The Stefan--Boltzmann law gives the thermal-radiation flux emitted by a surface as , where is its emissivity and is the Stefan--Boltzmann constant.
Emissivity is the ratio of the thermal radiation emitted by a surface to that emitted by an ideal black body at the same temperature.
A body is in radiative equilibrium when absorbed radiative power equals emitted radiative power. A surface absorbing flux and emitting as a grey body has equilibrium temperature .
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The Stefan-Boltzmann law describes the relationship between the temperature of a black body and the total energy radiated per unit surface area across all wavelengths per unit time (also known as the black body radiation). The law states that the power radiated per unit area \( j^* \) of a black body is proportional to the fourth power of its absolute temperature \( T \).