Thin Film on Principled BSDF for Cycles

First of all, what is Thin Film (and why is not a fancy name to call the Clearcoat feature). Thin Film is the simulation of a film with a custom width (in nanometers) along the material and its own IOR, this film interferes with the material and is used to simulate iridescent material and soap bubbles.

We will take a soap bubble to have a better understanding of the thin film. Let's take a sphere with a material with no roughness, full transparency and an IOR of 1.0, we'll end with a full transparent material. When we add the Thin Film with, for example, 400 nm, suddenly we are able to see the material again, resembling a soap bubble. What this does is simulates the thickness of the object surface using the faces, i.e. it makes objects hollow (like a bubble, not a glass ball). We have now our bubble... But with a strange effect, is iridescent by default, like it happens in real life. This happens in real life due the Thin-Film interference where, basically, the film is so thin that the light waves interferes with each other enhancing some wavelenghts and weakening another. This effect only appears in thin layers of film with widths of nanometers (0 to 3000). This width on the Thin Film would be able to have it's own texture to simulate uneven thickness because the interference varies depending of the thickness. This does not happens with the Clearcoat, because it doesn't have any thickness value and interference (because the clearcoat

And this can be applied to any material too:

This feature exists in other path-tracing engines (like Arnold ).

Currently, in cycles, you have to fake this feature with the fresnel option, but is not physically correct nor lets you have different thickness.