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

(1)

where is the flux at the surface of the emitter, and has units J m^-2 s^-1 steradian ^-1.

The emissivity is denoted , equals

(2)

and is measured in erg cm^-3 Hz^-1 s^-1 sterradian^-1.

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

(3)

and are denoted or and are measured in J m^-2 s^-1.

Flux densities are measured per wavelength or frequency interval

			(4)
			(5)

.

(6)

denoted L, and measured in J m^-2 s^-1.

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