An apparatus comprising at least one sealing aperture (40) comprising a hollow part (41), having an inner surface (42), extending at an interface between different zones (50;60) of the apparatus; and a member (43) positioned in the hollow part configured to substantially transmit EUV radiation and to substantially filter non-EUV radiation at the interface; wherein the inner surface of the hollow part has a surface treatment configured to increase absorption of the non-EUV radiation that is transferred by the member to the hollow part.