I'm triying to set up a mesh light only scene (AKA IDL emitter, isn't it?) in Poser Pro 2014 and I keep bumping into this problem.

The scene has three cubes with the default material shader and one set up as a mesh light with an ambient value of 10 and no shadows and visible in camer ticked off. Lights are turned off. The transition from light to dark is not smooth but full of that noise that makes the base cube looks as if it had a "sandy" bump map (seen more clearly on the second render)

These are my settings. I've tweaked again and again every little thing but the only difference I get is increasing irradiance catching reduces the "scale" of the noise but it keeps appearing. I'm running an up to date 64-bit Poser Pro 2014 on Windows 7 but the same scene and settings rendered in Poser 11 gives the same results. Can someone help me?