# Propagation du rayonnement en milieu diffusant. Etude de la transition entre le regime balistique et le regime diffusif et des fluctuations temporelles d'intensite

Abstract : We study the propagation of light pulses through scattering media
using the time-dependant radiative transfer equation (RTE). The
standard discrete-ordinate method is used to solve this equation in
the space-frequency domain in a slab geometry. We calculate both the
diffuse and ballistic transmission and the backscattered signal for
each direction.

We revisit the definition of the diffusion
coefficient D for light transport in scattering and absorbing
media. From an asymptotic analysis of the transport equation, we
present a novel derivation of the diffusion coefficient, which is
restricted neither to low absorption nor to a situation in which the
specific intensity is quasi-isotropic.

We study the transition between the diffusive and the non-diffusive
regimes. We observe a non-diffusive behavior for the systems whose
thickness L is smaller than $8 ltr$ where ltr is the transport mean-free
path. We show also that the value of the effective diffusion
coefficient in the non-diffusive region is strongly affected by
internal reflections.

Finally, we propose a model for the temporal correlation function of
scattered intensity, which is based on a random walk approach and the
solution of the time-dependant RTE. This model describes the evolution
of the correlation function during the transition from the
single-scattering regime to the diffusive regime. It increases the
range of validity of the previous QELS and DWS theories, wich were
valid for single-scattering and in the diffusive regime,
respectively. Our model permits us to describe recent measurements of
temporal fluctuations in the intermediate regime of multiple
scattering.
Mots-clés :
Document type :
Theses
Domain :

https://tel.archives-ouvertes.fr/tel-00003084
Contributor : Rachid Elaloufi <>
Submitted on : Thursday, July 3, 2003 - 6:14:51 PM
Last modification on : Friday, July 3, 2020 - 9:28:02 AM
Long-term archiving on: : Tuesday, September 11, 2012 - 9:05:31 PM

### Identifiers

• HAL Id : tel-00003084, version 1

### Citation

Rachid Elaloufi. Propagation du rayonnement en milieu diffusant. Etude de la transition entre le regime balistique et le regime diffusif et des fluctuations temporelles d'intensite. domain_other. Ecole Centrale Paris, 2003. Français. ⟨tel-00003084⟩

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