JP2814960B2 - Liquid crystal display - Google Patents

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.

[0002]

2. Description of the Related Art Conventionally, in a liquid crystal display device having a time-division driving characteristic, at least one of a background and a display is colored,
Black and white display could not be performed.

In recent years, however, improvement in image quality of liquid crystal display devices has
The demand for increasing the amount of displayed information is becoming more
Specifications are shifting to black and white display and even color display.
You. Therefore, as in the element structure shown in FIG.
The device has a two-layer structure, and the thickness d of the liquid crystal layer between the two devices is d.
(Μm) and the product Δn · d of the liquid crystal refractive index anisotropy Δn
And the twist angles representing the twist of the liquid crystal molecules are equal.
And the twist direction was reversed. As a result, FIG.
Incident on the liquid crystal element to indicate the polarization state of the liquid crystal element
Linearly polarized light produces a phase difference and becomes elliptically polarized light.
By disposing the liquid crystal element of
And linearly polarized light, enabling black-and-white display (Okumura, Eimura
Ta, Wada: Technical Report of the Institute of Television Engineers of Japan, 11, p79,
1987). On the other hand, using a birefringent film,
A method has been proposed to achieve black and white display by performing correction.
(Nagae, Square, Komura: Technical Report of the Institute of Television Engineers of Japan,1
2, P29-34, 1988).

[0004]

In the above prior art, it is necessary to use two liquid crystal elements as a liquid crystal display device, and the mass productivity is low. Also, when the liquid crystal element is viewed obliquely,
The contrast ratio, which is the ratio of the luminance of the background portion to the display portion, changes greatly, and the viewing angle characteristics deteriorate. FIG. 4 shows the viewing angle characteristics of the horizontal contrast ratio of the liquid crystal element, and FIG. 5 shows the viewing angle characteristics in the vertical direction. As the viewing angle increases, the contrast ratio decreases. At a viewing angle of 40 degrees or more, the contrast ratio becomes 1 or less, and black-and-white reversal development occurs.

In the case where a birefringent plastic film is used, the wavelength dependence of the phase difference is not considered, and the phase correction is completely performed as shown in FIG. However, there was a problem that the contrast ratio was low.

SUMMARY OF THE INVENTION An object of the present invention is to make the background and the display unit achromatic, thereby enabling color display, and at the same time, to improve the high contrast ratio and the viewing angle dependence of the display characteristics.

[0007]

According to the present invention, there is provided a liquid crystal device having a nematic liquid crystal layer having a helical structure sandwiched between electrode substrates and having a helical structure twisted in the thickness direction between the electrode substrates. A liquid crystal display device having a pair of polarizing plates, wherein a birefringent film of an optically biaxial crystal is disposed between the electrode substrate and the polarizing plate, and the thickness (d) of the birefringent film is Product of refractive index anisotropy difference (Δn), (Δ
n · d) ₂ , and the product (Δn · d) ₁ of the wavelength dependence of the phase difference caused by the thickness (d) of the liquid crystal element and the difference (Δn) between the anisotropy of the refractive index and the helical structure. This is a liquid crystal display device in which the wavelength dependence of the phase difference caused by the twist angle θ ₁ is almost equal.

[0008]

The twisted nematic type liquid crystal element having excellent time-division driving characteristics has a color on at least one of the background and the display. On the other hand, by laminating liquid crystal elements with a reverse twist and making the liquid crystal element a two-layer structure, it is possible to perform phase correction and display black and white, but the contrast ratio greatly changes when the liquid crystal element is viewed obliquely. ,
There is a problem in the viewing angle dependence of the display characteristics. When a birefringent film was used, the contrast ratio was low because the wavelength dependence of the phase difference was not considered.

The contrast ratio and the viewing angle characteristics have been improved by making the wavelength dependence of the phase difference between the liquid crystal element and the birefringent film substantially equal.

In order from the further improvement of the viewing angle characteristics, the stretching axis direction of the refractive indices n _x of the birefringent film, the refractive index n _y in the direction perpendicular to the stretching axis, the refraction of the thickness direction of the birefringent film An optically biaxial crystal having three different ratios _nz may be used. This is because the change in the phase difference due to the viewing angle is small.

[0011]

【Example】

[Embodiment 1] FIG. 1 is a perspective view of an element structure of a liquid crystal display element according to the present invention.

In FIG. 1, a twisted nematic liquid crystal having a twisted structure is arranged between an upper substrate 5 and a lower substrate 8 made of a translucent glass plate. Also, the upper substrate 5,
Polarizing plates 1 and 10 are arranged on the outer surface of the lower substrate 8, and
The birefringent film 3 is disposed between the upper substrate 5 and the upper polarizing plate 1. Further, on the outer surface of the lower polarizing plate 10, an external light source 12 is arranged to constitute a liquid crystal display element.

The liquid crystal is a nematic liquid crystal having a biphenyl liquid crystal and an ester cyclohexane liquid crystal as main components, to which 0.5% by weight of (S811 manufactured by Merck) is added as a rotatory substance, and Δn = 0.118, Liquid crystal layer thickness d
Was set to 6.5 μm, and Δn · d was set to 0.77 μm.

The twist direction and the twist angle α of the liquid crystal molecules 7 are
The rubbing direction 6 of the upper electrode substrate 5 and the rubbing direction 9 of the lower electrode substrate 8 and the optically rotating substance to be added are defined.

As the birefringent film 3, a polycarbonate film was used, and Δnd · 0.56 μm. The stretching axis 4 of the polycarbonate film and the upper rubbing axis 6
Is 90 degrees.

G1229DV manufactured by Nitto Denko having a polarization rate of 99.9% is used for the polarizing plates 1 and 10. The angle between the absorption axis (or the polarizing axis) 11 of the lower polarizing plate 10 and the lower rubbing axis 9 is 45.
The angle between the absorption axis (or polarization axis) 2 of the upper polarizing plate 1 and the stretching axis 4 of the polycarbonate film was 75 degrees.

Although a cold cathode tube is used as the light source 12,
A hot cathode tube, electroluminescence, or a type using external light using a reflection plate without using a light source may be used.

FIG. 7 shows the colors of the background and the display section in the present embodiment in CIE chromaticity coordinates. As shown in the figure, both the background color and the display color are close to the C light source, so that a substantially monochrome display is realized. The contrast ratio at this time is 15: 1.

The relational expression of the phase difference between the liquid crystal element and the birefringent film can be derived using a Jones matrix. The twist angle of the liquid crystal element is θ ₁ and the optical path difference is (Δn ·
d) ₁ , when the optical path difference of the birefringent film is (Δn · d) ₂ , the wavelength of light is λ, and m is an arbitrary integer, (Δn · d) ₂ = ｛(Δn · d) ₁ ² + λ ² (mπ + θ ₁ ² ) -2mπλ ² (θ ₁ ² + (Δn · d) shown ^{_{1 2 / λ 2) 1/}} 2} 1/2 composed of a relational expression.

In the range of λ = 450 nm to 650 nm, when the above expression is satisfied, the phase correction is completely performed,
A complete white or black display is obtained.

In general, the wavelength dependence of the refractive index of liquid crystal molecules is as follows:
Δn (λ) = A + B / (λ ² −λ ₀ ² ) In the present embodiment, A = 0.0765, B = 4443.8.
(μm ² ), λ ₀ = 263.5 (nm). FIG. 8 shows the wavelength dependence of Δn · d of the liquid crystal molecules and the wavelength dependence of Δn · d of the birefringent film that realizes black-and-white display derived from the above equation. The phase correction is performed in the entire wavelength range by using a film having a wavelength dependence substantially similar to that shown in FIG. However,
The phase difference changes depending on the value of the integer m in the above equation. FIG. 17 shows the wavelength dependence of the phase difference between the liquid crystal element and the birefringent film. When m = 1, the wavelength dependence of the phase difference between the film and the liquid crystal element approximately matches, and the phase correction becomes more complete, and a black-and-white display suitable for color display is realized. At that time, the contrast ratio is equivalent to that of the two-layer liquid crystal system. 30 to 1 are obtained.

In FIG. 8, there is a wavelength dependency of m = 1.
In order to obtain a birefringent film, a material having a so-called extraordinary dispersion having a large refractive index on the long wavelength side may be used. Regarding the anomalous dispersion of the refractive index, for example, “optical”,
This is described in detail in Kozo Ishiguro, (Kyoritsu Zensho), p.260. In FIG. 18, when an absorption peak occurs at an arbitrary wavelength, a divergence phenomenon occurs in the dispersion curve of the refractive index. When the absorption peak is in the near infrared, the refractive index increases in the visible light region on the long wavelength side.

For example, the above-mentioned anomalous dispersion can be obtained by using a polystyrene film shown in FIG. 19 or adding a dye such as naphthalocyanine or fuchsin having an absorption peak in the near infrared to the film. .

As another means for obtaining the wavelength dependence shown in FIG. 8, it is also possible to laminate films having different wavelength dependences.

FIG. 9 shows the viewing angle characteristics of the contrast ratio in the horizontal direction. The range where the contrast ratio is 2 to 1 or more is 40
In the case of the conventional two-layer liquid crystal system, it is wider than 30 degrees. FIG. 10 shows the viewing angle characteristics of the contrast ratio in the vertical direction. The black-and-white inversion phenomenon in which the contrast ratio becomes 1 or less does not occur, and the viewing angle characteristics are superior to those of the two-layer liquid crystal system.

The liquid crystal molecules were observed under a crossed Nicols by inserting a condenser lens in a polarizing microscope shown in FIG. While the polycarbonate film is an optically uniaxial crystal from the conoscopic image, the polycarbonate film is an optically biaxial crystal from the conoscopic image shown in FIG. FIG. 13 shows the viewing angle dependence of the transmittance when the polarizing plate is orthogonal Nicols and the parallel alignment liquid crystal element and the polycarbonate film alone are inserted. Assuming that the transmittance in the vertical direction is 1, in the direction of 45 degrees, the liquid crystal is 0.4, whereas the polycarbonate film is 0.6. That is, the optical biaxial crystal has little change in the phase difference depending on the viewing angle, and has excellent viewing angle characteristics such as display characteristics such as a contrast ratio. Since the combination of the optical biaxial crystal and the colorization has good viewing angle characteristics, a change in color tone due to a change in viewing angle is reduced.

Example 2 In FIG. 1, two polycarbonate films having Δn · d = 0.3 μm are used as the optically biaxial crystal 3 and laminated with their stretching axes shifted. The angle made was set to 10 degrees from the relationship of the contrast ratio shown in FIG. The angle between the stretching axis of the upper film and the absorption axis (or polarization axis) 2 of the upper polarizing plate is 40 degrees, and the other element configuration is the same as that of the first embodiment.

FIG. 15 shows the background color and display color in CIE chromaticity coordinates in this embodiment.

When compared with FIG. 7, the CIE chromaticity coordinates are as follows: the background color is (0.35, 0.35) to (0.33, 0.33).
In addition, the display color changes from (0.29, 0.36) to (0.33, 0.3).
4) approaching the C light source (0.31, 0.31) in all cases, and have realized a substantially monochrome display. The contrast ratio at this time is 20: 1.

[Embodiment 3] In the device configuration of Embodiment 1, the optical biaxial crystal 3 or the electrode substrates 5, 8 shown in FIG. Color display can be achieved by combining the color filters of the pattern electrodeposition type and the dye type. Although the color filter is arranged between the electrode substrate and the liquid crystal layer, the arrangement is not limited to this arrangement as long as the color filter is in close contact with the liquid crystal element. FIG. 16 shows the display colors in CIE chromaticity coordinates.

The region indicated by the dashed line in the figure indicates the range of color reproducibility of a color liquid crystal television using thin film transistors (TFTs). As shown in the figure, the range of color reproducibility of this embodiment is equivalent to that of the TFT color liquid crystal television.

[0032]

According to the present invention, since the phase correction is performed in the entire wavelength range, the background and the color of the display section can be made achromatic, so that black and white and color display can be performed. . Furthermore, by using an optically biaxial crystal,
There is an effect that the viewing angle characteristics are improved as compared with the conventional two-layer liquid crystal element.

[Brief description of the drawings]

FIG. 1 is a perspective view of a liquid crystal element of the present invention.

FIG. 2 is a perspective view of a liquid crystal element according to the related art.

FIG. 3 is a diagram illustrating a polarization state of a liquid crystal element according to the related art.

FIG. 4 is a view showing a viewing angle characteristic of a contrast ratio in a left-right direction of a liquid crystal element in a conventional technique.

FIG. 5 is a diagram showing viewing angle characteristics of a contrast ratio in a vertical direction of a liquid crystal element in a conventional technique.

FIG. 6 is a diagram showing wavelength dependence of transmitted light according to a conventional technique.

FIG. 7 shows the display color, background color, and C of the liquid crystal element in the present invention.
The figure shown on the IE chromaticity coordinates.

FIG. 8 is a graph showing wavelength dependence of Δn · d of liquid crystal molecules and a birefringent film for realizing monochrome display.

FIG. 9 is a view showing viewing angle characteristics of a contrast ratio in the left-right direction of the liquid crystal element according to the present invention.

FIG. 10 is a view showing viewing angle characteristics of a contrast ratio in a vertical direction of a liquid crystal element in the present invention.

FIG. 11 is a view showing a conoscopic image of liquid crystal molecules.

FIG. 12 is a diagram showing a conoscopic image of a polycarbonate film.

FIG. 13 is a diagram showing the viewing angle dependence of the transmittance of a parallel alignment liquid crystal element and a polycarbonate film.

FIG. 14 shows the relationship between the angle between the stretching axes of the phase plate film and the contrast ratio in the second embodiment of the present invention.

FIG. 15 is a view showing display colors and background colors in CIE chromaticity coordinates according to the second embodiment of the present invention.

FIG. 16 is a diagram showing a display color range in CIE chromaticity coordinates according to the third embodiment of the present invention.

FIG. 17 is a diagram showing wavelength dependence of a phase difference.

FIG. 18 is a view showing a phenomenon of anomalous dispersion.

FIG. 19 shows an absorption spectrum of polystyrene.

[Explanation of symbols]

1, 19: Upper polarizing plate, 2, 13: Absorption axis of upper polarizing plate, 3: Birefringent film, 4: Stretching axis, 5, 22: Upper electrode substrate of driving liquid crystal element, 6, 14, 16 ... Upper rubbing axis, 7, 15, 17: liquid crystal molecules, 8, 25: Lower electrode substrate for driving liquid crystal element, 9, 16, 24: Lower rubbing axis, 10, 26: Lower polarizing plate, 11, 18 ... Polarization axis of lower polarizing plate, 12, 27 ... Light source.

Continuation of the front page (56) References JP-A-64-23224 (JP, A) JP-A-63-261225 (JP, A) JP-A-63-239421 (JP, A) JP-A-2-37321 (JP) , A) Technical Report of the Institute of Television Engineers of Japan, Vol. 32, pp. 29-34 Technical Report of the Institute of Television Engineers of Japan, Vol. 49, pp. 61-66 (58) Field surveyed (Int. Cl. ⁶ , DB name) G02F 1/1335 510

Claims (2)

(57) [Claims] 1. A liquid crystal element comprising: a pair of upper and lower electrode substrates; and a nematic liquid crystal layer sandwiched between the electrode substrates and having a helical structure twisted in a thickness direction between the electrode substrates.
A liquid crystal display device having a pair of polarizing plates provided with the liquid crystal display element interposed therebetween, wherein one or two birefringent films are disposed between the electrode substrate and the polarizing plate. A birefringent film of an optically biaxial crystal in which the refractive index nx in the stretching axis direction and the refractive index ny in the direction perpendicular to the stretching axis and the refractive index nz in the thickness direction are different from each other; The wavelength dependence of the phase difference caused by the product (Δnd) _{2 of the} thickness (d) and the anisotropic refractive index (Δn),
The thickness of the nematic liquid crystal layer (d) and the anisotropic refractive index (Δn)
The liquid crystal display device is characterized in that the product (Δn · d) ₁ of the helical structure is substantially equal to the wavelength dependence of the phase difference caused by the twist angle θ of the helical structure. 2. A twist angle θ ₁ sandwiched between a pair of upper and lower electrode substrates and said electrode substrate in a thickness direction between said electrode substrates.
A liquid crystal display device comprising: a liquid crystal element having a nematic liquid crystal layer having a twisted helical structure; and a pair of polarizing plates provided with the liquid crystal display element interposed between the liquid crystal element and the liquid crystal layer. One or two birefringent films disposed between the electrode substrate and the polarizing plate, wherein a refractive index nx in a stretching axis direction and a refraction in a direction perpendicular to the stretching axis are provided. A birefringent film of an optically biaxial crystal having a different refractive index ny and a different refractive index nz in the thickness direction, respectively, and a product of a thickness (d) of the birefringent film and an anisotropic refractive index (Δn). Where (Δnd) ₂ , the product of the thickness (d) of the nematic liquid crystal layer and the anisotropic refractive index (Δn) is (Δnd) ₁ , and the wavelength of incident light is λ, _{Δn · d) 2 = {(} Δn · d) 1 2 + λ 2 (m π + θ 1 2) 2 -2mπλ 2 (θ 1 2 + (Δn · d) 1 2 / λ 2) 1/2} ^1/2
A liquid crystal display device substantially satisfying a relational expression (m is an arbitrary integer).