Bare gold has high reflectance in the near, mid and far IR regions. Gold coating does not oxidize, so it is not necessary to have a protective dielectric overcoat. However, Bare gold layer is soft and easy to be scratched, and can only be cleaned by flow-washing with solvents, or by blowing the surface clean with a low pressure stream of dry air. Durability is greatly enhanced with a protective overcoat.
Protected gold can be cleaned regularly using standard organic solvents. In addition, gold is chemically inert, thus it is well suited for harsh environments, such as DF and HF laser cavities.

BK7 or B270 glass substrates with single-side gold coating
Bare gold coated
Lambda/4 surface flatness
R > 98% 1um to 5um
R > 95% from 800nm to 10um