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Chapter 2:
Thermal Imaging: We All Glow in the Dark
As we progress past the near-infrared and SWIR wavebands along the wavelength scale of the electromagnetic spectrum to longer wavelengths, we encounter the Midwave Infrared band (MWIR), which starts around 2.5 microns (abbreviated μm) and extends out to about 7 μm in wavelength; and the Longwave Infrared band (LWIR), which is generally defined as light in the 7–15-μm waveband. Figure 2.1 shows the relationships between the visible waveband and the infrared wavebands. For the sake of simplicity, I will refer to the MWIR and LWIR bands as the infrared or IR waveband, and the corresponding imaging technology as thermal imaging. The infrared band is one of the more interesting regions of the spectrum for imaging because objects at terrestrial temperatures (people, cars, animals) emit light in this waveband. The higher an object's temperature, the brighter it appears. For example, in a typical thermal image, a person standing in a room will appear to glow bright white against a dark background. The presence or absence of visible light does not change the appearance of the image at all. For this reason, thermal imaging technology facilitates the detection of many objects in total darkness and, in many cases, makes it possible to measure their surface temperatures. Midwave and longwave infrared light can also penetrate smoke and mist, facilitating observation in conditions that make visible-light imaging impossible. For instance, a person lost on a moonless night in the woods without a flashlight or signal flare would be very difficult to find from the air with visible-light imaging. However, this person (or any warm-blooded animal) displays a natural infrared flare in the form of his or her body, a flare of thermal energy against a dark (cold) background. Outdoor backgrounds appear darker than any warm objects in the thermal wavebands, since trees and grass are much cooler than warm-blooded animals.
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