue)

– protanopes and deuteranopes: about 2% of males, <1% of females

– tritanopes: <1%

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https://www.youtube.com/watch?v=FKSOe5NK_qQ

Color deficiency

•  Monochromacy – World appears truly black-and-white –  No useful color vision –  Cone monochromats - have just one type of

cone –  Rod monochromats - have only rods

Color tests

•  Pseudo-isochromatic (Ishihara) plates

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Color tests

•  Farnsworth-Munsell 100-hue test

Color tests

•  D15 test

Color tests

•  D15 test - scoring