Method for measuring the mean time of flight spent by photons inside a volume element of a highly diffusing medium

A method of measuring the mean time of flight, [ti], spent by photons insid e a generic volume element of a highly diffusing medium is presented. The m ethod comes from a general property of the radiative transfer equation and is based on relative measurements of cw attenuation that correspond to smal l variations of the absorption coefficient inside the volume element. By us e of a liquid phantom and small gels with known optical properties it was p ossible to measure [ti] with good accuracy, even when it was only a few pic oseconds long. The results were in good agreement with Monte Carlo results. (C) 1999 Optical Society of America. OCIS codes: 170.5280, 170.7050, 170.3 660.