Optical Surface Properties Behaviour

Physics Processes, Models and Cross Sections
4

Thank you for the reply.
If what you wrote is the intended behaviour, the documentation is wrong, as it also appears from the tests I made.

From the User’s guide:

“It is possible to specify that a given fraction of photons are absorbed at the surface, or transmitted without change in direction or polarization. This is applicable for dielectric_dielectric interfaces that are not backpainted. The material properties REFLECTIVITY and TRANSMITTANCE are used. By default, REFLECTIVITY equals 1 and TRANSMITTANCE equals 0. At a surface interaction, a random number is chosen. If the random number is greater than the sum of the values of REFLECTIVITY and TRANSMITTANCE at the photon energy, the photon is absorbed. Otherwise, if the random number is greater than the REFLECTIVITY value, the photon is transmitted. Otherwise, the usual calculation of scattering takes place.”

I also found this expaination in topics in this forum

Could this have been changed at some point or am I doing something wrong?

What I want to model are surface imperfections: if a photon is trapped by total internal reflection, it will propagate until it is absorbed. In reality the surfaces persent imperfections that cause a fraction of the trapped photons to escape even if they are over the critical angle. If the light guide has a large enough area this effect could be significant.