JP2892101B2 - Full color liquid crystal display - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal display device, and more particularly to a full-color liquid crystal display device capable of displaying all colors in the visible region.

[Background Art] In the color superhomeotropic (CSH) technology, the birefringence phenomenon of a liquid crystal having a homeotropic alignment perpendicular to a substrate is electrically controlled and used in combination with a compensator. The CSH technology enables full-color, high multiplicity (the number of lines in the display surface), and liquid crystal display with a wide viewing angle (an angle range in which the display can be viewed). CSH technology requires a homeotropic alignment that is more difficult to realize and stabilize than the planar alignment in which liquid crystal molecules used in conventional twisted nematic liquid crystal displays are aligned in a fixed direction parallel to the substrate. And

[Problems to be Solved by the Invention] Homeotropic alignment is a technique that is difficult to realize and stabilize as compared with planar alignment. The adoption of CSH technology for full-color display necessitates the use of this difficult homeotropic arrangement.

An object of the present invention is to provide a liquid crystal display device having performance comparable to that of the CSH technology, particularly with respect to full color display and a wide viewing angle, and realizable using a planar arrangement.

[Means for Solving the Problems] The liquid crystal cell has a planar arrangement.

A compensator having biaxial optical anisotropy is used in combination with a liquid crystal cell. The axis of intermediate refractive index points in the direction normal to the compensator.

The axis of the minimum refractive index of the compensator is arranged along the direction of the planar alignment of the liquid crystal molecules, and the axis of the maximum refractive index is arranged along the in-plane direction perpendicular to the direction of the planar alignment. To give an optical retardation opposite to the optical retardation of the liquid crystal cell.

The liquid crystal cell is a color type having a color filter and a black mask inside. The cell gap of the liquid crystal cell is independently selected for each color.

[Operation] When the liquid crystal cell is in the off state, no voltage is applied to the liquid crystal or only a voltage lower than the threshold is applied. In this off state, all the liquid crystal molecules are arranged in the planar direction. As a result of the compensation of the compensator, at normal incidence, the retardation of the liquid crystal is totally compensated by the opposite retardation of the compensator. Furthermore, by selecting the optical constant and the thickness of the compensating plate to appropriate values, the compensating effect can be extended over a wide viewing angle. For example, it covers an azimuth of ± 80 degrees. Placing an axis of intermediate refractive index in the direction normal to the compensator improves uniformity of compensation. The off state of the liquid crystal cell provided with the compensator appears black.

When the liquid crystal cell is in the ON state, a voltage higher than a threshold is applied to the liquid crystal cell. Although the optical retardation of the liquid crystal cell changes, the optical retardation of the compensator does not change. Therefore, the ON state of the liquid crystal cell including the compensator appears transparent. The transparency is controlled by the tilt angle of the liquid crystal molecules in the bulk controlled by the applied voltage.

Even if each optical constant of the liquid crystal and each optical constant of the film have dispersion with respect to wavelength, the optical retardation of the liquid crystal and the optical retardation of the compensator are almost equal in absolute value. The cell gap is independently selected for each color. Therefore, compensation for each color is performed in an optimal state, and full-color display is realized.

Embodiment FIG. 1 schematically shows a liquid crystal display device according to an embodiment of the present invention. In the figure, the liquid crystal cell has a structure in which a polarizer 1, a liquid crystal cell 3, a compensator 5, and an analyzer 7 are stacked from above. The liquid crystal cell 3 has a pair of glass substrates 11 and 12 with liquid crystal molecules 9.
Have a structure sandwiched between them. On one transparent substrate 11,
A red filter 13, a green filter 14, and a blue filter 15 are separated and arranged by a black stripe 17 to constitute a color display device. A plurality of segment electrodes 16 are arranged on one transparent substrate 11 side, and the other transparent substrate 12
A command electrode 18 is arranged on the upper side. The polarization axis P1 of the polarizer 1 and the polarization axis P2 of the analyzer 7 are orthogonal to each other and form an angle of 45 degrees with respect to the alignment direction of the liquid crystal.

The liquid crystal cell has a homogeneous planar arrangement. The major axis of the nematic liquid crystal molecule is the refractive index n _xe for extraordinary rays
And is parallel to the OX axis. The segment electrode 16 is deposited on the surface of the color filter,
18 is deposited on a transparent substrate. Compensator 5 has thickness d
Having. On the other hand, the thicknesses of the liquid crystal layers under the red, green, and blue color filters are G _R , G _G , and G _B , respectively, and are independently selected. This provides a solution to the fact that the optical constants of liquid crystals change with wavelength, and with changes in optical constants, the birefringence also changes with wavelength.

FIG. 2A is a graph showing a wavelength change of the birefringence of the liquid crystal cell. The vertical axis indicates the difference [Delta] n _x between the refractive index n _x0 to the refractive index n _xe an ordinary ray for the extraordinary ray, the abscissa indicates the wavelength λ in [mu] m. Three plots on the wavelength axis 0.45, 0.
Numerals 55 and 0.65 denote the transmission bands of the respective color filters as shown in FIG. 2C, that is, the center wavelengths of the blue, green and red bands. The change in birefringence caused by the wavelength change from blue to red has a magnitude of about 0.01 with respect to the average value.

The compensator 5 also changes the optical constant with respect to the wavelength. If the compensation plate 5, which has a negative uniaxial property that is formed, for example, polystyrene, if the difference between the refractive index n _ce with respect to the extraordinary ray and a refractive index n _c0 for ordinary ray and [Delta] n _c, the birefringence [Delta] n
_c changes as shown in FIG. 2 (B). Similar to FIG. 2 (A), the vertical axis represents the birefringence [Delta] n _c, the horizontal axis represents the wavelength in [mu] m. The birefringence of the compensator has a value about two orders of magnitude smaller than that of the liquid crystal. In addition, the amount of change in birefringence due to the change in wavelength is about 0.0001 or less.

2 (A) and 2 (B), the case where the compensating plate 5 is uniaxial has been described, but in the present embodiment, a biaxial compensating plate is used. In the case of biaxiality, the refractive indexes are n1, n2, and n3.

In both the uniaxial and biaxial cases, the axis having the lowest refractive index of the compensator 5 is set along the OX direction, which is the major axis direction of the liquid crystal molecules.

The liquid crystal layer has a positive dielectric anisotropy. That is, the dielectric constant ε‖ parallel to the major axis direction of the liquid crystal molecules is larger than the dielectric constant ε⊥ perpendicular to the major axis direction. Δε> 0.

Good compensation can be performed on such a liquid crystal by using a negative uniaxial compensator. Further, when a biaxial compensating plate is used, even better compensation can be performed.

Before describing the embodiment of the present invention, a case where a uniaxial compensator is used will be considered.

The compensator has negative uniaxiality. Here, the negative uniaxial optical constants n ₀ for ordinary ray means a large n _0> n _e than the optical constants n _e for extraordinary ray. The main axis of the compensator is arranged parallel to the direction in which the liquid crystal molecules are arranged. That is, the n _e shaft arranged parallel to OX in FIG. 1, OY, OZ
Parallel placing n ₀ to. Here is a n _e <n _0.

 The gap G of the liquid crystal cell is determined for each color as follows.

G = (n _c0 −n _ce ) d / (n _xe −n _x0 ) That is, since the refractive indices of the compensator and the liquid crystal change depending on the wavelength, the liquid crystal cell gap has a refractive index at each of the red, green and blue wavelengths. Is determined by substituting into the above equation. By selecting the gap of the liquid crystal cell for each color according to the above equation, the retardation magnitudes of the liquid crystal and the compensator are equal for each color, and the signs are reversed. Preferably, the refractive indices of the liquid crystal and the compensator for ordinary light are selected as close as possible. If the index of refraction for ordinary light is selected near the index of refraction of glass 1.5,
Reflection at the index interface is reduced and compensation is improved.

Example when Compensator has Negative Uniaxiality A uniaxial film is obtained by uniaxially stretching a polymer film having negative birefringence, for example, polystyrene.

FIG. 3 shows that the liquid crystal cell has n _x0 = 1.5, n _xe = 1.7, and G = 5μ.
m, and the compensator has n _c0 = 1.5, n _ce = 1.498, d = 500
The compensation effect at μm is shown. The wavelength considered is
0.5 μm.

FIG. 3 (A) shows an orthogonal view of a liquid crystal cell provided with a compensating plate on the right side of the liquid crystal display device shown in FIG. The transmittance T through the deflector is shown. Note that the azimuth from the direction which is the alignment direction of the liquid crystal molecules is fixed to 0 degree. That is, the incident surface is a surface including the OX axis and the OZ axis.

The left side of FIG. 3 (A) shows the change in the magnitude of the normalized retardation under the same conditions as the right side of FIG. 3 (A). Here, the normalized retardation is a value obtained by dividing a normal retardation (a product obtained by multiplying a difference in refractive index of birefringence by a thickness) by a wavelength (normalized by a wavelength) in degrees. The normalized retardation values show well-matched values for the liquid crystal and the compensator.

The right side of FIG. 3 (B) is a graph showing a change in transmittance when the azimuth is fixed at 90 degrees and the apex angle is changed from 0 degrees to 80 degrees. That is, the incident surface is a surface including OY and OZ.

The left side of FIG. 3B shows the magnitude of the normalized retardation under the same conditions as the right side of FIG. 3B. The normalized retardation between the liquid crystal and the compensator spreads as the vertex angle increases. For this reason, the transmittance also increases as the apex angle increases. Although the transmittance is higher than in the case of FIG. 3 (A), it is still 25% or less at the apex angle of 80 degrees. The transmittance of 100% corresponds to the transmittance when the light passes through a parallel polarizer.

The right side of FIG. 3 (C) is a graph showing a change in transmittance with respect to a change in apex angle from 0 ° to 80 ° when the azimuth is fixed at 45 °.

The left side of FIG. 3 (C) shows the normalized retardation under the same conditions. The amount of normalized retardation has different values for the liquid crystal and the compensator as the vertex angle increases.

On the other hand, as shown in the right side of FIG. 3C, the propagation modes of the liquid crystal and the compensator are not exactly parallel (incident angle
80 degree case is illustrated). However, the shift is very small, within one degree. The resulting transmittance is very low, less than 3% even at an incident angle of 80 degrees.

Next, examples of the present invention will be described. In an embodiment, the compensator is biaxial. This means that the compensator has different optical constants in the three axial directions. In this case, the optical medium has no principal axis. The axis of the smallest refractive index of the biaxial film is arranged in a direction parallel to the alignment direction of the liquid crystal molecules. The axis with intermediate refractive index is located along the OZ axis. That is, the direction is perpendicular to the compensator. The largest refractive index is arranged in the in-plane direction of the compensator and in the direction orthogonal to the alignment direction of the liquid crystal molecules. That is, nc1 <nc3 <nc
At 2, nc1‖OX, nc2‖OY, nc3‖OZ. The thickness of the liquid crystal layer is selected as G = (n _c2 −n _c1 ) d / (n _xe −n _x0 ) for each color. Since the refractive index changes with the wavelength, the gap of the liquid crystal cell is selected for the red, green, and blue filters in accordance with the value of the refractive index at each wavelength.

The above equation is used to calculate the gap G when a uniaxial compensator is used.
Compared to the formula giving, n _c2 corresponds to n _c0, n _c1 is n _ce
It can be seen that this corresponds to That is, retardation of the biaxial compensator can be thought to be multiplied by the thickness of the Δn _c = n _c1 -n _c2.

Preferably, the refractive index of the liquid crystal for ordinary light and the compensation plate
Choose n _c2 as close as possible. If the index of refraction of the liquid crystal for ordinary light and the value of n _c2 of the compensator are selected to be close to the index of refraction of glass of about 1.5, the compensation is improved and the reflection at the index interface is reduced.

The third index of refraction _nc3 of the compensator is located in the direction of the normal to the compensator and is chosen to be very close to and slightly closer to _nc2 . Uniformity of compensation when the n _c3 slightly lower than n _c2, is greatly improved. This result is quite surprising.

The medium for compensating the liquid crystal layer having the planar alignment and the positive uniaxial property is not limited to the negative uniaxial medium having the main axis along the alignment direction of the liquid crystal molecules. biaxial medium having n _c2 refractive index n _c3 slightly closer than is superior to the compensation effect.

Example where the compensator is biaxial A biaxial film can be obtained by stretching that is not completely uniaxial. That is, a biaxial medium can be formed by strongly stretching in a certain direction and weakly in the direction of direct light. For example, a polymer film having negative birefringence can be formed from polystyrene.

FIGS. 4A to 4C show the compensation effect in the case of biaxiality by retardation RT and refractive index T. FIG. The cell is n _x0 = 1.
5, n _xe = 1.7, G = 5 μm, and the compensator is n _c1 = 1.49
8, n _c2 = 1.5, n _c3 = 1.499875, d = 500 μm. The wavelength is 0.5 μm as in FIG.

The right side of FIG. 4 (A) shows the transmittance of the liquid crystal cell provided with the biaxial compensator at an azimuth of 0 degree and an apex angle of 0 to 80 degrees.
That is, the incident surface is a surface including OX and OZ.

The left side of FIG. 4 (A) shows the normalized retardation under the same conditions as the right side of FIG. 4 (A). The amount of normalized retardation is not exactly the same. The resulting transmission is close to the transmission in the two orthogonal polarizers. At an angle of incidence of 80 degrees, the transmission is 16%, compared to 12% in a crossed polarizer.

The right side of FIG. 4 (B) shows a liquid crystal cell provided with a biaxial compensator for an apex angle of 0 to 80 degrees when the azimuth angle is 90 degrees (the incidence plane is a plane containing OX and OZ). Shows the transmittance.

The left side of FIG. 4 (B) shows the normalized retardation under the same conditions. The amount of normalized retardation is
Although not exactly matched, they have very close values and the resulting transmittance is low for any angle of incidence. It can be seen that the transmittance is remarkably reduced from 22% to 13% at an incident angle of 80 degrees as compared with the case of FIG. 3B using a uniaxial medium.

The right side of FIG. 4 (C) shows the transmittance of the liquid crystal cell provided with the compensator for the apex angle of 0 to 80 degrees when the azimuth is 45 degrees.

The left side of FIG. 4 (C) shows the amount of normalized retardation under the same conditions. The normalized retardations do not exactly match, but have very low values.

As shown in the right side of FIG. 4C, the propagation mode is not completely parallel between the liquid crystal cell and the compensator, but the shift is extremely narrow, and is much smaller than 1 degree.

Compared with the uniaxial case, the transmittance is reduced from 3% to 1% at the incident angle of 80 degrees.

As described above, a liquid crystal display device having a wide viewing angle can be obtained by using a uniaxial compensator, and a further improved liquid crystal display device can be obtained by using a biaxial medium.

Although the present invention has been described with reference to the embodiments, the present invention is not limited thereto. For example, various changes,
It will be obvious to those skilled in the art that improvements, combinations, and the like are possible.

[Effects of the Invention] As described above, according to the present invention, an excellent full-color liquid crystal display device can be obtained by using a liquid crystal having a planar arrangement.

[Brief description of the drawings]

FIG. 1 is a perspective view showing an embodiment of the present invention, and FIGS. 2A, 2B and 2C are chromatic dispersion of birefringence of liquid crystal and birefringence of compensator in a full-color liquid crystal display device. 3A to 3C are graphs showing transmittance and normalized retardation when a uniaxial compensator is used, and FIG. (A)-(C) are graphs showing transmittance and normalized retardation when using a biaxial compensator. In the drawing, 1 ... polarizer 3 ... liquid crystal cell 5 ... compensator 7 ... analyzer 9 ... liquid crystal molecules 11, 12 ... transparent substrate 13 ... red filter 14 ... green filter 15 ... blue filter 16, 18 Electrode 17 Black stripe

 ──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-2-47629 (JP, A) JP-A-64-5019 (JP, A) JP-A-61-121033 (JP, A) JP-A-60-1985 217337 (JP, A) JP-A-64-15719 (JP, A)

Claims (2)

(57) [Claims] 1. A liquid-phase cell containing a nematic liquid crystal in a planar arrangement between a pair of transparent substrates, wherein a color filter for full-color display is provided on one of the transparent substrates, and the liquid crystal layer has a color filter. A liquid crystal cell having a thickness selected for each, and a compensator disposed in parallel with the liquid crystal cell and having biaxial optical anisotropy, wherein the axis of the minimum refractive index is an alignment of liquid crystal molecules. Parallel to the direction, the axis of the intermediate refractive index is along the normal line of the compensator, and the axis of the maximum refractive index is almost perpendicular to the alignment direction of the liquid crystal molecules in the plane of the compensator, A compensating plate, wherein the sign of the optical retardation of the plate is opposite to the sign of the optical retardation of the liquid crystal cell. 2. The liquid crystal layer of the liquid crystal cell has a thickness selected for each color so that the optical retardation corresponding to each color of the color filter is equal to each other. 2. The full-color liquid crystal display device according to claim 1, wherein the absolute value of the optical retardation of the compensator is substantially equal to the absolute value of the optical retardation of the compensator.