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Forum: Processes Involving Optical Photons
Re: Question Is it possible to set reflectivity of an optical surface dependent on the angle of incidence of an optical photon? (Maarten van Dijk)
Date: 31 Mar, 2014
From: Maarten van Dijk <Maarten van Dijk>

Hi Peter,

Sorry for taking a while to get back to you, work has been quite busy here. I spent some time today and last week looking at the solution you have suggested, and it seems like it would mostly do the trick. After diving into the matter some more, I have some additional questions.

First of all, if I read the code correctly, I will have to set the transmittance to 1 for the angles for which the total internal reflection criterion (classically speaking) is not met. Taking the Geant formalism, this would be between angles 0 and theta_c, the critical angle. For angles over this, I would then calculate the reflectivity based on the formula mentioned before, and make a lookup table to feed to the DichroicVector.

The first question I have is at which point the total internal reflection condition is checked? My understanding so far is, that if I choose to set the surface to Dichroic, then this would not be determined at all, but it would just take the information it needs from the reflection / transmittance table.

The main issues I have with this approach is that the available granularity in the angular part is fairly coarse - per degree. If I read the code on the levels below it correctly, this could be solved by not flooring the angle before calling the Value() function of the G4Physics2DVector and setting SetBicubicInterpolation (G4bool) to 1. This would allow the table to be as granular as desired, but the payoff would be in efficiency - the speed of the computation goes with the size of the table. To give you an idea of how precise I would like to be able to set this, about to 1 mrad precision (~1/50 of a degree).

However, the function that I would have calculate the reflectivity / transmittance is known and simple, as stated before. Would it be possible to use that formula instead to calculate the reflectivity "live"? It would also require either calculation of or looking up the phase refractive index of the material.

I would naively think that using the formula would be computationally more efficient than expanding the lookup table.

Finally, an issue raised by my colleagues is that there's a compound effect caused by the Fresnel effect that would change the reflectivity slightly again. This would require calculation of the s and p components of the wave, from the product of the plane normal vector and the photon polarization, and calculating the Fresnel reflectivity based on this (also using the refractive indices of the materials on both sides). Finally, the product of the Fresnel reflectivity and the roughness reflectivity (as above) would then decide the number that goes into the boolean decision maker. The decision on which process to take if it doesn't perfectly reflect, would be based on the relative size of the two reflectivity factors.

My apologies for making this so complicated, it took a while for me to sort out in my head and write it down. I hope it is clear like this, if you have any questions let me know.

I am happy to input this into Geant myself, and / or help write a separate type of reflection that is handled in the manner described above. As far as I can tell, Geant does not currently add in the Fresnel effect, is this correct? Apologies for any mistakes I have made.

Kind regards,

Maarten

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1 Feedback: Re: Is it possible to set reflectivity of an optical surface dependent on the angle of incidence of an optical photon?   (Gumplinger Peter - 11 Apr, 2014)
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