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

[Mostapha Sadeghipour]

Hi Peter,

Thank you so much for your informative email.

I hope the material provider has already the measurements done so I can use
their measurements for the study. I'm not sure though.

I have used .xml files generated by Window for a study before however in
that case the surfaces were planar so I generated the file for the glazing
construction and applied it to a single surface. In this case though I have
non-planar surfaces with a different pattern on top of each one which makes
it a different case.

Also thank you for the link to the paper. I looked at it when you sent it
out first and I found it really useful. It is a really comprehensive
review. Thank you so much for sharing.

Have a great weekend!

Best,
Mostapha

On Sat, Jul 20, 2013 at 7:01 AM, Peter Apian-Bennewitz <apian at pab.eu> wrote:

> 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<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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>>
>>
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>
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