Night Vision Goggles in Civilian Aviation
by G. J. Salazar, M.D.
and Van B. Nakagawara, O.D.
Article reprinted with permission of FAA Aviation
Night vision goggles? Aren't they for the military
and police? Not anymore! On January 29, 1999, the
FAA issued the first Supplemental Type Certificate
(STC) to permit use of night vision goggles by a
civilian helicopter EMS (emergency medical service)
operator. Since then several more have been issued
to other commercial operators. In addition, rulemaking
was initiated (but at the time of this writing is
temporarily on hold) for changes to FAR Part 91
that would permit use of this technology by general
aviation pilots. With this in mind, it will only
be a matter of time before pilots start hearing
more and more about these significant aids to night
flying. Therefore, it is important for pilots to
become aware of this technology and understand some
of the basic operational issues.
NIGHT VISION GOGGLES
Night vision devices include a variety of different
technologies, such as forward-looking infrared radar
(FLIR) and night vision goggles. The focus of this
article will be on night vision goggles, more commonly
known by the acronym NVG. The simplest analogy to
explain how NVG's work is a video camera. The basic
principle is the same in that the user is not directly
seeing what they look at, but rather is viewing
an electronic image of the scene.
NVG equipment may be monocular or binocular.
However, in aviation, binocular, helmet-mounted
equipment is almost exclusively used. Like a video
camera, an NVG is an electro-optical device. Electromagnetic
energy, both visible and infrared, reflected from
the terrain at night enters the NVG through the
objective lens. These photons of light energy are
directed to an electronic processing unit called
the image intensifier, which contains several components.
The photocathode element in the image intensifier
converts the light photons to electrons and moves
them to the microchannel plate (MCP) which accelerates
and multiplies them several thousand times. The
electrons then strike the phosphor screen, which
is ultimately responsible for emitting the visible
light the user will see through the eyepiece lens
as a focused image.
Unlike the video camera, the NVG does not require
much light to produce an image. Light as faint as
a starlight or low-level moonlight will suffice.
However, the efficiency of the equipment will be
degraded in total darkness or with too much light.
The image intensifier will increase what little
light energy there is on average several thousand
times. State-of-the-art NVG's are capable of intensification
on the order of 35,000 times or more. That amplified
or intensified energy is projected onto the phosphor
screen, which creates the visible image the user-sees
through the eyepieces. The NVG image is monochrome,
i.e., in one color, typically either green or amber
depending on the type of phosphor used. NVG equipment
lacks the ability to produce a multi-color representation
of a scene.
Aviation NVG models are helmet-mounted with electrical
power supplied by a battery pack attached to the
back of the helmet. As with any optical device,
the user has a variety of ways of adjusting fit
and focus. The NVG binoculars and mounting assembly
are cumbersome, weighing approximately one pound.
In addition, one must factor in the weight of the
helmet and battery pack.
ADVANTAGE OF NVG's
The advantages of this night vision aid technology
in aviation can be summed up as an increase in nighttime
situational awareness for pilots. This technology
does not turn night into day, but it does permit
the user to see objects that normally would not
be seen by the unaided eye. This would markedly
decrease the possibility of collisions with terrain
or man-made obstructions. Many other benefits exist,
but the bottom line is that this technology, when
properly used, has the potential to significantly
increase nighttime flying safety.
DISADVANTGAGES OF NVG's
Unfortunately, this increase in safety comes
with a significant price. Some of the disadvantages
of NVG's include:
- decreased field of aided view
- decreased visual acuity
- loss of depth perception
- lack of color discrimination
- neck strain and fatigue
- high initial cost to purchase
- require on-going maintenance
- need for recurrent training
- requires modification of aircraft lighting
Current NVG's provide approximately 40 to 60
degrees of aided nighttime circular field of vision,
although the user retains some unaided vision by
being able to look peripherally around or under
the goggles. With a reduced field of vision, effective
scanning techniques are even more important than
with unaided vision alone. Because one is looking
at an electronic image, depth perception is lost.
The user must learn to recognize terrain contrast
and shadowing to replace some of the lost depth
perception cues. Thus, the ability of the pilot
to determine precise closure on terrain or other
aircraft when these are first detected is limited.
Low-light level operations inherently produce
decreased visual resolution, acuity, and contrast,
thereby making hazard detection more difficult.
Visual acuity from NVG devices provides a vast improvement
over unaided human night vision, which can be 20/200
or worse. With properly focused goggles at starlight
or quarter moon, one can have nighttime visual acuity
equivalent to 20/40 or 20/30. The latest generation
of goggles can achieve 20/25; however, this is difficult
to accomplish in an operational setting. Enhanced
vision with NVG's is proportional to altitude and
airspeed. With NVG's, "lower and slower" improves
visual acuity. Therefore, a helicopter pilot would
have some advantage over his or her fixed-wing counterpart
in determining terrain features in low light conditions.
In addition, newer generation equipment provides
greater contrast detection, thereby improving situational
awareness. It is important to note that NVG-aided
acuity of 20/30 or 20/40 assumes proper cockpit
lighting, properly focused and well-maintained goggles,
and ideal environmental conditions.
As mentioned previously NVG's produce monochrome
images. Because the eye can differentiate more shades
of green than other phosphor colors, the night vision
phosphor screen is typically green. This allows
the user to see more detail, but with an inability
to detect differences in color. Changing illumination
can affect visual acuity. External incompatible
light from the ambient environment could result
in "washout" or halo effects, when using NVG's.
This could result in glare, flash blindness, and
afterimage for the pilot. Particularly troublesome
is ensuring aircraft and cockpit lights are NVG-compatible.
Incompatible lights make the outside scene less
visible with NVG's. Changing cockpit lights to be
NVG compatible is very complicated and expensive.
NVG's are sensitive to light ranging from yellow-green
to near-infrared wavelengths. FAA required aircraft
position and anti-collision lights could cause problems
for goggle wearers. NVG's are also subject to interference
by environmental factors, such as rain, clouds,
snow, mist, dust, smoke, and fog. In anything more
than very small amounts, any of these will tend
to severely degrade the performance of the equipment.
During prolonged use of helmet-mounted NVG devices,
the potential for neck discomfort and other problems,
such as increased general fatigue, exists because
of the weight of the helmet, battery pack, and NVG
In summary, while NVG and other night vision
technology are potentially great safety enhancements
for select nighttime flight operations, they are
an expensive and sophisticated pieces of equipment
requiring considerable effort to implement and maintain.
Night vision goggles do not turn night into day
and if not properly used, rather than preventing
accidents they could be the cause of one. Operational
use of these devices should be accomplished only
after pilots have received extensive, supervised
ground and in-flight training with the equipment.
Once trained pilots must strive to maintain proficiency
by ongoing use and recurrent training.
G. J. Salazar, M.D. is
the Regional Flight Surgeon in FAA Southwest Region,
Fort Worth, Texas. Van B. Nakagawara, O.D., is a
Research Optometrist at FAA's Civil Aeromedical
Institute in Oklahoma City, Oklahoma.