利用人眼對比敏感函數解決TFT-LCD 背光模組LED亮度不均勻問題

Abstract

LED在TFT-LCD背光模組的滲透率越來越高,許多側入式設計的背光模組(BLM),時常遇到LED亮度不均勻的問題,業界稱此為LED Hot-Spot現象.此篇論文則是利用人眼對於亮度對比的感知函數來討論一些關於解決Hot-spot現象的BLM設計概念. 理論上,人眼對於亮度對比的敏感度是定義成背光模組亮度對比的倒數.而Hot Spot 現象可以被人眼感知的必要條件是當時的人眼敏感度(Sensitivity value)低於人眼能感受的最小敏感數值(Threshold Value). 為了證明理論的可行性,在此設計藉由不同LED亮度而產生特殊亮度分布的背光模組.再以CCD量測實際的亮度分布,並分析這些亮度分佈所對應的敏感度數值與人眼最小敏感數值做比較,發現人眼感知函數確實能有效解釋Hot spot 現象的發生狀態. 為了進一步分析各種BLM的設計值對於Hot spot 現象的影響,此篇論文則採用了Trace Pro®光學模擬軟體針對導光板的入光結構以及LED亮度作完整分析.結果發現,消除Hot spot 現象的最佳入光稜鏡角度為40˚~60˚; 稜鏡的週期與Hot spot的產生無明顯關係; 最後,利用特徵指標d/p 值來描述LED的周期位置與TFT-LCD可視區的距離,並且發現d/p 值與LED各自的亮度差異會存在某一關係式來決定Hot spot發生與否的結果.

Questions of the Hot spot issue have long been of concern to both TFT-LCD makers and consumers. There is an awareness of the opportunity to eliminate this issue by utilizing the relation between parameters of LG optical design and human eye contract sensitivity function. Generally speaking, the sensitivity of human eye is the reciprocal of the object modulation, and the result of hot-spot could be detected comes from the sensitivity value is less than the threshold value which is calculated by the theorem. For the sake to verify the validity of theorem that could correctly estimate the hot-spot phenomenon, samples with and w/o hot-spot were arranged for the experiment. And these experiment results indeed prove the theorem validity directly. Further, the TracePro® optical simulation software is used for the advanced stud. There are various simulation parameters includes (a) the prism and R-cut type with various angle and pitch that the angle range of prism type is from 10 to 90 degree, the R-type structure with radius 100 um and pitch 200 um; (b) the pitch range of prism type of the LG in-coupling structure is from 150 to 350 um; (c) various LED rank arrangements of multi-LED quantities. Conclusions could be summarized briefly: (A) The prism pitch is not the key factor for hot-spot elimination.(B) The Hot spot phenomenon could be explained by Human contrast sensitivity function.(C) The prism angle is the key factor for hot-spot elimination, and the optimized angle is the range from 40˚ to 60˚.(D)R-cut and Prism type in-coupling structure could be viewed as the same for the efficiency of hot-spot elimination.(E) Table 9 is a useful reference of d/p ratio for eliminating Hot-spot.(F) The more LED intensity difference is, the more probability of Hot-spot issue will be.