Radiant flux Radiant flux, denoted Phi| ('e' for "energetic", to avoid confusion with
photometric quantities), is defined as \begin{align} \Phi_\mathrm{e} &= \frac{d Q_\mathrm{e}}{d t} \\[2pt] Q_\mathrm{e} &= \int_{T} \int_{\Sigma} \mathbf{S}\cdot \hat\mathbf{n}\, dA dt \end{align} where • is the
radiant energy passing out of a
closed surface in time interval ; • is time; • is the area of the surface ; • is the
Poynting vector, representing the
directional flow of energy per unit time, per unit area; • is the unit
normal vector to the differential area element . The rate of energy flow through the surface fluctuates at the frequency of the radiation, but radiation detectors only respond to the average rate of flow. This is represented by replacing the Poynting vector with the time average of its norm, giving \Phi_\mathrm{e} \approx \int_\Sigma \langle|\mathbf{S}|\rangle \cos \alpha\ dA , where is the time average, and is the angle between and .
Spectral flux Spectral flux in frequency, denoted Φe,
ν, is defined as \Phi_{\mathrm{e},\nu} = \frac{\partial \Phi_\mathrm{e}}{\partial \nu} , where is the frequency.
Spectral flux in wavelength, denoted , is defined as \Phi_{\mathrm{e},\lambda} = \frac{\partial \Phi_\mathrm{e}}{\partial \lambda} , where is the wavelength. ==SI radiometry units==