infrared photography - Film cameras, Digital cameras

Photography which uses wavelengths beyond visible red light. Black-and-white film has medical and forensic application; for example, it is used for camouflage detection, since chlorophyll in green living foliage reflects infrared strongly, unlike visually matching pigments. Multilayer colour film including an infrared sensitive emulsion gives a false colour rendering in which natural vegetation appears red or magenta; this is used in aerial surveying.

In infrared photography, the film or CCD sensor used is sensitized to infrared light. Usually an "infrared filter" is used: this lets infrared (IR) light pass through to the camera but blocks all or virtually all of the visible light spectrum (and thus looks black). There is a small contribution from chlorophyll fluorescence but this is extremely small and is not the real cause of the brightness seen in infrared photographs. The other attributes of infrared photographs include very dark skies and penetration of atmospheric haze, caused by reduced Rayleigh scattering and Mie scattering (respectively) in the atmosphere compared to visible light. The dark skies, in turn, result in less infrared light in shadows and dark reflections of those skies from water, and clouds will stand out strongly. depending on your taste) halation effect often seen in photographs is an artefact of Kodak High Speed Infrared film and not of infrared light per se.

False-color infrared photography became widely practiced with the introduction of Kodak Ektachrome Infrared Aero Film, Type 8443, in the 1960s.

Infrared photography was popular with 1960s recording artists, because of the unusual results; Infrared photography can easily look gimmicky, but photographers such as Elio Ciol have made subtle use of black-and-white infrared-sensitive film.

Film cameras

Many conventional cameras can be used for infrared photography, where infrared is taken to mean light of a wavelength only slightly longer than that of visible light. (Photography of rather longer wavelengths is normally termed thermography and requires special equipment.)

Some lenses have a dot with a red "R" that can be used to guide focusing when photographing infrared and normal light together. If a sharp infrared picture of an object at rest is wanted, it is absolutely necessary to use a tripod, a narrow aperture (like f/22) and a slow shutter speed. However, some cameras of the 1990s that used 35mm film have infrared sprocket-hole sensors that can fog infrared film (their manuals may warn against the use of infrared film for this reason).

Other film cameras are not completely opaque to infrared film. But arguably the greatest obstacle to infrared film photography is the increasing difficulty of obtaining infrared-sensitive film.

Digital cameras

Digital camera sensors are sensitive to infrared light, which would interfere with the normal photography by confusing the autofocus calculations or softening the image (because infrared light is focused differently than visible light), or oversaturating the red channel. Also, some clothing is transparent in the infrared, leading to unintended (at least to the manufacturer) uses of video cameras. Thus, to improve image quality and protect privacy, many digital cameras employ infrared blockers. Infrared photography is usually not practical with these cameras because the exposure times become overly long, creating noise and motion blur in the final image.

An alternative method of digital SLR infrared photography is to remove the infrared blocker in front of the CCD and replace it with a filter that removes visible light. This filter is behind the mirror, so the camera can be used normally - handheld, normal shutter speeds, normal composition through the viewfinder, and focus, all work like a normal camera.

Since the Bayer filters in most digital cameras absorb a significant fraction of the infrared light, these cameras are sometimes not very sensitive as infrared cameras.

Satellite sensors and thermographic cameras are sensitive to longer wavelengths of infrared, and use a variety of technologies which may not resemble common camera or filter designs.