JAKE HICKS, PETAPIXEL:
Nearly every professional studio I’ve ever used has these “polyboards” and you‘ve probably even seen them yourself but may not have known what they’re used for. Polyboards are polystyrene boards that usually measure 4 feet wide by 8 feet high and are normally 2 inches thick. One of the other defining characteristics is that they are often white on one side and black on the other.
Read more, here.