An optical diffuser including a fibre-optical piece (1) comprising: a first face (2) and a second face (3); a plurality of substantially parallel optical fibres (4) each extending along a respective optical axis from an input end of the optical fibre exposed at the first face to an output end of the optical fibre exposed at the second face. The optical axes of each of the plurality of optical fibres are inclined at a common oblique angle relative to at least one of the first face and the second face, such that light (7) incident substantially perpendicularly upon the at least one face enters the optical fibres in a direction oblique to the optical axes.