Radiant energy

The radiant energy is a physical quantity of radiometry. It is the energy of the electromagnetic radiation, for example light.


Considering electromagnetic radiation ( in a row described as light ) is simplified as a stream of photons, the radiation energy is transported in this stream of energy.

The energy of a photon depends only on the frequency:

With the Planck's quantum of action and the frequency of the wave.

For monochromatic ( single color ) light, the radiation energy, therefore, is measured as the product of the number of photons and their energy:

Wherein the number of photons.

In general, however, consists of light photons of different energy ( it is polychromatic, that is multi-colored).

For a general description of the radiant energy, the spectral distribution one must therefore take into account:

For the radiation energy arises therefore:

Considering the time during which a certain amount of radiation energy is transported, the radiation power is given (also called irradiance ).

Use in photometry

In photometry is used instead of the energy. This is not a mistake: Since physics knows a lot more sizes than the Latin and the Greek alphabet characters have, it turns out every now and then that a sign is used differently in different contexts. In photometry, especially for the radiation energy ( above as referred ) uses the symbols. Here, the index s for an energetic measurand; the index V ( for visually ) is used in the photometry for measuring quantities which take account of the physiological properties of the human eye.

  • Physical size
  • Form of energy