An optical instrument (100; 200), comprising: a chamber (110; 210); an object (120, 130; 220) exposed to an interior of the chamber (110; 210); a source of low-pressure gas (140; 240), the gas comprising at least one of low-pressure molecular hydrogen gas, low-pressure molecular oxygen and a low-pressure noble gas, the source of low pressure gas being fluidly coupled to the chamber (110;210); a low voltage source (150; 250) electrically coupled between the object (120, 130; 220) and a remaining portion of the instrument that is exposed to the interior of the chamber (110; 210) so as to maintain the object (120, 130; 220) at a low voltage relative to the remaining portion; and an EUV/VUV light source adapted to direct EUV/VUV light through the low pressure gas in the chamber onto the object (120, 130), whereby when the EUV/VUV light source is activated ions of the low-pressure gas are formed and directed to the object (120, 130); characterised in that: wherein the chamber is configured with one or more exit ports (152, 154) which are at a lower pressure than the gas from the source of low-pressure gas (140;240) such that the low-pressure gas flows through the chamber (110; 210) from the source of low-pressure gas (140; 240) to the one or more exit ports (152, 154), wherein the source of low-pressure gas (140;240) is located at one end of the chamber (110; 210) and the one or more exit ports (152, 154) are located at the opposing end of the chamber (110;210) with the object (120, 130, 220) being located within the chamber (110, 210) between the source of low-pressure gas (140, 240) and the one or more exit ports (152, 154). a chamber (110; 210); an object (120, 130; 220) exposed to an interior of the chamber (110; 210); a source of low-pressure gas (140; 240), the gas comprising at least one of low-pressure molecular hydrogen gas, low-pressure molecular oxygen and a low-pressure noble gas, the source of low pressure gas being fluidly coupled to the chamber (110;210); a low voltage source (150; 250) electrically coupled between the object (120, 130; 220) and a remaining portion of the instrument that is exposed to the interior of the chamber (110; 210) so as to maintain the object (120, 130; 220) at a low voltage relative to the remaining portion; and an EUV/VUV light source adapted to direct EUV/VUV light through the low pressure gas in the chamber onto the object (120, 130), whereby when the EUV/VUV light source is activated ions of the low-pressure gas are formed and directed to the object (120, 130); characterised in that: wherein the chamber is configured with one or more exit ports (152, 154) which are at a lower pressure than the gas from the source of low-pressure gas (140;240) such that the low-pressure gas flows through the chamber (110; 210) from the source of low-pressure gas (140; 240) to the one or more exit ports (152, 154), wherein the source of low-pressure gas (140;240) is located at one end of the chamber (110; 210) and the one or more exit ports (152, 154) are located at the opposing end of the chamber (110;210) with the object (120, 130, 220) being located within the chamber (110, 210) between the source of low-pressure gas (140, 240) and the one or more exit ports (152, 154).