[Radiance-general] Using radiance for simulating light pipes

[Lars O. Grobe]

Hi Alejandro,

the topic of light pipes appears from time to time, and unfortunately
the simple answer is that Radiance (and any backwards-tracer) is not
really suiteable for the task, if

1) you consider sunny days (you probably want to do so)

2) you do not have a diffusor placed on the top aperture of the pipe.

There are still some promising approaches to solve the problem. One is
using descrete patch models of the sky, such as the Tregenza scheme,
with the direct sun contribution being distributed over larger patches.
This implies distributing the sun radiance over a larger part of the
sky, which smoothes out sharp peaks in the transmission - which may not
be crucial in many cases.

The other option is using a forward extension to Radiance, such as the
photon map. This requires a modified version of Radiance, but gives you
a tool which has been developed with exactly applications such as light
pipes in mind.

Finally, you may even try to generate the BSDF of a light pipe using
genBSDF and apply this to a surface representing the bottom aperture.

The latter two options keep the resolution of the incident radiance,
especially the "sharpness" from the sun, intact.

Cheers, Lars.

> Hi everyone,
> I am a newcomer in the radiance community. I am involved in a project to
> assess the light output from light pipes. We are gonna simulate the
> light output of 2 types of light pipes and then compared them to the on
> site measurements. I would appreciate any guiding or advise on where to
> start the simulations. I am considering backward retracing as a more
> suitable option than forward retracing for this particular case, what do
> you think?
> Thank you
> Ale