Midterm 2 March 9 th and 10 th Review Session Monday 7pm in this room (probably)

What Newton Found (and everyone believed) White light can be split into all wavelengths by a prism

Color Vision Theories of Color Vision “Blue” “Green” “Red” Blue Wavelength InputCone Signal to Brain

Color Vision Theories of Color Vision “Blue” “Green” “Red” Green Wavelength InputCone Signal to Brain

Color Vision Theories of Color Vision “Blue” “Green” “Red” Red Wavelength InputCone Signal to Brain

Color Vision Theories of Color Vision “Blue” “Green” “Red” Yellow Equal Parts Red and Green = Wavelength InputCone Signal to Brain

Color Vision Theories of Color Vision “Blue” “Green” “Red” Yellow Equal Parts Red and Green = Wavelength InputCone Signal to Brain

Color Vision Theories of Color Vision “Blue” “Green” “Red” Yellow Equal Parts Red and Green = Wavelength InputCone 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 Light Green 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 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 Light Green 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 long filter Camera splits image into maps 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 perceives desaturated hues including 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?

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

Color Constancy The “color” of objects is independent of the ambient light – even though light can vary dramatically Relative Intensity Wavelength Relative Intensity SunlightIncandescent Light

Color Constancy Because of our mechanism of color constancy we can even use completely artificial spectra

Color Constancy The “color” of objects is independent of the ambient light

Next Time ATTENTION!