That’s great Vlado!
you know I probably shouldn’t have put n and k in there as those values need to be normalized to fit 0-255 (from refractiveindex.info) as was said. The idea is to make it easier to have a complex fresnel but be able to tweak those colours by eye.
just have a drop-down with values from the refractiveindex website (copper, gold etc) so that you don’t have to type those each time…
so that’s why in another post I was wondering about a dropdown list in OSL 
even that thinFilm shader you wrote would be great to have some pre-sets for different materials.