Lenses

A solar concentrator comprising a curved linear Fresnel lens including a plurality of prismatic features, said prismatic features being arranged into at least two segments, wherein in each segment said prismatic features have a common apex angle, the common apex angle differing between adjacent segments.

An optical concentrator film comprising a plurality of prismatic features together forming a lens arrangement when the film is curved, said prismatic features being arranged into at least two segments, wherein in each segment said prismatic features have a common apex angle, the common apex angle differing between adjacent segments.

An optical film comprising a plurality of prismatic features together forming a Fresnel lens when the film is curved, said prismatic features being arranged into at least two segments, wherein in each segment said prismatic features have a common apex angle, the common apex angle differing between adjacent segments.

A method of manufacturing a mould for producing a Fresnel lens optical film;

• said method comprising rotating a mould blank, and forming a plurality of different prismatic features on said mould blank by repeatedly advancing a cutting tool onto and then withdrawing the tool away from a surface of the mould blank.

A method of manufacturing an optical film, the method comprising passing a continuous film of transparent material over a moulding drum, said moulding drum having surface relief adapted to impart corresponding linear features to the film passing over the drum, said features having a prismatic sectional profile and being arranged to cooperate to form a lens arrangement having a linear focal axis parallel with said linear features.