[Radiance-general] Problem with Metal Roughness Parameter

[Kevin Thuot]

Hello Radiance Community,

I'm having a problem with the metal material when it is both highly specular
and rough. In my room model, I have a vertical array of specular louvers at
the top of a glazed facade which bounce light onto the ceiling. The ceiling
for the first 6 m from the facade has a bumpy specular surface, using the
metal material. Material description:

void metal ceil_diff2
0
0
5 .856 .856 .856 .95 .125

The problem I'm seeing is that as I increase the roughness of the metal
ceiling from 0 (I've tried 0, .125, .2, and .9) the total amount of light in
the room increases significantly. This should not happen as the ceiling is
only redistributing the light that enters through the louver array. I found
that metal with 0 roughness produces essentially the same results as if I
used the mirror material on the ceiling, which we would expect. As I
increase the roughness, the area under the illuminance curve increases (to
absurd levels in the case of .9 roughness, but I know that .2 is about the
max for real objects). I  tested the illuminance profile at the centerline
of the room as well as nearer to the side walls and saw the same pattern, so
I don't think it's a matter of undersampling the room. The only thing I
changed between cases was the roughness of the ceiling.

Can anyone think of what might be causing this effect? Are there any known
problems relating to the roughness of the metal material? I've searched the
archives, but didn't find an answer to my problem.

If you are interested in helping, here is a link to the graph of the
illuminance results for my test room. The values are the aggregate of
individual time steps from an annual simulation. The legend shows the
roughness in parentheses.

https://docs.google.com/leaf? <http://goog_618206457/>
id=0B69GQAj_Aw_QNzZhNWMwOTktZjRjOS00NWJiLThhMTUtODNlNDQxYmMyYjJi&hl=en&authkey=CIC91fUF<https://docs.google.com/leaf?id=0B69GQAj_Aw_QNzZhNWMwOTktZjRjOS00NWJiLThhMTUtODNlNDQxYmMyYjJi&hl=en&authkey=CIC91fUF>

Thanks very much for your help. I can send more information on my model if
requested.

-Kevin



-- 
Kevin Thuot
Research Assistant
Massachusetts Institute of Technology
kthuot at gmail.com
(512) 497-8168
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