thanks guys, that was very helpful. I have taken on board your points and now
have a structure that avoids coincident interfaces by overlapping layers. I dont
need to start in air but I do need to end in it. I get around that problem by
adding strong absorption to a final air layer which stops the unwanted final
reflection.

My code is below and I think it is giving me reasonable results. However it is a
bit 'noisy'. Any ideas which settings could improve the uniformity of my result?

#default {finish {ambient 0 diffuse 0}}

global_settings
{
   assumed_gamma 1

   photons{count 9000000 max_trace_level 15 adc_bailout 0}

}

#macro PhotonTarget(Reflect, Refract, IgnorePhotons)
 photons {
  target
  reflection Reflect
  refraction Refract
  #if(IgnorePhotons) collect off #end
 }
#end

camera
{
   orthographic
   location <0,0,1>
   look_at  <0,0,-1>
   angle 90
}

light_source{<0,0,2.5>, 1 cylinder radius 10 falloff 10 tightness 0 point_at
<0,0,4> parallel}

object{

box
{
   <-100,-100,-2> <100,100,50>
   pigment{rgbt 1}
   finish
   {
     reflection{0,1 fresnel}
     conserve_energy
   }
   interior{ior 3.2 fade_distance 1000 fade_power 1000}
   PhotonTarget(yes, yes, no)
}

}

object{

box
{
   <-100,-100,8> <100,100,10.5>
   pigment{rgbt 1}
   finish
   {
     reflection{0,1 fresnel}
     conserve_energy
   }
   interior{ior 3.2 fade_distance 25 fade_power 1000}
   PhotonTarget(yes, yes, no)
}

}

object{
box
{
   <-100,-100,13> <100,100,14.5>
   pigment{rgbt 1}
   finish
   {
     reflection{0,1 fresnel}
     conserve_energy
   }
   interior{ior 2.68 fade_distance 1000 fade_power 1000}
   PhotonTarget(yes, yes, no)
}
}

object{
box
{
   <-100,-100,14> <100,100,15>
   pigment{rgbt 1}
   finish
   {
     reflection{0,1 fresnel}
     conserve_energy
   }
   interior{ior 1 fade_distance .01000 fade_power 1000}
   PhotonTarget(yes, yes, no)
}
}

disc
{
   <0,0,-1> <0,0,1> 100
   pigment{rgb 1}
   finish{diffuse 1.0}
}

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