exam 2 !!!
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EXAM 2 !!!. Monday, Tuesday, Wednesday, Thursday of NEXT WEEK. EXAM 2 !!!. Will cover everything from: taste/touch/smell/balance To Colour Vision (Thursday this week). COLOR VISION. Color is an illusion. What color is this box?. Wavelength and Color. - PowerPoint PPT PresentationTRANSCRIPT
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EXAM 2 !!!
Monday, Tuesday, Wednesday, Thursday of NEXT WEEK
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EXAM 2 !!!
Will cover everything from:
taste/touch/smell/balanceTo
Colour Vision (Thursday this week)
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• What color is this box?
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Wavelength and Color• Recall that light is electromagnetic radiation
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Wavelength and Color• Recall that light is electromagnetic
radiation• Light waves have a
frequency/wavelength
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Wavelength and Color• Recall that light is electromagnetic
radiation• Light waves have a
frequency/wavelength• Frequency/wavelength is the physical
property that corresponds (loosely) to the perception called color
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Color Vision• Different wavelengths correspond
roughly to the “colors” of the spectrum
Wavelength and Color
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Color Vision• White light is a mixture of wavelengths
– prisms decompose white light into assorted wavelengths
Wavelength and Color
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Color Vision• Primary colors
Perceiving Color
What are the primary colors?
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Color Vision• Primary colors
Perceiving Color
Red Green Blue
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Color Vision• Primary colors
Perceiving Color
What makes them primary?
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Color Vision• Primary colors
• Every color (hue) can be created by blending light of the three primary colors in differing proportions
Perceiving Color
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Color Vision• Primary colors
• Every color (hue) can be created by blending light of the three primary colors in differing proportions
• Led to prediction that there must be three (and only three) distinct color receptor types
Perceiving Color
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Color VisionTrichromatic Theory of Color Vision
“Blue”
“Green”
“Red”
Blue
Wavelength Input Cone Signal to Brain
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Color VisionTrichromatic Theory of Color Vision
“Blue”
“Green”
“Red”
Green
Wavelength Input Cone Signal to Brain
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Color VisionTrichromatic Theory of Color Vision
“Blue”
“Green”
“Red” Red
Wavelength Input Cone Signal to Brain
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Color VisionTrichromatic Theory of Color Vision
“Blue”
“Green”
“Red”
Yellow
Equal Parts Red and Green =
Wavelength Input Cone Signal to Brain
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Color VisionTrichromatic Theory of Color Vision
“Blue”
“Green”
“Red”
Yellow
Equal Parts Red and Green =
Wavelength Input Cone Signal to Brain
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Color VisionTrichromatic Theory of Color Vision
“Blue”
“Green”
“Red”
Yellow
Equal Parts Red and Green =
Wavelength Input Cone Signal to Brain
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Color Vision• Trichromatic theory of color vision:
– brain interprets the relative amounts of signaling from each of these cone types
Theories of Color Vision: Trichromatic Theory
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Color Vision• Trichromatic theory of color vision:
– brain interprets the relative amounts of signaling from each of these cone types
• This means that some colors can be matched by a pair of wavelengths– metamers: colors that have no definite single
wavelength (e.g. yellow)
Theories of Color Vision: Trichromatic Theory
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Color Vision• Trichromatic theory of color vision:
– brain interprets the relative amounts of signaling from each of these cone types
• This means that some colors can be matched by a pair of wavelengths– metamers: colors that have no definite single
wavelength (e.g. yellow)• This also means that any color can be
matched by mixing (not more than) three different wavelengths
Theories of Color Vision: Trichromatic Theory
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• What color can only exist as a metamer (an additive mixture of wavelengths)? In other words, what color cannot be made with a single wavelength?
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• What color can only exist as a metamer (an additive mixture of wavelengths)? In other words, what color cannot be made with a single wavelength?
MagentaThink about why!
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• Subtractive mixing is different!• Subtractive mixing is commonly used in color printers
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• Subtractive mixing is different!• Subtractive mixing is commonly used in color printers
• In subtractive mixing, pigments are used to REMOVE wavelengths
• The resulting colour is a mixture whatever wavelengths don’t get absorbed by the pigment
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• Problem with Trichromatic Theory:
Theories of Color Vision: Trichromatic Theory
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• Problem with Trichromatic Theory:
YELLOW
Theories of Color Vision: Trichromatic Theory
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• Problem with Trichromatic Theory:– most people categorize colors into four primaries:
red, yellow, green, and blue
– some colors simply cannot be perceived as gradations of each other• redish green !?• blueish yellow !?
– It is as if these colors are opposites
Theories of Color Vision: Trichromatic Theory
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• Opponent-Process Theory– color is determined by outputs of two
different continuously variable channels:• red - green opponent channel• blue - yellow opponent channel• “yellow” = red + green
• Your visual system balances input to either end of each channel
Theories of Color Vision: Opponent-Process Theory
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• Everything you’ve learned so far is wrong.
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• Everything you’ve learned so far is wrong.
• Well, not really wrong, just far from complete.
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What Newton Found (and everyone believed)
• White light can be split into all wavelengths by a prism
• According to previous theories: two wavelengths combine to yield intermediate color and no others
Red LightGreen Light
Red + Green = YELLOW
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What Newton Found (and everyone believed)
• White light can be split into all wavelengths by a prism
• According to previous theories: two wavelengths combine to yield intermediate color and no others
• Red + Green light can never yield blue
• Blue + Green light can never yield red
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What Land found:
• Two bands (colors) of the spectrum recombine to produce all the possible colors– provided the appropriate relative amount of
each wavelength is projected
transparency slides
Red LightGreen Light
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How did Land project the “appropriate” ratio of
wavelengths?
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medium/“green” light
long/“red”light
Projector combines “longer” and “shorter” wavelengths using the maps to get the appropriate amounts of each
Viewer perceivesdesaturated huesincluding blues
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Why would the visual system have evolved this way?
• Hint: “Within broad limits, the actual values of the wavelengths make no difference, nor does the over-all available brightness of each”
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Color Constancy• The “color” of objects is independent of the ambient
light – even though light can vary dramatically
Rel
ativ
e In
tens
ity
Wavelength
Rel
ativ
e In
tens
itySunlight Incandescent Light
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Color Constancy• Because of our
mechanism of color constancy we can even use completely artificial spectra
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Next Time
• ATTENTION!