[Radiance-general] Modeling glare from light fixtures

[Thomas Bleicher]

On Thu, May 12, 2011 at 12:27 AM, Chien Si Harriman <CHarriman at gb-eng.com>wrote:

> 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.
>

Radiance is not the right tool to simulate the internal reflections of the
fitting. You could do it with a lot of rendering time and high accuracy
values. However due to inaccuracies in your material descriptions etc I
expect that the final result would not be more accurate than the IES data
you already have.

Since you're focused on video conferencing you also have to model the video
screens accurately. You can look at the various workshop presentations on
this topic.

Also keep in mind that you may be able to "prove" that there is no glare
caused by the light fittings but your client will still be disappointed by
the reflections of the fitting. Glare is a subjective experience and all you
can do with Radiance is to show that you meet some existing standard.

As a lighting designer I would recommend to reduce the potential for glare
as far a possible (different fitting, louvre accessories, different layout
and fitting type etc). If your clients don't want to spend money on good
lighting they have to live with the consequences.


> 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)
>

IES data (and LDT) does indeed not reflect the physical characteristics of
certain light fittings very well. If the light source is small and directly
visible (think LEDs, halogen or discharge lamps) you will almost certainly
experience glare when you look directly into the lamp. The intensity of a T5
lamp can also be perceived as disturbing. However, if that is an issue here
you should look into diffusers etc. to cover the light source from direct
views.

Glare that results from the average intensity of the fitting is accurately
represented in the IES file and its geometry (disc, rectangle or box). Some
lighting design apps can generate a UGR diagram from the IES file. If you
use the IES file in Radiance I think you will also loose some angular
resolution because the conversion to a Radiance *.dat file has a fixed
number of vertical angles (although I'm not sure about this).


> It seems my most accurate option is to model a T5 fixture as a glow object,
> giving off the appropriate luminous flux inside of a reflective housing.  I
> would likely have to take some guesses as to the geometry of the fixture
> just because, at least at this time, I don't have a physical sample of the
> fixture, nor do I have any sort of exploded orthgraphic projection showing
> the inner details of the fixture.  I'd likely take just a section of the
> fixture (found on the cut sheet) and extrude it the appropriate length.
>

If you are only interested in the source visibility you can use an extruded
cross section to simulate the fitting. If you want to use a mirror material
for the extrusion you have to change the "glow" to a "light" or you won't
get any secondary light sources. You can also reduce the ambient bounces a
lot because you only want to simulate the visibility of the light source.
This should speed up your calculation but it requires that you have a good
representation of the reflector geometry.


> My biggest concern is in regard to modeling such a high intensity of light
> in such a small confined space with so many bounces of light.  I am not sure
> if there are "standards" in terms of simulation settings, or other helpful
> pointers, that would normally be applied to simulate the fixture in this
> way.
>

There is no limit for the intensity you can define. Keep in mind that a
glare analysis is done without depreciation of the lamp and fitting (new
lamp and no dirt).

You are also less interested in ambient bounces than source reflections
("direct relays"). The bounces are only necessary if you want to show the
fitting within the conference room ambience. For your video conferencing
situation it should be enough to produce a reflection of the light source on
the video screen.


> The other option, far simpler though possibly just as effective (and this
> is a huge possibly in my mind), is to simply place a square luminous glow
> object over the fixture where the light would normally exit the fixture.
>

This is exactly what ies2rad does.


> I suppose I could build up my own mathematical representation of how the
> glow object should distribute its light (in effect build my own .IES file
> (of sorts).  Though, I truly have never done this before and I would assume
> there is a chance of error.
>

Exactly. You will not only introduce some errors due to inaccurate
information but you also have hardly a way to verify your model. If
something is wrong with your calculation results you will have a hard time
to prove it wasn't your hand made distribution. That's hardly worth it.

Regards,
Thomas
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.radiance-online.org/pipermail/radiance-general/attachments/20110513/36a9edbd/attachment.html>