JPH09326205A - Backlight and liquid crystal display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To intensify brightness and contrast. SOLUTION: Only P polarized light is caused to come into a light guide plate 2 by combining a polarization beam splitter 19, a reflection element 18, and the like, and only P polarized light is caused to come into a liquid crystal cell by combining them further with the light guide plate 2 capable of keeping a plane of polarization. With this, brightness and contrast of a liquid crystal display element are improved by enabling light from a light source 1 to come into the liquid crystal cell without a loss caused by a polarization filter.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a backlight of a liquid crystal display device (back lighting device) and a liquid crystal display device using the same.

[0002]

2. Description of the Related Art In recent years, miniaturization of personal computers has been promoted, and a portable type called a laptop type has become widespread. In this laptop type,
A liquid crystal device is usually used for the display, but with the recent color display, a light source and a light guide plate are provided behind the liquid crystal display plate to illuminate the entire display surface from the back side. Shaped display devices are becoming popular.

The conventional structure of the liquid crystal display device will be described below.

First, FIG. 8 shows a sectional view of the structure of a conventional backlight and a TFT-LCD color liquid crystal display device using the same. The backlight 8 includes a light source 1 which is a cold cathode tube, a light guide plate 2 formed of a transparent resin such as acrylic resin, and print dots formed on a lower surface of the light guide plate for scattering light guided to the light guide plate. 6, and the reflection sheet 7,
It is composed of a diffusion sheet 3 installed on the upper surface of the light guide plate 2 for scattering light, a first prism sheet 4 and a second prism sheet 5 for condensing the scattered light in the x direction and the y direction, respectively. There is.

That is, in the conventional backlight 8, the light source 1
Light emitted from the light guide plate 2 is guided to the printing dot 6
A part of the scattered light is again incident on the light guide plate 2 by the reflection sheet 7, and then passes through the diffusion sheet 3 to be applied to the liquid crystal element. Here, since the diffusion sheet 3 is used, the polarization direction of the light with which the liquid crystal element is irradiated is random.

Further, as shown in FIG. 8, the liquid crystal panel of the TFT-LCD color display device has a polarizing filter 9 attached to the lower surface of the glass 10 substrate, a TFT (thin film transistor) 11 formed on the upper surface of the glass 10 substrate, It is composed of a transparent electrode 12, an alignment film 13, a liquid crystal 14, an alignment film 13, a counter transparent electrode 15, a color filter 16 and a polarization filter 17.

[0007]

The transmittance of the polarization filter 9 is one of the bottleneck for improving the brightness of the liquid crystal display device. The polarization filter 9 is an element that is disposed between the liquid crystal cell and the backlight and has a function of causing predetermined polarized light to enter the liquid crystal cell. The polarizing filter 9 can be generally manufactured by adsorbing a dichroic substance on a micelle tube of a polymer film in which micelles are arranged in a certain direction. Here, polyvinyl alcohol (PVA) is used as the polymer film, and this is stretched about 3 to 5 times in a certain direction between rollers that rotate at different speeds. The drawn PVA micelles are arranged in the drawing direction, and the arranged film has strong birefringence. Examples of the substance for imparting dichroism include a halogen substance such as iodine and a dye, and the polarizing property is exhibited by adsorbing the substance on a stretched film. The polarization filter can easily obtain the polarization separation function, but since its transmittance uses a dichroic substance in principle, it absorbs polarized light that is orthogonal to the transmitted polarized light, so The rate is as small as 50% or less. Therefore, 50% or more of light energy is lost in the polarization filter, and the brightness of the liquid crystal display device is significantly reduced under the present circumstances.

As a means for improving this, an attempt has been made to suppress the reflection of the polarization filter and improve the transmittance. For example, there is a method of coating an antireflection film to reduce the reflectance and improve the transmittance. However, the effect of these attempts is at most several percent, and the effect is small.

To improve the brightness, it is easy to increase the brightness of the light source. However, in a laptop personal computer or the like, a battery or the like is used as a driving source, so that the brightness of the light source is increased due to the power consumption. There was a limit and there was no effective method.

The present invention has been made to overcome such a situation, and a backlight capable of improving the brightness of a liquid crystal display device without improving the brightness of the light source by improving the conventional defects, Another object of the present invention is to provide a liquid crystal display device using the same. In particular, it is an object of the present invention to provide a backlight and a liquid crystal display device having high brightness and contrast by improving the transmittance of the polarization filter.

[0011]

According to the present invention, a liquid crystal display device comprising a liquid crystal cell, a light guide plate arranged on the back surface of the liquid crystal cell, and a light source arranged on the side surface of the light guide plate is provided with a specific polarization. A polarization separation element having a function of transmitting light (for example, P-polarized light) and reflecting polarized light (for example, S-polarized light) orthogonal to the light is arranged in the optical path, and the reflected S-polarized light is scattered by a scattering plate or the like. A method of converting the light into circularly polarized light or random polarized light, re-entering the polarization separation element, and transmitting P-polarized light is adopted.

Specifically, the backlight of the liquid crystal display device according to the present invention is a backlight for supplying light from the back surface of the liquid crystal cell to the liquid crystal cell by a light source, and an optical path between the light source and the liquid crystal cell. Inside, a polarized light separating element which transmits a predetermined polarized light from the light source and reflects a polarized light substantially orthogonal thereto, and a polarization direction of the polarized light reflected by the element is converted into random, elliptical or circular polarized light. A polarization conversion element is provided, and the polarization plane converted light by the polarization conversion element is re-incident on the polarization separation element.

The "polarization separating element" is, for example, a "dielectric multilayer film polarization beam splitter" in the embodiments described later, and the "polarization conversion element" is, for example, a "diffuse reflection plate" in the embodiments described later. 18 "" Phase plate 3
3 ”and“ diffuse reflection film 31 ”.

According to the structure of the present invention, compared with the conventional structure, the transmittance of the polarizing film can be greatly improved, and as a result, the brightness and contrast of the liquid crystal display device can be improved.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION Preferred embodiments of a backlight according to the present invention and a liquid crystal display device using the same will be described in detail below.

(First Embodiment) FIG. 1 shows a first embodiment of the present invention.
It is a configuration diagram of a backlight device showing an embodiment of,
This is an example in which a dielectric multilayer film polarization beam splitter is used as an element that transmits a predetermined polarized light of the light from the light source and reflects the polarized light substantially orthogonal thereto.

The backlight of this embodiment is shown in FIG.
As shown in FIGS. 1A and 1C, the light source 1, the dielectric multilayer polarization beam splitter 19, the diffuse reflection plate 18, and the light guide plate 2 are included. The dielectric multilayer polarization beam splitter 19 and the light guide plate 2 are bonded with an adhesive 20 as needed.

FIG. 1B is a diagram schematically showing a ray trace near the light source. In FIG. 1B, the light emitted from the light source 1 which is a cold cathode tube in this case is a diffuse reflection plate 18. And is incident on the dielectric multilayer polarization beam splitter 19 formed on the surface of the prism sheet. The light emitted from the light source 1 is randomly polarized. P of the above incident light
Only (parallel) polarized light passes through the dielectric multilayer film polarization beam splitter 19 and enters the light guide plate 2. On the other hand, P
The S (Zenklit) polarized light orthogonal to the polarized light is reflected by the dielectric multilayer film polarization beam splitter 19 and then returns to the light source 1 direction. The returned light is the diffuse reflection plate 18 or the light source 1.
Is reflected and scattered by the cold cathode tube of and converted into random polarization,
After entering the dielectric multilayer polarization beam splitter 19 again, only the P-polarized component enters the light guide plate 2. With this configuration, the total amount of light transmitted from the light source 1 to the light guide plate 2 can be increased.

In FIG. 1A, reflective dots 21, a reflective film 22 and a protective film 23 are formed on the bottom surface of the light guide plate 2. The light incident on the light guide plate 2 proceeds while being totally reflected on the bottom surface. On the other hand, the light incident on the reflective dots 21 formed on the bottom surface of the light guide plate is specularly reflected by the reflective dots 21, changes its traveling direction, is emitted upward, and enters the liquid crystal cell to become illumination light. Since the bottom surface of the light guide plate is mainly specularly reflected, the polarization plane is not disturbed by the reflection, and the light guide plate incident light is emitted from the light guide plate 2 while the polarization plane is preserved. The bottom surface of the light guide plate 2 is inclined because the mass (weight) of the light guide plate 2 is reduced by gradually reducing the thickness of the light guide plate 2.
This is also to reduce the space. Further, by inclining the bottom surface, the reflection angle of the light in the light guide plate can be gradually changed (increased) to improve the emission efficiency. In addition, in this specification, such a light guide plate is referred to as a reflection type light guide plate. Details of the reflective light guide plate will be described later.

FIG. 2 is an overall configuration diagram of a liquid crystal display device using the above backlight device. On the front surface (upper part in FIG. 2) of the backlight, there are a polarization filter 9 formed by being attached to the glass 10, a transparent conductive film 12, an alignment film 13, and a TFT 11 formed on the glass 10, which are formed on another glass. The liquid crystal 14 is sealed between the alignment film 13, the counter transparent electrode 15, the color filter 16 and the polarization filter 17. In this liquid crystal display device, since the predetermined polarized light is emitted from the backlight 8 toward the polarization filter 9, the transmittance of the polarization filter 9 has a high value, and the luminance can be significantly improved as compared with the conventional device. .

As described above, only the P-polarized light is made incident on the light guide plate 2 by combining the element for converting the polarized light into the randomly polarized light, the dielectric multilayer polarization beam splitter as the polarization separating element, and the reflecting element. It is possible,
By combining with a light guide plate that can preserve the polarization plane,
Only P-polarized light can enter the liquid crystal cell. From this, the light can be incident on the liquid crystal cell without the intensity of the light source being lost by the polarization filter. Further, it is possible to remove the polarization filter 9 which is indispensable in the conventional configuration, if necessary.

If necessary, a member having a function of expanding the viewing angle can be arranged on the front surface of the liquid crystal cell.

The individual members will be described below.

The dielectric multilayer polarization beam splitter 19 formed on the prism sheet used in the above embodiment is formed by the method shown in FIG. Specifically, a film 40 in which a ZnS film having a refractive index of 2.3 and an ice crystal thin film having a refractive index of 1.35 are alternately formed is provided on a prism sheet 24 having a refractive index of 1.5.
The dielectric multilayer film forming prism sheet thus formed is obliquely cut into a predetermined size, and the sheet is adhered to the light guide plate 2 with an adhesive 20 having a refractive index of about 1.5.

FIG. 4 shows the wavelength dependence of the polarization separation characteristic of the polarization beam splitter thus formed. Wavelength 40
It can be seen that good polarization separation characteristics are exhibited in the range of 0 to 700 nm. That is, P-polarized light has a transmittance of 90% or more, and S-polarized light has a transmittance of several%, that is, a reflectance of 90% or more. Therefore, by using the polarization beam splitter 19 formed of a dielectric multilayer film, only P-polarized light can be transmitted with high efficiency. Moreover, the reflected S-polarized light is returned to the direction of the light source and scattered there as shown in FIG. 1 (B), thereby randomly changing the polarization direction and again entering the polarization beam splitter to only P-polarized light. Since it is possible to enter the light guide plate 2,
It is possible to extract P-polarized light with low loss, the transmittance of the polarization filter is increased, and the brightness of the liquid crystal display device is improved.

(Second Embodiment) FIG. 5 shows a second embodiment of the present invention.
3 is a cross-sectional view of the backlight device showing the embodiment of FIG.

In FIG. 5, the light emitted from the light source 1 which is a cold cathode tube is reflected by the diffuse reflection plate 18 and enters the microlens array 25, and then the dielectric multilayer film polarized beam formed on the surface of the triangular prism. It enters the splitter 19. The light emitted from the cold cathode tube is randomly polarized light. The feature is that the collimated light is incident on the dielectric multilayer polarization beam splitter 19 by the microlens array 25 as compared with the embodiment of FIG. This improves the polarization separation characteristics and the polarization characteristics of the light emitted from the light guide plate.

(Third Embodiment) FIG. 6 shows a third embodiment of the present invention.
3 is a cross-sectional view of the backlight device showing the embodiment of FIG.

In FIG. 6, the light emitted from the light source 1 is diffused and reflected by the diffuse reflection plate 18 and is incident on the collimating prism sheet 26, and then the dielectric multilayer film polarized light formed on the surface of another triangular prism 26 '. Beam splitter 19
Incident on. After that, the light enters the second triangular prism 26 ′ arranged on the right side of the dielectric multilayer polarization beam splitter 19, and further enters the second prism sheet 26,
It is incident on the light guide plate 2. With such a configuration, the angles of the incident light from the light source 1 and the incident light to the light guide plate 2 can be made substantially the same. Compared to the embodiment of FIG. 1, the prism 26, 26 ′ is characterized in that the collimated light is incident on the dielectric multilayer film polarization beam splitter. This improves the polarization separation characteristics and the polarization characteristics of the light emitted from the light guide plate.

(Fourth Embodiment) FIG. 7 shows a fourth embodiment of the present invention.
3 is a cross-sectional view of the backlight device showing the embodiment of FIG.

In FIG. 7, the light emitted from the light source 1 which is a cold cathode tube is diffused and reflected by the diffuse reflection film 31 and enters the microlens array 25, and then the polarization beam splitter 30 formed on the surface of the glass substrate 27. Incident on.
The light emitted from the cold cathode tube is randomly polarized light. Of the incident light, only P-polarized light is polarized beam splitter 30.
And is incident on the light guide plate 2. On the other hand, the S-polarized light is reflected by the polarization beam splitter 30, then reflected by the side reflector 28, and then again reflected by the polarization beam splitter 30 to return to the cold cathode tube direction. The returned light is reflected and scattered by the diffuse reflection plate 31 or the cold cathode tube to be converted into random polarized light, passes through the microlens array 25 again, and is incident on the polarization beam splitter 30. Incident on 2.

The light incident on the light guide plate 2 is specularly reflected by the reflection dots 21 formed on the bottom surface of the light guide plate to change its traveling direction, is emitted upward in the drawing, and is incident on the liquid crystal cell. Since the bottom surface of the light guide plate is mainly specularly reflected, the polarization plane is not disturbed by the reflection, and the light guide plate incident light is emitted from the light guide plate 2 while the polarization plane is preserved.

In this way, by combining the element for converting polarized light into random polarized light, the polarizing beam splitter, and the reflecting element, only P-polarized light can be made incident on the light guide plate 2, and the polarization plane can be preserved. By combining with 2, only P-polarized light can enter the liquid crystal cell. As a result, the light intensity of the light source can be incident on the liquid crystal cell without being lost by the polarization filter.

(Fifth Embodiment) FIG. 9 shows a fifth embodiment of the present invention.
2 shows a configuration of a backlight according to the embodiment.

The light source 1, the diffuse reflection film 31 arranged on the lower surface of the light guide plate 2, the transmissive light guide plate member 29 arranged on the upper surface of the light guide plate 2 (detailed later), and the upper surface of the member. It is composed of the polarized beam splitter 30. The light emitted from the light source 1 enters the light guide plate 2 and proceeds while being diffusely reflected by the diffuse reflection film 31 disposed on the bottom surface of the light guide plate. Of these, the light that has entered the transmission window 32 of the transmissive light guide plate member 29 enters the polarization beam splitter 30 upward. Light having a specific polarization plane among the incident light of the polarization beam splitter is reflected and returns to the light guide plate 2 through the transmission window 32 of the transmission type light guide plate member 29.

The light returning to the light guide plate 2 is reflected by the diffuse reflection film 3
The light is diffusely reflected at 1, and the polarization plane becomes random, and the light is again incident on the polarization beam splitter 30 through the transmission window 32 of the transmission type light guide plate member 29.

When the backlight having such a structure is used, only light having a desired polarization plane can be made incident on the liquid crystal display element arranged on the upper surface of the backlight with a low loss, so that the light transmitted through the polarization filter is particularly transmitted. The rate is improved, and the brightness of the liquid crystal display device can be improved.

(Sixth Embodiment) FIG. 10 shows a structure of a backlight according to a sixth embodiment of the present invention.

The light source 1, the reflection film 34 arranged on the lower surface of the light guide plate 2, the transmissive light guide plate member 29 arranged on the upper surface of the light guide plate 2, the polarization beam splitter 30 arranged on the upper surface of the member, and the transmission. It is composed of a phase plate 33 arranged between the mold light guide plate member 29 and the polarization beam splitter 30. As the phase plate 33, for example, a sheet imparted with birefringence by stretching a polymer film can be used. Since the phase plate 33 can convert linearly polarized P or S polarized light into random polarized light, the phase plate 33 shown in FIG.
In, the light reflected by the polarization beam splitter 30 is converted into random polarized light and returned to the light guide plate 2. This increases the amount of P-polarized light that passes through the polarization filter.

In this embodiment, the reflection film 34 is formed of a metal film or the like so as to be regularly reflected by the reflection film 34. Further, by disposing the phase plate 33, the polarization plane of the light returning to the light guide plate 2 can be changed.

When the backlight having such a structure is used, only light having a desired polarization plane can be made incident on the liquid crystal display element arranged on the upper surface of the backlight with low loss, so that the light transmitted through the polarization filter is particularly transmitted. The rate is improved, and the brightness of the liquid crystal display device can be improved.

(Seventh Embodiment) FIG. 11 shows a structure of a backlight according to a seventh embodiment of the present invention.

A light source 1 and a reflection type light guide plate (detailed later) 2
And a polarizing beam splitter (dielectric multilayer film) 30 arranged on the upper surface of the light guide plate 2.

In the present embodiment, the light incident on the light guide plate is diffused and reflected by the diffuse reflection film 31 provided on the bottom surface of the light guide plate, and becomes the randomly polarized light and travels through the light guide plate 2. Of the guided light, the light that has entered the reflection dots 21 of the light guide plate 2 is reflected on the upper surface and enters the polarization beam splitter 30. Light having a specific polarization plane of the incident light of the polarization beam splitter is reflected and returns to the light guide plate 2. This return light is reflected by the diffuse reflection film 3
The light is diffused and reflected at 1 and the polarization plane becomes random, and is reflected again by the reflection dot 21 to enter the polarization beam splitter 30.

When the backlight having such a structure is used, only light having a desired polarization plane can be made incident on the liquid crystal display element arranged on the upper surface of the backlight with a low loss. The rate is improved, and the brightness of the liquid crystal display device can be improved.

(Eighth Embodiment) FIG. 12 shows a structure of a backlight according to an eighth embodiment of the present invention.

A light source 1 and a reflection type light guide plate (detailed later) 2
The reflective film 34 is disposed on the lower surface of the light guide plate 2, the phase plate 33 is disposed on the upper surface of the light guide plate 2, and the polarization beam splitter (dielectric multilayer film) 30.

In the present embodiment, the light incident on the light guide plate travels in the light guide plate 2 while being regularly reflected by the reflection film 34 disposed on the bottom surface of the light guide plate. Of the guided light, the reflection dots 2 of the light guide plate 2
The light entering 1 is reflected on the upper surface and enters the phase plate 33 and the polarization beam splitter 30. Light having a specific polarization plane of the incident light of the polarization beam splitter is transmitted and acts as illumination light of the liquid crystal display element. On the other hand, the reflected light returns to the light guide plate 2 via the phase plate 33. The polarization plane of the return light is changed by the phase plate 33, is reflected again by the reflection dots 21, enters the polarization beam splitter 30, and the light having a specific polarization plane is transmitted to illuminate the liquid crystal display element. Work as light.

When the backlight having such a structure is used, only light having a desired polarization plane can be made incident on the liquid crystal display element arranged on the upper surface of the backlight with a low loss. The rate is improved, and the brightness of the liquid crystal display device can be improved.

(Description of Individual Members) The elements constituting the present invention will be specifically described below.

The "polarization separating element that transmits a predetermined polarized light and reflects the polarized light orthogonal to the predetermined polarized light" used in the present invention is (a) prism (b) transmission type polarizer (c) polarizing beam splitter. (D) There is a polarization separation element using a birefringent material.

When the prism (a) is used, Brewster's angle is used. That is, the incident light from the light source is incident on the prism at Brewster's angle, the P-polarized light is transmitted, and the S-polarized light is reflected. In this case, the transmittance of P-polarized light is 100% in principle. On the other hand, the reflectance of S-polarized light is generally 10% or more, and the rest is transmitted,
In order to improve efficiency, it is preferable to use several prisms in combination to improve the reflectance of S-polarized light.

As the transmission type polarizer (b), an element in which a large number of parallel flat plates are arranged so as to satisfy the Brewster's angle with respect to incident light, generally, is a Pile of Pla.
An element called tes can be used. Quartz, various transparent plastics, and transparent films can be used as the material.

(C) As the polarization beam splitter, an element generally called a dielectric multilayer polarization beam splitter can be used. The dielectric multilayer polarization beam splitter can be manufactured, for example, by alternately forming two types of thin films having different refractive indexes on a glass substrate or a film, a prism-formed substrate, or a film.

Among these polarization separation elements, the method using the dielectric multilayer film of (c) is convenient in the present invention in terms of efficiency, and it is used in the embodiment of FIG. 1 and the like described above.

Next, the manufacture and configuration of the polarization beam splitter will be specifically described.

The polarizing beam splitter used in the present invention may be one obtained by forming a multilayer film of an optical thin film on a polymer film such as a polyester film or a glass substrate and cutting this into a predetermined size. The manufacturing method is based on the principle of optics 1 (M. Born et al. Tohru Kusagawa et al., Tokai University Press) P78-99, Optical Technology Handbook (Hiro Kubota et al. Asakura Shoten) P87-9
4. Optical measurement handbook (Toshiharu Tayuki et al., Asakura Shoten) P451 can be used. In addition, as outlined in FIG. 3, a triangular prism is formed on a polymer film such as a polyester film or a glass substrate, and the dielectric multilayer film 40 is formed on the triangular prism, and then the prism is formed into a predetermined size. It can be obtained by cutting. 3B, 3C and 3D show an example of a sectional view after cutting. The cutting direction can be perpendicular or oblique to the longitudinal direction of the triangular prism depending on the preferred polarization plane direction. Since the incident surface is changed by using the prism sheet cut in the oblique direction, the polarization direction of the light emitted from the polarization beam splitter can be changed.

The "polarization conversion element for converting the polarization direction of the reflected polarized light into random, elliptical or circular polarization and re-incident on the polarization separation element" used in the present invention includes a diffusion sheet, a diffusion plate and a scattering plate. , Scattering sheet, diffusion type reflection sheet, scattering type reflection sheet, and birefringence sheet. An element that causes light scattering when transmitted or reflected may be used.

Next, the diffuse reflection plate or the diffuse reflection film (reflector) will be described.

The diffuse reflection plate is a reflection film or a reflection plate having a light scattering and light diffusion function, and is required to have flat spectral reflectance and high reflectance. The light is diffusely reflected by the diffuse reflection film and the polarization direction changes. In particular,
It can be manufactured by coating a polycarbonate or PET (polyethylene terephthalate) film with beads formed of an acrylic resin. Also, ground glass, a glass having a reflective film formed thereon, a film having a roughened surface, a sheet having a reflective film formed on the top or bottom surface of the film, inorganic or organic particles such as MgO or MgCO3 titanium white There is a sheet coated with or screen-printed, a sheet having a reflection film formed on the upper surface or the bottom surface of the sheet, and the like. The diffusion sheet and print dots used for the conventional light guide plate correspond to this.

A reflection sheet or a prism can be used as the "traveling direction conversion element" used in the present invention to re-enter the polarization plane conversion light into the polarization separation element. Of course, it is also possible to perform polarization plane conversion and re-incident at the same time by using the diffusion type reflection sheet and the scattering type reflection sheet.

The "light guide plate" used in the present invention for irradiating the liquid crystal panel with uniform light while preserving a predetermined polarization direction is formed by reflection, refraction or diffraction.
A light guide plate that changes the traveling direction of light is used.
A specific example will be described below.

(Specific Example of Light Guide Plate) The light guide plate used in the present invention is a light guide plate capable of irradiating a liquid crystal cell with guided light while preserving the polarization plane.

Specific examples will be shown below.

(Reflective Light Guide Plate): The light guide plate shown in FIG. 13 is referred to as a reflective light guide plate in the present invention.

FIG. 13A is a perspective view of the light guide plate, and the reflective dots 21 and the reflective film 2 are provided on the bottom surface of the light guide plate.
2. The protective film 23 is formed. FIG. 1B is an enlarged view of the bottom surface, and FIG. 13C is a ray trace.
In FIG. 13C, the light incident on the light guide plate 2 travels while being totally reflected on the bottom surface. On the other hand, the light incident on the reflective dots 21 formed on the bottom surface of the light guide plate is specularly reflected by the reflective dots 21, changes its traveling direction, and is emitted upward in FIG. Become light. Since the bottom surface of the light guide plate is mainly specularly reflected, the polarization plane is not disturbed by the reflection, and the light guide plate incident light is emitted from the light guide plate 2 while the polarization plane is preserved. Note that, as described in the seventh embodiment, a method of changing the polarization plane by forming a diffuse reflection film on the bottom surface of the light guide plate 2 is also effective as described in the seventh embodiment.

The plurality of reflective dots 21 are basically randomly arranged to prevent the occurrence of moire. Further, since the light intensity from the light source 1 generally decreases in the light guide plate 2 as the distance from the light source increases, the density, height or size of the reflective dots is changed accordingly, and the intensity distribution of the reflected light, that is, the brightness is derived. Make it uniform over the entire surface of the light plate. In the present invention, in the case of a single light source, it is preferable that the density of the reflective dots be formed so as to increase exponentially or exponentially from the light source side end face toward the opposing light guide plate end face.

(Transmissive Light Guide Plate Member): The light guide plate shown in FIGS. 9 and 10 is referred to as a transmissive light guide plate member in the present invention.

Hereinafter, the transmission type light guide plate member of the present invention will be described with reference to FIG.
This will be described in detail based on FIG.

FIG. 14A is a perspective view of the transmissive light guide plate member 29 of the present invention, and FIG. 14B shows the light guide plate 2 made of transparent resin. FIG. 14C is a perspective view of the light guide plate in which the transmissive light guide plate member 29 and the light guide plate are bonded together with an adhesive or the like.

This light guide plate member includes the light guide plate 2 and the transmission window 32.
Is used as a minimum component, and the light guide plate 2 and the film or plate having the transmissive window 32 are optically coupled at the plane of the tip of the transmissive window 32. The upper surface and the lower surface of the light guide plate 2 are generally mirror surfaces. The transmissive windows 32 are basically randomly arranged.

FIG. 14D shows a ray trace of the light guide plate guided light 37 traveling in the light guide plate of the present invention.
In FIG. 14D, the light emitted from the light source 1 (not shown) is the light guide plate 2 as the light guide plate incident light 37 on the left end surface of the light guide plate 2.
To the other end face while repeating total reflection on the light guide plate upper surface 38 and the light guide plate lower surface 39. Of the guided light, the light that has entered the transmission window 32 is emitted there upward and becomes illumination light for the liquid crystal display element. Here, the transparent window 3
By optimizing the size, the surface density and the inclination angle of the cross section of the transmission window, the guided light can be gradually emitted from the light guide plate 2 to illuminate the liquid crystal display element. That is, gradually increase the size of the transmission window 32 and the density of the transmission window 32 (the number of the transmission window 32 per unit area) from the light source side (the left side in FIG. 14D) to the opposite side. Thereby, it is possible to supply light of uniform intensity to the surface of the liquid crystal display element. Also, by optimizing the cross-sectional inclination angle of the transmission window,
It is possible to supply light at an angle closer to vertical to the liquid crystal display element surface. When such a light guide plate is used, its traveling direction is changed by specular reflection and refraction, so that the emitted light 37 from the light source irradiates the liquid crystal cell with the polarization plane preserved and serves as illumination light for the liquid crystal display device. .

The other members will be described below.

Specific examples of the light source include cold cathode tubes, hot cathode tubes, tungsten lamps, xenon lamps, metal halide lamps, and the like. Usually, a low temperature light source such as a cold cathode tube is desirable.

The liquid crystal element or liquid crystal cell used in the present invention is not particularly limited, and known elements and panels can be used. Typical liquid crystal cells include twisted nematic type, super twisted nematic type, homogeneous type, thin film transistor type, active matrix driven type and simple matrix driven type.

A luminance uniformizing mask (not shown) used as necessary is for compensating for unevenness in luminance due to a difference in distance from the light source. For example, a sheet having a changed light transmittance is used. Formed as. The brightness uniforming mask can be arranged at any position on the light guide plate.

A particularly wide viewing angle is required for a desktop type liquid crystal display device of a personal computer or a television monitor. In this case, a diffusing plate that scatters the illumination light to expand the viewing angle is placed at an appropriate position. Can be placed. After arranging a prism sheet and irradiating the liquid crystal cell with illumination light with higher directivity, a sheet with a light diffusion effect is arranged to widen the viewing angle, or the light transmitting surface is processed to provide a light scattering function. You can also extend the viewing angle by adding.

[0078]

As described above, according to the present invention, by combining a polarizing beam splitter, a reflecting element, and the like, it is possible to make only P-polarized light incident on the light guide plate with a low loss and to reduce the polarization plane. By combining with a storable light guide plate, only P-polarized light can enter the liquid crystal cell. As a result, the intensity of the light source can be incident on the liquid crystal cell without loss by the polarization filter, and the brightness of the liquid crystal display element
The contrast can be improved. Furthermore, it becomes possible to remove the polarizing filter, which was indispensable in the conventional configuration, if necessary.

[Brief description of drawings]

FIG. 1 is a perspective view of a backlight according to a first embodiment of the present invention.

2 is a cross-sectional view showing a configuration of a liquid crystal display device of the present invention using the backlight of FIG.

FIG. 3 is a perspective view of a polarization beam splitter used for the backlight of FIG.

FIG. 4 is a diagram showing characteristics of the polarization beam splitter of FIG.

FIG. 5 is a perspective view of a backlight according to a second embodiment of the present invention.

FIG. 6 is a sectional view of a backlight according to a third embodiment of the present invention.

FIG. 7 is a perspective view of a backlight according to a fourth embodiment of the present invention.

FIG. 8 is a cross-sectional view showing a configuration of a conventional liquid crystal display device.

FIG. 9 is a perspective view of a backlight according to a fifth embodiment of the present invention.

FIG. 10 is a perspective view of a backlight according to a sixth embodiment of the present invention.

FIG. 11 is a perspective view of a backlight according to a seventh embodiment of the present invention.

FIG. 12 is a sectional view of a backlight according to an eighth embodiment of the present invention.

FIG. 13 is a perspective view of a light guide plate used for the backlight of the present invention.

FIG. 14 is a perspective view of another light guide plate used for the backlight of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Light source, 2 ... Light guide plate, 3 ... Diffusion sheet, 4 ... First prism sheet, 5 ... Second prism sheet, 6 ... Print dots, 7 ... Reflection sheet, 8 ... Backlight, 9 ... Polarization filter, 10 ... Glass, 11 ... TFT, 12 ... Transparent electrode,
13 ... Alignment film, 14 ... Liquid crystal, 15 ... Opposite transparent electrode, 16
... Color filter, 17 ... Polarization filter, 18 ... Diffuse reflection plate, 19 ... Dielectric multilayer film polarization beam splitter, 20
... Adhesive, 21 ... Reflective dots, 22 ... Reflective film, 23 ... Protective film, 24 ... Prism sheet, 25 ... Microlens array, 26 ... Collimator prism sheet, 27 ... Glass substrate, 28 ... Reflector plate, 29 ... Transmission Type light guide plate member, 30
... polarizing beam splitter, 31 ... diffuse reflection film, 32 ... transmission window, 33 ... phase plate, 34 ... reflection film, 35 ... triangular prism, 36 ... polymer film or glass substrate, 37 light guide plate incident light, 38 ... light guide plate Upper surface, 39 ... Lower surface of light guide plate, 40
…film

Claims (13)

[Claims] 1. A backlight for supplying light from a back surface of a liquid crystal cell to a liquid crystal cell by a light source, wherein predetermined polarized light from the light source is transmitted in an optical path between the light source and the liquid crystal cell. A polarization splitting element for reflecting polarized light substantially orthogonal thereto, and a polarization converting element for converting the polarization direction of the polarized light reflected by the element into random, elliptical or circularly polarized light. A backlight characterized in that converted light is re-incident on the polarization separation element. 2. The backlight according to claim 1, further comprising a traveling direction conversion element for re-incident the polarization plane conversion light on the polarization separation element. 3. The backlight according to claim 1, further comprising a light guide plate for irradiating the liquid crystal cell with the polarized light while preserving the polarization direction of the predetermined polarized light. Method backlight. 4. The backlight according to claim 1, 2 or 3, wherein the polarization separation element is a dielectric multilayer film polarization beam splitter. 5. The backlight according to claim 3, wherein the polarization separation element is arranged between the light source and an end surface of the light guide plate. 6. The backlight according to claim 3, wherein the polarization separation element is arranged between the surface of the light guide plate and the liquid crystal cell. 7. The backlight according to claim 5, wherein a microlens array and a polarization beam splitter are arranged between the light source and the end surface of the light guide plate. 8. The backlight according to claim 5, wherein a prism and a polarization beam splitter are arranged between the light source and the end surface of the light guide plate. 9. The backlight according to claim 5, wherein a microlens array, a prism, and a polarization beam splitter are arranged between the light source and the end face of the light guide plate. 10. The backlight according to claim 5, wherein
It has a diffusion or scattering reflection type reflection sheet arranged so as to cover the light source, and a microlens array, a prism, and a polarization beam splitter are arranged between the light source and the end surface of the light guide plate. Back light. 11. The backlight according to claim 6, wherein:
A backlight characterized in that a phase plate is disposed between the light guide plate and the liquid crystal cell, and a reflective film is provided on the back surface of the light guide plate. 12. The backlight according to claim 6, wherein
A backlight characterized in that a diffuse reflection film is provided on the back surface of the light guide plate. 13. A liquid crystal display device comprising a liquid crystal cell, a light guide plate arranged on a back surface of the liquid crystal cell, and a light source arranged on a side surface of the light guide plate, wherein the light source is arranged between the light source and the liquid crystal cell. In the optical path of, a polarized light separating element which transmits a predetermined polarized light from the light source and reflects a polarized light which is almost orthogonal thereto, and a polarization direction of the polarized light reflected by the element is changed to random, elliptical or circular polarized light. A liquid crystal display device, comprising: a polarization conversion element for conversion; and polarization plane converted light by the polarization conversion element is re-incident on the polarization separation element.