利用調制或擷取影像的空間頻率來提升影像品質已成為現代重要的研究及應用課題。任何的影像或圖片在傅氏光學影像處理系統中可看成一系列空間頻率的組成，空間濾波就是在空間頻率中，選擇某些空間頻率，或濾除某些空間頻率，舉例來說，雜訊通常是傅氏光學影像處理中最高頻的成份，如果我們可以將這最高頻率的成份濾除，便可對雜訊做出某種程度的抑制，進而增進影像品質。
本論文主要探討利用各種液晶元件來做為空間濾波器，首先為利用聚合物-液晶混合薄膜 (Polymer Dispersed Liquid Crystal，簡稱PDLC)做出可電控的空間濾波元件，此方法主要利用PDLC在不同聚合光強下，會導致相分離出不同大小液晶顆粒的薄膜，一般光強越高，液晶顆粒越小。因此對應不同強度的空間頻率即時的產生對應的不同液晶顆粒聚合物-液晶薄膜，由於較小液晶顆粒的薄膜具有較高的驅動電壓，我們即可外加不同的電壓來控制其空間頻率通過的成份。
其次為利用參雜偶氮染料液晶薄膜做為可偏振調制之空間濾波器，利用不同光強激發偶氮染料在基板上之吸附，進而對液晶產生不同的配向效果，在空間頻率的分佈中產生了水平配向及扭轉向列型二種結構，如此利用控制影像的偏振態即可控制其空間頻率。
最後為使用參雜偶氮染料之平面結構膽固醇液晶薄膜做為半反穿之空間濾波器，因其具布拉格反射特性，因此會選擇性的反射光波。由於在偶氮染料受光激發後，會由trans state轉變為cis state，彎曲狀之cis染料會使膽固醇液晶薄膜亂度增大，以致於相變點降低，如降至室溫下時，膽固醇液晶變為各方同向 (isotropic)而失其布拉格反射特性，如此可利用空間頻率的不同強度分佈來達成即時的分離為強度互補之穿透部份和反射部份的空間頻譜。
論文中，針對不同濾波功能的液晶元件皆模擬其各自對應的濾波結果，並與實驗結果比較，兩者相當的吻合。這些液晶元件能即時的對應空間頻率分佈來產生對應的薄膜，並可加以調制。這些元件不只容易製作，使用上也很方便，未來具有極佳的應用潛力。

The manipulation of spatial frequencies of two dimensional images in the Fourier optical processing is well studied and applied for uses in many areas, such as edge enhancement, character recognition, image correlation and more recently medical image processing.
Conventionally, a spatial filter is placed at the Fourier plane to block the undesired spatial frequencies of the object. An inverse Fourier transform of the transmitted spatial orders displays the processed image. Technically, it is difficult to exactly filter the desired spatial orders in real time using the conventional techniques, since the filter is not all-optical and continuously controllable. Hence there is a need for spatially filtering in real time. Moreover, such a real-time spatial filtering technique can be controllable by an external means. This dissertation demonstrates the feasibility of using liquid crystal films as real-time and controllable spatial filters.
Firstly, we use polymer-dispersed liquid crystal (PDLC) films as electrically switchable spatial filters in the Fourier optical signal process. The fabrication relies on the fact that the size of the LC droplet formed in a PDLC film is inversely proportional to the intensity of curing. Also, the driving voltage is lower as the LC droplet size is lager. Controlling the driving voltage on the PDLC sample can filter certain particular spatial frequencies in the Fourier optical signal process.
Then, we use the surface-assisted photoalignment effect in dye-doped liquid crystal (DDLC) films as polarization controllable spatial filters in the optical signal process. The fabrication relies on the fact that different intensity of a diffracted order causes different change of the polarization state by the photo-aligned DDLC film. By controlling the polarization state of the diffracted orders, particular spatial orders in the Fourier optical signal process can be filtered with the use of an analyzer placed behind the sample.
Finally, we exploit the photoisomerization effect in azo-dye-doped cholesteric liquid crystal (DDCLC) films with a concomitant decline of the phase transition temperature from the cholesteric to an isotropic phase (TCh-I) as a spatial filter. The fabrication depends on the fact that the various intensities of the diffracted orders are responsible for the various degrees of transparency associated with the photoisomerized DDCLC film. High- and low-pass images in the Fourier optical signal process can be simultaneously observed via reflected and transmitted signals, respectively.
Notably, a simulation is performed for each of the above three cases. The results are highly consistent with those of obtained from experiments.

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