A radiation system comprises a fuel emitter configured to provide fuel to a plasma formation region, a laser arranged to provide a laser beam at the plasma formation region incident on the fuel to generate a radiation emitting plasma, and a reflective or transmissive device (30) arranged to receive radiation emitted by the plasma and to reflect or transmit at least some of the received radiation along a desired path, wherein the reflective or transmissive device comprises a body configured to reflect and/or transmit said at least some of the radiation, and selected secondary electron emission (SEE) material (34) arranged relative to the body such as to emit secondary electrons in response to the received radiation, thereby to clean material from a surface of the device.