There is provided an optical system, including a mechanical body (110), a light-transmitting substrate (20) having two major surfaces and edges, embedded in the mechanical body, an optical element (90) for coupling light into the substrate by total internal reflection and a plurality of partially reflecting surfaces (22) carried by the substrate, wherein the partially reflecting surfaces are parallel to each other and are not parallel to any of the edges of the substrate. The system also includes an image capturing device (112), a display source (4), and an image-processing unit (114). The image-capturing device (112) is connected via the image-processing unit (114) to the display source (4).