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/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^[1] $${\displaystyle H_{\mathrm {e} }={\frac {\partial Q_{\mathrm {e} }}{\partial A}}=\int _{0}^{T}E_{\mathrm {e} }(t)\,\mathrm {d} t,}$$ where

• ∂ is the partial derivative symbol;
• Q_e is the radiant energy;
• A is the area;
• T is the duration of irradiation;
• E_e is the irradiance.

Spectral exposure in frequency of a surface, denoted H_e,ν, is defined as^[1] $${\displaystyle H_{\mathrm {e} ,\nu }={\frac {\partial H_{\mathrm {e} }}{\partial \nu }},}$$ where ν is the frequency.

Spectral exposure in wavelength of a surface, denoted H_e,λ, is defined as^[1] $${\displaystyle H_{\mathrm {e} ,\lambda }={\frac {\partial H_{\mathrm {e} }}{\partial \lambda }},}$$ where λ is the wavelength.

• Exposure (photography)
• Irradiance
• Radiant energy