HOW TO BUY NIGHT VISION
HOW
NIGHT VISION WORKS
NIGHT
VISION TERMINOLOGY
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 protect 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.
Auto-Gated Power Supply
When the power supply is "auto-gated," it means
the system is turning itself on and off at a
very rapid rate. This, combined with a thin
film attached to the microchannel plate (an
ion barrier) reduces blooming. While
"blooming" can be noticeably less on systems
with a thin film layer, systems with thicker
film layers can be perfectly acceptable
depending on the end user's application.
Deciding which night vision goggle is better
should not be based solely on blooming.
Black Spots
These are common blemishes in the image
intensifier of the NVD or can be dirt or
debris between the lenses of the NVG. Black
spots that are in the image intensifier do
not affect the performance or reliability of
a night vision device and are inherent in
the manufacturing processes. Every night
vision image intensifier tube is different.
They are like diamonds. See image to the
right. See also - How to Buy Night Vision
Equipment
Bright Spots
These can be defects in the image area
produced by the NVG. This condition is
caused by a flaw in the film on the
microchannel plate. A bright spot is a
small, non-uniform, bright area that may
flicker or appear constant. Bright spots
usually go away when the light is blocked
out and are cosmetic blemishes that are
signal induced.
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.
Blooming
Loss of the entire night vision image, parts
of it, or small parts of it, due to
intensifer tube overloading by a bright
light source. Also, known as a "halo"
effect, when the viewer sees a "halo" effect
around visible light sources. When such a
bright light source comes into the night
vision device's view, the entire night
vision scene, or parts of it, become much
brighter, "whiting out" objects within the
field of view. Blooming is common in
Generation 0 and 1 devices. The lights in
the image to the right would be considered
to be "blooming".
Bright-Source Protection (BSP) -
High-Light Cut-Off
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, it also has the
effect of lowering resolution when
functioning.
Boresighting
The alignment of a weapon aiming device to the
bore of the weapon. See also Zeroing.
C-Mount br /> 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.
COMSPEC (Commercial Specification)
A term used to describe image tube quality,
testing and inspection done by the original
equipment manufacturer (OEM).
Chicken Wire
An irregular pattern of dark thin lines in
the field of view either throughout the
image area or in parts of the image area.
Under the worst-case condition, these lines
wwill form hexagonal or square wave-shape
lines.
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. Usually, adjustments to an optical
eyepiece accommodate for differences in
individual eyesight. Most ITT systems
provide a +2 to -6 diopter range.
Distortion
There are two types of distortion found in
night vision systems. One type is caused by
the design of the optics, or image
intensifier tube, and is classical optical
distortion. The other type is associated
with manufacturing flaws in the fiber optics
used in the image intensifier tube.
- Classical Optical Distortion:
Classical optical distortion occurs when the
design of the opticsor image intensifier
tube causes straight lines at the edge of
the field of view to curve inward or
outward. This curving of straight lines at
teh edge will cause a square grid pattern to
start to look like a pincushion or barrel.
This distortion is the same for all systems
with the same model number. Goodoptical
design normally makes this distortion so low
that the typical user will not see the
curving of the lines.
- Fiber Optics Manufacturing
Distortions: Two types of fiber
optics distortions are most significant to
night vision devices: S-distortion and shear
distortion:
- S-Distortion:
Results from the twisting operation in
manufacturing fiber-optic inverters.
Usually S-distortion is very small and
is difficult to detect with the unaided
eye.
- Shear Distortion:
Can occur in any image tube that use
fiber-optic bundles for the phospor
screen. It appears as a cleavage or
dislocation in a straight line viewed in
the image area, as though the line were
"sheared".
Equivalent Background Illumination
(EBI)
This is the amount of light you see through
a night vision device when an image tube is
turned on but no light is on the
photocathode. EBI is affected by
temperature; the warmer the night vision
device, the brighter the background
illumination. EBI is measured in lumens per
square centimeter (lm/cm2). The lower the
value the better. The EBI level determines
the lowest light level at which an image can
be detected. Below this light level, objects
will be masked by the EBI.
Edge Glow
There is a defect in the image area of the
NVG. Edge glowis a bright area ( sometimes
sparkling) in the outer portion of the
viewing area.
Emission Point
A steady or fluctuating pinpoint of bright
light in the image area that does not go
away when all light is blocked from the
objective lens. The position of an emission
point within the field of view will not
move. If an emission point disappears or is
only faintly visible when viewing under
brighter nighttime conditions, it is not
indicative of a problem. If the emission
point remains bright under all lighting
conditions, the system needs to be repaired.
Do not confuse an emission point with a
point of light source in the scene being
viewed.
Eye Relief
The distance a person's eyes must be from
the last element of an eyepiece in order to
achieve the optimal image area.
Field-of-View
The diameter of the imaged area when viewed
through an optic
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 2 and Gen 3 systems if the
light level is high enough.
Footlambert(fL)
A unit of brightness equal to one footcandle
at a distance of one foot.
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. Tube gain is measured as the light
output (in fL) divided by the light input
(in fc). This figure is usually expressed in
values of tens of thousands. If tube gain is
pushed too high, the tube will be "noiser"
and the signal-to-noise ration many go down.
U.S. military Gen 3 image tubes operate at
gains of between 20,000 and 45,000. On the
other hand, system gain is measured as teh
light output (fL) divided by the light input
(also fL) and is what the user actually
sees. System gain is usually seen in the
thousands. U.S. military systems operate at
2,000 to 3,000. 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 the Gen 3 photocathode. GaAs
photocathodes have a very high
photosensitivity in the spectral region of
about 450 to 950 nanometers (visible and
near-infrared region).
Generations
Two technologies are referenced as night
vision; image intensification and thermal
imaging (see definitions). Because of cost
and the fact that image intensifier scenes
are easier to interpret than thermal
(thermal images show targets as black or
white - depending upon temperature - making
it more difficult to recognize objects), the
most widely used night vision aid in law
enforcement is image intensification (l²)
equipment. Developments in the l² technology
are categorized in terms of "generations".
To date, there have been four generations of
l² devices, identified as Gen 0, Gen 1, Gen
2, and Gen 3. Developmental laboratory work
is on-going, and the U.S. military may
designate the resulting as Gen 4. However,
no definition for Gen 4 presently exists.
Generation 0
The first night vision aids (also called
Generation Zero or Gen 0) were sniper scopes
that came into use during World War II and
the Korean conflict. These were not true
image intensifiers, but rather image
converters, which required a source of
invisible infrared (IR) light mounted on or
near the device to illuminate the target
area.
Generation 1
The "starlight scopes" developed during
the early 1960's for use in Vietnam were the
first Generation (Gen 1) of image
intensifier devices. In Gen 1 night vision
units, three image intensifiers were
connected in a series, making the units
longer and heavier than future night vision
units would be. Gen 1 equipment produced an
image that was clear in the center of the
field of view but suffered from large
optical distortion around the periphery. Gen
1 equipment was also subject to "blooming".
Most low-cost imported night vision units
use Gen 1 technology, though often under the
guise of a higher "generation".
Generation 2
The development of the microchannel plate,
or MCP, in the late 1960s brought on the
second generation (Gen 2) in l² night
vision. The MCP accelerated and multiplied
electrons which provided the gain previously
supplied by coupling three image
intensifiers together (Gen 1). The
introduction of the MCP significantly
reduced size and weight for image
intensifier tubes, enabling design of
smaller night vision goggles and hand-held
devices. The MCP also provided much more
robust operation when bright lights entered
the field of view. The Gen 2 tubes used the
same tri-alkali photocathode as the Gen 1
devices. This generation was implemented to
reflect the change in how the light was
amplified (MCP versus three-stage coupling).
Generation 3
Third-generation (Gen 3) image
intensifiers were developed in the mid-1970s
and became available during the early 1980s.
Gen 3 introduced two major technological
improvements: the gallium arsenide (GaAs)
photocathode and the ion barrier coating to
the microchannel plate. The GaAs
photocathode increases the tube's
sensitivity to light from the near-infrared
range of the spectrum, enables it to
function at greater detection distances, and
improves system performance under low-light
conditions. Application of a metal-oxide ion
barrier to the MCP increases the life of the
image tube. The operational life of Gen 3
tubes is in excess of 10,000 hours, compared
to that of Gen 2 tubes which is about 2,000
to 4,000 hours. This generation was
implemented to reflect the change in the
photocathode (tri-alkali replaced with
GaAs).
Generation 4
Generation 4 or Gated Filmless technology
was created in 1998, but without the
reliability required for military delivery.
By removing the ion barrier film and
"gating" the system power supply, Gen 4
technology demonstrated substantial
increases in target detection range and
resolution. In the process of Generation 4
development, however, it was discovered by
ITT, that the same performance results could
be achieved using a Generation 3 tube, but
with a thinner ion barrier film and an
auto-gated power supply, without sacrificing
reliability and life-span of the intensifier
tube.
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.
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
The distance between the user's pupils
(eyeball centres). The 95th percentile of US
military personnel falls within the 55 to
72mm range of IPD.
IR Illuminator
Many night vision devices incorporate a
built-in infrared (IR) diode that emits
invisible light or the illuminator can be
mounted on to it as a separate component. IR
light cannot be seen by the unaided eye;
therefore, a night vision device is
necessary to see this light. IR Illuminators
provide supplemental infrared illumination
of an appropriate wavelength, typically in a
range of wavelengths (e.g. 730nm, 830nm,
920nm), and eliminate the variability of
available ambient light, but also allow the
observer to illuminate only specific areas
of interest while eliminating shadows and
enhancing image contrast.
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.
I2 (Image Intensification)
Collects and intensifies the available light
in the visible and near-infrared spectrum.
Offers a clear, distinguishable image under
low-light conditions.
IR (Infrared) Area
outside the visible spectrum that cannot be
seen by the human eye (between 700
nanometers and 1 millimeter). The visible
spectrum is between 400 and 700 nanometers.
Ip/mm (Line Pairs per
Millimeter) Units used to measure image
intensifier resolution. Usually determined
from a 1951 U.S. Air Force Resolving Power
Test Target. The target is a series of
different-sized patterns composed of three
horizontal and three vertical lines. A user
must be able to distinguish all the
horizontal and vertical lines and the spaces
between them. Typically, the higher the line
pair, the better the image resolution.
Generation 3 tubes generally have a range of
64 - 72 lp/mm, although line pair
measurement does not indicate the generation
of the tube. Some Generation 2+ tubes
measure 28-38 lp/mm, while a Generation 1+
tube may have measure at 40 lp/mm.
Lumen: Denotes the
photons perceptible by the human eye in one
second.
Monocular
A single channel 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
mA/W (Milliamps per Watt):
The measure of electrical current (mA)
producted by a photocathode when exposed to
a specified wavelength of light at a given
radiant power (watt).
MCP (Microchannel Plate):
A metal-coated glass disk that mulitplies
the electrons produced by the photocathode.
An MCP is found only in Gen 2 or Gen 3
systems. MCPs eliminate the distortion
characteristic of Gen 0 and Gen 1 systems.
The number of holes (channels) in an MCP is
a major factor in determining resolution.
ITT Industries' MCPs have 10.6 million holes
or channels compared to the previous
standard of 3.14 million.
Near-Infrared:
The shortest wavelengths of the infrared
region, nominally 750 to 2,500 nanometers.
Also see IR (infrared).
Photocathode:
The input surface of an image intensifier
tube that absorbs light energy (photons) and
in turn releases electrical energy
(electrons) in the form of an image. The
type of material used is a distinguishing
characteristic of the different generations.
Photocathode Sensitivity:
Photocathode sensitivity is a measure of
how well the image intensifier tube converts
light into an electronic signal so it can be
amplified. The measureing units of
photocathode sensitivity are
micro-amps/lumen (µA/lm) or microamperes per
lumen. This criterion specifies the number
of electrons released by the Photocathode
(PC). PC response is always measured in
isolation with no amplification stage or ion
barrier (film). Therefore, tube data sheets
(which always carry this “raw” figure) do
not reflect the fact that over 50% of those
electrons are lost in the ion barrier. While
for most latest 3rd generation image
intensifiers the photoresponse is in the
1800 µA/lm (2000 µA/lm for the latest Omni
VI Pinnacle tubes), the actual number is
more like 900 µA/lm. The 4th generation DOES
NOT use ion barrier and while its “raw”
photoresponse is the same as 3rd, the actual
number is actually 100% higher.
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
Stereoscopic Night Vision
When two views or photographs are taken
through one device. One view/photograph
represents the left eye, and the other the
right eye. When the two photographs are
viewed in a stereoscopic apparatus, they
combine to create a single image with depth
and relief. Sometimes this gives two
perspectives. However, it is ususally not an
issue because the object of focus is far
enough away for the perspectives to blend
into one.
System Gain
Equal to tube gain minus losses
induced by system components such as lenses,
beam splitters and filters.
Variable Gain Control
Allows the user to manually adjust the
gain control ( basically like a dim control
) in varying light conditions. This feature
sets the
PVS-14 apart from other popular
monoculars that do not offer this feature.
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.
