Irradiance

In radiometry, irradiance is the radiant flux received by a surface per unit area.

Irradiance of a surface is also, according to the definition of radiant flux, equal to the time-average of the component of the Poynting vector perpendicular to the surface: where For a propagating sinusoidal linearly polarized electromagnetic plane wave, the Poynting vector always points to the direction of propagation while oscillating in magnitude.

This assumption is typically valid in transparent media in the optical frequency range.

In astronomy, stars are routinely treated as point sources even though they are much larger than the Earth.

For instance, the irradiance of Alpha Centauri A (radiant flux: 1.5 L☉, distance: 4.34 ly) is about 2.7 × 10−8 W/m2 on Earth.