DISTRIBUTION OF THE PATHS OF EARLY-ARRIVING PHOTONS TRAVERSING A TURBID MEDIUM

We describe experiments to measure the spatial and the temporal distri bution of photons traversing a turbid medium in the early-arriving reg ime in which the photons are multiply scattered but are not completely randomized. The photon paths are resolved temporally by a streak came ra and spatially by an adjustable absorbing screen with a small apertu re. The results are compared with predictions of a theory based on pat h integrals (PIs) and with the standard diffusion approximation. The P I theory agrees with the data for both long and short times of flight; this agreement is in contrast to the diffusion approximation, which f ails for short times. An alternative PI calculation, based on the use of an effective Lagrangian, also agrees with the experiments. PI theor y succeeds because it preserves causality. The implications for optica l tomography are discussed. (C) 1998 Optical Society of America.