[Radiance-general] Radiance

[Germán Molina Larrain]

Ikrima,

I am going to try to build the puzzle of CFS, Phases, BSDF and
calculations.... at least the way I understand it.

*CFS* are those systems that, via interreflection or other light transport
phenomena, redirect light (or solar radiation). Thus, in order to get a
reliable result you will have to consider all the phenomena involved.
Then, *common
simple performance indexes*, such as the miss-used Shading Coefficient and
the Aperture Percentage, *always loose a lot of information, trying to
reduce all the complex behavior of a CFS to one single number*. We all know
that venetian blinds are more "transparent" from certain viewing directions
than from others, but these performance indexes do not tell you that.

Now... Radiance can certainly perform calculations of spaces with CFS using
its "common" Ray-tracing. However, this may be slow for some purposes (i.e.
annual simulations and climate-based daylight modelling), and *this is why
2, 3 and 5 phase methods have been developed*. The *BSDF* representation, I
would say, goes in the same direction... It allow summarizing all the
bounces, reflections, refractions, etc.  that occure withing the CFS in a
single matrix or tensor.* By using BSDFs*, Radiance itself and other tools
(i.e. EnergyPlus) can treat CFS as blackboxes, avoiding all the opcits
within the system. A BSDF that uses the Klems Full representation has
21,045 numbers (instead of one, such as the Shading Coefficient).

Being said all that, I would not trust a calculation method unless it can
actually deal with the optics of a CFS that is drawn and/or it can use BSDF
(or similar) information.

Lets remember that a perforated screen, a venetian blind, a light diffusing
device can all have a Shading Coefficient of 50%, but all of them will
behave very differently. I made some presentations about this on my
previous work (we sold complex Shading Devices), trying to promote the use
of BSDF in EnergyPlus calculations... the differences (in solar heat gains)
were more than considerable.

I hope that someone else gives us his/her perspective on this topic...
there are a lot of concepts that I might be misunderstanding.

Best!



2015-06-18 10:50 GMT-03:00 Ikrima Amaireh <ezxia at nottingham.ac.uk>:

> Hi all,
>
> I am really sorry for writing on the same point again but really in-need
> for clear and complete answer, if possible.
> Here, in my institution, people are in doubt if simple daylight
> calculation tools like Ecotect (both standalone Ecotect as well as Ecotect
> using Radiance engine methods) are efficient/enough for characterising
> different complex fenestration systems (CFS) and most importantly
> differentiate them for purpose of classification and rating?
> Many are saying there's no need to go through Radiance to get almost the
> same thing that can be achieved using other simple tools (like Ecotect).
> For me, as a beginner with some readings and research, I found that Ecotect
> is still not efficient tool to differentiate different CFS (especially if
> they are very close in geometry, etc.) so we need something that's more
> robust like Radiance to do the Job; does that sound right?
>
> Your reply is highly appreciated.
>
> Regards
> Ikrima
>
>
>
>
>
>
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