A high-reflectivity double-layer liquid crystal polymer film for eliminating automotive head-up display ghost image

Windshields in automotive augmented reality head-up displays (AR-HUDs) typically employ wedge-shaped films to eliminate ghosting caused by dual reflections from their front and rear surfaces. However, constrained by the requirement for high environmental light transmittance, the reflectivity of such wedge films for the light from picture generation unit (PGU) is generally below 20 %, resulting in significant energy waste. This paper proposes a double-layer liquid crystal (LC) polymer film structure composed of a cholesteric LC layer and a LC waveplate layer. The cholesteric LC layer efficiently reflects circularly polarized light from the PGU into the eyebox, while the waveplate modifies the polarization state of light reflected from the windshield’s rear surface, preventing it from re-entering the eyebox through the cholesteric LC layer. For green light displays, the LC layer structure was optimized using rigorous coupled-wave analysis (RCWA) and experimentally fabricated. Results demonstrate that the proposed structure achieves 80 % reflectivity for PGU light, over 70 % transmittance for environmental light, and a primary-to-secondary image intensity contrast exceeding 100:1. This double-layer LC polymer film meets the ghosting elimination requirements for automotive AR-HUD windshields while improving HUD energy efficiency by more than threefold compared to the general wedge-shaped film structure.