Irradiance

Irradiance is a term in radiometry. It is defined as the change in flux over the change in area:

$E={\frac {d\Phi }{dA}}$

Typical units are [${\frac {W}{m^{2}}}$]. Irradiance typically varies as a function of wavelength.

The term "exitance" is used when talking about the irradiance leaving a surface. (E.g. the exitance from an area source).