Color Vision

Module 1.6b (Part 2)

Learning Targets

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• LT 1.6-6

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Explain how we perceive color in the world around us.

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Color Vision

• Color exists in our mind
• Our visual system interprets differences in the wavelength of light as color
• Rods are color blind, but with the cones we can see different colors
• This difference occurs because we have only one type of rod but three types of cones`

Properties of Color

• Hue—property of wavelengths of light known as color; different wavelengths produce different colors in our brain
• Intensity—perceived brightness of a color, corresponds to amplitude (height) of the light wave.
• The higher the wave, the brighter the color.
• Saturation or purity—property of color that corresponds to the purity of the light wave.
• 1 wavelength is considered more saturated than many.
• Highly saturated color is rich & vivid.
• Less saturated color is faded & dull

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Do objects possess the color we see?�NO!

• An object gets its color from the wavelength of light it REFLECTS.
• It’s not possessing color, it’s reflecting it!
• Something that is white is reflecting all the wavelengths of color to your eye.
• Something that is black is absorbing all the wavelengths of color.

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A red apple is actually absorbing all other wavelengths of light and reflecting back only the red wavelength to your eye.

Color Mixing

• Two basic types of color mixing
• additive color mixture
• example: combining different color lights

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• subtractive color mixture
• example: combining different color paints

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Subtractive Color Mixing

• When mixing colored paints, each new color SUBTRACTS (soaks up) another wavelength.
• Red, blue, and yellow combine to make black paint.

Additive Color Mixing

• When mixing colored lights, each new color ADDS another wavelength.
• Red, green, and blue combine to make white light.
• Examples:
• red + green = yellow
• red + blue = purple
• green + blue = cyan

Young & Helmholtz Trichromatic (three-color) Theory

• Theory of color vision that says cones are “tuned” to be sensitive to red, green and blue light
• All the colors we see are a combination of these three colors.
• Also known as the Young & Helmholtz Theory
• Similar to the design of a color TV

Trichromatic Theory �of Color Vision

• Researchers found that by mixing only three primary lights (usually red, green and blue), they could create the perceptual experience of all possible colors
• This lead Young and Helmholtz to propose that we have three different types of photoreceptors (cones), each most sensitive to a different range of wavelengths

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Trichromacy and TV

• All color televisions are based on the fact that normal human color vision is trichromatic
• Although we perceive the whole range of colors from a TV screen, it only has three colored phosphors (red, green, and blue)
• By varying the relative intensity of the three phosphors, we can fool the visual system into thinking it is seeing many different colors

Color Deficient Vision

• People who lack one of the three types of cones
• Usually the red or green receptors are missing
• Usually referred to as color blindness
• 1 in 50 people are color blind. Inherited and found more in males (sex-linked trait).
• Trichromats – people with normal color vision
• Monochromats – no color vision
• Dichromats – blind to red/green or yellow/blue

Normal vision

Red/Green Color Blindness

Opponent Process Theory �of Color Vision

• Some aspects of our color perception are difficult to explain by the trichromatic theory alone
• Example: afterimages
• if we view colored stimuli for an extended period of time, we will see an afterimage in a complementary color when we view something white.

Opponent-Process Theory of Color

• Theory that says color is processed in opponent pairs of color:
• Red-green, yellow-blue, black-white
• Light that stimulated one half of the pair inhibits/hinders the other half
• Ewald Hering developed this

Afterimage Effect�Stare without blinking at the white dot

Put the Bird in the Cage! Stare at the bird then look at the cage.

Opponent-Process Theory

• Explaining the afterimage effect:
• In the retina and thalamus are ganglion cells that are “turned on” by red but “turned off” by green and vice versa.
• A single ganglion cell handles both but only one at a time. (you can’t see “greenish-red”)
• By tiring out green receptors staring at the image, the other opponent red receptors turn on when you view white (reflects all colors)

Current View of Color Vision

• Our current view of color vision is that �it is based on both the trichromatic and opponent process theories
• Color Vision is a two stage process
• The Retina’s red, green, & blue cones respond in varying degrees to different colors (trichromatic theory)
• Their signals are then processed by ganglion cells that encode color in terms of opposing pairs while they are sending the info to the thalamus and visual cortex which also encode the info by opponent pairs (opponent-process theory)

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