The broken lines are for the purposes of illustrating portions of the article and form no part of the claimed design. The arrows depicted in broken lines indicating light emission are for environmental purposes only and form no part of the claimed design. The regions of oblique surface shading do not imply any particular reflectivity of the surface of the illumination sheet, only that light is emitted from light-emitting surface regions thereon. The back surface is plain and unornamented.