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Forum: Processes Involving Optical Photons
Re: Question difference between polished and polishedfrontpainted (Athreya Shankar)
Re: Feedback Re: difference between polished and polishedfrontpainted (Gumplinger Peter)
Re: Question Re: difference between polished and polishedfrontpainted (Athreya Shankar)
Date: 18 May, 2012
From: Gumplinger Peter <Gumplinger Peter>

Dear Athreya,

> For my convenience, (and hopefully for other users'), I have made a
> diagram summarizing the types of optical surfaces available in the
> unified model. I have provided the details as I understand them. Could
> you please go through it and let me know if it is right?

Yes, it looks quite correct! Thank you for producing this diagram. Would you allow me to include it with my documentation? I will certainly credit it to you when I post it.

> Also, I have two more questions:
> 
> 1. In one of your replies to another user, you have written,
> 
> 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.
> 
> My doubt is about the last statement - even when sigma alpha is not 0,
> and if SS=1, then shouldn't the reflection be exactly like from a
> polished surface? Because, SS=1 (I think) implies a spike reflection
> about the global surface normal always, and therefore the facetNormal
> (and consequently sigma alpha) does not come into the picture at all?

Well, it does come into the picture, for precisely the reason I wrote. Your diagram also shows this to be the program flow. As the code is presently written, the program first decides based on Snell's Law whether we have reflection or refraction. It has to do this based on some normal. It does this based on a normal sampled with sigma_alpha. If sigma_alpha is zero, the sampled normal is the nominal normal. If sigma_alpha is not zero, the decision will be based on a facet normal. If based on the normal used, reflection results, the logic will decide on the type of reflection. If not, refraction will be simulated again based on the normal used.

You argue that the program should always be using the nominal normal if SS=1 and completely ignore sigma_alpha in that case. That's one argument, but that's not what the program does at the moment. Right now it is the user's responsibilty to set (actually leave) sigma_alpha at 0 (zero is the default) when s/he desires spike reflection (and spike refraction). It's pretty contrived for a user to set SS=1 and at the same time purposely set sigma_alpha > 0.

> 2. What is the role of SetPolish in the glisur model? that is, is it the
> same as sigma alpha in the UNIFIED model?

The glisur model has a different parameter with which you define the surface's roughness. Please, see the G4 documentation which in turn refers to the original Geant3 documentation:

http://wwwasdoc.web.cern.ch/wwwasdoc/geant_html3/node240.html#SECTION097033000000000000000000

about the meaning of this parameter.

Best regards, Peter

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1 Feedback: Re: difference between polished and polishedfrontpainted   (Athreya Shankar - 19 May, 2012)
(_ Agree: Re: difference between polished and polishedfrontpainted   (Gumplinger Peter - 22 May, 2012)
(_ Feedback: Re: difference between polished and polishedfrontpainted   (Athreya Shankar - 23 May, 2012)
(_ Disagree: Re: difference between polished and polishedfrontpainted   (Gumplinger Peter - 23 May, 2012)
(_ Ok: Re: difference between polished and polishedfrontpainted   (Athreya Shankar - 23 May, 2012)
(_ Question: Re: difference between polished and polishedfrontpainted   (Erik Dietz-Laursonn - 10 Sep, 2012)
1 Feedback: Re: difference between polished and polishedfrontpainted   (Gumplinger Peter - 18 Sep, 2012)
1 None: Re: difference between polished and polishedfrontpainted   (Erik Dietz-Laursonn - 19 Sep, 2012)
2 Ok: Re: difference between polished and polishedfrontpainted   (Athreya Shankar - 03 Oct, 2012)
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