# Factor of 3 in Photon Diffusion coefficient

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.

• Diffusion of what? Please give a reference as to where your formula comes from, and what it is supposed to apply to. – mike stone Sep 6 at 14:54