Prism sheet

A prism sheet is a clear film whose top surface is covered in tiny parallel ridges, each shaped like a long triangular prism. They are so small, often tens of micrometers apart, that the sheet just looks shiny and finely striped.

It's made from a roll of clear PET film. A thin layer of liquid acrylate resin is spread on the film, then pressed against a metal roller engraved with the reverse of the prism pattern. While the resin touches the roller, ultraviolet light shines through the film and hardens it in seconds. The film peels away carrying an exact copy of the prisms.

Inside the backlight unit, light reaches the prisms from many angles. Light heading roughly forward passes through and is bent toward the viewer. Light arriving at steep angles is reflected back down, where the reflector sheet sends it up again for another try. The result is a brighter picture straight ahead without using more power. TVs often use two sheets with their ridges at right angles, one for each direction. 3M's Brightness Enhancement Film is a well-known example, which is why these are often called BEF.