[Radiance-general] material models [Was: Daylight simulation for Multi-Layer ETFE Pillows]

[Peter Apian-Bennewitz]

Hi Mostapha

as Lars said, it depends on the required exactness of your simulation.
Btw, 'glass' has the index of refraction as an optional 4th parameter.

As an approximation, choose a material and its parameters and/or use function 
files "by eye". If the resulting image looks right, - job done. However, that 
leaves the question why bother with Radiance and its complexity in the first 
place, considering that other renderers are perceived as being much friendlier 
to the user.

Measurements on the light scattering (BSDF) of a material take out the guess 
work, but are generally associated with costs. A number of companies sell 
different equipment and a number of companies offer measurement services (me 
included). Choosing a Radiance material is then based on solid data and the 
resulting model comes with a known error budget.

Or you can enquire about existing scattering data with the manufacturer of the 
product,- which you probably already have done in your case.

Otherwise, LBNL is building a database on window materials, so your product may 
already be in it.

And, if I may say so, the recently posted review 
(http://www.pab.eu/docs/Review_of_simulating_four_classes_of_window_materials_for_daylighting_with_non-standard_BSDF_using_simulation_program_Radiance.pdf) 
has all currently available Radiance window materials in one place. Including 
the every-now-and-then asked question about the difference between trans and 
transfunc. "Window" materials in this context means anything transmissive or 
translucent, so it is not specific to the usual coated, non-scattering panes . 
This compilation was started in March, when I realized I had already asked the 
same specific question to Greg in April 1995. So it started as a notebook to 
remind me of details, - maybe this list is useful to others too.

hope that helps, have a nice weekend,
best
Peter


On 07/19/13 10:16, Lars O. Grobe wrote:
> Hi Mostapha,
>
> it depends on what results you are looking for.
>
> If you need a useful model of the transmission through your fenestration, you may consider using the glass modifier. There is an error in this, as the refractive index of ETFE is lower (should be 1.4) than that of glass, however, for a clear, thin ETFE film, this may be rather ok for near-normal angles.
>
> In some of the images, the ETFE did not seam to be clear. Probably it hat be printed, or some surface treatment had been applied to roughen it. In this case, you need to model the surface as it is, which has little to do with the pure material properties of ETFE. Probably you need some data on the transmission and reflection, provided by the manufacturer or by your own measurements. That would allow you to approximate a trans / transfunc material to your sample.
>
> If you want to model something like patterns, where parts of the surface are printed and others are not, you can use mixfunc and mix the surface material with a clear ETFE material.
>
> The multiple internal reflections would be accounted for when directly looking at the pillow. Bright spots by such reflections e.g. on a ceiling or wall would not appear. That would require secondary light sources (not very useful for the complex geometry), the pmap, or maybe a precalculated BSDF of one pillow.
>
> Cheers, Lars.
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