PENETRATION DEPTH FOR DIFFUSING-WAVE SPECTROSCOPY

The depth at which diffusing photons are assumed to be deposited in a random scattering medium has traditionally been treated as a phenomeno logical parameter comparable to the photon transport mean free path. W e show how to average properly over an exponential distribution of dep ositions weighted additionally by the transmission probability, and co mpare our prediction for the autocorrelation of intensity fluctuations in the transmitted light with experimental data on an ideal system. T he improved correlation function, where distinguishable from the prior form, provides slightly better agreement with data as long as the sam ple is thicker than approximately 10 transport mean free paths. Howeve r, in contrast with static transmission, proper averaging over a range of penetration depths does not extend the validity of diffusing-wave spectroscopy to significantly smaller slab thicknesses. The most signi ficant errors in the theory must therefore arise from approximations o ther than the treatment of the source of diffusing photons. (C) 1995 O ptical Society of America