>    As said, I don't really know *why* this is so. As I have mentioned,
> with my CRT a 'rgb 0.5' as produced by pov3.7 by default looks about
> 50% gray when compared to a test pattern, so *in theory* a linear
> gradient produced by pov3.7 should look linear.

No it shouldn't - your eye/brain does not see light in a linear fashion. 
  50% actual brightness from something (as measured in cd/m^2 or 
whatever) will *not* look half as bright as 100%.  It's no different to 
a real scene, light bulbs, or an image on your monitor.  It's just the 
way your eye/brain works.

An analogy is hearing, a sound with twice the physical power (in Watts) 
does not sound twice as loud to a human.

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