Post on 16-Dec-2015
Midterm 2
• March 9th and 10th
• Review Session Monday 7pm in this room (probably)
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 VisionPerceiving Color• Four absorption peaks in retina: 3 cone types plus
rods
Abs
orpt
ion/
Con
e re
spon
se
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
“Blue”
“Green”
“Red”
Yellow
Equal Parts Red and Green =
Wavelength Input Cone Signal to Brain
Theories of Color Vision: Trichromatic Theory
Color Vision
“Blue”
“Green”
“Red”
Yellow
Equal Parts Red and Green =
Wavelength Input Cone Signal to Brain
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
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?
Magenta
Think about why!
Color Vision
• 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: Trichromatic 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
Theories of Color Vision: Opponent-Process Theory
Color Vision
• White light is a mixture of wavelengths– prisms decompose white light into assorted
wavelengths– OR recompose a spectrum into white light
Wavelength and Color
• 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 = yellowred+blue = magentablue+green = cyanred+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”
• The result of a mixture depends on what wavelengths don’t get absorbed by the two pigments
wavelength
Am
ount
of
refl
ecti
on
blue green yellow red
• Both yellow and blue pigments reflect a bit of green
wavelength
Am
ount
of
refl
ecti
on
blue green yellow 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?