Design of an off-axis four-mirror system for automotive head-up display

It has been a challenge to design an automotive head-up display (HUD) that has large eyebox while maintain a compact structure. In this paper, we present a combiner-type HUD design using freeform mirrors. The HUD is an off-axis fourmirror system consisting of 4 mirrors, which are enclosed in a box to achieve a compact form. The tilt angle of the combiner is adjustable, which can be adapted to different heights of exit pupil. We initially propose an off-axis, fourmirror system which only uses spherical surfaces, then we optimize it by replacing with freeform surfaces, which enables us to balance the residual asymmetrical aberrations. Finally, we obtain an HUD system with good imaging performance. The virtual image is projected at 5 meters away from observer, and the eye box is 106 × 66 mm2 .