For all those who - just like me - have wondered what the most realistic 
setting for the radiosity "brightness" parameter should be:

After having done some experimenting and comparison with MCPov (see 
povray.unofficial.patches), I can conclude with confidence:

   "brightness 1.0" is absolutely right.

With "recursion_limit 1", it may even make sense to increase the 
brightness a bit above 1.0, to compensate for the "missing" multi-bounce 
diffuse illumination.

Any dissimilarities with MCPov results are pretty likely due to an error 
in MCPov (which gets the brightness wrong by a factor of 2), and any 
overly bright materials are pretty likely to be due to gamma issues, bad 
material settings, or inferior lighting setup. Or possibly even other 
bugs in conjunction with radiosity - but the default brightness value is 
absolutely right.

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