In radiometry, radiant energy density is the radiant energy per unit volume.
[1] The SI unit of radiant energy density is the joule per cubic metre (J/m3).
Radiant energy density, denoted we ("e" for "energetic", to avoid confusion with photometric quantities), is defined as[2] where Because radiation always transmits the energy,[2] it is useful to wonder what the speed of the transmission is.
If all the radiation at given location propagates in the same direction, then the radiant flux through a unit area perpendicular to the propagation direction is given by the irradiance:[2] where c is the radiation propagation speed.
Contrarily if the radiation intensity is equal in all directions, like in a cavity in a thermodynamic equilibrium, then the energy transmission is best described by radiance:[3] Radiant exitance through a small opening from such a cavity is:[4] These relations can be used for example in the black-body radiation equation's derivation.