Whilst expirimenting with the media feature, I've noticed something odd. 

A media with absorption 1.0 blocks all passing light, given that the way
of the light through the object is 1 unit long. That makes sense. 
When adding emission rgb <0,0,1> one would expect the obstacle to appear
uniformly blue. 
However, I've noticed that where the background contains more blue, the
intensity of the obstacle is higher too. But the absorption 1.0 should
have blocked all background light, no ?

I've created a small sample scene that shows this. It renders a
background of red, blue and green hexagons, and a box with the described
media. 
Comment away the emission value and the object will appear completely
black (except for the edges). With emission, the blue hexgons in the
scene appear brighter through the object. 

This makes no sense in my head. Is it just my head or is this something
the POV-team has overlooked ? If all you brains out there don't get it
either, I'd better notify the POV-team.

// ---------------- poser scene ----------------

global_settings { assumed_gamma 2.2 }

#include "colors.inc"

camera {  
   location <0, 0, -7>
   direction <0, 0, 2>
   look_at <0,0,0>
}

object {light_source{<0,1000,-1000> color White}}

// hexagon wall
plane { z, 10
   pigment { hexagon color Green color Blue color Red rotate <90,0,0>}
   hollow
}

box { <-1, -1, -0.5>, <1, 1, 0.5> 
   hollow 
   material
   {
      texture {
         pigment {color rgbf 1.0}
         finish {diffuse 0 ambient 0}
      }
      interior
      {
         media
         {
            samples 3,10
            absorption rgb 1
            emission rgb <0,0,1>
            /* scattering {0, rgb <0,0,5>
               extinction 0.0} */
         }
      }
   }
}

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