An optical element comprising: synthetic diamond material; and a flattened lens surface structure in the form of a zone plate, Fresnel lens, or a spherical lens formed directly in at least one surface of the synthetic diamond material, wherein the synthetic diamond material has an absorption coefficient measured at room temperature of ≦̸0.5 cm−1 at a wavelength of 10.6 μm, and wherein the synthetic diamond material has a laser induced damage threshold meeting one or both of the following characteristics: the laser induced damage threshold is at least 30 Jcm−2 measured using a pulsed laser at a wavelength of 10.6 μm with a pulse duration of 100 ns and a pulse repetition frequency in a range 1 to 10 Hz; and the laser induced damage threshold is at least 1 MW/cm2 measured using a continuous wave laser at a wavelength of 10.6 μm.