[Radiance-general] Some trans oddities

[Rob Guglielmetti]

I am struggling to understand some seemingly inconsistent results I'm 
getting with the trans material.  We are doing some fairly complex 
studies of the daylight penetration in a bunch of art galleries.  These 
galleries receive daylight that is filtered through some diffuse glass, 
and further modulated with sun control shades. It is the material 
properties of these shades that is giving me fits at the moment.  In 
brief, certain shade transmissions give wacky results, while most 
perform as expected.  To reduce the possibility of error, I have 
constructed a simple box room (16'x16'x9'), with a single 3' square 
aperture in the ceiling.  this aperture has two panes of diffuse glass 
(as the real building will), between which I have a polygon that I 
assign various shade materials (using trans) to.  I am then computing 
the horizontal illuminance on the floor with rtrace for each shade.

Results:

Test1 (no shade): EhFloor - 222 Lux
Test2 (10% shade): EhFloor - 23 Lux (10.41% of Test1, 4% off)
Test4 (5% shade): EhFloor - 12 Lux (5.4% of Test1, 8% off)

So far, so good!

(in all cases, the exterior horizontal illuminance is being predicted to 
within 10 Lux, so it's not even worth weighting the interior predictions 
for each individual calculation, IMHO.)

Now, we have two shades in the real building, both of which can be 
deployed at a time.  So next I tried simulating a 10% and a 1% shade 
down, but I "cheated", by assigning the net transmission of the two 
shades to the single polygon (IOW, .10 * .01 = .001).

Test3 (10% + 1% shade as a single polygon): EhFloor = 0.4434 Lux (0.20% 
of test1, or 100% off!)

Thinking that maybe the aforementioned cheat was ill advised, I added a 
second polygon to the model, so I had two physical objects to apply 
shade materials to:

Test8 (10% + 1% shades, as two separate polygons): EhFloor - 0.5509 Lux 
(0.25% of Test1, or 148% off)

So much for that theory.  thinking maybe there's a limit to how much you 
can remove from the daylight before the calculation goes awry (hey, I'm 
grasping at straws at this point), I tried another scenario:

Test7 (10% + 5% shades, as two separate polygons): EhFloor - 0.9656 Lux 
(0.43% of Test1, 13% off)

This is certainly close enough for Government work.  But then:

Test9 (single 01% shade): EhFloor - 1.189 Lux (0.54% of test1, 46% off)

So, the 10% + 5% was accurate, which is cutting out more light together 
than a single 1% shade, but the 1% shade calc was fairly inaccurate.  I 
thought maybe it was the inherent variability in the indirect 
calculation, so I ran test9 again and got exactly the same numbers! 
(I'm using rad with Q=M D=M V=H and -ab 3 for these tests, and it's a 
clear sky at noon at a latitude that pretty much places the sun directly 
over this aperture.)

The reason this entire test came about is because I can't seem to affect 
any further light reduction in my model past a certain point.  I even 
tried making the shades totally opaque, and when I do that my model goes 
completely dark, as I expected, so the model seems to be "tight", and 
have no light leaks.  Everything points to my shade material definitions 
but I can't find anything wrong with them, except for the fact that 
certain ones seem far less acurate than others.  I have been trying to 
use Radiance as a relative performance evaluation tool on this project, 
but for this last part of the project where we are trying to knock out a 
very high percentage of the available light, I seem to be getting 
counter-intuitive results.  Does anyone have any insight as to why these 
results are occurring?

Here are the trans parameters for the various shade materials (all are 
assumed to have a 95% transmitted specularity, as they are view 
preserving shades, even tho it probably does not matter too much because 
they are between two pieces of diffuse glass.):

## 10% shade
void trans shade.10
0
0
7 .4 .4 .4 0 0 .25 .95

## 05% shade
void trans shade.05
0
0
7 .25 .25 .25 0 0 .2 .95

## 01% shade
void trans shade.01
0
0
7 .21 .21 .21 0 0 .047619 .95

## 10%+01% shades in single polygon
void trans shade.001
0
0
7 .201 .201 .201 0 0 .004975 .95

## 10%+5% shades as single polygon
void trans shade.005
0
0
7 .205 .205 .205 0 0 .02439 .95

Any help appreciated.


----

      Rob Guglielmetti

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