NIGHT VISION

Generally, most pilots are poorly informed about night
vision. Human eyes never function as effectively at
night as the eyes of animals with nocturnal habits, but
if humans learn how to use their eyes correctly and
know their limitations, night vision can be improved
significantly. There are several reasons for training to
use the eyes correctly.

One reason is the mind and eyes act as a team for a person
to see well; both team members must be used
effectively. The construction of the eyes is such that to
see at night they are used differently than during the
day. 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 10-1]

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, but in the
absence of normal light, the process of night vision is
placed almost entirely on the rods.

The fact that the rods are distributed in a band around
the cones and do not lie directly behind the pupils
makes off-center viewing (looking to one side of an
object) important during night flight. 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 difficult to see anything until the eyes become
adjusted to the darkness. Most everyone has experienced
this after entering a darkened movie theater. In
this process, the pupils of the eyes first 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

Figure 10-1. Rods and cones.

times more sensitive to the light than they were before
the dark room was entered. Much more time, about 30
minutes, is needed for the rods to become adjusted to
darkness, but when they do adjust, they are about
100,000 times more sensitive to light than they were in
the lighted area. After the adaptation process is complete,
much more can be seen, especially if the eyes are
used correctly.