JPH04215619A - Liquid crystal display device - Google Patents

Abstract

PURPOSE:To display high-quality images on a liquid crystal display device using a CTN-type liquid crystal cell by eliminating leakage of light at the OFF portion perfectly, and making the OFF portion blacker to increase the contrast. CONSTITUTION:Two phase plates 31, 32 arranged between a STN-type liquid crystal cell 10 and an outgoing side polarizing plate 22 are bonded to each other by a transparent adhesive 41, and thus the index of refraction of the portion (a layer of adhesive 41) for light between the respective phase plates 31, 32 is brought near the index of refraction of the phase plates 31, 32 to eliminate the reflection of light on the faces of the phase plates for transmitted light to pass through the respective phase plates 31, 32 only once respectively. Thus, most of the transmitted light of the OFF portion comes into the outgoing side polarizing plate 22 as straight polarizing light to eliminate leakage of light at the OFF portion.

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 using an STN type liquid crystal cell.

0002

2. Description of the Related Art A liquid crystal display device includes a liquid crystal cell in which liquid crystal is sealed between a pair of transparent substrates each having a transparent electrode and an alignment film formed thereon, and the molecules of this liquid crystal are arranged in a twisted manner between the two substrates. The liquid crystal cell consists of a pair of polarizing plates placed before and after the liquid crystal cell.
Generally, a TN type is used in which liquid crystal molecules are arranged in a twisted manner at a twist angle of approximately 90°.

[0003] Recently, liquid crystal display devices for displaying television images, etc., have been increasing the number of pixels in order to have larger screens and higher resolutions. It is becoming.

However, in a liquid crystal display device using the TN type liquid crystal cell, the response time (response) of the liquid crystal to the application of a display drive voltage is as fast as about 100 msec, but when time-divisionally driven at high duty, the ON (light) The problem is that the light transmittance at the time of transmission (transmission) is extremely reduced and the display becomes dark, so liquid crystal display devices that are driven in a time-division manner at high duty have to twist the liquid crystal molecules at a larger twist angle ( ST twisted arrangement at 180°~270°)
An N-type liquid crystal cell is used.

[0005] A liquid crystal display device using this STN type liquid crystal cell has a liquid crystal layer thickness larger than that of a TN type liquid crystal cell, so the display driving speed is lower than that of a liquid crystal display device using a TN type liquid crystal cell. Although the response time of the liquid crystal to the application of voltage is slow at about 300 msec, the retardation of the liquid crystal cell is large, and the optical rotation of the transmitted light is large due to the large twist angle of the liquid crystal molecules, so it is driven in a time-division manner at high duty. Also, it is possible to obtain a bright display by increasing the light transmittance when turned on.

However, although a liquid crystal display device using an STN type liquid crystal cell can provide a bright display, it also transmits a certain amount of light even when it is OFF (light blocked), so the contrast (brightness and darkness between ON and OFF areas is Furthermore, since the retardation and optical rotation of the liquid crystal cell are large, the wavelength dependence of the retardation and optical rotation is large, resulting in coloring of the display.

[0007] This is because in the OFF part, the light that passes through the polarizing plate on the incident side becomes linearly polarized light, and in the process of transmitting through the liquid crystal cell, a phase difference is generated in each wavelength light due to the wavelength dependence of this liquid crystal cell. This is because the elliptically polarized light exits the liquid crystal cell and enters the polarizing plate on the output side. When the elliptically polarized light enters the polarizing plate on the output side in this way, the polarized light component in the absorption axis direction of this polarizing plate does not pass through the polarizing plate, but the polarized light component perpendicular to the absorption axis direction passes through the polarizing plate, so
This leaked light reduces the degree of darkness of the OFF portion that should be black, resulting in a decrease in contrast. In addition, in the conventional liquid crystal display device described above, since elliptically polarized light enters the output side polarizing plate, both when ON and OFF, light of a specific wavelength is absorbed by the output side polarizing plate, or when the output side polarizing plate is Therefore, the light transmitted through the output side polarizing plate becomes colored.

For this reason, in a liquid crystal display device using the STN type liquid crystal cell, a phase plate is disposed between the liquid crystal cell and one of the polarizing plates, and the polarizing effect of this phase plate allows the OFF
The transmitted light in that part is compensated so that it becomes almost linearly polarized light. In addition, in this liquid crystal display device, it is difficult to compensate the transmitted light to linearly polarized light using only one phase plate, so generally, multiple phase plates (usually two) are used to change the polarization state of the transmitted light in stages. Compensated.

FIG. 11 is a sectional view of a conventional liquid crystal display device equipped with the above phase plate. This liquid crystal display device is STN
A pair of polarizing plates 21 and 22 are arranged on both sides of the liquid crystal cell 10, and the liquid crystal cell 10 and the pair of polarizing plates 21,
In this liquid crystal display device, as shown in the figure, the phase plates 31 and 32 are arranged between the liquid crystal cell 10 and the output side polarizing plate (in the figure). It is arranged between the right polarizing plate) 22.

The liquid crystal cell 10 has a pair of transparent substrates 11 and 12 made of glass bonded together via a frame-shaped sealing material 13, and a sealing material 1 between the two substrates 11 and 12.
A liquid crystal 18 is sealed in a gap surrounded by 3, and a large number of striped transparent scanning electrodes 14 are arranged parallel to each other on the surface of the input side substrate (the left substrate in the figure) 11 facing the liquid crystal layer. , on the surface of the output side substrate 12 facing the liquid crystal layer,
A large number of striped transparent signal electrodes 15 are arranged in parallel to each other and perpendicular to the scanning electrodes 14 . Further, alignment films 16 and 17 whose surfaces are subjected to alignment treatment by rubbing are formed on the electrode formation surfaces of both substrates 11 and 12, respectively, and molecules of liquid crystal 18 sealed between both substrates 11 and 12 are formed. is 180° to 2 between the substrates 11 and 12 due to the alignment regulating force of the alignment films 16 and 17.
Twisted array with a twist angle of 70°.

On the other hand, among the two phase plates 31 and 32 disposed between the liquid crystal cell 10 and the output side polarizing plate 22, the first phase plate 31 on the liquid crystal cell 10 side The second phase plate 32 arranged on the output side has a polarization effect that reduces the phase difference between each wavelength of the elliptically polarized light. It is said to have a polarizing effect that further reduces the phase difference between the two.

That is, this liquid crystal display device transmits the elliptically polarized light emitted from the liquid crystal cell 10 to the first phase plate 31 and the second phase plate 31.
According to this liquid crystal display device, the polarization effect of the phase plate 32 causes the light to approach linear polarization in stages.
Although it uses an STN type liquid crystal cell, the elliptically polarized light emitted from the liquid crystal cell 10 can be made into almost linearly polarized light and input to the output side polarizing plate, reducing the occurrence of light leakage at the OFF part. This OFF portion can be made close to black, and coloring of the display can also be suppressed.

[0013]

However, in the conventional liquid crystal display device described above, the two phase plates 31 and 32 disposed between the liquid crystal cell 10 and the output side polarizing plate 22 are simply superposed. Therefore, the liquid crystal cell 10 emits light and the phase plate 31,
A part of the light transmitted through the output side polarizing plate 22
This causes a problem in that the degree of blackness in the OFF portion is low and the display is also colored to some extent. This is the first phase plate 31
A part of the light transmitted through the second phase plate 32 is reflected by the first
After returning to the phase plate 31 side, some of the light is further polarized again by the first phase plate 31 and transmitted to the second phase plate 32.
This is because it is incident on .

That is, FIG. 12 shows the path of transmitted light in the above liquid crystal display device. The light passes through the plate 31 and enters the second phase plate 22 , but in this case, some of the light emitted from the first phase plate 31 is reflected by the incident surface of the second phase plate 22 . This is because there is a small air layer between the first phase plate 31 and the second phase plate 22, and the refractive index of this air layer (approximately 1
) and the second phase plate 22 have different refractive indexes of light. Therefore, the light emitted from the first phase plate 31 has
As shown in FIG.
There is light that passes through the phase plate 22, and light that is reflected by the incident surface of the second phase plate 22 and returns to the first phase plate 31 side.Furthermore, the light that returns to the first phase plate 31 side includes There are two types of light: the light beam reflected by the output surface of the first phase plate 31, and the light beam B, which is reflected by the incident surface after entering the first phase plate 31.

Of these lights A, B, and C, lights A and B only pass through the first phase plate 31 and the second phase plate 22 once each, The light B becomes almost linearly polarized light and enters the output side polarizing plate 22, but the light C is repeatedly subjected to the polarization effect by the first phase plate 31. FIG. 13 shows an expanded view of the path of the light C, in which the light C passes through the first phase plate 31 after exiting the liquid crystal cell 10, and then passes through the first phase plate 31 twice. The light then enters the second phase plate 22. Therefore, this light H is polarized by the first phase plate 31 three times and by the second phase plate 2.
2 will be subjected to the polarization effect once.

FIG. 14 shows each of the above-mentioned light sources in the OFF state.
(a) to (d) show the changes in the polarization state of B and C.
shows the polarization state at each position a to d shown in FIGS. 12 and 13. The polarization state at position a where the light is emitted from the liquid crystal cell 10 is elliptically polarized as shown in FIG. The polarization states of each light A, B, and C are shown in Figure 1.
The light is elliptically polarized as shown in 4(b). Of the lights A, B, and C, the lights A and B pass through the first phase plate 31 only once and enter the second phase plate 22, so the second phase plate for the lights A and B passes through the first phase plate 31 only once and enters the second phase plate 22. The polarization state at position c where the light emitted from 22 becomes linearly polarized light as shown in FIG. 14(c). In addition, in FIG. 14(c), 22a indicates the transmission axis of the output side polarizing plate 22, and as mentioned above, the light A and B that become linearly polarized light and enter the output side polarizing plate 22 are transmitted from this output side. Polarizing plate 2
Absorbed in 2. On the other hand, the light C reflected by the second phase plate 22 remains in the polarized state as shown in FIG.
Since the light enters the phase plate 31 and undergoes two degrees of polarization during the process of reciprocating through the first phase plate 31, the polarization state at position d when the light exits from the first phase plate 31 again is as shown in FIG.
As shown in d), elliptically polarized light is obtained in which the axial direction of the ellipse is shifted from the polarization state shown in FIG. 14(b). Then, since this light B enters the second phase plate 22 after this, the polarization state of this light B at the position e where it exits the second phase plate 22 is as shown in FIG. 14(e).
As shown in FIG. 14(d), the elliptically polarized light has a longer single axis length than the state shown in FIG. 14(d). Therefore, since this light B does not become linearly polarized light and enters the output side polarizing plate 22, the transmission axis 2 of the output side polarizing plate 22
The wavelength light of the vibration component along the direction 2a is emitted from the output side polarizing plate 22.
It passes through and becomes leaked light.

[0017] The present invention was made in view of the above circumstances, and its object is to arrange a plurality of phase plates between an STN type liquid crystal cell and one polarizing plate. Although it compensates for the polarization state of transmitted light,
Most of the transmitted light in the OFF state is made into almost linearly polarized light and is incident on the polarizing plate on the output side, making it possible to almost completely prevent light leakage in the OFF part.The OFF part is made closer to black. To provide a liquid crystal display device capable of displaying high-quality images with high contrast and without display coloring.

[0018]

[Means for Solving the Problems] The liquid crystal display device of the present invention comprises an STN type liquid crystal cell in which liquid crystal molecules are arranged in a twisted manner at a twist angle of 180° to 270°, and a pair of liquid crystal cells arranged before and after this liquid crystal cell. a polarizing plate, and a plurality of phase plates are arranged between the liquid crystal cell and one of the polarizing plates, and each of these phase plates is bonded to each other with a transparent adhesive. It is.

[0019]

[Operation] That is, in the present invention, by bonding a plurality of phase plates arranged between a liquid crystal cell and one polarizing plate to each other with a transparent adhesive, the light of the part (adhesive layer) between each phase plate is The refractive index of the phase plate is made close to the refractive index of the phase plate.In this way, the light that passes through each phase plate sequentially passes through each phase plate one by one without being reflected by the phase plate surface.
Since the transmitted light only passes through the same polarizing plate twice, it does not receive any unnecessary polarization effect.
Most of the transmitted light of the portion can be converted into substantially linearly polarized light and made incident on the polarizing plate on the output side.

[0020]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIGS. 1 to 10.

FIG. 1 is a sectional view of the liquid crystal display device of this embodiment. This liquid crystal display device rotates liquid crystal molecules from 180° to
A pair of polarizing plates 21 and 22 are arranged on both sides of an STN type liquid crystal cell 10 arranged in a twisted manner at a twist angle of 270°, and the liquid crystal cell 10 and the pair of polarizing plates 21 and 2
In this embodiment, as shown in the figure, two phase plates 31 and 32 are arranged between one of the phase plates 31 and 2.
, 32 are arranged between the liquid crystal cell 10 and the output side polarizing plate (the right polarizing plate in the figure) 22. Note that the liquid crystal cell 10 has the same structure as that shown in FIG. 11, so the description thereof will be omitted by attaching the same reference numerals to the figure. However, even if this liquid crystal cell 10 displays a black and white monochrome image, each scanning electrode 14 or each signal electrode is placed under the alignment film (below or above the electrode) on one of the substrates 11 and 12. It may also be a color image display cell in which red, green, and blue color filters (not shown) are alternately arranged to correspond to the color filters 15 and 15, respectively.

In this liquid crystal display device, the opposing surfaces of the phase plates 31 and 32 are bonded to each other with a transparent adhesive 41 over the entire light transmitting area. Furthermore, in this embodiment, among the phase plates 31 and 32, the liquid crystal cell 1
The incident surface of the first phase plate 31 on the 0 side is adhered to the outer surface of the output side substrate 12 of the liquid crystal cell 10 with a transparent adhesive 42, and the polarizing plate 2
The output surface of the second phase plate 32 on the second side is adhered to the input surface of the output side polarizing plate 22 with a transparent adhesive 43, and then the output side of the second phase plate 32 on the input side
The output surface of the liquid crystal cell 10 is bonded to the outer surface of the input side substrate 12 of the liquid crystal cell 10 with a transparent adhesive 44.

Further, the value of retardation Δn10·d10 (product of refractive index anisotropy Δn10 of liquid crystal and liquid crystal layer thickness d10) of the liquid crystal cell 10 is 700 nm to 1000 nm.
m, and in this embodiment, a liquid crystal having a refractive index anisotropy Δn10 of 160 nm or more is used as the liquid crystal 18 sealed in the liquid crystal cell 10, and the refractive index anisotropy Δn10 of this liquid crystal 18 is
The layer thickness (cell gap) d10 of the liquid crystal layer is set according to the above, and the retardation Δn10·d10 of the liquid crystal cell 10 is set to the above value.

On the other hand, among the two phase plates 31 and 32, the first phase plate 31 on the liquid crystal cell 10 side is a 1/4 wavelength phase plate, and the second phase plate 32 on the polarizing plate 22 side is a 1/4 wavelength phase plate. It is considered to be a one-wavelength phase plate. Then, the retardation Δn31·d31 of the first phase plate 31 (refractive index anisotropy Δ
The value of the product of n31 and plate thickness d31 depends on the retardation Δn10・d10 of the liquid crystal cell 10.
When the light that has been emitted and transmitted through the first phase plate 31 is placed on the Poincare sphere and viewed, the wavelength light in the visible light band follows the peripheral line of the cut surface when the Poincare sphere is cut by a plane. It is said that the value is distributed around that area. In this example,
The retardation Δn31・d31 of the first phase plate 31 is
It is 125 nm to 150 nm.

Furthermore, the retardation Δ of the second phase plate 32
The values of n32 and d32 are determined depending on the retardation Δn10 and d10 of the liquid crystal cell 10 and the retardation Δn31 and d31 of the first phase plate 31. When the light transmitted through the phase plate 32 is placed on the Poincare sphere and viewed, the wavelength light in the visible light band is concentrated and distributed near one point on the equator of the Poincare sphere. In this example, the second
The retardation Δn32・d32 of the phase plate 32 is 40
It is 0 nm to 700 nm.

The first phase plate 31 and the second phase plate 32
The incident-side polarizing plate 21 and the exit-side polarizing plate 22 are arranged with their optical axes aligned in the following directions. FIG. 2 shows the liquid crystal molecule alignment direction (rubbing direction of the alignment films 16 and 17) on both substrates 11 and 12 of the liquid crystal cell 10, the direction of the fast axes of the phase plates 31 and 32, and the direction of the polarizing plate 21.
, 22, and as shown in FIG. Twist angle θ of liquid crystal molecules clockwise with respect to the liquid crystal molecule orientation direction 11a on the substrate 11 surface (θ=180° to 270°)
The orientation directions 11a and 12a of liquid crystal molecules on both substrates 11 and 12 respectively intersect at the same angle with respect to the horizontal line Z in the viewing angle direction of the liquid crystal display device. Further, as shown in FIG. 2(b), the fast axis 31a of the first phase plate 31 on the liquid crystal cell 10 side rotates counterclockwise with respect to the liquid crystal molecule orientation direction 12a on the output side substrate 12 surface of the liquid crystal cell 10. 90
The fast axis 31a of the second phase plate 32 on the polarizing plate 22 side is at an angle α1 of 100° to 100° counterclockwise with respect to the liquid crystal molecule alignment direction 12a.
The directions intersect at an angle α2 of 150°. moreover,
As shown in FIGS. 2(c) and 2(d), the polarization axis 21a of the incident side polarizing plate 21 is rotated counterclockwise by 110 degrees with respect to the liquid crystal molecule alignment direction 11a on the incident side substrate 11 surface of the liquid crystal cell 10.
The polarization axis 22a of the output side polarizing plate 22 is in a direction that intersects with the output side substrate 12 of the liquid crystal cell 10 at an angle β of 160°.
70° counterclockwise with respect to the liquid crystal molecule orientation direction 12a on the surface
The directions intersect at an angle γ of 190°. Here, clockwise rotation and counterclockwise rotation both refer to directions when viewed from the light output side.

Note that the liquid crystal display device of this example has an OF
A so-called negative mode in which light is blocked when the liquid crystal molecules in the liquid crystal cell 10 enter the initial alignment state by application of the F voltage, and light is transmitted when the liquid crystal molecules rise and enter the alignment state by application of the ON voltage. It is a display type, and the polarization axis direction 21a of the input side polarizing plate 21 and the output side polarizing plate 22 are
The polarization axis direction 22a is within the range of the above angles β and γ,
The angle is set so that the display on the liquid crystal display device is in negative mode.

In this liquid crystal display device, two phase plates 3 are disposed between the STN type liquid crystal cell 10 and the output side polarizing plate 22.
1 and 32, and these two phase plates 31,
32, the value of the retardation Δn31・d31 of the first phase plate 31 on the liquid crystal cell 10 side is calculated as the retardation Δn10・d10 of the liquid crystal cell 10 (Δn10・d10=7
00 nm to 1000 nm), when the light emitted from the liquid crystal cell 10 and transmitted through the first phase plate 31 is arranged on the Poincaré sphere, the wavelength light in the visible light band crosses the Poincaré sphere in a plane. Values distributed around the peripheral line of the cut surface when cutting (Δn31・d31=125 nm ~
150 nm), and the value of retardation Δn32·d32 of the second phase plate 32 on the polarizing plate 22 side is set to
When the light that has been emitted, transmitted through the first phase plate 31, and then transmitted through the second phase plate 32 is placed on the Poincare sphere, the wavelength light in the visible light band is reflected at a point on the equator of the Poincare sphere. Values concentrated in the vicinity (Δn32・d32=40
(0 nm to 700 nm), the liquid crystal cell 10
The emitted elliptically polarized light can be converted into linearly polarized light by the polarization action of the first phase plate 31 and the second phase plate 32. Therefore, according to this liquid crystal display device, light leakage is reduced when OFF (light is blocked), and light transmittance is also increased when ON (light is transmitted), thereby increasing contrast.
Furthermore, high-quality images can be displayed without coloring the display.

FIG. 3 shows the spectrum of transmitted light of the liquid crystal display device, and as shown in this spectrum,
This liquid crystal display device has high transmittance when ON, and also has high transmittance when ON.
The transmittance at F is almost 0, so a high contrast display can be obtained. In addition, in this liquid crystal display device, the transmittance when turned off is almost constant over almost all wavelengths in the visible light band, and the transmittance when turned on is also at a high level over almost all wavelengths in the visible light band. Coloration due to wavelength dependence of the liquid crystal cell 10 does not occur. Note that the spectrum in FIG. 3 is based on the liquid crystal molecule twist angle θ and retardation Δn10・d10 of the liquid crystal cell 10.
θ=220°, Δn10・d10=750 nm, and the retardation Δn31・d31 of the first phase plate 31
130 nm, retardation Δn of the second phase plate 32
32·d32 is set to 575 nm, and the first phase plate 31 and second phase plate 32 and polarizing plates 21 and 22 are
The axial angles α1, α2, β, and γ shown in Fig. 2 are α1 = 75°, α2 = 100°, and β = 16, respectively.
5°, γ=135°.

To explain the change in the polarization state of transmitted light in the liquid crystal display device using the Poincare sphere, FIG. 4 shows the Poincare sphere which is generally used as a method of expressing the polarization state. In FIG. 4, C is the spherical axis of the Poincaré sphere (an axis corresponding to the earth's axis), and E is the equator. Both poles of this Poincaré sphere represent circularly polarized light R, the area on the equator E represents linearly polarized light S, and the other regions represent elliptically polarized light. A point on the equator E with a longitude of 0° is linearly polarized light along the equator E, a point with a longitude of 90° is linearly polarized light along the spherical axis C, and a point with a longitude of 45° is the equator E passing through this point. Linearly polarized light in a direction 45° to a parallel tangent, a point at longitude 135° is 135° to a tangent parallel to the equator E passing through this point
It is linearly polarized light in the direction (direction perpendicular to the linearly polarized light at a point at 45° longitude). Also, the upper hemisphere represents right-handed polarized light,
The lower hemisphere represents left-handed polarized light. The latitude τ and longitude ψ of the distribution point of each wavelength of elliptically polarized light on this Poincaré sphere are:
The relationship between the ellipticity ε of polarized light and the angle ξ in the major axis direction of the ellipse (the angle with respect to the horizontal line Z shown in FIG. 2) is expressed by the following equation.

FIG. 5 is a diagram showing the distribution state of each wavelength of light when the light emitted from the liquid crystal cell 10 is arranged on the Poincare sphere as seen from the upper pole side of the Poincare sphere. Here, the liquid crystal molecule twist angle θ
is 240°, retardation Δn10・d10 is 78
It shows the distribution of light emitted from a 0 nm liquid crystal cell. As shown in FIG. 5, the light A1 emitted from the liquid crystal cell 10 is elliptically polarized light in which each wavelength of light is distributed as if diagonally crossed over a Poincaré sphere.

On the other hand, the phase plate changes the distribution state of each wavelength of light on the Poincaré sphere with a straight line (fast axis of the phase plate) passing through the center of the sphere and a certain point on the equator E as the center. It has the function of rotating, and therefore, the light that is emitted from the liquid crystal cell 10 and transmitted through the first phase plate 31 has the following polarization state.

FIG. 6 is a diagram showing the distribution state of each wavelength light when the light emitted from the liquid crystal cell 10 and transmitted through the first phase plate 31 is arranged on the Poincare sphere, as seen from the upper pole side of the Poincare sphere. Here, the retardation Δ is used as the first phase plate 31.
A phase plate with n31 and d31 of 137 nm is used, and the first phase plate 31 is connected to a straight line (liquid crystal cell 10 shows the distribution of light of each wavelength when arranged in a direction of 92.5° with respect to the liquid crystal molecule alignment direction 12a on the emission side substrate 12 surface of No. 10. When the phase plate 31 of the first phase plate 31 is arranged in this way and the light emitted from the liquid crystal cell 10 is passed through the first phase plate 31, the distribution state of each wavelength light emitted from the liquid crystal cell 10 is changed to the first phase plate 31. The phase plate 31 rotates around the sphere at 137 nm/λ from the state shown in FIG. As a result of the rotation, each wavelength light in the visible light band is distributed in the vicinity along the peripheral line L of the cut plane when the Poincaré sphere is cut along a certain plane, as shown in FIG.

FIG. 7 shows the distribution state of each wavelength light by distributing the light emitted from the liquid crystal cell 10, passing through the first phase plate 31, and then passing through the second phase plate 32 on the Poincaré sphere. This is a diagram seen from the upper pole side of the Poincaré sphere, and here,
Retardation Δn32・d32 as the second phase plate 32
A phase plate with a wavelength of 610 nm is used, and the second phase plate 32 is connected so that its fast axis 32a is a straight line passing through the center of the Poincaré sphere and a point at a longitude of 90° on the equator E (the output side substrate of the liquid crystal cell 10). 1 for the orientation direction 12a of liquid crystal molecules on the 12th plane
It shows the distribution of each wavelength light when arranged in accordance with the 20° direction). When the second phase plate 32 is arranged in this way and the light that has been emitted from the liquid crystal cell 10 and transmitted through the first phase plate 31 is passed through the second phase plate 32, the light is emitted from the liquid crystal cell 10 and passes through the first phase plate 31. The second phase plate 32 further changes the distribution state of each wavelength light transmitted from the state shown in FIG. The light of each wavelength in the visible light band is rotated around the sphere at 610 nm/λ around a straight line passing through the sphere, and as shown in Fig.
2.5°). Since the area on the equator E of the Poincare sphere represents linearly polarized light as described above, each wavelength of light that is concentrated and distributed near one point on the equator E of the Poincare sphere as shown in Figure 7 becomes almost linearly polarized light. .

FIGS. 8 to 10 show light emitted from the liquid crystal cell 10, light transmitted through the first phase plate 31, and light transmitted through the second phase plate 32 when an OFF voltage is applied to the liquid crystal cell 10. The latitude τ and longitude ψ of the distribution point of each wavelength light on the Poincaré sphere are shown, and the light emitted from the liquid crystal cell 10 has the latitude τ and longitude ψ of each wavelength light as shown in FIG. It is elliptically polarized light with a distribution of ψ. However, when the light emitted from this liquid crystal cell 10 is passed through the first phase plate 31, this light is polarized by the first phase plate 31, and each wavelength light is distributed at latitude τ and longitude ψ as shown in FIG. be in a state of doing so. Then, when this light is further passed through the second phase plate 32, the light transmitted through the second phase plate 32 falls within the visible light band (here, 450nm to 650nm), as shown in FIG.
The wavelength light of m) is distributed almost on the line of latitude τ = 0°, that is, on the equator E, and the longitude ψ where the wavelength light of this visible light band is located.
(in FIG. 10, ψ=approximately 90°), that is, the distribution is concentrated near a point on the equator E. And Figure 1
As shown in Figure 0, the fact that the latitude τ and longitude ψ of the distribution points of each wavelength light are almost the same means that the phase difference between each wavelength light has almost disappeared. The light transmitted through the two-phase plate 32 is approximately linearly polarized light.

Note that the polarization states shown in FIGS. 8 to 10 are as follows:
As the liquid crystal cell 10, the twist angle of the liquid crystal molecules is 240°.
, a liquid crystal cell with a retardation Δn10·d10 of 780 nm is used, and the retardation Δn10 is used as the first phase plate 31.
Phase plate with n31 and d31 of 140 nm, second phase plate 3
2, the retardation Δn32・d32 is 610n
These are the measured values when an OFF voltage of 20.6 V was applied between the electrodes 14 and 15 of the liquid crystal cell 10 using a phase plate of m.

Furthermore, in the above liquid crystal display device, the two phase plates 31 and 32 disposed between the liquid crystal cell 10 and the output-side polarizing plate 22 are bonded to each other with a transparent adhesive 41. The optical refractive index of the layer of adhesive 41 is
Since the refractive index of the phase plates 31 and 32 is much closer to that of the air layer, the light that passes through each of the phase plates 31 and 32 sequentially passes through each phase plate 31 and 3 without being reflected by any of the phase plate surfaces.
You only have to go through each of 2 once.

That is, in the conventional liquid crystal display device shown in FIG. 11, since there is a small air layer between the first phase plate 31 and the second phase plate 22, Of the light beams B and C shown in FIG. 12, the lights B and C shown in FIG. The light C that is reflected by the incident surface after being incident on the first phase plate 31 is subjected to an extra polarization effect by the first phase plate 31. However, in the liquid crystal display device of the above embodiment, the transparent adhesive 41 between the phase plates 31 and 22 Since the refractive index of the layer is close to the refractive index of the phase plates 31 and 32, the first phase plate 3
The light emitted from the second phase plate 22 enters the second phase plate 22 without being reflected by the second phase plate 22. Note that surface reflection of light decreases as the ratio of refractive indexes at the interface approaches 1. Therefore, as for the transparent adhesive 41, the refractive index n thereof is equal to the refractive index n1, n2 of the phase plates 31, 22. for,

0
039]

[Math 1]

If we use something that has the following relationship, we can obtain the second
Reflection of light on the incident surface of the phase plate 22 can be almost completely eliminated. Therefore, the transmitted light only passes through each of the phase plates 31 and 32 once, and therefore, the transmitted light does not pass through the same polarizing plate twice and undergo unnecessary polarization, so the transmitted light in the OFF portion Most of the light is converted into linearly polarized light by each of the phase plates 31 and 32, and then enters the output side polarizing plate 22.

Therefore, according to the liquid crystal display device, O
In the FF state, most of the light transmitted through the liquid crystal cell 10 can be input to the output side polarizing plate 22 as linearly polarized light, and if the light incident on the output side polarizing plate 22 is linearly polarized, the Since light leakage can be almost completely prevented, it is possible to make the OFF part closer to black, increasing the contrast, and displaying a high-quality image without coloring the display. This is true not only for liquid crystal display devices that display black and white monochrome images, but also for liquid crystal display devices that display full-color images. It does not take on the color of red or green, and is colored with a color filter. The blue display will not be tinged with other colors and color shift will not occur.

Furthermore, in this embodiment, two phase plates 31
, 32 to each other with a transparent adhesive 41, the first phase plate 31 and the liquid crystal cell 10, the second phase plate 32 and the output side polarizing plate 22, and the input side polarizing plate 21 and the liquid crystal cell 1.
0 are also adhered with transparent adhesives 42, 43, and 44, respectively, so that reflection of light between them can also be eliminated. Therefore, the light emitted from the liquid crystal cell 10 is reflected by the first phase plate 31 and returns to the liquid crystal cell 10, further reflected by the incident side substrate 11 or the incident side polarizing plate 21 of the liquid crystal cell 10, and then emitted from the liquid crystal cell 10 again. or phase plate 31,
The light that has passed through 32 is reflected by the output side polarizing plate 22 and returns to the phase plates 31 and 32, and is further reflected by the first phase plate 31 or the liquid crystal cell 10 and may be emitted through the phase plates 31 and 32 again. Therefore, the OFF portion can be made closer to black, and coloring of the display can be further prevented.

Note that in the above embodiment, the phase plates 31 and 32
Although the side on which the phase plates 31 and 32 are arranged is defined as the light output side, the above liquid crystal display device can also use the side on which the phase plates 31 and 32 are not arranged as the light output side. The incident light passes through the second phase plate 32 and the first phase plate 31 and is polarized to compensate for the polarization effect of the liquid crystal cell 10, and then passes through the liquid crystal cell 10 to become linearly polarized light.

Furthermore, in the above embodiment, two phase plates 31
. By adhering them to each other, the same effect as in the above embodiment can be obtained.

Furthermore, in the above embodiment, the first phase plate 31
and the liquid crystal cell 10, the second phase plate 32, and the output side polarizing plate 22,
The incident-side polarizing plate 21 and the liquid crystal cell 10 are also bonded together using transparent adhesives 42, 43, and 44, but these do not necessarily need to be bonded together.

[0046]

Effects of the Invention The liquid crystal display device of the present invention has a plurality of phase plates disposed between a liquid crystal cell and one polarizing plate, which are bonded to each other with a transparent adhesive. The refractive index of the light in the phase plate (layer) is made close to the refractive index of the phase plate to prevent the light that passes through each phase plate in sequence from being reflected on the phase plate surface. Since the transmitted light only passes through each phase plate once, the transmitted light does not pass through the same polarizing plate twice and undergoes an unnecessary polarization effect. According to this liquid crystal display device,
Although multiple phase plates are arranged between the STN type liquid crystal cell and one of the polarizing plates to compensate for the polarization state of transmitted light, most of the transmitted light in the OFF state is converted into almost linearly polarized light and output. It is possible to almost completely prevent light leakage at the OFF part by letting it enter the polarizing plate on the side, and therefore,
It is possible to make the OFF part closer to black to increase the contrast, and to display a high-quality image without coloring the display.

[Brief explanation of the drawing]

FIG. 1 is a sectional view of a liquid crystal display device according to an embodiment of the present invention.

FIG. 2 is a diagram showing the alignment direction of liquid crystal molecules on both substrate surfaces of a liquid crystal cell and the direction of optical axes of a phase plate and a polarizing plate.

FIG. 3 is a diagram showing a spectrum of transmitted light of a liquid crystal display device.

FIG. 4 is a perspective view of the Poincaré sphere.

FIG. 5 is a distribution state diagram of light of each wavelength when the light emitted from a liquid crystal cell is arranged on a Poincaré sphere.

FIG. 6 is a diagram showing the distribution of light of each wavelength when the light emitted from the liquid crystal cell and transmitted through the first phase plate is arranged on a Poincaré sphere.

FIG. 7 is a diagram showing the distribution of light of each wavelength when the light emitted from the liquid crystal cell and transmitted through the first and second phase plates is arranged on a Poincaré sphere.

FIG. 8 is a diagram showing the latitude and longitude of distribution points of light of each wavelength on the Poincaré sphere of light emitted from a liquid crystal cell.

FIG. 9 is a diagram showing the latitude and longitude of distribution points of light of each wavelength on the Poincaré sphere of light that has been emitted from a liquid crystal cell and transmitted through a first phase plate.

FIG. 10 is a diagram showing the latitude and longitude of distribution points of light of each wavelength on the Poincaré sphere of light that has been emitted from a liquid crystal cell and transmitted through first and second phase plates.

FIG. 11 is a diagram showing the spectrum of transmitted light of a liquid crystal display device using a conventional STN type liquid crystal cell.

FIG. 12 is a diagram showing the path of transmitted light in a conventional liquid crystal display device.

FIG. 13 is a developed view of the optical path of C in FIG. 12;

FIG. 14 is a diagram showing changes in the polarization state of transmitted light in a conventional liquid crystal display device.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 10... Liquid crystal cell, 11, 12... Transparent substrate, 14... Scanning electrode, 15... Signal electrode, 16, 17... Alignment film, 18... Liquid crystal, 21, 22... Polarizing plate, 31... First phase plate, 32... Third 2
Phase plate, 14, 42, 43, 44...transparent adhesive.

Claims (1)

[Claims] 1. A liquid crystal cell comprising: an STN type liquid crystal cell in which liquid crystal molecules are arranged in a twisted manner at a twist angle of 180° to 270°; and a pair of polarizing plates disposed before and after the liquid crystal cell; A liquid crystal display device characterized in that a plurality of phase plates are disposed between one polarizing plate and the phase plates are bonded to each other with a transparent adhesive.