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High-efficiency trichromatic LED backlight for mobile LCDs

Trichromatic LED backlights render higher color gamut and panel transmittance to the liquid crystal displays (LCDs) than yellow phosphor-converted white LED backlights can possibly do at their best. In realization, however, several technical challenges arise, such as color mixing, minimizing the total number of chips, and maintaining the color balance. We designed and demonstrated a backlight unit for 2.2 inch TFT LCD using two RGB 3-chip LEDs to assess the feasibility and the technical hurdles to overcome. The average brightness of the backlight is 2509cd/m2 at the input power of 200mW. The power efficiency is lower than but comparable to commercially available white LED backlights. The color gamut of the LC panel is increased from 53% to 78% when its conventional white LED backlight is replaced by the trichromatic LED backlight. Panel transmittance is expected to be enhanced as well by about 8%. The ambient temperature change was found to be the most significant cause of the color shift of the trichromatic LED backlight. The forward bias voltage can be used in the feedback, since it changes linearly with temperature.