CHAPTER 1
INTRODUCTION TO NIGHT VISION EQUIPMENT
Throughout the history of wars, the cover of
darkness has provided a tactical advantage for one
force or the other. In this chapter, we will begin with
a brief explanation of how the human eye “sees”
during daytime and nighttime. We will then introduce
and explain the various devices used in today’s
military to enhance the night vision capabilities of
military personnel.
THE HUMAN EYE
The primary components of the eye that control
day vision and night vision are the rods and the cones,
found in the retina. The retina translates light energy,
absorbed by the rods and cones, into nerve impulses
to be carried to the brain by the optic nerve. The brain
then converts the nerve impulses into images.
The eyes of some animals that hunt at night and
sleep in the daytime (such as the opossum) have
retinas composed almost entirely of rods; other
animals that sleep in the nighttime (such as most
birds) have retinas composed almost entirely of
cones.
The human eye has a retina composed of both
rods and cones.
CONES
The primary factor in both color vision and acute
daytime vision is the cones. During the day, the eye
tends to rotate, to center the image nearer to the area
of the retina where the cones are most concentrated.
The center three degrees of the retina is made up
entirely of cones. This area is called the fovea.
RODS
The rods, being very sensitive to low illumination
levels, control night vision. At night, the eye switches
from cone vision to rod vision and becomes color
blind. Since only rods can adjust to the low-light
levels available at night, the center three degrees of
solid cones becomes a blind spot in the center of the
field of view.
1-1
The average field of view for the human eye is
about 80 degrees vertical by 170 degrees horizontal.
OFF-CENTER VISION
To compensate for the blind spot in the center of
the field of view, an observer should use off-center
vision. Off-center vision involves looking off to the
side of the object of interest, scanning its periphery
using short three-second movements. Three seconds
allows the rods adequate stimulation time. In the
dark, extra stimulation time is needed for the rods to
gather sufficient information to form visual images.
DARK ADAPTATION
The cones and rods are turned on and off by the
secretion of photosensitive pigments called iodopsin
and rhodopsin.
Cones adapt (secrete iodopsin) very quickly to
high-light levels. However, the rods require several
minutes to adjust (secrete rhodopsin) to low-light
levels. In extremely dark situations, the normal eye
may require up to 40 minutes to become fully
dark-adapted.
Now that we have discussed how the human eye
reacts to different light levels, let’s move on to
electro-optical devices.
HISTORICAL BACKGROUND
Early efforts to remove the cover of darkness
included the use of torches, brush fires, flares, and
rockets. During World War I, formal research work
was begun in the realm of night vision, but those
efforts were limited to developing and refining
searchlight illumination.
Research by an electronics company in the 1930’s
led to development of an image tube that could be
used to convert infrared images to visible displays.
The military significance of this quickly became
apparent. Further development led to the production
of a small arms unit—the “Sniperscope.”
Use of the sniperscope involved pointing the
device toward some sound heard in the darkness,
