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"Thermal radiation " is electromagnetic radiation emitted by matter in a state of thermal excitation. The energy density of such radiation in an en-closure at constant temperature is given by the well known Planck formula. The importance of thermal radiation in physicai problems increases as the temperature is raised; at moderate temperatures (say, thousands of degrees Kelvin) its role is primarily one of transmitting energy, whereas at high temperatures (say, millions of degrees Kelvin) the energy density of the radi-ation field itself becomes important as well. If thermal radiation must be considered explicitly in a problem, the radiative properties of the matter muat be known. In the simplest order of approximation, it can be assumed that the matter is in thermodynamic equilibrium "locally " (a condition called local thermodynamic equilibrium, or LTE), and all of the necessary radiative properties can be defined, at least in principle. Of course whenever thermal radiation must be considered, the medium which contains it inevitably has pressure and density gradients and the treatment requires the use of hydro