The Design of Direct LED Backlight
In the demand of the future display, wide screen size, full digitalization and high definition are the most crucial requirements. Liquid crystal device (LCD) display undoubtedly may be taken as one of the potential candidate owing to its extra high resolution and pure digitalization and moreover, it might be the most competitive since it is of thin, light and high brightness. The present LCD displays commonly utilize the Cold Cathode Fluorescent Lamp (CCFL) as the light source, however due to the luminant characteristic of phosphor the chromatic range where the display can show is smaller than that of CRT display. And also, it will be an eco-threat whenever in the phase of manufacturing and recycling since mercury is not avoidable in fabrication of CCFL. Altering the light source to light emitting device (LED) is a good solution to enhance the color range to the level of CRT display, or moreover, better than CRT display. In this thesis, we concentrated on the design of back-light module with LEDs. To achieve a better uniformity of illumination and color mixing quality, an optical element was designed to shape emitting field of the Lambertian LED to side-emitting distribution and on the basis of the modification of emitting field of light source, the correlation between the optical performances on the exit surface of back-light module, including uniformity and chromatic mixing quality, and the design parameters, such as the locations of LEDs and the relative density corresponding to different sizes were explored. Consequently, the optimal design parameters were obtained. In the further analysis of tolerance, we studied the bearing capability of the proposed design under the assumption of specified devices malfunctioned. We also designed an optical element being able to reshape the emitting field from Lambertian distribution to side-emitting with the assistance of OSLO and built the complete model of back-light model in TracePro and furthermore, simulated it. We have obtained the uniformity about 92% within whole exit surface of back-light module in which good color mixing quality and great tolerance have been verified by numerical simulations.
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