A night vision camera, or night vision system, is an optical technology that permits observation and photography in extremely low-light or no-light conditions. These cameras are commonly used among the military, police, and other security forces, but civilians do use night vision for recreation and wildlife observation. Night vision is categorized into GEN-I, GEN-II, GEN-III, and GEN-III OMNI-VII technologies, depending on their sophistication. The most recent, GEN-III OMNI-VII, was developed in October 2007. Though these generation designations are set by the US military, they have been adopted by the civilian night camera community as a matter of convenience.
There are two primary technologies used for a night vision camera. The first, and most common, is a photomultiplier tube, or "conventional night vision," operating in the near-infrared frequency range, picking up light waves about 1 micrometer wide (human vision can only see light with a frequency between 0.4 and 0.7 micrometers). The second is thermal imaging, which allows a night vision camera that can take pictures even in cases where light is absent. This is because thermal cameras can see the electromagnetic radiation released by blackbody heat that emanates from every physical object. The newest types of night vision camera use a blend of both technologies.
Though the first night vision devices, bulky gadgets invented for snipers during World War II, only multiplied the ambient light by a few times, a modern night vision camera multiplies light by about 10,000-50,000X. This is enough to take pictures with a minimum of starlight, even if the moon is absent or obscured. One downside of most night vision systems is that the field of view is relatively narrow -- you cannot see in your peripheral vision, and your head and the device must be turned to scan an area. Panoramic night vision cameras are currently under development by the US Air Force, but they remain in limited use.
The basic principle of operation of a night vision camera is to intercept incoming photons, convert them to electrons using a very thin layer of gallium arsenide used as a photodiode, the electrons are accelerated and their energy boosted, which impacts another layer and causes a secondary emission cascade. The secondary emission cascade of electrons is then accelerated just enough to impact a phosphor screen and cause the emission of amplified light, which is viewed by the user. This light is monochromatic, and is usually portrayed as green because the human eye is most sensitive to this wavelength.