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Page 1 (General) Page 2 (Generations) Page 3 (Low Light, Objektive Lens) Page 4 (Image Intensifier Tube) Page 5 (Eyepiece Lens, IR Light Sources)
Page 1 (General) Page 2 (Generations) Page 3 (Low Light, Objektive Lens) Page 4 (Image Intensifier Tube) Page 5 (Eyepiece Lens, IR Light Sources)

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4. Generations

The 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.

The 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.

Generation 0

Devices 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 30's.
Structure, image converter tube:

An 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.

Generation 1 (Generation 1+)

With 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:

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.

Generation 2

By 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:

Compared 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.

Generation 2+ and Super-Gen

The 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').

Generation 3

The 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:

Compared 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.

'Generation 4' and future developments

At 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.

New 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.

0.Generation Working Range

0.Gen (750-950nm): Wide working range from the lower to the upper IR-spectrum - unfortunately very little luminous gain

1.Generation Working Range

1.Gen (750-800nm): Only the lower IR spectrum is used, working range is partly located in the visible spectrum (not pictured)

2.Generation Working Range

2.Gen (780-850nm): Significantly more IR-radiation can be used - working range is more remote from the visible spectrum

3.Generation Working Range

3.Gen (780-920nm): nearly full utilization of the IR-spectrum - essentially more IR-light can be used

Generation 0 - Sectional View
Generation 1 - Sectional View
Generation 1 - comparative pictures
Generation 2 - Sectional View
Generation 2 - comparative pictures
Generation 3 - Sectional View
Generation 3 - comparative pictures