An optical device, including a light waves-transmitting substrate has 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. The partially reflecting surfaces (22a, 22b) are parallel to each other and are not parallel to any of the edges of the substrate, one or more of the partially reflecting surfaces (22a, 22b) being an anisotropic surface. The optical device has dual operational modes in see-through configuration. In a first mode, light waves are projected from a display source through the substrate to an eye of a viewer. In a second mode, the display source is shut off and only an external scene is viewable through the substrate.