The interference node group is mostly adapted from Pruster's own OSL interference script found in this Blenderartists thread: https://blenderartists.org/forum/showthread.php?403299-UPDATE-v1-6-Cycles-PBR-thin-film-interference-and-metals
The interference node group inside calculates the Fresnel Reflectance of metals and thin-films of up to two layers. These calculations take into account both a material's real index of refraction, n, and imaginary index of refraction, k (together known as the complex refractive index), allowing for more accurate Fresnel and Interference colors.
Included with the interference node group are various materials made with it including: metals (e.g. gold, silver, copper, and osmium), 12 car paints, oxide coated metals (e.g. tarnished silver and heated iron), coated glasses, and more. Other materials can be made using the known n and k values for each material, many of which can be found here: https://refractiveindex.info/
Only the interference node group is under a CC BY-NC-SA 4.0 license, but the output can be used for any purpose.