Quantitative measurements in scattering media: Photon time-of-flight analysis with analytical descriptors

A method for the separate determination of the absorption and scattering: properties from time-resolved photon distributions is described. Time distributions inherently contain information on the absorption and scattering processes. We have investigated the use of analytic descriptors such as derivatives and moments on the time distributions to facilitate the extraction of these optical coefficients. The descriptors were specifically chosen on the premise that circuits can be constructed to process the signal. Selection of the descriptor processing techniques and estimates of the absorption and scattering coefficients were made with the use of stepwise multilinear regression (STMLR). Estimates employing the analytical descriptors were evaluated experimentally with the use of photon time-of-flight in both transmittance and reflectance geometries. Results for both transmittance and reflectance indicate that the principal parameters, which describe the absorption and scattering coefficient, are the mean fall and rise times, respectively. Absorption estimates were primarily related to the trailing edge of the time profiles and scattering, the rising edge. The results suggest that practical instruments may be developed which would allow for a robust quantification in highly scattering media.