(This article has been adapted from the excellent
USAF Special Report, AL-SR-1992-0002, "Night Vision
Manual for the Flight Surgeon", written by Robert
E. Miller II, Col, USAF, (RET) and Thomas J. Tredici,
Col, USAF, (RET))
structure of the eye is shown in Figure 1. The anterior
portion of the eye is essentially a lens system,
made up of the cornea and crystalline lens, whose
primary purpose is to focus light onto the retina.
The retina contains receptor cells, rods and cones,
which, when stimulated by light, send signals to
the brain. These signals are subsequently interpreted
Most of the
receptors are rods, which are found predominately
in the periphery of the retina, whereas the cones
are located mostly in the center and near periphery
of the retina. Although there are approximately
17 rods for every cone, the cones, concentrated
centrally, allow resolution of fine detail and color
discrimination. The rods cannot distinguish colors
and have poor resolution, but they have a much higher
sensitivity to light than the cones.
VERSUS NIGHT VISION
to a widely held theory of vision, the rods are
responsible for vision under very dim levels of
illumination (scotopic vision) and the cones function
at higher illumination levels (photopic vision).
Photopic vision provides the capability for seeing
color and resolving fine detail (20/20 of better),
but it functions only in good illumination. Scotopic
vision is of poorer quality; it is limited by reduced
resolution ( 20/200 or less) and provides the ability
to discriminate only between shades of black and
white. This limitation can be easily confirmed by
noting that, at dusk, the different colors of a
flower garden become virtually indistinguishable.
However, the scotopic system provides enhanced sensitivity
and low detection thresholds under markedly reduced
system allows the human eye to maintain sensitivity
over an impressively large range of ambient light
levels. Between the limits of maximal photopic vision
and minimal scotopic vision, the eye can function
rather effectively to changes in brightness of as
much as 1,000,000,000 times. The sensitivity of
the eye automatically adjusts to changes in illumination.
The dimmest light in which the rods can function
is equivalent to ambient conditions of an overcast
night with no moonlight. The dimmest light in which
the cones can function is roughly equivalent to
a night with 50% moonlight. Thus a white which can
just barely be seen by the rods must be increased
in brightness 1,000 times before it becomes visible
to the cones. The light intensity of the sun is
approximately 30,000 times that of the moon, yet
the eye can function well in bright sunlight as
well as in dim moonlight. Although the human eye
can function over a vast range of brightness, the
retina is sensitive to damage by light, e.g., from
lasers or unprotected sun gazing. This potential
for light injury exists because the optics of the
eye can concentrate light energy on the retina by
a factor of 100,000 times.
a common misconception that the rods are used only
at night and the cones only during the day. Actually,
both rods and cones function over a wide range of
light intensity levels and, at intermediate levels
of illumination, they function simultaneously. The
transition zone between photopic and scotopic vision
where the level of illumination is equivalent to
twilight or dusk, is called mesopic vision. Neither
the rods nor the cones operate at peak efficiency
in this range, but both actively contribute to visual
perception. Mesopic vision may be of primary importance
to the aviator at night because some light is often
present during night operations. Below the intensity
of moonlight, the cones cease to function and the
rods alone are responsible for what is pure scotopic
is an independent process during which each eye
adjusts from a high-luminance setting to a low-luminance
setting. The exact mechanisms are unclear, but they
are known to include biochemical, physical, and
and cones contain light-sensitive chemicals called
photopigments. The photopigment in the rods is called
rhodopsin. There are three different types of cone
photopigments that are composed of opsins only slightly
different from rhodopsin. Upon exposure to light,
photopigments undergo a chemical reaction that converts
light energy to electrical activity, initiating
visual impulses in the retina that are conducted
by nerve fibers from the eye to the brain. The initial
chemical reaction is called light adaptation and,
in this process, the photopigments are decomposed.
Intense light will decompose the photoreceptor pigments
rapidly and completely, thus reducing retinal sensitivity
to dim light. Regeneration of the photopigments
occurs during dark adaptation.
dark-adapted eye, in which photopigment regeneration
is complete, restores retinal sensitivity to its
maximal level. Rods and cones differ markedly, however,
in their rate of dark adaptation. Cones attain maximum
sensitivity in 5-7 minutes, while rods require 30-45
minutes or longer of absolute darkness to attain
maximum sensitivity after exposure to bright light.
have a faster rate of photochemical regeneration
because they function in greater light than the
rods. The cones, however, do not achieve the same
level of sensitivity as the rods. The rods slowly
adapt to dim illumination, but eventually achieve
a much greater sensitivity than the cones. Depending
on the preadaptation to light, dark adaptation is
about 80% complete within 30 minutes, but it may
take hours, or even days, to acquire total dark
to adaptation caused by changes in photopigment
concentrations, the eye has other mechanisms for
adapting to changing light conditions. Retinal adaptation
can be affected by physical changes in the size
of the pupil. The diameter of the pupil can contract
to 1.5mm and expand to 8mm, which equates to a 30-fold
range in the quantity of light entering the eye.
mechanism is neural adaptation, which is generated
by retinal neurons at successive stages of the visual
chain in the retina. A change in "neural gain" occurs
in seconds and can improve night vision by a factor
of 10 or more. Neural adaptation is rather like
having low-speed and high-speed film simultaneously
available in your camera. Furthermore, a large share
of the inherently greater sensitivity of rod dark
adaptation is a result of retinal summation. As
many as 100 rods, or more, converge onto a single
nerve fiber in the retina, which greatly increases
sensitivity. Thus, if the rods are slightly stimulated,
the summation of several low-level stimuli might
be enough to initiate a light signal to the brain.
Unlike the photoreceptor chemical changes, these
mechanisms occur instantaneously.
BLIND SPOT AT NIGHT
of the retina responsible for the keenest visual
acuity (VA) is the fovea, which corresponds to the
center of the visual field. The foveola, or center
of the fovea, possesses a high degree of cones,
but is completely devoid of rods. Thus, if the ambient
light is below cone threshold, when any small object
is fixated centrally, it cannot be seen because,
at light levels below dim starlight, a blind spot
exists in the central 1 degree of the visual field.
This central blind spot corresponds to the foveola,
which is rod-free; it cannot function in diminished
present outside the central 1-degree foveolar area.
The rods increase gradually with eccentricity from
the foveola, and finally reach a maximum concentration
at a point some 17 degrees from the fovea. Since
the rods have a lower threshold than the cones,
they are much more sensitive to light. A person
attempting to see in scotopic illumination, light
dimmer than moonlight, has to depend entirely on
rods. To best detect small targets with the rods
under such circumstances, the individual must look
approximately 15-20 degrees to one side, above,
or below an object to place the object of regard
on the part of the retina that possesses the highest
density of rods. Individuals can be taught to fixate
to one side of an object to avoid the central blind
spot and to scan, utilizing the most sensitive part
of the retina to improve target detection at night.
Therefore, proper education and training are helpful
in maximizing visual function at night.
of the electromagnetic energy spectrum which stimulates
the photoreceptors in the retina is known as visible
light. Visible light includes violet, indigo, blue,
green, yellow, orange, and red, i.e., a range of
wavelengths extending from about 380nm to 760nm.
Adjacent portions of the spectrum, although not
visible, can affect the eye. Ultraviolet wavelengths
extend from 180nm to 380nm. Exposure of the eyes
to ultraviolet radiation can produce eye tissue
damage. Acute overexposure can cause photokeratitis,
commonly called snow blindness, while chronic exposure
has been implicated as a possible cause of cataracts.
Infrared wavelengths occur from 760nm up to the
microwave portion of the spectrum. Infrared, or
thermal radiation, can also damage ocular tissue.
However, infrared does not contain as much energy
as shorter wavelength ultraviolet. Most night vision
devices are sensitive to portions of the infrared
cones are not equally sensitive to visible wavelengths
of light. Unlike the cones, rods are more sensitive
to blue light and are not sensitive to wavelengths
greater than about 640nm, the red portion of the
shift is the relatively greater brightness of blue
or green light, compared with yellow or red light,
upon shifting from photopic to scotopic adaptation.
For example, in a darkened room, if one looks at
two dim lights of equal illumination (one red and
one green) that are positioned closely together,
the red light will look brighter than the green
light when the eyes are fixating centrally. If one
looks to the side of the dim lights about 15-20
degrees, the green light will appear brighter than
the red. Central fixation involves the cones and
photopic vision while fixating eccentrically involves
rods and scotopic vision. The cones are more sensitive
to yellow and red, but the rods are more sensitive
to light of the blue and green wavelengths. The
most sensitive wavelength for cones is 555nm (yellow-green).
That is why the "optic yellow" tennis and golf balls
are, in fact, easier to see under photopic conditions.
The most sensitive wavelength for rods is 505nm
(blue-green). Thus, blue-green lights will generally
look brighter at night than red lights. The sensitivity
of the eye changes from the red end of the visible
spectrum toward the blue end when shifting from
the photopic to scotopic vision.
demonstration of the difference between photopic
and scotopic sensitivity is to slowly decrease the
intensity of a colored light until the cone threshold
is reached. This is the point at which the color
will disappear, but not the sensation of light.
When this procedure is performed with any color
except red, the color will disappear at the cone
threshold, but the light will still be perceived
by the rods as dim gray. If the intensity is further
decreased until the rod threshold is reached, the
light will disappear entirely. With red light, the
color and sensation of light disappear at the same
between the level of illumination at which the color
of a light disappears (cone threshold) and that
at which the light itself disappears (the rod threshold)
is known as the photochromatic interval. There is
a photochromatic interval for every color of the
spectrum, except for the longer red wavelengths.
blindness is unusual. Night blindness can be caused
by long-term vitamin A deficiency such as may occur
from chronic starvation, alcoholism, deficient fat
absorption, and diseases of the liver. Retinal conditions
that may cause night blindness are glaucoma, drug
toxicity and numerous hereditary disorders. Although
not true night blindness, night myopia may also
reduce night vision. Considerable individual variability
exists in retinal sensitivity to light among normals.
Also, as people age, night vision decreases.
also has a physiologic blind spot. Unlike the central
blind spot that is only present in low light, the
physiologic blind spot is always present. It is
caused by the position of the optic nerve in the
rear of the eye. The optic nerve is the confluence
of retinal nerve fibers leaving the eye. There are
no retinal receptors overlying the optic nerve.
Fortunately, the physiologic blind spots occur in
a different position in each eye. Thus, when both
eyes are open, the physiologic blind spots are not
ASPECTS OF NIGHT VISION
of decreased illumination on operational visual
function can be dramatic. Visual acuity may be reduced
to 20/200 or less, color vision is lost, blue-green
lights will appear brighter while red lights will
appear dimmer, problems may occur with night myopia,
depth perception is degraded, glare is a factor,
and a central blind spot is present. The potential
effects of these factors on the operational aspects
of night vision will now be considered.
is reduced at night under low illumination and 20/20
vision cannot be sustained below a level of deep
twilight. Objects can be seen at night only if they
are either lighter or darker than their background
and can be discriminated by subtle differences in
contrast. Because VA at night is a function of small
differences in the brightness (luminance contrast)
between objects and their background, any transparent
medium through which the flyer must look should
be kept spotlessly clean. Contrast discrimination
may be reduced by light reflected from windshields,
visors, spectacles, fog, or haze.
of the importance of contrast at night was used
by pilots during World War II to detect enemy planes,
as well as to hide their own positions. Pilots would
fly below the enemy when passing over dark areas,
such as land, or when flying over an area illuminated
by multiple points of light, such as a large city
at night. Conversely, they would fly above the enemy
when passing over white clouds, desert, moonlit
water, or snow. Following another aircraft from
above or below, rather than from directly behind,
will maintain the largest retinal image and lessen
the likelihood of losing sight of the other aircraft
in the darkness.
do not normally wear spectacles, or those who wear
visual correction, may have a small myopic shift
under extremely reduced illumination. This near-sighted
or myopic shift is called the dark focus of the
eye. The dark focus of the eye may become a problem
whenever there is a lack of adequate distance objects
upon which to focus. For most people, night myopia
has a relatively minor effect because no visually
resolvable distant target is present when it occurs.
When a target does become visible, the eye rapidly
readjusts. Since night myopia does not occur in
the cockpit when the crew is operating under photopic
conditions, it is probably more of a theoretical
Goggles or Spectacles
utilization of scotopic vision, 20 to 30 minutes
in total darkness are required to attain satisfactory
retinal dark adaptation. An alternative is to have
the aircrew member wear red goggles for 20 to 30
minutes before flying. When worn in normal illumination,
red goggles will not interfere significantly with
the ability to read most maps, charts, manuals,
etc., as long as the printing is not in red ink.
Red goggles block all light except red, which enhances
rod dark adaptation because red light does not stimulate
the scotopic system.
some drawbacks to wearing red goggles or using red
cockpit lighting. When reading maps, markings in
red on a white background may be invisible. Red
light also creates or worsens near-point blur in
older far-sighted, presbyopic (decreased near focusing
ability due to age), and pre-presbyopic aircrew.
Under red light or using red goggles in normal light,
red light is focused behind the retina due to the
optics of the eye and more "near focusing" than
average must be used to provide a clear image when
reading at near.
adaptation of the rods develops rather slowly over
a period of 20 to 30 minutes, it can be lost in
a few seconds of exposure to bright light. Accordingly,
during night operations aircrew members should be
taught to avoid bright lights, or, at least, protect
one eye. Dark adaptation is an independent process
in each eye. Even though bright light may shine
into one eye, the other eye will retain its dark
adaptation if it is protected from the light. This
is a useful bit of information, because a flyer
can prevent flash blindness and preserve dark adaptation
in one eye by simply closing or covering it. The
instrument panel should be kept illuminated at the
lowest level consistent with safe operation, and
the flyer should avoid looking at exhaust flames,
strobes, searchlights, etc. to avoid temporary flash
blindness. If light must be used, it should be as
dim as possible and should only be used for the
shortest possible period.
to ordinary sunlight can produce temporary but cumulative
aftereffects on dark adaptation and night vision.
Both civilian and military studies have documented
significantly diminished rod performance after prolonged
sunlight exposure at, for example, the beach or
ski slope. Two or three hours of bright sunlight
exposure has been shown to delay the onset of rod
dark adaptation by 10 minutes or more, and to decrease
the final threshold, so that full night vision sensitivity
could not be reached for hours. After 10 consecutive
days of sunlight exposure, the losses in night vision
were reported to cause a 50 % loss in visual acuity,
visibility range, and contrast discrimination. Repeated
daily exposures to sunlight prolong the time to
reach normal scotopic sensitivity, so that eventually
normal rod sensitivity may not be reached.
several means for providing eye protection during
the day and conserving night vision. First, aircrew
members should remain inside during the day of a
night flight, if at all possible. Sleeping during
the day in a darkened room is highly recommended.
While outside, aviators should wear their sunglasses
and a hat with a brim, which will block a great
deal of ambient solar radiation. Dark sunglasses
that transmit only 15% of the visible light will
prevent degradation of night vision. In general,
one day of protection from sunlight exposure was
usualy sufficient to recover normal vision sensitivity.
However, in certain individuals, it may take days
to weeks to recover full night vision capability.
to be effective, all visible light must be attenuated,
not just portions of the visible spectrum. Thus,
colored or yellow visors or spectacles are not protective.
To protect night vision, provide the best comfort,
allow for scanning close to the sun, and provide
normal color vision, dark sunglasses with a neutral
gray tint are recommended. Tinted sunglasses that
are too dark may reduce visual acuity unless the
ambient brightness remains excessively high. Lighter
sunglasses may not be dark enough to protect retinal
sensitivity for night vision. Therefore, it has
long been recognized that military flyers should
be required to use sunglasses on sunny days. These
sunglasses should have a visible luminance transmission
of 15%. Anyone who is unusually sensitive should
remain indoors during the afternoon before a night
mission. Pilots should ensure that they wear sunglasses
or sunvisors when flying on sunny days.
was used for illumination of the cockpit in post-World
War II aircraft because it, like red goggles, did
not degrade dark adaptation. The intent was to maintain
the greatest rod sensitivity possible, while still
providing some illumination for central foveal vision.
However, red cockpit lights interfered with reading
maps and log books, especially for pre-presbyopic
and presbyopic aviators. With the increased use
of electronic and electro-optical devices for navigation
the importance of the pilot's visual efficiency
in the cockpit has increased and new concerns have
white cockpit lights are often used now because
they afford a more natural visual environment within
the aircraft, without degrading the color of objects.
ILLUSIONS AT NIGHT
references, due to low ambient light levels, can
lead to several types of visual illusions that may
cause spatial disorientation. There are two forms
of visual processing, central (foveal) and peripheral
(ambient). Whereas central vision is predominately
concerned with object recognition and discrimination,
peripheral vision provides motion detection and
spatial orientation information. At night, under
reduced illumination levels, the normal peripheral
vision cues may be degraded or absent, and spatial
disorientation may be more difficult to overcome.
Spatial disorientation may arise from labyrinthine,
proprioceptive, or visual mechanisms.
or the autokinetic effect, is the phenomenon of
perceived movement exhibited by a static dim light
when it is stared at in the dark. This effect can
be demonstrated by staring at a lighted cigarette
in a dark room. The dim light will appear to move
about, even though it is stationary. Although the
exact cause of this illusion is not known, it is
related to tiny fixation movements of the eye and
the loss of the surrounding references which normally
stabilize visual perception. This illusion will
also occur when there are two dim lights in darkness,
but it normally disappears when three or more lights
are present. Pilots on night flights have mistaken
stars or ground lights for other aircraft and have
become disoriented, with fatal consequences. Therefore,
pilots should be aware of this visual illusion.
The autokinetic effect can be reduced by maintaining
good visual scanning techniques rather than staring
at the light source, or by increasing the intensity
of the lights, if that is possible.
hole illusion may occur on a dark night over water
or unlighted terrain where the horizon is not easily
discernible. The worst case occurs when there are
no visual references except for runway lights. Without
peripheral cues for orientation, the pilot tends
to perceive that his/her aircraft is stable, but
that the runway itself is out of position, usually
sloping down. The black hole illusion makes the
landing approach dangerous, and often results in
a landing far short of the runway. A particularly
hazardous type of black hole approach occurs when
the earth appears to be totally dark except for
the runway and the lights of a city on rising terrain
beyond the runway. By maintaining a constant vertical
visual angle on the distant city lights, the pilot's
approach may fall below the intended glideslope
as the aircraft gets closer to the runway.
illusion phenomenon is a dangerous illusion (DIP)
that can occur when one aircraft is trailing another
in a black hole environment with few peripheral
vision cues. A likely scenario for the DIP occurs
the trailing aircraft pilot, for position reference,
places the image of the lead aircraft on a certain
spot in his canopy. The pilot orients the lead aircraft
in exactly the same spot on the canopy, but, over
time, inadvertently falls back to a greater separation
distance. If the trailing aircraft started out 2
nautical miles behind and 300 feet below the lead
aircraft and then fell back to 4 nautical miles,
that would mean that the trailing aircraft was now
600 feet below the lead. If the lead aircraft gradually
descends to 600 feet or less, the trailing aircraft
may impact the ground. The potential mishap exists
only if the pilot maintains the lead aircraft in
exactly the same spot on the canopy, but fails to
monitor the actual altitude or fails to realize
that the separation distance has increased.
especially susceptible to misperception of the horizon
while flying at night. To a pilot, isolated ground
lights can look like stars and create the false
impression of a nose-high pitch or wing-low attitude.
If no stars are visible because of overcast conditions,
unlighted areas of terrain may blend with the dark
overcast to create the illusion that the unlighted
terrain is part of the sky. An extremely hazardous
takeoff occurs over an ocean or other body of water
that cannot be distinguished visually from the night
sky; pilots who have falsely perceived that the
shoreline receding beneath them was the horizon
have met with disastrous consequences.
if runway lights viewed from an approaching aircraft
are displaced laterally, the pilot may have the
impression that the runway is closer than it really
is, because it appears wider. This false perception
may result in an early flare and a tendency to land
short. Also, the intensity of runway lights may
appear to vary, depending on their color and the
adaptation states of the eyes. These differences
in the brightness of runway lights may lead to false
perceptions regarding altitude.
aware of certain vestibular illusions that may be
more difficult to overcome during nighttime operations.
Somatogravic illusions are false perceptions of
the body's orientation to gravity. Somatogyral illusions
are experienced by pilots during maneuvers of sustained
angular motion such as coordinated turns, spins,
or rolls. These illusions result from the inability
of the semicircular canals of the inner ear to register
sustained angular velocity. The lack of visual cues
at night may make these illusions more troublesome.
of smoking tobacco products on night vision are
controversial. Early studies showed a significant
decrease in scotopic dark adaptation with smoking,
which was attributed to the hypoxic effects of carbon
monoxide (CO). Later studies found that smoking
seemingly improved night visual performance on some
psychophysical tests. This improvement was presumed
to be a result of the stimulant effect of nicotine.
More recent studies have reported that smokers have
reduce mesopic vision when compared with nonsmokers.
the literature is somewhat confusing, smoking is
discouraged for several reasons. First, there is
some evidence that it may degrade mesopic and night
vision. Second, although many night flights are
low level, the hypoxic effect of CO is additive
with altitudinal hypoxia. Third, secondary smoke
is a significant irritant for aircrew who wear contact
lenses or for those with dry eyes. Fourth, smoke
forms filmy deposits on windscreens, visors, and
spectacles that can degrade contrast at night. Fifth,
the effects of smoking withdrawal during long missions
may be dangerous. Finally, the chronic long-term
effects of smoking are hazardous to overall health.
of altitudinal hypoxia on night vision is primarily
one of an elevation of the rod and cone threshold.
Although decreased cone function is clearly demonstrated
by the loss of color vision at hypoxic altitudes,
the decrement in central VA is usually insignificant.
However, scotopic night vision at altitude can be
significantly reduced. Scotopic vision has been
reported to decrease by 5% at 3,500 feet, 20% at
10,000 feet, and 35% at 13,000 feet, if supplemental
oxygen is not provided. Thus, the use of oxygen,
even at low pressure altitudes, can be very important
are some ways for aviators to protect, improve,
or maintain their operational night vision.
Complete a training course that emphasizes the
inherent limitations of night vision
Keep spectacles, visors, and windscreens clean
Wear proper spectacle correction.
When practical, dark-adapt or use red goggles
before night flying
Avoid bright lights, or at least protect one
Do not fixate centrally, but scan and look 15-20
degrees to the side of the visual target.
Regularly wear sunglasses on sunny days, especially
on days of night missions.
Eat an adequate diet that includes vitamin A.
Do not smoke.
Consider use of 100% oxygen at night, even at