Irradiance, radiant emittance, and radiant exitance are radiometry terms for the power per unit area of electromagnetic radiation at a surface. "Irradiance"'s used when the electromagnetic radiation's incident on the surface. "Radiant exitance" or "radiant emittance"'s used when the radiation's emerging from the 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… (More on Irradiance)