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Chapter 6:
Spectral and Polarimetric Imagery
Abstract
The discussion of remote sensing up to this point has focused on panchromatic (black and white) imagery. Beyond recording obvious features such as size and shape, remote sensing excels in capturing and interpreting color. Color systems also yield some spectacular imagery. For example, the early “true-color” image from Landsat 7, shown in Fig. 6.1, depicts the green hillsides and muddy runoff of the upper San Francisco Bay. The reflectance of most materials varies with wavelength, which allows spectral imagers, such as those on the Landsat missions, to distinguish different materials. This task is a fairly common goal for such work. Figure 6.2 illustrates different aspects of reflective spectra. Spectra are the fingerprints of elements, deriving from their fundamental atomic characteristics, as indicated in the previous discussion of Bohr’s model of the hydrogen atom. One of the more important, and dramatic, spectral features found in remote sensing is the “red edge” or “IR ledge” at 0.7 mm, as found in Fig. 6.2. This dramatic rise in reflectance with wavelength makes vegetation appear bright in the infrared. Military organizations design camouflage to mimic this behavior. The panchromatic sensors on Landsat, SPOT, IKONOS, and Quickbird extend well into the infrared; as a result, vegetation is bright in their imagery.
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CHAPTER 6
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