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Understanding Color-Infrared Photographs

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Understanding Color-Infrared Photographs 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 US Department of the Interior USGS Fact Sheet 129-01 US 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, use The Federal Government has color- the Ask.USGS fax service, which is avail- infrared photographic coverage of the able 24 hours a day at 703-648-4888, or entire United States from a high altitude visit the general interest publications Web (40,000 feet) and is obtaining similar cov- site on mapping, geography, and related erage at a lower altitude (20,000 feet) for topics at mac.usgs.gov/mac/isb/pubs/ many States. pubslists/. Color-Infrared Composite Images For additional information, visit the ask.usgs.gov Web site or the USGS home Another type of color-infrared image is page at www.usgs.gov. 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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