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Forum: Processes Involving Optical Photons
Re: None polishedbackpainted calculation order (Robert Penny)
Date: 16 Apr, 2010
From: Gumplinger Peter <Gumplinger Peter>

Hi Rob,

Thanks for taking the time to post your thorough investigation. Let me first warn you of a recent bug I found in the code; please see the thread below:

http://hypernews.slac.stanford.edu/HyperNews/geant4/get/opticalphotons/313/1/1.html

And also caution you of a wrong explanation in the manual:

The 'ground' and 'polished' part of 'backpainted' actually does refer to the wrapping!!! - The Application Developers Manual is wrong in stating that the external reflector is always a diffuse reflector for those photons which are refracted (transmitted) through the surface interface into the thin dielectric layer.

Now, with the fix above, the logic should be the following:

(1) If you want 'groundbackpainted with a polished surface between scintillator and air use: SPECULARSPIKECONSTANT=1, all others =0, for the polished surface and the Lambertian reflector

(2) If you want 'polishedbackpainted' with depolished surface between scintillator and air use: SPECULARLOBECONSTANT=1, all others =0, for a depolished surface and a polished reflector

You are using 'polishedbackpainted' and hence you should expect always spike reflection off the wrapping.

> So.... how would I simulate the reflections from inside a high-index

> perfectly smooth optical surface, wrapped with an infinitesimal air gap

> then backed with a lambertian reflector?

see above (1)

Independent of what you specify for the backing, you can assign the Unified constants of the initial (scint/air) interface at will. So, for example, you can combine SPECULARLOBECONSTANT=1 with 'groundbackpainted' etc.

With SPECULARLOBECONSTANT you need the sigma_alpha surface roughness parameter to specify just how rough your surface is. The back reflector can only be either SPIKE (polished) or Lambertian (ground). The code is set up to allow multiple encounters for the transmitted photon between the (now) air/scint and back reflector surfaces.

The reply took time because I was away from computers since you posted.

Hope this helps, Peter

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1 None: Re: polishedbackpainted calculation order   (Robert Penny - 29 Jun, 2010)
(_ None: Re: polishedbackpainted calculation order   (Robert Penny - 29 Jun, 2010)
(_ Warning: Re: polishedbackpainted calculation order   (Gumplinger Peter - 29 Jun, 2010)
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