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Forum: Processes Involving Optical Photons
Re: None Questions about spike constant and sigmaalpha (Paloma)
Date: 30 Apr, 2010
From: Gumplinger Peter <Gumplinger Peter>

Dear Paloma,

The SpecularSpike constant refers ONLY to what type of reflection is being simulated.

When you specify 'ground' and give a sigma-alpha, the code calculates internal reflection, Fresnel reflection and Fresnel refraction probabilities based on theFacetNormal return from GetFacetNormal. Now, if Snell's Law comes up with a reflection, that reflection will be simulated off the global normal; see the code in ChooseReflection:

http://www-geant4.kek.jp/lxr/source/processes/optical/include/G4OpBoundaryProcess.hh#L299

I agree this is convoluted but it was adopted from the original logic of the UNIFIED model in the DETECT program. It could make sense to check for the Spike constant in GetFacetNormal and only return theGlobalNormal. However, why treat Spike differently from BackScatter or Lambertian. Only the method ChooseReflection uses the reflection probability constants.

So, yes, you should see different behaviour when you change the sigma-alpha. In general, it is best that you specify polished when you want polished and not try to arrive at the same thing with special combinations of additional parameters further down in the logic.

Best regards, Peter

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