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)
• 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
Wavelength
and Color
• Different wavelengths correspond
roughly to the “colors” of the spectrum
Color Vision
Wavelength
and Color
• White light is a mixture of wavelengths
– prisms decompose white light into assorted
wavelengths
Color Vision
Perceiving
Color
• Primary colors
What are the primary colors?
Color Vision
Perceiving
Color
• Primary colors
Red Green Blue
Color Vision
Perceiving
Color
• Primary colors
What makes them primary?
Color Vision
Perceiving
Color
• Primary colors
• Every color (hue) can be created by
blending light of the three primary colors
in differing proportions
Color Vision
Perceiving
Color
• 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
Color
Vision
Trichromatic
Theory
of Color Vision
Wavelength Input
Cone
“Blue”
“Green”
“Red”
Signal to Brain
Blue
Color
Vision
Trichromatic
Theory
of Color Vision
Wavelength Input
Cone
Signal to Brain
“Blue”
“Green”
“Red”
Green
Color
Vision
Trichromatic
Theory
of Color Vision
Wavelength Input
Cone
Signal to Brain
“Blue”
“Green”
“Red”
Red
Color
Vision
Trichromatic
Theory
of Color Vision
Wavelength Input
Cone
Signal to Brain
“Blue”
“Green”
Equal Parts
Red and
Green =
Yellow
“Red”
Color
Vision
Trichromatic
Theory
of Color Vision
Wavelength Input
Cone
Signal to Brain
“Blue”
“Green”
Equal Parts
Red and
Green =
Yellow
“Red”
Color
Vision
Trichromatic
Theory
of Color Vision
Wavelength Input
Cone
Signal to Brain
“Blue”
“Green”
Equal Parts
Red and
Green =
Yellow
“Red”
Color
Vision
Theories of Color
Vision:
Trichromatic Theory
• Trichromatic theory of color vision:
– brain interprets the relative amounts of signaling
from each of these cone types
Color
Vision
Theories of Color
Vision:
Trichromatic Theory
• 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)
Color
Vision
Theories of Color
Vision:
Trichromatic Theory
• 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
• 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?
Magenta
Think 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
Theories of Color Vision: Trichromatic Theory
• 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: Opponent-Process 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
• 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 + 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
How did Land project the
“appropriate” ratio of
wavelengths?
Projector combines “longer” and “shorter” wavelengths
using the maps to get the appropriate amounts of each
long/“red”
light
medium/
“green” light
Viewer perceives
desaturated hues
including 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
Sunlight
Relative Intensity
Relative Intensity
• The “color” of objects is independent of the ambient
light – even though light can vary dramatically
Wavelength
Incandescent Light
Color Constancy
• Because of our
mechanism of
color constancy
we can even use
completely
artificial spectra
Next Time
• ATTENTION!