Optical elements

An optical element 1 comprises a body 2 of radiation converting monocrystalline material, (e.g. of a luminescent or scintillator material), and an extraction structure 4, 6 applied to at least one output and/or input surface of the body of radiation converting monocrystalline material; wherein the extraction structure is constructed and configured such that radiation at an output 19 of the body of radiation converting monocrystalline material is directionally modified, as compared with radiation at the output of the body of radiation converting monocrystalline material in the absence of said extraction structure, by interaction of radiation entering and/or propagating within and/or exiting the body of radiation converting monocrystalline material with the said extraction structure, (e.g. by modifying the energy or intensity of the radiation at the output of the body via interactions of radiation entering and/or propagating within and/or exiting the body with the surface, and/or modifying ...