US Patent 10564430 Compact head-mounted display system

There is provided an optical device, including a light-transmitting substrate having at least two major surfaces and edges, an optical element for coupling light waves into the substrate to effect total internal reflection, and at least one flat reflecting surface having at least one active side located between the two major surfaces of the light-transmitting substrate for coupling light waves out of the substrate, characterized in that light waves trapped inside the substrate are reflected at least twice by the active side of the reflecting surface before being coupled out from the substrate.