Night vision devices enable vision in low-light conditions.
A night vision device (NVD) is an optical device that makes it possible to see in conditions of almost complete darkness. Night vision devices were developed by the military for use on the battlefield, but they are now widely used in law enforcement and available to the general public.
The technology, which came in to widespread military use in the mid-1960s during the Vietnam war, has evolved considerably over the years through three stages -- Generation I, II and III night vision technology -- with each generation operating more efficiently and using less light than its predecessors.
Types of Night Vision Devices
Night vision technology falls into two main categories: thermal imaging and image-intensifying, or near-infrared (IR).
Thermal sensors work by detecting the differences in heat signatures produced by objects, such as vehicles, people, animals, and their environment. Infra-red devices use electro-optical technology to amplify distant light sources such as the moon, the stars or man-made light (cities) through intensifiers that capture the image on a phosphor display. The screen is green because the human eye can differentiate between more shades of green than any other color. Night vision devices include scopes (for hand-held use or attached to weapons), binoculars, and goggles.
Generation I Devices
Passive night vision was developed in the 1960s, but the technology was bulky and difficult to transport. Full moonlight was required to intensify an image about 1,000 times, though the quality was still very poor in comparison to today's technology. The intensifier tube had a life of around 2,000 hours, meaning the equipment was extremely expensive to replace.
Generation II Devices
In the 1970s, with the introduction of the microchannel plate, night vision technology became more efficient. It allowed the ambient light source to be amplified by up to 20,000 times using a quarter of the light required by Generation I devices. The operating life increased to around 5,000 hours.
Generation III Devices
The most recent technology -- developed in the 1980s and available as of 2011 -- amplifies light between 30,000 to 50,000 times. It is also capable of utilizing near-infrared light, a wavelength that is not picked up by the naked eye. It provides images at three times the distance and clarity of Generation II technology.
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