PSY 102 Prof. Stuart Anstis

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PSY 102Prof. Stuart Anstis

• Today
• CAPE !!
• COLOR VISION

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Physics of color(dont panic)
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How can I remember that?

• Long red hair
• Perhaps wearing blue shorts?

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Spectrum on prismatic lighthouse lens
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Diamond
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Salt crystals under polarized light
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Muscle fibers under polarized light
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Animal color vision
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Evolution of red/green color vision allows us to
find the ripe fruit among the green leaves
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Colored candies
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The colors of the rainbow
The colors of the rainbow
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Color mixture

• Additive for lights (Tells you about response of
  visual system)
• Subtractive for paints (Physics of the stimulus)

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Additive color mixture
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Figure 3. Additive colour mixtures of blue, green
and red to produce cyan, magenta, yellow and
white.
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Additive mixing of 3 primaries (R, G, B) to
match spectral colorsWEB Demonstration

• Google Brecher Lite

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R
G
R G

• GR

G R

• R
• G

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R
G
R G

• GR

G R

• R
• G

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Blue cones?
Red cones?
Green cones?
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Spectral sensitivity of the combined cones
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Subtractive color mixture Paints
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Why does an apple look red?

• Because it absorbs red? Or reflects red?

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Answer reflects-

• The apple absorbs all short light and reflects
  long light (Physics)
• If you measure the light coming off the apple it
  will have a long wavelength (Physics)

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Figure 4. Subtractive colour mixtures of cyan,
magenta and yellow to produce blue, green and red.
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Blue object under yellow light Subtractive color
mixture
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DemonstrationColored umbrella under colored
light
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Subtractive color mixing in color printing

• Clowns face

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Halftone picture Composed of Yellow, Cyan
Magenta dots
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Theories of color vision

• 3 retinal cone types (R, G, B)
• then
• Opponent cells
• (R G-) (R-G)
• (BY-) (B-Y)

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How does our eye turn long wavelength into red?
Trichromacy Theory

• Trichromacy Physiology
• (3 monochromatic lights) (3 receptor classes)

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3 types of retinal cone (R, G, B)feed into 2
types of Opponent cells(R/G, B/Y)
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• Hermann von Helmholtz
• 1821-1984
• Developed Thomas Youngs idea of 3 color
  receptors
• (retinal cones)

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Opponent color channels
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Perceiving color
Finding fruit
Finding mates
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Relative responsiveness of cones
Wavelength (nm)
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First images of the three kinds of cones in the
living human retina
Colors used here are just the popular labels for
each kind of cone Actually opposite to the
colors reflected by the cones
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perceived color varies with ratio of responses of
the three cones
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Color has only 3 dimensions
Infinite set of possible wavelength distributions
Human vision measures only 3 values, one for each
cone non-invertible cant recover the
wavelength distribution
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different spectra, same color
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Other factors illumination, lecture 15
Mystery of brown
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3. Color blindness
Lose r or g cones (1/20 males, 1/400 females)
Lose b cones (very rare)
Lose two cones (cone monochromat, rare)
Lose all cones (rod monochromat, rare, albinism)
Brain injury, cortical color blindness, very rare
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Color vision tests The Isihara plates
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Original
Protan -R Deutan -G Tritan -B
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Original
Protan -R Deutan -G Tritan -B
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Original
Protan -R Deutan -G Tritan -B
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Original
Protan -R Deutan -G Tritan -B
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Protan confusion lines
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Deutan confusion lines
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Tritan confusion lines
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Thank you.