[Radiance-general] Modeling glare from light fixtures

[Randolph M. Fritz]

On 2011-05-11 22:27:33 -0700, Chien Si Harriman said:

> A question to the radiance community.  I've been tasked with modeling a 
> T5 light fixture in its housing.  This is new for me as a project in 
> Radiance, as I've largely used it to model natural daylight only, with 
> the use of imported .IES files when needed.  I believe that I have to 
> model the light fixture and its associated housing because, for this 
> high-end conference room, the client for whom I am working is concerned 
> about glare emanating from the fixture during video conferencing.  The 
> .IES files just don't have that type of information, as they are really 
> (at least in this case) files that really just describe the flux as a 
> mathematical model, and does not physically model the interactions of 
> the light bulb in its housing.  As a result, I can't really show glare 
> emanating from the fixture at all!  (these are all things I'm sure you 
> are already aware of)

If the IES file is accurate and of sufficient resolution and you use 
the Radiance glare tools, the IES file will be sufficient to find 
sources of glare, even though the lamps and internal elements of the 
fixture will not be rendered correctly.

Radiance does not model rays emitted from a light source, and will not 
reliably compute the internal reflections of a fixture, even if the 
details of its materials and geometry are available.

For presentation, however, you may want to model the lamp and luminaire 
itself, as well as the IES file.  This will not be physically accurate, 
but is helpful in visualizing the glare.  To do this, you wrap the 
luminaire geometry in an illum (for fluorescents this is usually a long 
narrow box), to which you attach the IES data.  This is called an 
"impostor surface," because it is an impostor for the actual luminaire. 
 You then use glows within the illum to provide a visual representation 
of light within the luminaire.  Light from a glow does not pass through 
an illum and hence does not affect the lighting of the scene geometry 
or the glare calculation.

I wrote a program to do some of this work as part of my thesis.  It's 
not very finished and I haven't open-sourced it, but take a look at the 
thesis; the issues involved are covered there.  See:
  http://dmg.caup.washington.edu/pdfs/Thesis.RandolphFritz.2010.pdf
Be very careful, going through this process, to check your modeled 
luminaire against the IES data.  It is very easy to incorrectly place 
the IES data or fail to relate it correctly to the luminaire geometry, 
and this must be carefully checked.  I describe my procedures for this 
on pp. 53-57 of my thesis.

I also have a Python script which reads IES files that might be helpful 
in working with this; I've been meaning to put it out for a while.  If 
you think it might be helpful, drop me a note.

-- 
Randolph M. Fritz • RFritz at lbl.gov
Environmental Energy Technologies Division • Lawrence Berkeley Labs