A method of measuring the mean time of flight, , spent by photons inside a generic volume element of a highly diffusing medium is presented. The method comes from a general property of the radiative transfer equation and is based on relative measurements of cw attenuation that correspond to small variations of the absorption coefficient inside the volume element. By use of a liquid phantom and small gels with known optical properties it was possible to measure with good accuracy, even when it was only a few picoseconds long. The results were in good agreement with Monte Carlo results.
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