Thanks very much I have now got a much better picture.

I would like to ask about another issue which does apply to imaging, to a first
approxiation for a single photon energy the transmission through an object is
given by

Transmission = exp -(constant * thickness)

Is there a way to set the texture of the objects (cone, sphere, torus) so that
they attenuate light in this way. Also is it possible to set them up so for red
light the constant is different to green and blue light.

While for X-rays the spectrum of a typical tube is very wide (close to white)
for a gamma source such as Ir-192 it emitts two different wavelengths of
photons. So for this alternative light source it would be great to be able to
have two constants operating at once.

here is my current source code

  #include "colors.inc"
  #include "textures.inc"
#declare Floor_Texture     = texture {
                            pigment {color rgb <1,1,1> }
                            finish { diffuse 0.6,0.3 } }
camera {
    location  <0, -20, 0>
    look_at   <0, 0, 0>
    angle 48
  }
  light_source {
    <0, 100, 0>
    color White
  }
  plane {
    y, -1
    texture { Floor_Texture }
    }
  torus {
    1.5, 0.5
    texture { Brown_Agate }
    rotate <0, 150, 0>
    translate <-2, 1, 0>
  }
  cone {
    <0,1,0>, 0, <0,0,0>, 1
    texture { PinkAlabaster }
    scale <1, 3, 1>
    translate <1, 1, 2>
  }
  sphere {
    <0,1,0>,1
    texture { Sapphire_Agate }
    translate <1.5, 0, -2>
  }

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