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Glossary of Terminology
Acuity - The human eye has a
nominal resolution of 1 minute of arc. The common measure of visual
acuity is based on reading letters with 1 minute line width, Snellen
letters, or patterns with similar detail, suc h as Landolt rings.
Visual acuity is reported as a fraction. The denominator is the test
distance (usually 20 feet). The numerator is the relative size of line
that can be resolved. That is, 20/40 indicates that the resolution was
2 minutes of arc, twice the nominal value. In other words, that
individual can resolve at 20 feet what a "normal" person can at 40 feet
Automatic Brightness Control (ABC)
- An electronic feature that automatically reduces voltages to the
microchannel plate to keep the image intensifier's brightness within
optimal limits, and protects the tube. The effect of this can be seen
when rapidly changing from low-light to high-light conditions; the
image gets brighter and then, after a momentary delay, suddenly dims to
a constant level Biocular - Viewing a single image source with both eyes (example: watching a television set)
Binocular - Viewing a scene through two channels; i.e. one channel per eye
Black Spots - Blemishes - Cosmetic
blemishes in the image intensifier or dirt/debris between the lenses.
Black spots in the image intensifier do not affect the performance or
reliability of the device and are inherent in the manufacturing
processes Boresighting - The alignment of a weapon aiming device to the bore of the weapon. See also Zeroing
Brightness Gain - When
referring to an image intensification tube, brightness gain is the
ratio of the brightness of the output in units of foot-Lambert,
compared to the 10
illumination of the input in foot-candles. A typical value for a
GEN III tube is 25,000 to 30,000 Fl/fc. A tube gain of 30,000 Fl/fc
provides an approximate system gain of 3,000. This means that the
intensified NVG image is 3,000 times brighter to the aided eye than to
the unaided eye Bright Source Protection (BSP) -
An electronic function that reduces the voltage to the photocathode
when the night vision device is exposed to bright light sources such as
room lights or car lights. BSP protects the image tube from damage and
enhances its life. However, BSP may have the effect of lowering
resolution when it is functioning
Bright Spots. These are signal-
induced blemishes in the image area caused by a flaw in the film on the
MCP. A bright spot is small, non- uniform, bright area that may flicker
or appear constant. Bright spots usually go away when the light is
blocked out. Not all bright spots make the ANVIS unserviceable. A test
can be performed as follows: Place a cupped hand over the lens to block
out all light. Make sure any bright spot is not simply a bright area in
the viewed scene. If the bright spot remains, an emission point exists
and needs to be checked.
C-Mount - A standard still and video camera lens thread size for mounting to the body of a camera. Usually 1/2" or 3/4" in diameter.
Collimation - The act of
making rays of light travel in parallel lines. Also the process of
aligning the various internal optical axes of a system with each other.
Convergence. The shifting of an observer's eyes inward to view a nearby object i.e. crossing the eyes.
Chicken Wire - An
irregular pattern of dark lines in the Field-of-View (FOV) throughout
the image area or in parts of the image area. Under the worse
condition, these
lines will form hexagonal or square-wave shaped lines.
COMSPEC (Commercial Specification) - A term used to describe image tube quality, testing and inspection done by the original equipment manufacturer (OEM).
Counterweight System - Counterweight
systems are used to adjust the center of gravity of the pilot’s flight
helmet with goggles installed. Without counterweights, there can be a
fatiguing forward and downward force on the pilot’s neck. The
counterweight system may consist of a weight bag and counterweights.
The Army’s recommended initial weight is 12 ounces for one of its
systems. Pilots are instructed to add or remove weight to achieve the
best balance and comfort, not to exceed 22 ounces. Attachment of the
weight bag is below the back of the helmet with the battery pack
mounted vertically above it. The adjustment of the weight is to be made
with the binoculars attached and flipped down.
Cycles per Milliradian (cy/mr) - Units
used to measure resolution. A milliradian is the angle created by one
yard at a distance of 1,000 yards. This means that a device that can
detect two 1/2 yard objects separated by 1/2 yard at 1,000 yards has a
resolution of 1.0 cy/mr.
Daylight Lens Cover -
Usually made of soft plastic or rubber with a pinhole that allows a
small amount of light to enter the objective lens of a night vision
device. This should be used for training purposes only, and is not
recommended for an extended period of time.
Daylight Training Filter-
A glass filter assembly designed to fit over the objective lens of a
night vision device. The filter reduces light input to a safe
(night-time) level, allowing safe extended daytime use of the night
vision device. Diopter - The unit of measure
used to define eye correction or the refractive power of a lens.
Adjustments to an optical eyepiece accommodate for differences in
individual eyesight. Most systems provide a +2 to -6 diopter range.
Dipvergence - The shifting of an observer's eyes vertically, one up and one down.
Distortion - Three types of distortion are most significant to night vision devices: geometric, "S" and sheer.
- Geometric Distortion - is inherent in all Gen 0 (infrared Tubes ie.
B-20) and Gen I image intensifiers and in some Gen II image
intensifiers that use electrostatic rather than fibre-optic inversion
of the image. Geometric distortion is eliminated in image tubes that
use a microchannel plate and fibre-optics for image inversion, however,
some S-distortion can occur in these tubes.
- S-Distortion - results from the twisting operation in
manufacturing fibre-optic inserters (fibber optic twister - Russian
tubes do not use a fibre optic twister). Usually S-Distortion is very
small and is difficult to detect with the unaided eye. Gen III tubes
manufactured to U.S. military standards since 1988 have nearly no
perceptible S-Distortion.
- Sheer Distortion - can occur in any image tube that uses
fibre-optic bundles for the phosphor screen. It appears as a cleavage
or dislocation in a straight line viewed in the image area as through
the line were sheered.
*NOTE: Non-inverting image intensifiers that use microchannel plates
and clear glass for the optics are free of distortion. The 2nd Gen.
image intensifiers that Russians make are distortion free, since they
do not have fiberoptics twister. Divergence - The shifting of an observer's eyes outward.
Electronic Noise - Also known as
Scintillation. A faint, random, sparkling effect throughout the image
area. Scintillation is a normal characteristic of microchannel plate
image intensifiers and is more pronounced under low-light-level
conditions Equivalent Background Illumination (EBI) -
This is the amount you see in an image tube that is turned on but there
is no light at all on the photocathode; it is affected by temperature
where the warmer night-vision device, the brighter the background
illumination. EBI is measured in lumens per square centimetre (lm/cm2)
wherein the lower the value the better. The EBI level determines the
lowest light level at which you can detect something. Below this light
level, objects will be masked by the EBI. Eyepiece Lens
- The eyepiece lens focuses the image from the fiber optic inverter on
to the eye by adjusting for individual eye acuity. There are two
eyepiece lens assemblies in current systems; the 15 mm and the 25 mm
eyepiece lens assembly. Tests show the larger eyepiece is more
effective. This lens assembly is designed to provide some adjustment
for the user to compensate for minor vision deficiencies (i.e. diopter
adjustment). However, the assembly does not correct for all eye
deficiencies and does not replace the need for wearing prescribed
spectacles or contact lenses. Eye Relief - The focus-range distance
between the user's eye and the eyepiece lens of an optical device at
which the viewed scene can clearly be seen.
Exit Pupil - In an optical system, the
rays of light passing through the system will be limited by either the
edges of one of the components such as the eyepiece lens, or by an
internal aperture. The image passing through the entrance side of the
optical system is the entrance pupil. The image passing out the exit
side is the exit pupil. This image forms a small disk containing all of
the light collected by the optics from the entire field-of-view. Fiber Optic Inverter - A bundle of microscopic light transmitting fibers twisted 180 degrees.
Field-of-View (FOV) - The width or
spatial angle of the outside scene that can be viewed through the
intensifier tubes measured laterally and vertically. Typical NVGs have
a 40° FOV. There are NVGs in development that attempt to increase this
FOV significantly in an effort to enhance pilot performance. An example
of a wide FOV NVG would be the Panoramic NVGs'. Figure of Merit (FOM) - Image Intensification tube specification designation, calculated on line pair per mm X signal to noise.
Fixed Pattern Noise (FPN) - A faint
hexagonal (honeycomb) pattern throughout the image area that most often
occurs under high-light conditions. This pattern is inherent in the
structure of the microchannel plate and can be seen in virtually all
Gen II and Gen III systems if the light level is high enough. Focus Range - The range within which an optical device can be adjusted or focused on a target.
Gain - Also called brightness gain or
luminance gain. This is the number of times a night vision device
amplifies light input. It is usually measured as tube gain and system
gain. In any night vision system, the tube gain is reduced by the
system's lenses and is affected by the quality of the optics or any
filters; therefore, system gain is a more important measurement to the
user. Gallium Arsenide (GaAs) - The
semiconductor material used in manufacturing of the Gen III
photocathode. GaAs photocathodes have a very high photosensitivity in
the spectral region of about 450 to 950 nanometers (visible and
near-infrared region). Generations of Image Intensifiers -
Image intensifier tubes are classed by generation (GEN) numbers
assigned by the US Army's Night Vision Laboratory. Each GEN denotes a
new technology or manufacturing process that improved since the
previous generation.
- GEN I - Vietnam Era technology. GEN I has the most distortion and
the least tube life. GEN I image tubes are most commonly found in
imported night vision devices.
- GEN II - 1960s development of microchannel plate (MCP)
amplifier component provided the needed breakthrough for high gain and
small size.
- GEN II+ - 1970s development increased image tube bias
voltage to improve gain. Additionally, a glass faceplate was added to
improve resolution.
- GEN III - 1980s development provided substantially improved
gain and a bandwidth that reached farther into the near IR region.
Improvements in the microchannel plate (MCP) and a gallium-arsenide
photocathode greatly enhanced tube life and performance.
- GEN III+ - Generally referred to as GEN III Enhanced, 1990's
improvements in the photocathode and MCP resulted in increased gain and
resolution. Not a generally accepted term, but used herein for brevity.
Highlight Shutoff - An image
intensifier protection feature incorporating a sensor, microprocessor
and circuit breaker. This feature will turn the system off during
periods of extreme bright light conditions. Honeycomb - See Fixed-Pattern Noise.
Image Disparity - This
condition may exist when there is a difference in brightness between
the two image intensifier assemblies within the same binocular.
Image Distortion - This
problem is more easily detected in high- light conditions. Image
distortion is evidenced by vertical objects, such as trees or poles
appearing to wave or bend when the user moves his head vertically or
horizontally when looking through the goggles. Ground surfaces in the
direction of hover may appear to swell or sink. Distortion does not
change during life of an image intensifier. Limits on allowable
distortion are an important part of performance specifications since
excess distortion can interfere with viewing the image and thus with
the operator’s ability to perform necessary flight maneuvers.
Image Intensifier Tube (IIT - I2) -
A high-vacuum electro-optical device that collects photons and
amplifies these as electrons. This amplification of photons allows the
viewer to see more light than without a night vision device. Being an
(tube) which detects & amplifies light to produce a visual image.
Interpupillary Adjustment - The
distance between the user's eyes (pupils) and the adjustment of
binocular optics to adjust for differences in individuals. Improperly
adjusted binoculars will display a scene that appears egg-shaped or as
a reclining figure-8.
Interpupillary Distance -
(IPD). Interpupillary distance is the distance between the centers of
the pupils of the eyes when the eyes are parallel. Adjustment
provisions for
variable IPD should be a feature of the NVG to allow the full
image to be seen by the NVG user. The recommended range of adjustment
should be at least 57-70mm to
accommodate an estimated 90% of the potential user population. If
no adjustment is provided, then the exit pupil must be large enough for
the user to get a full field of view.
IR Illuminator- Many night vision
devices incorporate an infrared (IR) diode that emits invisible light.
IR light cannot be seen by the unaided eye; therefore, a night vision
device is necessary to see this light.
IR Laser - High-power devices providing
long-range illumination capability. Ranges of several thousand meters
are common. Most are not eye-safe and are restricted in use. Each IR
laser should be marked with a warning label like the one shown here.
Consult FDA CFR Title 21 for specific details and restrictions.
Light Interface Filter (LIF).
An optical filter that protects the NVG device and its user from some
laser hazards by the LIF. The LIF’s, if installed, are mounted on an
adapter attached to the end of the objective lens. Light-Secure Eyeguard (Shuttered Eyeguard) -
A rubber eyecup fitted to an eyepiece of a night vision device that
incorporates a shutter that closes when not depressed by the user's
eye. Prevents face glow that can compromise the user's location Line Pairs per Millimetre (lp/mm) -
Units used to measure image intensifier resolution. Usually determined
from a 1951 Air Force Resolving Power test target. The target is a
series of different sized patterns composed of three horizontal and
three vertical lines. The lines and spacing between lines in each of
the different patterns differ in width; the narrower the width, the
greater the resolution is needed to distinguish the lines in a given
pattern. Human test subjects must be able to clearly distinguish all
the horizontal and vertical lines of a particular pattern in order for
an image intensifier to achieve the resolution represented by that
pattern. Low Voltage Indicator - A warning device in a night vision system that signals low power.
Lumen - The unit denoting the photons (light) perceivable by the human eye in one second.
Lux - A unit measurement of illumination.
The illuminance produced on a surface that is on meter square, from a
uniform point source of one-candela intensity, or one lumen per
square meter.
Magnification - The
magnifying power of the lens. Four power (4X) indicates that the image
will appear four times larger than if viewed with a 1X lens. Microamps per Lumen (A/lm) - The measure of electrical current (A) produced by a photocathode when it is exposed to a measured amount of light (lumens).
Microchannel Plate (MCP) - A
metal coated glass disk that multiplies the electrons produced by the
photocathode. An MCP is found only in GEN II and GEN III systems.
These devices normally have anywhere from 2 to 6 million holes
(or channels) in them. Electrons entering the channel strike a wall and
knock off additional electrons which in turn knock off more electrons,
producing a cascading effect. MCPs eliminate the distortion
characteristic of GEN I systems. The number of holes in an MCP is a
major
factor in determining resolution.
MILSPEC - Term for Military
Specification. It is the minimum acceptable requirements for products
procured by the US Department of Defence. Use of the term MILSPEC
indicates that the product meets applicable military specifications Monocular A singlechannel op-tical device. The American Eagle in this catalogue is an example of a monocular
NATO-STANAG- Term for the North Atlantic Treaty Organization STANdard AGreement. This can be described as an international MILSPEC.
Near-Infrared - The shortest
wavelengths of the infrared region, normally 750 to 2,500 nanometers
(nm). GEN II operates from around 440 to 950 nanometers.
Objective Lens - The
objective lens assembly collects the available light energy and focuses
it on the photocathode (front end of the image intensifier tube). It is
housed in
an assembly that is used for distance focusing. A coating is
placed on the inside portion of the lens that filters out specific
wavelengths, thus allowing the use of properly
modified interior lighting.
Output Brightness Variation
- This condition is evidenced by areas of varying brightness in or
across the image area. The lower contrasts do not exhibit distinct
lines of
demarcation nor do they degrade image quality. This condition should not be confused with shading.
Phosphor Screen -The
phosphor screen converts electrons into photons. A very thin layer of
phosphor is applied to the output fiber optic system, and emits light
when struck by electrons. See also Photocathode.
Photocathode - The input surface
of an image intensifier that absorbs light energy and in turn releases
electrical energy in the form of an electron image. The type of
material used is a distinguishing characteristic of the generations of
image intensifiers. Photoresponse (PR) - See Photosensitivity.
Photosensitivity (Photoresponse)
- Also called photocathode sensitivity or photoresponse. The ability of
the photocathode material to produce an electrical response when
subjected to light waves (photons). The higher the value, the better
the ability to produce a visible image under darker conditions. Usually
measured in microamps of current per lumen (A/lm). Resolution - The ability of an
image intensifier or night vision system to distinguish between objects
close together. Image intensifier resolution is measured in line pairs
per millimetre (lp/mm) while system resolution is measured in cycles
per miliradian. For any particular night vision system, the image
intensifier resolution will remain constant while the system resolution
can be affected by altering the objective or eyepiece optics by adding
magnification or relay lenses. Often the resolution in the same night
vision device is very different when measured at the centre of the
image and at the periphery of the image. This is especially important
for devices selected for photograph or video where the entire image
resolution is important. Measured in line pairs per millimetre (lp/mm).
Reticle (Reticle Pattern) - An adjustable aiming point or pattern (i.e. crosshair) located within an optical weapon sight
Signal-to-Noise Ratio (SNR) - A
measure of the light signal reaching the eye divided by the perceived
noise as seen by the eye. A tube's SNR determines the
low-light-resolution of the image tube; therefore, the higher the SNR,
the better the ability of the tube to resolve objects with good
contrast under low-light conditions. Because SNR is directly related to
the photocathode's sensitivity and also accounts for phosphor
efficiency and MCP operating voltage, it is the best single indicator
of an image intensifier's performance Scintillation - Also known
as electronic noise. A faint, random, sparkling effect throughout the
image area. Scintillation is a normal characteristic of microchannel
plate image intensifiers and is more pronounced under low-light-level
conditions Screen- The image tube output that
produces the viewable image. Phosphor (P) is used on the inside surface
of the screen to produce the glow, thus producing the picture.
Different phosphors are used in image intensifier tubes, depending on
manufacturer and tube generation. P-20 phosphor is used in the systems
offered in this catalogue System Gain- Equal to tube gain minus losses induced by system components such as lenses, beam splitters and filters
Weaver Mounting System - A US weapon
mounting system used for attaching sighting devices to weapons. A
Weaver Rail is a weapon-unique notched metal rail designed to receive a
mating throw-lever or Weaver Squeezer attached to the sighting device Zeroing - A method of
boresighting an aiming device to a weapon and adjusting to compensate
for projectile characteristics at known distances.
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