JPH0713151A - Liquid crystal display device - Google Patents

Abstract

PURPOSE:To obtain an achromatic high contrast display. CONSTITUTION:A liquid crystal element 6 is disposed between polarizing plates 2, 3, and two phase plates 4, 5 are disposed between the polarizing plate 2 and the liquid crystal element 6. The wavelength dispersion B of the phase plates 4, 5 is 50 (30<=B<=250) of double refractive anisotropy DELTAn, and the optical axes of the phase plates make 45 deg. angle (5 deg.<=phi<=60 deg.). By this method, the polarized state of light is made same to give a high compensation effect. Thus, achromatic and high contrast display can be obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal display device used as a display means for a word processor, a personal computer or the like, and more particularly to a super twisted nematic liquid crystal display device.

[0002]

2. Description of the Related Art FIG. 10 is a sectional view showing the structure of a conventional normally white display type liquid crystal display device. Figure 10
(1) shows a super twisted nematic (hereinafter abbreviated as "STN") type liquid crystal display device 21 using a polarizing plate for color compensation, and FIG. 10 (2) shows an STN type liquid crystal display using a retardation plate. Device 33 is shown.

The liquid crystal display device 21 includes polarizing plates 22 and 23 and a liquid crystal element 24. The liquid crystal element 24 is the polarizing plate 22.
And the polarizing plate 23. In the liquid crystal display device 21, one of the polarizing plate 22 and the polarizing plate 23 serves as a color compensating polarizing plate. The liquid crystal element 24 is the transparent substrate 2
5, 26, transparent electrodes 27, 28, alignment films 29, 30 and a liquid crystal layer 31. The transparent substrates 25 and 26 are realized by glass, for example. A liquid crystal layer 31 is arranged between the substrates 25 and 26. The liquid crystal layer 31 includes the substrates 25 and 26.
It is realized with a super twisted nematic liquid crystal material in which liquid crystal molecules are twisted by 240 °. Transparent electrodes 27, 28 and alignment films 29, 30 are formed on the surfaces of the substrates 25, 26 on the liquid crystal layer 31 side, respectively. Transparent electrodes 27, 2
8 are realized by ITO (Indium Tin Oxide), for example, and are formed in a plurality of strips and parallel to each other. Further, the transparent electrodes 27 and 28 are arranged so as to be orthogonal to each other. Alignment films 29 and 30 are further formed on the liquid crystal layer 31 side surfaces of the transparent substrates 25 and 26 on which the transparent electrodes 27 and 28 are formed, respectively. Alignment film 29, 30
Is realized by, for example, a polyimide resin, and its surface is subjected to an alignment treatment such as a rubbing treatment.

On the other hand, the liquid crystal display device 33 includes a polarizing plate 22,
23, a liquid crystal element 24 and a retardation plate 32. Similar to the liquid crystal display device 21, a liquid crystal element 24 is arranged between the polarizing plates 22 and 23, and a retardation plate 32 is arranged between the liquid crystal element 24 and the polarizing plate 22, for example.

For example, linearly polarized light generated by passing through one polarizing plate on the incident side becomes elliptically polarized light that varies depending on the wavelength due to the birefringence effect of the liquid crystal when passing through the liquid crystal layer. Since this elliptically polarized light passes through the other polarizing plate on the emission side, the amount of transmitted light varies depending on the wavelength, and the display is colored. The liquid crystal display device 21 compensates the coloring with a color compensating polarizing plate, and the liquid crystal display device 33 includes a retardation plate 32.
To compensate.

FIG. 11 is a diagram showing a general spectral transmittance characteristic of a color compensating polarizing plate. A curve L1 shows the characteristics of one color compensation polarizing plate, and a curve L2P shows the characteristics when the optical axes of the two color compensation polarizing plates are arranged in parallel.
2C shows the characteristic when the optical axes of the two polarizing plates for color compensation are arranged so as to be orthogonal to each other. The transmittance at each wavelength is 1
The case is the highest, and the case where the two are arranged orthogonally is the lowest. As the above-mentioned color compensation polarizing plate, one having such characteristics is generally used.

The retardation plate 32 is made, for example, by stretching polycarbonate. The retardation value of the retardation plate prepared by stretching the polycarbonate is about 600 nm. The birefringence anisotropy Δn is 0.008, and the wavelength dispersion B of the birefringence anisotropy Δn is 360.

[0008]

In the liquid crystal display device 21 described above, the polarizing plate for color compensation having the characteristics shown in FIG. 11 is used. In this case, when the white color that is the background color is displayed, the color compensation effect is obtained and the achromatic white display is obtained, but when the black color that is the lighting color is displayed, the sufficient color compensation effect is not obtained. The display will be colored. Therefore, there arises a problem that good display characteristics cannot be obtained, such as a decrease in contrast ratio. For example,
The contrast ratio of such a liquid crystal display device is about 1: 3.
~ 1: 4.

On the other hand, in the liquid crystal display device 33, both the background color and the lighting color are close to the achromatic color, but a sufficient achromatic color cannot be obtained in the lighting color. The achromatic color is CIE.
In chromaticity coordinates (L *, a *, b *), it means a region where a * ≦ 5 and b * ≦ 5. Further, the lightness L * is related to the contrast ratio, and in order to obtain an achromatic color and a high contrast ratio, L *, a *, b *
Needs to be small. The liquid crystal display device 33 has an L
Although * is about 25 and | a * | · | b * | ≦ 5, it is relatively | b * |> | a * |. Therefore, a sufficient compensation effect cannot be obtained at the time of displaying the black color which is the lighting color, and especially on the short wavelength side, it is not sufficient, and the display becomes bluish. Therefore, similar to the liquid crystal display device 21, there arises a problem that good display characteristics cannot be obtained, such as a decrease in contrast ratio. For example, the contrast ratio of such a liquid crystal display device is about 1: 5 to 1: 6.
Is. Further, when it is used as a transmissive type, there is a problem that the visibility is lower than that of the reflective type.

An object of the present invention is to provide a liquid crystal display device which can obtain an achromatic color and a high contrast display.

[0011]

The present invention is directed to a pair of polarizing plates, a super twisted nematic liquid crystal element arranged between the pair of polarizing plates, and one of the pair of polarizing plates. Between the liquid crystal element and the liquid crystal element, the two retardation plates arranged in a laminated manner, the two retardation plates having the wavelength dispersion B of the birefringence anisotropy Δn,
A liquid crystal display device characterized by being selected in the range of 30 ≦ B ≦ 250, and the angle φ formed by each optical axis of the two retardation plates being set in the range of 5 ° ≦ φ ≦ 60 °. is there.

[0012]

According to the present invention, a liquid crystal element is arranged between a pair of polarizing plates, and two retardation plates are laminated between one of the pair of polarizing plates and the liquid crystal element. Will be placed. The wavelength difference B of the birefringence anisotropy Δn of both of the two retardation plates is selected within the range of 30 ≦ B ≦ 250.
The angle φ formed by each optical axis of the one retardation plate is 5 ° ≦ φ ≦ 60.
Set to the range of °.

In the STN type liquid crystal display device, the light which is incident as linearly polarized light has a birefringence anisotropy Δn of the liquid crystal material in the liquid crystal layer.
Is affected by the chromatic dispersion B and the optical rotatory dispersion, and becomes elliptically polarized light when emitted. Therefore, the display is colored. The liquid crystal display device of the present invention has a blue (450
nm), green (550 nm), and red (650 nm) light can have the same polarization state (azimuth angle of elliptically polarized light, ellipticity). From this, it was confirmed that, as compared with the conventional liquid crystal display device of the retardation film type, the color compensation effect was increased, and a relatively achromatic color and high contrast display was obtained. Therefore, reflective display, transmissive display, and gradation display with high display quality can be obtained.

[0014]

1 is a sectional view showing the structure of a transmissive liquid crystal display device 1 according to a first embodiment of the present invention. The liquid crystal display device 1 is a normally white type, that is, operates by a white display when not selected and a black display when selected, and includes the polarizing plates 2 and 3, the retardation plates 4 and 5, the liquid crystal element 6 and the back. Includes light 7. A liquid crystal element 6 is arranged between the polarizing plates 2 and 3, and retardation plates 4 and 5 are arranged between the liquid crystal element 6 and the polarizing plate 2, for example. The retardation plate 4 is disposed on the polarizing plate 2 side, and the retardation plate 5 is the liquid crystal element 6
Placed on the side. A backlight 7 is arranged on the side of the polarizing plate 3 opposite to the liquid crystal element 6.

The polarization degrees of the polarizing plates 2 and 3 are 99.95.
%, And a single transmittance of 41.5% is selected, and is realized by, for example, NPF-G1220Du (manufactured by Nitto Denko Corporation). The phase difference plates 4 and 5 are both birefringent anisotropy Δn.
Is selected as 0.006, the wavelength dispersion B of the birefringence anisotropy Δn is selected as 50, and the retardation value Re at a wavelength of 550 nm is selected as 390 nm, which is realized by, for example, stretching polyvinyl alcohol. .

The liquid crystal element 6 includes transparent substrates 8 and 9, transparent electrodes 10 and 11, alignment films 12 and 13, and a liquid crystal layer 14. The transparent substrates 8 and 9 are realized by glass, for example. A liquid crystal layer 14 is arranged between the substrates 8 and 9. The liquid crystal layer 14 is realized by a super twisted nematic liquid crystal material in which liquid crystal molecules are twisted by 240 ° between the substrates 8 and 9. The refractive index anisotropy Δn of this liquid crystal material and the liquid crystal layer 14
The product d · Δn with the thickness d of is selected to be 0.84 μm.
The liquid crystal material is realized by a mixture of cyclohexane liquid crystal, tolan liquid crystal, biphenyl liquid crystal, pyrimidine liquid crystal and the like.

Transparent electrodes 10 and 11 are formed on the liquid crystal layer 14 side surfaces of the substrates 8 and 9, respectively. Transparent electrode 10,
Reference numerals 11 are realized by, for example, ITO, and are formed in a plurality of strips and parallel to each other. In addition, the transparent electrode 1
0 and the transparent electrode 11 are arranged so as to be orthogonal to each other. Alignment films 12 and 13 are further formed on the liquid crystal layer 14 side surfaces of the substrates 8 and 9 on which the transparent electrodes 10 and 11 are formed. The alignment films 12 and 13 are made of, for example, a polyimide resin, and the surfaces thereof are subjected to alignment treatment such as rubbing treatment.

FIG. 2 is a diagram for explaining the positional relationship of the liquid crystal display device 1. The solid line R1 indicates the absorption axis of the polarizing plate 2, the solid line R2 indicates the optical axis (stretching axis) of the retardation plate 4, the solid line R3 indicates the optical axis (stretching axis) of the retardation plate 5, and the solid line R4 indicates the transmission axis. A solid line R5 indicates the alignment axis of the liquid crystal molecules on the light-transmitting substrate 8 side, and a solid line R5 indicates the alignment axis of the liquid crystal molecules on the light-transmitting substrate 9 side.
Reference numeral 6 denotes the absorption axis of the polarizing plate 3.

The angle α represents the angle formed by the alignment axis R4 of the liquid crystal molecules on the transparent substrate 8 side and the optical axis (stretching axis) R2 of the retardation plate 4, and the angle β is on the transparent substrate 8 side. The angle between the alignment axis R4 of the liquid crystal molecules and the absorption axis R2 of the polarizing plate 2 is shown,
The angle γ represents the angle formed by the alignment axis R5 of the liquid crystal molecules on the transparent substrate 9 side and the absorption axis R6 of the polarizing plate 3. Furthermore, the angle θ
Denotes an angle formed by the alignment axis R5 of the liquid crystal molecules on the transparent substrate 9 side and the alignment axis R4 of the liquid crystal molecules on the transparent substrate 8 side, and the angle φ
Indicates the angle between the optical axis (stretching axis) R2 of the retardation plate 4 and the optical axis (stretching axis) R3 of the retardation plate 5.

In the liquid crystal display device 1 of this embodiment, the above angles were set to the values shown in Table 1 below.

[0021]

[Table 1]

In the liquid crystal display device 1 of this embodiment, the wavelength dispersion B of the birefringence anisotropy Δn of the retardation plates 4 and 5 is selected to be 50, but the wavelength dispersion B is in the range of 30 ≦ B ≦ 250. I wish I had it. Further, although the angle φ formed by the optical axes of the phase difference plates 4 and 5 is set to 45 °, the angle φ is 5 ° ≦
It may be in the range of φ ≦ 60 °.

The above-mentioned chromatic dispersion B is expressed by the following equation (1).
(Cauchy's formula).

[0024]

[Equation 1]

Where Δn is birefringence anisotropy, and λ
Is the wavelength and A is a constant. This formula (1) is shown in FIG.
The constant A and the chromatic dispersion B are obtained based on the characteristic curve shown in FIG. For example, in FIG. 3, reference symbols L3 and L
In the case of the characteristic curves represented by 4 and L5 respectively, the reference symbol L3
The wavelength dispersion B is the largest, and the wavelength dispersion L5 is the smallest.

The range of the chromatic dispersion B and the range of the angle φ of the present invention are obtained by a simulation based on the following theory of and.

Berreman's 4 × 4 matrix method (DW Berreman, J. Opt. Soc.
m. , 62, 502 (1972)) Ericksen, Leslie's formula (BW Be.
rreman, J .; Appl. Phys. , 46, 37
46 (1975)) By using the above and the theory of the above, the transmittance at each wavelength can be obtained, and the contrast ratio and the blackness BL can be obtained from this transmittance. The blackness BL is BL = 100 − {(L *) ² +
(U *) ² + (v *) ² } ^1/2 is a value representing the blackness, and BL = 100 is an ideal value. Generally, B
If L is 60 or more, good display can be obtained. The L *, u *, and v * are values that represent CIE chromaticity coordinates.

FIG. 4 shows the wavelength dispersion B of the birefringence anisotropy Δn of the retardation plate obtained by the simulation.
FIG. 3 is a diagram showing the relationship between the contrast ratio and the blackness BL of the liquid crystal display device. Solid line L6 indicates chromatic dispersion B
And the contrast ratio, and the broken line L7 shows the relationship between the chromatic dispersion B and the blackness BL. The simulation results shown in FIG. 4 are obtained by setting the wavelength dispersion B of the two retardation plates to be the same and setting the angle φ formed by each optical axis to 45 °.

At present, examples of the material used as the retardation plate include polycarbonate, polyvinyl alcohol, polystyrene, polyarylate and the like. Wavelength dispersion B of polycarbonate, polystyrene, polyarylate
Is 300 to 400, and the wavelength dispersion B of polyvinyl alcohol is 50. As described above, the range of wavelength dispersion is limited, for example, the wavelength dispersion B is between 50 and 300, and no suitable material for a retardation plate has been found. However, the contrast ratio and the blackness BL in each wavelength dispersion B are obtained by the above-mentioned simulation.

From the results shown in FIG. 4, it can be seen that the contrast ratio and the blackness BL decrease as the chromatic dispersion B increases. From this, the wavelength dispersion B (300 to 400) of the conventionally used retarder material
In comparison with the range of chromatic dispersion B according to the present invention 30 ≦ B
When ≦ 250, contrast ratio and blackness BL
It is considered that an improved display of is obtained.

FIG. 5 is a diagram showing the relationship between the contrast ratio and the angle φ formed by the optical axes of the two phase difference plates obtained by the simulation. In the simulation result shown in FIG. 5, the angle α, that is, the angle α formed between the alignment axis R4 of the liquid crystal molecules on the transparent substrate 8 side and the optical axis R2 of the retardation plate 4 is set to 125 °, and two sheets are used. The wavelength dispersion B of the retardation plate of No. 2 was obtained as 50. When the angle φ formed by each optical axis of the retardation plate is increased from 5 °, the contrast ratio increases, and the maximum contrast ratio is obtained when the angle φ is around 50 °. When the angle φ is further increased, the contrast ratio is lowered. As described above, when the angle φ of the phase difference plates 4 and 5 is in the range of 5 ° ≦ φ ≦ 60 °, it is considered that display with a high contrast ratio can be obtained.

Next, the measurement result of the contrast ratio of the liquid crystal display device 1 of this embodiment will be described. FIG. 6 is a plan view showing the display screen 15 of the liquid crystal display device 1. A plurality of pixels 16 are formed on the display screen 15 of the liquid crystal display device 1. The pixel 16 includes the transparent electrodes 10 and 11 of the liquid crystal element 6.
Corresponds to the area where When the normally white type display of the present embodiment is performed in the transmissive type display, light leaks from the region between the plurality of pixels 16 shown by hatching, and the contrast ratio decreases. In particular, the effect of light leakage during black display is great, and the contrast ratio is significantly reduced. For this reason, a shading means generally called a black matrix is provided in the area. Such light shielding means is formed by a lift-off process of MnO ₂ or etching of Mo.

As the back light 7, a cold cathode tube having an emission spectrum shown in FIG.
When driven with 00 duty and 1/13 bias,
When the light shielding means is not provided, the contrast ratio is 1:
4 was obtained, and a contrast ratio of 1:10 was obtained when the light shielding means was provided. The contrast ratio in the case where the light transmitting means is provided is significantly higher than the contrast ratio (1: 5 to 1: 6) of the conventional liquid crystal display device using one retardation plate.

FIG. 8 is a sectional view showing the structure of a reflective liquid crystal display device 17 according to the second embodiment of the present invention. The liquid crystal display device 17 is configured almost the same as the liquid crystal display device 1, but a reflector 18 is provided in place of the backlight 7. The reflection plate 18 is realized, for example, by depositing aluminum on the surface of an insulating substrate. Further, as the polarizing plates 2 and 3, for example, the polarization degree is 99.5%,
A single transmittance of 44.5% is selected, for example NPF.
-G1225Du (manufactured by Nitto Denko Corporation) is used. This liquid crystal display device 17 is halved in the same manner as in the above embodiment.
When driven with 00 duty and 1/13 bias,
A contrast ratio of 1:13 was obtained, and the blackness BL of the lighting color (black display) was 89. The values of the contrast ratio and the blackness BL are values when the light shielding unit is not provided.

FIG. 9 is a sectional view showing the structure of a transflective liquid crystal display device 20 according to the third embodiment of the present invention. The liquid crystal display device 20 has a structure similar to that of the liquid crystal display device 1, but a transflective plate 19 is provided between the polarizing plate 3 and the backlight 7. The semi-transmissive reflection plate 19 is realized by, for example, forming a plate-shaped mixture of synthetic resin and scattering particles. As the polarizing plates 2 and 3, the same ones as used in the liquid crystal display device 17 are used. When this liquid crystal display device 20 was driven with 1/200 duty and 1/13 bias as in the above embodiment, a contrast ratio of 1:13 was obtained during reflective display and a contrast ratio of 1: 3. 5 was obtained. The contrast ratio in this embodiment is a value when the light shielding means is not provided.

As described above, the liquid crystal display devices 1, 17 and 20 according to the present invention use the two retardation plates 4 and 5,
The wavelength dispersion B of the birefringence anisotropy Δn of the phase difference plates 4 and 5 is 3
50 is selected from the range of 0 ≦ B ≦ 250, and the angle φ formed by the optical axes of the phase difference plates 4 and 5 is set to 45 ° from the range of 5 ° ≦ φ ≦ 60 °. By this, blue (450 nm), green (550 nm), red (650 n) at the time of lighting (black display)
The polarization state (azimuth angle, ellipticity of elliptically polarized light) of light m) is the same, a high compensation effect is obtained, and an achromatic display with a high contrast ratio is obtained.

The wavelength dispersions B of the phase difference plates 4 and 5 do not have to coincide with each other, and if the wavelength dispersions B of the respective phase difference plates 4 and 5 are both within the range of 30 ≦ B ≦ 250. The effect of the present invention can be obtained, and an achromatic and high-contrast display can be obtained. This is because, for example, polyvinyl alcohol (wavelength dispersion B = 50) is used as the retardation plate 4, and polycarbonate (wavelength dispersion B =) is used as the retardation plate 5.
360) was used to produce a liquid crystal display device, a liquid crystal display device using polyvinyl alcohol for both the retardation plates 4 and 5 and a liquid crystal display device using polycarbonate for both the retardation plates 4 and 5 were almost intermediate. Since it was visually confirmed that the display was obtained, it is considered that the effect of the present invention can be obtained even in the case of setting as described above. That is, for example, the wavelength dispersion B of the phase difference plate 4 may be 30, and the wavelength dispersion B of the phase difference plate 5 may be 30. The wavelength dispersion B of the phase difference plate 4 may be 30 and the wavelength dispersion B of the phase difference plate 5 may be The wavelength dispersion B of the phase difference plate 4 may be set to 200, and the wavelength dispersion B of the phase difference plate 5 may be set to 30.

Further, in this embodiment, the liquid crystal element 6 and the polarizing plate 2 are used.
Although the example in which the retardation plates 4 and 5 are arranged between the liquid crystal element 6 and the polarizing plate 3 has been described, the example in which they are arranged between the liquid crystal element 6 and the polarizing plate 3 also belongs to the scope of the present invention.

[0039]

As described above, according to the present invention, the wavelength dispersion B of the birefringence anisotropy Δn of the two retardation plates arranged between the polarizing plate and the liquid crystal element is 30 ≦ B. The angle φ formed by each optical axis is set in the range of 5 ° ≦ φ ≦ 60 °. Therefore, the polarization state (azimuth angle and ellipticity of elliptically polarized light) of light of each wavelength in black display is the same, and a high compensation effect is obtained. Therefore, a relatively achromatic color and high contrast display can be obtained, and high display quality can be obtained in reflective display, transmissive display, gradation display, and the like.

[Brief description of drawings]

FIG. 1 is a sectional view showing a configuration of a transmissive liquid crystal display device 1 according to a first embodiment of the present invention.

FIG. 2 is a diagram for explaining a positional relationship of the liquid crystal display device 1.

FIG. 3 is a diagram showing a relationship between wavelength λ and birefringence anisotropy Δn.

FIG. 4 is a diagram showing the relationship between the wavelength dispersion B of the birefringence anisotropy Δn of the retardation plate obtained by simulation, the contrast ratio of the liquid crystal display device, and the blackness BL.

FIG. 5 is a diagram showing the relationship between the contrast ratio and the angle φ formed by the optical axes of the two retardation plates, obtained by simulation.

6 is a plan view showing a display screen 15 of the liquid crystal display device 1. FIG.

FIG. 7 is a diagram showing an emission spectrum of a cold cathode tube used as a backlight 7.

FIG. 8 is a cross-sectional view showing a configuration of a reflective liquid crystal display device 17 which is a second embodiment of the present invention.

FIG. 9 is a sectional view showing a configuration of a semi-transmissive reflective liquid crystal display device 20 according to a third embodiment of the present invention.

FIG. 10 is a cross-sectional view showing a configuration of conventional liquid crystal display devices 21 and 33.

FIG. 11 is a diagram showing general spectral transmittance characteristics of a polarizing plate for color compensation.

[Explanation of symbols]

 1,17,20 Liquid crystal display device 2,3 Polarizing plate 4,5 Phase difference plate 6 Liquid crystal element

Claims (1)

[Claims] 1. A pair of polarizing plates, a super twisted nematic liquid crystal element arranged between the pair of polarizing plates, and a liquid crystal element between one of the pair of polarizing plates and the liquid crystal element. , And two retardation plates arranged in a laminated manner, wherein the two retardation plates are both selected such that the wavelength dispersion B of the birefringence anisotropy Δn is in the range of 30 ≦ B ≦ 250. , The angle φ formed by each optical axis of the two retardation plates is 5 ° ≦ φ
A liquid crystal display device, wherein the liquid crystal display device is set in a range of ≦ 60 °.