Title: How do we see what we see? How do we hear what we hear?
1How do we see what we see?How do we hear what we
hear?
- A question that has intrigued philosophers since
the beginning of recorded history
2Early Greek Philosophers
- Democritus (ca 460 B.C.) Epicurus (341-270 B.C.).
Eidola or film theory of vision. - Objects emit particles or copies of themselves
- For particles are continually streaming off from
surfaces of bodies, though no diminution of the
bodies is observed, because other particles take
their place. And those given off for a long time
retain the position and arrangement which their
atoms had when they formed part of the solid
bodies.
3Early Greek Philosophers Problems with Eidola
theory
- Why isnt matter used up?
- How does the image get in the eye?
- Why cant we see in the dark?
- Are copies given off in all directions?
- These problems led to other theories being
proposed.
4Early Greek PhilosophersScientific progress
Problems lead to new theories
- Extramission or visual touch theories.
- Plato (ca 427-347 B.C). Eyes emit visual fire
which coalesces with daylight to contact objects
and returns an impression of the object. - Problems with visual touch theories.
- Exactly what is emitted?
- How does it return impressions?
- Why cant you see in the dark?
5Early Greek PhilosophersExtramission theory
Theoretical development
- Euclids (300 B.C.) geometric theory of vision.
Assume - That the rectilinear rays proceeding from the eye
diverge indefinitely - That the figure contained by a set of visual rays
is a cone of which the vertex is at the eye and
the base at the surface of the object seen - That those things are seen upon which visual rays
fall and those things are not seen upon which
visual rays do not fall.
6Early Greek PhilosophersExtramission theory
Theoretical development
- That things seen under a larger angle appear
larger, those under a smaller angle appear
smaller, and those under equal angles appear
equal - That things seen by higher visual rays appear
higher, and things seen by lower rays appear
lower - Etc.
7Extramission theory Problems led to
modifications
- Galens answer (ca. 129-199 A.D.) as to why light
is needed. - When it (the air) has been illuminated by the
sun, it is already an instrument of vision of the
same description as the pneuma arriving from the
brain but until it is illuminated it does not
turn into a sympathetic instrument in accordance
with the change effected by the outflow of pneuma
into it.
8After the fall of the Roman Empire
- No further developments that we know of until
- The work of the Islamic scholars
- Al-Kinde (9th century) took elements of
extramission theory and Euclids geometry and
fused them into a coherent theory of vision. - Alhazen (969-1039 A.D.), however, took visual
theory to a whole new level.
9The Islamic Scholars Alhazen
- Proposed a new intromission theory.
- Took Euclid and turned it around.
- Points on a body radiate light in all directions.
- From each point of every colored body,
illuminated by any light, issue light and color
along every straight line that can be drawn from
that point.
10The Islamic Scholars Alhazen
- Only those rays that enter the pupil of the eye
produce a visual impression. - One problem.
- Superfluity of rays problem.
11The lens is the seat of vision.
Rays from different points in space fall on same
point on the lens.
12The Islamic Scholars Alhazen
Knew about refraction.
Hypothesized only non-refracted rays penetrate
the eye
13One point in space corresponds to one point on
the lens
14Alhazen theory was not overturned until the work
of Kepler
- Kepler (1571-1630) worked out the geometric
optics of the eye. - Light is reflected in all directions from each
point on a non-mirror surface. - He traced the rays and proved that an upside-down
and right-left reversed image should appear on
the retina.
15Thus, some 20 centuries later we finally solved
how images were sensed by the eye
- But now we had new problems to solve.
- Images on the retina change with distance
- Images on the retina change with orientation
- Images on the retina change with angle of view
16New problem Superfluity of Images
- For each object there are potentially an infinite
number of images that can be formed. - How does the observer recognize individual object
when the number of images is infinite? - Do they have an infinite number of templates to
match against the image?
17To answer this problem we need to consider how
signals are processed
- How do the eye and brain solve the superfluity of
images problem?. - Question How much information do we need to
characterize an image?
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33 n 3
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n 31 n 61 checkerboard
39- Figure 4.4
- The human eye, a simplified view.
40- Figure 4.2
- The lens gets its name from Latin for lentil,
referring to its shapean appropriate choice, as
this cross section of the eye shows. The names of
other parts of the eye also refer to their
appearance.
41- Figure 4.6
- The retina lies behind the vitreous humor, which
is the jelly-like substance that fills the
eyeball. Note that light does not fall directly
on the rods and cones. It must first pass through
the outer layers of the retina, made up of
additional nerve cells. Only about one half of
the light falling on the front of the eye reaches
the rods and conestestimony to the eyes amazing
light sensitivity. The rods and cones are much
smaller than implied here. The smallest receptors
are 1 micron (one millionth of a meter) wide. The
lower left photograph shows rods and cones as
seen through an electron microscope. In the
photograph the cones are colored green and the
rods blue.