Midterm 2
• March 9th and 10th
• 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
Theories
ofVision
Color Vision
Wavelength Input
Cone
“Blue”
“Green”
“Red”
Signal to Brain
Blue
Color
Theories
ofVision
Color Vision
Wavelength Input
Cone
Signal to Brain
“Blue”
“Green”
“Red”
Green
Color
Theories
ofVision
Color Vision
Wavelength Input
Cone
Signal to Brain
“Blue”
“Green”
“Red”
Red
Color
Theories
ofVision
Color Vision
Wavelength Input
Cone
Signal to Brain
“Blue”
“Green”
Equal Parts
Red and
Green =
Yellow
“Red”
Color
Theories
ofVision
Color Vision
Wavelength Input
Cone
Signal to Brain
“Blue”
“Green”
Equal Parts
Red and
Green =
Yellow
“Red”
Color
Theories
ofVision
Color Vision
Wavelength Input
Cone
Signal to Brain
“Blue”
“Green”
Equal Parts
Red and
Green =
Yellow
“Red”
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 = 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
How did Land project the
“appropriate” ratio of
wavelengths?
Short- and Long- “record”
Camera
• Capture two greyscale images of
the scene using
filters that allow
only the
film
wavelengths you
will project
“Long” filter
Object
“short” filter
Projector
“Long” filter
“short” filter
Image
Camera splits image into
maps of “longer” and “shorter”
wavelengths
long
filter
medium filter
Projector combines “longer” and “shorter” wavelengths
using the maps to get the appropriate amounts of each
long/“red”
light
medium/
“green” light
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?
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
Sunlight
Relative Intensity
Relative Intensity
• The “color” of objects is independent of the ambient
light – even though light can vary dramatically
Wavelength
Incandescent 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!