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Irradiance, radiant emittance, and radiant exitance are radiometry terms for the power of electromagnetic radiation at a surface, per unit area. "Irradiance" is used when the electromagnetic radiation is incident on the surface. The other two terms are used interchangeably for radiation emerging from a surface. The SI units for all of these quantities are watts per square metre (W/m<sup>2</sup>). These quantities are sometimes called intensity, but this usage leads to confusion with radiant intensity, which has different units.