[Radiance-general] Question on Setting ambient parameters

[Boettger, Richard G. (JSC-SF)[LM]]

Hi All,

We have been using Radiance in our lab for many years performing
lighting analyses mainly for exterior (on orbit) activities involving
the shuttle and the space station. We are now beginning to do more
interior lighting evaluations such as the space station laboratory
module. 

The interiors we evaluate tend to have a lot of white, non specular
surfaces. So diffuse interreflections seem to be important. I've been
looking into the parameters that control the ambient calculation in
Radiance and I am baffled by the "-ab" ambient bounces parameter. I
thought, by keeping all other parameters the same and increasing the
ambient bounces, the results should be more accurate. That is not what I
see from our test case.

We built a box 17" x 17" x 36" with white walls. The wall material
properties were measured. A calibrated light source was placed in a hole
at one end of the box. Illuminance measurements were taken at various
points within the closed box. We then modeled the environment in
Radiance and took illuminance values at the same points. After some
experimentation I settled on the following ambient settings:

      av: 0.0 0.0 0.0
      aa: 0.08
      ab: 3
      ad: 1024
      as: 512
      ar: 64
      
The errors between Radiance and measured values were less than 10%. 

But as I increased the ambient bounces beyond 3, the illuminance values
at the points kept increasing. By 5 bounces the errors were averaging
over 60%. 

It appears that I am missing something fundamental. I'm hoping the group
can help me out. 

Thanks,
rick


Rick Boettger
SF3/Habitability and Human Factors Branch
Lockheed Martin Space Operations
Graphics Research and Analysis Facility
Johnson Space Center