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Keywords: optical, efficiency
Forum: Processes Involving Optical Photons
Date: 19 Nov, 2004
From: Gumplinger Peter <Gumplinger Peter>

1) I've noticed that there are fluctations about the user specified efficiency value entered as an optical surface property. Specifically the efficiency appears NOT to be a simple multiplicative factor. I assume the fluctuations about this specified mean are Poisson in nature. Can this be confirmed as there is no documentation I have found that clarifies this?

No, the efficiency you specify should be the one used - just one simple random number for those photons which are not reflected. You should find that the number of photons 'detected' are e*(r-1), where r is the reflectivity you have specified and e is the detection efficiency you specify. However, there is one important caveat for rough surfaces, or rather, there WAS this caveat until G4 6.2 patch2 (what are you using?):

The value of the reflectivity you provide was for a 'single reflection' off the surface. What happens for very rough surfaces is that the photon, after being reflected from a micro-facet, whose normal happens to have a large angle with the nominal surface normal, still points (has a direction) out of your volume and hence within G4OpBoundaryProcess 'reinteracts' with the surface. The reflectivity is again applied and as a result, the rougher the surface the smaller the observed effective reflectivity. So, there was a correlation between roughness and observed reflection reflectivity and hence detection efficiency. - This correlation was fixed in the latest patch!

2) I understand that I can give the efficiency as a function of optical photon energy. Is the efficiency array treated as a histogram (constant mean between array entries) or is there interpolation between array values?

There is a simple linear interpolation between bin values as is the case for any G4MaterialPropertyVector (see G4MaterialPropertyVector:: GetProperty)

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