Night vision device

A night vision device usually consists of an image intensifier tube, a protective housing, and an optional mounting system. Many NVDs also include a protective sacrificial lens, mounted over the front/objective lens to prevent damage by environmental hazards, while some incorporate telescopic lenses. An NVD image is typically monochrome green, as green was considered to be the easiest color to see 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.