Chapter 5: Perception of Color (From Color of Lights to a World of Color…
Chapter 5: Perception of Color
Basic Principles of Color Perception
Color is not a physical property, its a psychophysical property.
Light we see is reflected
Sun, light bulb, fire
Part of EM spectrum, between 400 and 700 nm
Shorter Wavelengths = more blue; longer wavelengths = more red
: the color of any light is defined in our visual system by the relationship of 3 numbers -- the outputs of 3 receptor types now known to be the 3 cones.
Some times called the Young-Helmholtz theory
James Maxwell developed color matching technique still used today, where only 3 mixing lights are needed to match any light
2 lights insufficient, 4 too many
Additive color mixing: A mixture of lights
Subtractive color mixing: 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.
Cones in retina are neural substrate for detection of lights
LGN has cells that are maximally simulated by spots of light
Visual pathway stops in LGN on the way from retina to visual cortex
LGN cells have RFs with center-surround organization
cells with center surround organization
A neuron whose output is based on a difference between set of cones
Step 1: Color Detection
Wavelength must be detected in first place.
3 Types of cone photoreceptors
S-Cones: Short Wavelengths
M-Cones: Medium Wavelengths
L-Cones: Long Wavelengths
More accurate to refer them to short, medium, long, not "blue", "green", "red" since they respond to a variety of wavelengths
L-cone's peak sensitivity is 565 nm, which corresponds to yellow, not red.
Cones work at daylight/bright indoor lighting (
) light intensities
Bright enough to "saturate" the rod receptors to their maximum brightness
Rods work in dimmer (
) light intensities (moonlight, dim)0
Bright enough to stimulate rod receptors, too dim for cones
Step 2: Color Discrimination
Able to tell difference between one wavelength (or mixtures of wavelengths) and another
For M Cone; all wavelengths presented at same intensity
At 450 nm & 625 nm, you get the same response
Principle of Univariance
: Different wavelength-intensity combinations can elicit exactly the same response type of photoreceptor
One type of photoreceptor cannot make color discriminations based on wavelength
Human visual system solves this because we see color
Rods are sensitive to
Rods obey principle of univariance, cannot sense differences in color
All rods wave same sensitivity to various wavelengths of light
Under scotopic conditions, only rods active; that's why world seems drained of color.
Step 3: Color Appearance
Assign perceived colors to lights/surfaces in world and have those perceived colors be stable over time regardless of lighting conditions.
Color Space: a 3D space that describes all colors. There are several possible color spaces.
RGB color space: Defined by the outputs of long, medium, and short wavelength lights
HSB color space: defined by hue, saturation, and brightness
Hue: the chromatic (color) aspect of light
Saturation: The chromatic strength of a hue
Brightness: The distance from black in color space
Ewald Hering: noticed some color combos are legal while others are illegal
For example: can have bluish green or reddish yellow, you CANNOT have reddish green or bluish yellow
Opponent Color Theory
: perception of color depends on the output of three mechanisms, each of them based on an opponency between two colors: red-green, blue-yellow, and black-white
Individual Differences in Color Perception
: Private conscious experiences of sensation and perception
"Is my perception of blue the same as your perception of blue?"
Does everyone see colors the same way? -
About 8% of male pop and .5% of female pop has some form of color vision deficiency "color blindness"
Color blindness is a malfunction in one or more of the genes coding the 3 cone photopigments
Males have only one X so if defective on male, problem
: have 3 cone photopigments but 2 are so similar that they experience the world like those who have 2
Deuteranope: Absence of M-cones
Protanope: Absence of L-cones
Tritanope: Absence of S-cones
: Has only one cone type; truly color-blind
: Has no cones of any type; truly color-blind and very visually impaired in bright light
: Inability to see color due to cortical damage; sees the world drained of color
Basic Color terms
: Single words that describe colors and have meanings that are agreed upon by speakers of a language.
: basic perceptual experiences (e.g., color perception) may be determined in part by the cultural environment
Hard to ask someone to describe their experience of color blue and compare it to your own
Color perception is not influenced by culture and language - see same color boundaries even if not word for difference
From Color of Lights to a World of Color
When many colors are present, they can influence each other
: A color perception effect in which the color of one region induces the opponent color in a neighboring region
: A color perception effect in which two colors bleed into each other, each taking on some of the chromatic quality of the other.
Afterimages: A visual image seen after a stimulus has been removed
: An afterimage whose polarity is the opposite of the original stimulus
Light stimuli produce dark negative afterimages
Colors are complimentary
This is a way to see opponent colors in action
: The tendency of a surface to appear the same color under a fairly wide range of illuminants
Must discount illuminant (the light that illuminates the surface) and determine what the true color of a surface is regardless of how it appears
The visual system "knows" brightness changes under shadow but not hue