spectral - purity filters are disclosed that produce a stream of “ spf gas ” through which a beam of light , particularly a beam including extreme ultraviolet light , is allowed to pass . the spf can be located in a system that receives a beam of euv - containing light from a source and delivers the beam to a downstream euv - optical system , wherein the beam from the source passes through the spf to the optical system . the gaseous spf is formulated and configured to enrich the beam in at least one euv wavelength as the beam passes through the gaseous spf . for enrichment of euv wavelengths , an exemplary spf gas is zrcl 4 . the stream of spf gas can be sheathed in an inert “ sheath gas .” the gaseous spf is usable in a vacuum environment , in which used spf gas , and sheath gas if used , is collected .