I want to model the light propagation within biological tissues like apple tissue. I have read some papers about this, and found that the light source can only be a point light source located at one mean free path below the boundary of the tissue. How could I launch an actual beam with finite size onto the tissue? And, for the boundary conditions, both Dirichlet condition and Robin condition can be used. However, if I use the Dirichlet condition, i.e., set the fluence at the boundary as 0, then the fluence distribution of all domain is 0. Then I can not get the reflectance at the boundary. How could I deal with this? Thank you!