A luminaire comprising: a light source; an exit window for light from the light source; and a diffuser at the exit window for diffusing the light, wherein the diffuser comprises a plurality of micro-optical elements, each micro-optical element adapted to redirect light from the light source, so that the redirected light from each element emerges from the diffuser at a respective predetermined polar angle, wherein the plurality of micro-optical elements comprises first micro-optical elements, which redirect light from the light source into a first range of polar angles, wherein the first micro-optical elements occupy a greater proportion of the area of the diffuser in a first neighbourhood than in a second neighbourhood, wherein the first neighbourhood receives less light from the light source than the second neighbourhood and a local density of the first micro-optical elements over the surface of the exit window is proportional to the reciprocal of the luminous emittance at the exit window, wherein the plurality of micro-optical elements further comprises second micro-optical elements, which redirect light from the light source into a second range of polar angles, wherein the polar angles of the first range are greater than the polar angles of the second range, and wherein a spatial distribution of the second micro-optical elements complements a spatial distribution of the first micro-optical elements in that the second micro-optical elements are arranged to occupy area not occupied by the first micro-optical elements, and wherein the second micro-optical elements occupy a greater proportion of the area of the diffuser in the second neighbourhood than in the first neighbourhood. a light source; an exit window for light from the light source; and a diffuser at the exit window for diffusing the light, wherein the diffuser comprises a plurality of micro-optical elements, each micro-optical element adapted to redirect light from the light source, so that the redirected light from each element emerges from the diffuser at a respective predetermined polar angle, wherein the plurality of micro-optical elements comprises first micro-optical elements, which redirect light from the light source into a first range of polar angles, wherein the first micro-optical elements occupy a greater proportion of the area of the diffuser in a first neighbourhood than in a second neighbourhood, wherein the first neighbourhood receives less light from the light source than the second neighbourhood and a local density of the first micro-optical elements over the surface of the exit window is proportional to the reciprocal of the luminous emittance at the exit window, wherein the plurality of micro-optical elements further comprises second micro-optical elements, which redirect light from the light source into a second range of polar angles, wherein the polar angles of the first range are greater than the polar angles of the second range, and wherein a spatial distribution of the second micro-optical elements complements a spatial distribution of the first micro-optical elements in that the second micro-optical elements are arranged to occupy area not occupied by the first micro-optical elements, and wherein the second micro-optical elements occupy a greater proportion of the area of the diffuser in the second neighbourhood than in the first neighbourhood.