The inside surface of a cover for a skylight is molded to have a variable prism that directs low-angle light into the skylight tube and that reflects away some high-angle light, to achieve a more constant light output over the course of the day. The prism is established by a series of circular parallel grooves, with grooves nearer the apex of the cover having cross-sections that are different than the cross-sections of grooves nearer the periphery of the cover.