Transport length

The transport length in a strongly diffusing medium (noted l*) is the length over which the direction of propagation of the photon is randomized. It is related to the mean free path l by the relation^[1] :
$l^{*}={\frac {l}{1-g}}$
with: g: the assymetry coefficient. $g=<cos(\theta )>$ or averaging of the scattering angle θ over a high number of scattering events.
g can be evaluated with the Mie theory.
If g=0, l=l*. A single scattering is already isotropic.
If g→1, l*→infinite. A single scattering doesn't deviate the photons. Then the scattering never gets isotropic.

This length is usefull for renormalizing a non isotropic scattering problem into an isotropic one in order to use classical diffusion laws (Fick_law, Brownian_motion). The transport length might be measured by transmission experiments of backscattering experiments ^[2] ^[3]

Mean free path simple scheme

^ A. Ishimaru, Wave Propagation and Scattering in Random Media, Academic Press, New York, 1978.
^ Talanta, Volume 50, Issue 2, 13 September 1999, Pages 445-456
^ P. Snabre, A. Arhaliass, Anisotropic scattering of light in random media. Incoherent backscattered spot light,Appl. Optics 37 (18) (1998) 211 – 225.

[1] A. Ishimaru, Wave Propagation and Scattering in Random Media, Academic Press, New York, 1978.

[2] Talanta, Volume 50, Issue 2, 13 September 1999, Pages 445-456

[3] P. Snabre, A. Arhaliass, Anisotropic scattering of light in random media. Incoherent backscattered spot light,Appl. Optics 37 (18) (1998) 211 – 225.

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