高效率投影光學系統之設計及影像品質分析

Abstract

摘要 隨著電子資訊的發展，人們對於高資訊含量顯示系統（high information content displays）的需求也隨著增加。此顯示系統必須可以顯示高解析度的大型畫面，及擁有高畫質特性，以充分滿足人類視覺要求。拜微型光閥（miniature light valves）技術的快速成長之賜，投影顯示器的發展一日千里。投影顯示器結合先進投影光學及高畫素密度微型光閥，具有大畫面、高亮度、高對比度、全彩等的多重優點，預期將廣泛應用於高畫質電視、會議簡報、工作站螢幕、數位電影等各種用途。 本論文首先建立投影顯示器的理論模型分析基礎，透過光學系統的模型計算，探討光學結構參數與光源對於影像品質的影響，進而達到系統最佳化之目的。此外，我們推導在非等向性介質（optically anisotropic media）中的光線追跡公式，以探討於雙軸晶體（biaxial media）邊界上的多重穿透及反射現象。此理論可增加對光學補償膜及偏光元件的瞭解，進而拓展其相關應用。 投影光學系統是整個投影顯示器的核心，對於系統的顯示能力影響很大。本論文的第二部分將從光學架構作整體考量，發展高亮度的投影光學系統。在液晶投影顯示器中，由於液晶面板（liquid crystal displays）利用光偏振來控制光線的穿透或反射，僅用到1/2的光能量。為提昇液晶投影顯示器的光效率，本論文提出高效能的偏振轉換照明系統，利用特殊離軸（off-axial）的設計，可將某一偏振方向的光線轉換成另一偏振方向，使光效率提昇將近兩倍。同時，此系統具備光型轉換的功能，可產生均勻矩形分佈的照明光束於液晶面板，以提升更多的光能利用效率。 利用微機電（micro-electromechanical-system）技術製作於單矽晶板上的反射式光閥，由於其具有反應速度快、高通光率等優點，一直受到各界的重視。我們採用數位微型鏡面裝置（digital micromirror device）為光閥，設計具有高亮度及高對比度雙重優點之光學投影系統。在照明光學系統中，我們發展光分佈轉換技術，將圓形光分佈轉換成矩形分佈，達到增加光均勻度及提昇光效率之目的。由於入射至微型鏡面的光束與反射出來的光束十分的接近，為避免這兩束光線互相干擾，在成像系統中，我們特別針對微型鏡面的特殊角度，設計一種新型的光分離器（light separator）將反射光束偏折，不僅大幅改善投影系統的對比度，並同時獲得更佳之光效率。

Abstract The demand for the display systems with high information content is immediate in the information-booming era. Such displays shall exhibit the features of high resolution, large screen, and high image quality to fully convey the information into the human visual system. A promising display technology that has recently boomed is the projection display, which fuses the miniature light valves and the innovative projection optics to produce the images. The projection displays capable of displaying large, bright, seamless and vivid color images are expected for various applications, including high-definite televisions, conference presentation, workstation monitors, and digital electronic cinema. Establishment of the simulation models and optimization techniques for projection display applications are the first objective devoted in this thesis. The influences of individual optical components, light sources, and their combination upon the image quality were investigated to assist the development of optimal optical systems. Furthermore, a complete method was proposed for dealing with light propagation in optically anisotropic media. Generalized formulas were derived to determine the multiple reflection and refraction at an interface between arbitrary-oriented biaxial media. This simulation scheme is applicable for the analysis of birefringence films and optical devices incorporating polarization effects. The projection optical system is the heart of a projection display, which strongly impacts upon the system performance. The second objective of this research is to explore the optical systems with high optical throughput. The liquid crystal displays (LCDs) utilizes only one polarization for light modulation, leading to >50% light loss. To increase the light utilization, a novel polarized illumination system was proposed for LCD projectors. This optical system employs the unique off-axial design to convert random polarized light into linearly polarized light. Through this polarization conversion scheme, the optimized system can be nearly double the light throughput. Simultaneously, the optical system converts a circular-shaped beam into a rectangular one, and provides a uniform irradiant distribution on the light valve. The miniature light valves fabricated by the micro-electro-mechanical-systems (MEMS) technology are immerging for projection applications because of their inherent natures of high aperture ratio and fast switching speed. A novel optical system for the projection displays with the digital micromirror devices (DMDs) was presented to enhance light efficiency and image contrast. A high-efficiency rod integrator was adopted in the illumination optics to perform the shape conversion, yielding a uniform rectangular-shaped illumination beam on the DMD surface. In addition, a unique light separator was proposed in the imaging optics to adequately separate the incident and reflected beams. Because the light paths overlapping are eliminated, this optical system results in both high brightness and high contrast.