The transport length in a strongly diffusing medium (noted l*) is the length over which the direction of propagation of the photon is randomized.
or averaging of the scattering angle θ over a high number of scattering events.
g can be evaluated with the Mie theory.
A single scattering doesn't deviate the photons.
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).