What is Irradiance? Definition, Equations & Explanation

[Greg Bernhardt]

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 ($W.m^{-2}$), or cgs units of ergs per square centimeter per second ($erg.cm^{-2}.s^{-1}$, often used in astronomy).

Equations

surface area of star:
$$4\pi R^2$$
total power radiated (Luminosity):
$$L\ =\ 4\pi R^2\sigma T^4$$
power per area at distance d (Irradiance):
$$I\ =\ \frac{L}{4\pi d^2}\ =\ \frac{R^2}{d^2}\sigma T^4$$
($\sigma$ is the Stefan-Boltzmann constant, $5.67×10^{-8} \, W\cdot m^{-2}\cdot K^{-4}$)

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.

* This entry is from our old Library feature. If you know who wrote it, please let us know so we can attribute a writer. Thanks!

 

[Aaron Paulo]

Thanks for the overview of Irradiance!