> Hi all
>
> thanks for the help so far with Fresnel reflection and Photons. I have got some
> accurate results for some simple tests.
>
> I am trying now to get Povray to calculate the reflectance from a stack of two
> materials stuck together with no air gap between them. One material has ior =
> 3.2, the other ior=1.5. Using Fresnel equations I estimate reflectance of this
> structure to be ~36%.
>
> Can povray handle fresnel reflection from this sort of structure, and if so, how
> is it best to describe the two layers (I have tried union&  merge but both do
> not seem to give the results I expected).
>
> Thanks
> Stu
>
>

You have a coincident surface problem. Whenever 1 surfaces are at the 
same location, you can't reliably estimate the order in whitch they must 
be evaluated.
The end result is that for a given ray, you may evaluate the surface of 
the first object then of the second, but, for the next one, you may find 
yourself evaluating the surfaces in the oposite order. It's prety random 
and depends on floating point precision and rounding errors.

There are some attempt at finding a workable solution for that case, but 
it's not yet done.

The usual solution is to have a very thin gap between the surfaces. In 
this case, the finish of both surfaces need to be adjusted to have a 
lower reflection to compensate for having two reflecting surfaces.
Some will instead mage the two objects overlap by a very small amount.
The gap or overlaping is usualy set to less than 0.00001 unit.

Another possibility is to have only one surface from one object having 
an ior set to the difference between the two materials.

In your case, you can use ior = 1.7 for the high index material and 
remove the low index object. It may not be suitable as your ray may need 
to travel from the air, the low index material, the high index material 
then finaly back to the air.



Alain

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