[Radiance-general] Re: query about accurately modelling glazing
Zack Rogers
[email protected]
Thu, 15 Jan 2004 09:35:40 -0700
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