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The prevalent source of energy excitation used to illuminate natural targets is solar irradiation (also called insolation). The spectrum of sunlight is determined by the photospheric temperature of the sun (peaking near 5600 °C). Incoming solar radiation is mainly confined to the spectral interval between 200 and 3400 nm (0.2 and 3.4 µm), with the maximum power input close to 480 nm (0.48 µm) (in the visible green). As solar rays pass through the Earth's atmosphere, a fraction of the radiant energy is absorbed or backscattered, with the remainder transmitted.

On striking the land or ocean surface (ground targets) (or atmospheric targets - air, moisture, clouds), the transmitted radiation (technically, termed irradiance) will be partitioned into three modes of energy-interaction response: (1) transmittance - some radiation will penetrate into certain surface media such as water; (2) absorptance - some radiation will be absorbed through electron or molecular reactions within the medium encountered; a portion of the energy incorporated can then be re-emitted (as emittance), largely at longer wavelengths, so that some of the sun's radiant energy engages in heating the target giving rise then to a thermal response; (3) reflectance - some radiation will, in effect, be reflected (and scattered) away from the target at different angles (depending in part on surface "roughness" as well as on the angle of the sun's direct rays relative to surface inclination), and some being directed back on line with the observing sensor. Most remote sensing systems are designed to monitor reflected radiation.

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Code 935, Goddard Space Flight Center, NASA
Written by: Nicholas M. Short, Sr. email: nmshort@epix.net
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Updated: 1999.03.15.