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[EnergyPlus_Support] Tubular Daylighting devices - Beam visible transmittance

Dear all,

I am currently modeling a lightpipe (tubular daylighting device)  with 
E+.   However, I find a problem.  My modeled lightpipe is a vertical one 
with an aspect ratio of 11.2 and the pipe reflectance is 0.98.  It is 
noted that, the beam visible transmittance larger than 0 if and only if 
solar altitude larger than 55deg (i.e. incidence angle should be smaller 
than 35deg).

As I know that the built-in model is based on Swift and Smith (1995) 
model, and I has done a cross checking.  It is noted that the 
transmittance will approach zero only if it has a angle of incidence 
more than 85deg.  And even the angle of incident is 80deg, the 
transmittance should still about 20%.

So I would like to ask does the beam visible transmittance is account 
for the transmittance of light from direct sun?  Also does there is any 
assumption other than those mentioned in Engineering Document used for 
lightpipe calculation.  I have check across the source code, and I 
cannot find anything wrong in the code.


Regards,
Ernest Tsang


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