Problem: fade color seems to disappear on single sphere of union when using
photons. see povray.binary.images for image and longer description.

// begin scene

#version 3.5;

#declare Radiosity=on;
#declare Photons=off;
#declare Transmit=on;

global_settings {
  assumed_gamma 1.0
  max_trace_level 12
  #if (Photons)          // global photon block
    photons {
      //spacing 0.005                 // specify the density of photons
      count 3000000               // alternatively use a total number of
photons

      //gather min, max            // amount of photons gathered during
render [20, 100]
      //media max_steps [,factor]  // media photons
      //jitter 1.0                 // jitter phor photon rays
      //max_trace_level 5          // optional separate max_trace_level
      //adc_bailout 1/255          // see global adc_bailout
      save_file "excl_1.pho"       // save photons to file
      //load_file "excl_1.pho"       // load photons from file
      //autostop 0                 // photon autostop option
      //radius 10                  // manually specified search radius
      // (---Adaptive Search Radius---)
      //steps 1
      //expand_thresholds 0.2, 40
    }
  #end
  #if (Radiosity)
    radiosity {
      pretrace_start 0.08           // start pretrace at this size
      pretrace_end   0.04           // end pretrace at this size
      count 35                      // higher -> higher quality (1..1600)
[35]
      nearest_count 5               // higher -> higher quality (1..10) [5]
      error_bound 1.8               // higher -> smoother, less accurate
[1.8]
      recursion_limit 3             // how much interreflections are
calculated (1..5+) [3]
      low_error_factor .5           // reduce error_bound during last
pretrace step
      gray_threshold 0.0            // increase for weakening colors (0..1)
[0]
      minimum_reuse 0.015           // reuse of old radiosity samples
[0.015]
      brightness 1                  // brightness of radiosity effects
(0..1) [1]

      adc_bailout 0.01/2
      //normal on                   // take surface normals into account
[off]
      //media on                    // take media into account [off]
      //save_file "file_name"       // save radiosity data
      //load_file "file_name"       // load saved radiosity data
      //always_sample off           // turn sampling in final trace off [on]
      //max_sample 1.0              // maximum brightness of samples
    }
  #end
}

#default {
  texture {
    pigment {rgb 1}
    #if (Radiosity)
      finish {
        ambient 0.0
        diffuse 0.6
        specular 0.3
      }
    #else
      finish {
        ambient 0.1
        diffuse 0.6
        specular 0.3
      }
    #end
  }
}

// ----------------------------------------

camera {
  right x*image_width/image_height
  location  <0,0.65,-2>
  look_at   <0,0.35,0>
}

light_source {
  <500,500,-500>       // light's position
  color rgb <1, 1, 1>  // light's color
  photons {           // photon block for a light source
    refraction on
    reflection on
  }
}

sky_sphere {
  pigment {
    gradient y
    color_map {
      [0.0 rgb <0.6,0.7,1.0>]
      [0.7 rgb <0.0,0.1,0.8>]
    }
    #declare w1 = 8;
    #declare t1 = <0.34, 0.53, 0.43>;
    #while (w1 > 0)
     warp {repeat x flip y}
     warp {turbulence t1 }
     scale 1.1
     #declare w1 = w1 - 1;
    #end
  }
}

plane {
 -z, 0
 texture {
  pigment {rgb 1}
  normal {
  quilted 0.4 // amount
  control0 -1 // lower curvature (-1...1)
  control1 1  // upper curvature (-1...1)
  scale 0.0065
  }
 }
}

#declare t1 =  texture {
  pigment {
   rgb 1 //<1, 0.5, 0>*2
   #if (Transmit)
    transmit 1
   #end
   //filter 0.2
  }
  finish {
   specular 1
   roughness 0.0065
   diffuse 0.2
   reflection {
    0.1, 0.45
    fresnel on
   }
  }
 }

#declare i1 = interior {
  ior 1.65
    //dispersion 1.1           // needs a color_map in the light source
    //dispersion_samples 10  // accuracy of calculation (2..100) [7]
    fade_power 1003       // values larger than 1000 give realistic
exponential attenuation
    fade_distance 0.45       // distance where light reaches half intensity
    fade_color <1, 0.65, 0.0>
  }

#declare i2 = interior {
  ior 1.65
    //dispersion 1.1           // needs a color_map in the light source
    //dispersion_samples 10  // accuracy of calculation (2..100) [7]
    fade_power 1003       // values larger than 1000 give realistic
exponential attenuation
    fade_distance 0.05       // distance where light reaches half intensity
    fade_color <1, 0.65, 0.0>
  }

#declare m1 = material {
 interior {i1}
 texture {t1}
}

#declare m2 = material {
 interior {i2}
 texture {t1}
}

union {
 merge {
  cone {
   <0, 1, 0>, 0.25,
   <0, 0, 0>, 0.1
   hollow
  }
  sphere {
   <0, 0, 0>, 0.25
   scale <1, 0.5, 1>
   translate 1*y
   hollow
  }
  sphere {
   <0, 0, 0>, 0.1
   scale <1, 0.5, 1>
   hollow
  }
  rotate -x*(degrees(asin(0.25-0.1)))
  translate <0, 0, -0.1>
  material {m1}
 }
 sphere {
  <0, -0.35, 0>, 0.2
  translate <0, 0, -0.1>
  hollow
  material {m2}
 }


 photons {
  target
  reflection on
  refraction on
  collect on
 }
 rotate -10*z
 translate <0, 0.065, 0>
}

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