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NIGHT FLIGHT TECHNIQUES AND PROCEDURES
HEADQUARTERS, DEPARTMENT OF THE ARMY
DISTRIBUTION RESTRICTION: Distribution authorized to US
government agencies and their contractors to protect information
and technical data that address current technology in areas of
significant or potentially significant military application. This
determination was made on 5 August 1988. Other requests for this
document will be referred to Commander, US Army Aviation Center,
ATTN: ATZQ-DAP-SS, Fort Rucker, AL 36362-5035. DESTRUCTION NOTICE:
Destroy by any method that will prevent disclosure of contents or
reconstruction of the document.
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Training Circular * TC 1-204 No. 1-204 HEADQUARTERS
DEPARTMENT OF THE ARMY Washington, DC 27 December 1988
NIGHT FLIGHT TECHNIQUES AND PROCEDURES
TABLE OF CONTENTS
Page PREFACE v CHAPTER 1 NIGHT VISION
1-1. Night Vision Evaluation 1-1 1-2. Eye Anatomy and Physiology
1-1 1-3. Light Levels 1-2 1-4. Vision Types 1-3 1-5. Day Versus
Night Vision 1-4 1-6. Visual Problems 1-7 1-7. Dark Adaptation 1-8
1-8. Night Vision Protection 1-8 1-9. Self-Imposed Stress 1-10
1-10. Scanning Techniques 1-12 1-11. Distance Estimation and Depth
Perception 1-14 1-12. Visual Illusions 1-19 1-13. Aircraft Design
Limitations 1-23 1-14. Nerve Agents and Night Vision (Miosis)
1-23
CHAPTER 2 AVIATION NIGHT VISION AIDS Section I IMAGE-INTENSIFIER
SYSTEMS
2-1. Development 2-1 2-2. Operational Theory 2-2 2-3. AN/PVS-5
Series 2-3 2-4. AN/AVS-6 2-7 2-5. Adjustment Techniques 2-9 2-6.
Operational Considerations 2-10
Section II THERMAL-IMAGING SYSTEMS
2-7. Operational Principles 2-19 2-8. System Types 2-19
*This publication supersedes FM 1-204, 11 October 1983.
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2-9. Infrared Characteristics 2-20 2-10. Operational
Considerations 2-26
CHAPTER 3 HEMISPHERICAL ILLUMINATION AND METEOROLOGICAL
CONDITIONS
3-1. Light Sources 3-1 3-2. Meteorological Effects 3-2
CHAPTER 4 TERRAIN INTERPRETATION
4-1. Visual Recognition Cues 4-1 4-2. Interpretation Factors
4-4
CHAPTER 5 NIGHT OPERATIONS Section I PREMISSION PLANNING
5-1. Mission Briefing and Debriefing 5-1 5-2. Crew Duties 5-1
5-3. Common Terminology 5-1
Section II PREFLIGHT GUIDELINES
5-4. Preflight Inspection 5-2 5-5. Aircraft Lighting 5-3 5-6.
Aircrew Preparation 5-4
Section III NIGHT FLIGHT TECHNIQUES
5-7. Limitations 5-4 5-8. Hover 5-5 5-9. Takeoff 5-7 5-10. En
Route 5-8 5-11. Landing 5-9 5-12. Pathfinder Operations 5-14 5-13.
External Load Operations 5-15
Section IV EMERGENCY AND SAFETY PROCEDURES
5-14. Basic Considerations 5-17 5-15. Electrical Failure 5-17
5-16. Airport Traffic Control Light Signals 5-17 5-17. Visual Night
Signals 5-18 5-18. Emergency Landing 5-18 5-19. Ground Safety 5-19
5-20. Air Safety 5-19 5-21. Airspace Management 5-20
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CHAPTER 6 NIGHT TERRAIN FLIGHT Section I TERRAIN FLIGHT MODES
AND COMMAND CONSIDERATIONS
6-1. Terrain Flight Modes 6-1 6-2. Command Considerations
6-1
Section II PLANNING GUIDELINES
6-3. General Considerations 6-3 6-4. Cockpit Teamwork and
Coordination 6-6 6-5. Aircraft Preparation and Equipment 6-7 6-6.
Maps and Visual Aids 6-7 6-7. General Route and Air Control Point
Planning 6-8 6-8. Aided Night Mission Map Preparation 6-10 6-9.
Aided Night Mission Planning and Briefings 6-10 6-10. Route
Planning Cards 6-11
CHAPTER 7 MULTIHELICOPTER OPERATIONS Section I CONSIDERATIONS
AND RESPONSIBILITIES
7-1. Planning Considerations 7-1 7-2. Supported Ground Unit
Commander Responsibilities 7-2 7-3. Air Mission or Flight Commander
Responsibilities 7-3
Section II NIGHT FLIGHT FORMATIONS
7-4. Aircraft Separation 7-4 7-5. Night Formations 7-4 7-6.
Basic Night Formation Considerations 7-7 7-7. Formation Takeoff 7-8
7-8. Lead Changes 7-8 7-9. Formation Changes 7-9 7-10. Rendezvous
and Join-Up Procedures 7-9 7-11. Formation Breakup 7-9 7-12.
Formation Landing 7-11 7-13. Vertical Helicopter IFR Recovery
Procedures 7-11
Section III TACTICAL FORMATION FLIGHT
7-14. Free-Cruise Technique 7-12 7-15. Movement Techniques 7-14
7-16. Crew Teamwork 7-15 7-17. Mixed Aircraft Formations 7-15
CHAPTER 8 FIXED-WING NIGHT FLYING
8-1. Preparation 8-1 8-2. Taxi, Takeoff, and Departure Climb 8-1
8-3. Orientation and Navigation 8-3 8-4. Approaches and Landings
8-4
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CHAPTER 9 DROP FLARE EMPLOYMENT
9-1. Target Identification 9-1 9-2. Description 9-1 9-3. Fuse
Setting 9-2 9-4. Launch Procedures 9-4 9-5. Flight Pattern 9-5 9-6.
Wind-Drift Correction 9-7 9-7. Linear Target Illumination 9-7 9-8.
Safety Considerations 9-8 9-9. Training Program 9-9
APPENDIX A. ELECTROMAGNETIC SPECTRUM A-1 APPENDIX B. I2 SYSTEM
COUNTERWEIGHTS B-1 APPENDIX C. PNVS FLIR C-1 APPENDIX D. TRAINING
PROGRAMS D-1 GLOSSARY Glossary - 1 REFERENCES References - 1
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PREFACE
Night flight has assumed an increasingly important role in Army
aviation. The Threat trains around the clock. To counter it,
aviators must be able to conduct operations at night as well as
during the day. Technological advances in night vision devices are
enabling Army aviation to extend its operational capability to a 24
hour-a-day schedule. Ongoing improvements to these devices will
further enhance aircrew performance during night operations. This
publication provides aircrews a comprehensive document on night
flight. It is intended to serve as a reference for night vision,
unaided and aided night flight, and night vision training. The
proponent of this publication is HQ TRADOC. Submit changes for
improving this publication on DA Form 2028 (Recommended Changes to
Publications and Blank Forms), and forward it through the aviation
unit commander to Commander, US Army Aviation Center, ATTN:
ATZQ-ATB-O, Fort Rucker, AL 36362-5218. The provisions of this
publication are the subject of international agreements: STANAG
2999 (Edition One), Use of Helicopters in Land Operations STANAG
3627 (Edition One) and AIR STD 44/34B, Helicopter Day and Night
Tactical Formation Flying AIR STD 44/33B, Helicopter Tactical or
Non-Permanent Landing Sites Unless otherwise stated, whenever the
masculine gender is used, both men and women are included. This
publication has been reviewed for OPSEC considerations.
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CHAPTER 1
NIGHT VISION
Vision is the most important sense used in flight. During day or
night, IMC or VMC, vision is the sense that makes crew members
aware of the position of their aircraft in space. The eyes can
rapidly identify and interpret visual cues during daylight. During
darkness, however, visual acuity is decreased proportionally as the
level of illumination decreases. Night vision devices improve the
capability of the human eye to see at night. These devices include
the AN/PVS-5 and AN/AVS-6, which are commonly referred to as I2
systems or I2 devices. Night vision devices also include
thermal-imaging systems. This chapter provides a general discussion
of night vision and scanning techniques. FM 1-301 contains
additional information.
1-1. NIGHT VISION EVALUATION
a. During the initial flight physical examination, an aviator is
interviewed to determine if he has difficulty seeing at night. If
the inter-view indicates the aviator has adequate night vision,
visual testing is not required.
b. The ability to conduct night flight safely is based on how
well crew members can see at night and how well trained they are in
using their night vision. Although the limits of night vision vary
from person to person, most crew members never learn to use their
night vision to its fullest capacity. A crew member with an average
night vision capability who uses night vision techniques is more
effective than a crew member with superior night vision who does
not.
1-2. EYE ANATOMY AND PHYSIOLOGY
The eye is similar to a camera. The cornea, lens, and iris
gather and control the amount of light allowed to enter the eye.
The image is then focused on the retina. Functionally, the visual
receptive apparatus (retina) has two types of cells: the cones and
the rods. Vision is possible because of chemical reactions within
these cells. Figure 1-1 shows the anatomy of the human eye.
a. Cones. Cone cells are used primarily for day or
high-intensity light vision. The concentration of cones in the
central retina (fovea centralis) permits high visual acuity in high
illumination. The chemical iodopsin is always present in the cone
cells. Regardless of the ambient light condition, this chemical is
readily available so that the cones can immediately respond to
visual stimulation.
b. Rods. The rods are used for night or low-intensity light
vision. The peripheral retina is almost exclusively associated with
rods. Peripheral vision is less precise than central vision,
because the rods perceive only shades of gray and vague form or
shape. Rhodopsin, commonly referred to as visual purple, is the
photochemical found in rods. As the light level decreases, the
amount of rhodopsin in the rods
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builds and the rods become more sensitive. Rods are about
one-thousand times more sensitive to light than cones. When
illumination decreases to about the level of full moonlight (0.1
footcandle), the rods take over from the cones. The period of
highest light sensitivity usually occurs after 30 to 45 minutes in
a dark environment. The rod cells may become up to 10,000 times
more sensitive than at the start.
Figure 1-1. Anatomy of the eye
1-3. LIGHT LEVELS
Measuring light levels can be complex and confusing. Many
different units of light measurement and terms are used for various
scientific, engineering, and industrial applications. Terms of
measurement are usually familiar only to those who work directly
with light measurement problems. Some of the terms important to
aircrews are defined in the paragraphs below.
a. Illumination. Illumination is the amount of light that
strikes a surface at some distance from a source. The common unit
of measurement is the footcandle. A footcandle is the density of
light falling on the inner surface of a sphere of 1-foot radius
when a point source of light with an intensity of one international
candle is placed at the center of the sphere.
b. Luminance. Luminance is the amount of light per unit area
reflected from or emitted by a surface. It is an important
measurement for visual displays and is usually expressed in
millilamberts or footlamberts. Luminance is frequently called
brightness. However, brightness is influenced by contrast,
adaptation, and such factors as the physical energy in the
stimulus. Figure 1-2 shows examples of luminance levels found
during commonly experienced conditions.
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Figure 1-2. Commonly experienced light levels
c. Reflectance. Reflectance is the relationship between
illumination reaching a surface and the resulting luminance. A
perfectly diffusing and reflecting surface is one that absorbs no
light and scatters the illumination in the manner of a perfectly
flat surface. Such a surface has a reflectance of 100 percent. If
illuminated by 1 footcandle, it would have a luminance of 1
footlambert from all viewing angles. In actual practice, the
maximum reflectance of a nearly perfectly diffusing surface is
about 75 percent.
d. Contrast. Contrast is a measure of the difference in
luminance between an object and its background. Contrast can vary
from 100 percent (negative) to zero for objects darker than their
backgrounds and from zero to infinity (positive) for objects
brighter than their backgrounds. Contrast increases when the
difference in luminance between an object and its back-ground
increases. Contrast is zero when the luminance of an object and its
background is the same.
1-4. VISION TYPES
The three types of vision are photopic, mesopic, and scotopic.
Each type functions under different sensory stimuli or ambient
light conditions. Night vision involves mesopic and scotopic
vision. Photopic vision at night is possible only when sufficient
levels of artificial illumination exist.
a. Photopic Vision. Photopic vision is experienced during
daylight or when a high level of artificial illumination exists.
The cones concentrated in the fovea centralis of the eye are
primarily responsible for vision in bright light. Because of the
high
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light level, rhodopsin is bleached out and rod cells become less
effective. Sharp image interpretation and color vision are
characteristic of photopic vision.
b. Mesopic Vision. Mesopic vision is experienced at dawn, at
dusk, and during full moonlight. Vision is achieved by a
combination of cones and rods. Visual acuity steadily decreases as
available light decreases. Color perception changes because the
cones become less effective. As cone sensi-tivity decreases, crew
members should use off-center vision and proper scanning techniques
to detect objects during low light levels.
c. Scotopic Vision. Scotopic vision is experienced under low
light levels. Cones become ineffective, resulting in poor
resolution of detail. Visual acuity decreases to 20/200 or less.
This enables a person to see only objects the size of or larger
than the big "E" on visual acuity testing charts from 20 feet away.
(A person must stand at 20 feet to see what can normally be seen at
200 feet under daylight conditions.) Also, color perception is
lost. A night blind spot in the central field of view appears at
low light levels. The night blind spot occurs when cone-cell
sensitivity is lost.
1-5. DAY VERSUS NIGHT VISION
Differences between day and night vision involve color, detail,
and retinal sensitivity. Day vision is superior to night vision in
every respect.
a. Color. One major difference between night vision and day
vision is that color vision decreases or is lost at night. With
decreasing light levels, the eyes shift from photopic vision
(cones) to scotopic vision (rods). With this shift, the eyes become
less sensitive to the red end of the spectrum and more sensitive to
the blue part of the spectrum, as shown in Figure 1-3. Perception
of colors is not possible with the rods. Colors of nonilluminated
objects cannot be determined at night under very low illumination.
Light and dark colors at night can be distinguished only by the
intensity of reflected light. If, however, the brightness or
intensity of a color is above the threshold for cone vision, the
color can be perceived. This is why, for example, signal flares and
runway markers can be properly identified at night.
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Figure 1-3. Photopic (cone) and scotopic (rod) sensitivity to
various colors
b. Detail. Perception of fine detail is impossible at night.
Under low illumination, visual acuity is greatly impaired. At 0.1
footcandle (the level of full moonlight), acuity is one-seventh as
good as it is in average daylight. Therefore, objects must be large
or nearby to be seen at night. Identification of objects at night
is based on perceiving generalized contours and outlines, not on
small distinguishing features.
c. Retinal Sensitivity.
(1) Another important distinction between night vision and day
vision is the difference in the sensitivity of various parts of the
retina. The central part of the retina is not sensitive to
starlight illumination levels. During darkness or with low-level
illumination, central vision becomes less effective and a night
blind spot (5 to 10 wide) develops. This results from the
concentration of cones in the fovea centralis and para fovea, the
area immediately surrounding the fovea of the retina. The central
field of vision for each eye is superimposed for binocular vision.
Because the night blind spot for each eye occurs in the central
field of view, binocular vision cannot compensate for the night
blind spot. Therefore, an object viewed directly may not be
detected, as shown in Figure 1-4.
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Figure 1-4. Night blind spot
(2) The night blind spot should not be confused with the
physiological blind spot (the so-called day blind spot) caused by
the optic disk. The physiological blind spot is present all the
time, not only during the day. This blind spot results from the
position of the optic disk on the retina. The optic disk has no
light-sensitive receptors. The physiological blind spot covers an
area of approximately 5.5 by 7.5 and is located about 15 from the
fovea. Because of the overlap of binocular vision, this blind spot
is normally not noticed unless one eye is not used. The
physiological blind spot becomes an important consideration when
monocular night vision devices, such as the PNVS, are used.
(3) Because of the night blind spot, larger and larger objects
will be missed as distance increases. To see things clearly at
night, an individual must use off-center vision and proper scanning
techniques. Figure 1-5 shows the effect of distance on the night
blind spot.
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Figure 1-5. Effect of distance on the night blind spot
1-6. VISUAL PROBLEMS
Several visual problems or conditions affect night vision. These
include presbyopia, night myopia, and astigmatism.
a. Presbyopia. This condition is part of the normal aging
process, which causes the lens of the eye to harden. Beginning in
the early teen years, individuals gradually lose accommodation;
that is, the ability to focus on nearby objects. When individuals
are about 40 years old, their eyes are unable to reliably focus at
the normal reading distance without reading glasses. As presbyopia
worsens, instruments, maps, and checklists become more difficult to
read, especially with red illumination. This difficulty can be
corrected with certain types of bifocal spectacles that compensate
for the inadequate accommodative power of the eye lenses.
b. Night Myopia. Myopic individuals do not see distant objects
clearly; only nearby objects are in focus for them. At night, blue
wavelengths of light prevail in the visible portion of the
spectrum. Because of this, slightly nearsighted (myopic)
individuals will experience visual difficulty at night when viewing
blue-green light that could cause blurred vision. Also, image
sharpness decreases as pupil diameter increases. For individuals
with mild refractive errors, vision may become unacceptably blurred
unless corrective glasses are worn. Another factor to consider is
"dark focus." When luminance levels decrease, the focusing
mechanism of the eye may move toward a resting position and make
the eye more myopic. These factors are more important when the
aircrew looks outside the cockpit during unaided night flight.
Special corrective lenses can be prescribed to correct for
myopia.
c. Astigmatism. Astigmatism is an irregularity of the shape of
the cornea that may cause an out-of-focus condition. If, for
example, an astigmatic person focuses on
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power poles (vertical), the wires (horizontal) will be out of
focus in most cases. If the astigmatism is 1.00-diopter or greater,
the aviator must be individually evaluated before flying with I2
devices that preclude the wearing of eyeglasses. An example is
full-faceplate devices used with daylight filters.
1-7. DARK ADAPTATION
Going suddenly from bright light into darkness is a common
occurrence. For example, people experience this when they enter a
movie theater during the day or leave a brightly lit room at night.
At first they see very little, if anything. After several minutes,
they can see dim forms and large outlines. As time goes by, more
details of the environment become apparent as further dark
adaptation occurs.
a. Dark adaptation is the process by which the eyes increase
their sensitivity to low levels of illumination. Individuals
dark-adapt to varying degrees and at different rates. During the
first 30 minutes, the sensitivity of the eye increases roughly
ten-thousandfold. Very little increase in sensitivity occurs after
that time.
b. The lower the starting level of illumination, the more rapid
complete dark adaptation is achieved. For example, less time is
required to dark-adapt completely after leaving a darkened theater
than after leaving a brightly lit hangar.
c. Dark adaptation for optimum night visual acuity approaches
its maximum level in about 30 to 45 minutes under minimal light
conditions. If the dark-adapted eye is exposed to a bright light,
the sensitivity of that eye is temporarily impaired. The amount of
impairment depends on the intensity and duration of the exposure.
Brief flashes from a white (xenon) strobe light, commonly found on
aircraft, have little effect on night vision because the pulses of
energy are so short. On the other hand, exposure to a flare, a
searchlight beam, or lightning may seriously impair night vision.
In such cases, the recovery of a previous maximum level of dark
adaptation can take from 5 to 45 minutes in continued darkness.
d. Night vision devices affect dark adaptation. If a previously
dark-adapted crew member wearing an I2 device removes the device in
a darkened environment, a 30-minute dark adaptation level can be
regained in about two to three minutes. No dark adaptation period
is necessary before using the I2 device. Vision with I2 devices is
primarily photopic, but the low light levels produced by I2 devices
do not fully bleach out rhodopsin. Use of the device does not
seriously degrade dark adaptation.
1-8. NIGHT VISION PROTECTION
Night vision should be protected when possible. Some of the
steps crew members can take to protect their night vision are
described below.
a. Equipment.
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(1) Sunglasses. Repeated exposure to bright sunlight has an
increasingly adverse effect on dark adaptation. This effect is
intensified by reflective surfaces such as sand and snow. Exposure
to intense sunlight for two to five hours decreases scotopic visual
sensitivity for as long as five hours. Also, a decrease occurs in
the rate of dark adaptation and degree of night visual capacity.
These effects are cumulative and may persist for several days. If a
night flight is scheduled, crew members should wear military
neutral density (N-15) sunglasses or equivalent filter lenses when
exposed to bright sunlight. This precaution will increase the rate
of dark adaptation at night and improve night visual
sensitivity.
(2) Oxygen supply. Unaided night vision depends on optimum
function and sensitivity of the rods of the retina. Lack of oxygen
to the rods (hypoxia) significantly reduces their sensitivity. This
increases the time required for dark adaptation and decreases the
ability to see at night. Without supplemental oxygen, an
individual's night vision declines measurably at pressure altitudes
above 4,000 feet. Because I2 device output is photopic and central
vision is the last to be degraded by a lack of oxygen, aided night
vision is not significantly affected. At night, aviators should use
oxygen, if available, when operating unaided above a pressure
altitude of 4,000 feet.
b. Precautions.
(1) Airfield lighting. At a fixed airfield, light sources that
may impair the aircrew's dark adaptation should be eliminated.
Aircraft scheduled for night flight should be positioned, if
possible, on a part of the airfield where the least amount of light
exists. Maintenance and service crews should practice light
discipline. Hover lanes should be established and marked with
minimal appropriate lighting. This will preclude the use of the
landing light or searchlight during hover operations. Airfield
lighting should be reduced to the lowest intensity. The aviator
should select departure routes that avoid highways and residential
areas where artificial illumination can impair night vision. Runway
and takeoff-pad lights, when practical, should be reduced for
departing traffic.
(2) High-intensity lighting. During night missions, aircrews may
be exposed to high-intensity lighting such as city lights, flares,
searchlights, lightning, and artillery flashes. These may cause a
total or a partial loss of night vision. If a flash of
high-intensity light is expected from a specific direction, the
aviator should turn the aircraft away from the light source. When
such a condition occurs unexpectedly and direct view cannot be
avoided, a crew member can preserve his dark adaptation by shutting
one eye and using the other to observe. Once the light source is no
longer visible, the eye that was closed can provide the required
night vision. This is possible because dark adaptation occurs
independently in each eye.
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However, it should be remembered that problems with depth
perception can occur when the aviator views with one dark-adapted
eye. This is particularly true when an aviator hovers near terrain
obstacles or makes an approach. Techniques to counter expected
light conditions are discussed below.
(a) Lights in built-up areas. The aviator should select flight
routes that avoid built-up areas which may have heavy
concentrations of light. If this condition is inadvertently
encountered, the aviator should alter the flight route to avoid
overflying the brightly lit area. A decrease in dark adaptation
from a single light source, such as a farmhouse or an automobile,
can be minimized by looking away from the light.
(b) Flares. When a flare is used to illuminate the viewing area
or is inadvertently detonated nearby, the aviator should maneuver
to a position along the edge of the illuminated area. This
procedure reduces exposure to the light source.
(c) Weapon flashes. To reduce the effect of weapon flashes from
aerial weapon systems, the aviator should limit the time during
which the ordnance is expended. Rockets can be fired in almost any
combination without seriously impairing night vision. When firing
automatic weapons, the aviator should use short bursts of fire.
Closing an eye or looking away from the firing will also minimize
the loss of dark adaptation.
1-9. SELF-IMPOSED STRESS
Night flight is more fatiguing and stressful than day flight.
Many self- imposed stressors limit night vision. Crew members can
control this type of stress. The factors that cause self-imposed
stress are discussed below; crew members can remember them by the
acronym DEATH.
a. Drugs. Drugs can seriously degrade visual acuity during the
day and especially at night. A crew member who becomes ill should
consult a flight surgeon.
b. Exhaustion. If crew members become fatigued during a night
flight, they will not be mentally alert. Exhaustion causes crew
members to respond more slowly, even in situations requiring
immediate reaction. Exhausted crew members tend to concentrate on
one aspect of a situation without considering the total
requirement. Their performance may become a safety hazard,
depending on the degree of fatigue. Rather than use proper scanning
techniques, they are prone to stare.
(1) Illness. Increased temperature and a feeling of
unpleasantness usually are associated with illness. High body
temperatures consume a higher than
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normal rate of oxygen. As a result, hypoxia may be induced and
night vision may be degraded. In addition, the unpleasant feeling
associated with sickness distracts a crew member's attention and
degrades his ability to concentrate on night flying
requirements.
(2) Poor physical conditioning. To overcome this limitation,
crew members should participate in regular exercise programs. Crew
members who are physically fit become less fatigued during flight
and have better night scanning efficiency. However, too much
exercise in a given day may leave crew members too fatigued for
night flying.
(3) Inadequate rest. Adequate rest and sleep are important
before flying. Commanders should refer to the crew endurance
scheduling guide in AR 95-1 when developing crew work and rest
schedules.
c. Alcohol. Alcohol is a sedative. Its use impairs both
coordination and judgment. As a result, crew members impaired by
alcohol fail to apply the proper techniques of night vision. They
are likely to stare at objects and to neglect scanning techniques.
The amount of alcohol consumed determines the degree to which night
vision is affected. The effects of alcohol are long-lasting;
hangovers also impair visual scanning efficiency.
d. Tobacco. Of all the self-imposed stressors, cigarette smoking
most decreases visual sensitivity at night. Smoking significantly
increases the amount of carbon monoxide carried by the hemoglobin
in red blood cells. This reduces the blood's capacity to combine
with oxygen so less oxygen is carried in the blood. Hypoxia caused
by carbon monoxide poisoning affects peripheral vision and dark
adaptation. The results are the same as those for hypoxia caused by
high altitude. Smoking 3 cigarettes in rapid succession or 20 to 30
cigarettes within a 24-hour period may saturate from 8 to 10
percent of the capacity of hemoglobin. Smokers lose 20 percent of
their night vision capability at sea level. This equals a
physiological altitude of 5,000 feet.
e. Hypoglycemia and Nutritional Deficiency.
(1) Hypoglycemia. Missing or postponing meals can cause low
blood sugar, which impairs night flight performance. Low blood
sugar levels may result in stomach contractions, distraction, a
breakdown in habit pattern, a shortened attention span, and other
physiological changes.
(2) Vitamin A deficiency. Insufficient consumption of vitamin A
may impair night vision. Foods high in vitamin A include eggs,
butter, cheese, liver, apricots, peaches, carrots, squash, spinach,
peas, and most types of greens. A balanced diet usually provides
enough vitamin A. Excessive quantities of vitamin A will not
improve night vision and may be harmful.
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1-10. SCANNING TECHNIQUES
Dark adaptation is only the first step toward increasing
aircrews' ability to see at night. Applying night vision techniques
will enable aircrews to overcome many of the physiological
limitations of their eyes. Because the fovea centralis is
automatically directed toward an object by a visual fixation
reflex, scanning techniques require considerable practice and
concerted effort on the part of the viewer.
a. Scanning. Scanning techniques are important in identifying
objects at night. To scan effectively, crew members look from right
to left or left to right. They should begin scanning at the
greatest distance an object can be perceived (top) and move inward
toward the position of the aircraft (bottom). This scanning pattern
is shown in Figure 1-6. Because the light-sensitive elements of the
retina cannot perceive images that are in motion, a
stop-turn-stop-turn motion should be used. For each stop, an area
approximately 30 wide should be scanned. This viewing angle will
include an area approximately 250 meters wide at a distance of 500
meters. The duration of each stop is based on the degree of detail
that is required, but no stop should last longer than two to three
seconds. When moving from one viewing point to the next, crew
members should overlap the previous field of view by 10 . Other
scanning techniques, such as the ones illustrated in Figure 1-7,
may be used if appropriate to the situation.
Figure 1-6. Scanning pattern
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Figure 1-7. Alternate scanning pattern
b. Off-Center Viewing.
(1) Viewing an object using central vision during daylight poses
no limitation. If this same technique is used at night, however,
the object may not be seen because of the night blind spot that
exists during low illumination. To compensate for this limitation,
crew members must use off-center vision. This technique requires
that an object be viewed by looking 10 above, below, or to either
side of the object. In this manner, the peripheral vision can
maintain contact with an object. Figure 1-8 illustrates an example
of the off-center viewing technique.
(2) The technique of off-center vision applies only to the
surveillance of targets that are minimally illuminated or luminous.
Under these conditions, cone vision is not stimulated. Central
vision is best used when an object or a target is bright enough to
stimulate the cones and needs to be seen with considerable detail.
When the object or target begins to fade, it should be redetected
using off-center vision and retained until central vision recovers
sufficiently to permit further observation.
(3) With off-center vision, the images of an object viewed
longer than two to three seconds will disappear. This occurs
because the rods reach a photochemical equilibrium that prevents
any further response until the scene changes. This produces a
potentially unsafe operating condition. To overcome this night
vision limitation, crew members must be aware of the phenomenon and
avoid viewing an object for longer than two or three seconds. The
peripheral field of vision will continue to pick up the object when
the eyes are shifted from one off-center point to another.
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Figure 1-8. Off-center viewing technique
1-11. DISTANCE ESTIMATION AND DEPTH PERCEPTION
Distance estimation and depth perception cues are easily
recognized when crew members use central vision under good
illumination. As the light level decreases, the ability to judge
distances accurately is degraded and visual illusions become more
common. A knowledge of distance estimation and depth perception
mechanisms and cues will assist crew members in judging distances
at night. These cues may be monocular or binocular. Monocular cues
are more important for crew members than binocular cues.
a. Monocular Cues. The monocular cues that aid in distance
estimation and depth perception include motion parallax, geometric
perspective, retinal image size, and aerial perspective.
(1) Motion parallax. This cue to depth perception is a means of
judging distances under reduced illumination. Motion parallax
refers to the apparent motion of stationary objects as viewed by an
observer moving across the landscape. When the crew member looks
outside the aircraft, perpendicular to the direction of travel,
near objects appear to move backward, past, or opposite the path of
motion. Far objects seem to move in the direction of motion or
remain fixed. The rate of apparent movement depends on the distance
the observer is from the object. For example, as an aviator flies
low level, objects near the aircraft will appear to rush past the
aircraft while a mountain range near the horizon will appear
stationary. As the aviator flies across a power line that extends
to the horizon, that part of the power line near the aircraft will
appear to move swiftly, opposite the path of motion. Toward the
horizon, the same power line will
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appear fixed. Objects that appear to be fixed or moving slowly
are judged to be a greater distance from the aviator than objects
that appear to be moving swiftly.
(2) Geometric perspective. An object may appear to have a
different shape when viewed at varying distances and from different
angles. Geometric perspective cues include linear perspective,
apparent foreshortening, and vertical position in the field. They
are illustrated in Figure 1-9.
(a) Linear perspective. Parallel lines, such as runway lights,
tend to converge as distance from the observer increases. This is
illustrated in part A of Figure 1-9.
(b) Apparent foreshortening. The true shape of an object or a
terrain feature appears elliptical when viewed from a distance. As
the distance to the object or the terrain feature decreases, the
apparent perspective changes to its true shape or form. Part B of
Figure 1-9 illustrates how the shape of a body of water changes
when viewed at different distances at the same altitude.
(c) Vertical position in the field. Objects or terrain features
farther away from the observer appear higher on the horizon than
those closer to the observer. The higher vehicle in part C of
Figure 1-9 appears to be closer to the top and, thus, at the
greater distance from the observer. At night, crew members can
mistake lights on elevated structures or lights on low-flying
aircraft for distant ground structures because of the lights'
higher vertical position in the field.
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Figure 1-9. Geometric perspective
(3) Retinal image size. The brain perceives the actual size of
an object from the size of an image focused on the retina. Four
factors are considered in determining distance using the retinal
image. They are known size of objects, increasing or decreasing
size of objects, terrestrial associations, and overlapping contours
or interposition of objects.
(a) Known size of objects. The nearer an object is to the
observer, the larger its retinal image. By experience, the brain
learns to estimate the distance of familiar objects by the size of
their retinal images. Figure 1-10 shows how this process works. A
structure projects a specific angle on the retina based on its
distance from the observer. If the angle is small, the observer
judges the structure to be at a great distance. A larger angle
indicates to the observer that the structure is close. To use this
cue, the observer must know the actual size of the object. If the
observer is not familiar with the object, its distance would be
determined primarily by motion parallax.
Figure 1-10. Known size of objects
(b) Increasing or decreasing size of objects. If the retinal
image size of an object increases, the relative distance is
decreasing. If the image size decreases, the relative distance is
increasing. If the image size is constant, the object is at a fixed
relative distance.
(c) Terrestrial associations. Comparing an object, such as an
airfield, with an object of known size, such as a helicopter, helps
to deter- mine the object's size and apparent distance from the
observer. Objects ordinarily associated together are judged to be
at
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about the same distance. For example, a helicopter observed near
an airport is judged to be in the traffic pattern and, therefore,
at about the same distance as the airfield. Figure 1-11 illustrates
terrestrial association.
Figure 1-11. Terrestrial association
(d) Overlapping contours or interposition of objects. When
objects overlap, the overlapped object is farther away, as
illustrated in Figure 1-12. This overlapping is especially
important to consider at night during a landing approach. Lights
disappearing or flickering in the landing area indicate barriers
between the landing area and the aircraft. The flight path should
be adjusted accordingly.
Figure 1-12. Overlapping contour
(4) Aerial perspective. The clarity of an object and the shadow
cast by it are perceived by the brain and are cues for estimating
distance. Several aerial perspective factors are used to determine
distance.
(a) Variations in color or shade. Subtle variations in color or
shade are clearer the closer the observer is to an object. However,
as distance increases, these distinctions blur. For example, the
side of
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a hill from a distance will appear to be a uniform shade with no
distinguishable shape. As the aircrew flies closer to the hill, the
shades produced by individual trees and the spaces in between those
trees become noticeable. Thus under high light levels at night,
color or shade can provide cues for distance estimation.
(b) Loss of detail or texture. As a person gets farther from an
object, discrete details become less apparent. For example, when a
cornfield becomes a solid color and the leaves and branches of a
tree become a solid mass, the objects are judged to be far away.
Because reduced illumination also decreases resolution, these cues
will disappear shortly after sunset or be limited to close viewing
distances.
(c) Position of light source and direction of shadow. Every
object will cast a shadow from a light source. The direction in
which the shadow is cast depends on the position of the light
source. If the shadow of an object is toward the observer, the
object is closer than the light source is to the observer. Figure
1-13 illustrates light and shadows.
Figure 1-13. Light and shadows
b. Binocular Cues. Binocular cues depend on the slightly
different view each eye has of an object. Consequently, binocular
perception is useful only when the object is close enough to make
an obvious difference in the viewing angle of both eyes. In the
flight environment, most distances outside the cockpit are so great
that binocular cues are of little, if any, value. In addition,
binocular cues operate on a more subconscious level than monocular
cues and are performed automatically.
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1-12. VISUAL ILLUSIONS
Decreasing visual information increases the probability of
spatial disorientation. Reduced visual references also create
several illusions that can induce spatial disorientation. Many
types of visual illusions can occur in the aviation environment.
Included among them are autokinesis, ground light
misinterpretation, relative motion, reversible perspective
illusion, false horizons, altered reference planes, and height
perception illusion. Others include flicker vertigo, fascination
(fixation), structural illusions, and size-distance illusion.
a. Autokinesis. When a static light is stared at in the dark,
the light appears to move, as shown in Figure 1-14. This phenomenon
can be readily demonstrated by staring at a lighted cigarette in a
dark room. Apparent movement will begin in about 8 to 10 seconds.
Although the cause of autokinesis is not known, it appears to be
related to the loss of surrounding references that normally serve
to stabilize visual perceptions. This illusion can be eliminated or
reduced by visual scanning, by increasing the number of lights, or
by varying the light intensity. The most important of the three
solutions is visual scanning. A light or lights should not be
stared at for more than 10 seconds. This illusion is not limited to
light in darkness. It can occur whenever a small, bright, still
object is stared at against a dull dark or nondescript background.
Similarly, it can occur when a small, dark, still object is viewed
against a light, structureless environment. Anytime visual
references are not available, aircrews are subject to this
illusion.
b. Ground Light Misinterpretation. A common occurrence is to
confuse ground lights with stars. When this happens, aviators
unknowingly position aircraft in unusual attitudes to keep the
ground lights--believed to be stars--above them. For example, some
aviators have mistaken the lights along a seashore for the horizon
and have maneuvered their aircraft dangerously close to the sea;
they believed they were flying straight and level. Aviators have
also confused certain geometric patterns of ground lights. For
example, aviators have identified moving trains as landing zone
lights and have been badly shaken by their near misses. To avoid
these problems, aviators should cross-check aircraft instruments.
Also, position lights of other aircraft in formation can be
mistaken for ground lights and might be lost against the horizon
when another aircraft is at or below the altitude of the observer.
Figure 1-15 illustrates ground light and skylight illusion.
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Figure 1-14. Autokinetic illusion
Figure 1-15. Ground light and skylight illusion
c. Relative Motion. The illusion of relative motion can be
illustrated by an example. An aviator hovers an aircraft and waits
for hover taxi instructions. Another aircraft hovers alongside. As
the other aircraft is picked up in the first aviator's peripheral
vision, the aviator senses movement in the opposite direction. This
illusion may be encountered during multihelicopter operations.
Aircrews may mistake the motion of another aircraft for that of
their own. The only way to correct for this illusion is to have
sufficient experience to understand that such illusions do occur
and to not react to them on the controls. The use of proper
scanning techniques can help prevent this illusion.
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d. Reversible Perspective Illusion. At night, an aircraft may
appear to be going away when it is, in fact, approaching a second
aircraft. This illusion often occurs when an aircraft is flying
parallel to another's course. To determine the direction of flight,
aircrews should observe aircraft lights and their relative position
to the horizon. If the intensity of the lights increases, the
aircraft is approaching. If the lights dim, the aircraft is moving
away. Also, remembering the "3 Rs" will help identify the direction
of travel when other aircraft are encountered. If the red aircraft
position lights are on the right, the aircraft is returning (coming
toward the observer).
e. False Horizons. Cloud formations may be confused with the
horizon or the ground. Momentary confusion may result when the
aviator looks up after having given prolonged attention to a task
in the cockpit. Because outside references for attitude are less
obvious and reliable at night, aviators should rely less on them
during night flight. Using instrument cross-checks can help prevent
this situation. While hovering over terrain that is not perfectly
level, aviators might mistake the sloped ground in front of the
aircraft for the horizon and cause the aircraft to drift while
trying to maintain a stationary position. Figure 1-16 illustrates
false horizon illusion.
f. Altered Reference Planes. When approaching a line of
mountains or clouds, aviators may feel that they need to climb even
though their altitude is adequate. Also, when flying parallel to a
line of clouds, aviators may tend to tilt the aircraft away from
the clouds.
g. Height Perception Illusion. When flying over desert, snow,
water, or other areas of poor contrast, crew members may experience
the illusion of being higher above the terrain than they actually
are. This is due to the lack of visual references. This illusion
may be overcome by dropping an object, such as a chemical light
stick or flare, on the ground before landing. Another technique to
overcome this illusion is to monitor the shadows cast by near
objects, such as the landing gear, or skid shadows at a hover.
Flight in an area where visibility is restricted by haze, smoke, or
fog produces the same illusion.
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Figure 1-16. False horizon illusion
h. Flicker Vertigo. Much time and research have been devoted to
the study of flicker vertigo. A light flickering at a rate between
4 and 20 cycles per second can produce unpleasant and dangerous
reactions. Such conditions as nausea, vomiting, and vertigo may
occur. On rare occasions, convulsions and unconsciousness may also
occur. Fatigue, frustration, and boredom tend to intensify these
reactions. During the day, the problem can be caused by sunlight
flickering through rotor blades or propellers. At night, it can
also be caused by an anticollision light reflecting against an
overcast sky, haze, or the rotor system. This can be corrected by
turning the anticollision light off.
i. Fascination (Fixation). This illusion occurs when aviators
ignore orientation cues and fix their attention on a goal or an
object. This is dangerous because aircraft ground-closure rates are
difficult to determine at night; normal daylight peripheral
movement is reduced or absent. Target hypnosis is a common type of
fascination. For example, an aviator intent on hitting a target
during a gunnery run may delay pull-up so long that the aircraft
contacts the ground. Preventing this illusion requires increased
scanning by the aviator.
j. Structural Illusions. Structural illusions are caused by heat
waves, rain, snow, sleet, or other factors that obscure vision. For
example, a straight line may appear to be curved when seen through
a desert heat wave or a wing-tip light may appear to double or move
when viewed during a rain shower.
k. Size-Distance Illusion. This illusion results from viewing a
source of light that is increasing or decreasing in luminance
(brightness). The aviator may interpret the light as approaching or
retreating. For example, when an aviator, hovering near a second
aircraft, changes the position lights from DIM to BRIGHT, the other
aircraft may appear to jump toward him.
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1-13. AIRCRAFT DESIGN LIMITATIONS
The design of Army aircraft may degrade a crew member's ability
to see outside the aircraft. To minimize the loss of night vision
because of aircraft design shortcomings, an aircrew must properly
prepare the aircraft for night flight. Consideration should be
given to the aircraft limitations discussed below.
a. Windscreens reduce the ability to see outside the aircraft.
Dirt, grease, and bugs must be removed from the windscreen before
each night flight.
b. Aircraft instruments are easier to read under high levels of
instrument illumination. However, the level of illumination needed
for optimum reading interferes with maximum dark adaptation for
viewing dim objects outside the aircraft.
c. Interior lights also interfere with dark adaptation. They
reflect off the windscreen and reduce outside visibility. Also,
interior lights may be detected by the Threat. To reduce the
adverse effects of cockpit lights, the aviator should turn off
nonessential lights and keep the intensity of essential lights at
the lowest usable level.
d. Exterior lights are used to identify the aircraft. During
aided terrain flight, the illumination from these lights may
degrade the operation of the I2 system. To reduce the adverse
effect of exterior lights, the aviator should turn off all lights
not required by regulations. The remaining lights should be
operated in the DIM mode or properly taped or painted.
1-14. NERVE AGENTS AND NIGHT VISION (MIOSIS)
Night vision is adversely affected when eyes are exposed to
minute amounts of nerve agents. When direct contact occurs, the
pupils constrict (miosis) and do not dilate in low ambient light.
The available automatic chemical alarms are not sensitive enough to
detect the low concentrations of nerve agent vapor that can cause
miosis.
a. Exposure Time. The exposure time required to cause miosis
depends on the concentration of the agent. Miosis may occur
gradually as eyes are exposed to low concentrations over a long
period of time. On the other hand, exposure to a high concentration
can cause miosis during the few seconds it takes to put on a
protective mask. Repeated exposures over a period of days are
cumulative.
b. Symptoms.
(1) The symptoms of miosis range from minimal to severe,
depending on the dosage to the eye. Severe miosis, with the
resulting reduced ability to see in low ambient light, persists for
about 48 hours after onset. The pupil
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gradually returns to normal over several days. Full recovery may
take up to 20 days. Repeated exposures within the affected time are
cumulative.
(2) The onset of miosis is insidious because it is not always
immediately painful. Miotic persons may not realize their condition
even when they carry out tasks that require vision in low ambient
light. If the unit is attacked by nerve agents, especially the more
persistent types, commanders should assume that personnel otherwise
fit for duty will experience some loss of night vision. No
effective drug is available to remedy the effects of miosis without
causing other visual problems that may be just as severe.
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CHAPTER 2
AVIATION NIGHT VISION DEVICES
The two types of aviation night vision devices are
image-intensifier systems and thermal-imaging systems. I2 systems
amplify both visible and near infrared light energy. They greatly
improve night vision, but they require some degree of light to
function. I2 systems also do not work well under very low ambient
light and adverse weather conditions. The most recent advance in
night vision devices is thermal imaging, which detects infrared
energy. Thermal-imaging systems detect heat radiated by objects and
do not need light to function. They are less affected by weather
conditions than I2 systems. Thus far, only the AH-64 has a
thermal-imaging pilotage system, the PNVS. Appendix A discusses the
portions of the electromagnetic spectrum that are sensed by
image-intensifier systems and thermal-imaging systems.
Section I IMAGE-INTENSIFIER SYSTEMS
2-1. DEVELOPMENT HISTORY
a. A type of goggles for aviators to use during night helicopter
operations was first demonstrated in 1969. Because Army tactical
doctrine at that time did not require low-level or NOE night
flight, I2 system development stopped. In 1971, however, the Army
reevaluated tactical helicopter employment and determined that NOE
operations were necessary at night as well as during the day. As a
result, the AN/PVS-5 was adopted as an interim pilot's night vision
system because it was a significant improvement over unaided night
vision.
b. The AN/PVS-5 series contains second-generation
image-intensifier tubes. As use of the AN/PVS-5 expanded, the major
limitation of the full faceplate--lack of peripheral vision--became
apparent. Subsequently, the faceplate was modified (cut away) to
provide peripheral vision. The AN/AVS-6 was developed to overcome
or reduce the limitations of the AN/PVS-5 series. The AN/AVS-6 uses
third-generation image-intensifier tubes to increase light
amplification approximately two times more than the AN/PVS-5
series. The AN/AVS-6 also increases peripheral vision and has a
flip-up feature. Both the AN/PVS-5 series and the AN/AVS-6 are
being used. Figure 2-1 shows a comparison of peripheral vision for
the various models.
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Figure 2-1. Peripheral vision comparison of various I2
models
2-2. OPERATIONAL THEORY
An image intensifier is an electronic device that amplifies
light energy. The light enters into the I2 device and is focused by
the objective lens onto a photocathode that is receptive to both
visible and near infrared radiation. Figure 2-2 illustrates the
operation of an I2 device. The photons of light striking the
photocathode cause a release of electrons proportionate in number
to the amount of light projected through the lens. In turn, the
released electrons are accelerated away from the photocathode
surface by an electrical field that is produced by the device's
power source. The amount of light produced by the I2 tube is
proportional to both the number and the velocity of electrons that
strike the phosphor screen. The number of electrons striking the
phosphor screen is increased by means of the microchannel plate,
which is a thin wafer of tiny glass tubes. The glass tubes are
tilted in the microchannel plate approximately 8 . Electrons enter
these tubes and strike the walls of the tubes. As each electron
strikes a wall, more electrons are emitted. Each of these emitted
electrons strikes the wall again, producing even more electrons.
The accelerated electrons are directed through the microchannel
plate and against a phosphor screen placed on a flat plate opposite
and parallel to the photocathode surface. The phosphor screen emits
an amount of light proportional to the number and velocity of the
electrons that strike it. Voltage is applied between the
photocathode and the phosphor screen. This accelerates the
electrons, "brightening" the projected scene.
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Thus the picture delivered to the user is converted from a small
amount of light to accelerated electrons and back to an amplified
amount of light.
Figure 2-2. Image intensifier
2-3. AN/PVS-5 SERIES
a. Description.
(1) The AN/PVS-5 series is a self-contained, binocular
image-intensifier viewing system with no magnification. The device
is passive in normal operation. It measures about 6.5-inches
square, weighs 30 ounces, and has a 40 FOV. Power is supplied by a
2.7-volt DC mercury battery or a 3.0-volt DC lithium battery or two
1.5-volt AA alkaline batteries. The device operates satisfactorily
between 2.5 volts and 3.4 volts. The full-faceplate version of the
AN/PVS-5 completely surrounds the wearer's eyes. Only
through-the-tube viewing is possible, and eyeglasses cannot be
worn. Under ideal conditions, visual acuity of 20/200 or less
unaided can be improved to 20/50 with the AN/PVS-5 series. Figure
2-3 shows the AN/PVS-5 with full faceplate.
NOTE: Flying with full faceplate I2 systems is permitted only
during the daytime with daylight filters and with the second crew
member unaided.
(2) The weight of I2 devices shifts the CG of the head and
helmet system forward. This out-of-balance condition can be
relieved by attaching a counterweight to the back of the helmet.
Appendix B discusses I2 system counterweights.
(3) The AN/PVS-5 series operates by intensifying ambient light
750 to 1,500 times. This is sufficient to provide good imagery
under full moonlight conditions down to marginal imagery during
quarter moonlight conditions. Below quarter moonlight conditions,
artificial illumination (usually infrared) may be required to light
the helicopter's flight path.
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Figure 2-3. AN/PVS-5 series
b. Improvements and Modifications.
(1) AN/PVS-5A. This model has the same image intensifier as the
AN/PVS-5 but is constructed differently. The tubes are not
interchangeable. The ON-OFF switch on the AN/PVS-5A has a lift
requirement to turn on the infrared illuminator. All other features
and parts are the same.
(2) Modified faceplate. The faceplate of the AN/PVS-5A was
modified for aviation use by the US Army Aeromedical Research
Laboratory. The MFP was an interim quick-fix for the AN/PVS-5A to
make it safer for crew members to use. Figure 2-4 shows the
AN/PVS-5A with the MFP.
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Figure 2-4. Modified faceplate, AN/PVS-5A
(a) The lower portion of the faceplate was removed and its
electrical components were relocated to the top. The MFP enables
the user to view around the goggles. This permits an aviator to
view the cockpit, read maps, and discern the color of aircraft and
ground lights. Eyeglasses may be worn with the MFP.
(b) The MFP is mounted between the front of the helmet and the
visor cover. It is held in place by the standard vee and side
straps. A side strap of surgical tubing can also be used.
(3) GX-5 flip-up modification. The GX-5 uses a pivoting hinge
that locks the device in a stowed position or lowers it for use.
The hinge is hard-mounted to a section of the visor cover and
attached to the helmet visor cover by Velcro. A new frame holds the
I2 tubes and attaches to the lower portion of the hinge by a
bolt-and-nut assembly. This frame increases peripheral unaided
vision. The GX-5 flip-up modification requires the dual-battery
pack for power and ON-OFF switching. Figure 2-5 illustrates the
GX-5 flip-up modification.
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Figure 2-5. GX-5 flip-up modification, AN/PVS-5A
(4) AN/PVS-5B/C. The AN/PVS-5B/C uses the second-generation
intensifier tube of the AN/PVS-5A but with improved objective
lenses that gather more light. This results in a substantial
improvement in low light flight capability. The AN/PVS-5B/C with
the full faceplate is not employed in Army aviation.
WARNING
The AN/PVS-5B/C is designed for ground use only when the tube
assemblies are in the original full faceplate. A feature in the
faceplate automatically cuts off power if the tubes are exposed to
continuous high light levels for more than one minute. Aviation
personnel are not permitted to modify the AN/PVS-5B/C
faceplate.
c. Flight Helmet Attachment.
(1) A properly fitted flight helmet is essential for comfort and
to lessen fatigue. For the same reason, the flight helmet must be
correctly modified for attachment of the full-faceplate AN/PVS-5A,
MFP, or GX-5 modification. Properly adjusted helmet headbands and
nape straps will help prevent the helmet from rotating forward and
down during I2 device use.
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(2) Helmet straps are supplied with each device. The aviation
helmet kit (vee straps and side straps) provides four attaching
points (two male snaps and two 2-inch Velcro strips) that must be
attached to the helmet. Instructions for preparing the helmet are
described in TM 11-5855-238-10. The SPH-4 helmet is not configured
for external attachment of the AN/PVS-5 series without
modification.
2-4. AN/AVS-6
a. The AN/AVS-6 is a helmet-mounted, light-intensification
device. Figure 2-6 shows the AN/AVS-6. The AN/AVS-6 allows aircrews
to conduct operations at terrain flight altitudes during low
ambient light levels, to include overcast conditions. It has the
same 40 FOV as the AN/PVS-5 series. Under ideal conditions, visual
acuity of 20/200 or less can be improved to 20/40 with the
AN/AVS-6. Figure 2-7 illustrates the superiority of the AN/AVS-6
over the AN/PVS-5 series because of its improved sensitivity in the
red and near infrared region of the spectrum. Light in this portion
of the spectrum predominates at night, as shown in Figure 2-8. Not
only does the AN/AVS-6 provide greater light amplification but it
also amplifies light in that portion of the spectrum that is most
predominant at night.
NOTE: The small peak in the blue-green portion of the spectrum
in Figure 2-8 is caused by moon illumination. The size of this peak
will vary based on the amount of moon illumination.
b. The AN/AVS-6 operates by intensifying ambient light 2,000 to
3,500 times. It can provide sufficient imagery from overcast
starlight to moonlight conditions. However, below quarter moonlight
conditions, artificial illumination (usually infrared) may be
required to light the helicopter's flight path.
Figure 2-6. AN/AVS-6
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Figure 2-7. Relative sensitivity of AN/PVS-5 and AN/AVS-6
systems
Figure 2-8. Spectral distribution of starlight
c. The AN/AVS-6 comes in two versions: AN/AVS-6(V)1 and
AN/AVS-6(V)2. The AN/AVS-6(V)1 mounts directly to the standard
SPH-4 flight helmet. A special mount and offset binocular, the
AN/AVS-6(V)2, is available for SPH-4 helmets modified with a
helmet-mounted sight. The AN/AVS-6 is powered by batteries or
aircraft interface and has a 30-minute, low-voltage warning
indicator. The warning indicator is a dim red light emitted between
the binoculars above the FOV when 30 minutes or less of battery
life remains.
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d. The AN/AVS-6 is stowable on the helmet in a flipped-up
position, which automatically cuts off power to the tubes. Figure
2-9 shows the AN/AVS-6 in the stowed position. The AN/AVS-6 is
significantly lighter than the AN/PVS-5 series and has a breakaway
feature designed to separate the binocular from the helmet mount
under crash loads. The AN/AVS-6 also has an improved unaided
peripheral view. It incorporates a minus-blue filter that makes the
system insensitive to blue-green cockpit lights and their
reflections in the cockpit.
Figure 2-9. AN/AVS-6 in stowed position
2-5. ADJUSTMENT TECHNIQUES
a. Interpupillary Distance. If I2 device eyepieces are not
properly aligned with the eyes, less than optimum resolution with
the device will be obtained. Proper alignment of the eyepieces is
achieved when the distance between the tubes matches the distance
between the user's pupils. When the interpupillary distance of the
I2 device is properly adjusted, the edges of the images in both
tubes will be clear. When the edges are clear, the resultant
binocular view through the tubes may appear as a single circle or
as two circles. The circle or circles will be overlapped and
slightly displaced laterally. Interpupillary distance is adjusted
while the tubes are focused at infinity under dark-light conditions
with all lens caps removed. The procedure for adjusting
interpupillary distance is described below.
(1) Move the tubes away from the eyes as far as possible. This
makes edge clarity easier to judge.
(2) With both eyes open, move the tubes closer together and
farther apart. Observe the clarity of the edges of the circle in
each eye. If the outside edges are blurred, the tubes are too close
together. If the inside edges are
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blurred, the tubes are too far apart. If the upper or lower
edges are blurred, tilt the tubes.
(3) Move the tubes closer to the eyes as desired without the
eyelashes touching the eyepiece lenses. Recheck the tube tilt.
b. Binocular Focus. Each I2 device has a method for dioptric
adjustment. This adjustment is used to correct visual refractive
errors such as myopia (nearsightedness) and hyperopia
(farsightedness). For the AN/PVS-5 series, this is accomplished
with the diopter adjust ring. For the AN/AVS-6, it is accomplished
with the eyepiece focus ring. When setting the dioptric adjustment,
the user may achieve a clear image in each eye (monocular) and yet
have a blurred image or accommodative eyestrain when viewing with
both eyes (binocular). This occurs when the dioptric adjustment is
set for one eye while the other eye is closed or covered. In this
situation, the eyes tend to accommodate to a nearer distance than
infinity, typically 1 to 3 feet. Over-accommodation or focus
imbalance or both between the eyes can cause eyestrain and periodic
blurred vision. To achieve a clear and relaxed binocular focus, the
user should follow the procedure described below after focusing the
tubes for each eye and adjusting interpupillary distance.
(1) Focus at infinity and view a distant object.
(2) Slightly blur the image in one tube, left or right, with the
focus knob (AN/AVS-5 series) or objective focus ring (AN/AVS-6).
The amount of blur should allow recognition of general object
shapes but not fine details in the blurred tube.
(3) With both eyes open, adjust the diopter adjust ring or
eyepiece focus ring for the clearest image in the nonblurred
tube.
(4) Return the blurred tube to infinity focus, blur the other
tube, and repeat the process.
2-6. OPERATIONAL CONSIDERATIONS
a. Magnification. I2 systems do not amplify an image. An object
viewed through an I2 system will be the same size as if it were
seen with the unaided eye.
b. Lights.
(1) With an I2 device, individuals can detect light sources that
may not be visible to the unaided eye. Examples include lights from
other aircraft, flashlights, burning cigarettes, and chemical light
sticks. As the ambient light level decreases, aircrews can more
easily detect these light sources but are less able to estimate
distance correctly.
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(2) Performance of I2 systems is directly related to the ambient
light. During periods of high ambient light, resolution is improved
and objects can be identified at greater distances, although not to
the degree possible during daylight. To light the flight path of a
helicopter in low ambient light, the aviator may have to use an
additional light source. Night scenes viewed with I2 devices are
shown in Figure 2-10.
(3) I2 devices are adversely affected by bright lights and
periods of high ambient light. When exposed to a bright light
source, both the AN/AVS-6 and the AN/PVS-5 series are susceptible
to whiteout. Saturation of the I2 system appears on the tube as a
bright halo effect around the image of the light source. The halo
effect also degrades the contrast of adjacent portions of the
intensified image. This degradation of performance becomes worse
when several bright lights appear in the field of view.
Additionally, internal circuitry automatically adjusts output
brightness to a preset level to restrict peak display luminance.
When an area with bright lights is viewed, the display luminance
will decrease ("shut down"). In addition to the halo effect around
a bright light source, the overall display luminance of the rest of
the viewed scene will dim. The brighter the light source, the
dimmer the rest of the viewed scene. The crew member may also
experience the dimming effect when viewing in the direction of a
full moon at low angles above the horizon.
(4) Tunnel vision limits an individual's ability to see outside
an area lit by bright artificial lights such as flares, landing
lights, and lights with infrared filters. The ability to see
objects within the lighted area depends on the intensity of the
light and the distance of the object from the viewer. A crew member
should not look directly at a bright light source because it may
temporarily degrade the efficiency of the I2 system. When flying
with the landing light or searchlight with the pink light filter or
infrared band-pass filter on, the aircrew should avoid
concentrating on the area illuminated by the light. The aircrew
should also scan the area not illuminated by the light for hazards
and obstacles.
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Figure 2-10. Night scenes viewed with I2 devices
c. Depth Perception and Distance Estimation. Depth perception
and distance estimation are difficult with I2 systems. The quality
of an individual's depth perception in a given situation depends on
several factors. They include the available light, type and quality
of the I2 system used, degree of contrast in the field of view, and
viewer's experience. The aircrew must rely on the monocular cues
discussed in Chapter 1 for accurate depth perception and distance
estimation.
d. Color Discrimination. Color discrimination is absent when
scenes are viewed through I2 systems. The picture seen with I2
systems is monochromatic (single color). It has a green hue because
of the type of phosphor used on the phosphor screen of the I2 tube.
The green hue in I2 systems may cause crew members to experience a
pink, brown, or purple afterimage when they remove the device. This
is called chromatic adaption and is a normal physiological
phenomenon. The length of time the afterimage remains varies with
the individual.
e. Scanning Techniques. Although the basic principles of
scanning are the same for unaided and aided flight, crew members
must consider a few specific items when conducting operations with
I2 devices. Flight techniques and visual cues for unaided night
flight also apply to aided night flight. Use of the I2 device
improves ground reference but significantly reduces the field of
view.
(1) The FOV of I2 devices significantly reduces peripheral
vision as compared with unaided flight. Thus the crew member must
use a continual
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scanning pattern to compensate for the loss. Moving the eyes
will not change the viewing perspective; the head must be turned.
However, rapid head movement can induce spatial disorientation. To
view an area while using an I2 device, the crew member must rotate
his head and eyes slowly and continuously. When scanning to the
right, he should move his eyes slowly from the left limit of vision
inside the device to the right limit while moving his head to the
right. In this manner, the crew member will cover a 70 to 80
viewing field with only 30 or 40 of head movement. This technique
minimizes head rotation. However, maximum visual acuity can only be
attained when the crew member views through the center of the tube.
Acuity drops to 20/70 or worse in the periphery of the I2 device
FOV. The crew member should scan back to the left in reverse order
and avoid rapid head movements because they can induce vertigo. The
crew member must develop scanning techniques that involve a mix of
unaided and aided vision.
(2) The devices provide the primary source for detailed visual
information. When viewed through the devices, illumination sources,
such as aircraft position lights and ground lights, may not be
accurately interpreted according to intensity, distance, or color.
Unaided vision can provide this additional information. With the
newly modified I2 device-compatible aircraft cockpits, a slight
downward deflection of the eyes will provide all required visual
information inside the cockpit.
(3) Practice and experience are necessary to obtain maximum
visual information from both unaided and aided vision. Initially,
unaided peripheral vision may be somewhat distracting until the
crew member develops adequate experience combining through-the-tube
viewing with around-the-device scanning.
NOTE: Continuous flight with one lens focused inside and one
focused outside the aircraft is prohibited. This can cause spatial
disorientation, headaches, eyestrain, and reduced visual
acuity.
f. Obstruction Detection. Obstructions that have poor reflective
surfaces, such as wires and small tree limbs, are difficult to
detect. The best way to locate wires is to look for the support
structures. Hazardous wires in high-use areas should be marked with
reflective devices.
g. Spatial Disorientation. Maneuvers requiring large bank angles
or rapid attitude changes tend to induce spatial disorientation.
Therefore, the aviator should avoid making drastic changes in
attitude and bank angle and use proper scanning and viewing
techniques.
h. Airspeed and Ground Speed Limitations. Aviators using I2
devices tend to overfly their capability to see. To avoid
obstacles, they must understand the
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relationship between the device's visual range and forward
lighting capability and airspeed.
WARNING
The visual range of the I2 devices may not allow aviators enough
time to avoid obstacles. Therefore, aviators must exercise extreme
care when using the devices during terrain flight modes. Aviators
should reduce ground speed so that they can detect and avoid
obstacles when ambient light levels are low or visibility is poor
because of weather conditions.
(1) Different light levels affect the distance at which crew
members can identify an object. This, in turn, limits the ground
speed at which aviators can safely fly at terrain flight altitudes.
Ground speed limitations are not quantified because of continuously
changing variables affecting the limitations. Variables include the
type of aircraft, type and quality of I2 device, supplemental
lighting, vision obscurations, and ambient light conditions.
(2) Object acquisition and identification are related to ambient
light levels, visibility, and contrast between the object and its
background. For safety reasons, light levels required for training
may differ considerably from operational requirements. Variables
that affect the ability to see with I2 devices include--
o Type of I2 device. o Condition of aircraft windscreen. o Age
and condition of tubes and lenses. o Moisture content in the air
(humidity). o Individual's proficiency and capabilities. o Proper
care and maintenance of the I2 device. o Capabilities of infrared
band-pass filter used. o Visibility (haze, fog, rain, low clouds,
dust, smoke).
i. Aircraft Lighting. Various sources of lighting (especially
red) that are not compatible with I2 systems may degrade the
aviator's ability to see with the system. The adverse effects of
aircraft lighting on the I2 device are greatest during low ambient
light conditions.
(1) Cockpit lights. The initial I2 cockpit lighting used with
the full-faceplate AN/PVS-5 series was an "infrared cockpit
illuminator." It was a modified map light that flooded the
blacked-out instrument panel with a dim infrared light. The
instrument panel was readable when an I2 tube was focused inside.
Indicator, caution, and warning lights were dimmed to an acceptable
I2 level by covering them with tape. Night-Fix, Phase I, employed
blue-green lighting that had a negligible effect on I2 devices.
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Blue-green filters pass only blue-green light and block all
other light, especially red and infrared. All red lighting was
extinguished by an ON-OFF NVG light switch that activated the
blue-green lights flooding the instrument panel. Indicator,
caution, and warning lights were dimmed by blue-green filters. This
system was designed for compatibility with the AN/AVS-6 and is
therefore only partially compatible with the AN/PVS-5 series.
Night-fix, Phase II, employed the same blue-green filters, but each
instrument, radio, panel, and indicator light was illuminated or
dimmed individually. All red lighting was removed. Suggested ways
to improve I2 cockpit lighting are discussed on the next page.
(a) AN/PVS-5 series. The blue-green lighting should be dimmed to
the lowest readable level based on the ambient light level and the
aviator's dark adaptation level. This is required because the
AN/PVS-5 series is sensitive to all colors of light. Dimming them
reduces reflections on the windscreen, which limit I2 performance.
The AN/PVS-5 series does not have the minus-blue filter like that
used on the AN/AVS-6.
(b) AN/AVS-6. The AN/AVS-6 is designed to be operated with
blue-green cockpit lights. The combination of improved performance
in the red and near infrared portion of the spectrum and the
minus-blue filter makes red cockpit lights noncompatible and
blue-green cockpit lighting ideal. The use of red cockpit lighting
should be avoided or strictly limited. While the use of blue-green
cockpit lights will not degrade system performance, these lights
should be dimmed to the lowest readable level.
(c) Both systems. Light reflected on the windscreen of the
aircraft may degrade the aircrew's ability to see outside the
cockpit. Aircraft lights that cannot be controlled should be
covered with a filter compatible with I2 devices or with a
light-reducing material such as tape. Flight instruments should be
illuminated by interior lighting compatible with I2 devices. The
instruments can be read by looking beneath the I2 device.
(2) Supplemental external lights. During Night-Fix, Phase I, a
pink light filter was mounted on the standard landing light. It
provides supple-mental infrared lighting, useful for 100 to 200
feet, which can be directed forward in the helicopter's flight path
to detect obstacles. Figure 2-11 shows the pink light. The speed of
the helicopter is reduced based on the range of the light and
visibility restrictions. Further aircraft modification includes an
improved infrared band-pass filter mounted on the searchlight or
landing light. Different bulbs can vary the distance and beam
spread of the infrared light path. Figure 2-12 shows the infrared
band-pass filter in place.
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Figure 2-11. Pink light
Figure 2-12. Infrared band-pass filter
(3) External lights. I2 flights are degraded by aircraft
external lights unless they are properly modified. The lights
should be adjusted to the lowest level that will still allow
detection by other aircraft or the control tower. The top half of
the lower navigation lights and bottom half of the top navigation
lights should be painted or taped to allow aviators to see through
an unlit area surrounding the cockpit.
(a) Red navigation lights on the left side of the helicopter
produce more usable light with I2 systems than green lights. In
unaided
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flight, the opposite is true. Aviators switching seats should
anticipate this, especially during hovering. Other
lights--fuselage, formation, anticollision, electroluminescent
panels (slime lights), and infrared position lights--should be
turned off or subdued. Otherwise, their use should be based on
training, tactical, or airspace requirements. Figure 2-13 shows the
front of a UH-1 helicopter viewed through an I2 device. The red
light on the left side of the helicopter appears much brighter than
the green light on the right side.
(b) External aircraft lights aid the aircrew in interpreting
terrain close to the aircraft. To minimize the adverse effects of
external lights on night vision, the aviator should operate
navigation lights in the DIM position when they are required. The
anticollision light can be turned off to enhance training. The pink
light filter or infrared band-pass filter should be used for
training during periods of low ambient light. Exterior lights of
other aircraft will not degrade the vision of an aircrew using I2
systems if the lights are taped and operated properly.
Figure 2-13. Effect of position lights on I2 devices
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j. Weather. When using I2 systems, aviators may fail to detect
entry into IMC. This is because the I2 systems may enable aviators
to see through obscurations such as fog, rain, haze, dust, and
certain types of smoke.
(1) As the density of the visibility restriction increases,
aircrews will detect a gradual reduction in light and visual
acuity. When they recognize that their visibility is restricted,
they should try to determine the severity of the condition and take
appropriate action. This may include reducing airspeed, increasing
altitude, seeking areas of contrast, or landing. If visual flight
cannot be maintained, aviators should execute the appropriate IMC
recovery procedures.
(2) Certain visual cues will be evident when visibility
restrictions are encountered. A halo may form around sources of
illumination when devices are used and atmospheric obscurations are
present. The size of this halo effect around lights in the area of
operations should be noted. If the halo becomes noticeably larger,
a restriction could be developing. Also, an increase in "image
noise" may result when atmospheric obscurations are present and the
ambient light level is low. This is similar in appearance to the
"snow" seen on a television with poor reception.
k. Weapons.
(1) Tube-launched, optically tracked, wire-guided missiles. TOW
missile engagements at night should be made with unaided vision and
with the appropriate artificial illumination for target
acquisition. During TOW missile engagements, aviators may encounter
conditions that adversely affect the use of the I2 device. The most
critical of these conditions are listed below.
Initially, the target will not be visible because of the light
intensity produced by the missile's flight motor.
The aviator's ability to see the target will be impaired by the
missile's infrared source as the missile continues downrange.
Damage may occur to the lens of the telescopic sighting
unit.
(2) Rockets, cannons, and machine guns. When firing the
2.75-inch folding-fin aerial rocket, aviators will lose sight of
the target momentarily. After the rocket leaves the launcher, they
will immediately regain sight of the target. Firing the
20-millimeter cannon can cause some visual impairment. Firing the
7.62-millimeter machine gun will cause loss of sight with the
target during the entire firing burst.
NOTE: Recovery from bright flash illumination is more rapid with
I2 devices than with the unaided eye.
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Section II
THERMAL-IMAGING SYSTEMS
2-7. OPERATIONAL PRINCIPLES
a. Operation of thermal-imaging systems differs from that of I2
systems. Thermal systems operate passively and without regard to
levels of visible light. These systems do not transmit energy.
Rather, they sense and display the energy radiated from objects.
Thermal-imaging systems provide aviators with an image of an
infrared scene. This enables aviators to operate in environments
that could restrict or prohibit unaided operations.
b. The effectiveness of a thermal-imaging system depends on the
difference in detected infrared radiation between the object to be
detected and its background. Effectiveness also depends on
atmospheric considerations--the degree of obscuration present
between the system and the object. Thermal systems are most
effective when a great difference in infrared radiation exists
between an object and its background and when obscuration is
minimal.
2-8. SYSTEM TYPES
Four types of thermal devices are currently used on Army
aircraft. The PNVS, thus far, is the only type of pilotage thermal
system used. The other de-vices are target acquisition systems.
Aviators should consult the appropriate aircraft operator's manual
for specific operating instructions.
a. Pilotage System. The PNVS, which is mounted on the AH-64, is
a type of pilotage FLIR. Figure 2-14 illustrates the pilotage
system. Appendix C provides a detailed discussion of FLIR
components.
b. Target Acquisition Systems. A brief description of target
acquisition systems is given below. An example of a target
acquisition system, the TADS, is shown in Figure 2-15.
(1) C-NITE sight. C-NITE is a thermal sight for firing TOW
missiles from the AH-1.
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Figure 2-14. Pilotage system
Figure 2-15. Target acquisition system
(2) Target acquisition and designation sight. The TADS is a day
and night sight and laser designator and range finder for the
AH-64. The TADS FLIR can be used as a backup to the PNVS.
(3) Mast-mounted sight. The MMS is a day and night sight and
laser designator and range finder for the OH-58D.
2-9. INFRARED CHARACTERISTICS
Infrared is measurable electromagnetic energy and is part of the
electromagnetic spectrum. The infrared region occurs beyond the
visible light range. Infrared is, therefore, invisible to the human
eye.
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a. Infrared Radiation.
(1) A definition of reflectance, transmittance, absorptance, and
emissivity is given below.
(a) Reflectance--the ratio of radiant energy reflected by a body
to the radiant energy incident upon it.
(b) Transmittance--the ratio of radiant energy that, having
entered a body, reaches its farther boundary.
(c) Absorptance--the ratio of radiant energy absorbed by a body
to the radiant energy incident upon it.
(d) Emissivity--the relative power of a surface to emit heat by
radiation. It is the ratio of radiant energy emitted by a body--as
a consequence of its temperature only--to that emitted by a
reference body (blackbody) at the same temperature. This additional
characteristic has considerable significance regarding object
infrared radiation. A blackbody is a theoretical standard used for
the purpose of laboratory comparison. It is an ideal body or
surface that completely absorbs all radiant energy falling upon it
with no reflection. A blackbody absorbs 100 percent of infrared
energy incident upon it and emits 100 percent of its infrared
energy. Therefore, a blackbody is both a perfect absorber and a
perfect emitter.
(2) Reflectance, transmittance, absorptance, and emissivity
determine the amount of infrared energy that an object will radiate
when exposed to "x" level of thermal energy for "x" amount of time.
Incident infrared radiation on a body may be reflected, transmitted
through, and/or absorbed by the body. Absorbed energy may be
emitted over a period of time according to the emissivity of the
body. The total amount of infrared energy that an object will
radiate is the sum of reflected, transmitted, and emitted energy.
Figure 2-16 illustrates incident, reflected, absorbed, transmitted,
and emitted radiation.
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Figure 2-16. Infrared radiation
b. Minimum Resolvable Temperature. The FLIR can discriminate
objects from their respective backgrounds in the broad range of
environmental conditions in which the FLIR operates. FLIR
performance is measured by determining the MRT. A thermal-imaging
system discriminates an object from its background by measuring the
difference between the total infrared radiation of the object and
the total infrared radiation of its background. MRT is de-fined as
the lowest equivalent thermal difference between an object and its
background that can