Image Intensifier
An image intensifier (abbreviation: II or I²) is an electro-optical component that can produce an intensified monochrome image on a phosphor screen from a cone of incoming light, intended to intensify the signal beyond what optics and digital sensors are capable of.
In the field of night vision, image intensifier refers to image intensifier tubes which are miniaturized image intensifiers (usually in tubular shape) that form the core component of any night vision device.
Image intensifier tubes are inserted into a housing that otherwise only provides optics, power supply, and protection of the sensitive component. Many formats of image intensifier tubes are designed to be exchangable with limited tooling and know-how, originally intended to allow armies to replace damaged image intensifier tubes by an engineer during deployment.
Generations
The U.S. Army's Night Vision and Electronic Sensors Directorate (NVESD) categorizes image intensifiers into four distinct generations. With each generation, a change in the technology has lead to substantial performance improvements.[1]
Although the generation of any given image intensifier will quickly provide a rough understanding of what to expect from it, it is not a replacement for accurate performance parameters (e.g. FOM, SNR, Gain), as these can vary wildly for different image intensifiers of the same generation. The relevant agencies of the U.S. government have since recognized this circumstance and have mostly stopped using generations to refer to image intensifiers in their specifications, and instead use the Figure of Merit (FOM) for most purposes.
Some manufacturers and retailers use a + to indicate intra-generational improvements, e.g. Gen. 2+. This practice is not part of the official specification and can be defined differently by anyone.
Generation 0
Generation 0 was invented in 1929 by a Hungarian scientist in the UK. First uses where in World War II by the Germans, later the Soviets and the Americans.
Gen. 0 tubes don't have any or only very low gain of a around 10 and thus rely on strong IR illumination.
Gen. 0 tubes utilize S-1 photocathodes, that have a quantum efficiency of 0.5% in the IR spectrum and 1% in the UV spectrum.
Examples of Gen 0 devices
- Vampir
- M2 & M3 Sniperscope
- PNV57A
Generation 1
Generation 1, developed and patented in the 1960s, improved the gain to around 1000 fL/fc. This enabled the use of Gen. 1 devices under moonlight conditions without the use of IR illumination.
Later developments include the glass in the body being replaced with ceramic, improving the gain even further. These devices are sometimes referred to as Gen. 1+.
Some devices used multiple tubes in a cascade configuration, leading to improvements in gain of up to 100,000 fL/fc, but with very low SNR.
Examples of Gen 1 devices
- AN/PAS 2 Starlight
- Armasight Spark
Russian Gen. 1 tubes made by Ekran are also found in most cheap consumer devices.
Generation 2
The Generation 2 was developed in the 1970s and was the first generation using a Microchannel Plate (MCP), which substantially increased the gain to around 20k fL/fc. During this time, due to drastic innovations in the semiconductor space, the first widespread integration of the PSU with the tube in a single, modern package appears.
Generation 2 IITs are usually produced with either P20, P22, P43 or P45 phosphor screens.
Some manufacturers, like Photonis, have continued developing their Gen. 2 technology without ever developing a Gen. 3 production line. Nontheless they have succeeded in pushing the envelope with their image intensifiers matching Gen. 3 performance, most recently even producing image intensifiers with gain levels above 60k fL/fc.
Examples of Gen 2 devices
- GN-1 employed Gen 2 IIT upon release.
Generation 3
The photocathode is made using gallium arsenide and the refined MCP technology is coated with an Aluminum oxide layer called ion barrier film to prolong the tubes functional life by preventing the occasionally released ion to damage the MCP, a process called ion-poisoning which drastically diminishes the plates functional lifespan. The downside of this ion film is that it somewhat restricts the amount of electrons that pass through it thereby detracting some of the intensification. The so called haloing-effect was also increased by this film, something that lessens the practical performance since it may obscure parts of what is being observed.
Subsequent development of the technology led to "thin-film" tubes among other solutions to lessen the impact of the ion barrier on gain and resolution. Since the beginnings of thin-film IIT it has become so commonplace that instead of being singled out as having thin ion barrier film the earlier image intensifiers are nowadays referred to as "thick-film" instead.
Gen. 3 tubes are usually produced with either P43 or P45 phosphor screens. Adams Industries sold a Sentinel-CNV "color" night vision binocular where each optical channel had differently filtered input, each channel seeing different parts of the spectrum, and output as two colors, with one side having a red phosphor[2] screen, with green P43 on the other.
Gen 3 image intensifier gain is often set in the range of 40k to 80k fL/fc. High gain variants are set to 100k to 120k fL/fc.
Examples of Gen 3 devices
Filmless
As a logical next step, manufacturers such as Litton (now L3Harris) began developing third generation image intensifiers without an ion barrier film in the 1990s. Albeit successful in creating even higher performing image intensifiers, the early iterations suffered from shorter lifespans and less durability in addition to higher production cost. The first Omnibus containing filmless image intensifiers was Omni V. Both ITT and Litton (NGEOS) were given contracts for Gen 4 MX-10160B tubes. There is some evidence that Litton achieved filmless image intensifiers that reached SNR figures over 35, with the lifetime being around 7500 hours which was also the required minimum. It is unclear what exactly happened, and why the Gen 4 designation was dropped. On their webpage ITT wrote about the development of the so called Pinnacle technology[3], an autogated image intensifier tube with a thinner ion barrier film, where they mention receiving a $40mil Navy order for filmless F4949 sets. ITT writes they could not keep up with the requirements, and the U.S. Navy reverted back to buying the older Omni IV regular ion barrier image intensifier tubes. Litton, later L3, continued developing filmless technology. During the next 10 to 15 years the U.S. military bought L3 filmless image intensifier tubes for special use cases. Probably the best known early example is the AN/PVS-15 that used L3 manufactured filmless technology MX-10160Bs (also filmless MX-10160WG), NSN (5855-01-548-9651) assigned on 8 Mar 2007.
L3Harris stands out as the sole western producer of filmless image intensifier tubes, having patented the process. The only other eastern producer is Ekran FEP.
Generation 4
The official specification does not include Generation 4 any more, as even the most modern filmless image intensifier tubes still fall under the definition of Generation 3.
Terminology
While the American system for determining generations is quite clear cut, some systems like the Russian one aren't the same. The following table will shot the various definitions of tube generations. The Russian system exists because the Soviets never really had a generation system. The photocathode type was the only thing ever given, and even this is rare.
| Tube type | Western designation | Russian desigantion | Common Marketing term |
|---|---|---|---|
| Single chamber electrostatically focused with S-1 photocathode | Generation 0 | Generation 0 | Generation 0 |
| Single chamber electrostatically focused with S-20/25 photocathode | Generation 1 | Generation 0+ | Generation 1 |
| Single chamber electrostatically focused with S-20/25 photocathode and spherical photocathode | Generation 1 | Generation 0+ (sometimes the spherical nature is specified) | Generation 1+/CF super |
| Single chamber electrostatically focused with S-20/25 photocathode and spherical photocathode and fiber optic input/output | Generation 1 | Generation 0+ (usually the fiber optic is noted) | Generation 1+ |
| Multiple chamber cascade tube with S-1/20/25 input photocathode | Generation 1 (usually just called cascade) | Generation 1 | Cascade |
Formats
No single standard for the physical size and shape of image intensifier tubes exists. Instead, several different formats have been created over time.
Many older formats have since become partially or completely abandoned, as many have been designed to fit a specific housing or use case, as well as miniaturization allowing for ever smaller form factors.
Other formats, like MX-10160, have evolved to become somewhat of an industry standard, far exceeding their originally intended use case.
Manufacturers are still creating new formats, like Photonis with their 16mm format, which is even smaller and lighter than the commonly used formats.
- Mx9916 (Fat Anvis)
- MX-10130
- MX-10160 / Small Anvis / 37mm / 18mm
- MX-11769
- 16mm
Lifetime
Because image intensifiers rely on an electrochemical process to operate, they will slowly degrade with use.
The original MX-10160/AVS6 specification defines a mean time to failure of the image intensifier of 7500 hours under normal and 1500 hours under accelerated conditions. This includes degradation in performance as well as complete failure of the assembly.
Image intensifiers are considered to have "failed" if the FOM decreases below 50% of the initial value.Source?
Gen. 2 image intensifiers loose FOM almost linearly over their lifetime, while Gen. 3 image intensifiers have an almost constant FOM with a sharp decrease at the end of life.[4] This means that while used Gen. 3 image intensifiers are likely to meet their original specifications, Gen. 2 image intensifiers will have a more or less lower performance compared to when they were new.
References
- ↑ Night Vision Technologies Handbook https://www.dhs.gov/sites/default/files/publications/NV-Tech-HB_1013-508.pdf
- ↑ https://www.adamsindustries.com/
- ↑ Gen 4, Omni V and VI, and Pinnacle technology https://web.archive.org/web/20060324232309/http://www.nightvision.com/military/cs_gen3pinnacle.html
- ↑ Eric Garris - Harris - The Gen. 3 Advantage (https://dokumen.tips/documents/the-gen-3-advantage-harris-gen-3-advantage-night-vision-and-communication-systems.html?page=9)
