[Radiance-general] Translucent material modeling with Radiance

[Greg Ward]

Hi Thibault,

It seems like no one responded to this.  First, you might want to read through Peter Apian-Bennewitz's recent review of the various Radiance types, which he has posted at:

	http://www.pab.eu/docs/Review_of_simulating_four_classes_of_window_materials_for_daylighting_with_non-standard_BSDF_using_simulation_program_Radiance.pdf

Peter does an excellent job of explaining the different types, what they do, and when they are useful -- something that has been missing from the documentation and literature for decades, now.

What I think you will learn as it applies to your case is that without an interreflection calculation, the transdata and transfunc types will not give you a solar patch on the interior of your test space no matter what your material parameters.  The BSTDfunc type would, however.

Cheers,
-Greg

> From: T.Charles at maxfordham.com
> Date: November 14, 2013 11:01:49 AM PST
> 
> 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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