Night vision device
NVDs are mounted appropriately for their specific purpose, with more general-purpose devices having more mounting options. For instance, the AN/PVS-14 is a monocular night vision device in use with the US military as well as by civilians. It may be mounted on the user's head for handsfree use with a harness or helmet attachment, either as a monocular device, or in aligned pairs for binocular "night vision goggles" which provide a degree of depth perception as do optical binoculars. The AN/PVS-14 may also be attached to a rifle using a Picatinny rail, in front of an existing telescopic or red dot sight, or attached to a single-lens reflex camera. Other systems, such as the AN/PVS-22 or Universal Night Sight, are designed for a specific purpose, integrating an image intensifier into, for example, a telescopic sight, resulting in a smaller and lighter but less versatile system.
Night vision devices were first used in World War II, and came into wide use during the Vietnam War. The technology has evolved greatly since their introduction, leading to several "generations" of night vision equipment with performance increasing and price decreasing.

# 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.

# Function
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.
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.
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.

## 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. 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.
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.  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
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. 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. 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

## 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. 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. 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.

## Generation 1 (GEN I)
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, 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) with an S-25 photocathode, 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 Also improved were image resolution and reliability.
Examples:
- AN/PVS-4
- AN/PVS-5
- SUPERGEN

## Generation 3  (GEN III)
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 
Examples:
- AN/PVS-7
- AN/PVS-14
- XD-4, autogated or not

## Omnibus-VII
The US Army Night Vision and Electronic Sensors Directorate (NVESD) (/) 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. 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.  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 be 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 a supposed advancement(s) over the original specification's requirements.
Examples:
- AN/PVS-22
- XR-5 Autogated

# Other technologies
The US 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,MC-130 Combat Talon and AC-130U Spooky aircrews.
IN 2007 Xenonics Holdings, using newly patented technology, offered the first digital night seeing system, a hand held monocule device with 2-8X zoom capability branded Supervision. 
The PSQ-20, manufactured by ITT seeks to combine thermal imaging with image intensification, as does the Northrop Grumman Fused Multispectral Weapon Sight.

# Legality
Certain countries (e.g. Hungary and other European Union members) regulate possession of night-vision devices. 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.

# US Patents
- US D248860 - Night vision Pocketscope
- US 4707595 - Invisible light beam projector and night vision system
- US 4991183 - Target illuminators and systems employing same
- US 6075644 - Panoramic night vision goggles
- US 6158879 - Infra-red reflector and illumination system

# Manufacturers
- Aiptek Taiwan
- American Technologies Network Corporation USA
- ASELSAN Turkey
- Bushnell Corporation USA
- EG&G
USA
- Extreme CCTV
- Goodrich, SUI USA (formerly Sensors Unlimited, Inc.)
- Hensoldt USA
- ITT Corporation USA
- Northrop Grumman USA (acquired Litton Industries)
- Kongsberg Gruppen Norway (have acquired Simrad)
- Night Vision Systems USA
- Photonis Europe (have acquired DEP)
- Raytheon USA
- SELEX Galileo Italy
- Swarovski Austria
- Thales Group France