Eliminating sunlight backflow in AR-HUDs using a nonreciprocal optical isolator under the constraint of étendue

During daylight operation, sunlight propagates in reverse along the imaging path of a head-up display (HUD) through the windshield. Sunlight is then concentrated on the display panel, thereby degrading reliability. The sunlight backflow should be eliminated while not affecting the display function, especially in the visible spectrum. This study proposes a new, to the best of our knowledge, architecture incorporating a nonreciprocal optical isolator, i.e., a Faraday rotator, to eliminate sunlight backflow. Further, given the étendue constraint imposed by the small aperture of practical Faraday rotators, a time-multiplexing scheme doubles the effective étendue to create two distinct virtual image distances while fully utilizing the Faraday rotator's aperture-limited étendue. An LCD-based prototype demonstrated a sunlight-isolation rate of 3.23% in terms of irradiance (unit: W/m²) across the visible spectrum. Meanwhile, the Faraday rotator exhibited uniform, high transmittance across the visible band for forward light from the LCD, yielding a satisfactory forward luminous (unit: cd/m²) efficiency of 86.1% with negligible color cast.