A light diffuser is formed of a slab of material which has opposing first and second main surfaces, wherein each surface comprises an array of light directing facets. Each light directing facet of the first surface directs incident light through the slab predominantly to a corresponding paired light directing facet of the second surface, and at least some different light directing facets of the first surface direct incident light to their corresponding paired light directing facets of the second surface in different directions. The angular profile at the exit can be controlled, and for example does not need to diverge more at the output of the diffuser than at the input.