Hello! If one were to push uncollimated light through a tube of inner radius R and length L, coated with matte black - the maximum incidence angle it will allow through would be ATAN2(R, L). On the other end of the tube, only the "most collimated" fraction of light would exit - a very small fraction for a narrow tube. Is this principle available as an optical filter in sheet form? For example, simply drilling, etching, etc. a large number of very small holes (fraction of sheet thickness) in a thin sheet of absorbtive material would suffice and it would seem to be this is something that ought to be available - possibly going by a name i simply cannot figure out. Any ideas or tips would be most welcome.