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Source used for Scaled MC in VTS GUI

Mar 16, 2015 at 10:56 PM
I am using the Scaled Monte Carlo model in the VTS GUI to simulate spatially-resolved reflectance, and I was wondering what type of source is used in this model. Is it a point source at the origin?

Thanks,
Robert Wilson
Coordinator
Mar 16, 2015 at 11:36 PM
Hi Robert,

Yes, point source at the origin, pointed in the positive z-direction (downward into medium).

David
Developer
Mar 16, 2015 at 11:36 PM
Hi Robert,
The scaled MC, both Basic and NURBS, use a collimated point source at the origin.
Carole
Developer
Mar 16, 2015 at 11:39 PM
Jinx!
Mar 17, 2015 at 6:24 PM
Thanks! For that same model, how are the detected photons spatially binned? For instance, if you have 10 detectors with rho values from 0-9 mm, is the first detector collecting everything from 0-1 mm, the next detector collecting everything from 1-2 mm, and so forth (with the last detector collecting everything from 9 mm onward) or is it split up such that none of the photons with rho>9 mm are detected?

Thanks again,
Robert
Developer
Mar 17, 2015 at 10:16 PM
Hi Robert,
For the Basic MC results, using the baseline optical properties, the R(rho) results flatline at about 4cm. So I think photons beyond rho>4cm are not detected. This method uses uniform bins of drho=0.2mm, dtime=0.005ns. The NURBS MC results uses adaptive binning, so the bins in far-field and long times are adjusted so that the variance in those bins is reduced. Plotting this solver goes out to about 10cm and then there is no data, so no data collected beyond rho>10cm.
Carole
Mar 17, 2015 at 10:35 PM
Hi Carole,

Thanks for your reply. I'm still a bit confused though, because the Basic MC allows you to define the rho range and the number of rho bins in that range.
So if (last rho value - first rho value)/number of rho bins = something other than 0.2 mm, then the bin size drho would be something other than 0.2 mm, correct?

Thanks,
Robert
Developer
Mar 17, 2015 at 10:45 PM
Hi Robert,
I probably should have listed the rho (and time) bin sizes. The rho bin size for the baseline simulation is not really of importance to the user because the method scales the results to whatever the user specifies for rho locations. So the user can specify any Begin, End and Number. For example, if you specify Begin = 1mm, End = 6mm and Number=6, then the results are displayed for rho=1,2,3,4,5,6mm.
I hope this helps,
Carole
Mar 17, 2015 at 11:53 PM
Hi Carole,

Thanks, this does help. My follow-up question is: For the case you mentioned above (begin=1mm, end=6 mm, number=6), what is the range of spatial positions encompassed by "rho=1 mm"? Does everything between 1 mm and 2 mm get put into the "rho=1 mm" bin? Or is it everything from 1mm to 1.5 mm (or everything from 0-1.5 mm?), and then everything from 1.5mm to 2.5 mm gets put into the "rho=2 mm" bin, etc?

Thanks again,
Robert
Developer
Mar 18, 2015 at 12:28 AM
Hi Robert,

It doesn't really work like that. Once the baseline MC is executed, the baseline reflectance is taken at the midpoints of the bins. That reflectance is stretched or shrunk (scaled) depending on the optical properties specified and the specified rho is scaled accordingly to get the designed reflectance. So in the final reflectance prediction for a given rho, the reflection is determined at that rho point location.

I hope this makes sense. It might help to look at Kienle and Patterson's 1996 paper "Determination of the optical properties of turbid media
from a single Monte Carlo simulation" . This describes the Basic MC algorithm.

Carole