One of the important applications for compact optical elements is in head-mounted displays (HMDs) wherein an optical module serves both as an imaging lens and a combiner, in which a two-dimensional display is imaged to infinity and reflected into the eye of an observer. The image to be displayed is obtained directly from either a spatial light modulator (SLM), such as a cathode ray tube (CRT), a liquid crystal display (LCD), an organic light emitting diode array (OLED), or a scanning source and similar devices, or indirectly, by means of a relay lens or an optical fiber bundle. The display comprises an array of elements (pixels) imaged to infinity by a collimating lens and transmitted into the eye of the viewer by means of a reflecting or partially reflecting surface acting as a combiner for non-see-through and see-through applications, respectively. Usually, one of the most important issues to be addressed while designing an HMD is the brightness of the optical system. This issue is mostly important for see-through applications, where it is desired that the brightness of the display will be comparable to that of the external scene.
The strive for high brightness has led to several different complex optical solutions, all of which, on the one hand, are still not sufficiently bright for many practical applications, and, on the other hand, suffer major drawbacks in terms of fabrication procedures and operational conditions.