[Radiance-general] Trans Oddities Part Deux

[Rob Guglielmetti]

Jack de Valpine wrote:
> Hey Rob,
> 
> Do the shades all have the same reflectance or (Cr, Cg, Cb)?

No, and this is one of the things I have been wondering about.  It was 
my understanding that when specifying trans, that transmittance was 
soley determined by the Td and Ts parameters (talking about the "Ehrlich 
assumptions" that Chas has set up in RwR).  But perhaps that's it, the 
net transmission of the trans is calculated *after* the reflectance has 
been taken into account?  I did a test of that while testing all these 
shades, where I had a 5% transmissive shade, and ran it once with the 
reflectance at 20%, and another where the shade was 70% reflective, but 
the results were very similar, so I thought that my original assumption 
was true, that transmission was unaffected by reflectance.  But in 
looking at the data from the second pass of these tests, where Greg 
suggested the -lw bit, the values for the 70% reflective shade are much 
lower than the values for the 20% reflective shade.  Hmmm...

However, the difference in reflectance of my shades is not that great. 
the 10% shade has a 30% grey reflectance, and the 5% and the 1% shades 
are 20% reflective.  I don't see how a 10% change in reflectance could 
affect the calculated values by 100%, which is what's happenning.  Hmmm, 
again.

Still, thinking about it right now, it seems like that might be it.  It 
seems obvious.  Now.  I will test it out right now.  If this is the 
culprit, I owe you a beer.

> I think you indicated that the glass is translucent, what is the makeup? 
> How are you measuring it?

The glass is another story, I won't bore you with the details, I'm 
already way over my bandwidth limit on this list this month, but it was 
extensively measured and validated against a 1/2 full scale physical 
mockup, and inspected by Greg.  I'm confident in my glass material 
definitions, at least.

----

      Rob Guglielmetti

e. rpg@rumblestrip.org
w. www.rumblestrip.org