Design and fabrication of an off-axis four-mirror system for head-up displays

Head-up displays (HUDs), as one kind of the augmented reality (AR) applications, enable users to enhance the situational awareness. In this paper, we propose a design of a HUD employing the freeform off-axis four-mirror construction, in which the eyebox and eye relief as large as 120mm×80mm and 400 mm, respectively, are derived. In the part of system design, the biocular parallax is evaluated and corrected. An optimization strategy in forward ray-tracing mode is presented to directly govern the biocular parallax, and the convergence, divergence, and dipvergence are finally constrained within 3.5 mrad, 1.5 mrad, and 2.0 mrad, respectively. An overall design procedure of a HUD, including initial configuration, structure constraints, optimization method, and parallax correction, is illustrated in detail. After analyzing the tolerance, a proof-of-concept prototype with the field of 24∘×15∘ is developed to demonstrate the image quality.