Pupil expansion prism and beam splitter array waveguide for high luminance and high resolution head-mounted display

Head-mounted displays (HMDs) for augmented reality (AR) require optical systems that deliver high luminance, high optical see-through performance, and high image resolution. Conventional two-directional pupil expansion using beam splitter array (BSA) waveguides often suffers from low light coupling efficiency and degraded image quality due to narrow coupling mirrors and wavefront disturbances caused by exposed mirror edges along the total internal reflection surface. We propose a novel HMD optical system that combines a pupil expansion (PE) prism with a one-directional BSA waveguide. The PE prism features an enlarged coupling surface, efficiently capturing light from the light engine. Importantly, the PE prism is designed to avoid exposed mirror edges in the optical path, thereby eliminating wavefront disturbances and enabling high-resolution virtual image projection. Within the PE prism, multiple ray paths are generated through complex reflections and transmissions via overlapping multi-mirrors. This multi-path propagation not only expands the eye-box orthogonally to the BSA waveguide, but also ensures uniform output luminance. A prototype HMD implementing the proposed method achieves high luminance (over 5000 cd/m²), high image resolution (MTF > 58% at 10 cycles/degree), high optical see-through transparency (over 94%), and luminance uniformity (over 53%). These results demonstrate significant improvements over conventional BSA waveguide approaches.