1. Field of the Invention
This invention relates to display systems utilizing holographic lenses and more particularly to a display system incorporating a holographic lens constructed with aberrated wavefronts which is used in conjunction with other optical elements including cylindrical, tilted and decentered surfaces to provide correction of the aberrations in the holographic lens.
2. Description of the Prior Art
Previously, optical systems utilizing holographic lenses have been designed with good image quality over small fields of view and poor image quality over large fields of view. Also, optical systems utilizing holographic lenses have been required to operate at relatively small off-axis angles because of the aberrations inherent in a holographic lens utilizing large off-axis angles. Aberrations arise for the on-axis image point in a holographic lens designed for a large field of view because the wavefront used in the reconstruction process is not identical to either construction beam. This arises because there is a pupil-to-pupil relationship between the construction beam sources while an object-to-image relationship must hold upon reconstruction. The holographic lens is constructed with coherent sources located close to or at the desired entrance and exit pupil locations of the holographic lens so that the Bragg condition is met for the chief rays, thus providing a high reconstruction efficiency across a large field of view. The principal aberrations in a holographic lens constructed as above and operating at a relatively large off-axis angle are axial astigmatism, axial coma and an assymetric variation of coma and astigmatism across the field. In addition there is a considerable amount of field tilt and distortion. The assymetric variation of astigmatism arises principally because the field tilt for fans of rays traced in a vertical plane is different from the field tilt for fans of rays traced in a horizontal plane. Unless compensation is provided for the aberrations in the holographic lens operating at a relatively large off-axis angle, the display provides a very poor image quality that may be unsatisfactory for operations such as helmet-mounted displays or aircraft head-up displays.