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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.

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    $\begingroup$ Diffusion of what? Please give a reference as to where your formula comes from, and what it is supposed to apply to. $\endgroup$
    – mike stone
    Commented Sep 6, 2019 at 14:54

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Here is some information about this. The factor comes from an approximation and using spherical harmonics.

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The factor of three enters from the flux equation during the derivation. Not a full answer, but this was too large for a comment.

See here equation 5.

diffusion screenshot

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  • $\begingroup$ Do I understand it right that it depends on dimensionality of a problem, 2D, 3D, etc? $\endgroup$
    – Shantis
    Commented Sep 10, 2019 at 10:59

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