Electromagnetic radiation, which has a wavelength just above what we see as red light, is called infrared radiation. Film that is sensitive to this radiation is called infrared film. All film is sensitive to ultraviolet rays and infrared film is also sensitive to visible light. All normal sources of light have an infrared component in them. A bright sun has more infrared, and more at sunrise and sunset than the middle of the day. When using the film it's best to stay away from dull or wet situations as there will be minimal infrared present.
Flash is a good source of infrared along with tungsten light. To get the best effects with infrared film it should be exposed using a deep red or opaque black (87C) filter. To properly use infrared film you need to have infrared radiation falling on objects you are going to photograph as a light source. This is the same as using visible light in regular photography. Tree leaves are fine reflectors of infrared, but blue skies don't really reflect it. It is good film for scientific purposes and surveillance operations. The film can produce unusual and unreal tonality as clear skies can come out dark while clouds would retain their normal tones.
Infrared film is available in black and white and color and in various speeds. It costs about $10 for a black and white roll of 36 exposures.
