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