The Retina
The retina is a light-sensitive layer at the back of the eye that
covers about 65 percent of its interior surface. Photosensitive cells called rods
and cones in the retina convert incident light energy into signals
that are carried to the brain by the optic nerve. In the middle of the retina
is a thinned out area called the fovea or fovea centralis. It is
the center of the eye's sharpest vision and the location of most color
perception.
"A thin layer (about 0.5 to 0.1mm thick) of light receptor
cells covers the inner surface of the choroid. The focused beam of light is
absorbed via electrochemical reaction in this pinkish multilayered structure.
The human eye contains two kinds of photoreceptor cells; rods and cones.
Roughly 125 million of them are intermingled nonuniformly over the
retina."(Hecht) The ensemble of rods(each about 0.002 mm in diameter)
forms an exceedingly sensitive detector, performing in light too dim for the
cones to respond to. It is unable to distinquish color, and the images it
relays are not well defined.
"In contrast, the ensemble of 6 or 7 million cones (each
about 0.006 mm in diameter) can be imagined as a separate, but overlapping,
low-speed color film. It performs in bright light, giving detailed colored
views, but is fairly insensitive at low light levels."(Hecht)"
The
Fovea Centralis
Though the eye receives data from a field of about 200 degrees,
the acuity over most of that range is poor. To form high resolution images, the
light must fall on the fovea, and that limits the acute vision angle to about
15 degrees. In low light, this fovea constitutes a second blind spot since it
is exclusively cones which have low light sensitivity. At night, to get most
acute vision one must shift the vision slightly to one side, say 4 to 12
degrees so that the light falls on some rods.
Fovea provides our highest resolution vision.
"Just about at the center of the retina is a small depression
from 2.5 to 3 mm in diameter known as the yellow spot, or macula. There is a
tiny rod-free region about 0.3mm in diameter at its center, the fovea
centralis. (In comparison the image of the full Moon on the retina is about 0.2
mm in diameter.) Here the cones are thinner (with diameters of 0.0030mm to
0.0015mm) and more densely packed than anywhere else in the retina. Since the
fovea provides the sharpest and most detailed information, the eyeball is
continuously moving, so that light from the object of primary interest falls on
this region. ...the rods are multiply connected to nerve fibers, and a single
such fiber can be activated by any one of about a hundred rods. By contrast,
cones in the fovea are individually connected to nerve fibers. The actual
perception of a scene is constructed by the eye-brain system in a continuous
analysis of the time-varying retinal image."(Hecht)
Optic Nerve
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The optic nerve is the cable of
nerve fibers with carries the electrical signals from the retina to the brain for processing. The
point of departure of that optic nerve through the retina does not have any
rods or cones, and thus produces a "blind spot". |
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