A solar concentrator comprises a curved linear Fresnel lens (16) which has a plurality of prismatic features (10, 12, 14). The prismatic features are arranged into at least two segments, in each segment the prismatic features have a common apex angle, the common apex angle differing between adjacent segments. A method of manufacturing a mold (2) for producing a Fresnel lens optical film (16) is disclosed. The method comprises rotating a mold blank, and forming a plurality of different prismatic features (10, 12, 14) on the mold blank by repeatedly advancing a cutting tool (4) onto and then withdrawing the tool away from the surface of the mold blank.