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Forum: Processes Involving Optical Photons
Re: Question Specular spike and lobe (Daniel Bonifacio)
Date: 03 Apr, 2009
From: Gumplinger Peter <Gumplinger Peter>

Hi Daniel,

Unfortunately (of fortunately - depending on your point of view) the UNIFIED model has (too) many degrees of freedom. You should also note that when you specify 'groundbackpainted' the reflection off the wrapping will always be Lambertian. In the case of 'groundbackpainted', the other parameters (SPECULARLOBECONSTANT etc.) just refer to the interface between your LYSO and that thin air-layer. Moreover, sigma alpha is used - in the sampling of FacetNormals - only when the random numbers fall such that theStatus = LobeReflection wins out in this logic:

void G4OpBoundaryProcess::ChooseReflection()
                 G4double rand = G4UniformRand();

                 if ( rand >= 0.0 && rand < prob_ss ) {

                    theStatus = SpikeReflection;

                    theFacetNormal = theGlobalNormal;


                 else if ( rand >= prob_ss &&

                           rand <= prob_ss+prob_sl) {

                    theStatus = LobeReflection;


                 else if ( rand > prob_ss+prob_sl &&

                           rand < prob_ss+prob_sl+prob_bs ) {

                    theStatus = BackScattering;


                 else {

                    theStatus = LambertianReflection;



I am afraid the code is oblivious as to the actual values you choose for sigma_alpha, and the above prob_s. I don't have your answer but you could first specify prob_sl=1 and make the surface increasingly rough by simply changing sigma_alpha, where highly polished (sigma alpha = 0) you'll get spike reflection even though you had set prob_ss=0.

Hope this helps somewhat, Peter

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