How Night Vision Works - Terminology
Source: Night Vision Supply.com
How Night Vision Works
Night vision takes the small amount of light that's in
the surrounding area (such as moonlight or starlight), and converts
the light energy (scientists call it photons), into electrical energy
(electrons). These electrons pass through a thin disk (a microchannel
plate) that's about the size of a quarter and contains over 6 million
channels.
As the electrons go through the channels, the electrons
are multiplied thousands of times. These multiplied electrons then are
accelerated onto a phosphor screen which converts the electrons back
into photons and lets you see a bright nighttime view even under
extremely dark conditions.
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.
BlackSpots
These are cosmetic blemishes in the image intensifier or
can be dirt or debris between the lenses. Black spots that are in the
image intensifier do not affect the performance or reliability of a
night vision device and some number of varying size are inherent in
the manufacturing processes. Spots due to dirt or debris between the
lenses should be removed by careful cleaning if the system is designed
for interchanging optics.
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, it also has the effect of
lowering resolution when functioning.
Cycles per Milliradian (cy/mr)
Units used to measure system resolution. A milliradian
is the angle created by 1 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. Do not confuse
cy/mr with line pair per millimeter. For example, a system can have a
3X lens attached and increase the system resolution by a factor of 3,
yet the image intensifier's resolution (measured in lp/mm) has not
increased.
Diopter
The unit of measure used to define eye correction or the
refractive power of a lens. Usually adjustments to an optical eyepiece
accommodates for differences in individual eyesight. Many military
system provide a +2 to -6 diopter range.
Distortion
Three types of distortion are most significant to night
vision devices: geometric, "S", and sheer.
Geometric distortion is inherent in all Gen 0 and Gen I
image intensifiers and in some Gen II image intensifiers that use
electrostatic rather than fiber-optic inversion of the image.
Geometric distortion is eliminated in image tubes that use a
microchannel plate and fiber optics for image inversion; however, some
S-distortion can occur in these tubes.
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, Gen III image
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
fiber-optic bundles for the phosphor screen. It appears as a cleavage
or dislocation in a straight line viewed in the image area; as though
the line were "sheered."
Non-inverting image intensifiers that use microchannel
plates and clear glass for the optics are free of distortion. The
image intensifier ITT manufactures is distortion free.
Equivalent Background Illumination (EBI)
This is the amount of light 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 the night vision device,
the brighter the background illumination. EBI is measured in lumens
per square centimeter (Im/cm2) wherein the lower the value the better.
The EBI level determines the lowest light level at which you can
detect something and, below this light level, objects will be masked
by the EBI.
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 light source in the scene being
viewed.
Eye Relief
The distance your eyes must be from the last element of
an eyepiece in order to achieve the optimal image area.
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.
Footcandle (fc)
A unit of illuminance equal to one lumen per square
foot.
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 seen in values of tens of thousands. If tube gain is
pushed too high, the tube will be "noisier" and the signal-to-noise
ratio may go down. U.S. military Gen II and Gen III image tubes
operate at gains of between 20,000 and 37,000.
On the other hand, system gain is measured as the 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
III photocathode. GaAs photocathodes have a very high photosensitivity
in the spectral region of about 450 to 950 nanometers (visible and
near-infrared region).
Generation 0
GEN 0 night vision typically uses an S-1 photocathode
with peak response in the blue-green region (with a photosensitivity
of 60 uA/lm), high-voltage electrostatic inversion, and high-voltage
electron acceleration to achieve gain. Consequently, Gen 0 tubes are
characterized by the presence of geometric distortion and the need for
active infrared illumination.
Note: GEN 0 systems will usually fail if exposed to
bright light sources.
Generation 1
GEN-1 night vision typically uses an S-20 photocathode
(with photosensitivity of 180-200 uA/lm), high-voltage electrostatic
inversion, and high-voltage electron acceleration to achieve gain.
Because of higher photosensitivity, Gen I was the first truly passive
image intensifier.
Gen I is characterized by geometric distortion, poor
performance at low light levels, and blooming when exposed to bright
light sources.
Note: GEN I systems will usually fail if exposed to
bright light sources.
Generation 2
GEN-2 night vision is typically uses an S-25 (extended
red) photocathode (with photosensitivity of 240+ uA/lm and a
microchannel plate to achieve gain and either electrostatic or
fiber-optic inversion. Gen-2 tubes provide satisfactory performance at
low light levels and exhibit low distortion.
Generation 3
GEN-3 night vision uses gallium-arsenide for the
photocathode and a microchannel plate for gain. The microchannel plate
is also coated with an ion barrier film to increase tube life of
10,000+ hours. Produces more than 800 uA/lm in the 450 to 950
nanometer (near-infrared) region of the spectrum.
Gen-3 provides very good to excellent low-light-level
performance, long tube life.
Mil-spec quality tubes have no perceptible distortion.
Generation 4
GEN-4 is the latest development in image tubes and is
currently only available for military applications.
Line Pairs per Millimeter (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. You must be able to distinguish a
ll the horizontal and vertical lines and the spaces between them to
qualify for that pattern.
Lumen
The unit denoting the photons perceivable by the human
eye in one second.
Microamps per Lumen (uA/lm)
The measure of electrical current (uA) 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 a channel strike the
wall and knock off additional electrons which in turn knock off more
electrons producing a cascading effect. MCPs eliminate the distortion
characteristic of Gen 0 and Gen I systems. The number of holes in an
MCP is a major factor in determining resolution. ITT's new MCPs have
6.34 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. Gen III operates from around 450 to
950 nanometers.
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 different generations of image intensifiers.
Photoresponse (PR)
Also called photosensitivity or photocathode
sensitivity. The ability of the photocathode material to produce an
electrical response when subjected to light waves (photons). Usually
measured in microamps of current per lumen of light (uA/lm). The
higher the value, the better the ability to produce a visible image
under darker conditions.
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 millimeter
(lp/mm) while system resolution is measured in cycles per milliradian.
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 center of the
image and at the periphery of the image. This is especially important
for devices selected for photography or video where the entire image
resolution is important.
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 intensifiers performance.
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. Sometimes called "video noise." Not to be confused with
emission points.
Spectrum
The range of electromagnetic energy from cosmic rays to
extra-low frequency used in submersed submarine communication.
Electromagnetic frequency is measured in cycle per second and
wavelength in microns or nanometers. The ultraviolet region extends
from 100 to 400 nm with the near-ultraviolet nominally 300 to 400 nm.
The visible portion of the spectrum extends from 400 to 750nm. The
infrared region extends from 750 to 2xlO5 nm with the near-infrared
nominally 750 to 2,500 nm.
Night Vision Evaluation
There are four major characteristics that should be
evaluated when considering a night vision purchase: performance, human
factors, suitability to its application, and the overall cost of
ownership.
Performance
The very need for a night vision capability necessarily focuses on
performance as the most important factor in Night Vision Evaluation.
Can you see a clear image when it is dark and you cannot see the scene
or objects with your unaided eye?
Most night vision equipment available today will provide
an adequate image under higher night light conditions such as a full
moon. Evaluate the following parameters to determine how well a system
will perform when you need to see under truly dark conditions such as
starlight.
Photosensitivity
The ability of a night vision system to detect light
energy and convert it to an electron image is reflected in the image
intensifier's photosensitivity. Usually, the higher the value, the
better the ability to "see" under darker and darker conditions.
However, be aware that at night there is more light energy available
in the near-infrared region than in the visible region. Therefore, if
a device claims a high photosensitivity, make sure to find out where
in the spectrum this is measured. A high photosensitivity in the blue
or visible region may not perform as well as another system with a
lower overall photosensitivity, but a higher value in the
near-infrared region.
Signal-to-Noise Ratio (SNR)
This is probably the single most significant factor in
determining a system's ability to see when it gets dark. Be aware that
SNR can be computed many ways to get desired results. Be sure to find
out how SNR was computed. When measured according to U.S. mil specs,
the SNR takes into account the photosensitivity, as well as the
efficiency of the phosphor screen in reconverting the electron image
to visible light and the "noise" contribution of the microchannel
plate. Because the, SNR determines an image intensifier's
low-light-resolution, the higher the ratio, the clearer will be the
signal compared to the background noise, hence, the better the ability
to see under increasingly darker conditions.
Gain (System vs. Tube)
This tends to be a confusing parameter when evaluating
night vision devices. The most important gain measurement is the
system gain. Very high gain values for an image tube are not
especially significant - the U.S. military procures devices with the
tube gain ranging from 20,000 to 37,000. Look for the system gain.
U.S. military systems operate at 2,000 to 3,000. The higher the value
the better the ability of the device to amplify the light it detects.
A word of caution; gain is only part of the story. If a
system does not possess a good photosensitivity and SNR, a very high
gain value simply means that you will make a poor image brighter, not
better. Also, very high gain values could mean the tube is driven very
hard and the life of the tube will be reduced. The very best test is
field evaluation under very dark conditions.
Resolution
Usually this is measured as tube resolution (lp/mm) or
system resolution (cy/mr). The more significant measurement is system
resolution as this is what the viewer will actually experience and
takes into account the quality of the system's optics. If you are
evaluating systems with similar optical quality and filters, the tube
resolution is an important criteria. Resolution is often measured at
high and low-light conditions. Most systems produce an optimal
resolution at some point between very high light and very low light
conditions.
As long as resolution is measured the same way using the
same magnification and the same conditions (i.e., per U.S. mil specs)
the higher the value, the better the ability to present a sharp
picture. However, be aware that many devices will produce a sharp
image in the center of the viewing area, but less sharp as you look
toward the periphery. The lack of a sharp image, except at the center
of the viewing area, can be due to the presence of a Gen 0 image tube
or to the system's optics. Again, remember that many night vision
systems will produce adequate results under higher night-light
conditions, but perform poorly under darker conditions.
Human Factors
Here, such issues as weight, size, safe equipment, and
the ease of operation should be considered. Remember that the ease of
operation should be determined under dark conditions where the user
cannot see the device being used. What may appear to be an acceptable
level of operating ease under room lights may not be "user friendly"
at all when it is dark. Protracted use should also be considered when
evaluating weight. What may seem an acceptable weight when using a
device for a short time, may not be so when viewing for long periods
of time.
Additionally, consider such functions as the on/power
switch. Will you need to continually hold down the switch? - even
light pressure for one finger for a long time can produce fatigue. Do
you need to repeatedly press the switch to recharge the image tube? -
such devices usually produce an initially bright image which gradually
fades, reducing the ability to see and then shuts off unless you
repress the switch. This characteristic could cause you to lose an
image at a crucial moment.
Suitability to its Application
Within this category, characteristics such as field of
view (FOV), magnification, versatility, weather resistance, and image
distortion affect the ability of a night vision device to perform as
needed.
Magnification and FOV
Regarding magnification and FOV, consider the distance
you will need and the overall area you are observing or searching. For
most surveillance or search applications, the higher the magnification
or narrower the FOV, the greater the number of times you need to scan
an area to avoid missing important objects or events. Usually a 1:1
lens with a 400 FOV provides optimal performance.
For long range observation or weaponsight applications,
the amount of magnification needed will vary; however, be sure to
consider the other performance characteristics of the device; as the
magnification increases, FOV decreases and the F number increases, all
reducing the amount of light captured. Consequently, you will need an
image tube with excellent performance at very low-light levels and/or
high-performance lenses.
Another factor involves the versatility of a device if
it is used in situations that may require different magnification. How
easily and quickly can the magnification be changed? Is it necessary
to open the system to install the optics? In some cases, this may be
inescapable, and the susceptibility of internal components to damage
should be considered.
Distortion
Gen 0, Gen I, and 25-mm Gen II electrostatically
inverted image tubes produce a certain amount of geometric distortion
in the image. In Gen III and 18-mm Gen II systems, geometric
distortion is eliminated although it is possible to encounter some
perceptible "S" and sheer distortion. The degree of any distortion and
its interference with the application should be considered. When the
application involves photography, video work, or weaponsights, the
distortion and peripheral resolution are critical.
Weather Resistance
The ability of a night vision system to operate under
adverse environmental conditions is another important factor. Any
system built to U.S. mil specs for environmental factors will perform
suitably under almost any condition encountered. The major concern is
internal fogging that destroys the ability to see an image, hence, the
ability to resist humidity and moisture is vital.
In addition, when a night vision system is used on or
around rivers or bodies of water, floatability can be a determining
factor. ITT's Night Enforcer 150/160 monoculars and 250/260 binoculars
will float if dropped into water.
Overall Cost of Ownership
Evaluation factors that impact the actual cost of
acquiring a night vision capability are image tube life (referred to
as "reliability"), warranty coverage, repair availability, service
support, and overall workmanship as an indicator of quality. When
evaluating night vision equipment, the initial acquisition cost does
not equate to the cost of ownership. How often will you need a new
image tube? What is the likelihood for repairs? Are batteries
available? What about exposure to bright lights?
All image intensifiers will "wear out" over time due to
gases generated within the tube that migrate to the photocathode and
slowly kill it. Because of this, characteristics such as reliability,
a bright-source protection (BSP) circuit, and the presence or absence
of an ion-barrier film on the microchannel plate are important. U.S.
mil specs describe procedures for projecting reliability. You should
know what the reliability is for the tube you evaluate.
An important factor that can influence reliability is
the voltage used to produce gain. If an image tube is "driven" hard to
produce high gain, it will accelerate the production of gases and more
quickly kill the ability to convert light into electrons.
A final evaluation criteria is to determine whether or
not the night vision device incorporates automatic protection for the
image intensifier when it is exposed to high-light conditions or
bright-light sources. Image tubes manufactured by ITT have a BSP
circuit built into the image intensifier. This circuit automatically
reduces the voltage to the photocathode when the system is exposed to
bright light sources. The BSP feature protects the image tube and
enhances its life. If there is doubt, consult the warranty; does it
exclude exposure to high light or bright lights?
Note 1: Generation Classification:
Some night vision advertising has presented confusing
information listing Russian equipment as Gen I, Gen II, and Gen III,
when in fact, by worldwide classification it is Gen 0, Gen I, and Gen
II, respectively.
Note 2: Reconditioned Generation II:
While the prices of "reconditioned" Gen II systems may
be attractive, be aware that the hours of remaining life and
photosensitivity performance cannot be restored to Gen II tubes.
"Reconditioned" usually means the system has a new or repaired power
supply but the photosensitivity will be lower, the SNR will be lower,
and the remaining life will be less. Some reconditioned units may be
operating at below acceptable minimums and few companies possess the
necessary test equipment to evaluate the tube's level of performance.
The U.S. military specifications for Gen II require a
reliability of 2,000 hours of operating time (ITT's new Gen II image
intensifier has tested to well beyond the military specification).
This situation does not pertain to Gen III equipment. Due to the
presence of an ion-barrier film in Gen III devices, the
gallium-arsenide photocathode is protected from degradation and the
life and performance are extended many times longer than Gen II.
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