There is disclosed herein a waveguide comprising an optical slab and an optical wedge. The optical slab has a first refractive index, n1>1. The optical slab comprises: a pair of opposing surfaces and an input port. The pair of opposing surfaces are arranged in a parallel configuration. The input port is arranged to receive light into the optical slab at an angle such that the light is guided between the first and second opposing surfaces by a series of internal reflections. The optical wedge has a second refractive index, n2, wherein 1