Infrared photography

alt=This comparison shows two photographs of the same tree, with the top image in near infrared and the bottom image in visible light.|thumb|This comparison shows two photographs of the same tree, with the top image in near infrared and the bottom image in visible light.

In infrared photography, the photographic film or image sensor used is sensitive to infrared light. The part of the spectrum used 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. Film is usually sensitive to visible light too, so an infrared-passing filter is used; this lets infrared (IR) light pass through to the camera, but blocks all or most of the visible light spectrum. These filters thus look black (opaque) or deep red.

When these filters are used together with infrared-sensitive film or sensors, "in-camera effects" can be obtained, false-color or black-and-white images with a dreamlike or sometimes lurid appearance known as the Wood effect, an effect mainly caused by foliage (such as tree leaves and grass) strongly reflecting infrared in the same way visible light is reflected from snow. There is a small contribution from chlorophyll fluorescence, but this is marginal and is not the real cause of the brightness seen in infrared photographs. The effect is named after the infrared photography pioneer Robert W. Wood, and not after the material wood, which does not strongly reflect infrared.

The other attributes of infrared photographs include very dark skies and penetration of atmospheric haze, caused by reduced Rayleigh scattering and Mie scattering, respectively, 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 millimeters into skin and give a milky look to portraits, although eyes often look black.

History

File:Minnesota Lock and Dam 1 006-062-02-1994.jpg|False-color infrared image of the Mississippi River crossed by a bridge and a dam, between red foliage on left, and green parking lots and buildings on right

File:Visible Spectrum vs IR.jpg|Visible and infrared (900 nm LP) aerial photography of Old Hickory Lake, Tennessee. Taken from a passenger airplane within seconds apart using a Sony H-9 Digital camera.

File:Bnb train 2 bnw.jpg|A near-infrared photograph of a Ringling Brothers' circus train idling near MIT in Cambridge, Massachusetts

File:Pagny-le-Château 2013 08 21 01 IR M8.jpg|Digital infrared photograph showcasing the Wood effect on vegetation

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Until the early 20th century, infrared photography was not possible because silver halide emulsions are not sensitive to longer wavelengths than that of blue light (and to a lesser extent, green light) without the addition of a dye to act as a color sensitizer. The first infrared photographs (as distinct from spectrographs) to be published appeared in the February 1910 edition of The Century Magazine and in the October 1910 edition of the Royal Photographic Society Journal to illustrate papers by Robert W. Wood, who discovered the unusual effects that now bear his name.

The RPS co-ordinated events to celebrate the centenary of this event in 2010. Wood's photographs were taken on experimental film that required very long exposures; thus, most of his work focused on landscapes. A further set of infrared landscapes taken by Wood in Italy in 1911 used plates provided for him by C. E. K. Mees at Wratten & Wainwright. Mees also took a few infrared photographs in Portugal in 1910, which are now in the Kodak archives.

Infrared-sensitive photographic plates were developed in the United States during World War I for spectroscopic analysis, and infrared sensitizing dyes were investigated for improved haze penetration in aerial photography. After 1930, new emulsions from Kodak and other manufacturers became useful to infrared astronomy.

thumb|left|[[Album cover of Are You Experienced (1967) by The Jimi Hendrix Experience; the photographer captured the trio with a fisheye lens on color infrared film]]

Infrared photography became popular with photography enthusiasts in the 1930s, when suitable film was introduced commercially. The Times regularly published landscape and aerial photographs taken by their staff photographers using Ilford infrared film. By 1937, 33 kinds of infrared film were available from five manufacturers including Agfa, Kodak and Ilford.

Infrared movie film was also available and was used to create day-for-night effects in motion pictures. A notable example being the pseudo-night aerial sequences in the movie The Bride Came C.O.D., starring James Cagney and Bette Davis.

False-color infrared photography became widely practiced with the introduction of Kodak Ektachrome Infrared Aero Film and Ektachrome Infrared EIR. The first version of this, known as Kodacolor Aero-Reversal-Film, was developed by Clark and others at the Kodak for camouflage detection in the 1940s. The EIR film became more widely available in the form of 35 mm film in the 1960s but has been since discontinued.

Infrared photography became popular with a number of 1960s recording artists, because of the unusual results; Jimi Hendrix, Donovan, Frank Zappa and the Grateful Dead all issued albums with infrared cover photos. The unexpected colors and effects that infrared film can produce fit well with the psychedelic aesthetic emerging in the 1960s.

Techniques and special equipment

Infrared filters

thumb|Infrared-passing filters

Infrared light lies between the visible and microwave portions of the electromagnetic spectrum. Infrared light has a range of wavelengths, just like visible light has wavelengths that range from red light to violet. "Near infrared" light is closest in wavelength to visible light, ranging from approximately 700 to 5000 nm, and "far infrared" is closer to the microwave region of the electromagnetic spectrum, ranging from approximately 25 to 350 μm. The longer, far infrared wavelengths are about the size of a pinhead and the shorter, near infrared ones are the size of cells, or are microscopic.

Historically, black-and-white infrared films are sensitive to near infrared wavelengths shorter than approximately 860 nm, and retain significant sensitivity to blue wavelengths. Infrared-passing filters are used in black-and-white infrared photography to block blue wavelengths and limit the photograph to infrared wavelengths only. Without filters, infrared negative films look much like conventional negative films because the blue sensitivity lowers the contrast and effectively counteracts the infrared look of the film. Typically, a red filter (Wratten #25) is recommended as the best compromise, which removes blue wavelengths while still passing enough visible light for focusing.

! No. !! 50% (nm)

|-style="background:#f90;"

| 15 || 530

|-style="background:#f20;"

| 21 || 560

|-style="background:#f00;color:#fff;"

| 23A || 580

|-style="background:#d00;color:#fff;"

| 25 || 600

|-style="background:#a00;color:#fff;"

| 29 || 620

|-style="background:#600;color:#fff;"

| 70 || 675

|-style="background:#400;color:#fff;"

| 89B || 720

|-style="background:#200;color:#fff;"

| 88 || 735

|-style="background:#200;color:#fff;"

| 72B || 740