Understanding Color-Infrared Photographs

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					Understanding Color-Infrared Photographs
Images obtained by satellites and high-       apart and are analogous to light wave-
altitude aircraft give engineers and scien-   lenths and how closely they are spaced.
tists a tool to study landforms, vegetation
health patterns, environmental pollution,     Other parts of the spectrumCsuch as the
and other effects of human activities on      invisible near-infrared wavelengthsCcan
the planet’s surface.                         be recorded by either electronic sensors or
                                              special photographic films sensitive to
Satellites and high-altitude aircraft         these wavelengths. These sensors and
equipped to record scenes of the Earth use    films record the energy reflected by the
both visible and invisible parts of the       ground and the Sun’s spectral energy. The
electromagnetic spectrum. Near-infrared       color-infrared film images referred to in
light is invisible to the human eye, but      this fact sheet should not be confused
adding it to these images allows scientists   with electronic thermography (thermal
to “see” the surface of the Earth in other    recordings), a process in which long-wave
than natural colors. The result is “color-    or “far-infrared” radiation is electronically
infrared” photography.                        detected and subsequently displayed at
                                              visible wavelengths. Near-infrared and
The electromagnetic spectrum is the sci-      visible wavelengths that are simultaneous-
entific term for the collective types of      ly recorded combine to provide a unique
light and energy emitted from the Sun.        view of the Earth’s vegetation and other        NASA color-infrared photograph--New Orleans, La.

The part of the spectrum visible to the       features of the planet’s surface.
human eye is the normal rainbow of col-                                                       infrared film. Unhealthy or dormant vege-
ors we see every day. Passing sunlight        This unique aerial view, created by a           tation may appear light red or a light
shining through a prism separates white       combination of wavelengths, gives scien-        shade of blue-green (cyan), depending on
light into individual colors, just as sun-    tists a means to better understand what is      the plant’s degree of good health. These
light through raindrops creates a rainbow.    happening on the Earth’s surface. For           color distinctions make color-infrared
More technically, a prism divides light       example, leaves of healthy, growing vege-       photographs useful in assessing the health
into its component wavelengths. Ripples       tation reflect a high level of near-infrared    of plants. Water, on the other hand,
on a lake can be close together or far        wavelengths and appear red on color-            absorbs near-infrared wavelengths and
                                                                                              appears black in the image. Water with
                                                                                              varying amounts of suspended particles
                                                                                              appears as shades of blue. Also, near-
                                                                                              infrared wavelengths penetrate atmos-
                                                                                              pheric haze and result in clear, crisp
                                                                                              images. This is an important consideration
                                                                                              when collecting satellite images and high-
                                                                                              altitude aerial photographs.

                                                                                              Satellite electronic sensors and aerial
                                                                                              color-infrared films both record visible
                                                                                              and near-infrared wavelengths, but each
                                                                                              of these systems requires different labora-
                                                                                              tory processes. Here is how they work.

                                                                                              Color-Infrared Photographs

                                                                                              Both standard-color and color-infrared
                                                                                              films are manufactured to have three dis-
                                                                                              tinct layers, or emulsions. Each layer is
Color photograph--near Burlington, Vt.        Color-infrared photograph--same area.

U.S. Department of the Interior                                                                                                 USGS Fact Sheet 129-01
U.S. Geological Survey                                                                                                                 December 2001
sensitive to different wavelengths or ener-    How to Find More Information
gy. Standard-color film emulsions normal-
ly record the visible wavelengths as red,      For information about ordering color-
green, and blue. After the picture has been    infrared composites or color-infrared aeri-
taken, chemical processing of the film         al photographs, please contact any Earth
generates cyan, magenta, and yellow dyes       Science Informatioon Center, or call
proportional to the amount of exposure         1-888-ASK-USGS.
given each layer. Color pictures result
when the human eye views the varying           More information on color-infrared tech-
combinations of the three dye layers.          nology is available from many libraries.
Color-infrared film has a yellow filter
over the three emulsion layers to block        Among the many books exploring this
untraviolet (UV) and blue wavelengths.         subject are the following:
Processing color-infrared film after expo-     M.M. Thompson’s Maps for America,
sure produces yellow, magenta, and cyan        U.S. Geological Survey (Government
dyes. The near-infrared wavelengths and        Printing Office, Washington, D.C., 1987),
the lack of UV and blue wavelengths            and Robert K. Holtz’s The Surveillant         Landsat color-infrared composite--San Franscisco, Calif.
result in a clear, crisp color-infrared        Science: Remote Sensing of the
image. Green, healthy vegetation has a         Environment (John Wiley and Sons,
high reflection level of near-infrared         1985). For more technical information,
wavelengths and appears red on the             you could consult the Manual of Remote
processed film; red objects with very low      Sensing, published by the American
near-infrared reflection appear green;         Society of Photogrammetry and Remote
green objects with very low near-infrared      Sensing, 5410 Grosvenor Lane, Bethesda,
reflection appear blue; and blue objects       MD 20814-2160.
with very low near-infrared reflection
appear black.                                  For information on other USGS products
                                               and services, call 1-888-ASK-USGS, or
The Federal Government has color-              visit the general interest publications Web
infrared photographic coverage of the          site on mapping, geography, and related
entire United States from a high altitude      topics at
(40,000 feet) and is obtaining similar cov-
erage at a lower altitude (20,000 feet) for    For additional information, visit the
many States.                          Web site or the USGS home
                                               page at
Color-Infrared Composite Images

Another type of color-infrared image is                                                      NASA color-infrared photograph--San Diego, Calif.
the color-infrared composite of multispec-
tral data collected by electronic sensors
on satellites such as Landsat. These sen-
sors record the light levels of Earth's
reflected energy (from blue/green wave-
lengths through infrared wavelengths) and
transmit these data in digital format to the
ground in sets of four or seven wave-
length-dependent bands for each typical
Landsat scene. On the ground, the digital
image data may be converted to hardcopy
images similar in appearance to conven-
tional color-infrared photographs.
Computerized image-recording devices
process the bands of green, red, and near-
infrared digital data, exposing conven-
tional color film or paper with blue,
green, and red light, respectively. In the
resulting image, growing healthy vegeta-
tion appears red, clear water appears
black, sediment-laden water appears light
                                                                                             Landsat color-infrared composite--Williams, Calif.,
blue, and urban areas appear blue-gray.                                                       quadrangle.

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