www.nivitech.com
is hosted by
www.nivitech.com
is hosted by
Night
Vision Devices (NVD)
1.
Application / User
"Seeing
without being seen" - In such a way the slogan
reads itself primarily with special law enforcement units and military commandos.
For decades mankind tried to achieve this goal by means of modern technology.
The better this succeeded, the more complex became the technology of night vision
devices. For the actual initiator of this type
of equipment - the military - the costs of development
and production only played a subordinated role. It is to be noted in the following
that the night vision technology represents high-level technology and the devices
were designed primarily for military purposes.
Therefore application within the scientific or civilian range is sometimes
limited or even forbidden by law. However acquisition is always connected
with enormous costs.
Nevertheless
times are changing and the commercial market
for these devices grows from year to year. Keyword 'conversion'
: Since end of the 'Cold War' there are more ex-military low-light imagers
from eastern production on the commercial market than ever. If in the past
night vision were exclusively reserved for the military, then meanwhile simple
or superseded night vision devices are also available for civilian users e.g.
security companies, hunters, marine crew members or nature observers. But
of course it is still the military aviation, special
forces, secret services and authorities with
law enforcement tasks who mainly takes benefit from modern night vision
devices. Thereby the spectrum of utilized NVDs reaches from night vision monoculars
over night vision weaponsights up to night vision goggles (e.g. right: PVS-18
as mono-goggle).
Finally
our intention with this service is to establish an online
guide to night vision technology and provide some information to interested
individuals. Particularly for those, who are seriously concerning the acquisition
of a NVD, it should be mentioned that unfortunately some 'businessmen' on
the commercial market want to take advantage in selling overpriced devices
to uninformed customers in the prospect of a huge profit. The following pages
may help to prevent getting a white elephant.
2.
History / Future Systems
The
need for protection against an unpleasant surprise and the desire to be able
to notice an incident taking place through the cover of darkness has been
occupying the minds of ingenious people throughout history. Since 300 B.C.
the Romans had been using cackling geese in order not to be surprised at night
by an attack of the Gallians. However the actual history of opto-electronic
night vision devices (NVDs) began with the development of the first image converter
tube in the 30's of the last century. Since then every step in technology
is associated with the notion 'Generation'.
In World War 2 some few special forces already used first night vision devices
which utilized infrared converter tubes (Zero
Generation). But these devices (so-called 'active
night-vision devices') were quite unmanageable. They had
to be used with a powerful additional infra-red light source and were easily
detectable by other night vision devices.
More handy devices, which could also be used weapon-mounted as sniperscopes,
were sporadically deployed by the U.S. armed forces at the time of the Vietnam
war. In general the performance of these 1st
Generation devices was limited and practical restrictions were considered
to be too large to provide an effective advantage over the so-called 'night-blind'
opponent in combat or reconnaissance operations. Especially in areas with
particularly little low light (e.g. leafy forest, jungle) Gen1-NVDs again
required additional, position-revealing infra-red light.
In
the seventies the development of the micro
channel plate (MCP) meant a big increase in performance.
With the upcomming 2nd Generation of
night vision devices much higher gain than possible before with (the usually
single-stage) image intensifier tubes was achieved. However this advantage
first came along with an even poorer image resolution and a bad signal
to noise ratio (S/N). The basic principle of the 'proximity focusing'
made it possible for the first time to design small and lightweight devices
(especially important for the use as night vision goggles). With exception
of digital CCD night vision devices so far all modern NVDs functions still
after the basic principle of the proximity focusing and electron-multiplying
over MCP.
Due
to further success in research the 3rd
Generation characterized by the new Gallium-Arsenide-coating
(GaAs) of the light-sensitive photocathode was introduced by the American
night vision-industry at the end of the 80's. Up to now this generation represents
state of the art night vision technology in different performance levels for
applictions within battlefield environment and aviation. Today Gen3 image
intensifier tubes (IITs) are widely used by many western armed forces. First
deployment of this tubes in large quantities took place on allied side in
the Gulf War at the beginning of the 90's. In the process of the fighting
(at the beginning usually nocturnal) the technological projection in the opto-electronic
range significantly showed up. Under the cover of darkness the superiority
was distinctive and the own losses could be kept small. The European night vision-industry
also developed some successors to Gen2 called 'XD-4', 'XH-72' or 'XR-5' (comparable
to U.S. Gen3). On the basis of Gen2+
image intensifier tubes (other photocathode coatings, improved
control electronics, etc.) modern MCP-technology from Europe is very competetive
under the line, so that meanwhile it is difficult to determine who is ahead
in terms of performance and potential.
Also
thermal imaging devices become increasingly smaller
and more handier. They are capable to display images under no light conditions
rendered through nebulas and some optical obstacles (e.g. vegetation). In
the future and today's development of imaging devices this represents rather
an addition than a replacement for conventional NVDs. Especially since thermal
imaging devices show the environment only in temperature differences instead
of a 'common' view from a NVD they may not be as well suitable for orientation
in the darkness than in finding individuals. Current efforts in night vision
development incorporate higher image resolution, lower signal to noise ratio,
increased contrast and a wider field of view (FOV)
than usually 40° at simple power. In some cases night vision goggles with
50° FOV are already issued while other devices with up to four IITs and
100° FOV are still in development.
In
the long run it will probably come down to a combination
of both image-providing devices. The gathered visual information will
first be processed by a small computer (selective magnification and marking,
data-input interface, HUD, video-monitoring via radio, etc.) before in a second
step an actual image is generated from both pictures. In Future it is also
conceivable that the common image intensifier tube will become less important
in favor of other electronic signal proccessing devices (CCD-cameras, e.g.
'Land Warrior Program'). The questions is when sensitivity, reaction time
and image noise from uncooled CCDs are ahead of the specifications of classical
intensifier tubes. However until today an official 4th
Generation of IITs is not yet designated although the industry
is marketing 'filmless tubes' and 'autogated tubes' as Gen4.
3.
Structure / Working Principle NVD
Similar
to the term 'photomultiplier' the operational basics of an image intensifier
tube makes attentive to the physical working principle, the 'multiplication'
or 'amplification' of the existing 'low
light'.
Beside the small range of electromagnetic radiation visible for the human
eye (between 380 - 780 Nm wavelength) there is a lot of other (invisible)
electromagnetic radiation of higher and lower wavelength existing. Every radio-station
and also each warm object emits electromagnetic radiation in certain wavelengths.
While at night there is very little radiation 'detectable' for the human eye,
a varying quantity of infrared radiation (IR-radiation) is present in the
EM-spectrum starting from 700 Nm.
The
night vision device functions like 'correction eyeglasses', by catching the
radiation of this wavelength, amplifying / converting
it electronically and delivering it as light within the visible spectral
range.
Therefore one also speaks of 'opto-electronic devices',
which are either 'active' (i.e. use
an IR-light source for illuminating
the environment) or only use 'passively' the low light.
A
night vision device consists of three parts (optical - electronical - optical):
-
Objective Lens, collects and focuses
the low light - particularly permeable for IR-radiation
-
Image Intensifier Tube (IIT), converts
photons in electrons (photocathode), multiplies these and converts it back
again in light (phosphor screen)
-
Eyepiece Lens, magnifies the relatively
small image of the image intensifier tube
Zero
to 1st Generation: If the reflected
IR-radiation by the observed object meets the objective of the NVD, it is
bundled and focused on the photocathode (image
converting). Theoretically every hitting photon (light particle) drives out
one electron on the back of the photo-sensitive, chemical coated photocathode
- so called 'photoelectric effect' (actually using even the best IITs this
happens only in one out of five cases!). Due to the almost identical
distribution of projected IR-image and electron-output the image remains
as a 'stamp of electrons' within the tube. In
a high voltage circuit (15-36 kV) the electrons are accelerated by an anode
cone (the actual amplification) to meet on the phosphor
screen of the image intensifier tube. Here the electron distribution
is transformed back again into visible light by a special coating of the screen.
Now the usually greenish image on the back of the tube (for the human eye
different shading-levels in green are easier to distinguish) must only be
magnified. Since these IITs are not free from distortion and their amplification
is achieved by an electron acceleration only, today tubes of Zero and 1st
Generation are only produced for special applications (e.g. scientifical research).
Also one fact should not remain unmentioned that especially older
tubes of these first generations can produce a certain portion of
x-ray radiation due to the high accelerating voltage. Similar to a
television tube the manufacturer of a night vision device (ex-military devices
from Russia?) should take measures to avoid unnecessary
radiation of x-rays from the tube (e.g. lead additives in the eyepiece
glass). Particularly since the tube of a night vision device is usually very
close at the head / eye when using. Of course the personal radiation dose
depends on the overall exposure time (total working hours).
The
working principle of today's manufactured 2nd
and 3rd Generation is very similar
in general, but extended by using a micro
channel plate (MCP). The image gain is less a result of electron-acceleration
(approx. 'only' 5-6 kV) than more a matter of electron-multiplication within
the MCP. The two elements of an image intensifier tube, the more sensitive
photocathode (PC) and the greenish-yellowish phosphor screen (PS), are still
utilized (and continously enhanced in performance) but instead of the anode
cone the mentioned micro channel plate is added. The MCP is an extremely
thin glass plate with approx. 2-6 millions of smallest 'holes', which
are arranged in a slightly sloped position to the optical axis. If electrons
penetrate into these so-called micro-channels
they hit the special coated walls (because of the approx. 8° inclination
of the micro channels) and drive some further electrons out of the coating.
These meet for their part again the tube inner wall and extract
cascade-like electrons. In this way only one electron triggers some
hundred particles to be released at the back side of the MCP (current flows).
This technology makes also more compact systems possible, since the 'electron
image' is focused nearly distortion-free very
close behind the photocathode on the thin MCP (parallel projection). Due to
this structure these tubes are also called 'proximity
image intensifier'. Naturally the resolving power of such systems depends
on the number of micro channels. Their efficiency is substantially larger
than those of pure 'accelerator systems'.
Older German
Army 'infra-red sniperscope for small arms', mounted on a H&K G3 assault
rifle (Gen 0)
European
night vision goggles: GN-2 and TN-21
US-ANVIS
goggles - used for night vision flights
Modern
night vision goggles (here PVS-21) allow to display mission data input via
HUD - especially of importance in military aviation
With
beamcombiners (PVS-21)
the unaided FOV is almost not limited. Relatively comfortable to wear due
to small dimensions & little weight.
3rd
Generation Low Profile Night Vision Goggle AN/AVS-502 (AN/PVS-21) by Northrop
Grumman (LEOS) - The night vision enhanced image is displayed over the individuals
view like a Head-Up-Display. The system gain adapts automatically to twilight
conditions.
Objective lenses appears blue on the picture, located above the beamcombiners
and the large cylinders (left & right) are the IIT-housings with built-in
IR-sources.
