Theory 
The specific intensity
of light in a three-dimensional infinite medium with a point source at 
the origin obeys the following RTE.
where
Here we assume linear scattering and the scattering phase function is given by
The energy density U is obtained as
where v is the speed of light in the medium.  
Since U is spherically symmetric, we measure U along the 
z-axis.  
We have
See the report for furhter details.
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