[Radiance-general] Modelling glass lamellas in Radiance.

[Thomas Bleicher]

Hi Jacob.

I've not exactly used IES in our office to create Radiance input
but I have looked at it's scene description files once. There is
somewhere in your IES project directory are folder called 'obj' which
contains all the radiance input files. I think the material library
files are simply copied into that directory where you can modify the
material descriptions after you have exported/rendered your scene.
After you made your changes you can use the Radiance binaries
directly from the command line or a script to do your simulations
with the new materials.

I think you can change the library files (edit with text editor)
or even import an external material file. That allows you to assign
your material in the IES interface and control the rendering from
the interface (for very small values of control ...).

Now, what do you need to write into your files:
> Here’s the question:
> What is the best way to model the glass blinds in Radiance?

> The glass that I’ll be using for the project has the following  
> characteristics:
> Visual properties
> Ext. Reflectance, LR [%]        31
> Int. Reflectance, LR [%]        31
> Transmittance, LT [%]   59
> RA-index        97.3
>
> Solar properties
> Solar reflectance [%]   32
> Direct solar transmittance [%]  40
> Solar Absorption [%]    20/8
> g-vl/solar factor       0.48

I'm not an expert in material definitions. This thread from 3 years ago
suggests to use a BRTD material to define the glass and get a proper  
incident
angle dependent refraction:

http://www.radiance-online.org/pipermail/radiance-general/2004- 
January/001396.html

There is now a script that allows you to
convert Optics5 glass descriptions to Radiance BRTDfunc based materials.
So if your department has Optics5 running somewhere you could look
for a material that matches your glass for the blinds and try to
convert it (the script needs a Unix system, i think).


If you are interested in sun patterns created by reflections on the
blinds you have to use a 'mirror' material. Otherwise the ray tracing
is likely to miss the sun and therefore will not create a bright
spot where you expect it.

> I have been trying to use the material types “trans” and  
> “dielectrica”, in order to benefit from the angular dependent  
> properties of the glass, I would prefer to use a matrial type which  
> includes the refraction index.
>
> Here comes my dilemma:
> The material type “glass”: in IESve the type does not include the  
> refraction index which means that the only input is the  
> transmissivity of the glass.
'glass' is a specialized form of dielectric with a preset refraction
index of 1.52. It's refraction/reflection does not depend on the angle
of incident light, though, which is what your looking for. It was  
created
to avoid the calculation of reflections between the two sides of a glass
pane and you have to convert your 'Transmittance' value to the  
'transmittivity'
needed for the glass primitive definition if you go that way.

[Note: that's what I understand about glass reading the 'usman.pdf'
        document. Someone should correct me if I'm wrong.]
> The “trans” material type: same problem but at least I’m able to  
> define the reflectance of the glazing.
'trans' materials don't have an index of refraction (not that I know  
of).
> The “dielectrica” material type: IESve only allows me to enter  
> values between 1 and 2 for the refraction index which in this case  
> have no significant effect.
If you know how to define the Radiance 'dielectric' material (you can  
look
up the man pages and other texts on the web) you can define your own  
dielect
material in the IES library.
> I’m sure that the best thing I could do is to model the whole thing  
> over again in the “original” Radiance – but as I have a short  
> deadline, I prefer to get it done in IESve!
The IESve Radiance files are very simple (as the geometry tends to be).
You just have to redefine the material descriptions to suit your needs.


Regards,
Thomas