The discussion of remote sensing up to this point has focused on panchromatic (black & white) imagery. Beyond recording obvious features of size and shape, remote sensing excels in capturing and interpreting color. Color systems also yield some spectacular imagery. For example, in this early "true-color" image from Landsat 7, we perceive the green hillsides and muddy runoff of the upper San Francisco bay (Fig. 5.1).
5.1 Reflectance of Materials
The reflectance of most materials varies with wavelength. This allows spectral imagers, such as those on the Landsat missions, to distinguish different materials. Distinguishing between minerals is a fairly common goal for such work by geologists.
In Fig. 5.2, different aspects of reflective spectra are illustrated. Spectra are the fingerprints of elements, deriving from their fundamental atomic characteristics, as indicated in our discussion of Bohr's model of the hydrogen atom. One of the more important, and dramatic, spectral features found in remote sensing is the "IR ledge" at 0.7 μ¼m, as found in Fig. 5.2. This dramatic rise in reflectance with wavelength makes vegetation appear bright in the infrared (old-style, black-and-white infrared film would show vegetation as white, for example). The military designs camouflage to mimic this behavior. The panchromatic sensors on Landsat, SPOT, IKONOS, and Quickbird extend well into the infrared, and as a result, vegetation is bright in their imagery.
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