先进材料疑固实验室 Heat transfer Laboratory of Advanced Materials Solidification Thermal radiation is electromagnetic radiation generated by the thermal motion of charged particles in a substance. The characteristics of thermal radiation depend on various properties of the surface,including its temperature,spectral absorptivity and spectral emissive power. The radiation is not monochromatic,i.e.,it does not consist of just a single frequency,but comprises a continuous dispersion of photon energies,its characteristic spectrum. If the radiating body and its surface are in thermodynamic equilibrium and the surface has perfect absorptivity at all wavelengths,it is characterized as a black body. A black body is also a perfect emitter.The radiation of such perfect emitters is called black-body radiation.The ratio of any body's emission relative to that of a black body is the body's emissivity,so that a black body has an emissivity of unity. 上降充通大学 SHANGHAI JIAO TONG UNIVERSTTYHeat transfer Thermal radiation is electromagnetic radiation generated by the thermal motion of charged particles in a substance thermal motion of charged particles in a substance. • The characteristics of thermal radiation depend on various properties of the surface, including its temperature, spectral absorptivity and spectl ii ra emissive power. • The radiation is not monochromatic, i.e., it does not consist of just a singqy p p p le frequency, but comprises a continuous dispersion of photon energies, its characteristic spectrum. • If the radiating body and its surface are in thermodynamic equilibrium and the surface has perfect and the surface has perfect absorptivity absorptivity at all wavelengths it is at all wavelengths, it is characterized as a black body. • A black body is also a perfect emitter. The radiation of such perfect emi i ll d bl k itters is called black-b d di i Th i f b d ' body radiation. The ratio of any body's emission relative to that of a black body is the body's emissivity, so that a black body has an emissivity of unity