Frontlighting Technologies for Reflective Displays: e-PAPERs, MEMS, MEOMS, LCDs K. KÄLÄNTÄR Global Optical Solutions, 2-50-9, Sanda-Cho, Hachi-Oji-Shi, Tokyo 193-0832, Japan Keywords: Frontlight unit (FLU), stick light guide (SLG), edge-lit FLU, nano/micro-deflector, structured LGP/LGF. ABSTRACT The reflective displays, such as electronic papers (e-Papers), micro electro-mechanical system (MEMS) or micro electro-optical-mechanical system (MEOMS) based displays and reflective liquid-crystal displays (LCDs) have great advantages over the transmissive displays in terms of low power consumption, color gamut and high contrast under bright ambient illumination. However the optical characteristics are deteriorated under a dim illumination environment. Frontlighting a reflective display using an edge-lit light-guide plate is a unique method to modulate the displayed images and recover the optical characteristics under the dim environment. Since a frontlight unit (FLU) is an indispensable unit for a reflective display, the optical matching between the FLU and the display, the efficiency and the image quality on the unit are of great interest. In this paper the concepts of frontlighting and the recent developments of FLU are reported. 1. INTRODUCTION The liquid-crystal display (LCD) now plays the leading role in various flat-panel electronic display devices, because of its excellent features such as a low profile, lightweight, large area, low operating-voltage, low power-consumption, and a full color capabilities, and high resolution. However, in spite of high display quality of a transmissive LCD under bright environment, the image quality is degraded under dim illumination environment. In addition the power keen backlight unit (BLU) consumes most of the energy in the display. A necessity for reducing the power consumption of the transmissive or transflective LCD module led to development of not only the reflective LCD but also the development of reflective displays such as electrophoretic displays (e-papers or EPDs), micro electro-mechanical systems (MEMS) and micro-electro-optical-mechanical systems (MEOMS) that uses the front-ambient light instead of back light [1-7]. However, while the most reflective display technologies have shown significant power consumption advantages over existing displays the implementation of the reflective displays have been delayed because of their optical performances. The required characteristics are wide color gamut and high image contrast under all ambient illuminations. The viewing angle characterization of reflective displays is highly required than for transmissive displays, because the viewing angle is supposed to be omnidirectional unlike the transmissive displays that use almost a directional light from a BLU. Reflective displays have higher contrast under bright environment, however they require illumination systems under dim environment. There are several technologies applicable to illuminate reflective displays in a dim environment. The simplest method is to mount an external light source over the reflective display and illuminate the display from the front side. However, the light source should be over the display within tens of centimeter. The usage of external light source makes a complicated system around the display. Another method is to change the structure of reflective display, i.e. changing the pixel structure and making partially transmissive pixel. In dim regions the display functions as a transmissive while using the BLU and as a reflective display in the bright regions. However in this case there are two disadvantages compared to a conventional reflective type. Dividing a pixel reduces the reflectance of display. When a display (e.g. LCD) functions as a transmissive, the transmitted light passes through the color filter only once before arriving to the viewers. On the other hand, when the display functions as a reflective type the light passes color filter twice. There is a significant chromaticity difference between transmission and reflection images. However the optical pass difference can be solved by using the external reflector in the reflective display structure. A reflective display may have an internal or external reflector due to its optical function. The general solution to the issue of illuminating a reflective display in dim regions is frontlighting by using an edge-lit flat frontlight unit (FLU) as shown in Fig.1. An edge-lighting reflective display with a light-guide plate (LGP) and light-emitting diodes (LEDs) is technologically unique method [7-15]. An edge-lit FLU employs a single or few LEDs whether directly coupled to a LGP or coupled via a stick light guide (SLG). The LGP can be a prismatic optically transparent solid plate or a structured flexible light-guide film (LGF). In this paper the optical principle, the trends of frontlighting technologies, their issues are studied and discussed and the futuristic images of FLU is portrayed. 2. OPTICAL CONCEPT OF EDGE-LIT FLU A plane frontlight unit employs pseudo-white LEDs as light sources, a nano/micro-structured light bar or SLG, a reflector for the SLG, a partially light diffusing film and a nano/micro-structured LGP/LGF as shown the breakdown EP2 - 3 Invited ISSN-L 1883-2490/21/1166 © 2014 ITE and SID IDW ’14 1166