A method of forming a radiation filter for a lithographic system, the method including forming at least one structure in or on a filter body, wherein the at least one structure provides a filtering effect and least one of a), b), c) or d): a) the at least one structure includes a plurality of transmissive, reflective, absorbing or fluorescent structures, and the method includes providing a desired distribution of the structures to provide a desired filtering effect; b) forming the at least one structure includes forming at least one transmissive, absorbing, reflective or fluorescent layer that has a variable thickness; c) forming the at least one structure includes altering at least one optical property to provide a variation of the optical property with position; d) the at least one structure includes a fluorescent layer that provides variation of at least one fluorescence property with position and/or angle of incidence.