I'm sure I'm missing something obvious here, but I can't seem to figure 
out what it is.  I'm trying to render a scene using HDR lighting from a 
light probe, and am finding that semi (or even wholly) transparent 
objects are interacting with radiosity as if they were opaque.  I've 
tried cranking up max_trace_level and recursion_limit to no effect.

I've included a sample scene which shows the problem.  The scene 
contains a bright disc as a radiosity-based light source, a white plane, 
and a transparent disc slightly over the plane.  Despite the fact that 
the disc is completely transparent, it seems to be casting a shadow and 
a get this result: http://i.imgur.com/PME9OfO.png  I get similar results 
for partially-transparent objects where their shadows are much darker 
then they should be.

Anyone know what I'm doing wrong?  I'm using Povray version 
3.7.RC7.msvc.win64.

--------------

#version 3.7;


global_settings {
   assumed_gamma 2.2
   max_trace_level 10
   ambient_light rgb 1
   radiosity {
     // as far as I know, these should be really high-quality settings
     pretrace_start 0.08
     pretrace_end   0.004
     count 2000
     nearest_count 9
     error_bound 0.2
     recursion_limit 10
     low_error_factor 0.2
     gray_threshold 0
     minimum_reuse 0.01
     brightness 1.0
     adc_bailout 0.01/2
   }
}

camera {
   orthographic
   location <0, 0, 10>
   direction <0, 0, -1>
   right 2*x
   up 2*y
}

background {
   color rgb 0
}

// set up a `light source' to cast shadows via radiosity
disc {
   2*z, z, 1.0, 0.0
   pigment{ color rgb 1 }
   finish { diffuse 0 emission 5 }
   hollow
   no_image
   no_shadow
   scale 10000
}

// a plane for the shadows to be cast upon
plane {
   z, 0
   pigment { color rgb 1 }
   finish { ambient 0 diffuse 1.0 }
}

// This disc is completely transparent, so why does it cast a shadow?
disc {
   0.1*z, z, 0.9, 0.5
   pigment { color rgbt 1 }
   finish { diffuse 1 }
}

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