This invention relates to relay lenses for head-up optical displays suitable for use in vehicles such as aircraft and, more particularly, to a relay lens having a general aspheric plate with non-rotational symmetry for control of aberrations in images found in a head-up display having a curved reflecting combiner.
Head-up displays have a combiner which is at least semitransparent, and is formed of a curved semireflecting plate, or holographic optical element by which an image can be presented superposed upon a scene external to the aircraft, such external scene being of the "real world" as viewed through a window of the aircraft. The combiner presents both the image and the external scene simultaneously to a viewer or operator of a vehicle, the combiner permitting the viewer to see straight ahead to be able to pilot a vehicle. Typically, the display equipment includes a generator of the image, such as a cathode ray tube (CTR) plus a relay lens which focuses and directs optical rays from the CRT to the combiner for presentation to the viewer.
A problem arises in that the curvature and off-axis characteristic of the combiner affects the optical characteristics of an image by introducing aberration, primarily coma, in the image presented to the viewer. Attempts have been made to compensate for the aberration by the inclusion of tilted and decentered elements in the relay lens. However, these attempts have been disadvantageous in that the resulting compensation still allowed the presence of excessive image aberration. Furthermore, the compensation was obtained at a cost of complexity, difficulty of mounting the lens elements, and difficulty of alignment.