液晶透鏡於行動裝置之成像應用研究

Abstract

行動手持裝置已成為日常生活中不可缺少的電子用品，而手持裝置裝中的攝像鏡頭，往往成為在執行攝影、影像辨識、電腦視覺以及影像溝通時重要元件。在諸多的光學元件中，液晶透鏡由於擁有電控式變焦以及體積小等特點，泯除傳統機械式移動的特性，充分展現應用於體積輕巧的行動手持裝置優勢。然而，液晶透鏡目前仍無法克服的缺點，諸如較差的影像品質、反應速度過慢以及不切實際的高驅動電壓，都大大地降低液晶透鏡的實用性。 在本論文中，我們首先針對液晶轉角不易精準控制的問題，提出多電及控制結構，藉由多電擊的高控制自由度，可更精準控制液晶層轉角分布，以提高聚焦品質。在此項研究項目中，我們也實現了針對不同焦距給定不同電壓組的最佳化控制。第二，我們提出突破性的液晶透鏡控制結構—漸變式驅動液晶透鏡，在此項目中，我們應用高電阻層做為控制電極，不但能夠充分利用施加之電場，亦可創造出漸變性的電壓分布，一方面大大將控制電壓從原本的幾十伏特甚至上百伏特降低至五伏特以內，搭配爾後本團隊發展出來針對液晶透鏡的加強驅動方式，將原本大於三十秒以上的反應速度將低至六百毫秒內，這對液晶透鏡發展領域無疑是突破性的發展，再者，由於電阻及液晶層猶如一層RC電路，所以頻率上的可控制性，亦延續了先前多電極控制這種高自由度的控制概念，來提升聚焦品質。在論文最後，我們展示了結合所有成果於手機鏡頭的成像應用，實現了快速、低電壓驅動的液晶透鏡之近距離攝影(近拍)成果。

Mobile devices become necessary products for daily life’s use in recent years. When perform functions of photography, phantom identification, computer vision, and image communications, lens-heads play an important role to deal with the basic function of imaging. Liquid crystal (LC) lens has unique properties such as electrically tunable focal length. Since there are no moving mechanical parts, LC lens is smaller and lighter than conventional tunable glass lenses. In mobile devices, LC lens can directly combine with mobile lens-module in front of the lens-head, and easily perform auto-focusing (AF) function by driving appropriate voltage to focus objects at different distances. However, inferior optical quality, slow focusing time, and high driving voltage are current major issues of LC lenses’ study. These drawbacks make LC lenses unpractical and unfeasible in real usage for mobile devices. In this thesis, we first started from Multi-electrode Driven LC lens (MeDLC lens) to precisely control profiles of LC lens for different focal length. By the multi-electrode, we demonstrate highly controlled freedom for modifying the index distributions for each focal length to obtain similar and superior focusing profiles. In the second part, Gradient-driven LC lens (GDLC lens) was proposed to intrinsically improve the driving efficiency. We utilized a resistance layer connected with electrodes as the control layer. The benefit of this structure is that the applied electric field can be efficiently employed and a gradient distribution in voltage can be produced. By GD-LC lens, the driving voltage typically higher than tens root-mean-square voltages was reduced down to less than 5Vrms for imaging 6cm closed objects. In the third part, Over-drive (OD) method for LC lenses was proposed to investigate focusing behavior. A switching operation in this method according to the focusing profile was used to reduce the focusing time. Performing with GDLC lens, we yielded 600msec focusing time by a LC lens with 60um cell gap and 2mm aperture size for capturing the 6cm objects. By combining these results, we not only dramatically improved the performance for current LC lenses but also to break through the issues of the current studies. Our outcome, furthermore, make the use of LC lenses for commercial mobile devices more feasible and practical.