The Stefan-Boltzmann law states that the total energy radiated per unit surface area of a blackbody across all wavelengths per unit time is proportional to the fourth power of the absolute temperature. The constant of proportionality in this relationship is called the Stefan-Boltzmann constant. The Stefan-Boltzmann law can be written, in mathematical form, as:

 

    E = σT ⁴

 

where E is the energy radiated, T is the temperature, and σ.

 

The first measurements of the heat transferred by radiation between a body and its surroundings were made by John Tyndall. On the basis of these experiments, it was concluded by Josef Stefan (1835–1893) in 1879 that the heat radiated was proportional to the difference of the fourth powers of the absolute temperatures. This purely experimental result was later derived thermodyamically by Boltzmann, who showed that the radiant emittance of a blackbody at any temperature is given by the equation above.