Radiation is the phenomenon where heat is transferred as electromagnetic waves.

It is different from thermal conduction and convection because heat is transferred without any medium, in a vacuum for example. Radiation is expressed with the following equation:

where;
E : The amount of energy emitted by radiation [W/m2]
ε : Emissivity [-]
σ : Stefan-Boltzmann constant (= 5.67×10-8 [W/(m2∙K4)])
T : Object temperature [K]

An object with higher temperature and greater emissivity releases greater amount of energy.