Fluence

In radiometry, radiant exposure or radiant 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 or spectral fluence 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/m²), while that of spectral exposure in frequency is the joule per square metre per hertz (J⋅m⁻²⋅Hz⁻¹) and that of spectral exposure in wavelength is the joule per square metre per metre (J/m³)—commonly the joule per square metre per nanometre (J⋅m⁻²⋅nm⁻¹).

Radiant exposure of a surface, denoted H_e ("e" for "energetic", to avoid confusion with photometric quantities), is defined as

where

Spectral exposure in frequency of a surface, denoted H_e,ν, is defined as

where ν is the frequency.

Spectral exposure in wavelength of a surface, denoted H_e,λ, is defined as

where λ is the wavelength.

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