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Forum: Processes Involving Optical Photons
Re: Question Unified model ground surfaces (Andrzej Zuranski)
Date: 08 Apr, 2009
From: Gumplinger Peter <Gumplinger Peter>

Hi Andrzej,

> and specify a surface there with the air gap and properties

So, you have a light guide (dielectric), and an air gap, separating the former from the PMT window (glass?) Are these three G4 volumes with index of refraction r_lightguide, r_air=1, and r_pmt-window?

When you specify a surface as having an index of refraction, this surface can only be of type 'groundbackpainted' or 'polishedbackpainted'. If there is a thin air gap between two volumes, such as the light guide and the PMT-window, then you must implement all three volumes as physical volumes in Geant4.

G4double RindexCon[n]={1.,1.};

With 'unified', 'ground' and dielectric_dielectric, the index of refraction you are giving (above) to the surface is ignored.

> requiring that the PreStepPoint is in the light guide and the PostStepPoint is

>in the PMT window and the Step Status is fGeomBoundary

This is correct. A ray arriving at the boundary from the light guide will have this info at the step level.

> Suprisingly some of the statuses for the polished surface were Fresnel

> Reflection and Total Internal Reflection.

Why is that surprising to you? I presume the r_lightguide > r_pmtwindow

> Is this right that the PreStepPoint and PostStepPoint always are on the

> opposite sides of the surface even though the photon is reflected, so it stays

> in the same volume?

Yes, as the photon arrives this is the case. When there is reflection, an intermediate 'null' step is taken and the reallocation happens - the new PostStepPoint is back in the original volume and the PreStepPoint is swapped with the original PostStepPoint.

> Spike, Lobe, BackScatter or Lambertian but still majority of them were saying

> Fresnel Refraction.

Having these types of possible reflections does not change the original logic of when there is Fresnel Refraction. Only the reflected light can now be either one of these types.

> Shouldn't we expect for the ground surface that there are no longer Fresnel

> refractions, but only the above four types?

Why, 'ground' refers to how rough the surface is, not that it is totally reflecting.


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