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Color Perception

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Color Perception
• The Physical and Psychological variables
• Grassman’s Laws of color mixing
• Additive vs. Subtractive color mixing
• Color facts and findings
• Physiological findings
• Balance Theory
Sensation and Perception - color.ppt
В© 2001 Dr. Laura Snodgrass, Ph.D.
1
3 Physical And Psychological Variables
• PHYSICAL
– wavelength
– intensity
– purity
Sensation and Perception - color.ppt
• PERCEPTUAL
– hue
– brightness
– saturation
В© 2001 Dr. Laura Snodgrass, Ph.D.
2
Double Cone Color Solid
GRAPH of the 3 physical variables
Every point on the
graph represents a
color
Some colors are not on the graph:
•extra-spectral purple
•non-spectral - gold, silver, bronze, brown
Sensation and Perception - color.ppt
В© 2001 Dr. Laura Snodgrass, Ph.D.
3
Grassman’s Laws of Additive Color Mixture
• A normal person can match any hue with a combo of
3 non-complementary colors
• Complementaries
– equal mix produces gray
– unequal produces de-saturation of the dominant color
• Non - complementaries
– de-saturated intermediate hue
• Metamers
– appear the same, but composed of different
wavelengths
Sensation and Perception - color.ppt
В© 2001 Dr. Laura Snodgrass, Ph.D.
4
Additive vs. Subtractive Color Mixing
• Additive color mixing is a mix of lights
– stage lighting
– pointillism (paintings)
• Subtractive color mixing is usually from reflected
light
– most of our color experience is reflected light
Sensation and Perception - color.ppt
В© 2001 Dr. Laura Snodgrass, Ph.D.
5
Additive vs. Subtractive Color Mixing
Subtractive Color Mix
The paint absorbs or subtracts out
wavelengths and the color you see if
the wavelengths that were reflected
back to you (not absorbed)
Sensation and Perception - color.ppt
Additive mixture
The wavelengths are added
together so the final color
you see is the sum of the
wavelengths
В© 2001 Dr. Laura Snodgrass, Ph.D.
6
Color Facts
• Grassman’s laws, esp. 3 colors can make any hue
• Lack of reddish-green or bluish-yellow
• 3 cone types
• 4 primary colors
• Complementary afterimages
• Simultaneous contrast
• Color map of the retina
• Color blindness
Sensation and Perception - color.ppt
В© 2001 Dr. Laura Snodgrass, Ph.D.
7
Color Blindness
• Normal color vision - 3D trichromats
• Dichromats - 2D vision (2% males)
– Protanopes
• see blue and yellow (no red or green)
• blind to deep red light
– Deuteranopes
• see blue and yellow
• but can see all wavelengths
– Tritanope
• see red and green
• Monochromat - hue blind
Sensation and Perception - color.ppt
В© 2001 Dr. Laura Snodgrass, Ph.D.
8
Physiological Findings
• Wald - measures spectral sensitivity of cones
– three cone types short, medium, and long wavelength
detectors
• De Valois
– opponent process cells in LGN
– excitatory to one color and inhibitory to another R+G-;
G+R-; B+Y-; Y+B-
Sensation and Perception - color.ppt
В© 2001 Dr. Laura Snodgrass, Ph.D.
9
Hurvich and Jameson’s Balance Theory
• Retina
– three cone types (short, medium, and long wavelength
detectors)
• LGN
– opponent process cells
• Cortex
– “balances” that produce all spectral colors by
balancing inhibition and excitation
Sensation and Perception - color.ppt
В© 2001 Dr. Laura Snodgrass, Ph.D.
10
Hurvich and Jameson’s Balance Theory
Note the pattern of
excitatory and inhibitory
connections between the
cones in the retina and the
opponent process cells in
LGN
Sensation and Perception - color.ppt
В© 2001 Dr. Laura Snodgrass, Ph.D.
11
Balance Theory
Color is determined by the pattern of responses in the
three different cone types.
Sensation and Perception - color.ppt
В© 2001 Dr. Laura Snodgrass, Ph.D.
12
Hurvich and Jameson’s Balance Theory
• Balances in cortex are needed to account for colors
beyond four primaries
S
M+L
M
S
Blue
Yellow
Green
Red
E.g. Yellow & Red = Orange
Blue & Red = Purple
Sensation and Perception - color.ppt
В© 2001 Dr. Laura Snodgrass, Ph.D.
13
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