BRTF for glazing

For glazing definition we always use BRTDfunc material, which we define
partly automatically partly manually.
We export each glazing layer from Optics5 (I don't have idea what you
can do whan you don't have access as administrator), and then combine
them in BRTD func. If no data in Optics5 database, we use some estimated
values for transmittance and reflectance.

For transmittance/reflectance functions of each layer we use functions
used in glaze output. For coated glazing also. I think glaze.csh has
some functions for laminated glass too, but not sure.

Well, than the problem still persists concerning the limitations for
redirecting systems, but actually, if I did not get completely confused
by now (hopefully not) for the definition of the glass panes, the
lambertian model should be fine, right? So modelling a glazing with
integrated blinds, anything acting like a prisma etc. as brtf would not
make too much sense, while for glass and most homogenous materials there
should not arise any problems...?

CU Lars.

Hi Lars,

I don't know about redirecting systems (we haven't worked on them yet :slight_smile:
For now we simulated various glazings, usually double, with various
reflection/transmission values. For coated glazings we use functions from
glaze, as I already said. Note that double coated glazings usually are
created from one clear glass layer and one coated!

Other experiences?

Marija

···

2008/12/10 Lars O. Grobe <[email protected]>

> For glazing definition we always use BRTDfunc material, which we define
> partly automatically partly manually.
> We export each glazing layer from Optics5 (I don't have idea what you
> can do whan you don't have access as administrator), and then combine
> them in BRTD func. If no data in Optics5 database, we use some estimated
> values for transmittance and reflectance.
>
> For transmittance/reflectance functions of each layer we use functions
> used in glaze output. For coated glazing also. I think glaze.csh has
> some functions for laminated glass too, but not sure.

Well, than the problem still persists concerning the limitations for
redirecting systems, but actually, if I did not get completely confused
by now (hopefully not) for the definition of the glass panes, the
lambertian model should be fine, right? So modelling a glazing with
integrated blinds, anything acting like a prisma etc. as brtf would not
make too much sense, while for glass and most homogenous materials there
should not arise any problems...?

CU Lars.

Other experiences?

No, for me the result of this that for my current applications, modeling
the glass panes as brtf (as I also did, mostly using the glaze script &
Co.) should be fine - and for future applications, which will involve
measured data of more complicated redirecting structures, a different
solution will have to be found.

Thank you for sharing your experiences, CU, Lars.

Hi,

One more idea.
On Daysim site you can find very nice Reinhart paper about simulation of
translucent glazings (+ he added few translucent materials to materials
database in Daysim, and you can also use them in Radiance)
Also Reinhart has a paper about complex fenestration systems, maybe there is
something interesting there (I haven't read the paper )

CU, Marija.

Hi Lars and Marija,

the mentioned restriction occur also for glazings, which are defined as
BRTDfunc!

That means, the angular information of the BRTD is lost for the glow
material (= sky distribution). This means for example, the the
circumsolar range of a sun with high altitude will contribute in the
same way as other parts of the sky with lower angles of incidence. And
all the contributions from glow will be distributed lambertian from the
inner pane. So whenever you use a sky distribution (e.g. with gendaylit
or within DAYSIM), the occurred difference may be significant.
This modeling may be sufficient for many applications, but you must be
aware of loosing the angular information as soon as the ambient
calculation hits BRTDfunc.
My way is to use other standard material which do not show those
restriction and to mix them together according to the
transmission/reflection properties (which I get either from the
manufacturer or I generate them with window or WIS. The "mix-func-fix
takes me 10 min and then I'll get a model which works for (hopefully :wink:
) all situations.

The other comments are below:

Lars O. Grobe wrote:

A remark from me: I never use BRTF in RADIANCE now, since all the
angular information you put into your model is lost for the glow
material. That means, if you model a specific sky luminance distribution
and you are using BTDF-func, no angular information of your BTDF-model
is used! It is treated lambertian ! And this is especially hard, if you
want to model a system, which is intended to redirect the bright
zenith sky.
If I model advanced glazings, I use either standard glass and modify it
by brightfunc or for high reflective materials I use a mixfunc of glass
(+brightfunc) and metal. I always check the angular transmission and
reflection of the model by a virtual measuement - and I also test, if it
still works for the sky.

Hi Jan, hi list,

one more late reply to this. At the moment I am setting up the
materials for a model with known specifications for the glazing, but I
do not have samples to measure. So if I want to get a material
description e.g. for a double-glazing including coating and such, I
use the script glaze to have some reasonable values that I could not
produce else (and I can write this down as a reproduceable routine for
the docs). I could use Optics and access the glazing database, but for
some strange reason it requires administrator access to a Windows
maching that I do not have at the moment (did anyone manage to open
the database file using something like Openoffice?) - but also Optics
would give me brtf modifiers.

Did anyone try to model such glazings in radiance, using dielectric
and interface? What are you using for defining glass panes if no
samples to measure are available? Ah, and one more question (which
leads me back to the idea of modeling glazing), what about glazings
that consist of laminated glass panes + non-uniform laminate (such as
printed screens for sun protection or advertising), would it be
possible to model these using dielectric&interface, having one (very)
thin layer for the laminate modified by a pattern and two dielectric
layers of glass on both sides? Is it complete nonsense to try modeling
such layers geometrically (there will be a limit as the thinnest
layers surfaces may get too close to each other)? Or is all this
trouble about considering the coatings and such only introducing
error, and a simple clear glass definition ignoring the coating would
still be "better" to use?

A colleague of mine -Francesco Frontini - is currently just doing this
(=geometric modeling), even extending your ideas having printings at
very specific positions and having three layers (directed laminated
using interface, printings modeled with plastic or trans). In principle
it is possible to model it like that, but it is a rather hard job to get
a reliable model. My colleague is working on that issue for months and
is trying to get close to the laboratory measurements. As far as we are
now, we cannot get a potential contribution of multi-internal
reflections within the glass for the sun as light source (the plastic
stripes reflect the light and there should be a certain light transport
to the inner side). The ambient calculation will not find the sun - even
if the parameters for ad is increased radically. But this is as I
expected before. But the direct light transport and the sun blocking for
certain angles (that's the purpose of the system) works more or less
like in theory.

Cheers,

Jan

···

TIA&CU... Lars.

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Thermal Systems and Buildings, Lighting and Daylighting
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the mentioned restriction occur also for glazings, which are defined as
BRTDfunc!

Thank you for the clarification Jan, I had completely misunderstood this!

My way is to use other standard material which do not show those
restriction and to mix them together according to the
transmission/reflection properties (which I get either from the
manufacturer or I generate them with window or WIS. The "mix-func-fix
takes me 10 min and then I'll get a model which works for (hopefully :wink:
) all situations.

By standard material you mean glass (or trans or dielectric)? And you
use mix-func to describe the reflected component? Has this been
published somewhere? I never came across it so far...

is trying to get close to the laboratory measurements. As far as we are
now, we cannot get a potential contribution of multi-internal
reflections within the glass for the sun as light source (the plastic
stripes reflect the light and there should be a certain light transport
to the inner side). The ambient calculation will not find the sun - even
if the parameters for ad is increased radically.

Hm, is this something that could be better modeled using the pmap
extension? Or is the number of interreflections between the surfaces
(with their little distance) simply too high? I guess that you mean the
interreflections under low angles, right?

CU Lars.

Hi Jan, Lars:

It's important when discussing these material types to be aware of what works and what doesn't. While I concede that the BRTDfunc type does not account for the sky correctly in it's "directional-diffuse" component, the diffuse and specular components are done properly. In the case of a glazing system that has a mixture of specular peaks (reflected and transmitted) and diffuse (e.g., scattering from frit), the BRTDfunc does a perfectly adequate job. This is why it is employed in the glaze script and other places to represent glazings. You'll notice that the directional-diffuse part of the glaze output materials are zero.

Cheers,
-Greg

Hi List!

I could use Optics and access the glazing database, but for some strange reason it requires administrator access to a Windows maching that I do not have at the moment (did anyone manage to open the database file using something like Openoffice?) - but also Optics would give me brtf modifiers.

Just a note, the database files are MS Access file and can be opened e.g. using gmdb2 (GNOME MDB Viewer)...

CU Lars.

1 Like

The Optics5 IGDB database (glazing.mdb) is password protected, in order to
keep general users from changing the data.

If you want to get the spectral data information for records in the
glazing.mdb file, I would recommend getting it from the WINDOW 5 / 6 program
database -- the WINDOW Glass Library allows importing of records from the
glazing.mdb file, and all the spectral data information is copied into the
WINDOW database, which is not password protected (the default WINDOW
database is W5.mdb or W6.mdb, an MS Access database).

However, if you need the BRTF information, I suggest that you use the
function in Optics5 that writes a Radiance file (File menu / Export Radiance
file). Optics5 calculates the BRTF functions from the spectral data, so
getting the raw spectral data (directly from the databases) may not result
in information that is useful to Radiance.

Both Optics and WINDOW are available for download (no charge, courtesy of
the US Department of Energy) on our website:

http://windows.lbl.gov/software/

Robin

···

******
Robin Mitchell
Windows & Daylighting Group
Lawrence Berkeley National Laboratory

-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Lars O.
Grobe
Sent: Sunday, December 14, 2008 7:45 PM
To: Radiance general discussion
Subject: Re: [Radiance-general] BRTF for glazing

Hi List!

I could use Optics and access the glazing database, but for some
strange reason it requires administrator access to a Windows maching
that I do not have at the moment (did anyone manage to open the
database file using something like Openoffice?) - but also Optics
would give me brtf modifiers.

Just a note, the database files are MS Access file and can be opened
e.g. using gmdb2 (GNOME MDB Viewer)...

CU Lars.

_______________________________________________
Radiance-general mailing list
[email protected]
http://www.radiance-online.org/mailman/listinfo/radiance-general

1 Like

Thanks, Robin.

Just wanted to add a reminder to folks using Optics output that the Radiance "optics2rad" program fixes some compatibility issues and should be used to mediate the exported Optics description.

-Greg

···

From: "Robin Mitchell" <[email protected]>
Date: December 15, 2008 11:57:54 AM PST

The Optics5 IGDB database (glazing.mdb) is password protected, in order to
keep general users from changing the data.

If you want to get the spectral data information for records in the
glazing.mdb file, I would recommend getting it from the WINDOW 5 / 6 program
database -- the WINDOW Glass Library allows importing of records from the
glazing.mdb file, and all the spectral data information is copied into the
WINDOW database, which is not password protected (the default WINDOW
database is W5.mdb or W6.mdb, an MS Access database).

However, if you need the BRTF information, I suggest that you use the
function in Optics5 that writes a Radiance file (File menu / Export Radiance
file). Optics5 calculates the BRTF functions from the spectral data, so
getting the raw spectral data (directly from the databases) may not result
in information that is useful to Radiance.

Both Optics and WINDOW are available for download (no charge, courtesy of
the US Department of Energy) on our website:

http://windows.lbl.gov/software/

Robin