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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
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