In radiometry (measurement of electromagnetic radiation), irradiance is the radiant flux (power) received by a surface per unit area, and spectral irradiance is the irradiance of a surface per unit frequency or wavelength, depending on whether the spectrum is taken as a function of frequency or of wavelength. The SI unit of irradiance is the watt per square metre , while that of spectral irradiance is the watt per square metre per hertz (W·m^-2·Hz^-1) or the watt per square metre per metre (W·m^-3)—commonly the watt per square metre per nanometre . The CGS unit erg per square centimetre per second is often used in astronomy. Irradiance is often called "intensity" in branches of physics other than radiometry, but in radiometry this usage leads to confusion with radiant intensity.