> 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.
>

If your object have some thickness, then adding an interior block that 
define a fade_color, fade_distance and fade_power will do the trick.
This will model a substance that ONLY atenuate the transmited light 
without any scattering.
Normaly, you should use colour values in the 0..1 range. Small value 
increase the absorbtion, large reduce it.
If your desired colour is "out of gamut", then you may have some 
negative value.
It's possible to use values larger than 1. It could be concidered as 
something similar to fluoressance.

If you want some single scattering, then adding the "hollow" attribure 
and adding some scatering media could be closer to what you want.
Depending on the dencity of the media and the thickness of the object, 
it can go from barely noticeable to opaque.
Here, a value of zero mean that the media don't affect that colour.
Large values increase the opacity of the media. There is no upper limit.

If your material is supposed to cause multiple scattering, then you need 
to use version 3.7 and use the SSLT feature. Then, in the finish, you 
set the translucency of your material. If the translucency is high 
ehough compared to the thickness, you'll start to see through the object.
The translucency value can be related to the mean free travel of photons 
through the material.
There is no upper limit. Very large values makes the material transparent.

For all 3 cenarios, you provide an RGB colour vector.

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