radiantenergy

Radiant energy is the energy carried by electromagnetic radiation as it travels through space or a medium. It includes energy from visible light as well as other wavelengths such as infrared and ultraviolet. The energy of a single photon is E = hν, where h is Planck's constant and ν is the frequency. For a stream of photons or any radiative flux, the total radiant energy transferred over a time interval is the integral of radiant power P with respect to time: E = ∫ P dt. The instantaneous radiant power, measured in watts, is the rate at which energy crosses a surface.

Radiant energy is a central concept in radiometry and photonics. Radiant flux density (Watts per square meter) and radiance (Watts per square meter per steradian) describe how energy is distributed across surfaces and directions. Radiant energy transfer does not require a material medium and can occur through vacuum, distinguishing radiation from conduction and convection as methods of heat transfer.

In applications, radiant energy underpins solar energy, thermal radiation, infrared heating, and the operation of photosensitive devices such as photodetectors and solar cells. Blackbody theory and Planck's law describe how radiant energy is emitted by bodies at a given temperature and how it is distributed across wavelengths. The term is sometimes used historically in thermodynamics and radiometry, emphasizing the energy carried by electromagnetic radiation rather than the internal energy stored in matter.