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Photon diffusion approximates the spreading of radiation after many scattering events have largely randomized propagation directions. Individual photons travel along paths; their distribution can obey a diffusion equation on scales much larger than a transport mean free path. The longer arrival times arise from longer, repeatedly redirected paths.

For homogeneous, conservative isotropic scattering, the photon diffusion coefficient for isotropic scattering is $D=v\ell /3$, where $v$ is the transport speed and $\ell$ the scattering mean free path. Directional persistence replaces that scale by a transport mean free path in an appropriate transport model. Crossing a characteristic thickness $L$ then takes a time of order $L^2/D$, with geometry-dependent factors, which can greatly exceed the direct travel time $L/v$.

A diffuse photon time-of-flight experiment sends a short pulse into a scattering sample and measures the delayed transmitted or returning light. A pulsed source, a fast photodetector and time-correlated single-photon counting can build an arrival-time histogram from repeated pulses. The detector's timing response must be accounted for before attributing all measured broadening to the sample.^[ref]

The 1989 study by M. S. Patterson, Britton Chance and B. C. Wilson used a slab diffusion model to relate picosecond-pulse reflectance and transmittance to absorption and scattering, comparing predictions with preliminary measurements and Monte Carlo simulations.^[ref] The earliest nearly unscattered light, nearby boundaries and strongly directional transport can require a fuller radiative transfer description.