Modelling glass lamellas in Radiance.

I'm a relatively new user of Radiance and using it from within IESve
(the IES virtual environment program suite), which means there are
certain limitation for the input to the program. I'm currently working
on a research project on glass lamellas for daylight directing and solar
shading purposes at the technical university of Denmark.

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 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.
The "trans" material type: same problem but at least I'm able to define
the reflectance of the glazing.
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.
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!

Hope someone out there is able to help me out on this one!

Kind Regards

Jacob B Laustsen, DTU, Denmark

Hi Jacob,

It's difficult to answer your question without knowing anything about IESve or what it permits. You also didn't specify what you mean by "glass blinds." Is this just a window with some odd coating(s) on it, or does it have some structure? Is there some way to give a Radiance material description to IESve directly? When is your deadline?

-Greg

···

From: "Jacob Birck Laustsen" <[email protected]>
Date: January 5, 2007 6:43:04 AM PST
I’m a relatively new user of Radiance and using it from within IESve (the IES virtual environment program suite), which means there are certain limitation for the input to the program. I’m currently working on a research project on glass lamellas for daylight directing and solar shading purposes at the technical university of Denmark.

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 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.

The “trans” material type: same problem but at least I’m able to define the reflectance of the glazing.
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.

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!

Hope someone out there is able to help me out on this one!

Kind Regards

Jacob B Laustsen, DTU, Denmark

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

Just a minor correction from what Thomas wrote...

From: Thomas Bleicher <[email protected]>
Date: January 6, 2007 10:43:22 AM PST
...

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.]

Actually, the "glass" primitive does correctly model reflection and transmittance changes as a function of angle, and it is possible to alter the index of refraction. However, I don't think it's reasonable to set an index of refraction high enough to get a reflectance of 31% as Jacob specified.

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).

Correct. But you can apply a pattern such as "winxmit.cal" to incorporate the influence of incident angle.

-Greg