

irradiance
Definition/Summary

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 StefanBoltzmann constant, [itex]5.67×10^{8} \, W\cdot m^{2}\cdot K^{4}[/itex]) 
Recent forum threads on irradiance




Breakdown

Physics
> Astro Cosmo
>> Astronomy


Extended explanation

Other similar units:
"Ir" means "on", so irradiance relates to radiation on (received by) a surface, while radiance, and radiant intensity emittance excitance or flux, relate to radiation from a surface. 
Commentary
