KR100685915B1 - LCD Projector with Wide Cell Gap Tolerance LC Panel - Google Patents

Abstract

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal projector (LCD Projector), and to improve image quality by minimizing changes in brightness and color coordinates for each gray level. The three-panel liquid crystal projector separates the white light emitted from the light source 30 into red, blue and green, merges again through the liquid crystal panels 31, 32 and 33 which modulate the respective colors, and then enlarges the field lens 37. On the screen 38. Each color-modulating liquid crystal panel should have the same transmittance for all gray levels. However, in the process of making the liquid crystal panel, there is some nonuniformity of the cell gap of the liquid crystal cell, which causes the quality of the liquid crystal panel to be different due to different transmittance. According to the present invention, a slit pattern 20 or a floating electrode 21 in which each electrode is etched is disposed on a pixel electrode 5 or a common electrode 6 that applies a voltage to a liquid crystal layer. By using the lateral electric field properties according to the change, the image quality was improved by minimizing the change in transmittance of the liquid crystal panel with respect to the thickness change of the liquid crystal cell. The liquid crystal projector of the present invention can be applied as a display element in a part requiring high luminance and high image quality such as conference materials or HD TV.

LCD Projector, Slit Pattern, Floating Electrode, Cell Gap

Description

LCD Projector with Wide Cell Gap Tolerance LC Panel

1 is a schematic diagram of a single-plate liquid crystal projector.

Fig. 2 is a pixel structure of a single plate liquid crystal projector.

3 is a schematic diagram of a three-panel liquid crystal projector.

4 is a cross-sectional view of a TFT liquid crystal panel attached to a conventional three-panel liquid crystal projector.

5 is a change in transmittance of each gray scale with respect to a thickness change of the liquid crystal cell.

6 is a cross-sectional view of the TFT liquid crystal panel attached to the liquid crystal projector of the present invention.

7 is a plan view of a pixel electrode having a slit pattern.

8 is a cross-sectional view of a pixel electrode having a floating electrode.

9 is a graph showing transmittance of each gray scale with respect to a thickness change of the liquid crystal cell of the present invention.

Fig. 10 is an explanatory diagram for calculating the voltage distribution of the slit pattern.

  ※ Explanation of codes for main parts of drawing

1,2 Up and down glass substrate 3 Alignment layer 4 Liquid crystal molecule

      5 pixel electrode 6 common electrode 8 source electrode

      9 Drain Electrode 20 Slit Pattern 21 Floating Electrode

      30 Light source 31, 32, 33, 40 Liquid crystal panel

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal projector (LCD projector). The image quality is improved by minimizing the change in transmittance for each gradation stage of the liquid crystal panels 31, 32, and 33 with respect to the change in the cell gap of the liquid crystal cell.

The liquid crystal projector separates the white light emitted from the light source 30 into red, blue and green, passes through the liquid crystal panels 31, 32, and 33 which modulate the respective colors, merges again, and is enlarged in the field lens 37. On the screen 38. Liquid crystal projectors are small in size, can easily adjust the size of the screen, and have excellent color reproduction, and are widely used as large display devices such as HDTV or seminar presentation screens. Liquid crystal projectors can be roughly divided into three panel type and single plate type according to the number of liquid crystal panels. 1 is a schematic diagram of a single-plate liquid crystal projector. The liquid crystal panel 40 has a single liquid crystal panel projector attached thereto. Each pixel 10 of the liquid crystal panel selectively modulates light by absorbing red (R), green (G), and blue (B) as shown in FIG. 2, and the modulated light is enlarged in the field lens 37. An image is made on screen 38. The three-panel liquid crystal projector separates the white light emitted from the light source 30 into red, blue, and green, as shown in FIG. 3, and merges again through the liquid crystal panels 31, 32, and 33 which modulate the respective colors, and then the field lens ( Magnified at 37, it forms an image on screen 38. The white light from the light source is divided into red, blue and green in the dichroic mirror (34,36), and then gathers again in the color mirror through the liquid crystal panels (31, 32, 33) that control the light transmittance. Projected onto the screen 38 past lens 37. In FIG. 3, the lowercase letters of the alphabet are color mirrors representing respective colors, and the uppercase letters are liquid crystal panels representing each color.

The liquid crystal panel used in the liquid crystal projector of FIG. 1 or 3 is a TFT liquid crystal panel in which a TFT (Thin Film Transistor) is attached to most pixels. TFT liquid crystal panels can implement moving images and 256 gray levels relatively easily, and most liquid crystal projectors currently use TFT liquid crystal panels. 4 is a cross-sectional view of a TFT liquid crystal panel in a conventional liquid crystal projector. The operation principle of the TFT liquid crystal panel is as follows. The gate electrode 11 of the TFT in each pixel is connected to the scanning line, the source electrode 8 to the signal line, and the drain electrode to the pixel electrode 5, respectively. There is a liquid crystal layer 4 between the common electrode 6 and the pixel electrode. The alignment films 3 and 3 'are coated on the electrodes 5 and 6 which apply voltage to the liquid crystal layer so as to uniformly arrange the liquid crystals. In the selection period, a voltage higher than that of the signal line is applied to the gate electrode connected to the scan line, and the connection resistance of the channel between the drain electrode and the source electrode is reduced, so that the voltage applied to the signal line is applied to the liquid crystal layer through the pixel electrode. In the non-selection period, a voltage lower than that of the signal line is applied to the gate electrode connected to the scan line, and the drain electrode and the source electrode are electrically disconnected to maintain the charge accumulated in the liquid crystal layer during the selection period. While scanning the scan lines sequentially, each pixel electrode is charged through the signal lines to apply voltage to the liquid crystal layer. When the rms (root means square) voltage applied to the liquid crystal layer between the pixel electrode and the common electrode is adjusted, the polarization state changes as the linearly polarized light passes through the liquid crystal layer and the light passes through the liquid crystal layer. Display information as. The liquid crystal mode of the liquid crystal projector mainly uses a 90 ^o TN mode in the case of a transmissive type, an electrically controlled birefringence (ECB) mode in which the liquid crystal is aligned, or a TN mode having a twist angle of less than 90 ^o .

Since it is difficult to make the thickness of the liquid crystal cell constant in the process of making the liquid crystal cell, the thickness of the liquid crystal cell varies to some extent depending on the position of the same liquid crystal panel. In addition, since a large amount of infrared light is emitted from the light source, the thickness of the liquid crystal cell varies depending on the position of the same liquid crystal panel due to the difference in thermal expansion. When the temperature difference between the upper and lower glass substrates 1 and 2 of the liquid crystal panel is 1 ° C., the change in the thickness of the liquid crystal cell is about 0.1 μm. 5 is a change in transmittance of each gray scale with respect to the thickness change of the liquid crystal cell of the liquid crystal cell. FIG. 5 shows a case where the gamma is 2.0 and the relative brightness of each gray level is shown in Table 1. FIG. The injected liquid crystal was manufactured by Merck Germany, K ₁₁ was 12 pN, K ₂₂ was 8 pN, K ₃₃ was 16 pN, dielectric anisotropy was 7, and refractive index anisotropy was 0.12. Table 2 shows the change rate of maximum transmittance when the thickness of the liquid crystal cell is changed by ± 10% based on 4.0㎛.

Gradation One 32 64 96 128 160 192 224 256 Transmittance (%) 0 1.4 6.1 13.8 24.8 38.8 56.1 76.4 100

Gradation 32 64 96 128 160 192 224 256 Maximum transmittance change (%) 52 45 40 37 33 29 22 3.1

Although the difference in transmittance is small depending on the thickness of the liquid crystal cell in the bright state of the screen, the lower the gray scale, the higher the transmittance change rate is in accordance with the thickness of the liquid crystal cell. Since the distance between the R, G, and B pixels is 0.1 mm or less, even if there is a change in the thickness of the liquid crystal cell in the liquid crystal panel, the R, G, and B pixels forming one pixel are near. It can be seen that there is little change in the thickness of the liquid crystal cell. Therefore, even if the thickness of the liquid crystal cell is different in the same panel, the brightness of the screen screen of the different thickness of the liquid crystal cell is different, but the color coordinate (color coordinate) does not differ. In other words, the change in the transmittance of red, blue, and green has little change depending on the thickness of the liquid crystal cell. However, in the case of separately modulating the red, green, and blue light in each liquid crystal panel as shown in FIG. 3, the change in brightness depending on the thickness of the liquid crystal cell brings about a serious deterioration in image quality. For example, assume that the thickness of a liquid crystal cell that modulates green and blue that constitutes a pixel of a liquid crystal projector is 4.4 μm and the liquid crystal cell that modulates red is 3.6 μm. When the brightness is 20% white, the screen becomes slightly yellowish white. As described above, since the three-plate type liquid crystal projector changes both brightness and color coordinates according to the change in the liquid crystal cell gap of the R, G, and B pixels, the yield is known to be less than 60% due to the variation in the thickness of the liquid crystal cell.

The three-panel liquid crystal projector separates the white light from the light source into red, blue and green, merges back through the liquid crystal panels that modulate each color, and then magnifies the field lens to form an image on the screen. When the thickness of the liquid crystal cell of the liquid crystal panel that modulates each color varies, the brightness and color coordinates of the screen change together, resulting in low yield and poor image quality. According to the present invention, even if there is a variation in the thickness of the liquid crystal cell, the change in brightness for each grayscale is minimized to increase the yield and image quality of the liquid crystal projector at the same time.

According to the present invention, a slit pattern 20 or a floating electrode 21 in which each electrode is etched is disposed on a pixel electrode 5 or a common electrode 6 that applies a voltage to the liquid crystal layer, thereby changing the thickness of the liquid crystal cell. The image quality is improved by minimizing the change in transmittance with respect to the thickness change of the liquid crystal cell by using the lateral electric field property. Liquid crystal alignment when the slit pattern is placed on the pixel electrode has been studied since Lien of IBM published in SID in 1992 {SID 92 DIGEST p33}. Until now, the liquid crystal was made into a multi domain mainly using a slit pattern, and thus, the liquid crystal was mainly limited to a portion having a wider viewing angle. The inventors have filed a patent in 1999 to increase the process tolerance of the thickness of the TN liquid crystal cell using a slit pattern (Patent Application No. 10-1999-0016499).

6 is a cross-sectional view of the TFT liquid crystal panel used in the liquid crystal projector of the present invention. FIG. 6 differs only in the shape of the pixel electrode from FIG. 4, and the other parts are the same. 6 are cross-sectional views of the pixel electrode of FIG. 6. 7 is a cross-sectional view of a pixel electrode with a slit pattern, and FIG. 8 is a cross-sectional view of a pixel electrode with a floating electrode.

The voltage distribution in the slit pattern can be obtained using a simple equation. 10 is a cross-sectional view of a structure similar to that of FIG. The voltage distribution at A of the slit pattern assumes that a very small fine electrode at the A position is circular as shown in FIG. Electrodes with various voltage distributions (V1, V2, V3 ....) are placed around the microelectrode, and knowing the capacitances (C1, C2, C3 ...) formed by each electrode and the microelectrode , The induced voltage V (A) of the A portion can be obtained by the following equation.

In FIG. 10, it is assumed that V1 and V3 are voltages of the pixel electrode, and V2 is the voltage of the common electrode. If the voltage distribution of the microelectrode portion of A is different from the voltage distributions (V1, V3; V1 = V3) of the pixel electrode, a horizontal electric field of a voltage corresponding to the difference is applied between the A region and the pixel electrode. As the thickness of the liquid crystal cell decreases, the capacitance C2 between the microelectrode and the common electrode increases, so that V (A) moves toward the voltage of the common electrode. Moves toward the voltage distribution of the pixel electrode. If the dielectric anisotropy of the liquid crystal is positive and the lateral and vertical electric fields are hung simultaneously, the force of the liquid crystal molecules to align horizontally becomes stronger when the lateral electric field is counted. Get stronger. If the liquid crystal cell's thickness d is reduced, the horizontal electric field becomes larger, so that the refractive index anisotropy of the liquid crystal becomes larger. Therefore, refractive index anisotropy becomes small. Therefore, the transmittance of the liquid crystal cell is a function of Δnd. Since Δn and d move in opposite directions, the change in Δnd is reduced when there is a slit pattern, so that the change in transmittance of the liquid crystal panel with respect to the thickness change of the liquid crystal cell is reduced. .

Even when the floating electrode 21 is inside the pixel electrode as shown in FIG. 8, the voltage induced in the floating electrode is expressed by Equation (1). It is possible to know the voltage induced in the floating electrode by calculating the storage capacitance between the floating electrode and the peripheral electrode rather than the minute region. Similarly, in the case of the floating electrode, when the thickness d of the liquid crystal cell decreases, the voltage induced by the floating electrode moves toward the voltage of the common electrode, thereby increasing the horizontal electric field, thereby increasing the refractive index anisotropy of the liquid crystal. As the voltage increases, the refractive anisotropy decreases because the voltage induced in the floating electrode moves toward the voltage of the pixel electrode. Since the brightness of the liquid crystal cell is a function of Δnd, and Δn and d move in opposite directions, the transmittance of the liquid crystal panel is insensitive to the change in the thickness of the liquid crystal cell, similarly to the case of the slit pattern, even with the floating electrode.

9 is a change in transmittance of each gray scale with respect to a thickness change of the liquid crystal cell of the liquid crystal cell when there is a slit pattern. The liquid crystal cell condition of FIG. 9 is the same as that of FIG. 5 except for the slit pattern of the pixel electrode. The width t in the short axis direction of the pixel electrode 5 and the width s in the short axis direction of the slit pattern 20 were both 4.0 µm. Table 3 shows the rate of change of the maximum transmittance when the thickness of the liquid crystal cell changes by ± 10% based on the thickness of 4.0㎛.

Gradation 32 64 96 128 160 192 224 256 % Change in brightness 16.4 4.2 8.2 14.1 15.5 17.8 15.4 3.1

Comparing Table 2 and Table 3, it can be seen that the average change in the transmittance of the liquid crystal cell for all gray levels is reduced by 50% or more compared to the conventional Table 2 when the slit pattern is placed.

The slit pattern and the floating electrode may be placed on the common electrode. When the slit pattern 20 and the floating electrode 21 are placed on the pixel electrode 5, the slit pattern 20 and the floating electrode 21 can be made simultaneously in the process of making the pixel electrode, and thus there is no additional process. However, when the slit pattern 20 and the floating electrode 21 are provided on the common electrode 6, the common electrode is exposed, so that an additional step of exposure may be performed in some cases.

The liquid crystal projector of the present invention can be applied as a display element in a part requiring high luminance and high image quality such as conference materials or HD TV. In particular, it is possible to increase the yield of the liquid crystal projector by minimizing the deterioration in image quality due to the change in the cell thickness of the liquid crystal panel. In particular, the uniformity of brightness was increased in the single-plate type liquid crystal projector, and the uniformity of brightness and color coordinates was improved in the three-plate liquid crystal projector.

Claims (8)

White light emitted from the light source is separated into red, blue, and green, and liquid crystal panels for modulating respective colors are attached, and the liquid crystal dielectric anisotropy of the liquid crystal panel is positive. An LCD projector comprising an etched slit pattern and a floating electrode. delete The liquid crystal projector according to claim 1, wherein the liquid crystal mode of the liquid crystal panel is TN or ECB mode. A slit pattern in which the pixels for modulating the red, blue, and green colors of the white light emitted from the light source are on the same liquid crystal panel, the liquid crystal dielectric anisotropy of the liquid crystal panel is positive, and the electrode is etched on the pixel electrode which applies a voltage to the liquid crystal layer; A liquid crystal projector characterized by having a floating electrode. delete The liquid crystal projector according to claim 4, wherein the liquid crystal mode of the liquid crystal panel is TN or ECB mode. White light emitted from the light source is separated into red, blue, and green, and liquid crystal panels for modulating respective colors are attached, and the liquid crystal dielectric anisotropy of the liquid crystal panel is positive, and an electrode is connected to a common electrode which applies a voltage to the liquid crystal layer. An LCD projector comprising an etched slit pattern and a floating electrode. A slit pattern in which the pixels for modulating the red, blue, and green colors of the white light emitted from the light source are on the same liquid crystal panel, the liquid crystal dielectric anisotropy of the liquid crystal panel is positive, and the electrode is etched on a common electrode which applies a voltage to the liquid crystal layer; A liquid crystal projector characterized by having a floating electrode.

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