Sheet material, for example for a swimming pool cover 12, comprises an upper layer 14 of mid-IR absorbing material and a lower layer 16, with bubbles 18, which absorbs visible light. The material passes at least 25% of radiation in the near-IR region, thus promoting heating of the water while suppressing growth of algae. Inorganic particles are used to block visible radiation and pigments are used to block mid-IR radiation.