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In infrared photography, the film or CCD sensor used is sensitized to infrared light. This part of the infrared spectrum is referred to as near-infrared to distinguish it from far-infrared, which is the domain of thermal imaging. Wavelengths used for photography range from about 700 nm to about 900 nm. 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).
When these band-pass filter filters are used together with infrared-sensitive film or sensors, very interesting "in-camera effects" can be obtained: false-color or black and white images with a dreamlike or sometimes lurid appearance.
The effect is mainly caused by foliage (like tree leaves and grass) strongly reflecting in the same way light is reflected from snow. Chlorophyll is transparent at these wavelengths and so does not block this reflectance. 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. These wavelengths also penetrate a few millimetres into skin and give a milky look to portraits, although eyes often look black. The notable (or notorious ... 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.
Infrared photography was popular with 1960s recording artists, because of the unusual results; Jimi Hendrix, Donovan and the Grateful Dead all issued albums with infrared cover photos. Infrared photography can easily look gimmicky, but photographers such as Elio Ciol have made subtle use of black-and-white infrared-sensitive film.
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. But when the lens of an SLR camera is fitted with a filter that is opaque to visible light, the reflex system becomes useless for both framing and focusing; a tripod may be necessary depending on the effect the photographer is going for. 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. Wider apertures like f/8 might produce fuzzy, if not completely blurry, pictures.
With some patience and ingenuity, most film cameras can be used. 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. Certain infrared-sensitive films may only be loaded and unloaded in darkness, and they require special development. But arguably the greatest obstacle to infrared film photography is the increasing difficulty of obtaining infrared-sensitive film.
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. Thus, to improve image quality, 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.
Satellite sensors which are sensitive to infrared use a variety of technologies which may not resemble common camera or filter designs.
External links
- All you ever wanted to know about digital UV and IR photography, but could not afford to ask
- Enhance IR photos - Article in EPhotoZine
- Color Infrared Film: Still the real deal Welcome to the wild world of color infrared film.
Commercial links
"Infrared photography"