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Understanding Color Infrared (CIR)

Aerial Photography


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Images obtained by satellites and high-altitude aircraft give engineers and scientists a tool to study landforms, vegetation health patterns, environmental pollution, and other effects of human activities on the planet's surface.

Satellites and high-altitude aircraft equipped to record scenes of the Earth use both visible and invisible parts of the electromagnetic spectrum. Near-infrared light is invisible to the human eye, but adding it to these images allows scientists to "see" the surface of the Earth in other than natural colors. The result is "color-infrared" photography.

The electromagnetic spectrum is the scientific term for the collective types of light and energy emitted from the Sun. The part of the spectrum visible to the human eye is the normal rainbow of colors we see every day. Passing sunlight shining through a prism separates white light into individual colors, just as sunlight through raindrops creates a rainbow. More technically, a prism divides light into its component wavelengths. Ripples on a lake can be close together or far apart and are analogous to light wavelenths and how closely they are spaced.

sample color photograph (CIR) showing land, buildings and a river
NASA color-infrared photograph—New Orleans, La.

Other parts of the spectrum—such as the invisible near-infrared wavelengths—can be recorded by either electronic sensors or special photographic films sensitive to these wavelengths. These sensors and films record the energy reflected by the ground and the Sun's spectral energy. The color-infrared film images referred to in this fact sheet should not be confused with electronic thermography (thermal recordings), a process in which long-wave or "far-infrared" radiation is electronically detected and subsequently displayed at visible wavelengths. Near-infrared and visible wavelengths that are simultaneously recorded combine to provide a unique view of the Earth's vegetation and other features of the planet's surface.

This unique aerial view, created by a combination of wavelengths, gives scientists a means to better understand what is happening on the Earth's surface. For example, leaves of healthy, growing vegetation reflect a high level of near-infrared wavelengths and appear red on color-infrared film. Unhealthy or dormant vegetation may appear light red or a light shade of blue-green (cyan), depending on the plant's degree of good health. These color distinctions make color-infrared photographs useful in assessing the health of plants. Water, on the other hand, 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 atmospheric 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 laboratory processes. Here is how they work.

color photograph showing natural color land and river color-infrared photograph showing land and river
Color photograph—near Burlington, Vt. Color-infrared photograph—near same area

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Color-Infrared Photographs

color-infrared photograph showing water and land
NASA color-infrared color photograph—San
Diego, Calif.

Both standard-color and color-infrared films are manufactured to have three distinct layers, or emulsions. Each layer is sensitive to different wavelengths or energy. Standard-color film emulsions normally record the visible wavelengths as red, green, and blue. After the picture has been taken, chemical processing of the film generates cyan, magenta, and yellow dyes proportional to the amount of exposure given each layer. Color pictures result when the human eye views the varying combinations of the three dye layers. Color-infrared film has a yellow filter over the three emulsion layers to block ultraviolet (UV) and blue wavelengths. Processing color-infrared film after exposure produces yellow, magenta, and cyan dyes. The near-infrared wavelengths and the lack of UV and blue wavelengths result in a clear, crisp color-infrared image. Green, healthy vegetation has a high reflection level of near-infrared wavelengths and appears red on the processed film; red objects with very low near-infrared reflection appear green; green objects with very low near-infrared reflection appear blue; and blue objects with very low near-infrared reflection appear black.

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Color-Infrared Composite Images

color-infrared photograph showing land and water
Landsat color-infrared composite—San
Francisco, Calif.

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

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