The effects of aberrations to image quality in HMDs are similar to that in other optical systems. Aberrations such as axial chromatic aberration, spherical aberration, coma, astigmatism, and field curvature introduce blur. Aberrations such as distortion, coma, and lateral chromatic aberration induce warping. Aberration control is important in the design of AR HMD optics. An example all-reflective freeform design is shown in Figure 2, where (a) displays the system design, (b) shows the test object, (c) provides the image simulation results of an initial system, and (d) demonstrates the image after optimization. In this example, we can appreciate the power of optimization in reducing aberrations and distortions.
As It is extremely challenging to match both the FOV and the resolution of human eyes, tradeoffs are often made for specific tasks among FOV, weight (volume, number of optical element), resolution, pupil size (eye box), eye clearance, and the size of the micro-displays. Some of the tradeoffs can be addressed with improved technology. A few potential solutions are listed as below: the tradeoffs between FOV and resolution can be addressed with high-resolution insets, partial binocular overlap, spatial tiling, temporal division, and diffraction-order tiling. The tradeoffs between FOV and pupil size can be addressed by duplicating the exit pupil into an array and employing eye-tracking devices.