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
(the filter thus looks black or deep red).
Light spectrum chart courtesy
of "The Art of Color
Infrared Photography" by Steven H. Begleiter
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.
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, 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.
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 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
that emerged in the late 1960s.
For some, infrared photography can easily look gimmicky,
but many photographers such as Elio
Ciol have made subtle use of black-and-white infrared-sensitive
film. With the advent of digital infrared photography,
as a part of full
spectrum photography, the technique is gaining popularity
and is being sold as fine art photographs in a variety
of galleries worldwide.
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;
many autofocus lenses no longer have this 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 only if 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.
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, that is, lenses of fixed focal length;
for example, an infrared photo taken with a 50mm prime
lens may look more contrasty than the same image taken
at 50mm with a 28–80 zoom.
Some lens manufacturers such as Leica never put IR index
marks on their lenses. The reason for this is because
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.
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 be absorbed by the anti-halation layer in
conventional films. 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
has been the increasing difficulty of obtaining infrared-sensitive
film. However despite the discontinuance of HIE, other
newer infrared sensitive emulsions from EFKE, ROLLEI, and
ILFORD are still available, but these formulations have
differing sensitivity and specifications from the venerable
KODAK HIE that has been around for at least two decades.
Some of these infrared films are available in 120 and larger
formats as well as 35mm, which adds flexibility to their
application. With the discontinuance of Kodak HIE, Efke's
IR820 film has become the only IR film on the market with
good sensitivity beyond 750nm, the Rollei film does extend
beyind 750nm but IR sensitivity falls of very rapidly.
The above was taken from Wikipedia; Infrared
Photography. 2008.
For a good comparison of infrared films and some overall
good information regarding infrared photography, go to vividlight.com.
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