JPH04140721A - Liquid crystal display device - Google Patents

Abstract

PURPOSE:To obtain a high contrast and excellent visual angle characteristics by forming both 1st and 2nd phase difference plates for color compensation by drawing high polymer films. CONSTITUTION:A phase difference plate 12 which has a negative characteristic birefringence value is arranged above an STN type liquid crystal panel 13 and a phase difference plate 14 which has a positive characteristic birefringence value is arranged blow the panel; and those area arranged between a couple of polarizing plates, i.e. an upper polarizing plate 11 and a lower polarizing plate 15. A reflecting plate 16 for observing the display device as a reflection type is arranged below the lower polarizing plate 15. The phase difference plates 12 and 14 are made of a material with the positive characteristic birefringence value such as polycarbonate resin and cellulose diacetate resin and a material with the negative refractive index such as acrylic resin and styrene resin.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a birefringent liquid crystal display device that is made colorless by compensating for coloring with a retardation plate.

[Conventional technology]

This is an improved type of twist-type liquid crystal display device in which a nematic liquid crystal panel with a twist angle of 90° is interposed between a pair of polarizing plates.By increasing the twist angle to about 270°, the change in light transmittance with respect to the driving voltage can be made steeper. This is known as a 5TN (super twisted nematic) type liquid crystal display device (for example, U.S. Pat. No. 4,
634゜229).

In this STN type liquid crystal display device, the polarizing plate is arranged so that the polarization direction intersects with the orientation direction of the liquid crystal molecules, and high contrast is obtained by the resulting birefringence effect.

By the way, this birefringence effect has wavelength dependence, so
When white light passes through a birefringent liquid crystal display device, there is a drawback that the transmitted light is colored.

Therefore, in order to perform full-color display by combining color filters of the three primary colors R, G, and B with this liquid crystal display device, it is necessary to neutralize the coloring caused by the wavelength dependence of the birefringence effect described above. .

As a method of making the coloring in the STN type liquid crystal display device colorless, an optically anisotropic layer is arranged together with the STN type liquid crystal panel between a pair of polarizing plates, and the birefringence effect of the latter compensates for the effect of the former, making it colorless. It has been proposed to do so (see, for example, U.S. Pat. No. 4,844,569).

In this case, a second twisted liquid crystal panel or a retardation plate is used as the optically anisotropic layer, but a retardation plate made of a stretched polymer film is more advantageous in terms of cost.

When a polymer film is uniaxially stretched or anisotropically biaxially stretched, the polymer is oriented intensively in the direction of the largest stretching ratio, that is, the main stretching direction, resulting in a retardation plate that exhibits optical anisotropy. In this case, the refractive index in the stretching direction is ne, and the refractive index in the direction perpendicular to the stretching direction within the film plane, that is, the width direction, is no.
Then, the value of ne no −Δn is called refractive index anisotropy or orientational birefringence. This value is determined by the optical anisotropy, that is, the intrinsic birefringence value, of the monomer that is the constituent unit of the polymer film, and the degree of orientation of the polymer. In an alignment film formed by stretching a polymer material whose refractive index in the molecular axis direction is larger than the refractive index in the direction perpendicular to the molecular axis, that is, a polymer material having a positive intrinsic birefringence value, n@) no.

On the other hand, in the case of an alignment film formed by stretching a polymer material having a negative intrinsic birefringence value, ns<no.

The product Δn−d of the refractive index anisotropy Δn and the film thickness d is called the optical path difference. Further, the main stretching direction may be referred to as the optical axis for convenience.

The product Δn− of the refractive index anisotropy Δn of the liquid crystal and the thickness d of the liquid crystal layer
If the optical path difference Δn −, d of the retardation plate is optimally balanced with respect to d, and the optical axis of the retardation plate is placed at an optimal angle with respect to the alignment direction of liquid crystal molecules, the liquid crystal panel can be Coloring due to refractive effects can be compensated for and the liquid crystal device can be made colorless.

The above is a description of the case where the liquid crystal display device is observed from the front, but when viewed from an oblique direction, both the values of Δn-d of the liquid crystal layer and the optical path difference Δn-cl of the retardation plate change, causing coloration. The terms of compensation will be lost. In this case, Δn-d of the retardation plate is compared to Δn-d of the liquid crystal layer.
In order to eliminate the viewing angle dependence of the display device, it is necessary to eliminate the viewing angle dependence of the optical path difference of the retardation plate as much as possible, or to change the viewing angle dependence of Δn-d of the liquid crystal layer by changing the phase difference. It is necessary to adjust the viewing angle dependence of the plate.

From this point of view, a method of stacking a plurality of retardation plates such that their optical axes are oriented in a twisted manner (Japanese Patent Application Laid-Open No. 2-5
3025) has been proposed. However, in order to completely match the viewing angle dependence of the optical path difference Δn-d of the retardation plate to the viewing angle dependence of Δn-d of the liquid crystal layer, 3 to 5 retardation plates are required, which reduces the cost. However, another disadvantage is that the light transmittance decreases. On the other hand, although it is possible to achieve some degree of alignment with two retardation plates, the birefringence effect of the liquid crystal panel cannot be completely compensated for, resulting in a decrease in contrast.

It has also been proposed to completely compensate for the birefringence effect of the liquid crystal panel by arranging retardation plates with positive intrinsic birefringence values on both sides of the liquid crystal layer. It was not possible to achieve both contrast and viewing angle dependence.

Additionally, a method has been proposed in which two types of retardation plates with positive and negative refractive index anisotropy are stacked so that their stretching directions are parallel to each other (Japanese Patent Application Laid-open No. 2-67518). In such a parallel arrangement, the refractive index anisotropy Δn of the retardation plates mutually decreases, so a very large film thickness d is required to compensate for Δn-ct of the liquid crystal layer, which is not practical.

[Problems to be Solved by the Invention 2] Therefore, the purpose of the present invention is to completely compensate for the birefringence effect of a liquid crystal panel even by combining only two retardation plates, and to further reduce the Δn-d of the liquid crystal layer. An object of the present invention is to provide a colorless STN liquid crystal display device with high contrast and good viewing angle characteristics by completely matching the viewing angle dependence and the viewing angle dependence of the optical path difference of a retardation plate.

[Means to solve the problem]

In order to achieve the above object, the present invention provides a first film having a positive intrinsic birefringence that is created by stretching a polymer film.
A retardation plate having a negative intrinsic birefringence and a second retardation plate having a negative intrinsic birefringence were arranged on both sides of the STN liquid crystal panel, and both were arranged between a pair of polarizing plates.

[Effect]

By placing a pair of retardation plates on both sides of the liquid crystal panel, the coloring phenomenon caused by the wavelength dependence of the liquid crystal panel's birefringence is effectively eliminated, resulting in a virtually colorless black and white display with high contrast. It is realized by Furthermore, by arranging a retardation plate with a positive intrinsic birefringence on one side and a retardation plate with a negative intrinsic birefringence on the other side, viewing angle dependence can also be minimized. This occurs because the change in optical path difference due to viewing angle of a retardation plate with a positive intrinsic birefringence and the change in optical path difference due to viewing angle of a retardation plate with a negative intrinsic birefringence cancel each other out. It's for a reason.

〔Example〕

Here, an embodiment of a liquid crystal display device according to the present invention will be described with reference to FIGS. 1 and 2.

FIG. 1 is a developed perspective view showing the structure of an embodiment of a liquid crystal display device according to the present invention.

A retardation plate 12 having a negative intrinsic birefringence value is disposed above the STN liquid crystal panel 13, and a retardation plate 14 having a positive intrinsic birefringence value is disposed below. It is arranged between the upper polarizing plate 11 and the lower polarizing plate 150. A reflective plate 16 is arranged below the lower polarizing plate 15 for viewing the display device as a reflective type.

FIG. 2 is a front view showing the mutual relationship of the optical orientations of the constituent members shown in FIG. 1.

The liquid crystal panel 13 has a positive dielectric anisotropy, and a nematic liquid crystal with a refractive index anisotropy Δn=0.121 has a liquid crystal layer thickness d,
This is an STN type liquid crystal panel oriented to have a left-handed helical twist structure with a twist angle of 240° between = 7.0 μm, and its Δnd value is 84.7 mm.

In FIG. 2, a dashed-dotted line 21 indicates the orientation direction of liquid crystal molecules in the upper part of the liquid crystal layer, and a dashed-dotted line 22 indicates the orientation direction of the liquid crystal molecules in the lower part of the liquid crystal layer. The twist angle between the upper and lower orientation directions 2L22 is 24. O'', and the direction that divides this twist angle into two is the Y axis.
Determine Cartesian coordinates.

The optical path difference Δn-d of the negative retardation plate 12 is -400 nm,
Its main stretching direction is in the direction of the straight line 26, and the angle θ between it and the X axis is set to 60°.

The optical path difference of the positive retardation plate 14 is +400 nm, its main stretching direction is directed toward the straight line 24, and the angle θ2 with respect to the X axis is set to 70''.

The polarization axis of the upper polarizing plate 11 is set in the direction of the straight line 25, and the angle P between it and the X axis is set to 90''.

The polarization axis of the lower polarizing plate 15 is set in the direction of the straight line 26, and the angle P2 between it and the X axis is set to 8 m.

Here, the material of the retardation plate of the present invention will be explained. Materials with positive intrinsic birefringence include polycarbonate resins, cellulose diacetate resins, polyphenylene oxide resins, polyester resins such as polyethylene terephthalate, and polyvinyl alcohol resins, and materials with negative intrinsic birefringence include poly(meth)acrylic acid ester resins and polymers such as polyethylene. A retardation plate made of a transparent homopolymer copolymer such as a resin of a saturated aromatic compound, or a polymer alloy or a polymer alloy containing these as a main component can be used.

Particularly preferred are polycarbonate resins with excellent transparency, cellulose diacetate resins, acrylic resins containing ester methacrylate as a main component, and styrene resins containing styrene as a main component.

Next, FIG. 3 shows a viewing angle characteristic diagram obtained by the present invention. For comparison, FIG. 4 shows a viewing angle characteristic diagram when phase difference plates having positive intrinsic birefringence and having the same optical path difference are arranged on both sides. However, in order to be optically equivalent, the main stretching direction of the retardation plate is the main stretching direction 23 in FIG.
It is shifted by 90 degrees.

3 and 4 show viewing angle ranges in which the same contrast can be obtained, and the larger the shape, the better the viewing angle characteristics. Compared to the conventional FIG. 4, FIG. 3 of the present invention shows a much wider viewing angle range, and it can be seen that the viewing angle characteristics are improved.

Furthermore, when the contrast was measured, a value of 401 or higher was obtained, confirming that the birefringence effect of the liquid crystal panel was completely compensated for.

In addition, in this example, a retardation plate with a negative intrinsic birefringence was placed above the liquid crystal panel, and a retardation plate with a positive intrinsic birefringence was placed below. Similar results were obtained even when a retardation plate having an intrinsic birefringence was placed below the retardation plate having a negative intrinsic birefringence.

〔Effect of the invention〕

According to the present invention, by using only two retardation plates,
The birefringence effect of the liquid crystal panel is completely compensated for, and the viewing angle dependence of Δn-d of the liquid crystal panel and the viewing angle dependence of the optical path difference of the retardation plate can be matched, resulting in a black and white display with high contrast and good viewing angle characteristics. An STN liquid crystal panel can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a developed perspective view showing the configuration of an embodiment of the liquid crystal display device according to the present invention, and FIG. 2 shows the mutual relationship of optical orientations of each component in the liquid crystal display device of the present invention shown in FIG. In the front section, the third part is a viewing angle characteristic diagram of the liquid crystal display device according to the present invention, and FIG. 4 is a conventional viewing angle characteristic diagram. 11... Upper polarizing plate, 12... Negative retardation plate, 16... Liquid crystal panel, 4... Positive retardation plate, 5... ...Lower polarizing plate, 6...Reflector, 1...Upper liquid crystal molecule alignment direction, 2...
Lower liquid crystal molecule alignment direction, 3...Stretching direction of negative retardation plate, 4...Stretching direction of positive retardation plate, 5
...... Upper polarizing plate polarizing axis, 6...... Lower polarizing plate polarizing axis. Figure 1 top

Claims (4)

[Claims] (1) A liquid crystal panel consisting of a nematic liquid crystal layer having positive dielectric anisotropy and a pair of electrode substrates that sandwich the liquid crystal layer, and a pair of retardation plates for color compensation arranged on both sides of the liquid crystal panel. and a pair of polarizing plates disposed on both sides of the pair of retardation plates, wherein the retardation plate includes a first retardation plate having a positive intrinsic birefringence value and a first retardation plate having a negative intrinsic birefringence value. It consists of a combination of two retardation plates, the second retardation plate having a birefringence value, and the first and second retardation plates are both made by stretching a polymer film. LCD display device. (2) The liquid crystal display device according to claim 1, wherein the retardation plate is a uniaxially stretched polymer film or an anisotropically biaxially stretched polymer film. (3) The liquid crystal display device according to claim 1 or 2, wherein the first retardation plate is a stretched film of polycarbonate resin or cellulose diacetate resin. (4) The liquid crystal display device according to claim 1, 2 or 3, wherein the second retardation plate is a stretched film of acrylic resin or styrene resin.