Night vision goggles
From Wikipedia, the free encyclopedia
Night vision goggles (NVG), passive night goggles (PNG, for the passive type only), or night vision devices (NVD) are a type of optical instrument that allows one to see in the dark. They are most often used by the military and law enforcement.
Brought into widespread use during the Vietnam War, there have been multiple "generations" of this technology. Currently the military is using Generation 4 technology, a step beyond commonly available Generation 3 devices (which incorporate gallium arsenide (GaAs) as part of the technology.
The price of such technology varies widely. Generation 1 goggles can retail for as little as $200, while few high-quality Generation 3 goggles can be found for under $1,000. Some equipment, built around finely manufactured image intensifier tubes, can be as expensive as $6,000.
In addition to goggles, night vision technology is also used in rifle scopes, security cameras, binoculars, etc.
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[edit] Passive
Passive goggles pick up any light in the immediate area and amplify it several thousand times using an image intensifier. This works by 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 type is commonly used for war reporting, tinting the picture green. The color green is chosen because the human eye is most sensitive and able to discern the most shades in green. It also allows the eye to re-adjust more quickly than with other colours if the goggles are removed.
For warfare in a snowy and icy environment, experimental goggles have been designed that tint the image light blue. This is done to give a soldier a more realistic view of the variability of shades. Using green goggles tends to lead to "overshading" whereupon more tints appear in the eyepiece than on the actual landscape as seen by the naked eye.
[edit] Active
Active goggles project infrared light (the same type used by most television remote control), and produce an image from the light reflected back. These goggles may require an image enhancer to amplify the light. This feature is commonly found on home video cameras, as it suits most consumers' needs for seeing in the dark. Pointing a remote control (such as a television's) at this type of system produces a glaringly bright image, due to the infrared light emitter used in these devices. This is hardly used in military context, as the enemy may have instruments to detect the infrared spotlight.
There are currently goggles that combine both of these technologies to create images that are almost as clear as day.
