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Chapter 7

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Chapter 7: Color Vision
How do we perceive color?
Color Vision
Light
EYE + BRAIN
Stimulus
Vision consists of the perception of:
•Color
•Size
•Shape
•Depth
•Distance
•Speed
VISION
Features of Color Vision
1. Trichromacy:
For complete specification of any color, at
least three parameters are needed.
• HUE – Dominant wavelength, D
Intensity
• CHROMA – Saturation or Purity, p
• VALUE – Brightness, Y
пЃ¬D
пЃ¬ (nm)
Features of Color Vision
2. Color Constancy:
Ability of the eye to compensate for changes in
illumination and viewing conditions.
Candlelight
Reflects less blue
Sunlight
Reflects more blue
Any object
Object appears more or less the same since eye
becomes less sensitive to blue under sunlight!
Color Constancy (Contd.)
Note: Color Photography Does Not Have Color Constancy.
These images show how a standard daylight film responds to
different illuminants. The visual system is able to compensate
color appearances for illuminant shifts.
Features of Color Vision
3. Contrast Effects:
• Simultaneous B & W Contrast:
White = Brightest Grey
Black = Darkest Grey
Contrast Effects (Contd.)
• Simultaneous Color Contrast:
The appearance of a color changes based on the
colors surrounding it .
http://webexhibits.org/colorart/contrast.html
Contrast Effects (Contd.)
• Successive Color Contrast:
The appearance of a color changes based on the
colors seen previously.
For example, green may appear “bluish” after viewing
orange!
http://www.lifesci.ucsb.edu/~mrowe/SuccessiveColorContrast.html
Features of Color Vision (Contd.)
4. Afterimages:
Latency, persistence, and positive afterimages:
The photo-receptors do not respond immediately to light. The
electro-chemistry takes about 0.05 seconds to respond (the latency
period) and persists for some time after the stimulus (about 0.15 sec
for cones, anywhere from 0.2 to more than one second for rods).
These delays and the persistence in later cell communications
produce a positive afterimage. Such afterimages are visible after
glancing at the Sun or any other bright light.
Desensitization and negative afterimages:
Continued exposure to light desensitizes the photo-receptors over
time scales of 1 to 30 seconds and longer. This desensitization
produces negative afterimages when the image field is changed.
http://staff.washington.edu/chudler/chvision.html
Afterimages (Contd.)
Follow the movement of the rotating
pink dot, you will only see one color,
pink. If you stare at the black + in the
centre, the moving dot turns to
green. Now, concentrate on the black
+ in the centre of the picture. After a
short period of time, all the pink dots
will slowly disappear, and you will only
see a green dot rotating!
Features of Color Vision (Contd.)
5. Lightness Constancy:
Our judgment of lightness or darkness remains
unimpaired even under different illumination.
Color Vision Defects
Genetic factors or disease can eliminate one or
more types of cones.
Monochromats
(0.003%)
Have B & W vision due
to rods only. Cones do
not function.
Protanopes
R-G blind and
abnormal insensitivity
to long wavelengths.
Dichromats
(4% males; 0.4% females)
Unable to distinguish R
from G. Also called R-G
blindness.
Deuteranopes
R-G blind and
abnormal insensitivity
to middle wavelengths.
Tritanopes
(very rare)
Abnormal insensitivity
to short wavelengths.
Color Vision Defects (Contd.)
Protanopia and deuteranopia are red-green defects. Persons with redgreen defects have difficulty distinguishing between reds, greens and
yellows but can discriminate between blues and yellows. Protanopes
often can name red and green correctly because green looks lighter to
them than red. Hereditary tritanopia is a blue-yellow defect. Persons
with blue-yellow defects cannot see the difference between blues and
yellows but can distinguish between reds and greens.
Color Vision Defects (Contd.)
Color Vision Test
Normal Vision
R-G Blind
L
R
L
R
Top
25
29
Top
25
Spots
Middle
45
56
Middle
Spots
56
Bottom
6
8
Bottom Spots
Spots
Theories of Color Vision
Early Theories:
Newton – Particles of light excited vibrations of
different sizes on the retina.
Young -
Postulated three types of
photoreceptors, each responding to
some part of the spectrum.
For example, Y light excites R and G
receptors equally!
Note: Young’s “Trichromatic” theory forms the
basis for the “Component Theory”.
The Component Theory
•Adopted & modified by Helmholtz.
•Color perceived is uniquely determined by relative stimulation of
the three basic retinal receptors (Components).
•Explains positive afterimages and R-G color blindness.
•Drawbacks: Color constancy, lightness constancy, and negative
afterimages cannot be explained.
Achromatic Response
Seems to indicate that there are four
“psychologically pure” primaries.
The Opponent Theory
•Based on the Opponent Theory
by Herring.
•Cone photoreceptors are linked
together to form three opposing
color pairs: blue/yellow,
red/green, and
black/white. Activation of one
member of the pair inhibits
activity in the other.
•Explains many psychological
effects and color blindness.
Trichromatic Theory or Opponent Theory?
Both theories are needed to explain what is known
about color vision. The trichromatic theory explains
color vision phenomena at the photoreceptor level;
the opponent-process theory explains color vision
phenomena that result from the way in which
photoreceptors are interconnected neurally.
Component
Theory
L cone
M cone
S cone
L cone
Opponent
Theory
M cone
S cone
Outputs compared
in higher visual
centers
Perceptions
combined in higher
visual centers
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