Radiant exposure
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 (), while that of spectral exposure in frequency is the joule per square metre per hertz () and that of spectral exposure in wavelength is the joule per square metre per metre  ()—commonly the joule per square metre per nanometre (). Mathematical definitions. Radiant exposure. Radiant exposure of a "surface", denoted "H" ("e" for "energetic", to avoid confusion with photometric quantities), is defined as formula_1 where Spectral exposure. Spectral exposure in frequency of a "surface", denoted "H", is defined as formula_2 where "ν" is the frequency. Spectral exposure in wavelength of a "surface", denoted "H", is defined as formula_3 where "λ" is the wavelength.