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Pilot's Handbook of Aeronautical Knowledge
Aeromedical Factors

Vision in Flight

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Pilot's Handbook of Aeronautical Knowledge



Table of Contents

Chapter 1, Introduction To Flying
Chapter 2, Aircraft Structure
Chapter 3, Principles of Flight
Chapter 4, Aerodynamics of Flight
Chapter 5, Flight Controls
Chapter 6, Aircraft Systems
Chapter 7, Flight Instruments
Chapter 8, Flight Manuals and Other Documents
Chapter 9, Weight and Balance
Chapter 10, Aircraft Performance
Chapter 11, Weather Theory
Chapter 12, Aviation Weather Services
Chapter 13, Airport Operation
Chapter 14, Airspace
Chapter 15, Navigation
Chapter 16, Aeromedical Factors
Chapter 17, Aeronautical Decision Making




The eye’s blind spot.
Figure 16-13. The eye's blind spot.

While the cones and their associated nerves are well suited
to detecting fine detail and color in high light levels, the
rods are better able to detect movement and provide vision
in dim light. The rods are unable to discern color but are
very sensitive at low light levels. The trouble with rods is
that a large amount of light overwhelms them, and they take
a long time to "reset" and adapt to the dark again. There are
so many cones in the fovea that the very center of the visual
field hardly has virtually no rods at all. So in low light, the
middle of the visual field is not very sensitive, but farther
from the fovea, the rods are more numerous and provide the
major portion of night vision.

The area where the optic nerve enters the eyeball has no rods
or cones, leaving a blind spot in the field of vision. Normally,
each eye compensates for the other's blind spot. Figure 16-13
provides a dramatic example of the eye's blind spot. Cover
the right eye and hold this page at arm's length. Focus the left
eye on the X on the right side of the windshield and notice
what happens to the airplane while slowly bringing the page
closer to the eye.

Empty-Field Myopia
Empty field myopia is a condition that usually occurs when
flying above the clouds or in a haze layer that provides
nothing specific to focus on outside the aircraft. This causes
the eyes to relax and seek a comfortable focal distance which
may range from 10 to 30 feet. For the pilot, this means
looking without seeing, which is dangerous. Searching out
and focusing on distant light sources, no matter how dim,
helps prevent the onset of empty field myopia.

Night Vision
It is estimated that once fully adapted to darkness, the rods are
10,000 times more sensitive to light than the cones, making
them the primary receptors for night vision. Since the cones
are concentrated near the fovea, the rods are also responsible
for much of the peripheral vision. The concentration of cones
in the fovea can make a night blind spot in the center of the
field of vision. To see an object clearly at night, the pilot must
expose the rods to the image. This can be done by looking 5°
to 10° off center of the object to be seen. This can be tried in
a dim light in a darkened room. When looking directly at the
light, it dims or disappears altogether. When looking slightly
off center, it becomes clearer and brighter.

Refer to Figure 16-14. When looking directly at an object,
the image is focused mainly on the fovea, where detail is
best seen. At night, the ability to see an object in the center
of the visual field is reduced as the cones lose much of their
sensitivity and the rods become more sensitive. Looking off
center can help compensate for this night blind spot. Along
with the loss of sharpness (acuity) and color at night, depth
perception and judgment of size may be lost.

While the cones adapt rapidly to changes in light intensities,
the rods take much longer. Walking from bright sunlight into
a dark movie theater is an example of this dark adaptation
period experience. The rods can take approximately 30
minutes to fully adapt to darkness. A bright light, however,
can completely destroy night adaptation, leaving night
vision severely compromised while the adaptation process
is repeated.