The emission or transmission of energy in the form of waves through space or through a material medium; the term also applies to the radiated energy itself. Radiation includes electromagnetic, acoustic, and particle radiation, as well as all forms of ionizing radiation.

According to quantum mechanics, electromagnetic radiation may be viewed as consisting of up of photons. Acoustic radiation is propagated as sound waves. Examples of particle radiation are alpha and beta radiation in radioactivity, and cosmic rays.

Brightness, radiance and surface brightness are all terms used to describe the intensity of radiation. They can be denoted B, I, or R and are equal to

where 👁 Image
is the flux at the surface of the emitter, and has units J m s steradian 👁 Eric Weisstein's World of Math
.

The emissivity is denoted equals

and is measured in erg cm Hz s sterradian.

The exitance, irradiance, and radiancy are all equal to the flux

and are denoted 👁 Image
or 👁 Image
and are measured in J m s.

Flux densities are measured per wavelength or frequency interval

denoted L, and measured in J m s.

👁 Image
Electromagnetic Radiation, Nuclear Radiation, Radiative Transfer, Sound, Wave