Enhancement of Brightness and Contrast in Fluorescent Liquid Crystal Display Using Sensitizing Effect

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Emissive liquid crystal display (LCD) in which a dichroic phosphor is dissolved as a guest material have been reported. Brightness and contrast of the LCDsare enhanced has been demonstrated by using sensitizing effect.(1)The remarkable increase of the green fluorescence intensity from the coumarin-6 in the LC host can be obtained by adding the blue fluorescent dichroic dye or the fluorescent LC as a sensitizer. Applying the sensitization to the emissive type LCD of which color is switched from green to blue, the luminance of the green fluorescence and the color switching property are improved. The color switching between blue to red in the LCD assembled with the blue LED and the red fluorescent LC cell is also improved by adding the sensitizer. These results indicate that the light utilization of the excitation back light is also increased by using the sensitizing effect.(2)The increase of the green and red fluorescence intensity of dichroic dyes in the LC host can be obtained by the sensitizing effect. Relations between absorption and emission dichroic ratios are measured in the fluorescent LC cell with and without the sensitizer. When the voltage is applied to the cell, the fluorescence intensity which is excited by the polarized UV light parallel to the LC direction decreases and becomes lower than that excited by the polarized UV light perpendicular to the LC direction. Moreover, it is clarified that both the brightness and the contrast can be enhanced, if the dichroic ratio of the sensitizer is higher than that of the emitter.(3)Liquid crystal displays emitting white fluorescence are successfully demonstrated by mixing two or three fluorescent dyes in the homogeneous, TN and homeotropic alignment LC cell. The white fluorescence intensity changes by applying the voltage while the color maintains white. The steepness is improved by using TN orientation. The contrast ratio of 13.3 in the homeotropic alignment LC cell is the highest in three types of LC cell.