JPH09127507A - Surface light source device and liquid crystal display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To manufacture a surface light source device and liquid crystal display device at low prices relatively easily while obtaining superior light utilization efficiency. SOLUTION: The surface light source device 51 having a surface photoconductor 52, a light source 53 which is so arranged that light is made incident from a side end surface of the photoconductor 52, and a reflecting plate 54 which is provided on the reverse surface side of the photoconductor 52 uses a photoconductor whose projection light angle is principally 60-90 deg. to the normal of a light projection surface, and a polarized light converting member 55 is arranged on the side of the light projection surface 52a of the photoconductor or/and between the photoconductor 52 and reflecting plate 54 in contact with the photoconductor 52. On the top side of the surface light source 51 which is thus constituted, a transparent base material whose reverse surface side is smooth or/and a light direction control means 57 are provided and arranged on the back surface of the liquid crystal display element 41 so that the mean polarization axis direction of the projection light from the surface light source device 51 and the polarization axis direction of the polarizing plate on the light incidence side of the liquid crystal display device 51 nearly match each other, thereby constituting the liquid crystal display device 40.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface light source device used for a backlight of a liquid crystal display device 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 surface (back surface) side of a liquid crystal display element in order to make characters and images easy to see. From the viewpoint of increasing the efficiency, so-called edge light type surface light source devices are 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 a liquid crystal display device, a first polarizing plate is generally provided on the light incident side and a second polarizing plate is provided on the light emitting side of a liquid crystal display element having a liquid crystal layer held by two glass substrates.
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,
Since more than half of the incident light is absorbed by the polarizing plate when passing through the first polarizing plate, there is a problem that the light utilization efficiency is reduced accordingly.

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.

[0005]

However, in the surface light source device and the like described in the above publication, a polarization separator having a complicated structure, specifically, a substance having a high refractive index and a substance having a low refractive index are used. Since it is necessary to use a polarization separator obtained by alternately stacking the layers to form a multi-layered polarization separation layer and stacking this on the array-shaped portion of the above-mentioned columnar prism array, there is a problem that the cost is high. is there. Also,
When a polarization separator having such a complicated laminated structure is arranged between the light guide and the liquid crystal display element, light is reflected, scattered and even absorbed in the polarization separator, resulting in a relatively large amount of optical loss. Therefore, although the ratio of the light component of the same polarization direction as that of the first polarizing plate incident on the liquid crystal display element increases, the light utilization efficiency of all light rays is reduced by the optical loss due to the polarization separator.

The present invention addresses such a problem, and provides a planar light source device having excellent light utilization efficiency and a liquid crystal display device including the planar light source device with almost no optical loss. It is an object of the present invention to provide a device capable of increasing the degree of polarization of light emitted from a liquid crystal display device, that is, the ratio of linearly polarized light of a specific component that can be used as incident light of a liquid crystal display element, and can be manufactured relatively easily and inexpensively. .

[0007]

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.

That is, in the invention described in claim 1 (first invention), as illustrated in FIGS. 1 (a) to 1 (c), the surface light guide whose surface side is the light emitting surface 2a side is illustrated. 2, the light source 3 arranged so that light is incident from the side end surface 2b of the light guide 2, and the reflection provided on the surface (rear surface) side opposite to the light emitting surface 2a of the light guide 2. In the surface light source device 1 or 11 or 12 having the plate 4, as the planar light guide 2, the emission angle φ of the emitted light is mainly 60 ° to 90 ° with respect to the normal line m of the light emission surface 2a. A light guide existing in the range is used, and the light exit surface 2a side of the light guide is used (see FIG.
In the case of), or between the light guide 2 and the reflector 4 (in the case of (b) in the figure) or both (in the case of (c) in the figure), polarization conversion for changing the polarization state of the incident light. Member 5
Is arranged in close contact with the light guide 2.

Further, in the invention described in claim 2, a retardation plate for rotating the polarization axis direction is used as the polarization conversion member 5, and in the invention described in claim 3, such a retardation plate is used. A member that can convert S-polarized light into P-polarized light is used.

Further, as illustrated in FIG.
In the surface light source device 13 according to the invention described in 1), in any of the surface light source devices 1 or 12 or 13, a smooth back surface (lower surface in the drawing) is provided on the light emitting surface side of the planar light guide.
The transparent base material 6 having 6a is arranged so as to be located on the most front side (upper side in the drawing). The term "most front side" as used herein means the surface side of the polarization conversion member opposite to the light guide side surface when the polarization conversion member is present on the light exit surface side of the planar light guide body. .

Further, as illustrated in FIG. 3, the surface light source device 14 according to the invention described in claim 5 is the surface light source device 1 or 11-13 according to each of the above inventions, which is a planar light guide. A light direction control means 7 for directing the emitted light of the planar light guide in the normal direction of the light emitting surface is arranged on the light emitting surface side of the body so as to be located on the most front side. The term "most front side" as used herein means that when the transparent substrate 6 as shown in FIG. 2 is present on the light emitting surface side of the planar light guide, the transparent substrate 6 is further front side (in the figure). Above), that is, the surface side opposite to the light guide side.

Further, as shown in FIG.
A liquid crystal display device 30 according to the invention described in 1) includes the surface light source device 14 according to claim 5 and a liquid crystal display element 33 in which polarizing plates 31 and 32 are arranged on a light incident side and a light emitting side, respectively. In addition, in the surface light source device 14 of these, in a state in which the average polarization axis direction of the emitted light and the polarization axis direction of the light incident side polarization plate 31 in the liquid crystal display element 33 are substantially aligned, It is characterized in that it is arranged on the back surface of the liquid crystal display element 33.

Next, the components and the like of each of the above inventions will be described in more detail. (1) Regarding the planar light guide In the present invention, the outgoing angle of the outgoing light of the planar light guide is mainly 60.
Existence in the range of 90 ° to 90 ° means that the light emitted from the center of the light guide is cut in 5 ° increments from -85 ° to + 85 ° in a certain cross section (cross section in the direction orthogonal to the light emission surface). When the brightness is measured with, the highest brightness point is +60
It means in the range of ° to +90 ° or -60 ° to -90 °. As long as it is a planar light guide having such characteristics, its shape, material, etc. do not matter.

As a method of imparting such characteristics,
For example, reticulated diffusive reflector printing (so-called back printing) using a modified light guide using a scattering light guide.
There are various methods such as adjusting the diffusivity of the light guide, using a thin light guide.

When using the 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 the light-scattering property to the light guide, for example, JP-A-54-105562 and JP-A-59-8 are used.
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, when the above-mentioned material having a scattering property is used, the reason why the output angle of the output light is mainly in the range of 60 ° to 90 ° with respect to the normal line of the light output surface. Explain why the body is easy to get and why.

Light from a light source (lamp) is usually incident directly or after being reflected by a lamp holder or the like and then incident from a side end surface 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 the angular range of at most -45 ° to + 45 ° (this angle slightly varies depending on the refractive index) with respect to the horizontal plane. In the example shown in FIG. 5, such light impinges on the upper side or the lower side of the light guides 2 ′, 2 in the figure (or when considering the length / thickness ratio of the light guides, it is extremely rare). Hit the opposite side, but ignore this) Either. 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, in the case where the light guide has a scattering property, as shown in FIG. 2B, the incident angle when the light guide 2 hits the upper side of the light guide is determined by the scattering property and the inside of the light guide 2. 90 °-(α-β ₁ ) to 90 °-(α + β ₂ ) depending on the movement 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.

Since the outgoing angle of the light guide 2 fluctuates continuously due to the scattering property, the values of β ₁ and β ₂ can be made sufficiently small by appropriately adjusting the scattering property of the light guide 2. Therefore, it is possible to adjust the emission angle range so that the emission angle peak is located in the range of 60 ° to 90 ° when the front direction (upward direction in the drawing) is 0 °.

When Using Modified Light Guide As a modified light guide, for example, as shown in FIG. 6, a wedge-shaped light guide 21 in which a reflecting surface 21c is inclined with respect to an emitting surface 21a by a predetermined angle θ 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. 6 shows the back surface 21c of the light guide pair 21.
1 shows a state in which a reflection plate 4 is provided on the side.

Here, the reason why the above-mentioned required emission characteristics are obtained by the modified light guide (wedge shape) will be explained with reference to FIG. The light incident from the end surface 21b on the light source side of the light guide body 21 is in the angle range of −45 ° to + 45 ° at most, as in the case described above. In the example of FIG. 6, whether such light strikes the upper side or the lower side of the light guide 21 (or, considering the length / thickness ratio of the light guide, very rarely, it strikes the opposite side, but this is ignored. 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 end surface 21b of the light guide 21 on the light source side is +2 each time it 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 total reflection condition is satisfied and all the 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.

In the case of adjusting the diffusivity of reticulated diffuse reflection agent printing (back printing) In a normal light guide, it is formed by a method such as screen printing on the lower surface (the surface located on the side opposite to the light emission side). In many cases, a diffuse reflector layer is provided. In that case,
As shown in FIG. 7 (a), the diffuse reflection part 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 °.

Further, in the light guide of the type in which the diffuse reflection agent layer is not provided, fine unevenness is provided on the light emitting surface or the surface opposite to the light emitting surface, and the fine unevenness guides the light into the light guide. There is a light emitting device, but in this type of light guide as well, it is controlled so that the emission angle of the emitted light is mainly in the range of 60 ° to 90 ° depending on the design of the fine concavo-convex portion. be able to.

In the case of using a thin light guide body Generally, when the thickness of the light guide body is made thin, the emission angle becomes large. However, by making use of such a property to make the light guide body thin, 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, although the explanation that the above-mentioned predetermined emission characteristics are obtained when such a thin light guide is used is omitted, in any case, when the front direction is 0 °, the emission light is The light guide having the emission characteristics such that the emission angle of is mainly in the range of 60 ° to 90 ° can be obtained by various methods. (2) Regarding Polarization Conversion Member and Retardation Plate As the polarization conversion member, for example, various retardation plates, films and adhesives made of organic substances, coatings made of inorganic substances, and the like can be 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. These members are generally S-polarized to P-polarized.
The higher the conversion efficiency into polarized light, the more preferable. 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 is 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. (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 pair, or further, the whole surface may be closely attached. (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 only on one side end face of the light guide (so-called one-lamp type) or both side end faces (so-called two-lamp type). (5) About transparent substrate Examples of the transparent substrate include PET, TAC (triallyl cyanurate), PVA (polyvinyl alcohol),
PE (polyethylene), PSt (polystyrene), PC
Examples include organic thin films such as (polycarbonate) and transparent materials such as various glass plates. As such a transparent material, it is desirable to use a transparent material having a thickness of 1 mm or less, one or both surfaces being smooth, and light absorption of the base material being 5% or less in the visible light region.

Further, a spacer such as an adhesive material may be provided at an appropriate portion so that the transparent base material does not adhere to the light guide. In that case, the thickness of the spacer is preferably 200 μm or less, more preferably 100 μm or less. When the spacer is used, it is desirable that the transparent base material has rigidity such that it does not bend. This is because when the transparent substrate is bent, an undesirable phenomenon such as Newton's ring may occur. (6) Light Direction Control Means When the surface light source device is incorporated into a liquid crystal display device, it is necessary to direct the emission angle of the emitted light of the surface light source device in the direction normal to the light emission 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 thereof, in which light is directed in a predetermined direction by a prism effect (also called a Fresnel lens), etc. can be mentioned. Then, in addition to the known prism sheet, various kinds of light control plates or light control devices for directing emitted light to the front direction by combining other lenses, a reflection plate, and a transmission reflection plate 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. (7) Other Means In the surface light source device of the present invention, in addition to the above components, other means for increasing the ratio of P-polarized light can be used as necessary. Further, a diffusion plate or the like may be arranged as necessary. However, these are not essential in the present invention.

<Operation> Next, the present invention will be explained with respect to the reason why the ratio of P-polarized light increases when the polarization conversion member is arranged on the back surface side or the front surface side of the light guide having the above-mentioned predetermined emission characteristics. The principle will be described with reference to FIG. Although FIG. 1 shows an example in which the polarization conversion member 5 is arranged between the light guide 2 and the reflection plate 4, the polarization conversion member is the light emission surface (front surface or top surface) 2 a of the light guide 2. The same applies whether they are arranged on the side or on both sides.

It is generally known that the light emitted from the light guide 2 has more P-polarized light and less S-polarized light when the incident angle is near the critical angle. That is, even if the proportion of P-polarized light ( _IP ) and the proportion of S-polarized light ( _IS ) are initially equal, the incident angle is in the vicinity of the critical angle, such as in the range of the first exit angle of 60 ° to 90 °. Then, at this point, the emitted light contains a large amount of P-polarized light and a small amount of S-polarized light ( _IP >
I _S ), and conversely, the reflected light has a small amount of P-polarized light and a large amount of S-polarized light (I _P <I _S ). When this reflected light is then reflected on the lower surface side of the light guide 2 in the figure, the S-polarized light is converted into P-polarized light by the polarization conversion member 5, or the P-polarized light is converted into S-polarized light. Since more of the light is S-polarized light, the light reflected from the lower surface side of the light guide 2 has more P-polarized light component ( _IP > _IS ).

Now, this reflected light whose polarization component has been converted goes again to the upper side of the light guide 2 in the figure. The incident angle at this time is also likely to be in the vicinity of the critical angle due to the characteristics of the light guide 2, and at this time, more P-polarized light is emitted and S-polarized light is relatively emitted. When most of the light is emitted after repeating this operation and looking at the polarization components of the entire emitted light, it can be seen that the proportion of P-polarized light is higher than in the normal case.

At this time, it is preferable to use a material having a high refractive index as the main material for the light guide 2 because the proportion of P-polarized light generally obtained further increases. Moreover, the conversion efficiency of the polarization component can be further increased by using the retardation plate to bring the retardation closer to the theoretical calculated value.

As described above, the ratio of the P-polarized component of the light emitted from the light guide body is changed by combining the light guide body having a special output angle characteristic which can be easily and inexpensively manufactured with the polarization conversion member. You can increase. Therefore, the liquid crystal display device using this can reduce the light absorbed by the polarizing plate on the incident side, and the light utilization efficiency can be improved accordingly.

When a transparent base material having a smooth back surface (lower surface) or both surfaces or a prism sheet having a smooth lower surface is disposed on the front surface side of the light guide body, the same criticality as in the above case is used here. By repeating the reflection and reflection in the vicinity of the corner, the proportion of P-polarized light is further increased, so that a surface light source device having higher light utilization efficiency can be obtained.

Even after the above additional treatment, the prism sheet is generally used in the conventional surface light source device.
Since the transparent base material does not require any special processing and is inexpensive, the surface light source device of the present invention can increase the proportion of P-polarized light extremely inexpensively, easily and efficiently. Therefore, if the surface light source device of the present invention is used and the average polarization axis of the light emitted from the light guide provided therein and the polarization axis of the incident side polarization plate of the liquid crystal display device are substantially matched,
It is possible to significantly reduce the wasted light that was conventionally absorbed by the incident side polarization plate of the liquid crystal display device and converted into heat. As a result, in the liquid crystal display device equipped with the surface light source device of the present invention, the light utilization efficiency can be improved more than before.

[0040]

EXAMPLES Examples of the present invention will be described below along with comparative examples. <Embodiment 1> FIG. 9 shows a basic structure of a liquid crystal display device 40 including the surface light source device of this embodiment. The liquid crystal display device 40 has a rear surface (a lower surface in the drawing) of a liquid crystal display element 41. The surface light source device 51 is arranged 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).

In the surface light source device 51, a reflector 54, a planar light guide 52 whose upper surface (front surface) side in the drawing is the light emitting surface 52a side, and a prism sheet 33 are laminated in this order. The lamp 53 constituting the light source is arranged in close contact with the one end surface 52b of the light guide body 52, and the lamp cover 59 made of a reflector is provided on the back surface thereof.
Here, the prism sheet 57 is the liquid crystal display element 4 in the figure.
A sheet formed by arranging a large number of prisms (not shown) of an isosceles triangle having an apex angle of 40 ° on the surface located on the first side is formed. 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, the surface light source device 51 employs the following configuration as a characteristic part of this embodiment. That is, as the light guide 52, the emission angle of the emitted light is mainly 6 with respect to the normal line of the light emission surface 52a.
A light guide having characteristics such that it exists in the range of 0 ° to 90 ° is used, and polarization conversion is performed on the lower surface of the light guide 52 so as to be positioned between the light guide 52 and the reflection plate 54. The retardation plate 55 forming the member is attached in a predetermined state.

The liquid crystal display device 40 provided with such a surface light source device 51 is specifically manufactured as follows.
First, 0.25 parts by weight of benzoyl peroxide and 0.075 part 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 polymerized in a glass cell at 70 ° C. for 16 hours. Heat treatment was performed at 80 ° C. for 8 hours to obtain a light-scattering cured product. And this cured product is 160 mm x 210 m
After cutting and polishing to a size of m × 4 mm to form a light guide body 52, the front surface and the back surface of the light guide body 52 are mirror-finished, and then a retardation plate 55 having a thickness of 50 nm is predetermined on the back surface side. Pasted in the state of. On the other hand, the reflector 5
Polyethylene terephthalate (reflectance: 85%) having aluminum vapor-deposited on the surface was used for No. 4, and a 10 watt (W) cold cathode tube having a side length of 152 mm and a diameter of 3.5 mm was used for the lamp 53. Furthermore, the lamp cover 5
As for No. 9, a cylindrical type whose inner surface was a mirror surface was used.

The light guide 52 obtained as described above is
The prism sheet 57 is not yet set in the arrangement as shown in FIG. 9, and in the cross section perpendicular to the length direction of the lamp 53 (direction penetrating the paper in the figure), the center of the light emitting surface 52a ( (Center in the horizontal direction of the light guide body 52 in the figure)
When the luminance of the light emitted from the sample was measured from -85 ° to + 85 ° in steps of 5 °, the highest luminance point was in the direction of 80 °.

Next, the above-mentioned prism sheet (the apex angle of the prism is the apex angle of the prism is set so that the light emitted from the light guide body 52 is mainly directed in the direction of the normal line of the light emitting surface 52a). By arranging one 40 ° 57, the surface light source device 51 of this embodiment shown in FIG. 9 was obtained.

Further, the surface light source device 51 thus obtained
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, an RG having the number of pixels corresponding to VGA is used.
A B color TFT driven TN liquid crystal display cell was used. Further, when the surface light source device 51 is arranged on the back side of the liquid crystal display element 41, the polarization axis of the light emitted from the light guide body 52 and the polarization axis of the light incident side polarization plate (not shown) of the liquid crystal display element 41 are arranged. Were almost matched. Further, a light-absorption-type organic polarizing plate is used as the light-exiting-side polarizing plate in the liquid crystal display element 41, and its polarization axis is oriented at 90 ° with respect to the polarization axis of the light-incident-side polarizing plate.
Only the direction was rotated. <Example 2> 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 0.5 parts by weight of benzoguanamine resin fine particles having an average particle diameter of 1 μm, and the mixture was placed in a glass cell. Then, after polymerizing at 70 ° C for 16 hours, heat treatment was performed at 80 ° C for 8 hours to obtain a light-scattering cured product. This cured product was cut and polished to form a light guide having a size of 160 mm × 210 mm × 4 mm.

The light guide thus produced was subjected to the same conditions as in Example 1 to measure the brightness of the light emitted from the center of the light guide in the section perpendicular to the lamp in the range of −85 ° to + 85 ° in steps of 5 °. The highest brightness point was at 70 °.

Using this surface light source device, a liquid crystal display device was manufactured in the same manner as in Example 1. <Embodiment 3> A wedge-shaped planar light guide 62 as shown in FIG.
Was molded with acrylic resin. The front surface (light emission surface) 62a and the back surface 62c of the light guide body 62 are both mirror-finished. Using the light guide 62 thus obtained, under the same conditions as in Example 1, in a cross section perpendicular to the lamp, the light emitted from the center was measured for brightness from -85 ° to + 85 ° in 5 degree increments. The highest brightness point was at 85 °.

A liquid crystal display device was manufactured in the same manner as in Example 1 using the surface light source device of this example. <Comparative Example 1> The same procedure as in Example 1 was carried out except that the retardation plate 55 was removed in Example 1. <Comparative Example 2> The same operation as in Example 2 was carried out except that the retardation plate was removed in Example 2. <Comparative Example 3> The same procedure as in Example 3 was carried out except that the retardation plate was removed in Example 3. Comparative Example 4 In Comparative Example 4, as shown in FIG.
A light guide 72 made of a rectangular parallelepiped acrylic plate on which dots of a white diffuse reflection agent were printed on the back surface 72c was used. Similar to the first embodiment, in the light guide 72, in the cross section perpendicular to the lamp, the light emitted from the center thereof is -85 ° to +85.
When the luminance was measured in steps of 5 degrees to 0 °, the highest luminance point was at 0 °. In this comparative example, no retardation plate is used. <Comparative Example 5> The same as Comparative Example 4 except that a retardation plate was attached to the back surface 72c of the light guide 72 in Comparative Example 4. <Evaluation> For the liquid crystal display devices obtained in each of the above Examples 1 to 3 and Comparative Examples 1 to 5, the ratio of the brightness in the front direction was determined, and the results shown in Table 1 below were obtained. Was obtained.

[0050]

[Table 1]

From Table 1, in the liquid crystal display devices of Examples 1 to 3 of the present invention, compared with 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 20 to 30%. Thus, according to each of the embodiments of the present invention, it was confirmed that the light utilization efficiency is enhanced and the visibility is improved especially in the front direction.

[0052]

As described above, according to the present invention, it is not necessary to newly add an expensive and complicated member such as an independent polarization separator, and the required polarized light can be directly emitted from the light guide member easily and inexpensively. Therefore, the ratio of light absorbed by the polarizing plate in the related art can be reduced, and a liquid crystal display device with high light utilization efficiency can be obtained accordingly.

[Brief description of the drawings]

FIG. 1 is used to explain a structure of a surface light source device of the present invention, and FIG. 1A is a configuration diagram of a surface light source device showing an example in which a polarization conversion member is arranged on the front surface side of a light guide, FIG. 6B is a configuration diagram of a surface light source device showing an example in which a polarization conversion member is arranged between the back surface of the light guide and the reflection plate, and FIG. 9C is a polarization conversion member arranged on the front surface and the back surface of the light guide. Diagram of the surface light source device showing the example

FIG. 2 shows another surface light source device of the present invention.
The block diagram which shows the example which has arrange | positioned the transparent base material further to the front side of the surface light source device in any one of (a)-(c).

FIG. 3 shows still another surface light source device of the present invention, and an example in which a light direction control means is further arranged on the front side of any of the surface light source devices of FIGS. 1 (a) to 1 (c) and FIG. Configuration diagram showing

FIG. 4 is a configuration diagram illustrating a configuration of a liquid crystal display device of the present invention.

FIG. 5 is 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 in the present invention, and (a) has no scattering property. Explanatory diagram showing the light entering and reflecting state 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. 6 is an explanatory diagram used to explain the reason why required emission characteristics are obtained when a wedge-shaped modified light guide is used as an example of the planar light guide of the present invention.

FIG. 7 illustrates the reason why a planar light guide having the required emission characteristics used in the present invention can be obtained by adjusting the diffusivity of a diffuse reflection agent layer normally provided on the back surface side of the light guide. (A) is an explanatory view showing the directionality of reflected light when light is incident on an ordinary diffuse reflection agent layer whose diffusivity is not adjusted, and (b) is a diagram in which diffusivity is adjusted. Explanatory diagram showing the directionality of the reflected light when light is incident on the diffuse reflector layer

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

FIG. 9 is a configuration diagram showing a surface light source device and a liquid crystal display device including the same according to a first embodiment of the present invention.

FIG. 10 is a perspective view showing a wedge-shaped planar light guide used in a third embodiment of the present invention.

FIG. 11 is a perspective view showing a planar light guide used in Comparative Example 4 with respect to the example of the present invention.

[Explanation of symbols]

1, 11, 12, 13, 14, 51 ... Surface light source device 2, 21, 22, 52, 62 ... Planar light guide 2a, 21a, 52a, 62a ... Light emitting surface 3, 53 ... ..Light source (53 ... Lamp) 4,54 ... Reflector 5,55 ... Polarization conversion member (55 ... Phase difference plate) 6 ... Transparent base material 7, 57 ... Light Direction control means (57 ... Prism sheet) 30, 40 ... Liquid crystal display device 33, 41 ... Liquid crystal display element 31, 32 ... Polarizing plate m ... Normal line .phi.

 ─────────────────────────────────────────────────── ─── Continued Front Page (72) Inventor Yozo Nagai 1-2-1, Shimohozumi, Ibaraki City, Osaka Prefecture Nitto Denko Corporation

Claims (6)

[Claims] 1. 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 surface of the light guide, and a light emitting surface of the light guide. Is a surface light source device having a reflection plate provided on the side opposite to the surface side, and the emission angle of the emitted light of the light guide is mainly 6 with respect to the normal line of the light emission surface.
A polarization conversion member that exists in the range of 0 ° to 90 ° and changes the polarization state of the incident light on the light emitting surface side of the light guide, between the light guide and the reflector, or both. The surface light source device is characterized in that is disposed in close contact with the light guide. 2. The surface light source device according to claim 1, wherein the polarization conversion member is a retardation plate that rotates a polarization axis direction. 3. The surface light source device according to claim 2, wherein the retardation plate is made of a member capable of converting S-polarized light into P-polarized light. 4. A transparent base material having a smooth back surface is disposed on the light emitting surface side of the planar light guide member so as to be located on the most front side. The surface light source device according to claim 1. 5. A light direction control means for directing the light emitted from the planar light guide in a direction normal to the light exit face of the planar light guide is arranged on the most front side. The surface light source device according to claim 1, wherein the surface light source device is provided. 6. The surface light source device according to claim 5, and a liquid crystal display element in which polarizing plates are respectively arranged on a light incident side and a light emitting side, wherein the surface light source device emits light emitted from the surface light source device. A liquid crystal arranged on the back surface of the liquid crystal display element in a state where an average polarization axis direction and a polarization axis direction of a light-incident side polarization plate of the liquid crystal display element are substantially aligned with each other. Display device.