Question about detector behaviour with total internal and Fresnel reflection

[Theo Husby]

Hi Dr. Fang,

I'm simulating a semi-infinite tissue volume where an optical fiber placed in contact with the top surface is used as a detector. In MCX I'm using a detector and only counting photons with trajectories that lie within the fiber's NA (accounting for the tissue's index of refraction). I had some basic questions about how total internal reflection and Fresnel reflections interact with detectors.

1. Are photons only detected only if they exit the volume and are terminated? 
   • e.g. If a photon packet is incident with a detector surface, but is totally internally reflected such that it does not leave the volume, is it detected? I couldn't tell from your FAQ post. 
2. Do Fresnel reflections create new photon packets with their own initial weight and partial path length, or do they scale the weight of the packet by the fraction that's reflected and add to the packet's pre-reflection partial path length?
   • i.e. if I use mcxdetweight to calculate reflectance, will I have inaccurate results because the partial path length includes the Fresnel reflection component?

Thanks,

Theo Husby