On the diffusion coefficient of a photon migrating through a turbid   medium: a fresh look from a broader perspective
  Does the diffusion coefficient of a photon depend on time $t$ or the probability of absorption $k$? To find an answer to the question, photon transport in a medium of infinite extent is analyzed using the method of moments. It is pointed out that if $D$ is defined so as to make it depend on $t$ or $k$, it will also depend on the experimental conditions; that the parameter $k$ which enters the stationary diffusion equation is in general different from that entering the transient version; and that a hitherto unused non-Markovian partial differential equation may be used for treating photon transport. 
K. Razi Naqvi