Measuring photon pathlengths by quasielastic light scattering in a multiply scattering medium

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Abstract
To gain insight into the migration of photons in optically dense heterogeneously structured media, scattering models have been developed which consist of polystyrene latex particles suspended in solutions containing varying amounts of absorptive dye. The amount and distribution of the dye affect the mean pathlength of emergent photons, the latter being proportional to < (eta) >, the average number of collisions of photons with moving scatterers. The quantity < (eta) > has been determined by cumulant analysis of the measured time autocorrelation function for photons which are injected and detected with a pair of fiber optodes positioned at the surface of the scattering medium. In agreement with theoretical predictions, < (eta) > is found to vary linearly with the spacing between the optodes if the medium is homogeneous. Moreover, when the scattering medium is composed of layers whose optical absorptions differ, the detected autocorrelation functions indicate that the photons which successfully traverse the medium move along paths which minimize the absorption probability.

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