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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.<CENTER><P>

		<IMG SRC="LndSlrRad75p.gif" WIDTH=504 HEIGHT=366 BORDER=0></CENTER><P>

		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 &quot;roughness&quot; 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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