Augmented reality head-up display (AR-HUD) systems can project driving information in the form of images or text into a driver’s field of view, providing real-time and intuitive driving information and enhancing driving safety. The focal point of vision changes with vehicle speed according to the characteristics of the human eye. However, traditional AR-HUD systems can only project images at a fixed distance, which not only fails to achieve the ideal AR effect, but also leads to visual fatigue. Existing variable imaging distance AR-HUD systems use off-axis reflection structures to adjust the position of the first mirror or the position and size of the picture generation unit (PGU) to change the projection distance. However, they can not simultaneously satisfy the different imaging distance requirements for basic and interactive information. Therefore, it is necessary to propose a variable imaging distance AR-HUD system that can display basic and interactive information separately.

The initial structure of an off-axis three-mirror system is generally based on the design of a coaxial three-mirror system, but the final structure usually deviates significantly. This intermediate process also requires considerable time. This study constructed an aberration evaluation function for off-axis reflective systems based on vector aberration theory and used a global optimization algorithm to find the optimal solution for the evaluation function, directly obtaining the initial structure of the off-axis three-mirror system. Two PGUs were used to construct near- and far-field optical paths to design a dual-focal-plane AR-HUD system. Referring to the zoom principle of coaxial systems, to ensure that the positions of the PGUs remain unchanged during changes in the imaging distance, it is necessary to adjust the spacing of the two mirrors. However, in this study, the near- and far-field optical paths shared the same freeform mirror. To ensure that the changes in the imaging distance of the far-field optical path do not affect the near-field optical path, the shared freeform mirror must remain stationary. If an off-axis three-mirror structure is used in the far-field optical path, it is impossible to ensure that the positions of the PGUs remain unchanged without affecting the near-field optical path. Therefore, a flat mirror was added to the far-field optical path, which did not change the position of the freeform mirror. By changing the spacing between the two flat mirrors, a change in the imaging distance was achieved. Using Zemax’s “Macro” to obtain the relevant parameters during the change in imaging distance, curves of the movement distance of the mirrors and change in image size on the PGU during this process were plotted. The image quality was evaluated and a tolerance analysis was conducted.

The imaging quality of the dual-focal-plane AR-HUD system was analyzed at projection distances of 2.5 and 10 m, with a pupil size of 6 mm. At both distances, the size of the spot at the center and worst imaging position at the edge of the RMS radius of the spot diagrams are smaller than those of the Airy spot (Figs. 6 and 9). The MTF at 6 lp/mm is greater than 0.3 at both distances (Figs. 7 and 10), and the grid distortions are all less than 5% (Figs. 8 and 11). By adjusting the projection distance, the variation in the distance between the mirrors during the imaging-distance change process is plotted (Fig. 13), as well as the variation curve of the image size on the PGU (Fig. 15). During the change in imaging distance from 10 to 20 m, the movement distance of the two flat mirrors does not exhibit any abrupt changes and can be continuously adjusted, ensuring that the PGU position remains unchanged. Imaging distances of 15 and 20 m validate the imaging quality during the distance-change process. A field of view box of 120 mm×60 mm is obtained, with field angles of 5°×1° and 10°×5° and virtual image distances of 2.5 and 10-20 m for the dual optical path heads-up display system. The imaging quality meets the design requirements, and the tolerance analysis demonstrates the stability and manufacturability of the system.

The dual-path AR-HUD with variable projection distance not only reduces driver visual fatigue but also meets the different imaging distance requirements for basic and interactive information. Near-field path imaging is located on the hood, 2.5 m from the driver’s eyes, to prevent interference with the vehicle in the front, whereas far-field path imaging is positioned at 10-20 m, enabling better integration with the real scene. To ensure that changes in the far-field path imaging distance do not affect near-field path imaging, and to maintain the PGU position unchanged, an off-axis four-reflective structure is designed for far-field path imaging. This system can achieve multilevel, different-depth information displays according to the actual application, significantly enhancing the driver’s visual experience and information interaction effects.