Application of Taguchi Method in Light-Emitting Diode Backlight Design for Wide Color Gamut Displays

This paper utilizes the Taguchi design method to optimize the design parameters of a light-emitting diode (LED) backlight unit for wide color gamut liquid crystal displays (LCDs). In optimizing the design, the parametric analyses consider two particular regions of the backlight unit, namely, the color-mixing zone and the extractor zone. The Taguchi experiments are configured in L₉ (3⁴ ) orthogonal arrays and are designed to evaluate the effects of the design parameters on the color-difference, optical efficiency and luminance of the unit. The analysis of variance (ANOVA) results reveal that the optical efficiency and color-difference properties are determined primarily by the reflector design and the length of the color mixing zone, respectively, while the luminance to the LCD panel is affected principally by the taper angle of the optical microstructures in the extractor zone. The optimal design parameters of the color-mixing zone and extractor zone are estimated from the Taguchi S/N ratio data and the ANOVA results, and are verified via ray-tracing simulations. For an input flux of 1501 lm, the optimal design shows 0.01 of the color differences in CIE 1976 color space, 85% of the optical efficiency, and 10675 nits of luminance. Thus, the optimized backlight unit provides an ideal solution for the illumination of large-scale LCD display devices.

I. Introduction

Recent years have witnessed a strong shift in consumer demand toward slim, large-scale display units for such applications as TV viewing, gaming, computing, and so forth. In response to this requirement, manufacturers have invested huge amounts of R&D resources in developing high-performance liquid crystal displays (LCDs)[1]–[3]. Since liquid crystals are a non-light-emitting material, such devices utilize a backlight unit positioned beneath the LCD panel to illuminate the image. The characteristics of the backlight unit, e.g., the luminance uniformity and the brightness, have a fundamental effect upon the image quality, and thus manufacturers have explored the relative merits of many different backlight configurations. In general, however, modern backlight units can be classified as either “bottom lit” or “side lit”, depending on the position of the light sources relative to the display screen. In bottom-lit units, the light sources are configured directly beneath the LCD panel, and thus a sufficient distance must be preserved between the emitting surface and the light sources to ensure a uniform lighting effect. Consequently, bottom-lit backlight units are more commonly used in large-scale display devices with thicknesses of around. As implied by their name, the light sources in edge-lit backlight units are positioned to one side of the LCD panel, and the light is transmitted through a light guide containing a pattern of light-scattering dots carefully designed such that the light is distributed uniformly over the undersurface of the LCD panel. In the design process, the thickness of backlight units could be a major concern. In general, the edge-lit backlight units are implemented with light guide plates. They tend to have a much smaller thickness than the bottom-lit backlight units.