Light Guide Film With Internal Air‐Cavities for Large‐Sized Front Light of Reflective Displays

ABSTRACT The advent of reflective displays has given rise to the demand of a front light unit (FLU) due to readability in a dim environment. Herein, we demonstrate air‐cavity optics for the efficiently light‐emissive light guide plate (LGP), giving the largest size for the FLU reported so far. Our LGP was composed of a transparent plate integrated with the functional film with internal air‐cavities. A large difference in refractive index at the air‐cavity interface lowers the critical angle, making it more efficient for light to be redirected to the reflective display by total internal reflection (TIR), leading to high efficiency of light extraction. Simulation suggests that light extraction efficiency of embedded air‐cavity optics is much higher than that of conventional collimation optics, such as the LGPs reported so far. The effective extraction could also be attributed to a decrease in the size and/or distribution density of cavities, leading to high transparency. The optical design of the cavity shape could give a high contrast ratio in the field of view. A preliminary prototype of the LGP with an E‐paper display showed excellent readability in both environments with and without ambient light. A high uniformity of brightness of the large‐sized LGP was found.