Optical design optimization of high contrast light guide plate for front light unit

This study addresses the optical design of LED edge-lit light guide plate (LGP) for front light unit panel of high illuminating contrast. Front light displays have excellent readability in outdoor environment with bright ambient light. A LED light guide is still required as an auxiliary lighting, referred to as a front light unit (FLU), in a dim environment. The ray tracing of the FLU light guide plate can be roughly classified into the noise rays refracted out of LGP directly and the image rays refracted downward to illuminate the reading media. Only the image rays is effective for FLU, and the noise ray will result in glare problems. This study defines the Effective Contrast Ratio (ECR) as the percentage of the image rays over the total rays within the ± 30° viewing angle. The layout of cylindrical microstructures are analyzed to determine a preferred configuration design from either concave or convex features deployed on the emitting surface or the bottom surface of LGP. Thought the ray tracing simulations of both the convex bottom and the concave bottom feature designs show excellent ECR over 90%, the concave design is preferred for better durability and assembly concerns at a cost of lower illuminating efficiency. The optimum parameters of the truncated cone microfeature with maximum ECR are identified as the base diameter of 70 (μm), the aspect ratio of 0.46, and the feature sidewall angle of 78° using a sequential optimization of Kriging Surrogate. A preliminary prototype of the LGP has been fabricated using Polydimethylsiloxane (PDMS) molding process from a SU8 master. A qualitative comparison of the photographic pictures of a sample text demonstrates the excellent transparency and uniform illumination of the LGP with the proposed feature design.