Infrared photography

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 blue 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 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. 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. 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 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. and RM90 (900 nm cutoff) filters for infrared photography. B+W (Schneider Kreuznach) and Heliopan sell filters using glass sourced from Schott AG, including types RG695 (695 nm transition, considered approximately equivalent to Wratten #89B), RG715 (715 nm, #88A), RG780 (780 nm, #87), RG830, RG850, and RG1000. Focusing infrared ; the infrared index mark is the small red dot under one yellow hash mark for hyperfocal range at . Many manual focus lenses for 35 mm single-lens reflex cameras (SLR) 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 (like ); 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 ('APO') 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 (their manuals may warn against the use of infrared film for this reason). 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 (2006), Kodak High-Speed Infrared (2007), 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. Black-and-white infrared film Black-and-white infrared negative films are sensitive to wavelengths in the 700 to 900 nm near infrared spectrum, and most also have an inherent sensitivity to blue light wavelengths. Kodak High-Speed Infrared (HIE), which produced negatives for photographic prints, was one of the most common black-and-white infrared films used. Because HIE was so prevalent, black-and-white infrared photographs have been associated with a notable halation effect or glow often seen in the highlights, similar to the soft focus effect of uncorrected spherical aberration. • Fotokemika/Efke IR 820 (also sensitive to approximately 800 nm) • Ilford SFX 200 ("extended" sensitivity in the near-infrared range to 740 nm) • Konica Infrared 750 (sensitive to approximately 800 nm) Color infrared film Like HIE, the most commonly used infrared color reversal film, also called transparency or slide film, was manufactured by Kodak and sold as Ektachrome Infrared (EIR), under code 2236, packaged as a 36-exposure roll; in addition, Kodak made EIR in bulk lengths (for the motion picture industry) and the similar Aerochrome III Infrared for aerial photography (codes 1443 and SO-734). and was adopted by the military and scientific communities after the war. It was not marketed to consumers until the 1960s. The structure of all color reversal films (both standard and infrared-sensitive) contains at least three separate photosensitive layers. Each layer is specifically sensitized to respond to a different set of wavelengths; for instance, a standard color reversal film has red-, green-, and blue-sensitive layers. During the modern E-6 process of development, grains of silver halide that were sensitized by the appropriate wavelengths of light in each layer react with a reducing agent to form metallic silver particles. The unexposed grains are then sensitized chemically during a second development step and produce oxidized developer, which react with dye coupler compounds embedded in the film emulsion layers to form negative images in various color dyes, respective to how the silver halide was originally sensitized for each layer. In conventional color films, the topmost (blue-sensitive) layer gets exposed to light prior to the green- and red-sensitive layers stacked behind it. Since the green- and red-sensitive layers also retain an inherent sensitivity to blue light, a yellow filter layer is placed behind the blue-sensitive layer, in front of the green and red-sensitive layers. This serves to minimize undesired passthrough of shorter wavelengths that are not supposed to expose the bottom layers. During development, each emulsion layer forms a negative image in the appropriate subtractive color (cyan-magenta-yellow): the blue-sensitive layer forms a yellow-dyed ("minus-blue") negative image, the green-sensitive layer forms a magenta-dyed negative image, and the red-sensitive layer forms a cyan-dyed negative image. When the slide is viewed or projected by passing white light through these stacked layers, the visible wavelengths are filtered correspondingly with the reversed colors. For example, blue light will result in no yellow dye formation in the blue-sensitive layer, but cyan and magenta dye will form in the red- and green-sensitive layers. By projecting white light through the combined layers, blue is produced: cyan (aka negative-red) removes red and passes blue and green, and magenta (aka negative-green) removes green and passes blue and red; when these layers are stacked, only blue light is passed. Since silver halides are sensitive to wavelengths of light outside of the visible range of the electromagnetic spectrum, longer wavelengths corresponding with infrared light can be captured by using suitable dyes. Without specialized dyes, silver halides are only sensitive to a wavelength shorter than around 450 nm. Color infrared reversal films share a similar three-layer emulsion structure with conventional color reversal films, with the blue-sensitive layer replaced by an infrared-sensitive layer, and different dyes used for each of the layers. An external yellow photographic filter is used (Wratten #12 or equivalent) to block the blue and violet wavelengths, which results in a false-color image by translating or remapping the captured spectrum (from green through infrared) to the visible spectrum: Infrared wavelengths are mapped to the red color, even though the infrared wavelengths are not normally visible. Similarly, visible red wavelengths are remapped to green, and visible green band wavelengths are remapped to blue. The filter and color remapping means visible blue and violet wavelengths are not captured. The infrared-sensitive layer will form cyan dyes (negative-red), while the green-sensitive layer will form yellow dyes (negative-blue) and the red-sensitive layer will form magenta dyes (negative-green). The external yellow filter is used because each emulsion layer in color films (both conventional and infrared) has an inherent sensitivity for short-wavelength radiation (blue and violet visible wavelengths of light) due to the silver halide chemistry. Since there is no blue-sensitive layer, color infrared films also omit the internal yellow filter layer built into conventional color films to protect the following layers. This requires photographers to use an external blue-blocking filter to absorb blue and violet wavelengths of light, which gives the filter a yellow color. File:Hollywood Hills California.jpg|View of the Hollywood Hills. Kodak Infrared color slide film, no filter used and developed with E-6 process File:Color infrared SFOSEAYYZ-tree.jpg|An example of color infrared File:Dülmen, Lüdinghauser Tor -- 1981 -- 0011.jpg|Lüdinghauser Tor in Dülmen (1981); remapped false color represents infrared wavelengths (as reflected by foliage) as shades of red File:Ribeira da Janela Valley on Madeira Island, view from Levada, Infrared False-Color Film (10-79.c).jpg|Ribiera da Janela Valley on Madeira Island, using Kodak EIR (2003) File:Bluebeard's Castle, St Thomas 1980, Infra Red Film Analog.jpg|Bluebeard's Castle, Charlotte Amalie (1980) Early color infrared films were developed in the older E-4 process, but Kodak later manufactured a color transparency film that could be developed in standard E-6 chemistry, although more accurate results were obtained by developing using the AR-5 process. Like HIE, EIR uses a clear polyester film base and must be loaded in complete darkness. In general, color infrared does not need to be refocused to the infrared index mark on the lens. At the time it was discontinued, HIE Infrared 135-36 was available at a street price of around $12.00 a roll at US mail order outlets. Also in 2007, Kodak announced that production of the 35 mm version of their color infrared film (Ektachrome Professional Infrared/EIR) would cease as there was insufficient demand. In 2008, Los Angeles photographer, Dean Bennici started cutting and hand rolling Aerochrome color Infrared film. Most Aerochrome medium and large format which exists today came directly from his lab. The trend in infrared photography continues to gain momentum with the success of photographer Richard Mosse and multiple users all around the world. Since 2011, all formats of color infrared film have been discontinued. Specifically, Aerochrome 1443 and SO-734. ==Digital cameras==

File:Montreal-in-infrared.jpg|A view in infrared of Montréal as seen from Mont Royal, taken with filter made of a floppy disk File:SD10 IR Bending Tree.jpg|Bending Cypress: infrared shot by Sigma SD10 with B+W 093 filter, ISO 100, , 1/160 s File:Succulents in a CT Greenhouse II.jpg|Digital infrared using a 50mm lens, D810 and the program Exposure. Infrared photography typically produces false-color artifacts, such as turning greens into pinks and purples as shown in this example. Digital camera sensors are inherently sensitive to infrared light, which could interfere with normal photography by confusing the autofocus calculations, because infrared light wavelengths may focus at a different point than visible light wavelengths, or by softening the image, if the red channel becomes oversaturated. Also, some clothing is transparent in the infrared, leading to unintended (at least to the manufacturer) uses of video cameras. Similar effects can be achieved by taking two exposures, one infrared and the other full-color, and combining in post-production. A yellow (minus-blue) filter can also be used, which produces a single image that can also be post-processed to emulate the Ektachrome look. The color images produced by digital still cameras using infrared-pass filters are not equivalent to those produced on color infrared film. The colors result from varying amounts of infrared passing through the color filters on the photo sites, further amended by the Bayer filtering. While this makes such images unsuitable for the kind of applications for which the film was used, such as remote sensing of plant health, the resulting color tonality has proved popular artistically. Color digital infrared, as part of full spectrum photography is gaining popularity. The ease of creating a softly colored photo with infrared characteristics has found interest among hobbyists and professionals. Hot mirror removal One method of infrared photography using digital cameras is to remove the infrared blocker in front of the sensor and replace it with a glass cover that either removes or restricts visible light (infrared-only conversion) or one that passes infrared wavelengths ("full spectrum" conversion). This filter is behind the mirror of DSLRs, so the camera can be used normally - handheld, normal shutter speeds, normal composition through the viewfinder, and focus, all work like a normal camera. Metering works but is not always accurate because of the difference between visible and infrared refraction. When the IR blocker is removed, many lenses which did display a hotspot cease to do so, and become perfectly usable for infrared photography. Additionally, because the red, green and blue micro-filters remain and have transmissions not only in their respective color but also in the infrared, enhanced infrared color may be recorded. Since the Bayer filters in most digital cameras may also absorb a significant fraction of the infrared light, converted cameras are sometimes not very sensitive to infrared wavelengths and can sometimes produce false colors in the images. An alternative approach is to use a Foveon X3 sensor, which does not have absorptive filters on it; the Sigma SD10 DSLR has a removable IR blocking filter and dust protector, which can be simply omitted or replaced by a deep red or complete visible light blocking filter. The Sigma SD14 has an IR/UV blocking filter that can be removed/installed without tools. The result is a very sensitive digital IR camera. While it is common to use a filter that blocks almost all visible light, the wavelength sensitivity of a digital camera without internal infrared blocking is such that a variety of artistic results can be obtained with more conventional filtration. For example, a very dark neutral density filter can be used (such as the Hoya ND400) which passes a very small amount of visible light compared to the near-infrared it allows through. Wider filtration permits an SLR viewfinder to be used and also passes more varied color information to the sensor without necessarily reducing the Wood effect. Wider filtration is however likely to reduce other infrared artefacts such as haze penetration and darkened skies. This technique mirrors the methods used by infrared film photographers where black-and-white infrared film was often used with a deep red filter rather than a visually opaque one. Post-processing Another common technique with near-infrared filters is to swap blue and red channels in software (e.g. Adobe Photoshop), which retains much of the characteristic "white foliage" while rendering skies a glorious blue. Phase One digital camera backs can be ordered in a modified form suited for infrared photography. ==Applications and specific implementations==

File:Near infra-red kite aerial photo at Rufford Abbey, Nottinghamshire, UK.jpg|An example of the use of infrared photography in aerial archaeology to delineate sub-surface features File:Series 2009 Five Dollar Bill in Infrared.jpg|The reverse of the United States five-dollar bill has two rectangular strips that are blanked out when viewed in the infrared spectrum, as seen in this image taken by an infrared camera. File:Cooktop Ceran IR.JPG|Nightshot infrared photography of a stove with 600 nm red-filter and polarizing filter at daylight File:Wikipedia Coffee Shot.jpg|Thermographic image of coffee cup File:IC 10 (infrared, red, green).jpg|The starburst galaxy IC 10 is hidden behind dust in the Milky Way, but infrared light is less strongly absorbed by dust than visible light. Green patches in the galaxy are its star-forming H II regions. The health of foliage can be determined from the relative strengths of green and infrared light reflected using color infrared film; this shows in color infrared as a shift from red (healthy) towards magenta (unhealthy). Several Sony cameras had a feature branded as Night Shot, which physically moves the blocking filter away from the light path, making the cameras very sensitive to infrared light. Soon after its development, this facility was 'restricted' by Sony to make it difficult for people to take photos that saw through clothing. Remote sensing and thermographic cameras are sensitive to longer wavelengths of infrared (see ). They may be multispectral and use a variety of technologies which may not resemble common camera or filter designs. Cameras sensitive to longer infrared wavelengths including those used in infrared astronomy often require cooling to reduce thermally induced dark currents in the sensor (see Dark current (physics)). Lower cost uncooled thermographic digital cameras operate in the Long Wave infrared band (see Thermographic camera). These cameras are generally used for building inspection or preventative maintenance but can be used for artistic pursuits as well, such as this image of a cup of coffee. ==See also==