Kirchhoff's law of thermal radiation states that a body in thermal equilibrium has equal absorptivity and emissivity at each frequency and direction. A layer with infrared absorptivity therefore emits .
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Kirchhoff's law of thermal radiation states that, for a body in thermal equilibrium, the emissivity (the efficiency of a body in emitting thermal radiation) at a given wavelength and temperature is equal to the absorptivity (the efficiency of a body in absorbing thermal radiation) at that same wavelength and temperature. This relationship implies that good emitters of thermal radiation are also good absorbers.