Why does the sky look blue

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Why does the sky look blue?
Chapter 3 ColorThe Thinking Eye, The Seeing
Brain by James T. Enns
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Classical model

• the object in vision could be completely
  described and understood without ever appealing
  to the perceptual process

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constructivist theory

• nonrelational color cannot be completely
  described or adequately understood without
  appealing to the perceptual process

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Question Why is the sky blue?

• The High School answer
• Light is a form of electromagnetic energy of
  varying wavelengths
• human eyes sense wavelengths from 360 nm to 780
  nm
• When photons hit particles in the air, the short
  wavelength light reflects more than the medium or
  long so the longer ones pass through
• Thus, the air scatters more of the short
  wavelength light the blue.

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Evaluating the High School answer

• The HS answer isnt merely wrong, its pernicious
  and evil because it is a roadblock to
  understanding.
• It stresses the physical steps and ignores the
  neuro-physiological steps.
• In this case, too little knowledge is a dangerous
  thing.

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Color constancy

• the mapping among light, visual processing, and
  color experience is complex

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How many different shades of grey do you see on
the checkerboard surface?
http//web.mit.edu/persci/people/adelson/checkersh
adow_illusion.html
A and B are the same shade.
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Proof
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More Proof
The brain is optimized to construct the virtual
field of vision in a world of 3-D objects and
when the light comes from above and behind, those
objects cast a shadow unless they are
vampires. When objects look the same, they are
Color Constant so that, in ordinary situations,
they look the same. There it is again! In this
experiment, the brain removes the wrong color
and inserts the correct color. So, we cant see
the intrinsic color of the object. Does a cat, a
dog or a bird?
http//web.mit.edu/persci/people/adelson/checkersh
adow_illusion.html
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how many different shades of grey do you see?

• through the perceptual apparatus - the answer is,
  equally obvious
• 3

The box colors outside the shadow and inside the
shadow (maybe 4 more but lets not be picky)
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why is the sky blue is just as difficult to
answer as how many colors do you see the
checkerboard as having?
obviously an indefinite variation in degree of
shading

• the light grey patch inside the shadow is exactly
  the same shade (measured at your eye) as the dark
  grey patch outside the shadow but you cannot
  experience them as the same.

Experience isnt objective in the sense of
physics (the intrinsic color), it is
intersubjective.
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Analysis

• intrinsic color degree of surface
  reflectance, some surfaces absorb most
  wavelengths and reflect a small range so they
  appear red or green, blue is associated with
  short wavelength some reflect the entire
  spectrum and appear white or grey.

See next
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Analysis Part II

• lighting conditions light source, interference
  patterns (e.g., shadows), filters (tree leaves,
  smoke, water, clouds, smog).
• The sun emits broadband light, all of the
  wavelengths

See next
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Analysis Part III

• The light at the eye is a complicated mixture of
  the source, the surface, and lighting conditions
  (like shadows) and filters (smog, smoke, clouds,
  tree leaves)

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Figure 3.2. Color constancy involves analysis of
light in three places
Color constancy is the phenomenon of the
intrinsic color of a surface being visible
regardless of the color of the ambient light
       Light at source        Light at
reflecting surface        Light at eye
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Summary of analysis

• light at the source if we saw light in an
  otherwise black room, we would call it blue
• light at the surface how the surface would
  appear in sunlight (broadband, white light),
  intrinsic color
• light at the eyea combination of the first two

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why is the sky blue? how is the triplet of
reflectance profiles reconstructed by the brain

• light at the source light at the surface
  light at the eye

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why is the sky blue? what color do you see the
sky as?

• The brain must construct the essential color
  of the object by subtracting all the intrinsic
  color, surface reflectance and color at the eye

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Object Invariance

• if color vision were a slave to physics, we would
  see the dark patch outside the shadow and the
  light patch inside the shadow as the identical
  shade.
• it is impossible
• what we see it as turns out to be a good guess
  about what color the object ought to be
• Color constancy is the brains solution for the
  problem of object invariance

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Object invariance

• The intrinsic visual properties of an object
  such as its size, color, and shape regardless
  of the viewing conditions
• We want to be able to recognize an object in
  sunlight, shade, in the house no matter what the
  three other factors happen to be.
• The checkerboard shows how good the brain does
  the job, it shows the extent to which the brain
  can keep objects normal even when a mad
  scientist confuses the issue

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color constancy experiments

• http//www.squarefree.com/2004/03/05/color-constan
  cy-illusion/

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Figure 3.3. The sensitivity of three cone types
to light
Figure 3.3. The sensitivity of three cone types
to light        Each cone is sensitive to light
over a broad range of wavelength.        Cone
types are not evenly spaced across wavelength.
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other violations of direct mapping (camera vision)

• Machinery

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Figure 3.4. The sensitivity of an eye with only
two cone types
       The same relative sensitivity in two cone
types can be achieved by many combinations of
light. (a) the consequences of two different
light mixtures. One (solid lines) consists of
equal amounts of 530 nm and 375 nm light. The
second (dashed lines) consists of equal amounts
of 600 nm and 460 nm light. Although the stimuli
are physically different, they both lead to
equivalent patterns of activity in the two cones.
(b) The colors of the rainbow as perceived by a
individual with only two types of cones like a
dog or a cat or somebody colorblind (dichromacy).
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Figure 3.5. The retina of the human eye
Cones are nothing more than tansducers, i.e.,
they convert light energy to something usable.
Lateral communication is carried out by some
cells like B and D which take into account the
state of the neighborhood before forwarding a
signal. We dont know much about D and E yet.
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Figure 3.6. The receptive fields of ganglion
cells that are color sensitive
       Ganglion cells that are sensitive to
medium-wavelength (green) and long-wavelength
(red) light are spatially more selective than
ganglion cells that are sensitive to
short-wavelength (blue) light. Arrows from the
cones represent a signal to increase () neural
firing. Dots from cones a decrease. L-M opponent
ganglion cells are spatially more selective than
S-(LM) ganglion cells as illustrated by their
smaller center which means that we can detect red
and green color edges with greater spatial
resolution than blue and yellow color edges.
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Figure 3.7. Retinal ganglion cells terminate in
the lateral geniculate nucleus (LGN) and the
superior colliculus (SC)
       LGN and SC are both lower brain centers
(older centers, evolutionarily-speaking). Ganglion
cells that originate in the eye connect to two
evolutionarily old structures. We can ignore
SC, the color opponent ganglion axons from the
eye end in the LGNs top four layers. The
specialization continues into the visual cortex,
V1. Peter (97) was left profoundly blind after a
shock yet he could see all colors non-spatially.
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The visual cortexby Tutis Vilis
The plastinated brain

• http//www.physpharm.fmd.uwo.ca/undergrad/medsweb/
  L2V123/M2-V123.swf

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Figure 3.16. Trichromacy versus Tetrachromacy
       Three cone types (trichromacy) result in
the usual appearance of the rainbow.        What
might the rainbow look like to someone with four
cone types (a tetrachromat)? Until a couple of
years ago, the tetrachromatic superwoman was
dismissed as a woman merely being a girly girl or
too fashion conscious. Now it seems that they
have an extra set of cones. However, since most
vision researchers are men, nobody can conduct
experiments to figure out whats going on because
men cant see what women see. See next.
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Figure 3.17. The rainbow color test
       How many distinct bands of color do you
see?        Tetrachromats see a greater number
of color bands than do trichromats, who in turn
see a greater number of color bands than do
dichtomats. Tetrachromats cannot be males.
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Figure 3.18. Color contrast
        (a) Complementary background colors
make figure colors appear more vivid while colors
appear less vivid against gray.         (b)
Negative afterimages can induce the appearance of
complementary colors so staring at the black
cross for 30 seconds then switching your gaze to
the black cross on the empty background will
create a negative afterimage, meaning the regions
that were blue will be yellow. The grey patches
take on the color that was previously a surround.
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Figure 3.19. Color averaging
       small regions of color in close proximity
to one another produce an averaging effect. The
blue and yellow paints used in these panels are
the same. The only change is in the color of the
intervening stripes which are (a) white, (b)
grey, (c) black. The intervening neutral strips
have an averaging effect on the colored stripes
because, first the blue yellow stripes are
deeper against a darker background and second,
the contrast in brightness between blue and
yellow increases with the increase in thickness.
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Figure 3.20. Neon color illusion
       Color perception fills in empty regions
of a figure but it doesnt really fill in
anything. What we see it as doesnt quite match
our experience.
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Figure 3.21. The watercolor illusion
       Darker colored lines are assumed to be
edges, and lighter colored lines are assumed to
be the color that fills the space. (a) pairs of
differently colored lines create an illusion in
which regions enclosed by the lighter color
appear to be tinted the same even though no
actual color is present in the region. (b) When
lines are all the same color, spatial proximity
determines grouping. Observers see this as a
series of narrow regions separated by wider
regions (the backgrounds). (c) when lines are
bicolored, the region between the lighter colors
is seen as a figure even though the spatial
proximity between lines favors the grouping in
(b).
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What is color for?

• Keeps us out of the rain
• Tells us when the food is rotten
• Provides texture and detail