An optical element and associated methods for generating an optical element and apparatus comprising the optical element, wherein the optical element comprises a first surface (10), a second surface (15), and a side wall structure (25) between the first and second surfaces. The side wall structure has an internally reflecting profile such that optical radiation incident on the first surface at an angle less than or equal to an acceptance angle and then incident on the side wall structure is internally reflected to the second surface by the side wall structure. In a first cross section of the optical element, the side wall structure has a first internally reflecting profile and/or the first surface has a first cross sectional profile. In a second cross section that is rotated relative to the first cross section, the side wall structure has a second internally reflecting profile and/or the first surface has a second cross sectional profile, wherein the second internally reflecting profile of the side wall structure and/or the second cross sectional profile of the first surface is different from the first internally reflecting profile of the side wall structure and/or the first cross sectional profile of the first surface.