irradiance

n. brightness, shining; radiant energy

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-2), while the cgs units are ergs per square centimeter per second (erg·cm-2·s-1, often used in astronomy). These quantities are sometimes called intensity, but this usage leads to confusion with radiant intensity, which has different units.
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(n.)
Alt. of Irradiancy
  

Radiant power incident per unit area upon a surface. Note: Irradiance is usually expressed in watts per square meter, but may also be expressed in joules per square meter. (188 ) Deprecated synonym power density.