The manufacture of metal-mesh filters starts with photolithography of copper on a substrate, which allows fine control over the parameters a , g , and t . The metallic grids are made of thin copper film on top of a dielectric substrate such as mylar or polypropylene. The copper is ≈ ≈ .4 μ μ m thick, and the dielectric ranges from .9 μ μ m to 1.5 μ μ m .