IR photography is an abbreviation for infrared photography. Infrared photography captures images using infrared radiation, which is invisible to the naked eye and usually undetectable by standard cameras. Film-based cameras achieved IR photography by employing specialized filters, film, or both; digital cameras can detect infrared radiation, but have built-in devices to neutralize it. Infrared photography creates interesting and sometimes unintended visual effects. One kind of infrared photography detects the subject’s heat and depicts it visually, a technique called thermography.
Visible light is part of a wide range of omnipresent radiation known as the electromagnetic spectrum. The human eye can detect only the visible part of this spectrum; other segments, such as radio waves, can be detected with the proper equipment. The color of light depends on the wavelength of its radiation, with the longest wavelengths appearing red and the shortest ones appearing blue. Infrared radiation exists just beyond the red end of the visible spectrum; hence its name. Specialized devices can detect this radiation and convert it into a form visible to the human eye.
Film cameras require special filters to photograph IR images; these lenses appear completely black, because they filter out all visible light and detect only infrared radiation. Traditional IR photography also involved IR-sensitive film that required special handling, because it could be ruined by the infrared radiation in an ordinary darkroom. Most infrared film was discontinued in the early 21st century. Digital image sensors automatically detect infrared radiation, so digital cameras are usually designed with filters to remove it from an image. Digital camera users can practice IR photography through special techniques involving long exposures with a tripod; it is also possible to remove the IR filter, but this is recommended only for experts.
With a wavelength unlike that of visible light, IR radiation can create some unusual and aesthetically pleasing effects. For example, haze and fog scatter visible light, but not infrared radiation, so infrared photography can create a clear picture on hazy days. Leaves and grass appear to glow, a phenomenon called the Wood effect, after IR photography pioneer Robert Wood. Skies will turn black, but clouds will appear unnaturally bright and vivid. IR radiation can also penetrate cotton clothing and render it transparent, an unintended side effect that caused several hasty redesigns during the early years of digital cameras.
The effects described above apply to near-infrared photography, that is, photography based on the IR radiation closest to visible light. Far-infrared photography, on the other hand, detects the heat generated by the photo’s subject and assigns it a color so that the windshield of a car may appear blue while its engine compartment appears red. This kind of IR photography is called thermography, and it has a wide range of industrial and scientific uses. Military forces use it for covert surveillance, while firefighters can use it to navigate through burning buildings. In astronomy, thermography can detect distant celestial bodies that would be invisible to ordinary telescopes.