ABSTRACT
This chapter describes with an exposition of the essential characteristics of thermal radiation as related to both radiation exchange between surfaces and to radiation transfer through participating media. Radiant energy may be alternatively envisioned as being transported either by electromagnetic waves or by photons. Radiation travels at the speed of light. Thus, from the viewpoint of electromagnetic theory, the waves travel at this speed. Electromagnetic phenomena encompass numerous types of radiation, from the short wavelength γ rays and X rays to long wavelength radio waves. The wavelength range encompassed by thermal radiation is approximately 0.3 µ to 50 µ, where µ = 10-4 cm. In turn, this wavelength range includes three subranges, the ultraviolet, the visible, and the infrared. Thermal radiation is defined as radiant energy emitted by a medium that is due solely to the temperature of the medium; that is, it is the temperature of the medium which governs the emission of thermal radiation.
