Infrared photography

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Top: tree photographed in the near infrared range. Bottom: same tree in the visible part of the spectrum.

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

In infrared photography, the 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. Usually an "infrared filter" is used; this lets infrared (IR) light pass through to the camera but blocks all or most of the visible light spectrum (and thus looks black or deep red).

When these 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 known as the "Wood Effect."

The effect is mainly caused by foliage (such as tree leaves and grass) strongly reflecting in the same way visible 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 millimeters into skin and give a milky look to portraits, although eyes often look black.

1 History
2 Focusing infrared
3 Film cameras
- 3.1 Black-and-white infrared film
- 3.2 Color infrared film
4 Digital cameras
5 Notes
6 External links

[edit] History

Until the early 1900s, infrared photography was not possible because silver halide emulsions are not sensitive to infrared radiation without the addition of a dye to act as a color sensitizer^[1]. The first infrared photograph was published in 1910 by Robert W. Wood, who discovered the unusual color effects that now bear his name^[2]. Wood's photographs were taken on experimental film that required very long exposures; thus, most of his work focused on landscapes.

Infrared-sensitive photographic plates were developed in the United States during World War I for improved aerial photography.^[3]

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; 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.

A near-infrared photograph of a Ringling Brothers' train idling near MIT in Cambridge, Massachusetts

[edit] Focusing infrared

Most manual focus 35mm SLR and medium format SLR lenses have a red dot, line or diamond, often with a red "R" called the infrared index mark, that can be used to achieve proper infrared focus (some autofocus lenses no longer have the mark). When a single-lens reflex (SLR) camera is fitted with a filter that is opaque to visible light, the reflex system becomes useless for both framing and focusing, and a tripod is necessary for composition without the filter before the exposure is done with the filter attached. A sharp infrared photograph can be done with a tripod, a narrow aperture (like f/22) and a slow shutter speed without focus compensation, however wider apertures like f/2.0 can produce sharp photos if the lens is meticulously refocused to the infrared index mark.

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 require this adjustment because mirrors do not suffer from chromatic aberration.

Zoom lenses may scatter more light through their more complicated optical systems than prime lenses, i.e. an infrared photo taken with a 50mm prime lens may look more contrasty and sharper than the same image taken at 50mm with a 28 to 80 zoom.

[edit] Film cameras

Infrared negatives fogged by the frame counter of a Minolta Maxxum 4

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.

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 light.

[edit] 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 a sensitivity to blue light wavelengths. The notable halation effect or glow often seen in the highlights of infrared photographs is an artifact of Kodak High Speed Infrared (HIE) black-and-white negative film and not an artifact of infrared light. The glow or blooming is caused by the absence of an anti-halation layer on the back side of Kodak HIE film, this results in a scattering or blooming around the highlights that would usually absorbed by the anti-halation layer in conventional films.

Frank Lloyd Wright's Rudin House: panchromatic film on the left, infrared on the right

The majority of black-and-white infrared art, landscape, and wedding photography is done using orange (15 or 21), red (23, 25, or 29) or visually opaque (72) filters over the lens to block the blue visible light from the exposure. The intent of filters in black-and-white infrared photography is to block blue wavelengths and allow infrared to pass through. 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. Some photographers use orange or red filters to allow slight amounts of blue wavelengths to reach the film, and thus lower the contrast. Very dark-red (29) filters block out almost all blue, and visually opaque (70, 89b, 87c, 72) filters block out all blue and also visible-red wavelengths, resulting in a more pure-infrared photo that usually looks more contrasty.

Certain infrared-sensitive films like Kodak HIE must only be loaded and unloaded in total darkness. Infrared black-and-white films require special development times but development is usually achieved with standard black-and-white film developers and chemicals (like D-76). Kodak HIE film has a polyester film base that is very stable but extremely easy to scratch, therefore special care must be used in the handling of Kodak HIE throughout the development and printing/scanning process to avoid damage to the film.

Arguably the greatest obstacle to infrared film photography is the increasing difficulty of obtaining infrared-sensitive film; where it is still sold the price can be twice as much as it was before the digital camera era.

[edit] Color infrared film

Color infrared transparency films are commonly used with a yellow filter to correct the magenta color casts found when no filtration is used. Early color infrared films were developed in the rare E-4 process, but Kodak today manufactures a color transparency film that can be developed in standard E-6 chemistry. In general, color infrared does not need to be loaded in total darkness, or refocused to the infrared index mark on the lens.

[edit] Digital cameras

Bending Cypress: infrared shot by Sigma SD10 with B+W 093 filter, ISO 100, f/8, 1/160 s. Photo by Seng P. Merrill

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.^[4] 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. Some lenses will also show a 'hot spot' in the centre of the image as their coatings are optimised for visible light and not for IR.

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. Metering works but is not always accurate because of the difference between visible and infrared reflection.^[5]

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 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 result is a very sensitive digital IR camera .

A Nikau palm against a clear, sunny sky shows the high contrast that is often typical of outdoors infrared photography.

Several Sony cameras have the so-called Night Shot facility, which physically moves the blocking filter away from the light path, which makes the cameras very sensitive to infrared light. Soon after its development, this facility was 'crippled' by Sony to make it difficult for people to take photos that saw through clothing. To do this the iris is opened fully and exposure duration is limited to long times of more than 1/30 second or so. It is possible to shoot infrared but neutral density filters must be used to reduce the camera's sensitivity and the long exposure times mean that care must be taken to avoid camera-shake artefacts.

A Fuji camera has been produced for use in forensic criminology and medicine which has no blocking filter and extended ultraviolet sensitivity (digital sensors are usually less sensitive to UV than to IR). The camera, designated the S3 PRO UVIR is only available in the USA and is modified by Fuji US. Optimum UV sensitivity requires special lenses but ordinary lenses usually work well for IR. Stocks of the S3 are limited because it is due to be replaced by the S5 in 2007, which may limit availability of the UVIR version.

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. In particular, they often require cooling, since at these wavelengths, and room temperature, all objects (including the camera body, the optics, and the detector itself) are glowing all the time (see thermal radiation).