[Radiance-general] Translucent material modeling with Radiance

[T.Charles at maxfordham.com]

Dear Radiance users and experts,

As an engineer in a the building industry, I use Radiance to model different facades options in terms of daylighting. I also use Daysim for annual calculations to estimate annual illuminance exposure and other metrics (DA, etc.).

I'm currently working on a translucent glazing option and below is my understanding of the several possibilities I have to model it with Radiance.
Please feel free to correct me.

The easiest way is to use trans but I understand that this does not allow one to model an angle-dependant transmittance and reflectance.
To overcome this limitation, I could use transfunc or transdata but the angle-dependant transmittance/reflectance only applies to the specular components and not the diffuse ones.
I understand that BRTDfunc suffers from the same limitations even if it allows the user specifying more distribution functions (mirror, reflectance, transmittance lobes).
>From my understanding then only a BSDF material allows a complete material description. This can be achieve with Window and (or?) the genBSDF routine.

Moreover, on the use of transdata/transfunc, I understand from the Radiance documentation that the only modified value is the transmitted specular component so the last one called tspec. Is that correct ?
I've carried out some calculation with a trans and transdata material following the process described in Reinhart's paper (Development and validation of a Radiance model for a translucent panel).

trans:
void trans south_panel 
0
0 
7 0.40446 0.40446 0.40446 0.08 0 0.435635 0 

transdata
void transdata south_panel 
4 noop refl.dat rang.cal rang 
0
6 0.40446 0.40446 0.40446 0.08 0.435635 1

Reading the paper, I though that the values contained into the refl.dat file where multipliers of the transmitted specular fraction (tspec) set to 1 (last parameter of the transdata). Then I ran a simulation with a sunny sky with sun (simple square box with a window). 
In the case of the trans material, with no transmitted specular component, I was expecting to see a perfect diffuser which was the case.
In the transdata case, I expected to see a patch of light on the floor as the transmitted specular fraction was not zero (from my understanding at least). Instead, the transdata material seemed to behave as a trans material with no transmitted specular component.
Am I missing something ?
These results really confuse me.

Thank you all for your support.

Regards,

Thibault



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