An antireflection overcoat for opaque baffle coatings in the FIR/submillimeter region has been made from a simple Teflon spray-on lubricant. The Teflon overcoat reduced the specular reflectance of four different opaque coatings by nearly a factor of two. Analysis, based on the interference term of a reflecting-layer model, indicates that in the submillimeter region the reduced reflectance depends primarily on the refractive index of the overcoat and very little on its thickness.
Sheldon M Smith,
"A Simple Antireflection Overcoat For Opaque Coatings In The Submillimeter Region", Proc. SPIE 0675, Stray Radiation V, (3 June 1987); doi: 10.1117/12.939482; https://doi.org/10.1117/12.939482