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Question Models and Finishes in G4OpticalSurface 

Keywords: G4OpticalSurface, glisur, unified
Forum: Geometry
Date: 20 Jul, 2006
From: Ben Morgan <Ben Morgan>

Hi,

I've, for the first time, been adding optical surfaces into a Geant4 application, and I'm trying to understand which surface finish (ground, polished etc) to apply to different situations, how these affect the optical processes that can take place, and how the glisur and unified models deal with the finish.

From the Application Developers manual and the UNIFIED paper, I think I understand the main difference between the glisur and unified models - the former uses the 'polish' parameter to model surface roughness, whereas the latter uses the SPECULARLOBECONSTANT, SPECULARSPIKECONSTANT and BACKSCATTERCONSTANT property tables (diffuse reflection being implicit from the sum of these tables). Is that correct?

The dielectric_metal interface seems straightforward, but am I correct in thinking that

--For the glisur model, polished(ground) finish will produce pure spike(pure lobe) reflection or absorbtion.

--For the unified model, polished(ground) finish will produce spike(choice of spike, lobe, backscatter or lambertian) reflection or absorbtion.

However, dielectric_dielectric has more possibilities. In the case of frontpainted am I correct in the following

--For the frontpainted finish, only reflection and absorbtion are possible.

--For groundfrontpainted, reflection will be pure lambertian.

--For polishedfrontpainted, reflection will be pure spike.

--There doesn't appear to be any distinction between glisur and unified models.

--I'd use this for e.g., a diffuse or specular reflector coating painted onto a scintillator crystal.

If that last point is correct then it raises the question - what's the difference between a polishedfrontpainted dielectric_dielectric surface and a polished dielectric_metal surface?

I'm clear about the backpainted finish being for cases where there is a small gap between the main volume and reflective surface - so I'd apply this to e.g. a scintillator wrapped with Al foil, or packed in MgO powder, or wrapped with a glued on reflector, or painted with a powder mixed with optical epoxy? However, does the ground (or polished) finish refer to the surface of the crystal or the reflective wrapping? From previous posts here I think it refers to the wrapping, but in that case is a polished finish assumed for the crystal? Also, the code in G4OpBoundaryProcess suggests that reflection from the wrapping is treated in the same way as for dielectric_metal (with the same distinction between glisur and unified models) - is that the case?

Apologies if these are stupid questions, but I want to make sure I fully understand what's going on!

Thanks,

Ben Morgan.

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1 Feedback: Re: Models and Finishes in G4OpticalSurface   (Gumplinger Peter - 25 Aug, 2009)
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