JP2002513168A - Liquid crystal display device provided with birefringence compensation means - Google Patents

Abstract

(57) [Summary] The present invention relates to a liquid crystal display device including a birefringence compensator. SOLUTION: The liquid crystal display device according to the present invention has a twisted nematic liquid crystal display screen (1) classified into Type II. At least one surface (10, 11) of the screen has a biaxial birefringent film (2) having anisotropic anisotropy and a uniaxial birefringent film (2) having an optical axis inclined with respect to the liquid crystal display screen. 3) is provided. Preferably, said biaxial film (2) is a stretched plastic film and said uniaxial film (3) is holographic grating. Further, a biaxial film (2, 2 ') and a uniaxial film (3, 3') are provided on both sides of the screen. The unit composed of these is sandwiched between polarizing plates (4, 4 ′) whose polarization directions cross each other. The elements constituting the screen have a specified relative directionality. The viewing angle could be increased in the horizontal and vertical directions.

Description

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a liquid crystal display device having a birefringence compensating means and a remarkably wide viewing angle. Thin film transistor (TFT) technology and refractive nematic liquid crystal display cells (TN)
With the development of portable computers using LCDs, the use of liquid crystal display screens has been greatly increased. The use of liquid crystal display (LCD) screens is about to be further expanded by being used in office computers. For this purpose, there is a problem to be solved: the problem of the viewing angle. Due to the inherent birefringence of the liquid crystal material, the contrast decreases as the viewpoint moves away from the screen in a vertical direction, resulting in a gray level reversal when viewed from a particular area. This phenomenon can be tolerated on a screen of a portable computer intended for a single user, but compensation is indispensable on a screen of an office computer.

[0002] The viewing angle characteristics of a liquid crystal screen are evaluated using a polarizing mirror that expresses contour lines of contrast as a function of the viewing angle represented by (θ, φ). Here, θ is the argument of the observation direction from the perpendicular to the screen surface, and φ is the angle formed by the projection of the observation direction onto the screen plane and the horizontal axis. FIG. 1 is an evaluation of a non-compensated TN liquid crystal display screen with a polarizing mirror. Various methods have already been proposed for LCD screen compensation. The first method is a method of forming a plurality of regions in each of the basic cells (pixels) where the orientation (anchoring) of the liquid crystal material is different. As a result, the problem is largely resolved on average. To adopt this method, it is inevitable that the screen manufacturing method becomes very complicated.

Liquid crystal displays employing liquid crystal materials or addressing methods different from twisted nematic liquid crystal displays have been tested. Some of them are commercialized, for example, coplanar switching (IPS) cells have display quality comparable to twisted nematic liquid crystal displays and very wide viewing angles. However, manufacturing is very complicated and difficult to achieve.

The third method does not modify the structure of the cell, but compensates for the birefringent properties of the liquid crystal material with one or more optimized birefringent layers. The idea is as follows: The problem of birefringence in TN liquid crystal displays is due to the birefringence properties of liquid crystal materials whose polarization direction varies with the angle of incidence. Since it is premised that the polarization characteristics are linear in order to block light with a polarizing plate provided so that polarization directions are orthogonal to each other, black display is possible only in the vicinity of a position perpendicular to the screen. To that end, it is intended to offset the birefringence by adding a layer having "reverse" birefringence properties.

[0005] The properties of birefringent materials are represented by refractive index ellipses. This ellipsoid is defined by a characteristic surface representing the refractive index of light having a given polarization direction coming from a given direction: the major axis of the ellipse is the intrinsic axis of the material, and the length of the axis is in that direction Represents the refractive index of the polarized light. If the cross section perpendicular to the axis of the ellipsoid is circular, the material is a "uniaxial" material. For the element made of a uniaxial material, the refractive index of the rotation axis or the optical axis is a special refractive index n _e, the refractive index of ordinary index of refraction n ₀ for the two axes of the others (see FIGS. 2a and 2b ). If the special refractive index _ne is greater than the normal refractive index, the material is called a positive uniaxial medium, and the ellipse is a cigar-shaped elongated shape (FIG. 2a). The special axis corresponds to the slow axis. Conversely, if a special refractive index n _e is less than the normal refractive index n _0, the material is called a negative uniaxial medium, ellipse is flat shape, such as a cushion or dish (Figure 2b). In this case, the special axis is the fast axis. Even if the difference between the two refractive indices is as small as 0.1%, for example, it is enough to significantly affect the polarization.

[0006] The medium has two axes if the ellipse is not of a rotating shape, ie if the three axes orthogonal to each other have three different indices of refraction. The liquid crystal material is a positive uniaxial medium, and the optical axis corresponds to the direction of the liquid crystal molecules. Generally, a birefringent film is provided between the polarizer and the cell substrate. FIG. 3 shows an example in which films are provided on both sides of the cell. All (uniaxial “dish” or biaxial “cigar”) in-plane birefringence values by 1D or 2D polyvinyl alcohol (PVA) plastic film; The birefringence which is a perpendicular line of can be realized. French patent application 94-13623 discloses compensation by providing a POLATECHNO-LTD brand stretched PVA-type birefringent film on both sides of the cell. The film is a biaxial film in which the axis of the ellipsoid of the refractive index is located on the surface e of the substrate and perpendicular to the surface (FIG. 4). One of the axes in the plane of the substrate coincides with the transmission axis of the polarizing plate. This configuration is shown in FIG. In FIG. 5, it should be noted that the polarization direction of the polarizing plate is oriented at 90 ° with the rubbing direction of the liquid crystal molecules on that side. This configuration is referred to as a type II configuration. In the type I configuration, the polarization direction of the polarizing plate is the same as the orientation of the liquid crystal molecules on the side of the polarizing plate.

As shown in FIG. 6, the contours of the contrast obtained by compensating the TN liquid crystal cell as a function of the observation point represented by (θ, φ) are used to basically compensate for the east-west direction by using this film. Compensation for Type II displays can be provided. The horizontal viewing angle is very good (contrast 50 contours are ± 70 °), but the vertical viewing angle is very limited (contrast 50 contours are ± 15 °).

The biaxial refraction of a uniaxial “cigar” type of liquid crystal material (n _e > n ₀ ) is converted to a “dish” type film (n _e <n ₀ )
Compensation is relatively effective. However, in this case, it is necessary to use a "dish" type film having an optical axis inclined with respect to the substrate surface in order to provide effective compensation. European patent application EP 0 645 829 discloses a compensation film using polymerized liquid crystal molecules (having "dish" type birefringence properties) with inclined crystal axes. This film is commercially available. Another method of obtaining a tilted "dish" type birefringence property is to use holographic gratings with fringes tilted relative to the substrate. If the fringe spacing is small enough for the wavelength of light used, the hologram will have a "birefringent" effect, functioning like a "dish" uniaxial medium whose optic axis coincides with the normal of the fringe plane. . A correction method according to this method is described in French patent 2,754,609.

Note that for effective compensation, the optical axis of the “dish” located on each side of the cell must be in a plane defined by the normal to the substrate and the rubbing direction on the side corresponding to that surface. There is a need to. It is difficult to make a tilted "dish",
This is particularly difficult when the required birefringence value is large. European Patent Application EP064
The method described in US Pat.
A type medium is disclosed. This value is a large value. Optimal compensation requires the combination of an "inclined dish" on both sides with an "in-plane dish" generally obtained by film stretching.

A “cigar” type 2 is formed by deposition of a dielectric material called “gradient method” and “birefringence formation”.
A thin film having a tilted optical axis can be manufactured. US Patent No. 55046
No. 03 discloses that compensation is performed by variously combining a tilted biaxial film and an in-plane birefringent film.

An object of the present invention is to increase the viewing angle in the horizontal and vertical directions. Therefore, the present invention relates to a liquid crystal display device having a liquid crystal display screen provided with a liquid crystal material birefringence compensation means on at least one of the surfaces, wherein the compensation means comprises a biaxial birefringent film having a non-uniform refractive index and a liquid crystal. A uniaxial birefringent film having an optical axis inclined with respect to the surface of the display screen.

The present invention improves the horizontal and vertical viewing angles by a combination of a biaxial film and a uniaxial film having an inclined optical axis. The invention also relates to the relative orientation of the components of the device. Various objects and features of the present invention will become apparent from the following description, by way of example, and the accompanying drawings.

The present invention combines a TN cell, a stretched biaxial plastic film, and a uniaxial birefringent film tilted in a direction different from the direction known by known compensation methods according to a Type II configuration. It is. Thus, the compensation mechanism is a composite compensation consisting of two components coming from different technologies: a stretched plastic film with so-called "in-plane" birefringent properties and positive or negative tilted birefringence. The direction of the optic axis is the direction of the optimized angle θ, the sign of which is determined by the direction in which the majority of the molecules are oriented with the cell energized (crossed by polarizers). Black in the case of TN liquid crystal). For example, if the majority of molecules are tilted + θ:-In the case of "dish-shaped" tilt, the optical axis is tilted in the positive direction of θ-In the case of "cigar-shaped" tilt In, the optical axis is inclined in the negative direction of θ. A "dish" type tilt is obtained by holographic or polymerized liquid crystal material. A "cigar" type tilt is obtained by a tilted or polymerized nematic type liquid crystal material.

The tilted birefringence improves contrast contours, which represent vertical viewing angles, while maintaining excellent horizontal reagents in cells with in-plane biaxial films. The advantage of this type of structure is that effective compensation can be achieved by combining birefringent properties, typically with a small slope of 40 nm, with commercially available 80 nm in-plane retardation biaxial films. It should be noted that the gradient birefringence characteristic is difficult to realize, and the smaller the gradient birefringence, the easier it is technically to realize.

FIG. 8 shows a simplified display device according to the present invention. The device comprises: a liquid crystal display screen (or cell) 1; two liquid crystal display screens (or cells) 2 provided on both sides of the liquid crystal display screen, each having an anisotropic refractive index.
Two biaxial films 2 and 2 '; Films 2 and 2' having an optical axis inclined with respect to the plane of the liquid crystal display screen
Uniaxial birefringent films 3 and 3 'in contact with: polarizing plates 4 and 4 provided on respective sides of films 3 and 3' in mutually crossing directions
Having '. FIG. 8 shows a preferred embodiment. However, the following configuration is also possible. The biaxial birefringent film 2 or 2 'or the uniaxial birefringent property 3 or 3' may be provided on both sides or one side of the liquid crystal display screen; Two uniaxial birefringent films may be used in place of the film; two biaxial birefringent films and one uniaxial birefringent film may be used. The basic concept of the present invention is to combine at least one biaxial birefringent film and at least one uniaxial birefringent film with a liquid crystal display screen. The components of the embodiment of the present invention and their relative directions will be described below.

FIG. 9 is an exploded perspective view of this type of device. The direction of the device is described according to the east-west and north-south directions described at the bottom of the figure. The liquid crystal display screen 1 has a twisted nematic liquid crystal layer. The liquid crystal material is sandwiched between two glass plates that have been rubbed so that their respective surfaces 11, 11 'in contact with the liquid crystal material are oriented to the liquid crystal molecules in contact. The rubbing direction of the surface 11 is −45 ° with respect to the east-west direction. For the same east-west direction, the rubbing direction of the surface 11 ′ is + 45 °. According to what is referred to in the art as type II, the polarizer 4 mounted on the surface 11 is at a 90 ° angle to the rubbing direction of the surface. Similarly, the polarizing plate 4 'is also at 90 degrees to the rubbing direction of the surface 11'. Therefore, the two polarizing plates are oriented at about 90 ° to each other.

In the biaxial birefringent film 2, the axis corresponding to the maximum refractive index is oriented in a direction perpendicular to the rubbing direction of the surface 11. Similarly, in the direction of the biaxial birefringent film 2 ', the axis corresponding to the maximum refractive index is orthogonal to the rubbing direction of the surface 11'. The direction of the uniaxial film 3 ′ is: the optical axis of the film is 2 ° perpendicular to the film surface
Make an angle θ _{0 in} the range of 5 ° to 60 °; the projection of the optical axis of the film onto the film plane is east-west and 90 ° to 135 °
Form an angle in the range The uniaxial film 3 'is as above: the optical axis of the film makes an angle θ _{0 in} the range of 25 ° to 60 ° with the normal to the film plane; the optical axis of the film 3 ′ Projection from east to west and 45 ° to 90 °
Make an angle in the range of °. According to a preferred embodiment of the present invention, the biaxial film is a stretched plastic film.

The uniaxial film is a holographic film in which grating is recorded in volume. Figure
10a and 10b show this kind of grating and recording method. This species
In this film, the optical axis is perpendicular to the plane of stacking of the refractive index. In this example, the optical axis of the film is at an angle θ of 35 ° with respect to the normal to the film. ₀ And the projection of the optical axis of the film overlaps the axis Oy of the film plane. FIG. 10a shows a grating in which two lights propagating from opposite directions interfere in the film.
The recording method of the log is shown. The direction of this light depends on the direction of the perpendicular to the stack to be obtained.
Matches. The wavelength of this light is shorter than the wavelength used for the display device.

According to one embodiment of the present invention, the device includes a POLATECHNO brand in-plane 2
Using the "dish" type film inclined manufactured based on biaxial film and the following characteristics: Polatechno Film - optical path length difference within the film _{plane, (n e - n 0)} × d = 80nm - film cross-section the optical path length _{difference, (n z - n 0)} × d = -128nm -2 axial _{ratio, r = (n z - n} 0) / (n e - n 0) = - 1.6 - maximum refractive index axis with the perpendicular to the rubbing direction (the range of a few degrees ± depending on the configuration) total optical path length difference in the holographic film -25μm thickness _{film, n e - n 0 = -40nm} - tilt angles, -.phi = 90 ° to the north and south, -θ ₀ = 33 ° (to the maximum distribution direction of the molecule)

An inclined “dish” is provided between the biaxial film and the polarizer on both sides of the cell. FIG. 7 shows contrast contours in this configuration. To optimize the configuration, it is necessary to adjust the polarizing plate slightly (about several degrees). This contrast contour is
Compared to the contrast contours using only the axial film, it can be seen that the vertical viewing angle is significantly expanded (+ 30 °, −40 °) with respect to 50 contours.

In order to explain the effects of the present invention, examples and their contrast contours will be described. In each of the examples shown below, the liquid crystal display cell has a T type having the above-described type II configuration.
N liquid crystal display cells. The first device is a biaxially stretched plastic film (POLATECHNO type)
Are conventional examples provided on both sides of a liquid crystal display screen. -Polarizing plate 135 °-First biaxial film 140 °-Type II TN cell-Second biaxial film 40 °-Analyzer 45 ° Contrast contours obtained by the above-mentioned device are shown in FIG.

The following three devices are devices according to the present invention, and have a biaxial plastic film and a uniaxial holographic film on each side of the liquid crystal display cell (see FIG. 9). In the embodiments, the angle φ formed by the projection of the optical axis of the holographic film onto the film surface is different from the east-west direction. Configuration 1-Polarizing plate 4 135 °-Holographic film 3 φ = 90 °-Biaxial plastic film 2 132 °-Type II TN cell-Biaxial plastic film 2 48 °-Holographic film 3 φ '= 90 °-Polarizing plate 45 ° For this device, the projection of the optical axis onto the holographic film plane is in the north-south direction. The performance of this device is shown in FIG. 11a.

Configuration 2-Polarizer 4 135 °-Holographic film 3 φ = 90 °-2-axis plastic film 2 135 °-TN cell of type II-2-axis plastic film 2 45 °-Holographic film 3 φ '= 110 ° -Polarizer 45 ° The performance of this device is shown in Figure 11b.

Configuration 3—Polarizing plate 4 135 ° —Holographic film 3 φ = 55 ° —Two-axis plastic film 2 135 ° —Type II TN cell—Two-axis plastic film 2 45 ° —Holographic film 3 φ ′ = 125 ° -Polarizer 45 ° The performance of this device is shown in Figure 11c.

[Brief description of the drawings]

FIG. 1 is a contrast contour diagram of a twisted nematic liquid crystal cell (type II).

2a and 2b show the refractive index ellipses of a "cigar type" (2a) and a "dish type" (2b) uniaxial medium.

FIG. 3 shows a liquid crystal cell provided with compensation means.

FIG. 4 shows a refractive index ellipse of a biaxial film.

FIG. 5 shows a liquid crystal cell comprising a biaxial plastic film known in the prior art.

FIG. 6 is a contrast contour map of a twisted nematic liquid crystal cell compensated by an 80 nm biaxial film.

FIG. 7 is a contrast contour map of a twisted nematic liquid crystal cell compensated by an 80 nm biaxial film and a dish film having a 40 nm slope.

FIG. 8 is a simplified embodiment of a display device according to the present invention.

FIG. 9 is a disassembled view of the display device according to the present invention.

FIGS. 10a and 10b are holographic films with gratings that can be used in the apparatus shown in FIG.

11a to 11c are contrast contours of a display device according to the invention.

Claims (18)

[Claims] 1. A liquid crystal display device comprising a liquid crystal display screen (1) having a liquid crystal material birefringence compensation means on at least one of its surfaces (10, 11). A liquid crystal display device comprising: an axial birefringent film (2); and a uniaxial birefringent film (3) having an optical axis inclined with respect to the liquid crystal display screen surface. 2. The liquid crystal display device according to claim 1, wherein the film having biaxial birefringence is a plastic film stretched in the same direction as the direction of the liquid crystal display screen. 3. The plastic film according to claim 2, wherein both the axis corresponding to the maximum refractive index and the axis corresponding to the minimum refractive index are in the plane of the film.
3. The liquid crystal display device according to 1. 4. The liquid crystal display device according to claim 1, wherein the film having an inclined birefringence characteristic is a holographic film having a layered grating in a volume. 5. The liquid crystal display device according to claim 4, wherein a distance between the grating layers is smaller than a shortest wavelength of light illuminating the screen. 6. A first surface (10) of the liquid crystal display device (1) is on a user side,
A plastic film (2) is provided on a first surface, and a film (3) having an inclined birefringence is provided on a second surface (11) facing the first surface. Item 6. The liquid crystal display device according to any one of Items 1 to 5. 7. A liquid crystal display (1) having a first surface (10) facing a user,
A film (3) having a tilted birefringence is provided on a first surface, and a plastic film (2) is provided on a second surface (11) facing the first surface. Item 6. The liquid crystal display device according to any one of Items 1 to 5. 8. The liquid crystal according to claim 6, wherein the plastic film is provided with a film on which a birefringent grating having an inclined birefringence is formed. Display device. 9. The liquid crystal display according to claim 6, wherein an extended plastic film having refractive anisotropy is provided on the film having inclined birefringence. apparatus. 10. The liquid crystal display screen according to claim 1, wherein the plastic film and the film having an inclined birefringence are provided on the first surface of the liquid crystal display screen on the user side. 6. The liquid crystal display device according to any one of 5. 11. A plastic film and a film having an inclined birefringence (3) are provided on a second surface of the liquid crystal display screen opposite to the user-side surface (10). The liquid crystal display device according to any one of claims 1 to 5, wherein 12. The liquid crystal display screen according to claim 1, wherein the liquid crystal of the liquid crystal display screen is a twisted nematic liquid crystal, and the polarizing plates are provided on both sides of the display screen so that the polarization directions cross each other. The liquid crystal display device as described in the above. 13. The liquid crystal display device according to claim 12, wherein the liquid crystal display screen has a type II configuration, and the polarizing plate sandwiches a unit including the liquid crystal display screen and the compensating means. 14. The liquid crystal display device according to claim 13, wherein the axis of the birefringence of each of the stretched films is substantially parallel to the polarization direction of the polarizing plate on the same side. 15. The liquid crystal display device according to claim 14, wherein the degree of parallelism has a deviation of less than 10 °. 16. The holographic film (3) is such that the perpendicular to the laminating surface constituting the holographic film layer is 25 ° to 60 ° with respect to the perpendicular to the liquid crystal display screen surface.
The liquid crystal display device according to claim 1, wherein the angle is in the range of (1). 17. The liquid crystal display screen, wherein the maximum distribution direction of the liquid crystal molecules in contact with the liquid crystal container is substantially horizontal and forms an angle of 45 ° with the rubbing direction of the inner surface of the container, and the rubbing direction of the first surface is horizontal. And the direction of rubbing of the second surface is + 45 ° with respect to the horizontal direction, and is perpendicular to the laminating surface of the holographic film provided with respect to the first surface with respect to the liquid crystal screen. 17. The liquid crystal display device according to claim 16, wherein the projection makes an angle ([phi]) of 90. +-. 45.degree. With respect to the horizontal direction. 18. The liquid crystal display screen, wherein a projection of a perpendicular to the laminating surface of the holographic film provided on the first surface on the first surface forms an angle of 45 ° ± 90 ° with respect to the horizontal direction. The projection of the perpendicular to the laminating plane of the holographic film provided for the second surface onto the second surface is 9 relative to the horizontal.
The liquid crystal display device according to claim 17, wherein the angle ranges from 0 ° to 135 °.