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

[Qianqian Fang]

see my replies below

On 11/4/25 13:59, Theo Husby wrote:

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? 

yes

1. • 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. 

it is not detected, nor should it be detected.

1. 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?

neither 

MCX/MMC uses a random number generator to create a random number between 0-1. if this number is greater than reflection coeff R_total, then, the photon packet will transmit through the boundary to the next medium (following Fresnel's law); it will keep its weight 100%, and continue;

if it is less than R_total, then it take the reflection path, while keeping its weight 100%. no weight is scaled or lost during this process. Statistically, this approach matches exactly the energy split at the boundary.

1. • i.e. if I use mcxdetweight to calculate reflectance, will I have inaccurate results because the partial path length includes the Fresnel reflection component?

no. there is no scaling or loss at reflection/transmission.

Thanks,

Theo Husby

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[Theo Husby]

Perfect! Thanks for the clarifications!