A light guide of the tapered-waveguide type includes an input slab (30) for expanding a projected image between an input end and an output end (40); and a tapered output slab (10) arranged to receive rays from the said output end of the input slab, and to emit them at a point on its face that corresponds to the angle at which the ray is received. The taper is calculated so that all rays injected into the input end undergo the same number of reflections before leaving the output face. The thickness of the input slab light guide (30) is greater in the transverse direction away from the centre line C, so that light travelling at the critical angle from the input face of the slab waveguide towards the output waveguide (10) bounces the same number of times in the input slab, regardless of its fan-out angle, in order to further reduce image distortion.