In
radiometry,
radiant energy is the
energy of
electromagnetic radiation. The SI unit of radiant energy is the
joule (J). The quantity of radiant energy may be calculated by
integrating radiant flux (or
power) with respect to
time. The symbol
Qe is often used throughout literature to denote radiant energy ("e" for "energetic", to avoid confusion with photometric quantities). In branches of physics other than 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.