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 ⋅Hz), while spectral irradiance of a wavelength spectrum is measured in watts per square metre per metre (W⋅m), or more commonly watts per square metre per nanometre (W⋅m ⋅nm).