Question

You All know these sunglasse with gold and silver mirror colored glasses in them, you can perfectly see through them though nobody can see your eyes?
Well this isn't a post on a make or something but more of a question: If I would like to make a Helmet with a visor (Spartan from Halo, Daft Punkt Helmets....) what is the process called for that sunvisor coating? I have looked up various terms on coating and gold and silver coatings but all I get are non-seethrough or Opaque....well you're practically blind wearing the helmet or you have a clear visor, how do I do this? What is the process called? The only thing I could find is from Nasa where they actually coated the visor in a thin layer of gold but no How to's?

The STL is from @Big_red_frog 's Halo Visor