JPH1020125A - Surface light source device and liquid crystal display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a surface light source device whose manufacture cost can be reduced with simple constitution, which can improve the polarization degree of outgoing light and has high light use efficiency against all light beams by providing a high refractive index layer having a specified refractive index on the outgoing-side surface of a light transmission body. SOLUTION: The surface light transmission body 2 where a surface-side is set to be a light outgoing surface 2a-side, a light source 3 arranged so that light is made incident from the side end surface 2b of the light transmission body 2 and a reflection board 4 provided on the surface side opposite to the light outgoing surface 2a of the light transmission body 2 are provided. The surface light source device 1 is provided with the high refractive index layer 7 with the refractive index of more than 1.7 on the outgoing-side surface 2a. It is desirable that the refractive index of the high refractive index layer 7 is high as much s possible and transparency is desirable to be high as much as possible. Optical thickness can be suppressed to about 1/4 of a center wavelength. It is desirable to use titanium oxide as the high refractive index layer 7 when the refractive index, operability and cost are considered, for example.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a surface light source device for generating polarized light, and more particularly to a surface light source device used for a backlight of a liquid crystal display element and a liquid crystal display device using the same.

[0002]

2. Description of the Related Art For example, word processors, liquid crystal televisions,
In a liquid crystal display device used for a personal computer or the like, a surface light source device is usually provided on the back (back surface) side of the liquid crystal display element in order to make characters and images easy to see. From the viewpoint of, for example, realization of a so-called edge light type surface light source device, it is often used. In this method, a planar light guide (light guide plate) is provided on the back side of a liquid crystal display element, and a light source made of, for example, a cold-cathode tube is arranged close to a side end face of the light guide, and light from the light source is guided. After being introduced into the light guide from the side end face of the light body, the light is emitted from the surface located on the liquid crystal display element side.

In general, in a liquid crystal display device, a first polarizing plate is provided on a light incident side of a liquid crystal display element having a liquid crystal layer held by two glass substrates, and a second polarizing plate is provided on a light emitting side.
Are arranged such that the respective polarization axes intersect at a predetermined angle. Then, the light transmitted through the first polarizer is further displayed as an image after passing through the second polarizer through the liquid crystal layer. In this case, the light is incident on the liquid crystal display element. The light is randomly polarized,
When transmitting the light through the first polarizing plate, more than half of the incident light is absorbed by the polarizing plate, which causes a problem that the light use efficiency is reduced by that much.

Therefore, in order to deal with such a problem,
Conventionally, it has been proposed that the light to be incident on the liquid crystal display element from the surface light source device is preliminarily polarized, thereby increasing the light transmittance of the first polarizing plate and improving the light use efficiency. One example is disclosed in JP-A-6-2658.
No. 92, a prism sheet is provided on the light exit surface side of a planar light guide to condense the emitted light in the normal direction of the light guide surface, and a triangular cross section is provided on the prism sheet. A surface light source device in which a polarization separator formed by laminating a polarization separation layer on an array-like portion of a columnar prism array and a liquid crystal display device including the same are described. According to this, the P-polarized light component that has exited the light guide plate and passed through the polarization splitter enters the liquid crystal display element after passing through the prism sheet, but the S-polarized light component is reflected by the polarization splitter and returned to the light guide. Further, since the phase of each light changes while being repeatedly reflected on the surface of the light guide and is converted into the P-polarized component, the ratio of the P-polarized component in the light emitted from the light guide is increased. be able to.

Another example is disclosed in Japanese Patent Application Laid-Open No. Hei 7-26112.
In Japanese Unexamined Patent Publication (Kokai) No. 2, a polarizing beam splitter is disposed on the light extraction surface side of a parallel light beam forming element comprising a wedge-shaped light scattering light guide having uniform scattering properties, with an air layer interposed therebetween. A surface light source device having a configuration in which a light emitting direction correcting element is disposed with an air layer interposed therebetween, and a reflecting member is disposed on the back side of the parallel light beam forming element is described. According to this, by using the function of forming the parallel light flux of the light scattering light guide and the reflection / transmission characteristic of the polarization component related to the Brewster angle condition in combination, a predetermined energy use efficiency can be obtained with a relatively high energy use efficiency. It is possible to generate a light beam containing a large amount of polarization components in the directions.

[0006]

However, in the surface light source device and the like described in JP-A-6-265892, a polarization separator having a complicated structure, specifically, a material having a high refractive index and a material having a low refractive index are used. It is necessary to use a polarization separator obtained by alternately laminating substances with different ratios to form a polarization separation layer of a multilayer structure and laminating this on the array-like portion of the above-described columnar prism array. There is a problem that it is expensive. In addition, when a polarization separator having such a complicated laminated structure is disposed between the light guide and the liquid crystal display element, light is reflected, scattered, and absorbed by the polarization separator, and a relatively large amount of optical loss is generated. Therefore, although the ratio of the light component incident on the liquid crystal display device and having the same polarization direction as that of the first polarizing plate increases, there is also a problem that the light use efficiency of all light rays is reduced by the optical loss due to the polarization separator. is there.

Further, in the surface light source device described in Japanese Patent Application Laid-Open No. 7-261122, a device using a relatively inexpensive polarization separation plate such as a 1 mm optical glass plate has been proposed. In the case of using a device, there is still a problem in that optical loss occurs, and furthermore, the handling thereof is difficult, and it goes against the demand for thinning. In addition, such polarization separators are classified into film type and plate type. Especially, the film type has a contact surface and a non-contact surface between the light guide, and adversely affects the image. The plate-type type has a problem that the thickness is increased and the flexibility is poor, and the plate-type type is weak against impact. When a stress is applied, a phase difference is generated, and the polarization separation characteristics are apt to deteriorate.

The present invention addresses such a problem, and has a simple structure, can reduce the manufacturing cost, increases the degree of polarization of the emitted light, and has a high light use efficiency for all light rays. It is an object to provide a device and a liquid crystal display device provided with the device.

[0009]

Means for Solving the Problems In order to achieve the above object, each invention of the present application is characterized in that it is constituted as follows.

First, according to the first aspect of the present invention, as shown in FIG. 1, a planar light guide 2 having a light exit surface 2a on the front side and a side end surface 2b of the light guide 2 are provided. A light source 3 arranged to receive light, and a light exit surface 2 of the light guide 2
In the surface light source device 1 having the reflection plate 4 provided on the surface (rear surface) opposite to the surface a, the emission surface 2a of the light guide 2 is provided.
Is provided with a high refractive index layer 7 having a refractive index of 1.7 or more. The refractive index of the high refractive index layer 7 is preferably higher, and the transparency is preferably higher. Further, it is preferable that the optical thickness of the high refractive index layer 7 is controlled to about １ ／ of the center wavelength. As described above, for example, titanium oxide is preferably used for the high refractive index layer 7 in consideration of the refractive index, ease of handling and cost.

If there is a high-refractive-index layer, for example, a two-layer structure of a low-refractive-index layer / high-refractive-index layer or a multilayer structure of a low-refractive-index layer / high-refractive-index layer / low-refractive-index layer is used. Can also. In the invention according to claim 2, the polarization conversion means for changing the polarization state of the incident light is provided on at least one of the light exit surface side, the light reflection surface side, and the inside of the light guide of the light guide. The light guide is provided with a high refractive index layer having a refractive index of 1.7 or more on the light emission side surface of the light guide. For example, as shown in FIG.
Side (in the case of FIG. 3A), or the light guide 2 and the reflector 4
(In the case of FIG. 2B) or both (in the case of FIG. 2C), the polarization conversion means 5 for changing the polarization state of the incident light is disposed. Outgoing side surface 2a
Has a configuration in which a high refractive index layer 7 having a refractive index of 1.7 or more is provided.

According to the third aspect of the present invention, in addition to the configuration of the second aspect, the emission angle of the light emitted from the light guide is mainly 60 ° to 90 ° with respect to the normal to the light emission surface. It is configured to be in range. In this configuration, as shown in FIG. 3, as the planar light guide 2, the emission angle φ of the emitted light is mainly in the range of 60 ° to 90 ° with respect to the normal m of the light emission surface 2 a. A light body is used, and the light guide 2 has an emission surface 2a side (in the case of FIG. 3A), or between the light guide 2 and the reflection plate 4 (in the case of FIG. 3B), or both. (Fig. (C)
), A polarization conversion means 5 for changing the polarization state of the incident light is provided, and a high refractive index layer 7 having a refractive index of 1.7 or more is provided on the emission side surface 2a of the light guide 2. It has become.

In the invention described in claim 4, the polarization conversion means is a retardation plate for rotating a polarization axis direction,
The retardation plate is disposed on the light exit surface side of the light guide, or between the light guide and the reflection plate, or both, in a state of being in close contact with the light guide. .

Further, in the invention according to the fifth aspect, the polarization conversion means includes a light-reflecting surface side inside the light guide, the light guide located below the high refractive index layer, or The reflecting plate has a scattering property.

In the invention according to claim 6, as shown in FIG. 4, the planar light guide 21 has a wedge-shaped cross section in the light emitting direction, and the rear surface thereof has an inclined surface 21c.
In addition, instead of the above-mentioned polarization conversion means, by reflecting the light on the inclined surface 21c, the polarization of the light is converted into another polarization.

Furthermore, in the invention according to claim 7, a metal oxide is used for the high refractive index layer 7. The surface light source device 14 according to claim 8 is, as exemplified in FIG. 5, the light emitting surface side of the planar light guide in any of the surface light source devices 1 or 11 to 13 according to each of the above inventions. Is characterized in that the light direction control means 8 for directing the light emitted from the planar light guide in the normal direction of the light emission surface is arranged so as to be located closest to the front side.

Further, as exemplified in FIG.
The liquid crystal display device 30 according to the invention described in (1) includes the surface light source device (14) according to claim 8, and a liquid crystal display element 33 in which polarizing plates 31 and 32 are disposed on the light incident side and the light emitting side, respectively. The surface light source device 14 is set such that the average polarization axis direction of the emitted light and the polarization axis direction of the polarizing plate 31 on the light incident side of the liquid crystal display element 33 substantially match. It is characterized in that it is arranged on the back of the liquid crystal display element 33.

Next, the components of each of the above-described inventions will be described in more detail. (1) Planar light guide A high refractive index layer having a refractive index of 1.7 or more is provided on the emission side surface of the planar light guide used in the present invention. For example, the entire planar light guide may be made of plastic or glass having a refractive index of 1.7 or more, or a high refractive index layer may be formed on the exit surface side of the planar light guide by a known method such as evaporation or a sol-gel method. May be formed, or a film having an inorganic or organic high refractive index layer formed thereon may be adhered and fixed to the emission surface side of the planar light guide with an adhesive or an adhesive. Such a substance having a high refractive index includes, for example, polymethyl methacrylate as an organic compound,
Polybenzyl methacrylate, polyphenyl methacrylate, polydiallyl phthalate, polystyrene, poly P
-Dielectric materials such as bromophenyl methacrylate, polypentachlorophenyl methacrylate, polychlorostyrene, poly-α-naphthyl methacrylate polyvinyl naphthalene, polyvinyl carbazole, and polypentabromophenyl methacrylate having a refractive index of 1.7 or more (Note: These polymers Most have a refractive index of 1.6 to 1.7, but by appropriately devising a substituent, the refractive index can be increased to 1.0.
7 or more) can be suitably used alone or in combination. As the inorganic compound, titanium oxide, tantalum pentoxide, tin oxide, ITO, ZrO ₂ , Zns, germanium, silicon, LiNbO ₃ , LiTaO ₃ , BaTiO ₃ , GaAs, Zn
O 2 and the like can be suitably used alone or in combination. In addition, among these inorganic compounds, titanium oxide has a refractive index of 2 or more and has a track record of the sol-gel method. Therefore, it is considered that titanium oxide is the most suitable substance for forming a high refractive index layer.

These high-refractive-index layers may be formed uniformly over the entire exit surface, or the formed and unformed portions may be combined. The refractive index of the high refractive index portion may be uniform or non-uniform. Further, a gradient index material may be used.

If there is only a high refractive index layer, for example, a two-layer structure of low refractive index layer / high refractive index layer or a multilayer structure of low refractive index layer / high refractive index layer / low refractive index layer is used. Can also. In the present invention, the exit angle of the exit light of the planar light guide is mainly 60 °.
To be in the range of up to 90 ° means that, in a cross section of a light guide (a cross section in a direction perpendicular to the light emitting surface), the light emitted from the center is -85 ° to + 85 ° in increments of 5 °. When measuring the brightness, the point with the highest brightness is + 60 ° ~
+ 90 ° or in the range of −60 ° to −90 °. The shape, material, etc., of the planar light guide having such characteristics are not limited.

As a method of giving such characteristics,
For example, using a scattering light guide, using a deformed light guide, and using a net-like diffuse reflector printing (so-called back print)
There are various methods such as adjusting the diffusivity of the light guide, using a thin light guide.

In the case of using a scattering light guide In this case, the material of the planar light guide is not particularly limited, and all known materials can be used. For example, organic materials such as acrylic resins, polystyrene resins, and silicone resins can be used alone or in combination, and inorganic materials such as various glasses can be used.

In order to impart light scattering to the light guide, for example, JP-A-54-105562 and JP-A-59-8
As disclosed in Japanese Patent No. 1683 or the like, fine particles having different refractive indexes may be dispersed in a transparent resin as a main material. Further, JP-A-6-347616 and JP-A-6-347616.
As disclosed in Japanese Patent No. 324330, scattering properties can be imparted also by a micro phase separation structure formed by a polymer of a main material and a sub material by kneading a plurality of resins having different refractive indexes. Furthermore, even if a material having a scattering property itself such as an epoxy resin is used, a scattering light guide having required characteristics can be obtained.

Here, if a material having the above-mentioned scattering properties is used, the light guide whose emission angle is mainly in the range of 60 ° to 90 ° with respect to the normal to the light emission surface is used. Explain why you can get your body easily.

Light from a light source (lamp) usually enters directly or after being reflected by a lamp holder or the like, from the side end face of the light guide on the light source side. The light incident on the light guide has various angles as it is. The refractive index of air is 1, and the material that can be used for the light guide is approximately 1.
Since it has a refractive index of about 4 to 1.6, it is in an angle range of at most -45 ° to + 45 ° with respect to a horizontal plane (this angle slightly varies depending on the refractive index). In the example shown in FIG. 7, whether such light hits the upper side or the lower side of the light guides 2 ′ and 2 in the figure (or is extremely rare considering the length / thickness ratio of the light guides). , But ignore this). If the light guide has almost no scattering property, light emitted at a certain angle α (unit is degrees, the same applies hereinafter) is reflected at the upper side, and the incident angle is 90 ° −α, as shown in FIG. Become.

However, when the light guide has a scattering property, as shown in FIG. 3B, the incident angle when the light guide 2 hits the upper side is determined by the scattering property and the light guide 2. From 90 °-(α-β ₁ ) to 90 °-(α + β ₂ ) depending on the moving distance of
Are distributed with a certain probability in the range. And
If the incident angle at the time when the light reaches the upper side of the light guide 2 in the drawing is larger than the critical angle, the light is totally reflected and all light components are reflected, and go to the lower surface. If the incident angle is smaller than the critical angle, a part of the light is emitted.

As described above, since the emission angle of the light guide 2 fluctuates continuously due to the scattering, the values of β ₁ and β ₂ can be sufficiently reduced by appropriately adjusting the scattering of the light guide 2. Accordingly, when the range of the emission angle is set to 0 ° in the front direction (upward direction in the drawing), adjustment can be made so that the peak of the emission angle is located in the range of 60 ° to 90 °.

When a Deformed Light Guide is Used As a deformed light guide, for example, as shown in FIG. 4, a wedge-shaped light guide 21 in which a reflection surface 21c is inclined by a predetermined angle θ with respect to an emission surface 21a is used. Can be mentioned. In this case, the angle θ is desirably within 0.2 ° to 5 °, and 1 ° to 2 °.
Is more desirable. Such a modified light guide can be produced by injection molding. As the material in that case, an organic material such as an acrylic resin, a polystyrene resin, or a silicone resin can be used alone or in combination, or an inorganic material such as glass can be used. As described above, it is conceivable to control the emission angle of the emitted light mainly in the range of 60 ° to 90 ° depending on the shape of the light guide. FIG. 4 shows the back surface 21 c of the light guide 21.
1 shows a state in which a reflection plate 4 is provided on the side.

Here, the reason why the above-described required emission characteristics can be obtained according to the modified light guide (wedge shape) will be described with reference to FIG. Light incident from the light source-side end surface 21b of the light guide 21 is at most in the angle range of -45 ° to + 45 °, as in the above case. In the example of FIG. 4, such light hits the upper side or the lower side of the light guide 21 (or extremely rarely hits the opposite side in consideration of the length / thickness ratio of the light guide 21, but this is ignored). Yes) either. End surface 21b on the incident side of light guide 21
The light emitted at a certain angle α with respect to the normal line is reflected by the upper side of the light guide 21 in the drawing, and the angle after the reflection is −α.
Becomes Next, the light is reflected on the lower side of the light guide 21,
At this time, since the lower side is the hypotenuse of the angle θ, the angle after reflection is α + 2θ. Next, when hitting the upper side, the angle is-
(Α + 2θ), and for the next reflection, α + 4θ. Similarly, the light incident from the light source-side end surface 21b of the light guide 21 is increased by +2 every time the light is reflected by the lower side of the light guide 21.
It can be seen that the light travels in the light guide 21 while increasing the angle by θ (the same is true even if the light is first reflected on the lower side of the light guide 21).

Since the refractive index of the light guide 21 is larger than that of air, if the incident angle is larger than the critical angle, the condition of total reflection is satisfied, and all light is reflected. In the light guide 21,
Angles (α, α + 2θ, α + 4) that light makes with the horizontal plane in the figure
..) gradually increase, so that the incident angle [= 9
0 °-(the angle formed by the light with the horizontal plane)] becomes smaller in increments of 2θ, and when the critical angle is exceeded, a part of the light starts to be emitted. Therefore, for this ray, the angles of incidence at the time of exit are, in order, certain angles A, A-2θ, A near the critical angle.
-4θ, A-6θ..., And the output angles are sin ^-1 ｛(sin, respectively), where n ₁ is the refractive index of the light guide.
A) × n ₁ }, sin ^-1 [{sin (A-2θ)} × n
₁ ], sin ^-1 [{sin (A-4θ)} × n ₁ ],
・ ・Here, if θ is an appropriate value, when the front direction (upward direction in the drawing) is 0 °, the output angle range is set so that the peak of the output angle exists in the range of 60 ° to 90 °. Can be adjusted.

When Adjusting Diffusivity of Reticulated Diffuser Printing (Backprint) In a normal light guide, the lower surface (the surface located on the side opposite to the light exit side) is formed by a method such as screen printing. In many cases, a diffused reflective layer is provided. In that case,
As shown in FIG. 9A, the diffuse reflection portion 2 of the light guide 22 '
The light a 'entering 2c' is substantially completely diffusely reflected, and the output angle range is a wide range from -90 ° to + 90 °. But,
For example, if the diffusivity is restricted by reducing the concentration of the diffuse reflector, the light a entering the diffuse reflection portion 22c of the light guide 22 has a high rate of causing forward scattering as shown in FIG. In other words, the ratio of the specular reflection component in the reflected light increases. Therefore, even by using the light guide plate 22 in which the diffusivity is adjusted, the emission angle of the emitted light can be controlled so as to be mainly in the range of 60 ° to 90 °.

In the light guide of the type without the diffuse reflector layer, fine irregularities are provided on the light emitting surface or the surface on the opposite side, and the light is guided into the light guide by the fine irregularities. Some light is emitted, but even in this type of light guide, the emission angle of the emitted light is controlled so as to be mainly in the range of 60 ° to 90 ° depending on the design of the fine uneven portion. be able to.

In the case of using a thin light guide In general, when the thickness of the light guide is reduced, the emission angle is increased. The emission angle can be controlled so as to be mainly in the range of 60 ° to 90 °. When reducing the thickness, the thickness of the main portion of the light guide is preferably 3 mm or less, more preferably 2.5 mm or less.

It is to be noted that, when such a thin light guide is used, the description that the above-mentioned predetermined emission characteristics are obtained will be omitted, but in any case, when the front direction is set to 0 °, the emission light is obtained. A light guide having an emission characteristic such that the emission angle of the light guide is mainly in the range of 60 ° to 90 ° can be obtained by various methods. (2) Polarization conversion means and retardation plate In the invention described in claim 1, polarization conversion is not actively performed. This is based on the fact that some degree of polarization conversion occurs naturally in the light guide even if no special means for polarization conversion is provided.

Further, in the invention of claim 2 or the following, means for positively performing polarization conversion is provided. In the invention according to claim 4, the phase difference plate is provided. As a means for performing the polarization conversion, a film made of an organic substance, a pressure-sensitive adhesive, a coating made of an inorganic substance, or the like is used.
These polarization conversion members need to be in close contact or pressure contact with the light guide. Further, a metal surface serving also as a reflector described in detail later may be formed on the lower surface (back surface) side of the light guide. In general, these members are more preferable as the conversion efficiency from S-polarized light to P-polarized light is higher. From such a viewpoint, it is most preferable to use one or a plurality of retardation plates whose optical thickness is adjusted.

The retardation plate can be obtained by stretching an organic film such as polycarbonate. This type of member is preferably formed directly on the front surface (light emitting surface) or back surface of the light guide, or is adhered to the light guide using an adhesive. When an adhesive is used, it is preferable to use an adhesive having as little optical absorption as possible.

According to another aspect of the present invention, as another polarization conversion means, the light scattering surface of the light guide, the inside of the light guide located below the high refractive index layer, or the reflection plate has scattering properties. It has a given configuration. Such scattering is a known method such as forming fine irregularities, using a mixture of a plurality of materials having different refractive indices, or using a mixture of fine particles having a different refractive index from the substrate. Can be used.

Further, in the sixth aspect, reflection on the slope of the light guide is utilized. This is because, for example, by making the lower surface of the light guide a plane that is not parallel to the vibration direction of the S-polarized light to be converted, a part of the S-polarized light is converted to polarized light when the S-polarized light is reflected by this plane. .

Although not explicitly stated in the claims, in addition to these, films and adhesives of various organic substances, coatings of inorganic substances, the structure of the light guide itself, and the reflection plate may be made of metal. The polarization can be converted by using a plane. (3) Reflector The type of reflector is not particularly limited, but it is preferable to use one having a reflectance of 85% or more, and more preferably one having a reflectance of 90% or more. The material of the reflection plate is not particularly limited, but the reflection surface is preferably a metal surface such as aluminum or silver. However, white polyethylene terephthalate (PET) or the like may be used.

The reflector may be provided independently of the light guide. Further, the four corners or both sides or several places of the reflection plate may be fixed to the light guide, or may be adhered to the entire surface. (4) Light source As the light source, a lamp whose back is covered by a lamp holder can be used. The lamp and lamp holder in that case are not particularly limited. Those used for ordinary surface light sources and backlights (for example, cold cathode tubes) can be suitably used. When the light from the light source is incident on the light guide, it is desirable that the collimation in the lateral direction (the direction of introducing the light into the light guide) be performed because the light is efficiently introduced into the light guide. Such lateral collimation,
For example, it can be realized by combining various concave / convex lenses such as a known cylindrical lens, a reflecting mirror, an optical fiber, and the like.

The light source may be provided on only one side end face of the light guide (so-called single-lamp type) or may be provided on both side end faces (so-called two-lamp type). (5) Light Direction Control Means When incorporating a surface light source device into a liquid crystal display device, it is necessary to direct the exit angle of the light emitted from the surface light source device to the normal direction of the light exit surface. Therefore, the invention according to claim 5 includes a light direction control means, and mainly emits light from the light guide at 60 ° to 90 °.
Is directed in the normal direction by the light direction control means. As such a light direction control means, for example, a prism sheet used for a normal surface light source,
That is, a sheet formed by regularly forming a large number of prisms each having a triangular cross section on one side, such as a sheet in which light is directed in a predetermined direction by a prism effect can be given. In the present invention, a known prism sheet is used. In addition, various types of light control plates or light control devices that direct outgoing light toward the front by combining other lenses, reflection plates, and transmission / reflection plates are preferably used.

When a prism sheet is used, it is desirable that each prism has an apex angle of 20 ° to 60 °. Further, it is desirable that the lower surface (the light guide side) of the prism sheet or the like be a smooth surface. Further, the prism sheet is
One or more sheets may be used. (6) Other Means In the surface light source device of the present invention, other means for increasing the ratio of P-polarized light can be used as necessary, in addition to the above components. Further, a diffusion plate or the like may be arranged as necessary. However, these are not essential in the present invention.

<Operation> Next, as described above, the principle of the present invention will be described with reference to FIG. 8 regarding the reason why the provision of the high refractive index layer on the light emitting surface side of the light guide increases the proportion of P-polarized light. I will explain while. FIG. 2 shows the light guide 2 and the reflection plate 4.
The polarization conversion member 5 is disposed between the light guide 2 and the light exit surface (surface or upper surface) 2 a of the light guide 2.
It is the same whether it is arranged on the side or both.

The transmittance of the light emitted from the light guide 2 is slightly larger in the P-polarized light when each incident angle is near the Brewster angle or the critical angle, and even slightly when the incident angle is close to 0 °. S
It is generally known that there is less polarization. That is, even if the ratio of the P-polarized light (I _P ) and the ratio of the S-polarized light (I _S ) are equal at the beginning, the ratio of the P-polarized light to the S-polarized light in the emitted light at the time when the first emission occurs. More compared (I _P > I _S )
, Among the reflected light in reverse, the proportion of P-polarized light is low compared to the S-polarized light (I _S> I _P). This reflected light is then
When the light is reflected on the lower surface side of the light guide 2 in the figure, if there is a polarization converter, the S-polarized light is efficiently converted into the P-polarized light, or the P-polarized light is converted into the S-polarized light. The light reflected from the lower surface side of the light guide 2 is smaller than the original reflected light because S-polarized light is more present in the original reflected light. The proportion of P-polarized light increases.
[I _P (before conversion) <I _P (after conversion), I _S (before conversion)>
I _S (after conversion)] The reflected light whose polarization component has been converted again travels to the upper side of the light guide 2 in the figure. Also at this time, more P-polarized light is emitted, and S-polarized light is relatively not emitted. By repeating this operation and emitting most of the light and then examining the polarization component of the entire emitted light, the proportion of P-polarized light is increasing.

As described above, even when a normal light guide is used, the ratio of P-polarized light increases, though very slightly. Further, in the present invention, as means for further increasing the degree of polarization, a polarization conversion means is provided, and a high refractive index layer 7 is provided on the light exit surface side of the light guide 2. Titanium oxide was used as the high refractive index layer 7, and this titanium oxide was laminated on the light emitting surface side of the light guide 2 made of acrylic. FIG. 10 is a graph showing the reflectance of P-polarized light and S-polarized light for a normal acrylic plate and an acrylic plate having titanium oxide (refractive index: 2.1) laminated on the surface thereof. In this drawing, the horizontal axis represents the emission angle when emitted to the air surface, and the vertical axis represents the reflectance. As is apparent from this drawing, the difference between the reflectance of P-polarized light and the reflectance of S-polarized light is much larger when titanium oxide is laminated on the surface, as compared with a normal acrylic plate. In particular, the emission angle is 6
In the range of 0 to 90 degrees, the reflectance of P-polarized light decreases,
Since the reflectance of the S-polarized light increases, the ratio of the P-polarized light can be increased by controlling the emission angle of the emitted light within this range.

In order to increase the polarization conversion efficiency, it is necessary to use a phase difference plate to bring the phase difference closer to the theoretically calculated value.
More effective. However, if necessary, the present invention is not limited to this retardation plate, and the same applies to the case where other polarization conversion means is used.

As described above, a high-refractive-index layer is formed on the emission surface side of the light guide by combining the light guide having a special emission angle characteristic that can be easily manufactured with the polarization conversion means that can be manufactured relatively inexpensively. This alone can increase the ratio of the P-polarized light component of the light emitted from the light guide. Therefore, the liquid crystal display device using this can reduce the light absorbed by the polarizing plate on the incident side, and accordingly, the light use efficiency can be improved. In addition, if the average polarization axis of the light emitted from the light guide and the polarization axis of the incident-side polarizing plate of the liquid crystal display device are made substantially coincident with each other, the light is conventionally absorbed by the liquid crystal display device and converted into heat. Wasted light that has been used can be greatly reduced. As a result, the light use efficiency can be further increased as compared with the related art.

[0048]

EXAMPLES Examples of the present invention will be described below along with comparative examples. <Embodiment 1> FIG. 11 shows a basic structure of a liquid crystal display device 40 provided with the surface light source device of the present embodiment. This liquid crystal display device 40 has a rear surface (a lower surface in the figure) of a liquid crystal display element 41.
In this configuration, the surface light source device 51 is disposed on the side. Here, the liquid crystal display element 41 is provided with a polarizing plate (not shown) on each of a light incident side (upper side) and a light emitting side (lower side).

The surface light source device 51 has a reflector 54, a planar light guide 52 whose upper surface (front surface) in the drawing is on the light emitting surface 52a side, and a prism sheet 57 laminated in this order. The light guide 52 has a structure in which a lamp 53 constituting a light source is disposed on one end surface 52b of the light guide 52, and a lamp cover 59 made of a reflector is provided on the back surface thereof. Here, the prism sheet 57 is configured by a sheet in which a number of isosceles triangular prisms (not shown) are arranged on a surface located on the liquid crystal display element 41 side in the drawing. Then, the light emitted from the lamp 53 is directly or reflected by the lamp cover 59 and is introduced into the light guide 52 from the side end surface 52b. The introduced light is the light emission surface 52a and the opposite side in the drawing. The light is scattered while repeating total reflection between the lower surface or the reflection plate 54, and the reflected or scattered light is emitted from the light emitting surface 52a and is directed in the front direction (upward in the drawing) by the prism sheet 57. It is designed to be focused.

In addition to the above, in the surface light source device 51, the following configuration is adopted as a characteristic portion of the present embodiment. That is, as the light guide 52, FIG.
As shown in FIG. 2, a titanium oxide (refractive index = 2.
1) was formed. The thickness was measured to be about 80 nm.

The light guide 52 obtained as described above is
The prism sheet 57 is arranged as shown in FIG. 11 in a state where the prism sheet 57 has not been set yet, and in a section perpendicular to the length direction of the lamp 53 (the direction penetrating the paper surface in FIG. 11), the center of the light emission surface 52a ( When the luminance of the light emitted from the light guide 52 (the center in the horizontal direction in the figure) was measured from -85 ° to + 85 ° in increments of 5 °, the point with the highest luminance was in the direction of 10 °.

Next, the above-mentioned prism sheet (vertex angle of the prism is set on the light emitting surface 52A side) so that the light emitted from the light guide 52 is directed mainly to the direction of the normal to the light emitting surface 52a. By arranging one (90 °) 57, the surface light source device 51 of the present embodiment shown in FIG. 11 was obtained.

Further, the thus obtained surface light source device 51
Was disposed on the back side of the liquid crystal display element 41 to obtain the liquid crystal display device 40 of the present example. In this case, as the liquid crystal display element 41, RGB having the number of pixels corresponding to VGA is used.
A TN liquid crystal display cell driven by a color TFT was used. Also,
When arranging the surface light source device 51 on the back side of the liquid crystal display element 41, the polarization axis of the light emitted from the light guide 52 and the liquid crystal display element 4
The polarization axis of the light-incident-side polarizing plate (not shown) was substantially matched with that of the first light-incident side polarizing plate. Further, a light-absorbing organic polarizing plate was used as the polarizing plate on the light emission side in the liquid crystal display element 41, and the direction of the polarization axis was a direction rotated by 90 ° with respect to the polarizing axis of the light incidence side polarizing plate. . <Example 2> A phase difference plate 55 of Δnd = 120 nm was adhered to the back surface of the light guide of Example 1 in close contact with the lamp such that the optical axis was at 45 degrees with the lamp as polarization conversion means. The point with the highest luminance when the prism sheet of the light guide was not set was 10 degrees.

Embodiment 1 Using the surface light source device of this embodiment,
A liquid crystal display device shown in FIG. <Example 3> Instead of the light guide of Example 1, a light guide made by the following method was used.

First, 0.25 parts by weight of benzoyl peroxide and 0.075 parts by weight of n-butyl mercaptan were added to 40 parts by weight of methyl methacrylate and 10 parts by weight of vinylphenyl acrylate, and the mixture was placed in a glass cell at 70 ° C. for 16 hours. After polymerization, heat treatment at 80 ° C for 8 hours,
A light-scattering cured product was obtained. Then, the cured product is vertically 210
The light guide 52 is cut and polished so as to have a thickness of 160 mm, a width of 160 mm, and a thickness of 4 mm, and the front and back surfaces of the light guide 52 are mirror-finished. A titanium oxide layer having a thickness of 80 nm was formed by the method. When the luminance was measured at intervals of 5 degrees in the same manner as in Example 1 without setting the prism sheet of the light guide 52, the point with the highest luminance was at 80 °.

A liquid crystal display device was manufactured in the same manner as in Example 1 except that the light guide was used and the apex angle of the prism sheet was 40 °. <Embodiment 4> Δnd =
A phase difference plate 55 of 120 nm was stuck so that the optical axis was at 45 degrees with the lamp. The point with the highest luminance when the prism sheet of the light guide was not yet set was at 80 degrees.

Embodiment 3 Using the surface light source device of this embodiment,
A liquid crystal display device was manufactured in the same manner as described above. <Embodiment 5> FIGS. 14A and 14B show the structure of a surface light source device according to Embodiment 5, wherein FIG. 14A is a side view, FIG. 14B is a bottom view, and FIG. FIG. In Example 5, a light guide 82 as shown in FIG. 14 was made of acrylic resin. In this light guide 82, a large number of quadrangular pyramid grooves 82a are formed on the reflection surface side of the light guide 82 instead of the back print of the diffuse reflector, and the number of the grooves 82a increases as the distance from the lamp 83 increases. Is formed. A 70 μm-thick polypentabromophenyl methacrylate layer (not shown) is provided as a high-refractive-index layer on the exit surface side of the light guide 82. The point of the highest luminance when the prism sheet of the light guide is not set yet is 4 points.
It was at 0 degrees.

Example 1 Using the surface light source device of this example,
A liquid crystal display device was manufactured in the same manner as described above. <Comparative Example 1> The procedure of Example 1 was repeated except that the titanium oxide layer of the light guide was omitted. Comparative Example 2 Example 2 was the same as Example 2 except that the titanium oxide layer of the light guide was omitted. Comparative Example 3 Example 3 was the same as Example 3 except that the titanium oxide layer of the light guide was omitted. Comparative Example 4 Example 4 was the same as Example 4 except that the titanium oxide layer of the light guide was omitted. Comparative Example 5 Example 5 was the same as Example 5 except that the polypentabromophenyl methacrylate layer of the light guide was omitted. <Evaluation> For the liquid crystal display devices obtained in each of Examples 1 to 5 and Comparative Examples 1 to 5, the ratio of brightness in the front direction was determined. The results as shown in Table 1 below were obtained. Obtained.

[0059]

[Table 1]

From Table 1, it can be seen that the liquid crystal display devices of Examples 1 to 5 of the present invention are provided with a high refractive index layer on the exit surface side of the light guide, so that the liquid crystal display devices of Comparative Examples 1 to 5 It can be seen that the brightness in the front direction is increased by at least about 10 to 30%. As described above, according to each example of the present invention, it was possible to increase the light use efficiency, and it was confirmed that the visibility was improved particularly in the front direction.

[0061]

As described above, according to the present invention,
Since the high-refractive-index layer having a refractive index of 1.7 or more is provided on the emission-side surface of the light guide, the difference between the reflectances of the P-polarized light and the S-polarized light increases, and the ratio of the P-polarized light component of the emitted light increases. Can be increased. In addition, in addition to the above configuration, when a polarization conversion unit is provided on at least one of the light exit surface side or the light reflection surface side of the light guide, or inside the light guide, the polarization conversion efficiency can be increased. Unlike the prior art, there is no need to add an expensive and complicated member such as a polarization separator, and the configuration can be simplified. As a result, required polarization can be directly extracted from the light guide at low cost. Further, the angle of emission of the light emitted from the light guide is 60 ° to 9 ° with respect to the normal to the light emission surface.
When it is configured to be in the range of 0 °, it is possible to provide a very advantageous polarization characteristic that the reflectance of P-polarized light is reduced and the reflectance of S-polarized light is increased, and furthermore, the light utilization efficiency is excellent. Thus, a liquid crystal display device can be obtained.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a surface light source device of the present invention.

FIGS. 2A and 2B are diagrams used to describe the configuration of another surface light source device according to the present invention, and FIG. (B) is a configuration diagram of a surface light source device showing an example in which a polarization conversion member is disposed between a back surface of a light guide and a reflection plate, and (c) is a polarization conversion member on the front side and the back side of the light guide. Diagram of a surface light source device showing an example in which

FIGS. 3A and 3B are diagrams used to describe the configuration of still another surface light source device according to the present invention, and FIG. 3A is a configuration of a surface light source device showing an example in which a polarization conversion member is arranged on the surface side of a light guide; Figure,
(B) is a configuration diagram of a surface light source device showing an example in which a polarization conversion member is disposed between the back surface of a light guide and a reflection plate, and (c) is a configuration in which polarization conversion members are provided on the front side and the back side of the light guide. Configuration diagram of a surface light source device showing an example of arrangement

FIG. 4 is an explanatory diagram used to explain the reason why required emission characteristics are obtained when a wedge-shaped deformed light guide is used as an example of a planar light guide according to the present invention.

FIG. 5 is a view showing still another surface light source device according to the present invention, in which an example in which light direction control means is further disposed on the front side of any one of the surface light source devices of FIGS. 1, 2 and 3;

FIG. 6 is a configuration diagram illustrating the configuration of a liquid crystal display device of the present invention.

FIGS. 7A and 7B are used to explain the reason why required emission characteristics are obtained when a scattering light guide is used as an example of the planar light guide according to the present invention, and FIG. Explanatory diagram showing the incident and reflected state of light in the light guide in the case,
(B) is an explanatory view showing the incident / reflective state of the light guide when there is scattering.

FIG. 8 is an explanatory view used to explain the operation of the surface light source device of the present invention.

FIG. 9 explains the reason why a planar light guide having required emission characteristics used in the present invention can be obtained by adjusting the diffusivity of a diffuse reflector layer usually provided on the back side of the light guide. (A) is an explanatory diagram showing the directionality of reflected light when light enters a normal diffuse reflector layer in which diffusivity is not adjusted, and (b) is a diagram in which diffusivity is adjusted. Explanatory diagram showing the directionality of reflected light when light enters the diffused reflective agent layer

FIG. 10 is a diagram for explaining the function of the light guide applied to the present invention, and shows the relationship between the emission angle of emitted light and the reflectance for each of an acrylic plate and an acrylic plate having titanium oxide laminated on the surface. Diagram showing correlation

FIG. 11 is a diagram showing the configuration of a surface light source device according to Embodiments 1, 3, and 5 of the present invention and a liquid crystal display device including the same.

FIG. 12 is a perspective view showing the shape of a planar light guide applied to the surface light source devices according to Embodiments 1, 3, and 5 of the present invention.

FIG. 13 is a diagram showing a configuration of a surface light source device according to Embodiments 2 and 4 of the present invention and a liquid crystal display device including the same.

FIG. 14 is a diagram illustrating a surface light source device according to a fifth embodiment of the present invention.

[Explanation of symbols]

 1, 11, 12, 13, 14, 51 ... surface light source device 2, 21, 22, 52, 72, 82 ... planar light guide 2a, 21a, 52a ... light emitting surface 3, 53 83, lamp 4, 54, reflector 5, 55, polarization conversion means 7, high refractive index layer 8, 57, light direction control means 30, liquid crystal display device 33, 41: liquid crystal display element 31, 32: polarizing plate m: normal line φ: emission angle

Claims (9)

[Claims] 1. A planar light guide whose front side is a light emitting surface side, a light source arranged so that light enters from a side end face of the light guide, and a light emitting surface of the light guide And a reflector provided on a surface opposite to the surface light source device, wherein a high refractive index layer having a refractive index of 1.7 or more is provided on an emission surface of the light guide. Characteristic surface light source device. 2. A planar light guide whose front side is a light emitting surface side, a light source arranged so that light is incident from a side end face of the light guide, and a light emitting surface of the light guide. And a reflector provided on a surface opposite to the surface light source device, wherein at least one of the light exit surface side, the light reflection surface side, and the inside of the light guide of the light guide is incident on the light guide. A polarization conversion means for changing the polarization state of the light, and a high-refractive-index layer having a refractive index of 1.7 or more on the light-emitting side surface of the light guide. Light source device. 3. A planar light guide whose front side is a light emitting surface side, a light source arranged so that light enters from a side end face of the light guide, and a light emitting surface of the light guide. And a reflector provided on the surface opposite to the light guide, wherein the emission angle of the light emitted from the light guide is mainly 6 to the normal to the light emission surface.
A polarization converter that exists in a range of 0 ° to 90 ° and that changes a polarization state of light incident on at least one of the light exit surface side, the light reflection surface side, and the inside of the light guide of the light guide. And a high-refractive-index layer having a refractive index of 1.7 or more is provided on a light-emitting side surface of the light guide. 4. The method according to claim 1, wherein the polarization conversion means is a phase difference plate, and the phase difference plate is provided on the light exit surface side of the light guide, between the light guide and the reflection plate, or on both. The surface light source device according to claim 2, wherein the surface light source device is arranged in a state of being in close contact with the light guide. 5. The light-emitting device according to claim 1, wherein the polarization conversion means is provided with scattering properties to the light reflection surface side inside the light guide, the inside of the light guide located below the high refractive index layer, or the reflection plate. The surface light source device according to claim 2 or 3, wherein: 6. The planar light guide has a wedge-shaped cross section in the light emitting direction, has a back surface as an inclined surface, and reflects light on the inclined surface instead of the polarization conversion means. 4. The apparatus according to claim 2, wherein the polarization of the light is converted into another polarization.
A surface light source device according to claim 1. 7. The surface light source device according to claim 1, wherein the high refractive index layer is made of a metal oxide. 8. A light direction control means for directing the light emitted from the planar light guide in the direction normal to the light exit surface is disposed on the light exit surface side of the planar light guide so as to be located closest to the front side. The surface light source device according to claim 1, wherein: 9. A surface light source device according to claim 8, comprising a liquid crystal display element having a polarizing plate disposed on each of a light incident surface side and a light emitting surface side. The liquid crystal display device is disposed on the rear surface of the liquid crystal display element in a state where the average polarization axis direction of the emitted light and the polarization axis direction of the polarizing plate on the light incident surface side of the liquid crystal display element substantially match. Liquid crystal display device.