www.nivitech.com is hosted by
www.intas.org

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)

www.nivitech.com is hosted by
www.intas.org
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 If in the past night vision were exclusively reserved for the military, then meanwhile simple or superseded night vision devices are also available for civilian users e.g. security companies, hunters, marine crew members or nature observers.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'.

AN/PVS-18 (M983)

Older German Army nightvision weaponsight (B-8 V)
Older German Army 'infra-red sniperscope for small arms', mounted on a H&K G3 assault rifle (Gen 0)

GN2 / TN-21
European night vision goggles: GN-2 and TN-21

Aviator's Night Vision
US-ANVIS goggles - used for night vision flights

AN/AVS-502 view
Modern night vision goggles (here PVS-21) allow to display mission data input via HUD - especially of importance in military aviation

PVS-21 (AN/AVS-502)
With beamcombiners (PVS-21) the unaided FOV is almost not limited. Relatively comfortable to wear due to small dimensions & little weight.

AN/AVS-502
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.

NV-Principles