From definition of Diffusion coefficient:
$$D = c/3(\mu\_a+\mu\_s),$$
where $c$ is the speed of light front, $\mu\_a$ is absorption coefficient and $\mu\_s$ is scattering coefficient.

I wonder where does factor of $1/3$ comes from? I assume it is coming from dimensionality, but I didn't find arguments proving that.
UPD:
In this question I refer to diffusion approximation of Radiation Transfer Equation also known as Photon Diffusion Equation.