EXAM 2 !!!
Monday, Tuesday, Wednesday, Thursday of NEXT WEEK
EXAM 2 !!!
Will cover everything from:
taste/touch/smell/balanceTo
Colour Vision (Thursday this week)
• What color is this box?
Wavelength and Color• Recall that light is electromagnetic radiation
Wavelength and Color• Recall that light is electromagnetic
radiation• Light waves have a
frequency/wavelength
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
Color Vision• Different wavelengths correspond
roughly to the “colors” of the spectrum
Wavelength and Color
Color Vision• White light is a mixture of wavelengths
– prisms decompose white light into assorted wavelengths
Wavelength and Color
Color Vision• Primary colors
Perceiving Color
What are the primary colors?
Color Vision• Primary colors
Perceiving Color
Red Green Blue
Color Vision• Primary colors
Perceiving Color
What makes them primary?
Color Vision• Primary colors
• Every color (hue) can be created by blending light of the three primary colors in differing proportions
Perceiving Color
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
Color VisionTrichromatic Theory of Color Vision
“Blue”
“Green”
“Red”
Blue
Wavelength Input Cone Signal to Brain
Color VisionTrichromatic Theory of Color Vision
“Blue”
“Green”
“Red”
Green
Wavelength Input Cone Signal to Brain
Color VisionTrichromatic Theory of Color Vision
“Blue”
“Green”
“Red” Red
Wavelength Input Cone Signal to Brain
Color VisionTrichromatic Theory of Color Vision
“Blue”
“Green”
“Red”
Yellow
Equal Parts Red and Green =
Wavelength Input Cone Signal to Brain
Color VisionTrichromatic Theory of Color Vision
“Blue”
“Green”
“Red”
Yellow
Equal Parts Red and Green =
Wavelength Input Cone Signal to Brain
Color VisionTrichromatic Theory of Color Vision
“Blue”
“Green”
“Red”
Yellow
Equal Parts Red and Green =
Wavelength Input Cone Signal to Brain
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
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
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
• 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?
• 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!
• Subtractive mixing is different!• Subtractive mixing is commonly used in color printers
• 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
• Problem with Trichromatic Theory:
Theories of Color Vision: Trichromatic Theory
• Problem with Trichromatic Theory:
YELLOW
Theories of Color Vision: Trichromatic Theory
• 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
• 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
• Everything you’ve learned so far is wrong.
• Everything you’ve learned so far is wrong.
• Well, not really wrong, just far from complete.
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
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
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
How did Land project the “appropriate” ratio of
wavelengths?
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
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”
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
Color Constancy• Because of our
mechanism of color constancy we can even use completely artificial spectra
Next Time
• ATTENTION!