[Radiance-general] Glare analysis of BSDF representation of light redirecting glass

[Zack Rogers]

Hi Chris,

I think the higher resolution BSDF definitions, using the tensor tree
representation, will be important for accurate glare analysis.  In Andy and
Greg's presentation here:

http://www.radiance-online.org/community/workshops/2011-berkeley-ca/presentations/day1/GW3_Multires_BSDF.pdf


on page 15 it compares a Klems BSDF with a 2X and 4X version, resulting in
the glare index dropping from 9% to 2%.  They show the Klems resolution is
fine for predicting annual quantities of light.  But we should be careful,
or really just use the tensor tree versions, when using them for glare
analysis.  The 145 Klems patches just spread the lumens too much,
particularly for systems that may have a more focused or directional optic
with sharp cut-offs.

Regards,
Zack

On Mon, Jan 14, 2013 at 1:42 PM, Humann Chris <chris at coolshadow.com> wrote:

> Hi Greg,
> Thank you for the response.  Do you think the BSDF estimate used would
> give a fair approximation of glare were it to be an issue?
>
> Cheers!
> Chris
> Christian Humann ~ Associate
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> On Jan 14, 2013, at 11:41 AM, Greg Ward wrote:
>
> Hi Chris,
>
> Changing the -c setting in genBSDF may result in a more accurate BSDF
> estimate, but it will not increase the resolution of the recorded
> distribution, which is defined by the 145x145 Klems basis.  Near normal
> incidence, the resolution is about 5 degrees, but increases to 15 or 20
> degrees towards grazing angles.
>
> To obtain a higher-resolution BSDF, you will need to switch to the tensor
> tree representation, which means using the -t4 option, probably '-t4 5' or
> '-t4 6' in your case.  The '-t4 5' setting will yield a BSDF with 1024
> incoming and 1024 outgoing directions, which corresponds roughly to
> 2-degree resolution near normal incidence.  The higher setting of '-t4 6'
> gives you about twice the resolution, or 1-degree accuracy.  However, it
> does take about 4 times as long to calculate, and any tensor tree BSDF will
> take much longer to compute with genBSDF than the Klems basis.  Final
> rendering times should be similar, however.
>
> Hope this helps!
> -Greg
>
> From: Humann Chris <chris at coolshadow.com>
>
> Date: January 14, 2013 11:26:16 AM PST
>
>
> Dear group,
>
>
> I have a BSDF description of a light redirecting glass generated with the
> following command:
>
> genBSDF -n 8 -c 20000 -r -ab 6 -ad 1 +f +b -g- millimeter -dim 12.49 12.51
> 12.47449 12.52551 -3.158 0
>
>
> We are using this file to understand the effective light distribution of a
> particular product as well as calculating DGP.   We are wondering if the
> number of samples (20000) used per each of the 145 klems directions would
> be sufficient to give an accurate representation of glare when looking at
> the scene description from inside the space?  I've looked for published
> studies of comparative glare analysis using DGP and BSDF files but have
> come up short.
>
>
> Thanks as always,
>
> Chris
>
>
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-- 
Zack Rogers, P.E., LEED AP BD+C
Daylighting Innovations, LLC
211 North Public Road, Suite 220
Lafayette, CO 80026
(303)946-2310
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