A filter, method of formation thereof, and an image sensor

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