[Radiance-general] Re: query about accurately modelling glazing
Daniel LASH(SED)
[email protected]
Thu, 15 Jan 2004 10:48:22 +0000
Hi Zack and Phil.
Thank you very much for your most helpful suggestions. My experiments are in two parts. Firstly,
I am attempting to simulate as accurately as possible the lighting conditions under a real atrium
roof canopy, and comparing with illuminance measurements measured directly under the roof. In
terms of modelling it accurately, I think Optic5 (and associated database) will be a useful way to
derive transmittance information. I have still not quite decided whether to take the information
(calculated transmittance of multi-layer configurations) from the export and create a simple
'glass' primitive, or to use the BRTDFunc generated. Perhaps I will experiment and see whether it
makes much of a difference.
The second part involves a systematic varying of the transmittance of the glazing in the
simulation. Here I believe it would be appropriate to use the simple glass primitive, and vary the
r,g,b,n parameters (though I think n should stay constant at 1.52 (or whatever it is...), and I
think it said in the manual page that having only the r,g,b arguments has Radiance use the default
n, fine for my needs). I thought that the BRTDFunc may have included some function depending on
angle of incidence that would make it more accurate than the glass primitive, but as it doesn't, I
will stick with the glass (as I am dealing with overcast skies, the 'grazing angle' changes in
transmittance may not be as critical (though still a factor) as if I were looking at direct light).
Once again many thanks
Dan Lash
PhD researcher
Sheffield Hallam University, UK
[email protected] writes:
>Hi Zack, Daniel,
>
>The BRTDfunc definitions delivered by desktop radiance are a bit difficult. they do allow for
>different reflectance from front and back faces, but there are some major problems with it.
>
>firstly, to get it to work, you need two glass polygons for each window, one for the front and one
>for the back.
>
>secondly, what does this do to the transmittance? physically, the transmittance going one way
>should be the same as the transmittance going the other way. should the (rtrns, gtrns, btrns)
>variables then be the same for both front and back panes? or does it not matter, as long as the
>product of their transmittances is equal to that of the combination.
>
>finally, there are no functions built in. that means that there is no dependence on angle of
>incidence for either transmittance or reflectance. this is unrealistic, as transmittance
>generally reduces and reflectance generally increases with increasing angle of incidence.
>
>so, i never use the BRTDfunc materials delivered by desktop radiance. if i understand correctly,
>these definitions are provided by Optics 5 (or 4, or something like that). is this right? if it
>is, then i wouldn't use the Optics output either.
>
>i like the look of Zack's definition using glazing.cal. might have to give that a burl. what i
>have been doing is creating 'glass' primitives and setting the r,g,b components and refractive
>index to give me the desired normal reflectance and transmittance, and hoping that the resulting
>angular dependence is about right. have created a spreadsheet with Greg's equations to determine
>the (r,g,b,n) parameters.
>
>Phil.
>
>>>> [email protected] 15/01/2004 12:11:56 pm >>>
>Hi Daniel,
>
>>My question is this. To what extent will my simplified way of
>>describing the transmittance affect the accuracy of my results?
>>
>Defining glass using the BRTDFunc method just lets you more accurately
>define the front and back reflectance of the glass. Defining glass with
>just the glass primitive uses a default front and back reflectance.
>
>>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 recommend looking at Optics 5 from LBNL.
>
>http://windows.lbl.gov/materials/optics5/
>
> It has a very extensive database of just about every type of glass and
>glass compositions available and allows you to build up various double
>pane combinations and save it all as a Radiance input file. Rather than
>measuring a sample in the real world and defining it in Radiance, you
>could probably find the glazing in this database and use Optics 5 to
>define it.
>
>>
>>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?
>>
>>
>I've always been a little confused how desktop radiance defines glass.
>The front RGB reflectance is actually defined with this line under
>"clear3_front"
>
>0.07428 0.08322 0.08556
>
>and the back RGB reflectance is defined with this line under "clear3_back"
>
>0.07567 0.08418 0.08538
>
>So the reflectances are roughly 8% which does obey the rule.
>
>I am not completely clear what the other RGB values are but my guess is
>that they modify the transmittivity defined under clear3_glass somehow.
>Whenever I've manually defined glass I've used this form (from
>"glazing.cal") instead, its just a little clearer to me.
>
>mod BRTDfunc my_glazing
> 10 rrho grho brho
> rtau gtau btau
> 0 0 0
> glazing.cal
> 0
> 18 0 0 0
> 0 0 0
> 0 0 0
> FRRHO FGRHO FBRHO
> BRRHO BGRHO BBRHO
> RTAU GTAU BTAU
>
> where:
> FRRHO FGRHO FBRHO is front normal spectral reflectance
> BRRHO BGRHO BBRHO is back normal spectral reflectance
> RTAU GTAU BTAU is normal spectral transmittance
>
>Hope this helps!
>
>Zack
>
>--
>Zack Rogers
>Staff Engineer
>Architectural Energy Corporation
>2540 Frontier Avenue, Suite 201
>Boulder, CO 80301 USA
>
>tel (303)444-4149 ext.235
>fax (303)444-4304
>www.archenergy.com
>
>
>
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