There is provided an optical device, having a light transmitting substrate (20) including at least two major surfaces parallel to each other and edges; optical means (16) for coupling light into the substrate by internal reflection and at least one reflecting, surface (22) located in the substrate which is non-parallel to the major surfaces of the substrate (20) characterized in that the optical means (16) for coupling light into the substrate is a partially reflecting surface, wherein part of the light coupled into the substrate (20) passes through the partially reflecting surface (16) out of the substrate and part of the light is reflected into the substrate (20).