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 also available on the open market. The term generally refers to the complete assembly of the required components in any particular device, namely, the 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.

The device's mounting system generally defines its use, with the most flexible of devices are generally the ones with the most mounting options. For instance, certain of the PVS monoculars are night vision devices in use with the US military as well as 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 parallel with another device, for binocular "night vision goggles" which have the advantage of individually focusing the image to each eye to maintain depth perception. Some night vision devices may also be attached to a rifle using a Picatinny rail, in front of an existing telescopic or red dot sight, or affixed to the end of a Single-lens reflex camera. Other systems are designed for a specific purpose, integrating an image intensifier into a telescopic sight, resulting in a more compact and lightweight, but less versatile system.

Night vision devices were first used in World War II, and brought into wide use during the Vietnam War. The technology has evolved several times since their introduction, leading to multiple "generations" of night vision equipment with greatly varying performance and price.

Function

Night vision devices work in the near-infrared band at a wavelength of about 1 micrometer. For comparison, human visual range is about 0.4 to 0.7 micrometers. Unlike thermal imaging systems, which can operate in total darkness, night vision systems rely on ambient light often from the moon and stars. The intensifier tubes use the photoelectric effect. As a photon collides with a detector plate, the metal ejects several electrons that are then amplified into a cascade of electrons that light up a phosphor screen. Often a dim star in the sky is enough to illuminate an entire field. This image is tinted green because the human eye is most sensitive to it, and allows the eye to re-adjust more quickly if the goggles are removed.

There are two types of night vision systems, active and passive.

Active: The earliest night vision systems were active systems, obsolete in today's military context, and rely on an infrared illumination device to function properly. This type of night vision is still commonly found on home video cameras, as it suits many consumers' night vision needs. The military no longer uses these devices, as the large infrared light source, gives away the user's location to those with passive devices, placing him or her at a tactical disadvantage.

Passive: Passive systems may be used without any infrared illumination, although many modern passive systems include an IR illuminator for use in total darkness. This is the most common type of night vision system, and is usually assumed.

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.

Generation 1

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. 

Generation 2

Second generation devices featured an improved image-intensifier tube utilizing microchannel 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.

Generation 3

Third generation night vision systems use a photocathode made with gallium arsenide, which further improves image resolution. In addition, the microchannel plate is coated with an ion barrier for increased life. The light amplification is also improved, to around 30000-50000x^.

Generation 4

The military describes Generation 4 systems as Generation 3 Autogated systems, however autogated systems are generally accepted as "Fourth Generation" NVDs. They are also often referred to as Generation 3+. These devices differ from 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 increased the noise and image distortion.