What Newton Found (and everyone believed)

• White light can be split into all wavelengths by a prism

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

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 twist did Land do to this paradigm that confounds the

conventional understanding of color mixing?

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?

Short- and Long- “record”

• Capture two grey-scale images of the scene using filters that allow only the wavelengths you will project

Camera

“short” filter

“Long” filter

film Projector

Object

Image“Long” filter

“short” filter

medium filter

longfilter

Camera splits image intomaps of “longer” and “shorter” 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

What is Land’s interpretation? How do we perceive color?

Land’s interpretation:

• perception of color is a weighing of the ratio of shorter and longer wavelengths

Land’s interpretation:

• perception of color is a weighing of the ratio of shorter and longer wavelengths

Why would the visual system have evolved this way?

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”

What is color for?

• What is color vision used for?

What is color for?

• What is color vision used for?– Identification - what is this thing?– Discrimination - what other things is this like?– Communication - indicates this thing to others

What is color for?

• What is color vision used for?– Identification - what is this thing?– Discrimination - what other things is this like?– Communication - indicates this thing to others

• But in each case color refers not to the illuminating light, but to the surface of the object itself

What is color for?

• What is color vision used for?– Identification - what is this thing?– Discrimination - what other things is this like?– Communication - indicates this thing to others

• But in each case color refers not to the illuminating light, but to the surface of the object itself

Does the color of an object remain constant under different lighting conditions?

Color Constancy

• The “color” of objects is independent of the ambient light

– yellow bananas and green leaves look yellow and green regardless of whether they are viewed in direct sunlight or by the light of a fire

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• ATTENTION!