I was using a program for simulating optical elements; mirrors, gratings, foils, crystals, slits, and zoneplates are what it mentioned. Does anyone know what a foil is in terms of optics? I know what aluminum foil is, obviously, but in terms of an optical system I don't know what a foil, or foil, would be, and what it's unique optical properties are.
It's a black coating to stick on surfaces of devices of an optical setuo to reduce spurious reflections.
So it's very very black ("99,99%") on one side, and possibly sticky on the other side.
For an example, see Edmund Optics: Acktar Light Absorbent Foil