Color holographic HUD with eyebox expansion using a folding optical path module

Head-up displays(HUDs) are used to integrate assistance information with realistic road information to improve driving safety. The current HUDs generally present the virtual image at a near fixed distance. When the drivers' vision focus on a far distance to observe the road conditions, they need to zoom back to see the virtual image, which may causes accidents. In this paper, we designed a color holographic HUD, it can flexibly produce color image at any depth to adapt to the conditions need. Another issue of current HUD is the small eyebox caused by the limited spatial bandwidth product (SBP) of spatial light modulator (SLM). To expand the eyebox, a folding optical path module consisting of trans-reflective mirrors and reflective mirrors is designed. The folding optical path module has two functions. Firstly, it duplicates wavefront Liquid crystal on silicon(LCOS) into two sub-wavefront path to expand the eyebox by two times. Secondly, it makes it possible to observe a clear virtual image of the same depth by controlling the optical path. The feasibility of this system was demonstrated through experiments.