Sources in the optical region of the spectrum can be classified under two broad categories: (1) thermal and (2) nonthermal. The first category includes a wide class of natural and artificial sources whose radiation obeys blackbody radiation laws. The second class includes electronic sources such as various arc lamps, and quantum sources such as lasers. Laser radiators are available at various frequencies from millimeter waves up and into the ultraviolet and even x-ray region. We will begin with thermal sources.
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