Color-Infrared (CIR) Imagery


What is it?

The human eye can see electromagnetic radiation (a.k.a. light) from only a very small portion of the electromagnetic spectrum. To "see" beyond this range we need instruments and cameras that can detect and then translate invisible radiation into the familiar colors of the rainbow. Color-infrared (CIR) imagery uses a portion of the electromagnetic spectrum known as near infrared that ranges from 0.70 μm to 1.0 μm (0.7 to 1.0 micrometers or millionths of a meter), just beyond the wavelengths for the color red.

(See diagram at right)

Click on image above to see full-size version

 

Actual Colors

Blue

Green

Red

Near Infrared

Observed Colors

Black

Blue

Green

Red

There are millions of colors of visible light yet each is simply a different combination of the three primary colors: blue, green and red. The invisible near infrared light of CIR can be "seen" by shifting it and the primary colors over as shown at left.

Near infrared wavelengths become visible as red while red wavelengths appear as green and green as blue. Blue wavelengths are shifted out of the visible portion of the spectrum and so they appear as black. On CIR imagery vegetation appears red while water generally appears black with artificial structures like buildings and roads showing as a light blue-green.

 

Interpreting CIR Imagery

The following is a general summary of what the different colors in CIR aerial imagery represent:

Intense bright red- Bright tones of red typically represent vigorously growing, dense vegetation that is producing a large amount of chlorophyll. (See trees and field in the center and right center of the CIR photo at right.)

Lighter tones of red, magenta, pinks- These colors generally represent vegetation that does not contain as much chlorophyll such as mature stands of evergreens. Agricultural fields nearing the end of the growing season, and dead or unhealthy plants often appear in less intense reds, green, or tan. 

White, blue, green, or tan- These colors often represent soils. Darker shades of soil generally indicate higher moisture levels or organic matter. Soil composition also affects soil color appearance, with clayey soils appearing in darker tans and blue-greens, and sandy soils appearing white, gray, or light tan. Crops nearing the end of the growing season, or dead or unhealthy plants will appear in various light tones of red and pink, or greens and tans. Pale or light blue can also represent sediment-laden water. Buildings and manmade materials such as concrete and dry gravel generally appear white to light blue in CIR photos.

Dark blue to black- Water ranges from shades of blue to black depending on the clarity and depth. Usually, the clearer the water, the darker the color. However, shallow streams will often display the colors associated with the materials in their stream beds. If the stream bed is made of sand, the color will appear white or very light tan due to the high reflective property of sand. Asphalt roads generally appear dark blue to black.


Roll over the photo above with your mouse to see it in color-infrared

 

What is it good for?

CIR imagery is good at penetrating atmospheric haze and for determining the health of vegetation.

CIR imagery is also good for:

  • identifying plant species
  • estimating biomass of vegetation
  • assessing soil moisture
  • assessing water clarity (i.e. turbidity)

From: Aronoff, Stan, Remote Sensing for GIS Managers ESRI Press, 2005, p. 64.

Click here for more examples of how CIR imagery is used.

 

Where does it come from?

In World War II CIR film was developed to identify camouflaged military equipment. On the film green vegetation appeared red while green-painted equipment and cut vegetation used to cover equipment showed up as blue-green.



From: Aronoff, Stan, Remote Sensing for GIS Managers ESRI Press, 2005, p. 17.

 


Need more info?

See the following excellent links for more information on CIR imagery:

Return to MnGeo's first-stop Aerial Photography page.

 

 

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