[Radiance-general] mirror and virtual sources

[Christopher Rush]

I am modeling a daylight redirection system, something like a light
shelf type of geometry.  I have a two part question regarding the
specular component and virtual sources.
 
It's by no means a validated material definition, but I've been testing
with the following material meant to be something like brushed aluminum.
The intention is that the metal gives the best approximation of the
surface characteristics, while the mirror adds the virtual sources of
the specular reflection.
 
void metal aluminum
0
0
5 .7 .7 .7 .5 .01
 
void mirror aluminum_mirror
1 aluminum
0
3 .35 .35 .35

My first question is the value of the mirror material.  I want to
confirm that my value for the mirror of 0.35 is appropriate (0.35 = 0.7
* 0.5).  I've based this on my general estimate of 70% general
reflectance and 50% specular reflectance for the actual material. Or
should the value for the mirror be the full 0.5 to match the full
specular component?
 
The second part of the question is regarding the use of the alternate
material in the calculation.  From an old post I turned up the following
from Greg:
 
> If you are trying to reflect objects in a surface, the "mirror" 
> primitive allows you to specify any alternate material you choose when

> it is not participating in the virtual source calculation.

Does this mean that the alternate material will participate in the
indirect calculation? Or is the alternate material only used to produce
the appearance of the surface in direct view?  Or something else?

Also, is it reasonable that for a rough surface the mirror value should
be toned down a bit to account for roughness that is not part of the
mirror definition?


Any suggestions for appropriate definitions for actual metals found
commonly in the built environment are welcomed.

Thanks,
Chris
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