US Patent 10012844 Variable focal backlighting

A backlight unit includes a waveguide assembly having a first wedge and a birefringent wedge disposed adjacent to, and arranged nose-to-tail with, the first wedge, to define an interface of the waveguide assembly. The birefringent wedge has different indices of refraction for light propagating through the waveguide assembly in first and second polarization states. The first wedge is configured to propagate the light in the second polarization state at a different speed than the birefringent wedge. A liquid crystal layer is configured to selectively switch the light between the first and second polarization states.