There is provided an optical device, including a light waves-transmitting substrate having two major surfaces and edges, optical means for coupling light into the substrate by total internal reflection, and a plurality of partially reflecting surfaces (22a, 22b) carried by the substrate wherein the partially reflecting surfaces (22a, 22b) are parallel to each other and are not parallel to any of the edges of the substrate, and wherein one or more of the partially reflecting surfaces (22a, 22b) is an anisotropic surface.