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classification of night vision devices in 'Generations' (Gen) explains the
respective development step of the used image intensifier
tubes in general. However there is no uniform standardisation or protection
of the term 'Generation'. So for example the '3rd Generation' is a term, which
is used mainly by US manufacturers (and almost considered as a brand name),
in order to mark tubes with a particularly sensitive Galium Arsenid coating
(GaAs) of the photocathode. While for obvious reasons Russian manufacturers
with prospect of the large, commercial US night vision market meanwhile designate
tubes with GaAs-coatings also as 3rd Generation, European NV-companies brought
tubes with other coatings, and with their own model designations, on the market.
On the basis of the 2+Generation technology European MCP tubes are being developed,
which takes up the race for the best image quality head to head with the US
competition (but usually comparable US tubes are more cheaper due to a larger
mass production). The complexity of the term 'Generation' caused by different
views on quality (and origin of the tube) is
still increased by substantial performance differences
within a generation. Some Quality tolerances seem to be larger with
eastern tubes of the same generation than with western products. In addition
to this also slightly different performance measuring methods must be taken
into account. For example the resolution of an image intensifier tube is measured
with US manufacturers somewhat more generously than in Europe. Since the end
of the 80's the 3rd Generation has been continously developed further in America.
Therefore a qualitative classification of Gen3 tubes are expressed by government
orders for the armed forces in so-called 'Omnibus' contracts
(OMNI V & VI is currently up-to-date). Whether the US industry succeeds
in marketing tube improvements - as for example the so-called 'Autogated
Filmless Tubes' - (like they do also in Europe) as a new (fourth) generation
depends not at least on the means of the Pentagon: Since years it is US politics
to purchase each new generation exclusively for the own armed forces.
Despite these definition problems and comparison difficulties in terms of
'night vision generations' essentially five generations can be technically
justified. The development step from Zero to the 1st Generation consists less
in the design than in the use of a more photo-sensitive multi-alkali coating
of the photocathode. A very large step in the night vision technology meant
the introduction of the micro
channel plate (MCP) starting from Gen2.
This new type of image intensifier design made the tube smaller apart from
better image quality (by the way: In many export regulations of western countries
there is the presence of a MCP mentioned as a criterion for night vision devices
to be export restricted). In a manner analogous to the step Gen0 to Gen1 is
the step Gen2 to Gen3: No radically new design, but again a performance increased
photo-sensitive coating (and a better power supply) justifies the term of
the 3rd Generation. Because sometimes a Gen2+ tube offers better image quality
than an early Gen3 tube the quality of the image intensifier can more exactly
be determined with given technical specifications for resolution,
luminous sensivity and signal to noise ratio (manufacturers attach data
sheets with every tube delivered). After all each tube is an unique example
in its specifications and still such a high investment should be tested under
the present environmental conditions when considered to be purchased.
following diagrams show the average infrared radiation in the night. It is
clearly to see that far beyond the 800 nm wavelength a large portion of the
IR radiation exists. In contrast to NVDs of 3rd Generation this particular
radiation is not at the disposal of 1st Generation night vision devices (despite
the fact that the luminous gain of the 3rd Generation has significantly increased
to the 1st Generation). Remarkably the oldest generation has the widest working
range, but due to the very low sensitivity of the photocathode tubes of this
generation do not benefit much from natural IR-light.
of this generation have so little low light amplification that as a rule more
strongly, additional IR-illuminators
must be used for observation. Therefore they are also called 'active
night vision devices'. Basically in contrast to the other generations
a transformation instead of an intensification of (IR-) light is achieved
(one speaks here rather of image converter tube than an image intensifier
tube). By the use of an IR-illuminator the user has mostly two crucial disadvantages:
On the one hand the observation duration is depending on the usually big and
heavy power source and on the other hand the user of such a strong illuminator
is visible to other distant NVD-users (no real covert operations possible).
The advantage of image converter tubes of the Zero Generation is a wide sensitivity
in the deep infrared range (absolutely invisible illuminators can be used).
The construction principle of the image converter tubes goes back into the
Structure, image converter tube:
photocathode, coated with silver shifted
cone, ensures over high voltage acceleration focusing and turning
the image (i. stands within the tube up-side-down)
coating out of zinc and cadmium transferred phosphor
automatic protection of the photo-sensitive tube does not exist (danger of
damage with bright light, e.g. car headlights). Due to the chemical characteristics
of the coatings a clear afterglow (of bright
objects) is visible and the life span (service life) is limited.
If the user does not depend on the handiness of a system and do not mind the
own IR recognizability, these devices are for instance well suitable for wildlife
observations, although they are considered technically as outdated. The main
working range of this generation is between 750 and 950 nm wavelength.
1 (Generation 1+)
the introduction of the so-called multi-alkali photocathode (starting from
the mid 50's) higher luminous gain of the tube was achieved. Under specific
circumstances additional IR-illumination
was unnecessary. This image intensifer tube works in the lower IR spectrum
/ upper visible range.
The structure of a Gen1 tube corresponds in principle to that of Gen0:
photocathode, coating from potassium, sodium,
cone, ensures over high voltage acceleration
focusing and turning the image around by 180°
P-20 phosphor screen, stronger luminosity
than with Gen0
Although in general the luminous gain is better than with Gen0, it remains
nevertheless clear behind the achievements provided by tubes of current 2nd
or 3rd Generation, since the principle of image intensification by electron
acceleration is limited. In that way only a longer acceleration distance would
mean an improvement in low light amplification. But by doing this the system-dependent
distortions of the image would increase more and also the equipment would
become just less practicable in its dimensions. In some night vision devices
up to three Gen1 tubes were placed one behind the other (2-3
staged tubes) to achieve more gain in image intensification. One one hand
the image becomes brighter, but on the other hand details disapear and contrast
becomes more worse. This means that from stage to stage more (light-) information
get lost and errors multiply.
The service life of the 1st Generation image intensifier tube (approx. 1000
- 2000 h) was increased compared to Gen0. Another improvement was made concerning
a shorter time of afterglow within the phosphor screen. Generally an automatic
protection against bright light sources was missing with tubes of this generation
(because of engagements flashes this proved to be partly restrictive to military
applications). The designation 'Generation 1+'
refers to the use of glass fiber bundles (instead of glass windows) at the
input/output side of the tube - still technology is the same as Gen1. 1st
and 1+Generation are considered as technically outdated.
The working range is between 750 and 800 nm wavelength.
the introduction of the micro channel plate (MCP)
- starting from the mid 60's - the essential step to the modern image intensifier
tube was taken. The working principle of the tube was changed from acceleration
of electrons to multiplication of electrons. In night vision devices of 2nd
Generation and upward there is instead of the anode cone a very thin glass
plate, which multiplies electrons by an electro-chemical
coating. This so-called MCP is perforated with over 2 millions parallel
arranged micro tubes (micro channels), which
are easily sloped to the optical tube axle. Within these tiny channels the
primary electron meets - by the inclination of the micro channels - the coating
of the wall and extracts cascade-like further secondary electrons. In the
end a multiplication of the electrons by the factor of 100-1000 is emitted
on the tube's back side. The number of micro channels on the glass plate determines
the resolution of the image intensifier. Focusing and possibly needed image
flip by 180° is achieved by a fiberoptic (more rarely by an additional
anode cone - eastern design).
Structure, image intensifier tube:
only slightly improved to Gen1
Channel Plate (MCP),
multiplication of electrons, starting from 2 million channels, fiberoptic-twist
(glass fiber bundle twisted by 180°)
P-10-52 phosphor screen, variant of the
P-20 of phosphor screen
with the predecessor generation a substantially larger light amplification
is reached by the new operational principle of the MCP (generally no IR-illuminator
needed). In addition a systems-inherent protective function is provided
from the structure against cross fade: The MCP has a natural upper limit of
emitable electrons, so that a strong beam of light does not immediately damage
the image intensifier. Typically starting from Gen2 also control electronics
regulate the current depending on the actual low light situation - this adjustment
is called ABC (Automatic Brightness
Control). With the introduction of the MCP night vision devices became
smaller in dimensions and less heavy (particularly important with night vision
googles). The life span increased to approx. 2500 - 5000 h. Apart from the
elimination of the problematic afterglow also the image distortions disapeared
with the utilization of a MCP (anodeless design). The 2nd Generation works
mainly within the range between 780 and 850 nm wavelength.
2+ and Super-Gen
improved variants of the Gen2 tube incorporated changes of the MCP, the photocathode
and the phosphor screen (starting from mid 70's): The resolution was refined
by at least 4 million micro channels, while an optimized inclination of the
micro channels made it possible to display some background-image areas against
a partly blinding direct light-source (BSP,
Bright Source Protection). The background noise was also reduced. The
new S-25 photocathode showed up to be more sensitive
to infrared light. A changed phosphor mixture
of the screen reacted faster (less traces of glowing objects on the screen)
and provided brighter images (more contrast).
With image intensifiers of 2Super-Generation sensitivity was continued to
shift into the IR spectrum by the new S-20R (redshift)
photocathode. Altogether the new MCP and the P-22
phosphor screen provided such a substantial improvement, that - not
at least also due to less production costs - the Super-Gen tube displaced
a lot of the first Gen3 tubes.
'The European way': Seen from the structure the newest European tubes still
represents this generation, but now with up to 12 million micro channels.
However their performance is on such a high level that they are quite comparable
with the latest US-tubes of the 3rd Generation (and '4th Generation').
improvements of this generation are based - apart from refined control electronics,
MCP and the P-20 phosphor screen - on a new photocathode coating. A mixture
from the elements gallium and arsenic
(GaAs) showed an enormous level of luminous sensivity,
significantly higher than all known coating-mixtures before. Typical characteristic
of the GaAs-coating are so-called 'Halos': Large,
bright shining disc-shaped areas around any spotlight displayed in the image.
At the end of the 80's the first night vision devices with the gallium-arsenide
photocathode were produced (and were first deployed in the Gulf War
theater in 1991).
Structure, image intensifier tube:
Channel Plate MCP, 6-12 Mio. micro channels
P-43 phosphor screen, (older tubes with P-20)
to the Gulf War generation of 1991 the newest Gen3 image intensifier tubes
are far more than doubled in performance (luminous sensivity opposite 0.Gen
- three-figure factor!). An thin aluminium layer on the PC (ion
barrier) is used for an enhanced service life of approx. 10,000 working
hours. Unfortunately the ion barrier film does not only protect the sensitive
coating from misguided electrons, it also reduces the number of transmitted
electrons. Modern Gen3-tubes demonstrate thier superiority to the 2nd Generation
tubes particularly in low light level situations (e.g. wooded areas). The
working range of the 3rd Generation is between 780 and 920 nm wavelength.
Western image intensifier tubes of this generation are definately export
restricted and therefore require an valid export license. Generally
they can be traded within NATO countries. In some cases also authorities of
friendly states are approved to obtain an export license. In co-operation
with the Department of State the night vision industry established an upper
performance limit every exportable tube may not exceed as maximum:
1600 FOM (so-called 'Figure
OF Merit'). This maximum value of 1600
represents the product of resolution
and signal to noise ratio (S/N).
Due to years of concentrating on improving the 3rd Generation there are many
image intensifier tubes of different quality. For that reason there is a broad
range of tubes available on the US domestic market (only for US residents).
As a quality criterion the official procurement contracts for the armed forces
(OMNIBUS contracts, ONMI I, II, III, IV, V, VI)
by the US government are used.
4' and future developments
present, in the context of the OMNI V & VI contract, US armed forces are
issued so-called 'filmless'
and 'thin filmed' tubes,
which are very sensitive in the deep IR range. The mentioned protective film
within these tubes is strongly reduced or even missing while the power supply
is shuttered very fast ('gated', 'autogated').
According to the industry this feature guarantees a tube life of 15,000 hours
and protects the tube of being damaged from bright light exposure. Although
manufacturers are in prospect of new contracts and still another clear improvement
in performance is achieved, it is not completely clear whether these image
intensifiers represent officially the 4th Generation.
Technologically seen the term '4.Generation' would be apparently justified.
Export of this technology - even to friendly states - is very unlikly at present.
The newest European 'autogated' tubes (e.g. DEP XR5,
Photonis XH 72) seem to have a practical advantage
in that they can be operated even by day without problems (e.g. night vision
riflescope: day/night transitions). Because of their different photocathode
coating bright spotlights do not draw so large halos on the intensified image.
While at present these tubes are 'leading by a nose' in terms of measured
resolution values, their superiority in pure luminous gain is questionable.
generations of night vision devices will bring together
classic NV and thermal imaging (rendered, overlayed image) with an
option for data input (HUD-like) as well as an
increased field of view. Thinking of night vision goggles also low
profile by minimizing the device is an issue (comfortable to wear a long time).
However also the coating technology is not yet at the end of its possibilities:
At present experiments are made with layers from photo-sensitive
nano-particles toward more cheaply, smaller, better image intensifier
tubes. Comparable CCD systems are still not able
to replace all the benefits with classical NV-vacuum tubes by now. However,
one may be assured that some interesting prototypes are already tested by
manufacturers - and/or are under catch at government agencies.
working range from the lower to the upper IR-spectrum - unfortunately very
little luminous gain
Only the lower IR spectrum is used, working range is partly located in the
visible spectrum (not pictured)
Significantly more IR-radiation can be used - working range is more remote
from the visible spectrum
nearly full utilization of the IR-spectrum - essentially more IR-light can