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Forum: Geometry
Re: Question Models and Finishes in G4OpticalSurface (Ben Morgan)
Date: 25 Aug, 2009
From: Gumplinger Peter <Gumplinger Peter>

Sorry, I am very late to reply to this message. It was just brought to my attention.

>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?

Yes, correct! Moreover, in the unified model SPECULARLOBE reflection is simulated, e.g. the sampling of the facet normal is specified by the roughness parameter 'sigma_alpha':

        /* This function code alpha to a random value taken from the
           distribution p(alpha) = g(alpha; 0, sigma_alpha)*std::sin(alpha),
           for alpha > 0 and alpha < 90, where g(alpha; 0, sigma_alpha)
           is a gaussian distribution with mean 0 and standard deviation
           sigma_alpha.  */

For dielectric_metal interface:

>--For the glisur model, polished(ground) finish will produce pure spike(pure

>lobe) reflection

yes

>--For the unified model, polished(ground) finish will produce spike(choice of

>spike, lobe, backscatter or lambertian) reflection

yes

For dielectric_dielectric interface:

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

yes

>--For groundfrontpainted, reflection will be pure lambertian.

yes

>--For polishedfrontpainted, reflection will be pure spike.

yes

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

correct

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

>scintillator crystal.

correct

>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?

With the latest versions of Geant4 you can now specify a complex index of reflection for dielectric_metal surfaces but otherwise there is no difference.

>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,

yes

>or packed in MgO powder,

not necessarily - where is there a gap?

>or wrapped with a glued on reflector,

yes

>or painted with a powder mixed with optical epoxy?

see my comment for the MgO powder above

>However, does the ground (or polished) finish refer to the surface of the

>crystal or the reflective wrapping?

the surface of the crystal; the reflective wrapping is always assumed to be Lambertian!

>From previous posts here I think it refers to the wrapping, but in that case is

>a polished finish assumed for the crystal?

see above, I am sorry if previous posts are confusing in this respect.

>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?

no, the wrapping in 'backpainted' is always a Lamberian reflector, while the glisur and unified models have roughness parameters specifying the LobeReflection and the unified dielectric_metal also allows for the other types of reflections.

Hope this clarifies the situation some more.

Peter

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