JP4970663B2 - Light diffusion sheet and backlight unit using the same - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a light diffusion sheet incorporated in a backlight unit of a liquid crystal display device and a backlight unit using the same.
[0002]
[Prior art]
In the liquid crystal display device, a backlight system in which a liquid crystal layer is illuminated from the back surface is widely used, and a backlight unit is provided on the lower surface side of the liquid crystal layer. As shown in FIG. 6A, the backlight unit 50 generally includes a rod-shaped lamp 51 serving as a light source, and a rectangular plate-shaped light guide plate 52 that is disposed so that an end thereof is along the lamp 51. And a plurality of optical sheets 53 stacked on the surface side of the light guide plate 52. Each of the optical sheets 53 has specific optical properties such as refraction and diffusion. Specifically, the light diffusion sheet 54 disposed on the surface side of the light guide plate 52 and the surface of the light diffusion sheet 54 For example, the prism sheet 55 disposed on the side corresponds to this.
[0003]
The function of the backlight unit 50 will be described. First, a light beam incident on the light guide plate 52 from the lamp 51 is reflected by a reflection dot or a reflection sheet (not shown) on the back surface of the light guide plate 52 and each side surface. Emitted from the surface. Light rays emitted from the light guide plate 52 enter the light diffusion sheet 54, are diffused, and are emitted from the surface of the light diffusion sheet 54. Thereafter, the light beam emitted from the light diffusion sheet 54 enters the prism sheet 55 and is emitted as a light beam having a distribution having a peak in a substantially upward direction by the prism portion 55 a formed on the surface of the prism sheet 55. In this way, the light beam emitted from the lamp 51 is diffused by the light diffusion sheet 54 and refracted by the prism sheet 55 so as to show a peak in a substantially upward direction, and further the entire liquid crystal layer not shown above is Illuminate.
[0004]
Although not shown, there is also a backlight unit in which a light diffusion sheet and a prism sheet are further arranged on the surface side of the prism sheet 55 in consideration of the light condensing characteristics of the prism sheet 55 described above.
[0005]
As the light diffusion sheet 54 of the backlight unit 50 having the above-described structure, there are a so-called bead coating type and a surface on which many hemispherical convex lens portions are formed. Specifically, the bead coating type light diffusion sheet 54 is formed by laminating a base material layer 56 and a light diffusion layer 57 as shown in FIG. 6B, and the light diffusion layer 57 is made of a synthetic resin. The binder 58 has a structure in which beads 59 made of synthetic resin, glass or the like are dispersed.
[0006]
[Problems to be solved by the invention]
The bead coating type or convex lens type light diffusion sheet 54 shown in FIG. 6B has the following disadvantages. That is,
(1) A light diffusing layer coating solution is prepared by mixing beads 59 with the resin composition constituting the binder 58, and the coating solution is applied to the base material layer 56 to form a light diffusing layer 57. However, it is not easy to uniformly disperse the beads 59 in the binder 58, and it is difficult to impart uniform diffusibility. Therefore, it is not easy to produce a light diffusion sheet 54 that is uniform and has a predetermined diffusibility.
[0007]
(2) Since the bead coating type or convex lens type light diffusion sheet 54 has a bead 59 or convex lens portion protruding from the surface, the bead 59 or convex lens portion is inclined at a predetermined angle or more from the normal direction. The light emitted from the periphery of the light hits the adjacent bead 59 or the convex lens portion and cannot be emitted to the surface side. Thus, the viewing angle is narrowed by the amount of light that cannot be emitted by hitting the adjacent beads 59 and the like, and the total light transmittance is reduced.
[0008]
(3) Since the conventional light diffusing sheet 54 has the beads 59 and the convex lens portions protruding on the surface as described above, in the backlight unit 50 in which the light guide plate 52 and other optical sheets 53 are laminated, or a plurality of them. When storing and transporting the stacked sheets, stress concentrates on the protruding beads 59 and the like, and the stacked surfaces are likely to be damaged, and the light transmittance may be lowered due to the damage.
[0009]
(4) Since the beads 59 and the convex lens portion contributing to the light diffusion are in principle spherical, a gap is formed no matter how densely formed, and an area that does not contribute to the diffusion of the light diffusion layer 57 is generated. There is a predetermined limit to improving the diffusion efficiency.
[0010]
The present invention has been made in view of these disadvantages, and has a high light diffusibility, a large viewing angle and a large light transmittance, can be prevented from being scratched even when used repeatedly, and has excellent handleability. The object is to provide a sheet and a backlight unit using the sheet.
[0011]
[Means for Solving the Problems]
The invention made to solve the above problems is a light diffusing sheet in which the opening is polygonal and the concavely curved polygonal concave lens part is densely formed without a gap. According to the light diffusion sheet, the following effects can be exhibited.
[0012]
(1) Since the polygonal concave lens portion is densely formed in the light diffusion sheet, there is no gap and the entire sheet surface is covered with the polygonal concave lens portion. Therefore, the light diffusion sheet has no portion that does not contribute to light diffusion. In other words, the area contributing to diffusion is maximized, so that the diffusion efficiency can be dramatically improved.
[0013]
(2) Moreover, since the polygonal concave lens part which exhibits light diffusibility can be formed uniformly, the said light-diffusion sheet has no area difference in light-diffusion ability, and there are few brightness nonuniformities by the area difference of light-diffusion ability.
[0014]
(3) Furthermore, the light diffusion sheet has a light diffusing action due to the polygonal concave lens part, and does not have a protruding part such as a bead or a convex lens part of the conventional light diffusion sheet. No damage is prevented, and as a result, handling is dramatically improved.
[0015]
(4) Further, since the light diffusion sheet does not have protrusions such as beads or convex lens portions of the conventional light diffusion sheet on the surface as described above, outgoing light rays inclined by a predetermined angle or less from the normal direction are adjacent to each other. Therefore, the viewing angle and the total light transmittance can be significantly increased as compared with conventional bead coating type or convex lens type light diffusion sheets.
[0016]
(5) The light diffusion sheet can adjust the viewing angle by changing the curvature of the curved concave lens surface in the polygonal concave lens portion.
[0017]
The polygonal shape of the opening of the polygonal concave lens is preferably a regular hexagon. A regular hexagon is a polygon with the maximum size that can be formed densely with no gap, and can exhibit the light diffusing ability closest to the spherical lens part. The improvement effect and the diffusion uniformity improvement effect can be promoted.
[0018]
Further, as the shape of the opening of the polygonal concave lens portion, a flat hexagon extending in a certain direction is also preferable. In this way, by making the shape of the opening a flat hexagon extending in a certain direction, the curvature radius of the polygonal concave lens portion is based on the extension direction of the flat hexagon and the direction perpendicular to the extension direction of the flat hexagon Unlike the case of the above, the diffusing action is small in the extending direction of the flat hexagon, and a relatively large diffusing action is exhibited in the direction perpendicular to the extending direction of the flat hexagon. That is, according to the means, anisotropic diffusion becomes possible.
[0019]
The aspect ratio of the flat hexagon is preferably 1: 2 to 1: 5. Thus, by adjusting the aspect ratio of the flat hexagon within the above range, the anisotropy of the light output characteristics in the backlight unit can be improved while performing the light diffusion function, and the isotropic diffusion sheet is used. The amount of light emitted in the normal direction direction can be increased as compared with the case, and the viewing angle of the backlight unit can be controlled.
[0020]
The minimum width of the opening of the polygonal concave lens is preferably 1 μm or more and 100 μm or less. Here, the “minimum width of the opening” means the diameter of a polygonal inscribed circle formed by the opening. Thus, by setting the minimum width of the opening of the polygonal concave lens part within the above range, the light diffusibility can be enhanced while ensuring the formability of the polygonal concave lens part.
[0021]
The ratio of the depth to the minimum width of the opening in the polygonal concave lens is preferably 5% or more and 50% or less. Thus, by making the ratio of the depth of the polygonal concave lens portion to the minimum opening width within the above range, the light diffusibility and the total light transmittance can be balanced in a high dimension.
[0022]
The light diffusion sheet may include a transparent synthetic resin base layer and a light diffusion layer, and the polygonal concave lens portion may be formed on the light diffusion layer. According to this means, it is possible to select a synthetic resin that can secure the necessary strength as a light diffusion sheet by the base material layer and can easily form the polygonal concave lens portion for forming the light diffusion layer. The manufacture of the diffusion sheet can be facilitated.
[0023]
Therefore, the material for forming this light diffusion layer is preferably an ultraviolet curable resin capable of forming the polygonal concave lens portion accurately and easily.
[0024]
A synthetic resin layer having a refractive index different from that of the base of the polygonal concave lens part is laminated on the polygonal concave lens part side, and the sheet surface on the polygonal concave lens part side may be formed smoothly. According to this means, the concave and convex portions of the polygonal concave lens part are completely covered with the synthetic resin layer, and the light diffusion sheet is made smooth, so that it is overlapped with other sheets when the backlight unit is assembled or transported. Even if it uses, the perfection of damage prevention is accelerated | stimulated and the fall of the optical performance by damage can be prevented.
[0025]
The difference in refractive index between the base of the polygonal concave lens part and the synthetic resin layer is preferably 0.01 or more and 0.5 or less. According to this means, a sufficient diffusing ability as a light diffusion sheet can be exhibited, and a decrease in light transmittance due to reflection at the interface between the polygonal concave lens portion and the synthetic resin layer can be reduced.
[0026]
In the light diffusion sheet, a sticking prevention layer can be laminated on the front surface and / or the back surface. In this way, by sticking the anti-sticking layer to the light diffusion sheet, sticking with a light guide plate, a prism sheet or the like disposed on the front surface side or the back surface side can be prevented. Such an anti-sticking layer may have a structure in which beads are dispersed in a binder, or may have a structure in which an uneven shape is formed on the surface.
[0027]
Further, instead of the above sticking prevention layer, a protective layer having a flat surface can be laminated on the front surface and / or the back surface of the light diffusion sheet. Thus, by laminating the protective layer on the light diffusion sheet, it is possible to physically protect the front surface and / or back surface of the light diffusion sheet and to prevent the prism sheet and the like from being damaged. .
[0028]
Therefore, in a backlight unit for a liquid crystal display device including a lamp and an optical sheet that disperses light emitted from the lamp and guides it to the surface side, the light diffusion sheet of the present invention may be used as the optical sheet. . According to the backlight unit, since the diffusibility of the light diffusing sheet is high as described above, it is possible to prevent the occurrence of luminance unevenness and to control the viewing angle widely and arbitrarily.
[0029]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings as appropriate. 1A and 1B are a schematic plan view and a schematic cross-sectional view showing a light diffusion sheet according to an embodiment of the present invention, and FIGS. 2, 3, 4 and 5 show the light of FIG. It is a typical sectional view showing a light diffusion sheet concerning a form different from a diffusion sheet.
[0030]
1 (a) and 1 (b) has a single-layer structure composed of a base material layer 2, and a large number of polygonal concave lens portions 3 are densely formed on the surface of the base material layer 2 without gaps. Has been. The polygonal concave lens part 3 has a concave lens surface 5 in which the shape of the opening 4 is a regular hexagon and is curved in a concave shape, and the concave lens surface 5 exhibits light diffusing ability.
[0031]
The light diffusing sheet 1 can be formed densely without gaps by forming the polygonal concave lens portion 3 to be a regular hexagon as described above. As a result, the diffusibility and diffusion of the light diffusing sheet 1 can be reduced. Performance uniformity can be improved. In addition, since the polygonal concave lens portion 4 that exhibits light diffusivity is concave as described above, there is no protruding portion where stress concentration occurs on the surface, and damage can be prevented even when used repeatedly.
[0032]
Since the base material layer 2 is required to transmit light, it is made of a synthetic resin that is transparent, particularly colorless and transparent. The synthetic resin used for the base material layer 2 is not particularly limited, and examples thereof include polyethylene terephthalate, polyethylene naphthalate, acrylic resin, polycarbonate, polystyrene, polyolefin, cellulose acetate, and weather resistant vinyl chloride. . Although the thickness of the base material layer 2 is not specifically limited, For example, they are 10 micrometers or more and 500 micrometers or less, Preferably they are 75 micrometers or more and 250 micrometers or less. This is because when the thickness of the base material layer 2 is less than the above range, inconveniences such as curling are easily generated when the light diffusion layer 3 is formed, and handling becomes difficult. In addition, if the thickness of the base material layer 2 exceeds the above range, the brightness of the liquid crystal display device may decrease, and the thickness of the backlight unit becomes large, which is contrary to the demand for thinning the liquid crystal display device. Because it becomes.
[0033]
The minimum width D of the opening 4 of the polygonal concave lens portion 3 (diameter of the inscribed circle of the opening 4) D is preferably 1 μm or more and 100 μm or less, particularly preferably 1 μm or more and 10 μm or less. This is because if the minimum width of the opening 4 of the polygonal concave lens part 3 is smaller than the above range, the formation of the polygonal concave lens part 3 becomes difficult. Conversely, if the minimum width exceeds the above range, This is because the light diffusibility of the above cannot be obtained.
[0034]
The ratio (H / D) of the depth H to the minimum width D of the opening 4 in the polygonal concave lens portion 3 is preferably 5% or more and 50% or less, and particularly preferably 20% or more and 45% or less. This is because if the depth ratio H / D of the polygonal concave lens portion 3 is smaller than the above range, the light diffusibility required for the light diffusion sheet cannot be exhibited, and conversely, the depth ratio H / D is If the range is exceeded, total reflection increases on the concave lens surface 5 and the total light transmittance decreases.
[0035]
As a method for forming the light diffusion sheet 1 having the above structure, specifically,
(A) A method of forming a polygonal concave lens portion 3 on the surface of the base material layer 2 by laminating a synthetic resin on a sheet mold having a shape obtained by inverting the surface shape of the light diffusion sheet 1, and peeling the sheet mold.
(B) an injection molding method in which a molten resin is injected into a mold having an inverted shape of the surface of the light diffusion sheet 1;
(C) A method of transferring the shape by re-heating the sheeted resin and pressing between the same mold and metal plate as described above,
(D) There is an extrusion sheet molding method in which a molten sheet-like resin is passed through a nip between a roll having a reverse shape of the surface of the light diffusion sheet 1 on the peripheral surface and another roll, and the above shape is transferred.
[0036]
Accordingly, a rod-shaped lamp 51 as a light source as shown in FIG. 6A, a rectangular plate-shaped light guide plate 52 arranged so that the end thereof is along the lamp 51, and a surface side of the light guide plate 52 are provided. In the backlight unit 50 provided with the light diffusion sheet 54 and the prism sheet 55 disposed on the surface side of the light diffusion sheet 54, when the light diffusion sheet 1 is used as the light diffusion sheet 54, the light diffusion sheet 1 has high diffusibility and can prevent occurrence of luminance unevenness. Further, since there is no protruding portion on the surface of the light diffusion sheet 1, it is possible to reduce scratches on the back surface of the stacked prism sheets 55 and to prevent occurrence of luminance unevenness due to the scratches.
[0037]
The light diffusing sheet 11 of FIG. 2 includes a transparent film-like base material layer 12 and a light diffusing layer 13 laminated on the surface of the base material layer 12. The polygonal concave lens portion 3 is densely formed without a gap. The polygonal concave lens portion 3 is the same as the light diffusion sheet 1 of FIG. 1 and has the same effects as the light diffusion sheet 1 of FIG. The base material layer 12 is the same as the base material layer 2 of the light diffusion sheet 1 of FIG.
[0038]
As a method for forming the light diffusion sheet 11 having the above structure, specifically,
(A) A UV curable resin is applied to the surface of the base material layer 12, pressed onto a roll having the same inverted shape as described above, and the shape is transferred to an uncured UV curable resin. A curing method,
(B) There is a method of using an EB curable resin instead of the UV curable resin.
[0039]
Since the light diffusing sheet 11 can select the UV curable resin or the like as the base for forming the polygonal concave lens part 3 as described above, the polygonal concave lens part 3 can be easily formed, and the shape of the polygonal concave lens part 3 can be accurately determined. In addition, the light diffusibility and the uniformity of light diffusion can be promoted.
[0040]
The light diffusing sheet 21 in FIG. 3 is the same as the light diffusing sheet 1 in FIG. 1 in that the light diffusing sheet 21 is composed of the base material layer 2 having the polygonal concave lens portion 3 formed on the surface, and has the same effects. Since the light diffusion sheet 21 is formed by laminating the synthetic resin layer 22 on the polygonal concave lens part 3 side and forming the sheet surface 23 on the polygonal concave lens part 3 side smoothly, the integrity of scratch prevention is promoted, Even when used repeatedly, it is possible to prevent a decrease in optical performance due to scratches.
[0041]
As the synthetic resin constituting the synthetic resin layer 22, a synthetic resin having a refractive index different from that of the base material layer 2 is used. For example, acrylic resin, polyurethane, polyester, fluorine resin, silicone resin, polyamideimide An epoxy resin or the like is used. The refractive index difference between the synthetic resin layer 22 and the base material layer 2 is preferably 0.01 or more and 0.5 or less. This is because if the difference in refractive index is smaller than the above range, the diffusivity by the polygonal concave lens portion 3 becomes small, and the optical performance as a light diffusion sheet cannot be exhibited. This is because light rays transmitted through the interface between the resin layer 2 and the synthetic resin layer 22 are reflected, leading to a decrease in light transmittance.
[0042]
The light diffusing sheet 31 of FIG. 4 includes a base material layer 2 on which a polygonal concave lens portion 3 is formed as in the light diffusing sheet 1, and in addition, a sticking prevention layer 32 is laminated on the back side of the base material layer 2. Has been.
[0043]
The anti-sticking layer 32 includes a binder 33 and beads 34 dispersed in the binder 33, and prevents sticking with a prism sheet or the like disposed on the surface side by the beads 34 protruding from the binder 33. is there. Further, by forming such an anti-sticking layer 32, the surface of the light diffusion sheet 31 can be physically protected and the prism sheet can be prevented from being damaged.
[0044]
Examples of the polymer used for the binder 33 include acrylic resins, polyurethanes, polyesters, fluorine resins, silicone resins, polyamide imides, and epoxy resins. In addition to the above-described polymer, the binder 33 may contain, for example, a plasticizer, a stabilizer, a deterioration preventing agent, a dispersant, and the like. The binder 33 is transparent because it is necessary to transmit light, and colorless and transparent is particularly preferable.
[0045]
The beads 34 are substantially spherical, and examples of the material thereof include acrylic resin, polyurethane, polyvinyl chloride, polystyrene, polyacrylonitrile, and polyamide. The beads 34 are preferably transparent in order to increase the amount of light transmitted through the light diffusion sheet 5, and particularly preferably colorless and transparent.
[0046]
The average particle diameter of the beads 34 is preferably 1 μm or more and 30 μm or less, and particularly preferably 1 μm or more and 15 μm or less from the viewpoint of preventing sticking. The amount of the beads 34 is relatively small, the beads 34 are separated from each other and dispersed in the binder 33, and many of the beads 34 have their lower ends protruding from the binder 33. The thickness of the anti-sticking layer 4 (the thickness of the binder 33 portion excluding the beads 34) is not particularly limited, but is, for example, about 1 μm or more and 10 μm or less.
[0047]
In the above description, the case where the anti-sticking layer 32 in which the beads 34 are dispersed in the binder 33 is formed has been described, but the structure of the anti-sticking layer is not limited thereto. For example, an anti-sticking layer having an uneven surface formed on the surface by embossing or the like may be formed. In this case as well, as in the case where the anti-sticking layer 32 is formed, sticking with the prism sheet or the like disposed on the surface side can be prevented by the convex portions formed on the surface of the anti-sticking layer. .
[0048]
Furthermore, instead of forming the anti-sticking layer as described above, a protective layer having a flat surface may be formed on the surface of the light diffusion layer 31. By forming such a protective layer, the surface of the light diffusing sheet 31 can be physically protected, and the effect of preventing the prism sheet from being scratched compared to the case of forming the anti-sticking layer. Become bigger.
[0049]
The light diffusion sheet 41 of FIGS. 5A, 5B, and 5C has a single layer structure composed of the base material layer 2, and a large number of polygonal concave lens portions 42 are formed on the surface of the base material layer 2 without gaps. It is the same as the light diffusion sheet 1 of FIG. 1 in that it is densely formed. However, the polygonal concave lens portion 42 is a hexagon in which the shape of the opening 43 extends in a certain direction (the front-rear direction in the figure). Therefore, the concave lens surface 44 of the polygonal lens portion 42 has a small radius of curvature as shown in FIG. 5B when viewed perpendicular to the extension direction, and FIG. 5C when viewed parallel to the extension direction. ), The radius of curvature increases. Therefore, the light diffusing sheet 41 can exhibit an anisotropic diffusing ability with a small light diffusing ability in the extending direction of the polygonal lens portion 42 and a large light diffusing ability in the direction perpendicular to the extending direction.
[0050]
The light diffusing sheet of the present invention is not limited to the above embodiment. For example, the opening 4 of the polygonal concave lens portion 3 is not limited to a regular hexagon, and may be a rectangle or the like as long as it can be formed densely without a gap. Further, a light diffusion sheet including both a synthetic resin layer and a sticking prevention layer is also possible.
[0051]
【Effect of the invention】
As described above, according to the light diffusing sheet of the present invention, the light diffusibility is high, the viewing angle and the total light transmittance are large, and even when used repeatedly, scratches can be prevented and the handleability is excellent.
[Brief description of the drawings]
FIG. 1A is a schematic plan view showing a light diffusion sheet according to an embodiment of the present invention, and FIG. 1B is a schematic cross-sectional view of the light diffusion sheet of FIG.
FIG. 2 is a schematic cross-sectional view showing a light diffusion sheet according to a different form from the light diffusion sheet of FIG.
3 is a schematic cross-sectional view showing a light diffusing sheet according to a different form from the light diffusing sheet of FIGS. 1 and 2. FIG.
4 is a schematic cross-sectional view showing a light diffusing sheet according to a different form from the light diffusing sheets of FIGS. 1, 2, and 3. FIG.
FIGS. 5A and 5B show a light diffusing sheet according to a different form from the light diffusing sheets of FIGS. 1 to 4, in which FIG. 5A is a schematic plan view, and FIG. (C) is a schematic cross-sectional view in a direction parallel to the extending direction.
FIG. 6A is a schematic perspective view showing a general backlight unit, and FIG. 6B is a schematic cross-sectional view showing a general bead coating type light diffusion sheet.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Light diffusion sheet 2 Base material layer 3 Polygonal concave lens part 4 Opening part 5 Concave lens surface 11 Light diffusion sheet 12 Base material layer 13 Light diffusion layer 21 Light diffusion sheet 22 Synthetic resin layer 23 Sheet surface 31 Light diffusion sheet 32 Sticking prevention layer 33 Binder 34 Bead 41 Light diffusion sheet 42 Polygonal lens portion 43 Opening portion 44 Concave lens surface

Claims (9)

A light diffusion sheet in which the opening is polygonal, and the concavely curved polygonal concave lens part is densely formed without a gap,
The ratio of the depth to the minimum width of the opening in the polygonal concave lens is 5% or more and 50% or less,
A transparent synthetic resin base layer and a light diffusion layer are provided, and the polygonal concave lens portion is formed in the light diffusion layer,
The opening of the polygonal concave lens part is a regular hexagon, or a flat hexagon extending in a certain direction,
A light diffusing sheet for use as an optical sheet in a backlight unit for a liquid crystal display device comprising a lamp and an optical sheet that disperses light emitted from the lamp and guides the light to the surface side. The light diffusion sheet according to claim 1 , wherein the aspect ratio of the flat hexagon is 1: 2 to 1: 5. The light diffusion sheet according to claim 1 or 2 , wherein the minimum width of the opening of the polygonal concave lens portion is 1 µm or more and 100 µm or less. The light diffusion sheet according to any one of claims 1 to 3 , wherein the light diffusion layer is formed of an ultraviolet curable resin. Synthetic resin layers of different refractive index from the base of the further polygonal concave portion to the polygonal concave portion are stacked, any one of claims 1 to the sheet surface of the polygonal concave portion side is formed smoothly in claim 4 1 The light diffusion sheet according to item. The light diffusion sheet according to claim 5 , wherein a difference in refractive index between the base of the polygonal concave lens portion and the synthetic resin layer is 0.01 or more and 0.5 or less. The light-diffusion sheet of any one of Claims 1-6 by which the sticking prevention layer is laminated | stacked on the surface and / or the back surface. The light diffusion sheet according to any one of claims 1 to 7 , wherein a protective layer having a flat surface is laminated on the front surface and / or the back surface. 9. A backlight unit for a liquid crystal display device comprising a lamp and an optical sheet that disperses light emitted from the lamp and guides the light to the surface side, wherein the optical sheet is any one of claims 1 to 8. A backlight unit for a liquid crystal display device, wherein the light diffusing sheet described is used.

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