>Actually, if I read the source code for POV-Ray correctly, (max_ior/min_ior)
>is the dispersion.  When POV-Ray calculates the dispersion, it starts with:
>        ior = ior /sqrt(disp);
>        ior_mult = pow(disp, 1.0/(disp_nelems-1));

>Then, in the dispersion loop, it does this:
>
>          ior *= ior_mult;
>
>The net result is that the IOR will range from ior/sqrt(disp) to
>ior*sqrt(disp).

This is correct.

>Assuming max_ior = ior*sqrt(disp), min_ior = ior/sqrt(disp), then
>max_ior/min_ior = ior*sqrt(disp) / (ior/sqrt(disp)) = sqrt(disp)*sqrt(disp)
>= disp

And this is not. You forgot the light wavelenghts. Pov-Ray dispersion
calculation make simple rainbow for light which ior will range from
ior/sqrt(disp) to ior*sqrt(disp). To calculate correct dispersion value we
need several ior values and the light wavelenghts used to measure ior
values to approximate dispersion value.

Matti

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