Night-vision device

A night-vision device (NVD), also known as a night optical/observation device (NOD), night-vision goggle (NVG), is an optoelectronic device that allows visualization of images in low levels of light, improving the user's night vision. The device enhances ambient visible light and converts near-infrared light into visible light which can be seen by the user; this is known as I2 (). By comparison, viewing of infrared thermal radiation is referred to as thermal imaging and operates in a different section of the infrared spectrum. A night vision device usually consists of an tube, a protective housing, and may have some type of mounting system. Many NVDs also include a protective sacrificial lens, mounted over the front lens (ie. objective lens) on NVDs to protect the latter from damage by environmental hazards and some can incorporate telescopic lenses. The image produced by an NVD is typically monochrome green, as green was considered to be the easiest color to look at for prolonged periods in the dark. Night vision devices may be passive, relying solely on ambient light, or may be active, using an IR (infrared) illuminator to better visualize the environment. Night vision devices can be handheld but many are head-mounted and attach to helmets. When used with firearms, an IR laser sight is often mounted to the user's weapon. The laser sight produces an infrared beam that is only visible through an NVD and aids with aiming. Some night vision devices are specially made to be mounted to firearms. These can used in conjunction with weapon sights like rifle scopes or can be used as standalone sights; some thermal weapon sights have been designed to provide similar capabilities. These devices were first used in World War II and came into wide use during the Vietnam War. The technology has evolved greatly since its introduction, leading to several "generations" of night-vision equipment with performance increases and price reductions.