Night Vision
How Night Vision Works
In night vision systems, the objective lens collects light that
you cannot see with your naked eye and focuses it on the image
intensifier. Inside the image intensifier a photocathode absorbs
this light energy and converts it into electrons. When this highly
intensified electron image strikes the phosphor screen, it causes
the screen to emit light that you can see. Since the phosphor screen
emits this light in exactly the same pattern and degrees of intensity
as the light that is collected by the objective lens, the bright
night-time image you see in the eyepiece corresponds precisely to the
outside scene you are viewing.
Terminology
Automatic Brightness Control (ABC) - An electronic feature
that automatically reduces voltages to the micro channel plate (2nd
& 3rd Gen) of the intensifier tube 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 constant level.
Black Spots - 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 numbers of varying sizes
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 (usually 1st Gen) 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.
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 i.e. B-20) 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
micro channel 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 inserters (fiber optic twister - Russian tubes do not use
a fiber 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
fiber-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 micro channel
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 fiber optics twister.
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 centimeter (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.
Eye Relief - The distance your eyes must be from the last
element of the 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 micro channel
plate and can be seen in virtually all Gen II and Gen III systems if the
light level is high enough.
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 or system gain. Tube gain is measured
as the light output divided by the light input. 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.
On the other hand system gain is measured as light output divided by
the light input and is what the user actually sees. System gain is
usually seen in the thousands. In any night vision device, 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 photo cathodes have
a very high photosensitivity in the spectral region of about 450 to
950 nanometers (visible and near-infrared region).
Generation 0 - Typically uses an S-1 photocathode with peak
response in the blue-green region, electrostatic inversion, and electron
acceleration to achieve gain. Consequently, Gen 0 tubes are characterized
by the presence of geometric distortion and the need for active infrared
illuminator.
Generation I - Typically uses an S-20 photocathode (with
photosensitivity of 180-200mA/lm), electrostatic inversion and electron
acceleration to achieve gain. Because of higher photosensitivity, Gen
I was the first thoroughly passive image intensifier. Gen I is characterised
by geometric distortion, performance at low light levels, and blooming.
Generation II - Usually an S-20 (extended red) photocathode
(with photosensitivity of 240+mA/lm) and micro channel plate to achieve
gain. Can be found with either electrostatic or fiber-optic inversion.
Gen II tubes provide satisfaction, performance at low light levels and
exhibit low distortion.
Generation III - Uses gallium-arsenide for the photocathode
and a micro channel plate for gain. The microchannel plate is also
coated with an ion barrier film to increase tube life. Produces more
then 800mA/lm in the 450 to 950 nanometer (near-infrared) region of
the spectrum. Gen III provides very good to excellent low-light-level
performance, long tube life. Recent mil-spec quality tubes have no
perceptible distortion.
Micro Channel 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. MCP's 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.
Photocathode - The input surface to 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 in a distinguishing
characteristic of the different generations of image intensifiers.
Photosensitivity - Also called photocathode sensitivity.
The ability of the photocathode material to produce an electrical response
when subjected to light waves (photons). Usually measured in micro amps
of current per lumen of light. The higher the value the better the ability
to produce a visible image under darker conditions.
Resolution - the ability of the tube to distinguish between
objects. The higher the numerical value, the better the tube will
distinguish between objects. An image tube's resolution is measured
in line pairs per millimeter (lp/mm).
Infra-Red Illuminators - All scopes need some light to amplify.
This means that if you were in complete darkness you could not see.
Basically what an IRI does is throw out a beam of infra-red light that
is near invisible to the naked eye but your NVD can see it. Allowing
you to use your scope even in total darkness. The IRI works like a
flashlight and the distance you can see with it will be limited.
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