Study of Spatial Filters based on Liquid Crystal Devices and their Applications

• Main Authors: Tsung-Hsien Lin, 林宗賢
• Other Authors: Andy Y.-G. Fuh
• Format: Others
• Language: en_US
• Published: 2006
• Online Access: http://ndltd.ncl.edu.tw/handle/58714865020671218494

博士 === 國立成功大學 === 光電科學與工程研究所 === 94 === 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.