One important application for compact optical elements is in head-mounted displays (HMDs), wherein an optical module serves both as an optical collimating system and a combiner, wherein a two-dimensional image source is imaged to infinity and reflected into the eye of an observer. The display source can be 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), a micro-LED display, a scanning source or similar devices, or indirectly, by means of a relay lens or an optical fiber bundle. The display source comprises an array of elements (pixels) imaged to infinity by a collimating lens and is 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. Typically, a conventional, free-space optical module is used for these purposes. As the desired field-of-view (FOV) of the system increases, however, such a conventional optical module becomes larger, heavier and bulkier, and therefore, even for a moderate performance device, such as a system, is impractical. This is a major drawback for all kinds of displays and especially in head-mounted applications, wherein the system should necessarily be as light and as compact as possible. In addition, the flatness of the display source, used in conjunction with a collimating system having some field curvature aberration, may contribute to degradations in the image viewed by the observer.
The strive for compactness has led to several different complex optical solutions, all of which on the one hand, are still not sufficiently compact for most practical applications, and on the other hand, suffer major drawbacks in terms of manufacturability. Furthermore, the eye-motion-box (EMB) of the optical viewing angles resulting from these designs is usually very small, typically less than 8 mm Hence, the performance of the optical system is very sensitive, even for small movements of the optical system relative to the eye of the viewer, and does not allow sufficient pupil motion for comfortable reading of text from such displays.