|Message: Re: questions about the parameter sigma alpha and ground-, polishedbackpainted||Not Logged In (login)|
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> I have still problems in understanding some definitions, which describe > the reflection and tranmission of photons on the interface. If I have > understood it right then the value sigmaalpha is a parameter of the > surface roughness. 1 indicates maximal roughness and 0 stands for a > polished surface. First I thought, if the model is unified, the type is > dielectric-dielectric and the finish ground, that if sigmaalpha is 0 > than there should be only spike reflection, like on a polished > interface. But this is not the case, because sigmaalpha omly defines the > facet normal which replaces the global normal, if the finish is ground. > It does not have any influence on the kind of reflection: lobe, spike, > backscatter or lambertian.
When the surface is defined as 'polished', the code has it easy; theFacetNormal is theGlobalNormal and Snell's law can be calculated resulting in TotalInternalReflection, FresnelReflection or Refraction and the reflected or refracted ray can be calculated.
When the surface is defined as 'ground', the code, as it is written, samples theFacetNormal (using the sigma_alpha you specified) and then employs Snell's law to arrive at one of the two reflections or refraction. If a reflection occurs, the type of reflection is sampled from the probabilities you specified for UNIFIED model's reflection parameters, SS etc. This will not converge to the same thing as 'polished' even when SS==1 unless you also have sigmaalpha = 0.
With other words, the code needs a normal to calculated the three possibilities for what happens at a dielectric_dielectric surface from physics alone - Snell's Law - there are no random numbers involved. To summarize, the unified model's four possible reflection probabilities come into play only once Snell's Law has already decided on a reflection. Snell's law, however, is calculated from theFacetNormal sampled using sigma_alpha for ground surfaces.
> This is strange because s.o. expects that the > relation between these different reflections should vary with the degree > of roughness. If the model is unified, the type is dielectric-dielectric > and the finish is ground only lambertian reflection should occure.
The code does not do a sanity check on what the user has specified and/or overwrites the user's input. The user must make sure that his/her input is consistent with what is to be simulated. A dielectric_dielectric, ground surface simulates a surface with FacetNormals sampled from a distribution using sigmaalpha. It does not default to Lambertian!
> What > is the difference between the reflection with sigmaalpha=0 and > sgmaalpha=1?
The sampling of the FacetNormal.
What is the physical explanation?
The surface is assumed to be made up of facets, each facet has a normal distributed around the nominal normal of the surface and sampled from a distribution (see the Application Developers Guide), the width of which is defined by sigmaalpha.
> Is the explanation the > following? - sigmaalpha=0 means that there are no macro facets but there > exsists surface roughness, which generates lambertian reflection?
The resulting reflection is termed to be 'LobeReflection' (not Lambertian!)
> I have also other questions: Is the reflection of the wrapping at the > finish groundbackpainted always lambartian or can it be changed to spike > reflection?
The 'ground' and 'polished' part of 'backpainted' actually does refer to the wrapping. The Application Developers Manual was wrong in stating that the external reflector is always a diffuse reflector for those photons which are refracted (transmitted) through the surface interface into the thin dielectric layer.
(1) If you want 'groundbackpainted with a polished surface between scintillator and air use: SPECULARSPIKECONSTANT=1, all others =0, for the polished surface and the Lambertian reflector
(2) If you want 'polishedbackpainted' with depolished surface between scintillator and air use: SPECULARLOBECONSTANT=1, all others =0, for a depolished surface and a polished reflector
Hope this helps, Peter
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