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 
) 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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Dipl.-Ing. Jan Wienold
Project Manager
Fraunhofer-Institut für Solare Energiesysteme
Thermal Systems and Buildings, Lighting and Daylighting
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