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 or input surface of the body (2); wherein the extraction structure (4, 6) is constructed and configured such that radiation at an output (19) of the body (2) is directionally modified, especially in terms of energy or intensity or of directional distribution or of both, as compared with radiation at the output of the body (2) in the absence of said extraction structure (4, 6), by interaction of radiation entering and/or propagating within and/or exiting the body (2) with the said extraction structure (4, 6), e.g. such as to reduce or ameliorate the deleterious effects of TIR within the body (2).