Following the answers to my question, I give you here the solution I find 
very near to my intentions. Who knows, you may need it yourselves?

//---code started---
#version 3.6;

#include "colors.inc"
#include "rad_def.inc"

global_settings {
  assumed_gamma 1.0
  radiosity { Rad_Settings(Radiosity_Fast,off,off) }
}

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

camera {
  location  <0.0, 0.5, -4.0>
  direction 1.5*z
  right     x*image_width/image_height
  look_at   <-0.6, 0.0,  0.0>
}

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>]
    }
  }
}

light_source {
  0*x
  color rgb 1.0
  area_light
  <8, 0, 0> <0, 0, 8>
  4, 4
  adaptive 0
  jitter
  circular
  orient
  translate <-300, 30, 300>
}

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

plane {
  y, -1
  pigment { color rgb <0.7,0.5,0.3> }
}

sphere {
  0, 0.5
  texture {
    pigment {
      radial
      frequency 8
      color_map {
        [0.00 color rgb <1.0,0.4,0.2> ]
        [0.33 color rgb <0.2,0.4,1.0> ]
        [0.66 color rgb <0.4,1.0,0.2> ]
        [1.00 color rgb <1.0,0.4,0.2> ]
      }
    }
    finish{
      specular 0.6
    }
  }
  translate -0.5*y
}

cone { <0, 1, 0>, 0.0, <0, -1, 0>, 0.5
  pigment {rgb <1,0,0>}
  translate <-3, 0, 3.5>
}

//----------------------------------------------------------------------
#declare MatGlass =
material {
  texture {
    pigment {
      color rgbt <1.0, 1.0, 1.0, 0.4>
    }
    normal {
      granite , 0.05
      scale  0.001
    }
    finish {
      ambient 0
      diffuse 1
      brilliance 5
      specular 0.6
      roughness 0.001
      conserve_energy
      reflection {
        0.1 , 0.1
        fresnel  on
        falloff  1.0
        exponent 1.0
        metallic 0.0
      }
    }
  }
  interior {
    ior 1.5
    dispersion 1.02
    //fade_distance 1.0
    //fade_power 1001
  }
}

polygon {
  5, // number of points
  <-0.5, 0>, <0.5, 0>, <0.5, 1>, <-0.5, 1>, <-0.5, 0>
  material {MatGlass}
  double_illuminate
  scale 2
  rotate -45*y
  translate <-1, -1, 1>
}

//---code ended---


Thomas

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