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The explanation of an image contrast improvement technique (see Contrast improvement with circularly polarized light) advanced by Gilbert and Pernicka (1967) would imply that this improvement applies only to a TM in which single-scattering dominates. Indeed, if multiple scattering of light in the TM were substantial, the improvement in the image contrast should be moderate, as both the TM and the rough-surface object backscatter light of both handedness in equal amounts.
However, the situation is a somewhat more complex. First of all, the handedness of circular polarization of light scattered by a particle at a given scattering angle depends on its size and refractive index. For spherical homogeneous particles made of material that is not optically active, light scattered at small angles largely retains the handedness of the incident light, while the backscattered light tends to have an opposite handedness. If a dispersion is made of spheres with an asymmetric scattering function, and that dispersion supports multiple scattering, a substantial fraction of backscattered light is made of light that underwent many small-angle scattering events. This tends to preserve the handedness of light backscattered by such dispersion, an effect known as the polarization memory (for example, MacKintosh et al 1989).
Kim and Mososco (2002) shown, by modeling the multiple scattering of light in dispersions of polystyrene spheres in water (refractive index of polystyrene is close to that of water), that this is consistent with temporal behavior of the backscattered light polarization: the time delay corresponding to the emergence of light with maximally preserved handedness increases with the square of the average cosine of the scattering angle, g, if the particle size and refractive index are moderate. Indeed, with g increasing towards unity (the scattering function becomes more forward-pointing), more scattering events are needed for the reversal of the direction of propagation of the predominantly forward-scattered light. Light emerging from the dispersion much later underwent many more scattering events, including the low-probability large-angle events that tend to reverse the handednes. This light is depolarized.
| CITATION: Jonasz M. 2006. Polarization memory (www.tpdsci.com/Tpc/PolMem.php). In: Top. Part. Disp. Sci. (www.tpdsci.com). |
HISTORY: Published: 2006 Modified: 2006 Peer-reviewed: PENDING |
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