Infrared photography

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 light pass through to the camera, but blocks all or most of the visible light spectrum. These filters thus look black 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 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

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 without the addition of a dye to act as a color sensitizer. The first infrared photographs 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.
File:Are You Experienced - US cover-edit.jpg|thumb|left|Album cover of Are You Experienced 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

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 is recommended as the best compromise, which removes blue wavelengths while still passing enough visible light for focusing.

No.	50%
15	530
21	560
23A	580
25	600
29	620
70	675
89B	720
88	735
72B	740
88A	750
87	795
87C	850
87B	930
87A	1050

Some photographers use orange or red filters to allow slight amounts of blue wavelengths to reach the film, and thus lower the contrast. The majority of black-and-white infrared art, landscape, and wedding photography is done using orange, red or visually opaque filters over the lens to block the blue visible light from the exposure. Very dark-red filters block out almost all blue, and visually opaque filters block out all blue and also visible red wavelengths, resulting in a more pure-infrared photo with a more pronounced contrast.
Instead of the Wratten number, some manufacturers embed the transition or cutoff wavelength in the name of the filter. For example, Hoya markets the R72 and RM90 filters for infrared photography. B+W and Heliopan sell filters using glass sourced from Schott AG, including types RG695, RG715, RG780, RG830, RG850, and RG1000.

Focusing infrared

Many manual focus lenses for 35 mm single-lens reflex cameras and medium format SLR have a red dot, line or diamond, often with a red "R" called the infrared index mark, which can be used to achieve proper infrared focus; many autofocus lenses no longer have this mark. For these lenses, after visual focus is achieved for the intended subject, the distance indicated by the visual focusing mark is then re-set to the infrared index mark. Without refocusing, a sharp infrared photograph can be taken by proper hyperfocal settings, which generally requires a tripod, a narrow aperture ; however, wider apertures like can produce sharp photos when the lens is meticulously refocused to the infrared index mark, and only if this index mark is the correct one for the filter and film in use. Diffraction effects inside a camera are greater at infrared wavelengths so that stopping down the lens too far may actually reduce sharpness.
Some lens manufacturers such as Leica never put IR index marks on their lenses. The reason for this is that any index mark is only valid for one particular IR filter and film combination, and may lead to user error. Even when using lenses with index marks, focus testing is advisable as there may be a large difference between the index mark and the subject plane.
Most apochromatic lenses do not have an Infrared index mark and do not need to be refocused for the infrared spectrum because they are already optically corrected into the near-infrared spectrum. Catadioptric lenses do not often require this adjustment because their mirror containing elements do not suffer from chromatic aberration and so the overall aberration is comparably less. Catadioptric lenses do, of course, still contain lenses, and these lenses do still have a dispersive property.
When a SLR camera is fitted with a filter that is opaque to visible light, the reflex system becomes useless for both framing and focusing, one must compose the picture without the filter and then attach the filter. This requires the use of a tripod to prevent the composition from changing.
Zoom lenses may scatter more light through their more complicated optical systems than prime lenses, that is, lenses of fixed focal length; for example, an infrared photo taken with a 50 mm prime lens may have more contrast than the same image taken at 50 mm with a 28–80 zoom.

Film cameras

Many conventional cameras can be used for near-infrared photography, where the portion of the infrared is light of a wavelength only slightly longer than that of visible light. Photography of the far-infrared spectrum with longer wavelengths is called thermography and requires special equipment.
With some patience and ingenuity, most film cameras can be used. However, some cameras of the 1990s that used 35 mm film have infrared sprocket-hole sensors that can fog infrared film. Other film cameras are not completely opaque to infrared light.
Arguably the greatest obstacle to infrared film photography has been the difficulty of obtaining infrared-sensitive film. Kodak typically manufactured just one or two batches of infrared films per year. In addition, the popularity of digital photography has pushed film manufacturers to discontinue niche film products, including infrared-sensitive films. The discontinuance of Konica Infrared 750, Kodak High-Speed Infrared, and Efke IR820 Aura has now narrowed the selection for black-and-white infrared film to Agfa Aviphot films, made for areal photography. These films are not available directly to consumers and must be purchased in large quantities and non-consumer formats. This film is often bought by large companies and sold in consumer quantities and formats under different brands, such as Rollei Infrared 400 film.