US Patent 8016475 Prismatic film backlight

A selectively light-reflecting sheet (30) includes a prismatic film (1) with parallel prisms (10) on its face, a low-index film (3) on the prisms and a layer (5) on the low-index film, this layer having a substantially flat surface. This results in a low effective index, which means that light incident at steeper angles on the prismatic film (1) will pass through it. This “leaky” effect can be used in conjunction with a reflector sheet (20) in a tapered-waveguide arrangement to construct a lightweight backlight.