Midterm 2
• March 9th and 10th
• Review Session Monday 7pm in this
room (probably)
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
Perceiving
Color
Absorption/Cone response
• Four absorption peaks in retina: 3 cone types plus
rods
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
Wavelength Input
Cone
Signal to Brain
“Blue”
“Green”
Equal Parts
Red and
Green =
Yellow
“Red”
Color
Vision
Theories of Color
Vision:
Trichromatic Theory
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!
Color
Vision
Theories of Color
Vision:
Trichromatic Theory
• Trichromatic Theory can explain some
aspects of colorblindness:
– most of us are trichromats
– someone missing one of the three cone
types is a dichromat
– someone missing two is a monochromat
– someone missing all cone types is called a
rod monochromat (very poor vision!)
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
Theories of Color Vision: Opponent-Process Theory
•
Opponent-Process Theory
– Red opposes Green
– (Red + Green) opposes Blue
•
Opponent-Process Theory
explains color afterimages
Color Vision
Wavelength
and Color
• White light is a mixture of wavelengths
– prisms decompose white light into assorted
wavelengths
– OR recompose a spectrum into white light
• Additive vs. Subtractive
There are two different ways to mix colors.
• Additive vs. Subtractive
What do you get if you use a prism to combine
all wavelengths of light?
• Additive vs. Subtractive
What do you get if you use a prism to combine
all wavelengths of light?
• Additive vs. Subtractive
What do you get if you mix a bunch of paint?
• Additive vs. Subtractive
What do you get if you mix a bunch of paint?
• Additive vs. Subtractive
• Additive mixing is most intuitive:
ADD wavelengths:
red+green = yellow
red+blue = magenta
blue+green = cyan
red+green+blue=white
• Subtractive mixing is much less intuitive
(but much more common)
• Subtractive mixing happens when we
mix pigments (paint) together
• Different pigments subtract different
wavelengths:
– red subtracts all but red, blue all but blue,
green subtracts blue and red, etc…
• Example: blue + yellow = green
Technically it’s called “cyan”
Amount of reflection
• The result of a mixture depends on what
wavelengths don’t get absorbed by the
two pigments
blue
green yellow
wavelength
red
Amount of reflection
• Both yellow and blue pigments reflect a
bit of green
blue
green yellow
wavelength
red
• Subtractive mixing is commonly used in
color printers
• 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 color is this box?
• What color is this box?