[Radiance-general] simulating fritted glazing systems

[Jack de Valpine]

Hello:

I am interested in people's comments on how best to simulate frit 
glazing systems from a performance and appearance standpoint.   We are 
interested in comparing the appearance based on available performance 
data of a variety of fritted glazing systems on the exterior of a 
building system under daylight conditions. The fundamental challenge it 
seems to me is how to use the data that is readily available (from 
software such as LBL's Optics5 as well as from manufacturers themselves) 
to make an appropriate material in Radiance and whether the readily 
available data is in fact sufficient to describe the material behavior.

All the frit systems that we are looking at use the same basic 
insulating glass with the following makeup:

    * (exterior) 6mm clear glass with low-e coating on the second surface
    * (airspace) 13.2mm
    * (interior) 6mm clear glass

A Radiance material description (brtdfunc) for this makeup can be 
generated by the LBL Optics5 program. Note that Optics5 outputs material 
descriptions for the glass primitive as well as interior and exterior 
descriptions based on the BRTDfunc. In this case I believe that the 
"front" version of the BRTDfunc represents the exterior and is the 
appropriate description to use for this study.

void  glass       ve1-2m_glass
0
0
3     0.712     0.791     0.719

void  BRTDfunc    ve1-2m_front
10
      0.111     0.114     0.116
      0.653     0.726     0.659
      0 0 0
      .
0
9 0 0 0 0 0 0 0 0 0

void  BRTDfunc    ve1-2m_back
10
      0.112     0.125     0.122
      0.653     0.726     0.659
      0 0 0
      .
0
9 0 0 0 0 0 0 0 0 0


The information that we do know is as follows:

    * measured photopic transmittance, and exterior and interior
      reflectance of base insulating glass system (70%, 11% and 12%)
    * radiance material description of  base insulating glass system
    * measured photopic transmittance, and exterior and interior
      reflectance of selected composite frit glass systems (ie the
      composition of the frit silkscreen [white] onto the 2nd or 3rd
      surface of  the base insulating glass system, for example 45%,
      22%, and 24% based on a 50% frit on the 2nd surface) , note that
      as far as I can tell it is not possible to build up such as system
      in a program like Optics5 (unless additional baseline measured
      data can be obtained from the manufacturer)

Is it sufficient to figure out the percentage variations for the 
transmittance and reflectance values between the original glass and the 
composite frit glass and then modify the brtdfunc material description 
accordingly? I know that this will not completely account for the change 
in color of the glass due to the white silkscreen on the 2nd or 3rd 
surface. But is this a reasonable approximation to start with.

Another thing to consider is the 'modeling' of the actual frit pattern 
on the glass. My thinking is that this is only important for cases where 
the glass is very close to the viewer. Typically a lot of the frit 
patterns (such as dot and lines) tend to disappear the the farther away 
from the building you are. Now in the cases where the view is close to 
the glass, what is a good way to build a composite material in Radiance 
(a mixfunc is probably a good starting point)? We know the data for the 
base insulating glass but what is a good way to figure out the 
reflectance of the frit silkscreen as seen through the glass?

I will look forward to your suggestions and comments.

Regards,

-Jack de Valpine