KR100743540B1 - Liquid crystal display - Google Patents

Abstract

An LCD is provided to adjust light leakage of a polarizing plate such as an hourglass phenomenon without changing the inherent characteristic of the polarizing plate, by controlling the light transmittance of a diffusing plate. An LCD comprises an LCD panel and a backlight unit, wherein the backlight unit includes a polarizing plate attached onto the LCD panel, and a diffusing plate(24). The diffusing plate has predetermined areas(A,B,C,D), which face regions where an hourglass phenomenon is generated in the polarizing plate. The predetermined areas of the diffusing plate have lower light transmittances than the other area of the diffusing plate by 10-20%.

Description

Liquid Crystal Display

1 is a schematic structural diagram of a WV polarizing plate applied to a general liquid crystal display device;

2 and 3 are graphs showing the luminance according to the position of the liquid crystal panel in the normal operation of the conventional liquid crystal display and a front view of the liquid crystal panel;

4 and 5 are graphs showing the luminance according to the position of the liquid crystal panel when the hourglass phenomenon occurs in the conventional liquid crystal display and a front view of the liquid crystal panel;

6 and 7 are structural diagrams of a backlight unit applied to a liquid crystal display according to an embodiment of the present invention;

8 is a graph showing luminance according to a position of a liquid crystal panel during a normal operation of the liquid crystal display according to the exemplary embodiment of the present invention;

9 is a graph showing luminance according to a position of a liquid crystal panel when an hourglass phenomenon occurs in the liquid crystal display according to the exemplary embodiment of the present invention.

<Description of Major Symbols in Drawing>

10. WV polarizer 20. LED backlight unit

21. Light source lamp 22. Light guide plate

23. Reflector 24. Diffuser

The present invention relates to a liquid crystal display device, and more particularly, to a liquid crystal display device for correcting an hourglass phenomenon using a backlight unit.

Various types of thin displays are currently being developed and marketed, but liquid crystal displays (LCDs) are frequently used for various reasons. The LCD arranges two substrates on which the field generating electrodes are formed so that the surfaces on which the two electrodes are formed face each other, injects a liquid crystal material between the two substrates, and then applies a voltage to the two electrodes to form a liquid crystal. By moving the molecules, it is a device that represents the image by controlling the transmittance of the light that changes accordingly.

However, unlike other thin displays, such as plasma display panels (PDPs) and field emission displays (FEDs), the display by the liquid crystal display itself is non-light-emitting (light-receiving element) and thus cannot be used where there is no light. A backlight unit (BLU: Back Light Unit), which is a surface light source device having a high brightness, is required to compensate for these disadvantages and to enable use in a dark place.

On the other hand, in general, liquid crystal molecules have anisotropy, and the anisotropy of the liquid crystal cell or film composed of the molecules is changed by the distribution of liquid crystal molecules and the degree of tilt angle with respect to the substrate. . In addition, this feature is an important factor to change the polarization of light according to the viewing angle of the cell or film made of liquid crystal.

Recently, as high-definition and large-size LCDs have been promoted, wide-view polarizers (WVs) are applied to TN-mode LCDs in order to improve weaknesses of narrow viewing angles.

FIG. 1 schematically illustrates the structure of a WV polarizer applied to a general liquid crystal display. As shown in the figure, the WV polarizing plate 10 has a PVA film (Poly Vinyl Alcoholic) film 11, which is a polarizing film that controls the amount of light transmitted according to the degree of polarization of the incident beam, and a PVA film (on both sides of the PVA film 11). 11) Compensation film 14 for optical compensation of the liquid crystal cell on one side of the TAC (TriAcetyl Cellulose) film 12, T-SWV film 13, T-SWV film 13 attached to protect / support ), The upper protective film attached to the outside of the compensation film 14 for the attachment of the glass of the liquid crystal cell with the adhesive 15, the adhesive 15 and the TAC film 12 for the protection of the WV polarizing plate ( 16) and the lower protective film (17).

As shown, the WV polarizer 10 has a structure in which several layers of films are stacked, and each of the layers of films has different expansion and contraction coefficients for heat and humidity. Therefore, when exposed to external heat and humidity for a long time, the weak part occurs in the WV polarizer 10. This will be described below.

As shown in the drawings, the films are attached to the substrate by applying a stress in a diagonal direction opposite to each other. That is, when one film is applied by applying a stress in a diagonal direction, the next laminated film is applied by applying a stress at an opposite diagonal. As a result, the respective films are attached by applying stress in the form of 'X'. As described above, the WV polarizing plate 10 is attached to various films by applying stress in a diagonal direction, and is vulnerable to heat and humidity, so that when exposed to external heat and humidity for a long time, the WV polarizer 10 generates stress due to the difference in shrinkage and expansion between the films. There is a part. This fragile part transmits beyond the given light transmission conditions, causing light leakage, which is called an hourglass phenomenon.

2 is a graph showing luminance according to the position of the liquid crystal panel 50 in the normal operation, and FIG. 3 illustrates the liquid crystal panel 50 at this time. As shown in the drawing, the entire area of the liquid crystal panel 50 exhibits substantially the same luminance. That is, when the luminance in the center of the liquid crystal panel 50 is set to '100', the luminance of the edge also corresponds to '100'.

4 is a graph showing luminance according to the position of the liquid crystal panel 50 when an hourglass phenomenon occurs, and FIG. 5 illustrates the liquid crystal panel 50 at this time.

As described above, the weak areas are mainly generated near the top, bottom, left and right edges of the WV polarizer 10, and more weak areas are generated in the left and right sides than the top and bottom. Since the vulnerable area transmits more light than given conditions, an hourglass phenomenon occurs on the displayed screen corresponding to the vulnerable area. This hourglass phenomenon is hard to hear when entering a white pattern, but appears clearly when entering a dark screen. Applicant's repeated experiments show that the light transmittance of the area vulnerable near the edge has about 20-40% more light transmittance compared to the middle. That is, when the luminance of the light transmitted in the center of the liquid crystal panel 50 is set to '100', the luminance of the edge corresponds to approximately '120 to 140'.

On the other hand, the change in physical properties due to heat and humidity of the WV polarizing plate is due to the inherent characteristics of the polarizing plate, it is difficult to solve by the development of its own cell process.

SUMMARY OF THE INVENTION An object of the present invention is to provide a liquid crystal display device which corrects an hourglass phenomenon by using a diffuser plate of a backlight unit (BLU) without changing characteristics inherent to a polarizing plate.

In the liquid crystal display device according to the present invention for achieving the above object, the liquid crystal display device comprising a polarizing plate attached to the liquid crystal panel and a backlight unit having a diffusion plate, facing the hourglass phenomenon generating region of the polarizing plate The predetermined region of the diffusion plate is characterized in that the light transmittance is lower than the region other than the predetermined region.

According to a preferred embodiment of the present invention, the light transmittance of a predetermined region of the diffusion plate is preferably about 10 to 20% lower than the light transmittance of regions other than the predetermined region.

In addition, the predetermined region of the diffusion plate is a first region which is a region corresponding to 75 to 100% in the left and right side direction from the left and right center lines of the diffusion plate, and 50 to 100% in the vertical direction from the vertical center line of the diffusion plate. A second area, which is an area corresponding to the second area, is included, and the area in which the first area and the second area overlap each other is not included.

The above objects, features and other advantages of the present invention will become more apparent by describing the preferred embodiments of the present invention in detail with reference to the accompanying drawings. Hereinafter, a liquid crystal display according to an exemplary embodiment of the present invention will be described in detail with reference to the accompanying drawings.

The liquid crystal display includes a liquid crystal panel, a polarizing plate attached to front and rear surfaces of the liquid crystal panel, and a backlight unit (BLU) positioned behind the liquid crystal panel to which the polarizing plate is attached. As the liquid crystal display becomes higher and larger in size, an hourglass phenomenon occurs, which is caused by the polarizer as described above, and the upper, lower, left, and right edges of the screen appear to be relatively bright.

On the other hand, the backlight unit generally includes a light source lamp, a light guiding plate, a reflector sheet, a diffuser sheet, and a prism sheet.

6 and 7, a backlight unit 20 applied to a liquid crystal display according to an exemplary embodiment of the present invention includes a light source lamp 21, a light guide plate 22, a reflector plate 23, and a diffuser plate 24. ).

The light guide plate 22 is a component that converts light incident from the light source lamp 21 into uniform planar light, and mainly uses PMMA (Polymethymethacrylate), which is an acrylic resin. This is because the absorption of light in the visible region is the lowest of the polymers because of the good transparency and gloss.

The reflector 23 is positioned under the light guide plate 22, and reflects light incident from the light source lamp 21 toward the light exit surface of the backlight unit 20, that is, the liquid crystal panel, to increase light utilization efficiency and to provide total incident light. By adjusting the amount of reflection of the backlight unit 20 by adjusting the amount of reflection so that the entire light emitting surface of the backlight unit 20 has a uniform luminance distribution.

The diffusion plate 24 is positioned toward the liquid crystal panel 50 (see FIG. 3) on the light guide plate 22, and diffuses and scatters the light incident through the light guide plate 22, so that the luminance of the front surface of the backlight unit 20 is emitted. Increases and makes uniform.

Although not shown, a prism plate may be installed on the diffusion plate 24. The prism plate prevents light from the diffuser plate 24 from going out of the front surface of the light-emitting surface, improves light directivity, and narrows the viewing angle, thereby increasing the brightness of the backlight unit 20 toward the light-emitting surface front.

Referring to FIG. 7, the predetermined regions A, B, C, and D of the diffusion plate 24 according to the present invention have a lower light transmittance than the remaining regions except for the predetermined regions A, B, C, and D. Is formed. The predetermined areas A, B, C, and D are areas facing the hourglass phenomenon generating region of the polarizing plate 10 (see FIG. 1), that is, the upper, lower, left and right edge regions of the polarizer 10. The arrow shown in FIG. 7 shows the amount of light transmitted through the diffuser plate 24. As a result, by lowering the light transmittance of the predetermined regions A, B, C, and D of the diffusion plate 24, the amount of light transmitted through the predetermined regions A, B, C, and D is different from other regions, in particular, the middle region. The amount of light transmitted through is reduced.

Meanwhile, as described above, when the polarizer 10 (see FIG. 1) is exposed to heat and humidity for a long time, weak portions of the upper, lower, left, and right edges transmit over a given light transmission condition, causing an hourglass phenomenon. Here, predetermined regions A, B, C, and D of the diffusion plate 24 are regions facing the weak portions of the polarizing plate.

After all, according to the present invention, the diffusion plate 24 has the effect that can correct the hourglass phenomenon. That is, even if the hourglass phenomenon occurs in the weak portion of the polarizing plate, by reducing the amount of light transmitted through the corresponding region of the diffuser plate 24, the amount of light transmitted through the liquid crystal panel is reduced, thereby reducing luminance non-uniformity. Will be.

The light transmittance of the predetermined regions A, B, C, and D of the diffusion plate 24 is preferably designed to be about 10 to 20% lower than the light transmittance of regions other than the predetermined region (particularly near the center). . As described above (refer to FIG. 4), according to the existing liquid crystal display device, when the hourglass phenomenon occurs, the light transmittance of the weak area near the edge of the liquid crystal panel 50 is approximately 20-40% higher than that of the center. Will have Therefore, if the light transmittance of the predetermined areas (A, B, C, D) is designed to be about 20 to 40% lower than the center, even if an hourglass phenomenon occurs, the luminance unevenness hardly occurs. However, in this case, in the normal operation, the predetermined region of the liquid crystal panel has a lower light transmittance compared to the center, which causes another luminance nonuniformity. This will be described below.

 8 and 9 are graphs showing luminance according to the position of the liquid crystal panel as a result of applying the diffusion plate according to the exemplary embodiment of the present invention. FIG. 8 illustrates normal operation and FIG. 9 illustrates an hourglass phenomenon.

As shown in FIG. 8, the light transmitted from the liquid crystal panel 50 in the normal operation has a lower luminance near the edge than the center, and the ratio is approximately '80 to 100 'when the luminance of the center is set to' 100 '. Corresponds to That is, it has 10-20% lower luminance. Since the light transmittance near the edge of the diffuser plate 24 is lowered by about 10 to 20% compared to the center, the reduced transmittance is applied during normal operation, and the reduced amount of light is transmitted to the polarizing plate and the liquid crystal panel.

Referring to FIG. 9, when the hourglass phenomenon occurs, the light transmittance of the weak region near the edge of the liquid crystal panel 50 has a light transmittance of about 10 to 20% more than that of the center. This lowered the light transmittance near the edge of the diffuser plate 24 by 10-20% compared to the center, and the transmittance in the polarizing plate 10 (see FIG. 1) increased by 20-40% due to the hourglass phenomenon. About 10 to 20% more light is transmitted through the liquid crystal panel.

Therefore, the liquid crystal display device according to the present invention has an effect of reducing the luminance unevenness compared with the conventional. Of course, the brightness of the edge of the liquid crystal panel in the normal operation is slightly reduced compared to the center, but the reduction of about 10 to 20% will not be a problem since the viewer is not recognized significantly. However, when the edge of the liquid crystal panel is 20 to 40% higher than the center, the viewer can recognize the luminance unevenness greatly.

On the other hand, the method of lowering the light transmittance of the diffusion plate 24 may be performed by increasing the thickness of the corresponding region or by adding a material that lowers the transmittance. Since such a method is well known to those skilled in the art, a detailed description thereof will be omitted.

Referring to FIG. 7 again, predetermined regions A, B, C, and D of the diffusion plate 24 are first to 75% to 100% in a left and right side direction from the left and right center lines E of the diffusion plate. Regions C and D and second regions A and B, which are regions corresponding to 50 to 100% in the vertical direction from the upper and lower center lines F of the diffusion plate, wherein the first and second regions are included. It is preferable not to include a region where the regions overlap.

The light transmittance decrease rate and the transmittance decrease area are values calculated by the applicant's repeated experiments, but the present invention is not limited thereto. This is because the characteristics of the components of the liquid crystal display and the experimental results may vary.

As described above, in the liquid crystal display according to the present invention, even if an hourglass phenomenon occurs due to the polarizing plate, the hourglass phenomenon is controlled by adjusting the light transmittance of a predetermined region of the diffuser plate of the backlight unit lower than that of other regions. This can be prevented from being observed, and eventually, luminance unevenness can be prevented, thereby improving the screen quality.

Although preferred embodiments of the present invention have been described above, the present invention is not limited to the above-described specific embodiments. That is, those skilled in the art to which the present invention pertains can make many changes and modifications to the present invention without departing from the spirit and scope of the appended claims, and all such appropriate changes and modifications are possible. Equivalents should be considered to be within the scope of the present invention.

Claims (3)

In the liquid crystal display device comprising a polarizing plate attached to the liquid crystal panel and a backlight unit having a diffusion plate, And a predetermined region of the diffusion plate facing the hourglass developing region of the polarizing plate is formed so as to have a light transmittance lower than a region other than the predetermined region. The method of claim 1, And a light transmittance of a predetermined region of the diffusion plate is about 10 to 20% lower than that of a region other than the predetermined region. The predetermined region of the diffusion plate according to claim 1 or 2, A first region corresponding to 75 to 100% in a left and right side direction from the left and right center lines of the diffusion plate, and a second region corresponding to 50 to 100% in an up and down side direction from an upper and lower center line of the diffuser plate Wherein the first region and the second region do not include an overlapping region.

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