Researchers have resolved a 30-year debate by showing when the diffusion equation provides a consistent description of light transport, and when it fundamentally falls short

For over thirty years, the definition of the diffusion coefficient in light transport has been the subject of persistent debate (Credit: iStock/Mettus).

Diffuse optics studies how light propagates through strongly scattering materials, including biological tissue, powders, and disordered solids. In these materials, repeated scattering events make it impractical to calculate the trajectory of every photon individually.

Yet, after many scattering events, the more exact but more complicated radiative transfer equation is usually replaced with the simpler diffusion equation, which should become increasingly accurate as light propagates.

The diffusion equation contains a coefficient describing how quickly light spreads out: the diffusion coefficient.…