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5The Perception of Color
Chapter 5
5 The Perception of Color
• Basic Principles of Color Perception
• Trichromacy
• Opponent Processes
• Does Everyone See Colors the Same Way?
• From the Color of Lights to a World of Color
5 Basic Principles of Color Perception
Color: Not a physical property but rather a psychophysical property.
–Most of the light we see is reflected.
–Typical light sources: Sun, light bulb; emit a broad spectrum of wavelengths 400–700 nm.
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5 Photoreceptor Response
5 Basic Principles of Color Perception (cont’d)
Problem of univariance: An infinite set of different wavelength-intensity combinations can elicit exactly the same response from a single type of photoreceptor.
– One type of photoreceptor cannot make color discriminations based on wavelength
5 The Problem of Univariance
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5 Trichromacy
Scotopic: Referring to dim light levels at or below the level of bright moonlight
– Rods are sensitive to scotopic light levels.
– All rods contain same type of photopigment molecule: Rhodopsin.
– All rods have same sensitivity to wavelength, making it impossible to discriminate light.
5 The Moonlit World
5 Trichromacy
Cone photoreceptors: Three varieties
– S-cones: Cones that are preferentially sensitive to short wavelengths (blue cones).
– M-cones: Cones that are preferentially sensitive to middle wavelengths (green cones).
– L-cones: Cones that are preferentially sensitive to long wavelengths (red cones).
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5 Trichromacy
With three cone types we can tell the difference between lights of different wavelengths.
5 Trichromacy
Trichromacy: The theory that the color of any light is defined in our visual system by the relationships between a set of three numbers, the outputs of three receptor types now known to be the three cones (The Young-Helmholtz theory).
5 Reflected Light from Real-World Objects
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5 Trichromacy
Metamers: Different mixtures of wavelengths that look identical. More generally, any pair of stimuli that are perceived as identical in spite of chroma differences.
MetamericColors
5 Metamers
5 Trichromacy
Two warnings:
– Mixing wavelengths does not change the physical wavelengths!
– In order for a mixture of a red light and a green light to look perfectly yellow, you have to add just the right amount of red and just the right amount of green.
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5 Trichromacy (cont’d)
Additive color mixture: A mixture of lights. If light A and light B are both reflected from a surface to the eye, in the perception of color, the effects of those two lights add together.
5 Additive Color Mixture with Paints
5 Trichromacy
Subtractive color mixture: A mixture of pigments. If pigments A and B mix, some of the light shining on the surface will be subtracted by A, and some by B. Only the remainder contributes to the perception of color.
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5 Subtractive Color Mixture
5 Trichromacy
Color space: The three-dimensional space, established because color perception is based on the outputs of three cone types, that describes the set of all colors.
Color space:
– Hue: Chromatic aspect of color
– Saturation: Chromatic strength of a hue
– Brightness: Distance from black in color space
5 Color Space
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5 A Color Picker
http://www.visionconnection.org
5 Opponent Processes
Lateral geniculate nucleus has cells that are maximally stimulated by spots of light
– Rely on the pathway from the retina to the visual cortex.
– Cells with receptive fields that have center–surround organization.
– Similar antagonistic relationship when it comes to color.
– Some cells: Excited by L-cone onset in center, inhibited by M-cone onsets in their surround.
5 Opponent Processes
Color-opponent cell: A neuron whose output is based on a difference between sets of cones.
Opponent color theory: The theory that perception of color is based on the output of three mechanisms, each of them on an opponency between two colors; red–green, blue–yellow, and black–white.
Hering’s idea about some colors being “illegal”, (e.g., reddish green, or bluish yellow).
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5 Opponent Processes
Afterimage: A visual image seen after the stimulus has been removed.
–This is a way to see opponent colors in action.
5 Opponent Processes
Negative afterimage: An afterimage whose polarity is the opposite of the original stimulus. Light stimulus produce dark negative afterimages. Colors are complementary: Red produces green; yellow produces blue.
Achromatopsia: An inability to perceive colors that is due to damage to the central nervous system.
5 Does Everyone See Colors the Same Way?
Does everyone see colors the same way? – YES!
– General agreement on colors.
– Some variation due to age (lens turns yellow).
Does everyone see colors the same way? – NO!
– About 8% of male population, 0.5% of female population have some form of color vision deficiency: Color blindness.
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5 Does Everyone See Colors the Same Way?
Several types of color blind people
– Deuteranope: Due to absence of M-cones
– Protanope: Due to absence of L-cones
– Tritanope: Due to absence of S-cones
– Color-anomalous: Two of cones are so similar that they can’t make discriminations based on them.
5 Does Everyone See Colors the Same Way? (cont’d)
Several types of color blind people
– Cone monochromat: Only one cone type; truly color-blind.
– Rod monochromat: No cones of any type; truly color-blind, badly visually impaired in bright light.
5 Hue Cancellation Experiment
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5 Does Everyone See Colors the Same Way? (cont’d)
Various cultures describe colors differently
–Idea of cultural relativism
5 From the Color of Lights to a World of Color
Unrelated color: A color that can be experienced in isolation.
Related color: A color, such as brown or gray that is seen only in relation to other colors.
Some problems when studying the real world:
– Color constancy
– Reflectance
5 McCann, McKee, and Taylor Experiment (Part 1)
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5 McCann, McKee, and Taylor Experiment (Part 2)
5 From the Color of Lights to a World of Color (cont’d)
Physical constraints make constancy possible:
– Intelligent guesses about the illuminant
– Assumptions about light sources
– Assumptions about surfaces
5 Bloj, Kersten, and Hurlbert Experiment
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5 From the Color of Lights to a World of Color (cont’d)
Animals: Provide insight into color perception in humans.
– Advertisements for bees to trade food for sex (for pollination).
– Colorful patterns on tropical fish and toucans provide sexual signals.
5 Animal Coloration
5 Photopigments (Part 1)
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5 Photopigments (Part 2)