[Radiance-general] How to describe a diffuse illuminating pane?

[Lars O. Grobe]

Hi Per!

This is a translucent, diffusing panel. There have been lots of 
discussions on this on the mailing list, as well as a bunch of 
presentations on workshops and even papers. The basic answer would be - 
use the trans material in Radiance.

However, the material model for trans is very generic, while translucent 
panels are made applying very different approaches. There is volume 
scattering, scattering by etching / sanding / ... a surface (outer or 
inner), laminating films on or in between panes, ... and all show 
different characteristics. In your case, it is probably volume 
scattering, so I would guess reflection to have a strong specular peak 
due to the clear, smooth surface, with a strong diffuse component from 
light scattered back, and probably a nice diffuse transmission. However, 
it is difficult to know before having any data on it. Can you see a 
light source through it, when holding it in fron of e.g. a strong lamp? 
Or does the brightness appear more or less flat? Peter is offering 
measurements using his goniophotometer, as well as others do. Christoph 
Reinhart described modeling a translucent panel in his paper:

Reinhart C F, Andersen M, "Development and validation of a Radiance 
model for a translucent panel", Energy and Buildings 38:7 pp. 890-904, 2006

which can be found at EPFL:

http://infoscience.epfl.ch/record/88112/files/Development%20and%20validation%20of%20a%20Radiance%20model.pdf

and presented on a workshop:

http://www.radiance-online.org/community/workshops/2005-montreal/PDF/TranslucentPanelValidation.pdf

So a wide field to work on... Cheers, Lars.
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