Night vision device

A U.S. Army aviator uses a pair of helmet-mounted AN/AVS-6 vision goggles.

A night vision device (NVD) is an optical instrument that allows images to be produced in levels of light approaching total darkness. They are most often used by the military and law enforcement agencies, but are available to civilian users. The term usually refers to a complete unit, including an image intensifier tube, a protective and generally water-resistant housing, and some type of mounting system. Many NVDs also include sacrificial lenses, IR illuminators, and telescopic lenses.

Night vision devices were first used in World War II, and came into wide use during the Vietnam War.^[1]^[2] The technology has evolved greatly since their introduction, leading to several "generations" of night vision equipment with performance increasing and price decreasing.

1 Usage
2 Function
- 2.1 Passive and active
  - 2.1.1 Active
  - 2.1.2 Passive
3 Generations
4 Other technologies
5 Legality
6 US Patents
7 Manufacturers
8 See also
9 References
10 External links

Usage

Night vision devices were originally developed for military use, but have since spread into other areas, such as security and police work, rescue outfits and various amateur uses (for example animal watching or hunting).

Night vision goggles have been especially praised by the pilots of rescue helicopters, as they eliminate the need for a 'sterile light environment' (i.e. a dark cabin to allow the pilot to let his eyes naturally adjust to night-flying conditions). This will for example allow a medic in the cabin to work on a patient under bright lights while retaining the pilot's ability to fly safely under night conditions.^[3]

Function

How night vision works.

Night vision devices (NVD) work in the near-infrared band at a wavelength of about 1 micrometer. For comparison, the human visual system is sensitive to light wavelengths in the range of about 0.4 to 0.7 micrometers. Unlike thermal imaging systems, which operate in complete darkness by detecting heat radiation signatures in infrared wavelengths beyond 3 micrometers, NVDs work in near darkness by detecting ordinary ambient light, usually from the moon and stars, that is reflected by objects in the scene being viewed. NVDs contain an image intensifier tube that uses the photoelectric effect to amplify very weak light. As each photon of incoming light collides with a detector plate inside the intensifier tube, the plate ejects several electrons that are further amplified into a cascade of electrons. These electrons are accelerated by a strong electric field towards a phosphor screen which emits light at the point of impact of the electrons. A bright image is thus formed on the phosphor screen. Outdoor environments that are illuminated only by star light can be easily viewed using night vision devices.

A scene illuminated with a night vision device.

Most night vision devices do not detect color information, and hence a monochromatic phosphor screen is sufficient. A green phosphor (P22) display is generally used because the human eye is most sensitive to the color green, which falls in the middle of the visible light spectrum.^[4]

One of the drawbacks of almost all current NVDs is the lack of peripheral vision, meaning that the user needs to turn his head to change his rather narrow field of view.^[3]

Passive and active

There are two methods of operating night vision systems, passive or active. Passive systems amplify the existing environmental ambient lighting, while active systems rely on an infrared light source to provide sufficient illumination. Early NVDs were designed to be used as active systems, as they did not have the sensitivity to operate on ambient light. Active systems are often used today in closed-circuit television security applications and also on many consumer devices such as home video cameras.

Military applications generally require passive operation, as an active system's infrared illumination device is easily spotted and tracked by others equipped with night vision devices, placing the user at a tactical disadvantage.^[5] However, most modern NVG devices include an inbuilt active IR illuminator which can be toggled for use when ambient light is not available.

Active

Active infrared night vision combines infrared illumination of spectral range 700nm-1000nm - just beyond the visible spectrum of the human eye - with special CCD cameras sensitive to this light. The resulting scene, which is apparently dark to a human observer, appears as a monochrome image on a normal display device.^[6]

Because active infrared night vision systems can incorporate illuminators that produce high levels of infrared light, the resulting images are typically higher resolution than other night vision technologies.^[7]^[8] Active infrared night vision is now commonly found in commercial, residential and government security applications, where it enables effective night time imaging under low light conditions. However, since active infrared light can be detected by night vision goggles, it is generally not used in tactical military operations.

Passive

The passive GEN III AN/PVS-10 rifle scope mounted on the M110 SASS.

Night vision technology, which refers to the quality of the image intensifier tube housed by the NVD, is often classified into "Generations" following the pattern originated by the US Military. Referring to night vision in terms of its generation is purely for indicative and reference purposes only, even though this has spread to become common consumer terminology. The United States Army class their current in-service devices with the Generation Family Type followed by the device's version or awarded contract.^[9] The latest night vision device in service with the United States Army, as of October 2007, is the Gen III Omni VII, manufactured by ITT Corporation.^[9] However, due to the fact that it is an autogated tube, the consumer market generally refers to this as being a 'Gen IV' device.

Within the European Union, Australia, and New Zealand, night vision devices are not referred to in terms of 'Generations', as the most recent image intensifiers in service is the XR5 autogated filmless tube from Photonis-DEP, and hence this product would be considered a ‘Gen IV’ type device by the consumer market.

Generations

The classification below was initially introduced by the US manufacturers through the US government. European manufacturers do not abide by it.

Generation 0

The first night vision devices, the M1 and M3 infrared night sighting devices, also known as the "sniperscope" or "snooperscope", were introduced by the US Army in World War II, and also used in the Korean War, to assist snipers.^[1] They were active devices, using a large infrared light source to illuminate targets. Their image intensifier tubes function using an anode and an S-1 photocathode, made primarily of silver, caesium, and oxygen to accelerate the electrons.^[10] Parallel development of night vision systems by AEG occurred in Nazi Germany, and by the end of World War II, it had equipped approximately 50 Panther tanks, which saw combat on both the Eastern and Western Fronts, and produced the "Vampir" man-portable system for infantry soldiers equipped with Sturmgewehr 44 assault rifles.^[11]

Generation 1 (GEN I)

An M16A1 rifle fitted with the AN/PVS-2 Starlight scope.

First generation passive devices, introduced during the Vietnam War, were an adaptation of earlier active GEN 0 technology, and rely on ambient light instead of an infrared light source. Using an S-20 photocathode, their image intensifiers produce a light amplification of around 1000x,^[12] but are quite bulky and require moonlight to function properly.

Examples:

AN/PVS-2

Generation 2 (GEN II)

Second generation devices featured an improved image-intensifier tube utilizing micro-channel plate (MCP)^[13] with an S-25 photocathode,^[10] resulting in a much brighter image, especially around edges of the lens. This leads to increased illumination in low ambient light environments, such as moonless nights. Light amplification was around 20000x^[12] Also improved were image resolution and reliability.

Examples:

AN/PVS-4^[14]
AN/PVS-5^[15]
SUPERGEN^[16]

Later advancements in GEN II technology has brought tactical characteristics of GEN II devices into the range of GEN III ones, which has complicated the reasonable comparison.

Generation 3 (GEN III)

An early development version of the AN/PVS-7 goggle.

Third generation night vision systems maintain the MCP from Gen II, but now use a photocathode made with gallium arsenide, which further improves image resolution. In addition, the MCP is coated with an ion barrier film for increased tube life. The light amplification is also improved, to around 30000-50000x^[12]

Examples:

AN/PVS-7^[17]
NVS-7
AN/PVS-10
AN/PVS-14^[18]
NVS-14
XD-4 Autogated ^[19]

Omnibus-VII

The US Army Night Vision and Electronic Sensors Directorate (NVESD) (http://www.nvl.army.mil/) is part of the governing body that dictates the name of the generation of night vision technologies. Although the recent increased performance associated with the GEN-III OMNI-VII components is impressive, the US Army has not yet authorized the use of the name GEN-IV for these components.

GEN-III OMNI-VII devices can differ from standard Generation 3 in two important ways. First, an automatic gated power supply system regulates the photocathode voltage, allowing the NVD to instantaneously adapt to changing light conditions.^[20] The second, is a removed or greatly thinned ion barrier, which decreases the amount of electrons that are usually rejected by the Standard GEN III MCP, hence resulting in less image noise and the ability to operate with a luminous sensitivity at 2850K of only 700, compared to operating with a luminous sensitivity of at least 1800 for GEN III type image intensifiers.^[21] The disadvantage to a thin or removed ion barrier is the overall decrease in tube life from a theoretical 20,000 hrs mean time to failure (MTTF) for Gen III type, to 15,000 hrs MTTF for GEN IV type. However, this is largely negated by the low numbers of image intensifier tubes that reach 15,000 hrs of operation before replacement.

It is important to note that while the consumer market classifies this type of system as "Generation 4", the United States military describes these systems as Generation 3 Autogated tubes (GEN-III OMNI-VII). Moreover, as autogating power supplies can now be added to any previous generation of nightvision, 'autogating' capability does not automatically class the devices as a GEN-III OMNI-VII, as seen with the XD-4. Another point to note is that any postnominals appearing after a Generation type (ie: Gen II +, Gen III +) does not change the generation type of the device, but instead indicates an advancement(s) over the original specification's requirements.^[22]

Examples:

AN/PVS-22^[23]
NVS-22
XR-5 Autogated^[24]

Though image intensification technology employed by different manufacturers varies, from the tactical point of view night vision system is an optical device that enables vision at low light. The US government itself has recognised the fact that technology itself makes little difference as long as an operator can see clearly at night. Consequently the United States base their export regulation not on the generations, but on calculated factor called Figure of Merit (FOM). The method of FOM calculation and its implications for export are briefly described in the publication of the National Defense University, USA in a document named “The NATO Response Force”^[25] authored by Jeffrey P. Bialos, the Executive Director of the Transatlantic Security and Industry Program at the Johns Hopkins University and Stuart L. Koehl, a Fellow at the Center for Transatlantic Relations of the same university.

“... beginning in 2001, the U.S. implemented a new figure of merit (FOM) system for determining the release of night vision technology. FOM is an abstract measure of image tube performance, derived from the number of line pairs per millimeter multiplied by the tube's signal-to-noise ratio." US made tubes with a FOM greater than 1600 are not exportable outside the US.

Other technologies

Panoramic Night Vision Goggles in testing.

The United States Air Force is experimenting with Panoramic Night Vision Goggles (PNVGs) which double the user's field of view to around 95 degrees by using four 16 mm image intensifiers tubes, rather than the more standard two 18 mm tubes. They are in service with A-10 Thunderbolt II, MC-130 Combat Talon and AC-130U Spooky aircrews.^[26]

The PSQ-20, manufactured by ITT seeks to combine thermal imaging with image intensification, as does the Northrop Grumman Fused Multispectral Weapon Sight.^[27]^[28]

Legality

Certain countries (e.g. Hungary and other European Union members) regulate possession and or use of night-vision devices.^[29] Civilians are allowed to have Generation 1 and Gen1+ devices, but citizen's access to Gen 2 and up is outlawed by adopting International Traffic in Arms Regulations into national legislation. Generation 2 and higher devices are classified as military/law enforcement purpose and espionage tools.

New Zealand rescue helicopter services use several sets of 3rd-generation night vision goggles imported from the USA, and is required to restrict access to the equipment to comply with the strict regulations regarding their export.^[3]