Night-vision is seeing in the dark. There are two ways to accomplish this; biologically, and through technology. Technological night vision works on one of two principles. The first is by detecting infrared radiation, which is a form of energy emitted by all objects regardless of the ambient light conditions. A device based on this principle is called an infrared camera. The second is by intensifying the small amount of light present even at night, from the stars and the moon. A device based on this principle is called an image intensifier or starlight scope (SLS).

Biological night vision works on similar principles. Rhodopsin in the rods of the eye breaks as light hits it. The peak rhodopsin build up time for optimal night vision in humans is 30 minutes. Rhodopsin in the human rods isn't sensitive to the longer red wavelengths of light, so many people use red light to preserve night vision as it will not deplete the eye's rhodopsin stores in the rods and instead is viewed by the cones.

Some animals, such as cats, dogs, and deer, have a structure called the tapetum in the back of the eye that reflects light for even better night vision than humans. Their night vision likely falls between a Generation 1 and Generation 2 image intensifier.

The other method of biological night vision is that of detecting thermal emissions from a heat source. This is prevalent in some snakes such as the pit viper and boas. However, this is not "vision" per se, but more of a system of thermosensitive pits in their face that can detect the amount of heat and the distance to the heat source. There is still some debate as to what degree this information is perceived as "feeling" heat, and to what degree it is processed as an image by the snake's brain.

Cadillac introduced passive infrared night vision as an option on the 2000 Deville. It sold well initially, but sales dropped and the option was dropped early in the 2005 model year.

Infrared camera

Note: this is just a summary. For more details, see infrared camera.

All objects emit a certain amount of radiation based on their temperature. At regular temperatures, the radiation emitted is in the infrared part of the spectrum, and can be detected by a special camera in the same way that a normal camera detects visible light. An infrared camera shows hot areas as white, and cool objects as black. It is independent of the level of ambient light, and can work in total darkness. This makes it useful for rescue operations in smoke-filled buildings and underground.

Images from infrared cameras tend to be monochromatic, because the cameras are generally designed with only a single type of sensor aimed at a particular wavelength of infrared radiation.

This is not to be confused with "near" IR, as the thermal portion of the spectrum is quite different from the wavelengths that CCDs and remote controls operate in.

Image intensifier

The image intensifier is a vacuum-tube based device that amplifies visible light from an image so that a dimly lit scene can be viewed by a camera or by eye. For more information, see image intensifier.

Night vision goggles

See main article night vision goggles

Patents

• US 248860 - Night vision Pocketscope
• US 4707595 - Invisible light beam projector and night vision system
• US 4991183 - Target illuminators and systems employing same
• US 6075644 - Panoramic night vision goggles