Luminaires and optical elements for use therein

A luminaire comprises at least one light source 2, and an optical system comprising at least one optical element 10, 11, 12a, 12b for directing and/or distributing the light 5 emitted by the source 2 into a desired output light distribution pattern 7. The optical elements 10, 11, 12a, 12b, each comprise a thin foil or sheet substrate having at least one optically functional surface or surface layer thereon or on a portion thereof. At least a portion of the at least one optically functional surface or surface layer on the substrate of at least one of the optical elements 10, 11, 12a, 12b has an at least partially diffractive optical function, and/or at least a portion of the at least one optical elements 10, 11, 12a, 12b is shaped such that its substrate has a non-flat or non-planar shape in three dimensions.