應用時序驅動之發光二極體背光源於提升液晶顯示器之影像品質研究

Abstract

隨著液晶顯示器的普及，顯示器規格也朝著高畫質與低功耗的方向發展。發光二極體(Light emitting diode, LED) 由於具有高色彩飽和度、局部控制的自由度與快速響應的特性等優勢，遂成為取代傳統冷陰極管(Cold cathode fluorescence light, CCFL)於液晶顯示器背光源的重要技術。採用發光二極體為背光源其優點為薄型化、重量輕及低耗電量，目前已被廣泛使用在攜帶式顯示器與高階監視器的產品上。在進一步降低耗電量的LED背光顯示系統之發展上，如不需使用彩色濾光片的色場序列法(field sequential color)，由視覺暫留原理可感知彩色影像，能大幅提昇提升系統效率與色域範圍。然而，因眼睛視點與所視物體的相對運動導致各色場於視網膜上非完美疊合時，造成的色彩分離(color break-up)現象需要被克服和抑制。 為了解決色分離問題，在中小尺寸的液晶顯示器，本論文提出了藉由規律變換色場的方式，使在視網膜上連續影像積分無色分離的結果，能有效的解決人眼追跡運動物體所造成的動態色分離現象。在液晶反應時間的限制內，我們也提出了分配影像強度到白色色場來解決靜態色分離現象，稱為RGBW方法。然而若是影像內容僅由兩個三原色為主要構成，其色彩重分配的效果會受影響。於是我們進而提出，依據影像內容調整第四個色場為青綠或黃色的RGBC/RGBY的交換色場序列方式，用以改善色彩重分配的程度。基於此方式，並更進一步發展決定最佳色場之演算法。藉由動態調整背光源顏色與搭配液晶同步處理，能集中影像強度於單一色場降低色分離現象，使其影像品質與顯示功能獲得提升。本論文並發展以視覺感受(visual perception)基礎上評估影像品質的條件和迴授控制(feedback control)的顯示電子電路，來實現動態即時的影像運算。由觀賞者感受與色分離指標，皆證明此方法是可以有效改善色分離現象，以及量產實現化的設計與技術。 針對大尺寸顯示器，應用局部控制的LED背光系統，針對局部影像調整的彩色背光對各區域來作色分離抑制的優化，具有提升顯示器動態範圍與影像的對比度，同時達到進一步降低功耗的優點。然而，此局部控制的背光源存在著區域範圍之光分佈最佳化議題，而本論文由分析人眼視覺對比敏感度，找出不可察覺邊界的背光分佈條件，以視角定義區塊的大小得到適當區塊數目設計通式。並且對邊界察覺度與對比增強度的交換考慮，推導出最佳的高斯分佈函數。符合此推薦值的光分佈設計，能有效提升對比，保持高均勻性，同時避免邊界察覺的現象以確保影像品質。 論文中展現了時序法驅動LED背光之光學和電路設計於液晶顯示器產業的重大應用潛力，提供技術上進一步設計與解決方案來開發高畫質低功耗的顯像技術。而結合光學模擬與電路實現的動態優化影像品質技術，也將在未來顯示器產業中具有益形的重要地位。

The popularity of liquid crystal displays (LCD) has encouraged designers to develop the LCD for high image quality and low power consumption. Light emitting diodes (LED), as LCD backlights has advantage in high color saturation, locally controlled ability, and fast on/off response, thus becomes major light sources to replace typical cold cathode fluorescence light (CCFL). Currently, LED backlighting is widely utilized in the mobile and high-level monitors because of its features such as thin format, light weight, and low power consumption. In the further development of LED backlighting, the field sequential color (FSC) mechanism, requiring no color filters to form color images, can greatly improve the light efficiency and the color gamut in perception. However, the color breakup (CBU) is essential to be suppressed while the color fields are not overlapped precisely in retina by the relative motion between viewing points and objects. To resolve the CBU issue under the restriction of LC response time, on small and medium-sized LCDs we proposed a color field arrangement method to blur the integration of color fields in retina. The dynamic CBU induced by eye tracking objects can be eliminated. For the static CBU suppression, the RGBW method was described to redistribute the image intensity to a while field and discolor the primaries. The effectiveness of color redistribution depends on image content; therefore, we proposed an exchange of color sequence RGBC/Y to modify the CBU artifact. Based on this dynamic control method, the optimized color field instead of yellow and cyan is determined by intelligently adapting the incoming video content. To concentrate image intensity into a single field, further reductions in CBU visibility can be achieved. We presented the index of CBU evaluation according to visual perception and the feedback control algorithm of color field optimization in real time application. After the implementation and observation, these proposed methods were confirmed as practical ways of CBU reductions in FSC applications. For large-sized LCDs, the locally controlled LED backlight can be applied to adapt corresponding parts of image for local reduction of CBU visibility. Local adaptive backlighting also benefits the dynamic range, contrast, and power consumption. However, there is a design trade off on the light profile of each segment. Based on human contrast sensitivity function, we derived a threshold of insensible boundary and a general condition of segment number in angular unit. We evaluated the boundary visibility and contrast enhancement to identify the Gaussian light profile as a recommended one for contrast improvement, uniformity maintenance, and boundary imperceptions. In this thesis, we presented the optical and circuit design of sequential driving LCD and provided technical solutions for upgrading the image quality and power consumption. Combining the optical model and circuit implementation for adaptive image optimization is anticipated to play a substantial role of the incoming display industry.