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Detector: T(ρ,angle) [TOfRhoAndAngle]

Mar 20, 2012 at 12:32 AM


I am having a simple model (air-tissue-air) and I am using the TofRhoandAngle detector. When I look at the values of the results in MATLAB, I am getting numbers greater than 1. Are not these values the fraction of photons in that specific (Rho, Angle)? How these numbers related to the Radiance?

Thanks so much for your time.


Mar 20, 2012 at 12:39 AM

Unless there is a bug, the units should be the fraction of transmitted light per-mm^2-per-steradian. Integrating (summing) w/r/t your spatial and angular bins should give you a total transmission fraction. Don't forget that integrating along rho requires accounting for the area of each annular ring (eg PI * r2 * r2 - PI * r1 * r1).

Transmission should be qualitatively similar in behavior/magnitude to the internal radiance near the surface, but there's a big difference: the radiance contains the balance of all light transmitted through the boundary and reflected back from it (due to index mismatch). The transmittance is just part of that quantity.

Hope this helps - let us know. 


Mar 24, 2012 at 3:40 AM

Hi David:

Thanks so much for clarifying. I ran couple of cases, and I just wanted to make sure there is no error in my calculations, and it seems that the number in each element of the matrix T(rho, angle) can go over one. 

But if I multiply than number by the area of the designated ring, it is less than one. Do you think after this multiplication, I have the fraction of photons in that (area(rho1,rho2), angle)?

Thanks for your time,


Mar 24, 2012 at 5:11 PM

Hi Alireza,

The detector T(rho,angle) contains data that has been normalized by the surface area of the annular ring AND the solid angle spanned by each rho, angle bin.  So integrating (as David described above) to get total transmission fraction, you need to account for the area of the annular ring and the solid angle [ 2pi int sin(theta) dtheta ].

One way to debug your calculations is to add in TDiffuseDetectorInput into your infile "DetectorInputs" list (an example of this detector input is in infile_one_layer_all_detectors).  This detector calculates the total diffuse transmittance.  If you've performed the integration correctly over all rho and angle bins, you should get a value that agrees with the value in TDiffuseDetector.