[Radiance-general] query about accurately modelling glazing

[Daniel LASH(SED)]

Hello Radiance community,

I posted the following query on the Desktop discussion board, though as I don't really use Desktop
anymore I thought it may be more appropriate here;

I am using Ecotect to run Radiance simulations (apologies now for my Unixus ignoramus!), and
changing 
photometric information of glazing using textpad in windows.  Say for example, I 
am modelling clear glazing that admits light monochromatically (I am 
not concerned with colours, only illuminance values on a grid) at a transmittance of 80%
(transmissivity 
0.871538), then the glazing description may look something like this (simple stuff);

# glazing with a transmittance of 80%
void glass window1
0
0
3       0.871538 0.871538 0.871538

I am going to parametrically change the transmittance at say 10% increments from 10% through to
100%, hence I will simply change the rgb transmittance to the appropriate values.

When I open up some of the files in the glazing library included with 
Desktop Radiance, most of them seem to include the BRTDfunc, for 
example, the lbnl clear glass (glass6.rad).  The description looks 
like this;

void  glass       clear6_glass      
0 
0 
3      0.85852      0.97582      0.95237                              
      

void  BRTDfunc      clear6_front      
10      
      0.78802      0.89604      0.87443      
      0.07011      0.0822      0.08231      
      0 0 0 
      .      
0 
9 0 0 0 0 0 0 0 0 0      

void       BRTDfunc      clear6_back      
10      
      0.78802      0.89604      0.87443      
      0.07011      0.0822      0.08231      
      0 0 0 
.      
0 
9 0 0 0 0 0 0 0 0 0      

My question is this.  To what extent will my simplified way of 
describing the transmittance affect the accuracy of my results?  I 
don't completely understand the workings of the BRTDfunc (even after reading the manual page), and
think I 
would have trouble measuring any variable of glazing other than 
transmittance of the specimens I am sampling in the real world (Rendering with Radiance 
talks briefly of measuring glazing transmittance, 
but not things like reflectance.  I have just seen a citation for a paper by Peter Apian-Bennewitz
(1994, Designing an Apparatus for mearuring Bidirectional..., Freiburg) who appears to be a
contributor on here that I am in the process of requesting from my library).

I presume also that transmittance + reflectance (+ absorptance) of 
glazing can never exceed 1.  In Desktop Radiance, the description of 
Generic clear glass lbnl (clear3.rad) states a transmittance of 
89.90% and a reflection of 82.0%.  Is my understanding fundementally 
wrong, or is this a typo?

Many thanks

Daniel

PhD researcher, Sheffield, United Kingdom.