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

[Phillip Greenup]

Hi again,

I'm afraid i don't have on hand a good reference for transmittance being the same either way through a glazing system.  i like the explanation given on that web site.  such a glazing system would probably break the third law of thermodynamics.

about your comparison below...  the desktop radiance glass material file contains definitions of three different materials (clear3_glass, clear3_front, clear3_back).  but, when DR creates the associated geometry files, they refer only to one of these materials (clear3_glass in this example).  so, the created BRTDfunc materials are not used.  that means that you will not get different reflectances from different sides of the materials, and that you will get a normal reflectance that doesn't necessarily match what is measured.  the only way to get both the transmittance and reflectance you are after with the glass primitive is to alter the refractive index.  you still don't get different reflectances for the different sides.  but, normally, you are only concerned with one side of the glass (eg. creating visualisation or predicting illums inside and not outside), so just use the reflectance of that side (eg inside face).

the BRTDfunc you created with the inbuilt functions will likely work better than the glass primitive with an altered refractive index.  the new forms (glass1.cal, glass2.cal and glaze.csh) will probably be better yet.

Phil.

>>> zrogers@archenergy.com 16/01/2004 3:35:40 am >>>
Hello,

>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.
>
This is true for a symetric glass composition (ie. single pane no low-e, 
double pane, clear, no low-e) but not true for non-symetric glass which 
is often the case for glass with low-e and glass with one of the panes 
tinted.  I believe the front and back transmittance is always really 
close, (ie. it can't be 10% in one direction and 90% in the other) but 
they can vary slightly.  This is what desktop radiance and optics 5 
definition method allows you to do.

>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.
>
I do think Desktop Radiance and Optics 5 both make use of the angular 
transmittance function.  This is part of the glass primitive.  And in my 
example, the BRTDFunc calls glazing.cal which provides the angular 
transmittance function.  Also, from my comparisions a while ago now, the 
two methods gave me identical results.  That is:

void glass      clear3_glass
0
0
3    0.92189    0.98612    0.972
                       
void BRTDfunc    clear3_front
10   
    0.84636    0.90553    0.89251   
    0.07428    0.08322    0.08556   
    0 0 0
    .   
0
9 0 0 0 0 0 0 0 0 0
   
void BRTDfunc    clear3_back   
10
    0.84636    0.90553    0.89251
    0.07567    0.08418    0.08538
    0 0 0
    .   
0
9 0 0 0 0 0 0 0 0 0

is equivalent to:

void BRTDfunc clear3_glass
    10    rrho    grho    brho
          rtau    gtau    btau
          0    0    0
          glazing.cal
    0
    18    0    0    0
          0    0    0
          0    0    0
          0.07428    0.08322    0.08556
          0.07567    0.08418    0.08538
          0.84636    0.90553    0.89251

Also, you might want to check out glass1.cal and glass2.cal which is 
part of the newest HEAD radiance release.  It just distinguishes between 
single pane and double pane which have different angular dependance 
functions.  Also, check out glaze.csh in the latest release, I was just 
informed of this and have not checked it out personally, but I 
understand it helps create these definitions.

So, I think the BRTDFunc methods can provide greater accuracy than the 
glass primitive.

I hope I have not mistated any of this, anyone please correct me if so.

Regards,
Zack



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