• Read article by Land for Thursday
• Article by Anne Treisman coming up in
about two weeks
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
Theories of Color
Vision:
Trichromatic Theory
Wavelength Input
Cone
“Blue”
“Green”
“Red”
Signal to Brain
Blue
Color
Vision
Theories of Color
Vision:
Trichromatic Theory
Wavelength Input
Cone
Signal to Brain
“Blue”
“Green”
“Red”
Green
Color
Vision
Theories of Color
Vision:
Trichromatic Theory
Wavelength Input
Cone
Signal to Brain
“Blue”
“Green”
“Red”
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
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
• use color sliders to adjust R,G,B values
• 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
Theories
ofVision
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!)
Color
Theories
ofVision
Color Vision
• Trichromatic Theory can explain some aspects of
colorblindness:
– dichromats have only two primaries: any color they can see
can be matched with differing proportions of the two
wavelengths to which they are sensitive
Color
Vision
Theories of Color
Vision:
Trichromatic Theory
• Trichromatic Theory can explain some aspects of
colorblindness:
– dichromats have only two primaries: any color they can see
can be matched with differing proportions of the two
wavelengths to which they are sensitive
– most common is deuteranopia (~3% of men, <1% of women)
- missing “green” cones
Color
Vision
Theories of Color
Vision:
Trichromatic Theory
• Trichromatic Theory can explain some aspects of
colorblindness:
– dichromats have only two primaries: any color they can see
can be matched with differing proportions of the two
wavelengths to which they are sensitive
– most common is deuteranopia (~3% of men, <1% of women)
- missing “green” cones
– cannot see color difference between reds and greens - but
they can see luminance difference
Color
Vision
Theories of Color
Vision:
Trichromatic Theory
Ishihara Color Plates
can indicate color
blindness
Color
Vision
Theories of Color
Vision:
Trichromatic Theory
DON’T DO THIS !
…~3% of male readers will have trouble seeing it!
Color
Vision
Theories of Color
Vision:
Trichromatic Theory
But this is OK.
Color
Vision
Theories of Color
Vision:
Trichromatic Theory
So is this.
Color
Vision
Theories of Color
Vision:
Trichromatic Theory
Even this is good.
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?