[Radiance-general] Measuring materials

[Rob Guglielmetti]

Recently, Visarc Jack said:

>  I think that Rob G. could give everyone a really superb summary on  
> how to setup a measuring environment for glazing materials.

Then he went on to say:

> Depending on the application is might be worth checking out glaze.csh  
> in the latest release. This calculates models for both single and  
> double surface glazing materials. I am not sure how it compares to  
> glazing.cal in terms of the accuracy of the underlying model. Note  
> however that glaze.csh is built around a handful (5 or 6) sets of  
> reflectance and transmittance values as it "database". Some of these  
> were actually measure by me and looking back on it, I would prefer to  
> measure the "pvb" materials under a more precise protocol than I did  
> originally (again Rob G. would be the real resource here).

Ha.  Well, Greg's the real resource here.  All I did was come up with a  
cockamamie scheme to use a luminance meter and an illuminance meter to  
try and figure out a diffuse glass sample we had; Greg's the one who  
provided the brains to both verify that my scheme would actually work,  
and to implement the procedure.  The materials we measured were also  
mundane enough to be modeled with just a trans; we didn't need to use  
BRTDfunc.  But here's what we did:

I had a couple of glass samples, an ellipsoidal reflector spotlight  
("leko"), a Minolta illuminance meter and a Minolta luminance meter.   
What I did was aim the spotlight at the glass, take an illuminance  
reading incident on the front of the glass, then a luminance reading on  
the back of the glass.  Dividing the luminance by Pi, then by the  
illuminance, gave me a reasonable estimate of transmittance.  We took  
luminance readings at 15 degree intervals from nadir to +- 60 degrees,  
for incident angles of 0-45 (also in 15 degree increments).  There's a  
picture of the measurement rig here:

http://www.rumblestrip.org/stat-img/setup.jpg

The transmissions were then hastily emailed to Greg Ward, who did some  
trial & error with trans parameters, using bgraph to visualize the  
transmission models until the trans definitions matched the measured  
dataset.   Greg sent me a valid trans description, but I of course  
wanted to know how he accomplished this.  Greg's email that explained  
what he did included this shell script:

#!/bin/csh -f
#
# Plot one or more files and display them
#
set dest=/tmp/plot$$
bgraph $* | meta2tga -x 700 -y 700 -o $dest
open -a Preview ${dest}*.tga

... but I could never get this to work on my end.  If you or anyone  
else on the list wants to have a look at this and help me out, I'd  
appreciate it.  When Greg first sent me this stuff, I really didn't get  
it at all.  I just used his trans material definition.  But in  
reviewing it all for this email, I understand it now.  I just lack the  
understanding of bgraph to fully troubleshoot this script on my  
computer.  But it's a very useful technique!  WHat he did was take each  
dataset I sent him and used that for the "Adata", then used a  
mathematical model based on the trans parameters for the second  
dataset, the "B".  He played with the Ts Td and roughness (alpha)  
parameters and used bgraph to see how they matched up.  Here's a sample  
bgraph input file; this is for the 15 degree incident angle (reflected  
in the theta variable) dataset:

include=polar.plt
xmin=-90
xmax=90
Adata=
-60     0.086210394
-45     0.103198913
-30     0.174652116
-15     0.244228975
0       0.363959998
15      0.424915818
30      0.304880523
45      0.201275914
60      0.146151974
;
Acolor=2
Bcolor=4
theta:15*PI/180
sq(x):x*x
Ts:.31
Td:.345
alpha:.62
B(x)=Td/PI+Ts/(PI*sq(alpha)*sqrt(cos(theta)*cos(PI/180*x)))*exp(- 
sq(tan(PI/180*x-theta)/alpha))
Bnpoints=50

Unfortunately, I don't have any copies of the graphs here on my laptop,  
but basically this input file and the script above creates a polar  
transmission plot showing two curves, one for the measured data and one  
for the mathematical plot of the Radiance trans performance, based on  
the input parameters in the input file.  Greg played with the T  
parameters until the curves looked similar.  It's one of those  
applications of the Radiance suite that shows the beauty and power of  
Radiance.  I just wish I could get the script to run on my own  
machines, so I could experiment on my own.

I hope this all makes sense, and sheds some light on what we did.


=================
    Rob Guglielmetti
www.rumblestrip.org