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