[Radiance-general] Accurate definition of trans material

[Rob Guglielmetti]

Tim Hanson wrote:

> Please can you clarify the following re- trans material...

> Do I use specular transmission with diffuse transmissivity when setting the
> trans parameters?

Hi Tim,

To be honest, the one project I've done so far that made heavy use of 
trans was approached in a different manner.   I took front-side 
illuminance & back-side luminance readings of my samples, shining a 
theatrical spotlight at them.  These numbers were sent to Radiance 
Central High Command (Albany, CA) via secret email address 
(gward at lmi.net), and the Radiance Grand Poobah himself derived proper 
values for trans.  He did this by old-fashioned trial & error, trying 
various trans values and plotting the results against my datasets, 
tweaking until the curves more or less matched up.  I summarized this 
process at radiance-online.org recently, even included sample code that 
Greg used to create the plots.  Here's the post:

http://www.radiance-online.org/pipermail/radiance-general/2004-January/001415.html

I never could get that shell script to work, but the bgraph input file 
works if you feed it directly to bgraph.  That input file also contains 
the Rosetta Stone you're looking for, I think: (excuse the wrapping)

B(x)=Td/PI+Ts/(PI*sq(alpha)*sqrt(cos(theta)*cos(PI/180*x)))*exp(-
sq(tan(PI/180*x-theta)/alpha))

That's where Greg mathematically described the Radiance trans behavior, 
to be overlaid with my dataset from the actual sample measurements.  You 
will see the variables Ts and Td in there; those are Ts & Td from trans. 
  I should add that these samples did not have very much of a specular 
transmitted component. I really don't know if this method would work if 
there is a higher amount of specular transmission...

Hmmm, after all that, I didn't even answer your question!  In essence, 
we worked backwards to arrive at a valid description.  To definitively 
answer your speculodiffusetransmissionivity question will require more 
brain power (cue lights; Chas enters stage left, Schorsch from stage 
right, and Greg arrives, deus ex machina-style, in a large winged 
chariot from above).  =8-)

----

      Rob Guglielmetti

e. rpg at rumblestrip.org
w. www.rumblestrip.org