Radiometry is the science of measurement of radiant energy in terms of absolute power.

  • Radiant energy is the electromagnetic energy transported by electromagnetic waves.
  • Radiant energy can also be described in terms of elementary particles called photons.
  • Radiometric techniques characterize the distribution of the radiation’s power (transfer of energy per unit of time) in space.
  • The symbol Qe is often used to denote radiant energy (“e” for “energetic”, to avoid confusion with photometric quantities).
  • The SI unit of radiant energy is the joule (J).
  • Whilst radiometry deals with electromagnetic radiation, photometry deals with the interaction of light with the human eye.
  • Outside of the field of radiometry, electromagnetic energy is referred to using E or W. The term is used particularly when electromagnetic radiation is emitted by a source into the surrounding environment. This radiation may be visible or invisible to the human eye.