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Radiation flux is a measure of the amount of radiation received by an object from a given source. This can be any type of radiation, including electromagnetic, sound, and particles from a radioactive source.
Φ = L⁄4πr2 is the radiation flux, L is the Luminosity, or total power output of the source, and r is the distance from the radiation source. The units of radiation flux are W·m−2, or kg·s−3.
Radiation flux density is a related measure that takes into account the area the radiation flux passes through, and is defined as the flux divided by the area it passes through. The Radiation flux density is also known as Intensity where I = L⁄4πr2