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Chapter 7:
Thermal Infrared
To remote sensors, the infrared portion of the electromagnetic spectrum is quite different from the visible. In the IR, sensors provide more than just an image—IR can give non-literal information which may have different value than a comparable panchromatic (visible) image. A large range of tactical and strategic sensors operate solely in the IR, sensing the emitted radiation from targets and backgrounds, as opposed to the reflected radiation we have focused on in earlier sections. In the visible spectrum, we mostly see by reflected light—typically sunlight. In the IR, there is a reflected solar component (during the day), but much remote sensing is due to emitted IR energy, particularly in the midwave-infrared (MWIR, 3–5 μm), and longwave-infrared (LWIR, 8–13 μm) spectral ranges. Figure 2.12 showed how the location of the peak in the spectrum and the amplitude of the radiated energy changes with temperature. As a consequence of these curves, we can both see in the IR and obtain temperature measurements of remotely sensed nighttime scenes.
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