US Patent 8422840 Flat-panel optical projection apparatus

A light guide of the tapered-waveguide type includes an input slab for expanding a projected image between an input end and an output end, and an output slab arranged to receive rays from the said output end, and to emit them at a point on its face that corresponds to the angle at which the ray is received. The input slab and output waveguide are matched so that all rays injected into the input end undergo the same number of reflections before leaving the output surface. With the invention, the input slab is itself tapered slightly towards the output waveguide. This means that input and output waveguides can be made the same length, in the direction of ray travel, and can therefore be folded over each other with no wasted space.