2.关键术语 2.1.1.18 Kirchhoff's Law In general,a medium that absorbs radiation may also emit radiation at the same wavelength.Kirchhoff's law states that the absorptivity of a body is equal to its emissivity at the same temperature (Kirchhoff 1860),namely, dk=8 (2.17) where ak is the absorptivity of a body.A body that is a good radiator (or emitter) is also a good absorber.Therefore,an ability to emit is oppositely related to its ability to reflect.Kirchhoff's law requires thermodynamic equilibrium,implying that the surface is isothermal and radiates to an isothermal background at the same temperature (Salisbury et al.1994).This relationship is often used to measure emissivity of materials by reflectance in laboratory. 电子科
2.关键术语 2.1.Thermodynamic or kinetic temperature A 'true'temperature would be measured with an accurate,infinitesimal in situ thermometer that makes good thermal contact with the object of interest.For our purposes,thermodynamic and kinetic temperatures are equal even though they are defined differently.Thermodynamic temperature is a macroscopic quantity that is constant throughout any group of subsystems that are in thermodynamic equilibrium (thermodynamic equilibrium means no heat transfer).The equilibrium condition for this group of subsystems can be derived by maximizing the total entropy with respect to the ， 电子科技大学周纪