:Transport length

{{Short description|Measurement of photon propagation}}

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:{{cite book |first=A. |last=Ishimaru |year=1978 |title=Wave Propagation and Scattering in Random Media |publisher=Academic Press |location=New York}}

$l^*=\backslash frac\{l\}\{1-g\}$

with g: the asymmetry coefficient. $g=\; \backslash langle\; cos\; (\backslash theta)\; \backslash rangle$ 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 useful for renormalizing a non-isotropic scattering problem into an isotropic one in order to use classical diffusion laws (Fick law and Brownian motion). The transport length might be measured by transmission experiments and backscattering experiments.{{Cite journal | doi=10.1016/S0039-9140(99)00129-0| pmid=18967735| title=TURBISCAN MA 2000: Multiple light scattering measurement for concentrated emulsion and suspension instability analysis| year=1999| last1=Mengual| first1=O.| last2=Meunier| first2=G.| last3=Cayré| first3=I.| last4=Puech| first4=K.| last5=Snabre| first5=P.| journal=Talanta| volume=50| issue=2| pages=445–456}}{{Cite journal | doi=10.1364/AO.37.004017| pmid=18273374| bibcode=1998ApOpt..37.4017S| title=Anisotropic scattering of light in random media: Incoherent backscattered spotlight| year=1998| last1=Snabre| first1=Patrick| last2=Arhaliass| first2=Abdellah| journal=Applied Optics| volume=37| issue=18| pages=4017–26}}

Image:figure_mean_free_path.png|Mean free path simple scheme