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Warning Re: strange total internal reflection when using polishedbackpainted 

Forum: Processes Involving Optical Photons
Re: Question strange total internal reflection when using polishedbackpainted (Akira Okumura)
Re: Note Re: strange total internal reflection when using polishedbackpainted (Akira Okumura)
Date: 15 Feb, 2010
From: Gumplinger Peter <Gumplinger Peter>

Dear Akira,

Thanks for being so thorough with your simulation and with trusting Geant4!

You found a bug - and my explanation of 'polishedbackpainted' at

http://hypernews.slac.stanford.edu/HyperNews/geant4/get/opticalphotons/305/1.html

does not tell the full truth. It is true that the sigma_alpha parameter is NOT used at this stage to calculate the roughness of the boundary since the boundary is assumed to be polished but the probability parameters of the UNFIED model: SPECULARLOBECONSTANT, SPECULARSPIKECONSTANT and BACKSCATTERCONSTANT are NOT ignored. In fact, since they are by default zero, the reflection chosen is always the default, which is Lambertian.

The code at:

http://www-geant4.kek.jp/lxr/source/processes/optical/src/G4OpBoundaryProcess.cc#L813

should read:

if ( theModel == unified &&

     (theFinish != polished && theFinish != polishedbackpainted))

                                                        ChooseReflection();

Similarly, at:

http://www-geant4.kek.jp/lxr/source/processes/optical/src/G4OpBoundaryProcess.cc#L888

(the logic is becoming really spaghetti)

I have no explanation why photons escape your box.

As in N06PhysicsList.cc (examples/novice/N06) can you set:

G4int verbose = 1; theBoundaryProcess->SetVerboseLevel(verbose);

and pipe the output to a file. In your situation with n>>1 theBoundaryProcess (without the correction above) should only ever write:

LambertianReflection,

and with the correction:

TotalInternalReflection

besides

*** StepTooSmall ***

(for that Navigator 'reallocation step' which follows every reflection)

Peter

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