When does the diffusion approximation fail to describe photon transport in random media?

The transport of photons through a slab of random medium is shown to deviate from the diffusion approximation when z/${\mathit{l}}_{\mathit{t}}$ is small, where z is the thickness of the slab and ${\mathit{l}}_{\mathit{t}}$ is the transport mean free path. When z/${\mathit{l}}_{\mathit{t}}$=10 and z=10 mm, the average time of arrival is about 0.9 times that predicted by diffusion theory. Photons are found to arrive earlier than that predicted by the diffusion theory as z/${\mathit{l}}_{\mathit{t}}$ becomes smaller or the anisotropic scattering increases.