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

[Greg Ward]

A minor point.  Unless they changed the laws of physics since I was in 
college, transmittance has to be the same for a surface seen from the 
front as seen from the back.  This is called Helmholtz reciprocity, and 
is one of the basic tenets of reflection theory.  Total (hemispherical) 
transmittance must be the same, and specular (directional) 
transmittance must be the same in every direction.  It has to do with 
the time-reversibility of light, I think.

-Greg

> From: Zack Rogers <zrogers@archenergy.com>
> Date: January 15, 2004 8:35:40 AM PST
>
> 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.