A night-vision device (NVD), also known as night optical/observation device (NOD) and night-vision goggles (NVG), is an optoelectronic device that allows images to be produced in levels of light approaching total darkness. The image may be a conversion to visible light of both visible light and near-infrared, while by convention detection of thermal infrared is denoted thermal imaging. The image produced is typically monochrome green, because it was considered to be the easiest color to look at for prolonged periods in the dark. NVDs are most often used by the military and law enforcement agencies, but are available to civilian users. The term usually refers to a complete unit, including an image intensifier tube, a protective and generally water-resistant housing, and some type of mounting system. Many NVDs include a protective sacrificial lens, or optical components such as telescopic lenses or mirrors. An NVD may have an IR illuminator, making it an active as opposed to passive night-vision device. They are often used in conjunction with IR laser sights which project a beam onto the target that is only visible through an NVD.
Night-vision devices were first used in World War II and came into wide use during the Vietnam War. The technology has evolved greatly since their introduction, leading to several "generations" of night-vision equipment with performance increases and price reductions. Consequently, they are available for a wide range of applications, e.g. for gunners, drivers and aviators.