In radiometry, irradiance is the radiant flux received by a surface per unit area. The SI unit of irradiance is the watt per square metre (W⋅m⁻²). The CGS unit erg per square centimetre per second (erg⋅cm⁻²⋅s⁻¹) is often used in astronomy. Irradiance is often called intensity, but this term is avoided in radiometry where such usage leads to confusion with radiant intensity. In astrophysics, irradiance is called radiant flux.

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 two forms have different dimensions and units: spectral irradiance of a frequency spectrum is measured in watts per square metre per hertz (W⋅m⁻²⋅Hz⁻¹), while spectral irradiance of a wavelength spectrum is measured in watts per square metre per metre (W⋅m⁻³), or more commonly watts per square metre per nanometre (W⋅m⁻²⋅nm⁻¹).