Dear Greg,
Thanks for your thorough and clear response (as always)! I knew I must be missing something!
The reason for testing this ‘extreme’ case was actually understanding the material characteristics, so replacing it by a diffuse light source would not help with that.
Just to provide some further background: We had some trouble understanding the simulation results in our earlier simulations with a luminaire mounted on the walls, grazing the ceiling - so actually spreading out the light on the ceiling more, but still not the full room. When changing the specularity of the ceiling, we found quite a big difference with comparable simulations with the software LightTools (forward raytracer). Hence we setup a more extreme test case to first try to understand it ourselves, before reaching out to you guys.
To quantify it a bit, we are measuring ‘horizontal illuminance’ on a grid of points at a certain height beneath the ceiling. The maximum value in the case of a diffuse ceiling reflection is almost below the spot where the beam peak hits the ceiling. Which makes sense considering the diffuse reflection. When changing the specularity to (for example) 50% (which I believe means 50% of the reflected light is still diffusely reflected, and 50% is specular), you would expect the value at that same point to be roughly cut in half (as the diffuse component is half) and that the rest is sent into the space. This is roughly what we found in LightTools, but in Radiance this value became roughly 4-5x lower.
As said, thats a bit of the background on why we were creating this odd setup, to see what would happen in this more controlled case. However, in my enthusiasm to use radiance for all kinds of light-based evaluations apparently I am breaking a few of the assumptions.
Anyway, any additional thoughts on the above struggles are more than welcome, in the mean time I will study photon mapping a bit more. I actually did already look at it, but initially thought that it would support creating images, not the calculation points. But upon further investigation I see there the user guide also mentions rtrace, so will look into that a bit more.