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The spectral radiance L_{\Omega,
u} is the energy per area of the photons of frequency
u emitted at a solid angle d\Omega.

If the spectral radiance is integrated in the frequency, the total radiance is obtained:

equation


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The integration of the radiance L on the solid angle d\Omega gives us the radiative flux \Phi

equation


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Radiance is the derivative of radiative flux at the angle and projected surface section S\cos\theta

equation

The radiative flux is the radiative energy that by time is irradiated:

equation

The radiative intensity is the radiative flux per element of solid angle:

equation

The photon transport equation is

equation

where \mu_t is the absorption coefficient and scattering, c the velocity of light, P(\hat{n}',\hat{n}) is the phase function that gives the probability that a photon traveling in the direction \hat{n} is deflected in the direction \hat{n}'.