A quick correction to myself....
the second law of thermodynamics, not the third... entropy, as Georg said.
Phil.
[email protected] 16/01/2004 9:41:36 am >>>
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.
[email protected] 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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