Infrared radiation is part of electromagnetic spectrum which permeates our physical world. The major source of infrared used for photographic purposes is the sun. It produces by far the most infrared. There are also other sources that we can use for photography, however. These would include tungsten bulbs, flash bulbs, and electronic flash.
Infrared films are available in a variety of sizes.
The two most common of which are 35mm and 4x5in. Their color transparency version is called Aerochrome Infrared Film. Development times shorter than 5 minutes may produce unsatisfactory uniformity. For maximum contrast Color Infrared Transparency Film Color infrared film is available to the public only in 35mm transparency format. With color you can focus as normal because the film is imaging visible light as well as infrared.