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
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