Studies on Diffractive Mobile Display Backlights

Abstract

The digital convergence has brought about a new class of mobile devices that allows a compelling, visually rich multimedia experience to the user in a handheld product, such as a mobile telephone. The display in these “smart phones”, “feature phones”, or “multimedia computers”, is a strategically important component that defines the user experience for a great part. While the visual user experience provided by the display can be very good, the power dissipation in the display is the limiting factor in the length of the user interaction with the mobile multimedia content. As the proportion of time of interacting with the visually rich content increases over that of simple voice-driven communication, such commonly used techniques for reducing the overall energy consumption of the device as power-saving modes or time outs cannot be used during a multimedia session without compromising the quality of service to the user.The conventional mobile liquid-crystal display structure consists of a backlight unit with the associated light sources, the display panel itself, and various optical films that control the state of polarization and viewing characteristics of the display. The backlight unit itself has evolved in the last fifteen years to become a very efficient component to provide uniform illumination to the electro-optic spatial light modulator comprised by the liquid-crystal pixel array. In the conventional structure, the color filter array embedded in the liquid-crystal display panel limits the light throughput in the display system. The backlight unit itself in the conventional configuration is difficult to improve any further, and a system redesign is required to make the display system perform more efficiently than what is currently possible.One possible way to redesign a display system more effectively is to direct the appropriate primary bands of light through the respective subpixels in the display panel, instead of having these filter white light into the primary colors. This can be done by diffractive means, i. e. by placing a grating structure on the light guide plate of the backlight unit. Significant improvement in energy efficiency of a mobile display system can be achieved by this approach, and at the same time, cost savings can be expected due to the elimination of many beam-shaping films in the backlight unit, as compared to the conventional mobile liquid-crystal display configuration. Further cost savings can be achieved by removing the color filter array, provided that the color purity of outcoupled light is good.This thesis presents a new pixelated color-separating grating array concept that diffracts the light from red, green, and blue light-emitting diodes in the backlight unit through a subpixel array in a prospective mobile display module. A literature review was conducted and key research studies in the area of diffractive mobile display backlights were reviewed. Experimental studies to verify various constituent elements of this concept are presented, and conclusions are drawn on how the display industry could benefit from this new concept, and what should be taken into account when adopting a new display manufacturing paradigm based on diffractive backlights.