[Radiance-general] Re: query about accurately modelling glazing

[Phillip Greenup]

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.

>>> zrogers@archenergy.com 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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