## Stefan-Boltzmann Law

The Stefan-Boltzmann Law states that total spectral radiant exitance (W) leaving a blackbody is proportional to the fourth power of its temperature (T).

Radiant exitance is defined as the amount of radiant flux (radiant energy per unit time) leaving a surface per unit area. The total radiant exitance is the integration of all the area under a blackbody radiation curve. Examples of blackbody radiation curves for the sun and the Earth are illustrated in the figure below.

Note that the sun produces more spectral radiant exitance than the Earth. This demonstrates that as the temperature of a radiating body increases, the total amount of radiant energy increases and the radiant energy peak shifts to shorter wavelengths.