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Night-vision is seeing in the dark. Its technology 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.

Infrared camera

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

All objects emit a certain amount of infrared radiation. Generally speaking, objects that are hot by human standards emit more of it than those that are cold. This radiation 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.

Modern digital cameras are based on CCD chips. By the nature of the materials used in them, these chips are sensitive not only to visible light but also to infrared radiation. For example, the camera can 'see' the light of your TV remote control when pressed.

Many modern digital cameras have a filter to remove infrared light to avoid the colours being wrong.

Some other cameras have a special night mode or night vision mode, where an infrared LED illuminates the scene and the camera is in infrared (only) mode. The resulting picture is black-and-white.

A so-called X-ray effect has been described using an infrared filter (800-1000nm wave length) with night-mode in bright sunlight. Clothes then appear semi-transparent.

Other night vision devices (night vision goggles) are in military use and are also based on CCD chips.

Some animals, such as cats and some fish, use infrared vision to see at night.

Image intensifier

The image intensifier is a device that amplifies visible light from an image so that a dimly lit scene can be viewed by a camera or by eye. Unlike an infrared camera, an image intensifier does not work in total darkness, when there is no light to amplify. It does, however, create a more realistic image, because the intensities it shows are related to true optical intensity and not to temperature. This realism makes it more suitable for use by untrained operators.

One technique of accquiring an amplified image is to use digital equipment using ICCDs for image capturing.