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

• White light is a mixture of wavelengths– prisms decompose white light into assorted

wavelengths– likewise, adding all wavelengths together

recovers white light

What happens if you mix several different paints together?

Wavelength and Color

Color Vision

• Objects have different colors because they reflect some but not all wavelengths of light

– the surfaces of objects are like filters that selectively absorb certain 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 VisionTheories of Color Vision

“Blue”

“Green”

“Red”

Blue

Wavelength Input Cone Signal to Brain

Color VisionTheories of Color Vision

“Blue”

“Green”

“Red”

Green

Wavelength Input Cone Signal to Brain

Color VisionTheories of Color Vision

“Blue”

“Green”

“Red” Red

Wavelength Input Cone Signal to Brain

Color VisionTheories of Color Vision

“Blue”

“Green”

“Red”

Yellow

Equal Parts Red and Green =

Wavelength Input Cone Signal to Brain

Color VisionTheories of Color Vision

“Blue”

“Green”

“Red”

Yellow

Equal Parts Red and Green =

Wavelength Input Cone Signal to Brain

Color VisionTheories of Color Vision

“Blue”

“Green”

“Red”

Yellow

Equal Parts Red and Green =

Wavelength Input Cone Signal to Brain

Color VisionTheories of Color Vision• Trichromatic theory of color vision:

– brain interprets the relative amounts of signaling from each of these cone types

Color VisionTheories of 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)

Color VisionTheories of 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

Color VisionTheories of Color Vision• Trichromatic Theory can explain

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 VisionTheories of Color Vision• Trichromatic Theory can explain

colorblindness:– dichromats have only two primaries: any color

they can see can be matched with differing proportions of the two wavelengths that they are sensitive to

Color VisionTheories of Color Vision• Trichromatic Theory can explain

colorblindness:– dichromats have only two primaries: any color

they can see can be matched with differing proportions of the two wavelengths that they are sensitive to

– most common is deuteranopia (~3% of men, <1% of women) - missing “green” cones

Color VisionTheories of Color Vision• Trichromatic Theory can explain

colorblindness:– dichromats have only two primaries: any color

they can see can be matched with differing proportions of the two wavelengths that they are sensitive to

– most common is deuteranopia (~3% of men, <1% of women) - missing “green” cones

– cannot see difference between reds and greens

Color VisionTheories of Color Vision

Ishihara Color Plates can indicate color blindness

Color VisionTheories of Color Vision

DON’T DO THIS !

…~3% of male readers will have trouble seeing it!

Color VisionTheories of Color Vision

But this is OK.

Color VisionTheories of Color Vision

So is this.

Color VisionTheories of Color Vision

Even this is good.

Color VisionTheories of Color Vision

But this probably isn’t!

Theories of Color Vision

• Problem with Trichromatic Theory:

Theories of Color Vision

• Problem with Trichromatic Theory:

YELLOW

Theories of Color Vision

• 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 !?

Theories of Color Vision

• 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– Red opposes Green– (Red + Green) opposes Blue

• Opponent-Process Theory explains color afterimages