Study of Photon Migration Depths with Time-resolved Spectroscopy

In this study a light-shielding plate with a hole was placed in an intralipid emulsion. The probability distribution for photons emitted from a surface light source, passing through the hole at different depths, and reaching a surface detector at the other side of the plate was experimentally assessed. We provide qualitative verification for a model derived by Weiss et al. [J. Mod. Opt. 36, 349 (1989)] that the migration depths for the measured photons follow a distribution in depth and that this distribution has a maximum probability at a describable depth beneath the surface. This agreement, corroborated by a parallel study, suggests that we may have assessed the maximum migration depth distribution of photons that reached the detector and that the random walk model may describe the maximum migration depth distribution. The experimental results indicate that photons with the same path lengths within the medium reach a wide range of depths and suggest difficulties in resolving optical structure with time-resolved measurement. The results also provide experimental evidence that, for a given source-detector separation, the photons that migrate deeper have longer mean path lengths with larger variation in their path lengths.