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2. Biological Eye
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EYE DESIGN BOOK
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3. EYE DESIGN ILLUSTRATIONS
Birds need more
complex vision systems than many land animals. Some birds use the sun
and maybe even star patterns to navigate. Some bird eyes provide very
good distant vision. Large hunting birds, such as vultures, can spot an
animal carcass from 3,000 or 4,000 meters. Eagles may see a fish or
evidence of a fish at the same distance. At that height, most humans
cannot even see the bird. Most birds are far-sighted, and accommodation
of the eye is superior to most eyes mentioned so far. Of all the larger
animals, birds have the highest density of photoreceptors. For example,
the eyes of the hawk have 1 million photoreceptors per square
millimeter. This is a higher density than many of the CCD image
detectors used in today’s video cameras. Smaller sparrow eyes
have 400,000 photoreceptors per square millimeter, double the density
of photoreceptors in the larger human eye.
Eagles need to see
long distances with good resolution in order to hunt. They have closer
sensor spacing than human eyes, so they are able to see more details at
a given distance. Their image processing is carried out using a much
smaller brain than that of humans. They can respond quickly to what
they see, such as when they swoop down to pick up a small rodent.We
have heard that eagles can see a fish at five miles, but they are
probably seeing a reflection of a fish jumping out of the
water. One expects they would see less overall color than humans. Even
with a variety of pigments in their eyes, they can sense polarized
light. Eagles have excellent vision near their central viewing
remote-piloted vehicles have been patterned after the eagle. They are
not as versatile as an eagle's seeing capability for equivalent
tions and equivalent overall size. Most do not have the dynamic range
of sensing and are usually only relaying images to a command area, not
fully pro- cessing images for internal decision making. (Newspaper Ad,
unknown source, Trad. Photo)
The remarkable Hawk
Eye is probably similar to that of the Eagle and other similar birds.
Figure 3.38a illustrates a very sophisticated optical design in a very
small package for specific needs of the Hawk. (Reference: Figure 5.12,
p. 91, Animal Eyes,
Michael F. Land, Dan-Eric Nilsson, Oxford Animal Biology series, Oxford
University Press, 2002- Please see their book for more details )
3.38 Eagle Eyes.
flowers at a distance and very small parts of a flower at close range.
This indicates they have good focusing ability for near and far
objects. They probably have extended UV spectrum color vision like
insects, as well as vision to sense polarized light. Their brain has
the ability to control three-dimensional navigation and coordinate
rapid movements of their wings to their eyes, so they can take full
advantage of their eyesight. (P. 185, Readers Digest, Exploring
the Secrets of Nature, 1994)
3.39 Hummingbird Eyes.
3.39b Hummingbird Eyes.
Owls locate their target or
meals with their precise directional sensing using the difference of
the time it takes for sound to reach each of their two ears. They then
turn their eyes to the direction from which the sound originated. Owls
learn to do this while quite young. However, as they grow,
the capability has to be modified as the ear separation grows. Here
there must be some special compensation, in the brain, indicating
intelligence to compensate for increasing ear separation.
Owls have sensitive eyes for
hunting at night, they tend to have a large aperture (large
NA or small f/number) eye. large for their overall body size. Owl eyes
do not rotate as much as human eyes do, but an owl's head can
turn a considerable angle to accomplish the same purpose. They also
have good stereo vision and depth perception because of the owl's eyes
being separated by a significant distance. (Fig. 3.40 from P. 93,
Readers Digest, Exploring the
Secrets of Nature, 1994 and fig.
3.41 adapted from p. 105, fig 40 Vision
in the Animal World, R. H.
Smythe, Macmillan Press, 1975)
The ostrich has the
largest eye of any bird. It also has the longest neck of any bird. This
allows it to move its eyes close to smaller targets. It does not have
the same needs for extremely high resolution or acute eyesight as the
high flying eagle and other smaller birds, since it does not fly. (Pg.
126 The Illustrated
Encyclopedia of the Animal Kingdom,
1970, Danbury Press)
3.40a Owl Eyes.
3.40b Owl Eyes.
3.41 Optical Cross
section of Owl Eyes
capability to Focus
3.42 Ostrich Eyes.
3.42b Ostrich Eyes.
The cormorant eye
must have the ability for optical correction in a land based
camera eye. When it dives in water it must focus to catch a fish. It
can maintain good focus over a wide range in air and water. Its brain
can make fine control decisions based on rapid processing of the
eyes-output. This is especially true as it tracks a fish in the water
from the air and then dives after it, moving to readjust its vision
very quickly as it swims under the water. The adjustment requires
optical design adaptation when in the water because of variable
pressure and different focus requirements. (P. 323, Readers Digest, Exploring
the Secrets of Nature, 1994)
3.43. Cormorant Eyes.
The Falcon eyes must
have the ability for high-resolution and dynamic correction while
diving at almost 200 miles per hour. Some have said that it has eight
times the resolution of human eye where it is able to see at 160 feet
what humans see at 20 feet. It is somewhat similar to the eagle. It
achieves the best combination of speed and visual resolution. It is
able to target distant prey such as other birds and intercept them in
flight. As it intercepts them it uses its clenched claws to disable the
prey and then it swoops under them to recover the kill before it falls
to earth. For many years people have trained Falcons as hunters. Figure
3.43f illustrates the Falcon (From Today's
Chemist, page 11, Jan. 2002).
3.43f Falcon Eyes.