A new diffusion approximation to the radiative transfer equation for scattering media with spatially varying refractive indices

Traditionally, the diffusion approximation is derived for a medium with a spatially constant refractive index (Ishimaru 1978 Wave Propagation and Scattering in Random Media vol 1 (New York: Academic)). In this paper, we derive a new diffusion approximation to the radiative transfer equation for a medium with a spatially varying refractive index.