[Radiance-general] CFS with Radiance

[Guglielmetti, Robert]

Some very quick replies and clarifications within:

On 7/9/15, 10:26 AM, "Ikrima Amaireh" <ezxia at nottingham.ac.uk> wrote:

>Hi G. Larrain,
>
>Thanks for your detailed clarification (below).
>
>I wonder if you kindly can help me toward better understanding and
>answering the following:
>
>We agreed that for daylight modelling of spaces with CFS, Radiance is a
>suitable tool as it applies ray-tracing method. And for annual and/or
>climate-based daylight calculations, as calculation time becomes a
>critical factor, using BSDF data for CFS is highly recommended to tackle
>the task with radiance (using phases methods). However:


BSDF data in an annual simulation context is generally limited to Klems
basis BSDF data, which may not be high enough resolution for some CFS. And
the 5-phase method, which can circumvent this, is not necessarily "quick".
This is all still very much a quandary and the newest daylight metrics
have added confusion to all of this, IMO.


>
>- if someone needs to perform daylight calculations for given space with
>different CFS (different cases for comparison purposes) to get
>illumination levels for horizontal grid points (and not pictures nor
>scenes), is Radiance still the most convenient option?


Absolutely. The lack of an image-as-output requirement does not change the
fact that ray tracing is a good/convenient algorithm option for daylight
simulation problems, especially when dealing with diffusing media and CFS
in general. BSDFs allow you to do lots of "what-ifs", relatively quickly,
but are beholden to the limitations of the resolution of the BSDF.


>- is it still needed to use any of the phase methods (2, 3 or 5)?


Is what still needed? BSDF data? Ray tracing? I don't know of other
lighting simulation tools that can employ the multiphase methods. I would
say BSDF data is optional for 2-phase, required for 3-phase, and optional
for 5-phase. 

>- most importantly, is it still needed to get BSDF data for CFS or just
>can model the detailed CFS in Radiance (and, if yes, how?)


Again I'm confused as to what "it" is. A couple of approaches are
available to you in general. If you have a geometric model of the CFS you
can use Radiance (genBSDF) to make a BSDF of the CFS and use that in a 3-
or 5-phase context. However in an annual/climate-based simulation context,
you will be stuck with a Klems basis BSDF and that will not be very good
resolution for a lot of CFS; here you may want to use the 5-phase method
and stick the actual CFS geometry in the building model.

Some things to consider here are the photon map, now a part of Radiance
proper, or using the 2-phase method where appropriate. By 2-phase method I
mean generating a daylight matrix for your calculation points (or view),
and modeling the window material as-is. If the "CFS" is a shade cloth, you
can approximate that with a Radiance "trans". Same for translucent panels.
With this single daylight matrix, you can throw a vector of sky matrices
at it and get an annual climate-based result very quickly. Problem is, we
want to do stuff to the windows, sometimes at the time step level, so:

Blinds and compact daylight redirection devices (e.g. Lightlouver) are
best represented as BSDF, and if most of the redirected flux is headed up
and away from the points of interest, a Klems basis BSDF is good enough
IMO. In these cases you could use the 3-phase method. Problem is when you
have a BSDF for blinds, and you also want to simulate the blinds-up
condition (i.e. clear, specularly transmitting glass). Using a Klems basis
BSDF for this is sub-optimal. You end up needing to do two annual
simulations, one as 2-phase for the clear glass scenario, and again as a
3-phase (with a blinds BSDF for the transmission matrix) for the blinds
down condition. If you have a lot of different window groups, your
simulation space can get large in a hurry. But it's still do-able and you
can get results that tell a story you simply couldn't tell 5 years ago,
informing the newest daylight metrics as well.