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information that causes pilots to become disoriented.
Eyes
Of all the senses, vision is most important in providing
information to maintain safe flight. Even though the
human eye is optimized for day vision, it is also capable
of vision in very low light environments. During the day,
the eye uses receptors called cones, while at night, vision is
facilitated by the use of rods.
Both of these provide a level
of vision optimized for the
lighting conditions that they
were intended. That is, cones
are ineffective at night and
rods are ineffective during
the day.
Rods, which contain rhodopsin
(called visual purple), are
especially sensitive to light
and increased light washes out
the rhodopsin compromising
the night vision. Hence, when
strong light is momentarily
introduced at night, vision
may be totally ineffective as
the rods take time to become
effective again in darkness.
Smoking, alcohol, oxygen
deprivation, and age affect
vision, especially at night. It
should be noted that at night,
oxygen deprivation such as one
caused from a climb to a high
altitude causes a significant
reduction in vision. A return
back to the lower altitude will
not restore a pilot’s vision in the same transitory period used
at the climb altitude.
The eye also has two blind spots. The day blind spot is the
location on the light sensitive retina where the optic nerve
fiber bundle (which carries messages from the eye to the
brain) passes through. This location has no light receptors,
and a message cannot be created there to be sent to the brain.
The night blind spot is due to a concentration of cones in an
area surrounding the fovea on the retina. Because there are
no rods in this area, direct vision on an object at night will
disappear. As a result, off-center viewing and scanning at
night is best for both obstacle avoidance and to maximize
situational awareness. [See the Pilot’s Handbook of
Aeronautical Knowledge and the Aeronautical Information
Manual (AIM) for detailed reading.]
The brain also processes visual information based upon color,
relationship of colors, and vision from objects around us.
Figure 1-1 demonstrates the visual processing of information.
The brain assigns color based on many items to include an
object’s surroundings. In the figure below, the orange square
on the shaded side of the cube is actually the same color
as the brown square in the center of the cube’s top face.
1-3
Figure 1-2. Shepard’s Tables.
Isolating the orange square from surrounding influences
will reveal that it is actually brown. The application to a real
environment is evident when processing visual information
that is influenced by surroundings. The ability to pick out an
airport in varied terrain or another aircraft in a light haze are
examples of problems with interpretation that make vigilance
all the more necessary.
Figure 1-2 illustrates problems with perception. Both tables
are the same lengths. Objects are easily misinterpreted in
size to include both length and width. Being accustomed to
a 75-foot-wide runway on flat terrain is most likely going
to influence a pilot’s perception of a wider runway on
uneven terrain simply because of the inherent processing
experience.
Vision Under Dim and Bright Illumination
Under conditions of dim illumination, aeronautical charts and
aircraft instruments can become unreadable unless adequate
flight deck lighting is available. In darkness, vision becomes
more sensitive to light. This process is called dark adaptation.
Although exposure to total darkness for at least 30 minutes is
required for complete dark adaptation, a pilot can achieve a
moderate degree of dark adaptation within 20 minutes under
dim red flight deck lighting.
Red light distorts colors (filters the red spectrum), especially
on aeronautical charts, and makes it very difficult for the
eyes to focus on objects inside the aircraft. Pilots should
use it only where optimum outside night vision capability is
necessary. White flight deck lighting (dim lighting) should
be available when needed for map and instrument reading,
especially under IMC conditions.
Since any degree of dark adaptation is lost within a few
seconds of viewing a bright light, pilots should close one eye
when using a light to preserve some degree of night vision.
During night flights in the vicinity of lightning, flight deck
lights should be turned up to help prevent loss of night vision
due to the bright flashes. Dark adaptation is also impaired by
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