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Radiant energy density

Radiant energy per unit volume From Wikipedia, the free encyclopedia

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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).

Mathematical definition

Radiant energy density, denoted we ("e" for "energetic", to avoid confusion with photometric quantities), is defined as[2]

where

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Relation to other radiometric quantities

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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.

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SI radiometry units

More information Quantity, Unit ...
  1. Standards organizations recommend that radiometric quantities should be denoted with suffix "e" (for "energetic") to avoid confusion with photometric or photon quantities.
  2. Alternative symbols sometimes seen: W or E for radiant energy, P or F for radiant flux, I for irradiance, W for radiant exitance.
  3. Spectral quantities given per unit frequency are denoted with suffix "ν" (Greek letter nu, not to be confused with a letter "v", indicating a photometric quantity.)
  4. Spectral quantities given per unit wavelength are denoted with suffix "λ".
  5. Directional quantities are denoted with suffix "Ω".
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References

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