Light scattering from mesoscopic objects in diffusive media |
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Authors: | JM Luck TM Nieuwenhuizen |
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Institution: | (1) CEA Saclay, Service de Physique Théorique, 91191 Gif-sur-Yvette Cedex, France, FR;(2) Van der Waals-Zeeman Laboratorium, Valckenierstraat 65, 1018 XE Amsterdam, The Netherlands, NL |
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Abstract: | No direct imaging is possible in turbid media, where light propagates diffusively over length scales larger than the mean
free path .The diffuse intensity is, however, sensitive to the presence of any kind of object embedded in the medium, e.g. obstacles or defects. The long-ranged effects of isolated objects in an otherwise homogeneous, non-absorbing medium can be
described by a stationary diffusion equation. In analogy with electrostatics, the influence of a single embedded object on
the intensity field is parametrized in terms of a multipole expansion. An absorbing object is chiefly characterized by a negative
charge, while the leading effect of a non-absorbing object is due to its dipole moment. The associated intrinsic characteristics
of the object are its capacitance Q or its effective radius ,and its polarizability P. These quantities can be evaluated within the diffusion approximation for large enough objects. The situation of mesoscopic
objects, with a size comparable to the mean free path, requires a more careful treatment, for which the appropriate framework
is provided by radiative transfer theory. This formalism is worked out in detail, in the case of spherical and cylindrical
objects of radius R, of the following kinds: (i) totally absorbing (black), (ii) transparent, (iii) totally reflecting. The capacitance, effective
radius, and polarizability of these objects differ from the predictions of the diffusion approximation by a size factor, which
only depends on the ratio .The analytic form of the size factors is derived for small and large objects, while accurate numerical results are obtained
for objects of intermediate size .For cases (i) and (ii) the size factor is smaller than one and monotonically increasing with ,while for case (iii) it is larger than one and decreasing with .
Received: 7 August 1998 / Accepted: 3 September 1998 |
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Keywords: | PACS 42 30 -d Imaging and optical processing - 42 25 Bs Wave propagation transmission and absorption - 42 68 Ay Propagation transmission attenuation and radiative transfer |
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