Derivation of the scalar radiative transfer equation from energy conservation of Maxwell's equations in the far field.

In this paper the expression for the radiative transfer equation (RTE) commonly used when describing light propagation in biological tissues is derived directly from the equation of energy conservation of Maxwell's equations (Poynting's theorem) by making use of a volume-averaged expression for the time-averaged flow of energy. The derivation is presented step by step with Maxwell's equations as the starting point, analyzing all approximations taken in order to arrive at the expression of the scalar RTE employed in biomedical applications, which neglects particle nonsphericity and orientation, depolarization, and coherence effects.