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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)