In radiometry, radiant exposure or fluence is the radiant energy received by a surface per unit area, or equivalently the irradiance of a surface, integrated over time of irradiation, and spectral exposure is the radiant exposure per unit frequency or wavelength, depending on whether the spectrum is taken as a function of frequency or of wavelength. The SI unit of radiant exposure is the joule per square metre (J/m), while that of spectral exposure in frequency is the joule per square metre per hertz (J⋅m ⋅Hz) and that of spectral exposure in wavelength is the joule per square metre per metre (J/m)—commonly the joule per square metre per nanometre (J⋅m ⋅nm).