irradiance

Irradiance is the amount of electromagnetic radiation (EMR) per unit time (quantified as the EMR's power, i.e., energy per unit time) striking a surface per unit area, from any direction. It is conceptually the sum of the intensity (i.e., radiance) from all directions. Typical units:

• W/m² (watts per meter-squared; SI units).
• ergs/s/cm² (ergs per second per centimeter-squared; CGS units).

Spectral irradiance is irradiance per wavelength or frequency.