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This editable Main Article is under development and subject to a disclaimer.

[edit intro]

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

E = d Φ d A {\displaystyle E={\frac {d\Phi }{dA}}}

Typical units are [ W m 2 {\displaystyle {\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).