應用於光學身份辨識之可調變微型貓眼回射器陣列

Abstract

本研究中提出兩種不同設計的可調變貓眼回射器陣列，回射器在光學辨識系 統(Optical ID system)中扮演重要角色—光學標籤(Optical ID tag)，回射器顧名思義能將入射光沿著原方向將光傳回，所以透過回射器的回射特性能增加資料傳輸的安全性及辨識準確率。貓眼回射器是由前聚焦微透鏡和後反射凹面鏡所構成的，而前後模組之間的間距將決定是否能產生回射光的重要參數，本論文先研究以高分子分散型液晶(Polymer-dispersed liquid crystal, PDLC)薄膜配置於貓眼回射器陣列，透過施加電壓於元件產生一電場使液晶分子轉向，進而改變PDLC 薄膜的狀態，此時回射光將可藉由PDLC 的狀態而自由調控，實驗分別測試單顆貓眼及陣列型元件的特性，也建置了一套簡單的光學字元辨識(Optical character recognition)來辨別光學標籤所產生的動態圖形，我們所製作的PDLC 貓眼回射器陣列能夠運作於特定角度內，確保元件所夾帶的資訊安全。 然而PDLC 貓眼有些先天材料所造成的缺點如回射效率、灰階反轉及在OFF狀態下回射效率不夠低等。接著我們利用微機電製程技術(MEMS)製作貓眼回射器的可翻轉後模組，使用磁性粒子附著在後模組的調變小載台上，並透過永久磁鐵 的移動來改變後模組的旋轉角度，此磁驅動方式可以達到12°的角度旋轉，當凹面鏡的位置改變產生一個角度的傾斜，回射器的回射條件就因此被破壞，破壞的程度視凹面鏡的角度大小而定，此變動將影響回射光的回射特性。相較於以PDLC的散射光方式為調變回射光機制，MEMS 貓眼能夠真正破壞回射光來提高元件對比度。

This study presents two different types of tunable retroreflectors which act as an optical ID tag. Retroreflector plays an important role in optical identification system because it can direct the light back along its original path to the reader directionally. The retroreflection can secure the transmitting data compares to the scattering property of the barcode. The main structure of the cat’s eye retroreflector is the front side focusing unit and the back side reflecting unit. The front-side-back-side gap is an important factor to determine whether the retroreflection can produce. We first design and fabricate cat’s eye retroreflector array with polymer dispersed liquid crystal (PDLC) film. The status of the PDLC film varies as an external electric field applied on the device. This mechanism provides an independently-addressed ability of cat’s eye retroreflector. We characterize the single cat’s eye and cat’s eye array and also build a preliminary optical character recognition (OCR) to detect dynamic images. Next, we report a micro-electro-mechanical system (MEMS) tunable back side unit of cat’s eye. The rotational concave mirror is employed in the retroreflector to achieve modulability of the retroreflected optical intensity. The magnetic force between the permanent magnet and magnetic beads on the back side unit enables magnetic actuation and accomplishes modulation. The magnetically actuated mirror achieves a large tilt angle. Compare with the scatter mechanism of PDLC cat’s eye, MEMS cat’s eye can destroy the retroreflection condition of the retroreflector. Therefore, MEMS cat’s eye improves the shortcoming of PDLC cat’s eye such as retroreflection efficiency and contrast ratio.