Reaserch On Key Techniques Of Large-size LED Backlight Module With High Performance

At present, flat panel display market is dominated by liquid crystal display (LCD). In an LCD, backlight module is an important component. Since2008, light emitting diodes (LED) backlight has rapidly taken the place of conventional cold cathode fluorescent lamp (CCFL) backlight due to its advantages including high luminous efficiency, long work life, high color rendering capacity, compact volume, nonuse of mercury etc., and become the mianstream technical solution. LED backlight module can be classified into edge and direct configurations according to the way light sources are placed. With the consideration of characteristics of these two types of configurations, full-array direct LED backlight was adopted for large-size LCD in this paper. Several key techniques of large-size full-array direct LED backlight module with high performance were investigated in this paper.Firstly, based on former research, in which uniform lighting condition for generalized Lambertian LED array lighting system was studied on the basis of Sparrow’s Criterion, uniform lighting condition for LED array lighting system made up by LEDs with arbitrary spatial radiation distribution was investigated. If certain types of LED with wide viewing angle were used, uniforimity of the LED array lighting system was demonstrated to vary non-monotonically with system thickness or LED pitch. By taking consideration of the abnormal uniformity condition demonstrated in this study, a general design method of LED array lighting system with uniform lighting was proposed based on Sparrow’s Criterion.Then contrast sensitivity function in human vision was adopted to investigate the uniformity condition of LED array lighting system, and uniform lighting condition for Lambertian and batwing type LED array lighting system were derived again. Optimum structural parameters achieved through contrast sensitivity function were found to be more accurate and practical than those achieved through Sparrow’s Criterion. Normal and abnormal uniformity condition were analyzed systematically based on contrast sensitivity function. Finally, design method of LED array lighting system with uniform lighting was proposed based on human vision.Significant light loss is produced by brightness enhancement films (BEF) while they increase the on-axis luminance by reducing the viewing angle in backlight module, which prevents on-axis luminance from being further increased. To solve this problem, a19inch direct LED backlight module was actually fabricated. Transmittances of the light propagating onto the BEF(s) with different spatial directions were investigated angle by angle via simulations and a petal-like spatial transmittance characteristic of BEF(s) was achieved. Then a type of freeform lens was designed to match up the angle range of the light emitting from LED and spatial transmittance characteristic of BEF(s). Resultantly, on-axis luminance can be significantly enhanced with approximately unchanging viewing angle. The experimental results showed that on-axis luminance was enhanced by22.8%while viewing angle was only reduced slightly even though a degree of optical performance degradation was caused by the diffuse film.Local dimming, which is another key technique of full-array direct LED backlight with high performance, can enhance the contrast ratio of output image and reduce the power consumption. Functions and corresponding computational complexity of three main process steps, as dimmed backlight determination, actual backlight distribution simulation and gray level compensation, in local dimming were detailedly investigated and effect of different diming algorithms and dimming block mumbers on performances of distortion and power consumption reduction were analyzed. Resultantly, increasing dimming block number was demonstrated to be the only way to essentially improve distortion and power consumption reduction performances simultaneously. In this paper, a technical solution adopting a type of gridded reflector was proposed to completely move away the process step of actual backlight distribution simulation, reduce the total computational complexity by several times and make total computational complexity considerably uncorrelated with dimming block number. With the aid of gridded reflector, more dimming blocks and an auxiliary dimming algorithm entitiled "critical exceeding method" proposed in this paper were utilized, which resulted in improved performances of distortion rate and power consumption reduction.