>I was wondering what is the source (from a physical/scientific perspective) 
>of
> the formulas for k that are related to the various scattering types and 
> how
> closely they resemble the actual scattering that would occur in a real 
> world
> context (with a focus on the Mie and Rayleigh types).
>
> I am simulating a dust cloud and I was going through the source code to 
> see if I
> can make a clear correspondence between the parameters used in POV-Ray, 
> namely k
> and alpha, and those used when discussing scattering phenomenon such as
> extinction and backscatter ratios. For example, is alpha the extinction
> coefficient and k the backscatter? What is the relation between the real
> parameters and the ones in POV-Ray? Thanks.

Sorry it took a while to respond, I needed a while to think
about it.

From a scientific point of view POV doesn't even attempt
to simulate actual RL scattering. Instead rays pass from
the camera through a media without bending or scattering.
Instead it diminishes the intensity of the ray, while adding
in the color provided. In general this follows the scattering
type model only, the other parameters like extinction are
artistic only.

So the only real controls of the scattering media are the
type of model, the color, and the density (provided in
the density statement).

Type 5 provides some control of the Henyey-Greenstein
model by the eccentricity parameter, which I think is g
in the equations, but I'm not sure 100%.
http://www.nmr.mgh.harvard.edu/~adunn/papers/dissertation/node54.html

So you have 5 models to pick from, and the 5th one is
adjustable by a single parameter. Artistically that's perty
good, but if you are looking for exact results based on
some real world formula there's no good analog for which
setting is which because there are no settings beyond selecting
which type to use.

Note: there are a variety of settings having to do with the
sampling of the media, but those control the accuracy of
the media based on the model. Internally there are random
numbers involved in sampling the media repeatedly for
each ray, those settings have to do with how close POV is
getting to the model, but because the model isn't perfect
it usually doesn't matter much.

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