There is provided an optical device, having a light-transmitting substrate (20) having at least two major surfaces parallel to each other and edges; a display light source; optical means for coupling light from the light source into the substrate (20) by internal reflection, and at least one partially reflecting surface (22) located in the substrate (20) which is non-parallel to the major surfaces of the substrate wherein the source emits light waves located in a given field-of-view, that the light waves are collimated, that an angular resolution is defined for the optical device, and wherein the angular deviation between any two different rays located in one of the collimated light waves, is smaller than the angular resolution.