Cones
Fovea
(All Cones)
Rod
Concentration
{
{
{
{
{
Figure 13-1. A camera is able to focus on near and far objects
by changing the distance between the lens and the film. You
can see objects clearly at various distances because the
shape of your eye’s lens is changed automatically by small
muscles.
Figure 13-2. This illustration provides a dramatic example of
the eye’s blind spot. Cover your right eye and hold this page
at arm’s length. Focus your left eye on the X in the right side
of the visual, and notice what happens to the aircraft as you
slowly bring the page closer to your eye.
13-2
function well in darkness, which explains why you
cannot see color as vividly at night as you can during
the day. [Figure 13-3]
RODS
The rods are our dim light and night receptors and are
concentrated outside the fovea area. The number of
rods increases as the distance from the fovea increases.
Rods sense images only in black and white. Because
the rods are not located directly behind the pupil, they
are responsible for much of our peripheral vision.
Images that move are perceived more easily by the rod
areas than by the cones in the fovea. If you have ever
seen something move out of the corner of your eye, it
was most likely detected by your rod receptors.
Since the cones do not function well in the dark, you
may not be able to see an object if you look directly at
it. The concentration of cones in the fovea can make a
night blindspot at the center of your vision. To see an
object clearly, you must expose the rods to the image.
This is accomplished by looking 5° to 10° off center of
the object you want to see. You can try out this effect
on a dim light in a darkened room. When you look
directly at the light, it dims or disappears altogether. If
you look slightly off center, it becomes clearer and
brighter. [Figure 13-4]
How well you see at night is determined by the rods in
your eyes, as well as the amount of light allowed into
your eyes. The wider the pupil is open at night, the better
your night vision becomes.
NIGHT VISION
The cones in your eyes adapt quite rapidly to changes in
light intensities, but the rods do not. If you have ever
walked from bright sunlight into a dark movie theater, you
have experienced this dark adaptation period. The rods
can take approximately 30 minutes to fully adapt to the
dark. Abright light, however, can completely destroy your
night adaptation and severely restrict your visual acuity.
There are several things you can do to keep your eyes
adapted to the dark. The first is obvious; avoid bright
lights before and during the flight. For 30 minutes
before a night flight, avoid any bright light sources,
such as headlights, landing lights, strobe lights, or
flashlights. If you encounter a bright light, close one
eye to keep it light sensitive. This allows you to see
again once the light is gone. Light sensitivity also can
be gained by using sunglasses if you will be flying from
daylight into an area of increasing darkness.
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