JPH09105929A - Liquid crystal display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To lighten a device or to thin the whole device by arranging a polarization separator and a polarization converter so that contrast of light viewing through a liquid crystal panel becomes highest based on the refractive index of the polarization separator and the phase difference of the polarization converter and setting the polarization axis direction of light. SOLUTION: The polarization separator 28 and the polarization converter are arranged so that the contrast of light viewing through the liquid crystal panel 31 becomes highest based on the refractive index of the polarization separator 28 and the phase difference of the polarization converter. A fluorescent lamp 21 is brought into close contact with one side 24a of an acrylic plate light transmission body 24. Further, a λ/4 phase plate 25 is provided on the rear surface of the light transmission body 24 and the side surface of the light transmission body 24 opposite to the fluorescent lamp 21, and a reflection surface 26 is formed on the body 24. The polarization separator 28 is loaded on the light outgoing surface side of the light transmission body 24 so that its advance phase axis becomes parallel to the longitudinal direction of the fluorescent lamp 21. Further, a prism array 29, and a λ/2 phase difference plate 30 on the outside of the array 29 are set up. An incident side polarizing plate 32 and an outgoing side polarizing plate 33 are provided on the liquid crystal panel 31.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a planar light source provided behind a liquid crystal display device using a liquid crystal display system for modulating the polarization state of linearly polarized incident light, which is used in liquid crystal televisions, liquid crystal displays for computers, and the like. And a direct-view liquid crystal display device using the same.

[0002]

2. Description of the Related Art In recent years, the technological progress of liquid crystal display devices, especially liquid crystal display devices using color display devices, has been remarkable, and CR
Many displays with display qualities comparable to those of T have come to be seen.

Until a few years ago, a reflection type liquid crystal display device which does not use a backlight, which is a flat lighting device, was the mainstream.
At present, even in black and white display, it is almost replaced by a transmissive liquid crystal display element using a backlight. In addition, laptop computers have entered the popularization stage, and backlight-equipped models have swept the market. A color display liquid crystal display does not serve as a display without a backlight, and the backlight is an essential device in a direct-view liquid crystal display device.

Color liquid crystal display devices are roughly classified into two types: active matrix drive TN liquid crystal display devices using TFTs and multiplex drive STN liquid crystal display devices.
There are two types, a liquid crystal display system that modulates the polarization state of linearly polarized incident light by installing polarizing plates on the light incident side and the light emitting side of an element that holds a liquid crystal layer on a glass substrate. Is to do.

However, since the polarization directions of the incident light of the liquid crystal display element are not uniform and are random polarized light, in the case of both TN type and STN type liquid crystal elements, a polarizing plate mounted on the incident side of the display element causes More than half of the light was absorbed and the light utilization efficiency was low, resulting in a dark display screen. Alternatively, there is a problem that power consumption increases in order to make it bright.

The brightness level required for a backlight varies depending on its application, but especially in a color notebook computer, not only the required brightness but also thinning, weight reduction, and power saving (assuming battery drive) is a top priority. .

There are various methods for producing a flat lighting device, but they are roughly classified into two types. Generally, the most common method is an internal illumination method or a method directly below, in which the light source is inside the illuminated surface. On the other hand, in the edge light type, the light source is arranged outside the illuminated surface, and a fluorescent lamp (often a cold cathode discharge tube) or the like is provided on one or two sides of the light guide body made of a transparent acrylic resin plate or the like which is the illuminated surface. This is a system in which a substantially linear luminous body is brought into close contact and a lamp cover made of a reflector is provided to introduce light into the light guide body.

Since the color notebook personal computer is required to be particularly thin and lightweight, the edge light type backlight is effective. Necessary functions required for the light guide of the edge light type backlight are a function of sending the light incident from the end portion forward and a function of emitting the sent light to the liquid crystal display element side.

The former function depends on the material used and the interface reflection characteristics, and the latter function depends on the shape of the light guide surface that avoids the condition of total internal reflection. Regarding the shape of the light guide surface that avoids this total reflection condition, a method of forming a white diffusion material on the light guide surface and a method of forming a lenticular or prism Fresnel shape on the light guide surface are known.

On the other hand, when the liquid crystal display device is used as a light modulator for a transmissive projector and the tolerance for the depth of the device is large, in order to improve the light utilization efficiency of the light source lamp, A polarization separator for separating unpolarized light into polarized light orthogonal to each other is interposed between the lamp and the liquid crystal display device, one light is directly emitted from the polarization separator, and the other light is focused on the light source lamp. It has been proposed to use it again as a light source (for example, Japanese Laid-Open Patent Publication No. Hei 4).
-184429).

However, if this method is simply applied to a direct-view type liquid crystal display device, the thin and compact features of the direct-view type liquid crystal display device are impaired, which is not preferable.

In order to solve the above drawbacks, the inventors have proposed to use a multilayer polarizing plate as a polarization separator (Japanese Patent Laid-Open No. 7-49496).

[0013]

The polarization separator easily transmits light having a polarization axis in a specific direction, but easily reflects light having a polarization axis perpendicular to the specific direction. Have characteristics. The light reflected by the polarization splitter is
If the polarization axis is rotated by 90 ° so that the light enters the polarization separator again, the light passes through the polarization separator. As a result, the light that has passed through the polarization separator becomes light with polarized light that is polarized in a specific direction, and the amount of light in the specific direction increases. If the polarization axis transmitted through the polarization separator and the axial direction of the polarizing plate on the polarization separator side of the liquid crystal panel are substantially aligned, almost 100% of the light emitted from the polarization separator can be used, Use efficiency of light from the light source is increased.

The polarization axis of the light emerging from the polarization separator was determined by the light source located on the side of the planar light guide. Light polarized in a direction biased in the direction perpendicular to the linear light source is emitted. Therefore, in order to maximize the amount of light used by the liquid crystal panel, the axial direction of the polarizing plate on the polarization separator side of the liquid crystal panel had to be aligned with the polarization axis direction of the light emitted from the polarization separator. .

The liquid crystal panel has a high contrast direction and a low contrast direction depending on the viewing angle, and is usually designed to maximize the contrast in the viewing direction. This viewing angle is also affected by the angle of the polarizing plate and the like. If the angle of the polarizing plate on the polarization separator side of the liquid crystal panel is restricted by the lighting device, the viewing angle direction cannot be freely determined.
There was a problem that the device design was restricted.

In order to solve the drawbacks of the above-mentioned example, the inventors have proposed to further use a polarization rotator (JP-A-6-337413).

In the case where the polarization separator is a transparent support of at least one layer of dielectric interference film, and the transparent support is optically isotropic, almost all the problems described above are proposed by the above-mentioned JP-A-6-337413. Although the point is solved, in the case where the translucent material is a birefringent material, it is not always the case in JP-A-6-3374.
No. 13 couldn't solve it. In addition to the polarization separator, a multilayer structure having a structure in which a transparent material layer having a relatively large refractive index and a transparent material layer having a relatively small refractive index are stacked, or a flat material in a homogeneous transparent material is used. A structure in which various bubble layers are dispersed in layers is conceivable. Also in this case, when the material used exhibits birefringence, it could not always be solved by JP-A-6-337413.

[0018]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and includes a light source, a planar light guide arranged so that light from the light source enters from a side surface, A planar polarization converter provided on the light reflecting surface side of the planar light guide, a polarization separator provided on the light emitting surface side of the planar light guide, and a liquid crystal provided outside the polarization separator. In a liquid crystal display device including a panel, the polarization separator and the polarization separator are arranged so that the contrast of light viewed through the liquid crystal panel is highest based on the refractive index of the polarization separator and the phase difference of the polarization converter. Provided is a liquid crystal display device in which a converter is arranged and a polarization axis direction of light is set.

In a preferred embodiment, a polarization rotator is provided between the polarization separator and the liquid crystal panel, and in addition to the phase difference of the polarization separator and the phase difference of the polarization converter, the phase difference of the polarization rotator is added. Based on the above, the polarization axis direction of the light is set.

In a further preferred embodiment, the polarization separator comprises a birefringent transparent support having at least one layer of a dielectric interference film, and the fast axis direction of the support is the plane. It is characterized in that it substantially coincides with or is substantially orthogonal to the light emission direction of the light guide.

In yet another preferred embodiment, the polarization converter comprises a λ / 4 retardation plate, the polarization rotator comprises a λ / 2 retardation plate, and the support of the polarization separator is uniaxial. Is a resin film exhibiting birefringence.

In still another preferred embodiment, a polarization converter made of the λ / 4 retardation plate is provided on the back surface of the planar light guide, and a metal reflection film is formed on the back surface of the polarization converter. A prism array for increasing the amount of light in the direction perpendicular to the light guide is provided between the polarization separator and the polarization rotator composed of the λ / 2 retardation plate, and the liquid crystal panel has birefringence on both sides thereof. It is characterized by comprising a TFT liquid crystal display cell for color display in which films having the above are laminated.

In still another preferred embodiment, the polarization separator is formed by laminating titanium oxide on both surfaces of a polycarbonate film, and the λ / 2 retardation plate is
It is characterized in that it is made of polycarbonate, and its fast axis direction is inclined by 45 ° with respect to the fast axis direction of the polarization separator. It is generally known that the polarization axis of light can be rotated by transmitting light through a medium having birefringence or transmitting a medium having optical rotatory power. In addition, the polarization axis rotates even if the medium having the birefringence is superposed in multiple layers while rotating the optical axis.

In particular, when linearly polarized light enters a substance having birefringence, elliptically polarized light is obtained as the emitted light. The ellipticity and the major axis direction of the ellipse are determined by the size of the birefringence of the birefringent medium and the optical axis direction.

Therefore, when the polarization separator is a transparent support of at least one layer of dielectric interference film, a transparent material layer having a relatively large refractive index and a transparent material layer having a relatively small refractive index are used. Polarization rotation when the material used is a multilayer structure with a laminated structure or a structure in which flat bubble layers are dispersed in a layer in a homogeneous translucent material and the material used has birefringence. The polarization converter, so that the polarization axis of the polarized light emitted from the device has the highest contrast in the direction in which the liquid crystal panel is viewed,
The direction of the fast axis of the transparent support, which exhibits the phase difference and the birefringence of the materials used in the polarization separator as well as the polarization rotator, must be chosen. Therefore, it may be possible to omit either the polarization converter or the polarization rotator.

In particular, the polarization converter has a wavelength of λ of 1
When the polarization rotator has a phase difference of / 4 and the polarization rotator has a phase difference of ½ of the incident light wavelength λ, the polarization separator has at least one dielectric interference film. A transparent support exhibiting refraction, in which the direction of the fast axis of the transparent support exhibiting birefringence is substantially the same as or substantially orthogonal to the light emission direction of the planar light guide. Is preferred.

The wavelength region required for the liquid crystal display device is all visible light, and it is usually preferable to use a flat film as a retardation film, judging from the viewpoints of lightness, thinness, cost and the like. . Since it is not possible to satisfy the phase difference condition for all visible light, a wavelength of 550 nm is generally used.

The birefringence of the film is generally uniaxially stretched to obtain a uniaxial index ellipsoid. A difference between the refractive index in the stretching axis direction and the refractive index in the stretching axis perpendicular direction occurs, and birefringence occurs in the thickness direction.

[0029]

Embodiments of the present invention will be described with reference to FIGS.

The fluorescent lamp 21 (cold-cathode discharge tube) is closely attached to one side (end) 24a of the transparent acrylic resin plate light guide 24 which is the illuminated surface, and the lamp cover 22 including the reflector 23 is provided.
An edge light type backlight was used in which light was introduced into the light guide body.

The fluorescent lamp 21 has a length corresponding to the side length (125 mm) of a general-purpose notebook computer,
A 2 W cold cathode discharge tube having a tube diameter of 3 mm was used. Also,
The lamp cover 22 is a cylindrical or elliptic cylindrical reflecting mirror that wraps the cold cathode discharge tube, and the light guide 24 is a translucent light guide plate made of acrylic resin (n = 1.4).
In 9), a size of 128 mm × 225 mm × 2.8 mm was used. White dots were printed on the back surface of the translucent light guide plate to extract light to the liquid crystal display element side.

Further, a λ / 4 retardation plate 25 was provided on the back surface of the light guide 24 and the side surface of the light guide facing the fluorescent lamp installation surface, and a reflection surface 26 made of an Al metal reflection film was formed thereon.

As the polarization separator 28, two layers of titanium oxide (TiO ₂ : n = 2.35) are formed on the surface of a polycarbonate film having a phase difference of 70 nm, and the light emitting surface of the light guide 24 is formed. It was mounted on the side such that its fast axis was parallel to the longitudinal direction of the cold cathode discharge tube.

The polarization of the light emitted from the polarization separator used this time is biased in a direction substantially perpendicular to the linear light source.

As the prism array 29, an isosceles triangular prism array having a vertical cross section of 65 ° is used, and the prism array is arranged so that the vertical angle faces the polarization separator 28. The prism array plate had a thickness of 200 μm and the prism array pitch was about 30 μm. This makes it possible to increase the amount of light in the direction in which the maximum amount of light travels is perpendicular to the light guide plate.

Further on the outer side thereof, a λ / 2 retardation plate 30 is provided.
Was installed. The fast axis direction of the λ / 2 retardation plate was set at θ = 45 ° with respect to the light source vertical direction, that is, inclined as shown in FIG. Polycarbonate was used as a material, and had a birefringence of λ / 2 when measured at a wavelength of 550 nm. In FIG. 2, 41 is a fluorescent lamp, 42 is a light guide plate (light guide 24 of FIG. 1), 43 is a fast axis direction of a λ / 2 plate, 44 is an incident side polarization plate polarization axis, and 45 is an emission side. The polarization axis of the polarizing plate, 46 is the rubbing direction on the incident side, and 47 is the rubbing direction on the emitting side. Reference numeral 48 indicates the traveling axis direction of the support of the polarization separator 28.

The liquid crystal panel 31 uses a TFT liquid crystal display cell for color display in which two films having birefringence are laminated. As the incident side polarization plate 32, a normal light absorption type organic polarization plate was used. The polarization axis is θ = 90 °. As the emitting side polarizing plate 33, a normal light absorbing type organic polarizing plate was used. The polarization axis is θ = 0 °. The incident side rubbing direction is θ = 90
And the rubbing direction on the emission side is θ = 0 °.

The light emitted from the polarization separator is linearly polarized light in a direction substantially perpendicular to the light source, and the light utilization efficiency is improved, and the optimum viewing direction of the liquid crystal panel (direction of arrow C in FIG. 1).
It was designed to have the highest contrast.

In the present embodiment, λ /
2 The thickness of the surface of the retardation plate 30 (planar light source portion) is 6.5 m.
m. Thus, the present invention makes it possible to reduce the size of the device. The thickness of the planar light source unit can be set within 15 mm, preferably within 10 mm.

[0040]

According to the present invention, it is possible to obtain a liquid crystal display device in which the viewing angle direction is not restricted by the position of the illumination device and the light utilization efficiency is high.

In particular, in the present invention, in a liquid crystal display device using a polarization separator in order to improve the light utilization efficiency, when the polarization separator has birefringence, the polarization axis of the light is taken into consideration by taking the refractive index into consideration. By setting the direction, it can be configured so that the contrast of the light passing through the liquid crystal panel becomes the highest in the viewing direction. Therefore, as a substrate of the polarization separator, a resin film having birefringence other than glass is used. Can be used, and the weight of the device can be reduced. Further, the planar illumination device or the planar light source used in the present invention can be formed extremely thin, and the liquid crystal display device as a whole can be made thin.

[Brief description of the drawings]

FIG. 1 is a sectional view showing an embodiment of the present invention.

FIG. 2 is an optical axis arrangement diagram of an embodiment of the present invention.

[Explanation of symbols]

 21: Fluorescent lamp 22: Lamp cover 23: Reflector 24: Light guide 25: λ / 4 phase difference plate 26: Reflection surface 28: Polarization separator 29: Prism array 30: λ / 2 phase difference plate 31: Liquid crystal panel 32: Incident side polarization plate 33: Emission side polarization plate 41: Fluorescent lamp 42: Light guide plate 43: λ / 2 fast axis direction of plate 44: Incident side polarization plate polarization axis 45: Emission side polarization plate polarization axis 46: Incidence Side rubbing direction 47: emission side rubbing direction 48: fast axis direction of support of polarization separator

Claims (6)

[Claims] 1. A light source, a planar light guide arranged so that light from the light source enters from a side surface, and a planar polarization converter provided on the light reflecting surface side of the planar light guide. A liquid crystal display device comprising: a polarization separator provided on the light emission surface side of the planar light guide; and a liquid crystal panel provided outside the polarization separator, wherein the refractive index of the polarization separator and the polarization Based on the phase difference of the converter, the polarization separator and the polarization converter are arranged to set the polarization axis direction of the light so that the contrast of the light viewed through the liquid crystal panel becomes the highest. Display device. 2. A polarization rotator is provided between the polarization separator and the liquid crystal panel, and based on the phase difference of the polarization separator and the phase difference of the polarization converter, the phase difference of the polarization rotator,
The liquid crystal display device according to claim 1, wherein a polarization axis direction of the light is set. 3. The polarization separator comprises a transparent birefringent support having at least one layer of a dielectric interference film, and the fast axis direction of the support is the planar light guide of the planar light guide. 3. The liquid crystal display device according to claim 1, wherein the liquid crystal display device has a substantially same or substantially orthogonal relationship with the light emitting surface. 4. The polarization converter is composed of a λ / 4 phase difference plate, the polarization rotator is composed of a λ / 2 phase difference plate, and the support of the polarization separator is a resin exhibiting uniaxial birefringence. The liquid crystal display device according to claim 3, wherein the liquid crystal display device is a film. 5. A polarization converter comprising the λ / 4 phase difference plate is provided on the back surface of the planar light guide, and a metal reflection film is formed on the back surface of the polarization converter, and the polarization separator and the λ The liquid crystal display device according to claim 4, wherein a prism array for increasing the amount of light in the direction perpendicular to the light guide is provided between the polarization rotator formed of a / 2 phase difference plate. 6. The polarization separator is formed by laminating titanium oxide on both surfaces of a polycarbonate film, the λ / 2 retardation plate is made of polycarbonate, and the fast axis direction thereof is the polarization separator. 4 to the fast axis direction of
The liquid crystal display device according to claim 5, wherein the liquid crystal display device is arranged at an angle of 5 °.