Thermal Imagery

Thermal Radiation Principle

Any objects on Earth's surface and in the atmosphere at a temperature above absolute zero in [Page 2820]degrees Kelvin can emit thermal radiation due to kinetic energy of molecules within the objects. They are sources of thermal infrared radiation. The thermal radiation obeys Planck's function, which describes the relationship between the object (blackbody) temperature and its spectral radiant exitance. A blackbody is an object that is a perfect emitter and absorber. Theoretically, two physical laws can be used to describe the behaviors of thermal radiation: the Stefan-Boltzmann law and Wien's displacement law. The former is related to the Planck's function and is used to calculate the maximum spectral radiant exitance with a known blackbody temperature, and the latter describes the relationship between the true temperature of a blackbody in degrees Kelvin and its peak spectral exitance or dominant wavelength. However, no objects in nature are true blackbodies, but rather are graybodies such as vegetation, soil, rock, and so on. Therefore, when applying the two physical laws to describe the thermal radiation, an emission efficiency factor called emissivity (e) should be considered. The e is generally a function of wavelength, and it is a factor that describes how efficiently an object radiates energy compared with a blackbody. When e = 1, the object is a true blackbody. So e for all graybodies is less than 1. When sensors record the thermal radiation emitted by objects, thermal infrared atmospheric transmittance has to be considered because of its absorption. Those wavelength regions that allow the major portion of thermal energy to pass through the atmosphere are called atmospheric windows. Currently, there are two major thermal infrared atmospheric windows that can be used for imaging thermal energy: (1) 3 to 5 μm and (2) 8 to 14 μm; especially the 8- to 14-μm window is used by most orbital thermal sensors.

Thermal-infrared imaging sensors on NASA's Ikhana unmanned research aircraft recorded this image of the Grass Valley/Slide Fire near Lake Arrowhead/Running Springs in the San Bernardino Mountains of Southern California just before noon on October 25, 2007. The three-dimensional processed image is a colorized mosaic of images draped over terrain, looking east. Active fire is seen in yellow, while hot and previously burned areas are in shades of dark red and purple. Unburned areas are shown in green hues.

Source: NASA/U.S. Forest Service.

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