Simulating light pipe performance

Hello all:

Some papers point out Radiance cannot be used to simulate light pipe or
anidolic system performance. Do you have any suggestion about how to
simulate these systems that require multiple reflections?

Thank you,

Jia

Hi Jia!

I have some unfinished work on this lying around for almost one year now, and this topic arises again and again on this mailing list. It is true that the standard backwards-raytracing approach of Radiance is not suitable for such systems. There are several ways to simulate these if you are aware of what is happening, and, more important even, what kind of results you need.

In general, if you do not need high angular resolution, the tools developed for patch-based sky models are working, and you can even calculate the BDSF now using the Radiance-toolchain as long as you are more interested in the integrated transmission then in high angular resolution. So - if you need to know how many hours a year you can reach a certain illuminance on a work plane, that should work. If you want to perform glare studies, I would question such results.

For high angular resolution, switching from backwards to forward raytracing is more efficient, and here the photon map extension is what we have available right now. Unfortunately, the development status is not really clear, and at least the latest available versions had some bugs that can heavily influence results depending on sky conditions / models.

One entirely different approach is to use some optical modeling tool to get the transmission distribution or, even easier, to get luminaire distributions for certain times of day / dates / weather conditions.

And then, you are free to circumvent the whole ambient calculation and try do keep everything in the direct calculation, which works fine with such setups. So for calculating horizontal irradiance, you would place a fisheye-type camera with a full hemisperical view on you point of interest looking up and integrate the radiance - as long as you reflector is completely specular w/o roughness.

Cheers, Lars.

Hi Lars:

Thanks for your suggestions. I search the mail archive but still have many
queries. Please see bellow, sorry for so many questions...

Hi Jia!

I have some unfinished work on this lying around for almost one year now,
and this topic arises again and again on this mailing list. It is true that
the standard backwards-raytracing approach of Radiance is not suitable for
such systems. There are several ways to simulate these if you are aware of
what is happening, and, more important even, what kind of results you need.

In general, if you do not need high angular resolution, the tools developed
for patch-based sky models are working, and you can even calculate the BDSF
now using the Radiance-toolchain as long as you are more interested in the
integrated transmission then in high angular resolution. So - if you need to
know how many hours a year you can reach a certain illuminance on a work
plane, that should work. If you want to perform glare studies, I would
question such results.

How about the accuracy of calculated illuminance? Is that accurate or
acceptable? If I use Radiance toolchain (rtcontrib), do I have to use metal
material for tubular (round) pipe, and mirror or metal for square pipe?

For high angular resolution, switching from backwards to forward raytracing
is more efficient, and here the photon map extension is what we have
available right now. Unfortunately, the development status is not really
clear, and at least the latest available versions had some bugs that can
heavily influence results depending on sky conditions / models.

The extension is developed for Radiance 3.7. It seems not updated for
several years. Is that suitable for annual simulation?

One entirely different approach is to use some optical modeling tool to get
the transmission distribution or, even easier, to get luminaire
distributions for certain times of day / dates / weather conditions.

I know little about these tools. Is the optical modeling tool capable of
doing annual simulation? Could you recommend some tools? I just heart about
Photopia and only tried once. It would be better if the tool is free, but
this does not matter.

And then, you are free to circumvent the whole ambient calculation and try
do keep everything in the direct calculation, which works fine with such
setups. So for calculating horizontal irradiance, you would place a
fisheye-type camera with a full hemisperical view on you point of interest
looking up and integrate the radiance - as long as you reflector is
completely specular w/o roughness.

Do you mean I separate direct and ambient calculations? Could you tell me
how to do direct calculation only? If I set ab =0, it underestimates the
direct sunlight illuminance. I do not know how to let the direct sunlight
enter in to the space through the tubular light pipe. What materials should
I have to use for the light pipe?

Thank you very much.

Jia

ยทยทยท

On Sun, Jul 24, 2011 at 3:28 AM, Lars O. Grobe <[email protected]> wrote: