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Irradiance is the power per area of the radiation received by a surface.

It is measured in SI units of watts per square meter ([itex]W.m^{-2}[/itex]), or cgs units of ergs per square centimeter per second ([itex]erg.cm^{-2}.s^{-1}[/itex], often used in astronomy).

Equations

surface area of star:

[tex]4\pi R^2[/tex]

total power radiated (Luminosity):

[tex]L\ =\ 4\pi R^2\sigma T^4[/tex]

power per area at distance d (Irradiance):

[tex]I\ =\ \frac{L}{4\pi d^2}\ =\ \frac{R^2}{d^2}\sigma T^4[/tex]

([itex]\sigma[/itex] is the Stefan-Boltzmann constant, [itex]5.67×10^{-8} \, W\cdot m^{-2}\cdot K^{-4}[/itex])

Extended explanation

Other similar units:

"Ir-" means "on", so irradiance relates to radiationon(received by) a surface, while radiance, and radiant intensity emittance excitance or flux, relate to radiationfroma surface.

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# What is irradiance

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