Diffuse-diffuse photon coupling via nonscattering void in the presence of refractive index mismatch on the void boundary.
It is well established that the diffusion approximation (DA) is valid for light propagation in highly scattering media, but it breaks down in nonscattering void regions. The methods that handle the void with refractive index matched boundary were previously developed. For the void with refractive index mismatched boundary, no theory is yet available. We derived the finite element method that can handle the void problem with refractive index mismatch on its boundary. The method uses the ray tracing to account for the multiple reflections of rays within the void due to refractive index mismatch on the void boundary. We applied the method to the four-layer adult head model and the good agreements with Monte Carlo simulation results were found in the mean time of flight data.