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mark some supporting structures of overhead transmission
lines that stretch across rivers, chasms, and gorges. These
high intensity lights are also used to identify tall structures,
such as chimneys and towers.
As a result of technological advancements in aviation,
runway lighting systems have become quite sophisticated
to accommodate takeoffs and landings in various weather
conditions. However, the pilot whose fl ying is limited to VFR
needs to be concerned only with the following basic lighting
of runways and taxiways.
The basic runway lighting system consists of two straight
parallel lines of runway-edge lights defi ning the lateral limits of
the runway. These lights are aviation white, although aviation
yellow may be substituted for a distance of 2,000 feet from
the far end of the runway to indicate a caution zone. At some
airports, the intensity of the runway-edge lights can be adjusted
to satisfy the individual needs of the pilot. The length limits
of the runway are defi ned by straight lines of lights across the
runway ends. At some airports, the runway threshold lights are
aviation green, and the runway end lights are aviation red.
At many airports, the taxiways are also lighted. A taxiwayedge
lighting system consists of blue lights that outline
the usable limits of taxi paths. See the Pilot’s Handbook
of Aeronautical Knowledge for additional information on
airport lighting.
Night Vision
Generally, most pilots are poorly informed about night vision.
Human eyes never function as effectively at night as the eyes
of nocturnal animals, but if humans learn how to use their
eyes correctly and know their limitations, night vision can
be improved signifi cantly. The human eye is constructed so
that day vision is different from night vision. Therefore, it is
important to understand the eye’s construction and how the
eye is affected by darkness.
Innumerable light-sensitive nerves called cones and rods are
located at the back of the eye or retina, a layer upon which all
images are focused. These nerves connect to the cells of the
optic nerve, which transmits messages directly to the brain.
The cones are located in the center of the retina, and the rods
are concentrated in a ring around the cones. [Figure 12-6]
The function of the cones is to detect color, details, and
faraway objects. The rods function when something is seen
out of the corner of the eye or peripheral vision. They detect
objects, particularly those that are moving, but do not give
detail or color—only shades of gray. Both the cones and the
rods are used for vision during daylight.
Although there is not a clear-cut division of function, the rods
make night vision possible. The rods and cones function in
daylight and in moonlight; in the absence of normal light,
the process of night vision is almost entirely a function of
the rods.
The fact that the rods are distributed in a band around the
cones and do not lie directly behind the pupils makes offcenter
viewing (looking to one side of an object) important
during night fl ight. During daylight, an object can be seen
best by looking directly at it, but at night a scanning procedure
to permit off-center viewing of the object is more effective.
Therefore, the pilot should consciously practice this scanning
procedure to improve night vision.
The eye’s adaptation to darkness is another important aspect
of night vision. When a dark room is entered, it is diffi cult to
see anything until the eyes become adjusted to the darkness.
In the adaptation process, the pupils of the eyes fi rst enlarge
to receive as much of the available light as possible. After
approximately 5 to 10 minutes, the cones become adjusted to
the dim light and the eyes become 100 times more sensitive
to light than they were before the dark room was entered.
About 30 minutes is needed for the rods to become adjusted
to darkness; when they do adjust, they are about 100,000
times more sensitive to light than in the lighted area. After
the adaptation process is complete, much more can be seen,
especially if the eyes are used correctly.
12-6
Figure 12-6. Rods and cones.
The rods and
cones (film) of
the retina are
the receptors
which record
the image and
transmit it
through the
optic nerve to
the brain for
interpretation.
Rods and
Cones
Fovea
(All Cones)
Cones active
Area of best day vision
Rods active
Night blind spot
Area of best
night vision
Lens
Retina
Optic nerve
　
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