Dynamic characteristics of the PDLC-based electrooptic modulator for TFT-LCD inspection

To detect the electrical fault of a thin film transistor (TFT) panel for the LCD, a polymer-dispersed-liquid-crystal (PDLC) modulator is used to convert the electric field of TFT substrate to an image. The PDLC changes its light transmittance proportional to electric field strength so that electric faults can be detected without physically contacting to the surface. Specific pattern signals are applied to the data and gate electrodes of the panel to charge the pixel electrodes and the image sensor detects the change of transmittance of PDLC that is positioned in proximity distance above the pixel electrodes. The image represents the status of electric field of the TFT panel reflected on the PDLC so that the characteristic of PDLC itself plays an important role to accurately quantify the defects. In this paper, a sample PDLC modulator is manufactured and compared with the commercially available one. The dynamics of PDLC modulator is analyzed and the image of electric field of the arrayed electrodes on TFT panel is acquired. The signal pattern to the electrodes of TFT panel and modulator should be selected based on the response characteristics of the PDLC for better image quality. The retention time of PDLC is a key factor for the determination of signal pattern.