EXAM 2 !!!

Monday, Tuesday, Wednesday, Thursday of NEXT WEEK

EXAM 2 !!!

Will cover everything from:

taste/touch/smell/balanceTo

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• 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• 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 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• 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?

MagentaThink 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

• 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• “yellow” = red + green

• Your visual system balances input to either end of each channel

Theories of Color Vision: Opponent-Process Theory

• 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 LightGreen Light

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

Red LightGreen Light

How did Land project the “appropriate” ratio of

wavelengths?

medium/“green” light

long/“red”light

Projector combines “longer” and “shorter” wavelengths using the maps to get the appropriate amounts of each

Viewer perceivesdesaturated huesincluding 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• The “color” of objects is independent of the ambient

light – even though light can vary dramatically

Rel

ativ

e In

tens

ity

Wavelength

Rel

ativ

e In

tens

itySunlight Incandescent Light

Color Constancy• Because of our

mechanism of color constancy we can even use completely artificial spectra

Next Time

• ATTENTION!