Jim Charter wrote:

<snip>

> Thanks for replying
> 
> So I take it if you give the finish
> reflection { .96 fresnel on } with interior { ior 1.5 }
> then .96 is taken as the high reflectivity value as angle of incidence
> approaches 0 and the low reflectively value, at angle of incidence 90,
> is derived from the ior value?

Jim,

frankly spoken, I am not so familiar with SDL. The only thing I can do for
sure is to help you understand the Fresnel reflection.

The angle of incidence is usually measured between the direction of the
incident light and the surface normal. Therefore, a 90 degree angle of
incidence means light parallel to the surface, 0 degree angle of incidence
means light parallel to the suface normal, i.e. perpendicular to the
surface. The Fresnel reflection coefficients describe the amount of light
being reflected off a surface, depending on the angle of incidence. In two
cases the values for the Fresnel coefficients can be determined easily.

The first case is the 90 degree angle of inciedence. This is not really a
reflection, since the light is parallel to the surface. In this case no
light enters the surface and the reflectivity equals 1, i.e. all light is
"reflected".

The second case is the 0 degree angle of incidence. In this case some light
enters the surface and some light is reflected. The amount of reflected
intensity is
        R = | (n-1)/(n+1) |**2,
where n ist the index of refraction (ior). This is easy to calculate for
transparent media. For absorbing media, n becomes a complex number but the
formulae do not change. For all intermediate angles the expression for the
Fresnel coeffitients becomes more difficult and the reflection coeffitients
are different for different polarization states of light.

In my opinion, the Fresnel reflection should be well-defined by just giving
the ior. Since povray cannot handle abssorbing media by simply taking a
complex ior, it might be neccessary to give an additional parameter.
However, did you consult the povray manual?

I am not sure, if this does help you.

Alf

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