This is caused by the way that Radiance calculates ambient light (i.e. light that bounces off of surfaces, rather than emanating from a light source like the sky).
Try to adjust Radiance parameter settings (click “Show Quality Details” under the “Quality Settings” drop-down menu) as follows:
- Decrease ambient accuracy (-aa; but keep it above 0)
- Increase ambient resolution (-ar)
- Increase ambient divisions (-ad; typically in multiples of 2)
- If all else fails, you can set -aa = 0, but you must also set the limit weight (-lw) to something very small (typically approx. 0.0000001). You can continue to increase -ar and -ad until you achieve the desired accuracy.
The Radiance parameters -aa, -ar, and -ad, combined with the size of the scene (listed alongside Radiance parameters in the quality details drop-down), determine the minimum ambient spatial scale. If this value is larger than your wall thickness or grid point spacing, ambient light can “leak” between points.