JP2008122817A - Optical sheet and surface light source device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical sheet capable of reducing warping due to water absorption without using material having a low coefficient of water absorption, and to provide a surface light source device using the optical sheet. <P>SOLUTION: The optical sheet comprises: a first layer 161 close to an emission surface 163 formed of prism shapes; and a second layer 162 close to an incident surface 164, wherein the coefficient of water absorption of the first layer 161 is less than the coefficient of water absorption of the second layer 162. Further, when the surface area of the incident surface and the surface area of the emission surface per unit sheet area of a light control sheet 16 are respectively denoted by X, Y, the coefficient of saturation water absorption of material forming the first layer 161 is denoted by A% and the coefficient of saturation water absorption of material forming the second layer 162 is denoted by B%, the optical sheet satisfies the relation: 0.8≤AX/BX≤1.25. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an optical sheet and a surface light source device used for a liquid crystal display device or the like.

In an optical sheet in which a plurality of layers are formed using a plurality of materials having different water absorption (moisture absorption) rates, the amount of deformation of each material differs when water is absorbed due to the difference in water absorption rate of each layer, and the entire optical sheet warps. There was a problem that.
In addition, even an optical sheet made of a single material has a concavo-convex shape on its surface, particularly a lenticular lens shape, a so-called fly-eye lens shape (microlens array shape), a prism shape, or the like. In this case, there is a problem that the side on which the concavo-convex shape is formed extends due to water absorption and warps so that the side on which the concavo-convex shape is formed becomes convex.

Patent Document 1 discloses a technique for reducing the warpage of an optical sheet by defining the water absorption rate within a specific range.
However, even if the water absorption rate is set to a low value as in the technique described in Patent Document 1, there is no difference in the elongation due to water absorption between the front and back of the optical sheet, and warpage occurs.
It is also conceivable to use a material that hardly absorbs water. However, in that case, there is a problem that the optical sheet is inferior in terms of heat resistance, moldability, cost, secondary workability, and the like as compared with a material having high water absorption such as acrylic.
JP 2005-49795 A

  The subject of this invention is providing the optical sheet which can reduce the curvature by water absorption, and a surface light source device using the same, without using a material with a low water absorption.

The present invention solves the above problems by the following means. In addition, in order to make an understanding easy, although the code | symbol corresponding to the Example of this invention is attached | subjected and demonstrated, it is not limited to this.
The invention of claim 1 includes a first surface (163) having an uneven shape, a first layer (161) provided on the side where the first surface is formed, and the first surface. A second surface (164) having a surface area smaller than that of the first surface and a side on which the second surface is formed with respect to the first layer. And an optical sheet (16) comprising a second layer (162) formed of a material having a higher water absorption rate than that of the material forming the first layer.
According to a second aspect of the present invention, there is provided the optical sheet according to the first aspect, wherein the concave-convex shape is formed by arranging a plurality of unit optical shapes in a one-dimensional or two-dimensional direction. It is.
A third aspect of the invention is an optical sheet (16) characterized in that the optical sheet according to the first or second aspect is formed of a thermoplastic resin.
According to a fourth aspect of the present invention, in the optical sheet according to any one of the first to third aspects, the optical sheet is viewed from a normal direction of the sheet surface defined as a planar direction of the sheet when viewed as the entire sheet. The surface area of the first surface per unit sheet area of the optical sheet is X, the saturated water absorption of the material forming the first layer is A%, and the first sheet per unit sheet area When the surface area of surface 2 is Y and the saturated water absorption of the material forming the second layer is B%, the relationship of 0.8 ≦ AX / BY ≦ 1.25 is satisfied, This is an optical sheet (16).
Invention of Claim 5 is used for the surface light source device arrange | positioned in the back surface of a transmissive display apparatus (11) in the optical sheet of any one of Claim 1- Claim 4. It is the optical sheet (16) characterized.
A sixth aspect of the present invention is a surface light source device that illuminates a transmissive display unit from the back side, and is provided with a light source unit (12, 13, 14) and an emission side from the light source unit. An optical sheet (16) according to any one of Items 5 to 5, wherein the optical sheet is disposed such that the second surface (164) is on the light source unit side. It is the surface light source device (12, 13, 14, 15, 16, 17) characterized.
The invention according to claim 7 is the surface light source device according to claim 6, wherein a diffusion sheet (15) is arranged between the light source part (12, 13, 14) and the optical sheet (16). It is a surface light source device (12, 13, 14, 15, 16, 17) characterized by being.

According to the present invention, the following effects can be obtained.
(1) The optical sheet includes a first surface on which an uneven shape is formed, a first layer provided on the side on which the first surface is formed, and a surface opposite to the first surface. And the second surface having a smaller surface area than the first surface, and the water absorption rate of the material forming the first layer, provided on the side where the second surface is formed rather than the first layer. And the second layer formed of a material having a high water absorption rate, warpage due to water absorption can be reduced without using a material having a low water absorption rate.

(2) Since the concavo-convex shape of the optical sheet is formed by arranging a plurality of unit optical shapes in a one-dimensional or two-dimensional direction, it is possible to reduce warpage due to water absorption of the optical sheet capable of controlling light in a one-dimensional or two-dimensional direction. .

(3) Since the optical sheet is formed of a thermoplastic resin, it can be easily manufactured.

(4) Since the optical sheet satisfies the relationship of 0.8 ≦ AX / BY ≦ 1.25, warpage due to water absorption can be reliably reduced.

(5) Since the optical sheet is used in the surface light source device disposed on the back surface of the transmissive display device, the surface light source device having high environmental resistance can be obtained.

(6) Since the optical sheet of the surface light source device is disposed so that the second surface is on the light source side, the viewing angle control of the emitted light can be performed effectively.

(7) Since the diffusion sheet is disposed between the light source part of the surface light source device and the optical sheet, the viewing angle of the emitted light can be limited to a necessary range and emitted.

  Even if a material with low water absorption is not used, the optical sheet is formed of a plurality of layers having different water absorption for the purpose of reducing warpage due to water absorption, and the water absorption of the layer on the side close to the surface on which the uneven shape is formed This was realized by making the rate lower than the water absorption rate of the layer far from the surface on which the uneven shape was formed.

FIG. 1 is a diagram showing an embodiment of a transmissive display device according to the present invention.
In addition, each figure shown below including FIG. 1 is the figure shown typically, and the magnitude | size and shape of each part are exaggerated suitably for easy understanding.
In addition, the terms “plate”, “sheet”, “film” and the like are used, but these are generally used in the order of thickness, “plate”, “sheet”, “film”. Although it is used by copying, there is no technical meaning in properly using this, so the description in the claims is used as a sheet. Accordingly, the terms “sheet”, “plate”, and “film” can be appropriately replaced. For example, the diffusion sheet may be a diffusion plate or a diffusion film.

The transmissive display device 10 in this embodiment includes an LCD panel 11, a reflecting plate 12, an arc tube 13, a milky white plate 14, a first diffusion sheet 15, a light control sheet 16, a second diffusion sheet 17, and the like. This is a transmissive liquid crystal display device that illuminates and displays video information formed on the panel 11 from the back. In addition, the reflecting plate 12 and the light emitting tube 13 form a light emitting portion. By adding a milky white plate 14 to the light emitting portion, a light source portion that emits completely diffused light becomes a light source portion that illuminates the LCD panel 11 from the back side. For example, the reflecting plate 12, the arc tube 13, the milky white plate 14, the first diffusion sheet 15, the light control sheet 16, and the second diffusion sheet 17 are applicable.
Note that the completely diffused light has a luminance of 80% or more in a direction inclined by 30 degrees from the normal direction of the emission surface, assuming that the luminance in the normal direction (front direction) of the emission surface is 100%, and The diffused light having a luminance of 60% or more in a direction inclined by 60 degrees from the normal direction of the emission surface is indicated.

The LCD panel 11 is formed of a transmissive liquid crystal display element, the size of which is 40 inches diagonal (885 mm × 498 mm), and display with a resolution of 1920 × 1080 can be performed. The direction along the longitudinal direction of the arc tube 13 is used as the horizontal direction, and the direction in which the arc tubes 13 are arranged is used as the vertical direction.
The arc tube 13 is a cold cathode tube of a line light source that forms a light source part of the surface light source device. In this embodiment, 20 tubes are arranged in parallel at regular intervals of approximately 20 mm. A reflection plate 12 is provided on the back surface of the arc tube 13.
The reflector 12 is provided over the entire surface of the arc tube 13 on the side opposite to the light control sheet 16 (back side), and diffusely reflects illumination light traveling toward the back side toward the light control sheet 16 (outgoing direction). It works to make the incident light illuminance uniform.

The milky white plate 14 is a complete diffusion sheet having non-directional light diffusion characteristics and emitting transmitted light as substantially complete diffused light, and is disposed between the arc tube 13 and the first diffusion sheet 15. Yes. Matting treatment is performed on both surfaces of the milky white plate 14, and a diffusing agent is dispersed and mixed in the base material, thereby generating a diffusion effect. As the base material of the milk white plate, polycarbonate, MS (methacryl styrene: copolymer of acrylic resin and styrene resin) material, and MBS (methyl methacrylate butadiene styrene) material in which a rubber component is mixed in MS material Etc. are used. The thickness is about 1 mm to 3 mm. The base material of the milky white plate 14 of this embodiment is formed to a thickness of 2 mm using an MBS material, has a total light transmittance of 60%, and has a luminance of 95 in a direction inclined 30 degrees from the normal direction of the exit surface. The luminance in the direction inclined by 60 degrees from the normal direction of the exit surface is 82%.
Moreover, the milky white board 14 can also be made into a multilayer structure in order to improve moisture resistance. For example, in the case of MS material or MBS material, sandwiching with a styrene-rich material can be considered. Furthermore, in order to remove the ultraviolet rays generated from the light source, a UV absorber can be mixed in the base material, or the surface of the light source can be coated with a UV absorber.

  The purpose of arranging the milky white plate 14 is to reduce tube unevenness. Here, the tube unevenness means that since the light source is an arc tube, light and darkness occurs in a portion where the arc tube is present and a portion where the arc tube is absent. In order to solve this problem, it is preferable to spread the light by increasing the distance from the arc tube and using a diffusion effect or a lens effect. However, since the surface light source device is desired to be as thin as possible, the distance from the LCD panel can only be about 20 mm. Therefore, the diffusion degree and lens effect of the milky white plate (complete diffusion sheet) and the light control sheet are important. The same can be said when a point light source such as an LED (light emitting diode) light source is used instead of the arc tube.

  In the present invention, a milky white plate, a light control sheet, and a diffusion film are combined, and tube unevenness must be evaluated with these composites. As a result of various combination evaluations, especially for tube unevenness, the degree of diffusion of the milky white plate has the greatest influence, and in order to reduce tube unevenness by the milky white plate, the milky white plate has a total light transmittance. It has been found desirable to be between 50% and 70%.

The first diffusion sheet 15 is a sheet having a light diffusing action that is disposed between the milky white plate 14 and the light control sheet 16 and has a fine convex shape formed on the emission side.
The second diffusion sheet 17 is disposed between the light control sheet 16 and the LCD panel 11 and is a sheet having a light diffusing action in which a fine convex shape is formed on the emission side.
As the diffusion sheet, there are a type in which the surface of the transparent substrate film is coated with a diffusion material, a type in which the diffusion material is kneaded into the substrate, a type in which the surface is roughened, and a combination thereof. .

  Among these, the diffusion sheet of a present Example is a type which coated the diffusion material on the surface of the transparent base film. Specifically, the first diffusion sheet 15 is formed by coating a diffusion base layer in which fine beads are kneaded in a binder on a transparent base layer (film) made of PET (PolyEthylene Terephthalate) resin and having a thickness of 188 μm. is doing. The diffusion layer has a fine convex shape formed on the surface thereof by causing fine beads to protrude from the binder. The first diffusion sheet 15 used in this example is a light diffusion film BS-702 manufactured by Eiwa Co., Ltd., the haze value is 89.2% (manufacturer catalog value), and the total thickness is 220 μm. is there.

  The second diffusion sheet 17 is formed by coating a diffusion base layer in which fine beads are kneaded in a binder on a transparent base layer (film) made of PET resin and having a thickness of 188 μm. The diffusion layer has a fine convex shape formed on the surface thereof by causing fine beads to protrude from the binder. The second diffusion sheet 17 used in this example is a light diffusion film PBS-072 manufactured by Eiwa Co., Ltd., the haze value is 47.9% (manufacturer catalog value), and the total thickness is 210 μm. is there.

A diffusion sheet having a surface coated with a diffusion material (microbeads) exhibits a lens effect because the round portion of the top of the microbeads protrudes. Therefore, when diffused light with a wide viewing angle is incident, it exhibits a light condensing effect, and when diffused light with a narrow viewing angle is incident, it exhibits a diffusing effect. Has a different characteristic action.
In addition, the lens effect (condensing and diffusing effect) can be adjusted by changing the amount of the diffusing material coated on the surface. When the amount of the diffusing material is small, the density of the diffusing material is small, so that the scattering effect due to the rough surface becomes larger than the lens effect, and the condensing effect for the diffused light having a wide viewing angle becomes small.
In addition, the kneading type and the diffusion sheet having a rough surface type exhibit only the diffusion effect not by the lens effect but by the scattering effect and the refraction effect, so that the first diffusion sheet 15 and the second diffusion sheet 15 of the present embodiment. This is different from the diffusion sheet 17.

The light control sheet 16 is an optical sheet provided between the first diffusion sheet 15 and the second diffusion sheet 17 and having a prism shape with an apex angle of 90 degrees protruding as a unit optical shape on the emission side. . The prism shape formed on the light control sheet 16 is a shape extending in the horizontal direction while maintaining the cross-sectional shape of a substantially isosceles triangle, and a large number of prisms are arranged in the same direction as the direction in which the arc tubes 13 are arranged (horizontal direction). Has been placed.
The light control sheet 16 is more incident than the exit surface 163 on which the prism shape is formed, the first layer 161 provided on the side closer to the exit surface, the substantially flat entrance surface 164, and the first layer 161. And a second layer 162 formed of a material having a higher water absorption rate than that of the material forming the first layer 161. The second layer 162 is provided on the side close to the surface 164.
The first layer 161 is made of an MBS material, has a saturated water absorption rate of 0.2%, and an average layer thickness of 0.3 mm. Since the first layer 161 is formed along a prism shape, the average thickness here is an average thickness in a direction perpendicular to the planar portion of the prism shape.
The second layer 162 is made of an MS material, has a saturated water absorption rate of 0.3%, and an average layer thickness of 0.2 mm. Note that the average thickness of the second layer 162 is an average thickness in a direction orthogonal to the incident surface 164.
In addition, the light control sheet 16 of the present embodiment is formed by extruding two layers of MBS material and MS material.

Here, the surface area of the incident surface per unit sheet area of the light control sheet 16 when viewed from the normal direction of the sheet surface defined as the planar direction of the sheet when the light control sheet 16 is viewed as a whole sheet. Is X, the surface area of the emission surface is Y, the saturated water absorption rate of the material forming the first layer 161 is A%, and the saturated water absorption rate of the material forming the second layer 162 is B%. When satisfying the following formula (1), it is desirable to prevent warpage of the light control sheet 16 due to water absorption.
0.8 ≦ AX / BY ≦ 1.25 Formula (1)
When a material absorbs water, the larger the surface area, the easier it is to absorb water. Therefore, in a single layer, the side with the larger surface area absorbs more water and expands than the side with the smaller surface area, and warps so that the side with the larger surface area becomes convex.
However, by satisfy | filling said Formula (1), the difference in the water absorption which arises when a surface area differs in the front and back of an optical sheet can be offset by the difference in the water absorption rate of material, and generation | occurrence | production of curvature can be suppressed.
In this embodiment, if A = 0.2%, B = 0.3%, and Y = 1, the cross-sectional shape of the exit surface is an isosceles triangle, so X = 1 / √ (2) + 1 / √ (2) = 1.414. Therefore, AX / BY = (0.2 × 1.414) / (0.3 × 1) = 0.94, which satisfies the above-described formula (1).

In order to confirm the amount of warpage of the light control sheet 16 of this example during water absorption, the amount of warpage was measured by leaving the light control sheet 16 in an environment of 40 ° C. and 90% humidity for 100 hours. Moreover, the comparative example which extruded and formed in the same shape as the light-control sheet | seat 16 with the single layer only of resin of saturation water absorption 0.2% was prepared, and the amount of curvature was compared. The amount of warpage was evaluated by a test piece cut into a square having a side of 500 mm. Here, the amount of warpage is a value obtained by placing the test piece on a horizontal plane and measuring the floating amount of four corners.
As a result of the evaluation, the amount of warpage of the light control sheet 16 of this example was 5 mm or less. On the other hand, the warpage amount of the comparative example was 50 mm.

  According to the present embodiment, the first layer 161 close to the exit surface 163 where the prism shape is formed and the second layer 162 close to the entrance surface 164 are formed, and the water absorption rate of the first layer 161 is Since the water absorption rate of the second layer 162 is smaller, the occurrence of warpage due to water absorption can be suppressed.

(Modification)
The present invention is not limited to the embodiments described above, and various modifications and changes are possible, and these are also within the equivalent scope of the present invention.
(1) In the present embodiment, the light control sheet 16 shows an example in which one type of unit optical shape is arranged on the emission side. However, the present invention is not limited to this. For example, a combination of a plurality of types of unit optical shapes is used. You may arrange | position on the output side.

(2) In the present embodiment, an example in which line light sources are arranged in the light emitting unit is shown, but the present invention is not limited thereto, and for example, a light emitting unit in which many point light sources such as LEDs are arranged may be used. A light emitting unit that emits light such as organic EL (electroluminescence) or inorganic EL may be used.

It is a figure which shows the Example of the transmissive display apparatus by this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Transmission type display apparatus 11 LCD panel 12 Reflector 13 Light emission tube 14 Milky white board 15 1st diffuser sheet 16 Light control sheet 161 1st layer 162 2nd layer 163 Output surface 164 Incident surface

Claims (7)

A first surface on which an uneven shape is formed;
A first layer provided on the side on which the first surface is formed;
A second surface opposite to the first surface and having a smaller surface area than the first surface;
A second layer which is provided on a side where the second surface is formed with respect to the first layer, and is formed of a material having a higher water absorption rate than that of the material forming the first layer; ,
An optical sheet comprising: The optical sheet according to claim 1,
The concavo-convex shape is formed by arranging a plurality of unit optical shapes in a one-dimensional or two-dimensional direction,
An optical sheet characterized by In the optical sheet according to claim 1 or 2,
Formed of a thermoplastic resin,
An optical sheet characterized by In the optical sheet according to any one of claims 1 to 3,
The surface area of the first surface per unit sheet area of the optical sheet when viewed from the normal direction of the sheet surface defined as the planar direction of the sheet when viewed as the entire sheet is X, and the first surface The saturated water absorption of the material forming the layer is A%, the surface area of the second surface per unit sheet area is Y, and the saturated water absorption of the material forming the second layer is B%. When
0.8 ≦ AX / BY ≦ 1.25
Satisfying the relationship
An optical sheet characterized by In the optical sheet according to any one of claims 1 to 4,
Used in a surface light source device disposed on the back of a transmissive display device;
An optical sheet characterized by A surface light source device that illuminates a transmissive display unit from the back,
A light source unit;
The optical sheet according to any one of claims 1 to 5, wherein the optical sheet is provided on an emission side from the light source unit.
With
The optical sheet is disposed so that the second surface is on the light source side,
A surface light source device. The surface light source device according to claim 6,
A diffusion sheet is disposed between the light source unit and the optical sheet,
A surface light source device.

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