Viewing a glow through a completely transparent object changes its
brightness. This seems odd since the object is completely transparent and
should not affect anything that is behind it.
  Here is an example. A completely transparent cylinder is partially in
front of a glow (and a sphere) so you can clearly see the difference.
  Note that you can see part of the shape of the cylinder in the reflection
in the floor due to the dimmed glow.
  Filtering has the exact same effect.

  Oddly enough, this same effect happens even if the transparent object
is BEHIND the glow instead of being in front of it.
  You can test it by changing the 'Dist' identifier in the code, which sets
the position of the cylinder in the z-axis.
  Change it to '2' to see that the effect persists.
  Change it to '10' for another good example of the cylinder being behind the
glow.

  I think it has something to do with the length of the ray being calculated
(and perhaps it's a known problem).

#version unofficial MegaPov 0.6;

camera { location <-1,2,-14> look_at -y angle 35 }
light_source
{ <50,200,-100>,1
  spotlight point_at 0 radius 1 falloff 2
}
plane { y,-2 pigment { checker rgb 1, rgb z } finish { reflection 1 } }

sphere { 0, .75 pigment { rgb x } finish { specular .5 } }
glow
{ type 1 location 0 size .1 radius 2 fade_power 3
  color <1,1,0>
}

#declare Dist = -2;

cylinder { <1,-1,Dist>,<1,2,Dist>,1 pigment { rgbt 1 } }


-- 
main(i,_){for(_?--i,main(i+2,"FhhQHFIJD|FQTITFN]zRFHhhTBFHhhTBFysdB"[i]
):_;i&&_>1;printf("%s",_-70?_&1?"[]":" ":(_=0,"\n")),_/=2);} /*- Warp -*/

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