JP4317378B2 - Optical sheet and backlight unit using the same - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical sheet having various functions such as condensing, diffusing, and refraction in a normal direction, and particularly suitable for a backlight unit of a liquid crystal display device, and a backlight unit using the same. is there.
[0002]
[Prior art]
In the liquid crystal display device, a backlight system that illuminates a liquid crystal layer from the back is widespread, and a backlight unit such as an edge light type (side light type) or a direct type is provided on the lower surface side of the liquid crystal layer. As shown in FIG. 4A, the edge light type backlight unit 20 generally has a rod-like lamp 21 as a light source, and a rectangular plate shape that is disposed so that the end of the lamp 21 is along the end. The light guide plate 22 and a plurality of optical sheets 23 stacked on the surface side of the light guide plate 22 are provided. The optical sheet 23 has specific optical functions such as refraction and diffusion. Specifically, (1) the optical sheet 23 is disposed on the surface side of the light guide plate 22 and mainly has a light diffusion function and a light collecting function. The bead coating type light diffusing sheet 24 has (2) a prism sheet 25 disposed on the surface side of the light diffusing sheet 24 and having a function of refraction in the normal direction side.
[0003]
The function of the backlight unit 20 will be described. Light rays that are incident on the light guide plate 22 from the lamp 21 are reflected by reflection dots, a reflection sheet (not shown) on the back surface of the light guide plate 22, and each side surface. It is emitted from. The light beam emitted from the light guide plate 22 enters the light diffusion sheet 24, is diffused, and is emitted from the surface. Thereafter, the light beam emitted from the light diffusion sheet 24 enters the prism sheet 25 and is emitted as a light beam having a distribution having a peak in a substantially upward direction by the prism portion 25a formed on the surface. In this way, the light beam emitted from the lamp 21 is diffused by the optical sheet 23, refracted so as to show a peak in a substantially upward direction, and further illuminates the entire liquid crystal layer (not shown) above.
[0004]
Although not shown, there is also a backlight unit in which more optical sheets such as a light diffusion sheet and a prism sheet are arranged in consideration of the light guide characteristics of the light guide plate 22 and the optical function of the optical sheet 23 described above. .
[0005]
As the conventional light diffusing sheet 24, generally, as shown in FIG. 4B, a transparent synthetic resin base material layer 26 is laminated on the surface of the base material layer 26 and is light diffusive. (See, for example, Japanese Patent Laid-Open No. 2000-89007). The light diffusion layer 27 generally has resin beads 29 in a binder 28. Due to the presence of the resin beads 29, lens-shaped fine irregularities are formed on the surface of the light diffusion sheet 24. The light diffusing sheet 24 exhibits optical functions such as diffusion, condensing, refraction in the normal direction (angle change) by refraction at the lens-shaped fine irregularities and the resin bead 29 interface.
[0006]
[Patent Document 1]
Japanese Patent Laid-Open No. 2000-89007
[Problems to be solved by the invention]
In the conventional light diffusing sheet 24, since the binder 28 covers the surface of the resin beads 29, the resin beads 29 do not protrude sufficiently, and it is difficult to form microlenses having an intended shape. Therefore, the light diffusion sheet 24 has a certain limit in improving optical functions such as light collection, diffusion, and deflection. Therefore, the conventional backlight unit 20 needs to include one or two prism sheets 25 despite being expensive and difficult to handle.
[0008]
The conventional light diffusion sheet 24 can control optical functions such as light collection by changing the particle size, blending amount and coating amount of the resin beads 29. As a result, since uniform coating is difficult due to aggregation or the like, it is impossible to accurately control the optical function.
[0009]
Further, since the conventional light diffusion sheet 24 exhibits a light diffusion function by the resin beads 29 in the light diffusion layer 27, it has a uniform light diffusion function over the entire sheet surface and in all directions. On the other hand, light rays emitted from the surface of the light guide plate 22 have different luminance distributions in the lamp 21 in the vertical direction and in the parallel direction. Therefore, the light diffusing function of the light diffusing sheet 24 is adapted to either the luminance distribution in the vertical direction or the luminance distribution in the parallel direction to the lamp 21 so that the emitted light amount in the normal direction in the unidirectional luminance distribution is increased. If set, the amount of light emitted in the normal direction cannot be increased efficiently in the luminance distribution in the other direction. In other words, the conventional light diffusion sheet 24 cannot efficiently increase the amount of light emitted in the normal direction in consideration of the anisotropy of light emitted from the surface of the light guide plate 22.
[0010]
Further, in the conventional light diffusion sheet 24, the height of the fine irregularities formed on the surface by the resin beads 29 in the light diffusion layer 27 is random. For this reason, the light diffusing sheet 24 may be damaged by stress concentration with respect to other optical members to be overlaid.
[0011]
The present invention has been made in view of these inconveniences, and has extremely high optical functions such as condensing, diffusing, and bending, and the optical functions have anisotropy and high controllability. Optical sheet that suppresses damage to other components that are used, and a backlight unit that uses this to promote improvement in quality such as high brightness in the front direction, uniform brightness, proper viewing angle, thinning, etc. It is intended to provide.
[0012]
[Means for Solving the Problems]
An invention made to solve the above-mentioned problems is an optical sheet for a backlight unit of a liquid crystal display device having at least a light collecting function for transmitted light and a function of refraction in a normal direction side, and includes a plurality of microlenses. The microlens array is provided on the surface, the microlens has an elliptical partial shape, the major axis of the elliptical surface is positioned substantially parallel to a predetermined plane direction, and the height (H) of the microlens height ratio to the length axis radius of the elliptical surface (R _L) of (H / R _L) is 3/4 or more and 1 or less, the planar direction substantially parallel to the minor axis of the major axis radius of the elliptical surface (R _L) The flatness ratio (R _L / R _S1 ) with respect to the radius (R _S1 ) is 1.05 or more and 1.7 or less, and the refractive index of the material constituting the microlens array is 1.3 or more and 1.8 or less. It is a featured optical sheet
[0013]
Since the optical sheet is provided with a microlens array composed of a plurality of microlenses having a lens-like refractive action on its surface, it has excellent optical functions (condensing function, diffusion function, variable angle toward the normal direction side). And the optical function is easily and reliably controlled. The optical sheet has an anisotropy in the optical function because the microlens has a partial shape of an elliptical surface and the major axis of the elliptical surface is positioned substantially parallel to a predetermined plane direction. Specifically, the optical function in the direction perpendicular to the major axis is greater than the optical function in the direction parallel to the major axis of the microlens. Furthermore, in the optical sheet, by adjusting the height of the microlens to be constant, stress concentration is suppressed, and damage to other members that are superimposed is prevented. In addition, the optical sheet constitutes a microlens array in which the height ratio (H / R _L ) of the height (H) of the microlens to the major axis radius (R _L ) of the ellipsoid is 3/4 or more and 1 or less. Since the refractive index of the material to be adjusted is 1.3 to 1.8, optical functions such as light collection, diffusion, and refraction in the normal direction are further enhanced. In addition, the optical sheet has a flatness ratio (R _L / R _S1 ) of 1.05 or more and 1.7 to a minor axis radius (R _S1 ) substantially parallel to the plane direction of the major axis radius (R _L ) of the ellipsoid. As shown below, while maintaining high optical function, it develops anisotropy of the optical function, and as a result, efficiently directs the emitted light from the light guide plate surface having anisotropy in the normal direction. Can do.
[0014]
In the optical sheet, the major axis radius (R _L ) of the microlens is preferably 10 μm or more and 1000 μm or less, the filling ratio of the microlens is 40% or more, and the surface roughness (Ra) of the microlens is preferably 2 μm or less. By setting the major axis radius (R _L ), filling factor, and surface roughness (Ra) of the microlens within the above ranges, optical functions such as light collection, diffusion, and refraction in the normal direction are further enhanced.
[0016]
The arrangement pattern of the microlenses in the microlens array is preferably a rhombus pattern or a random pattern. This rhomboid pattern allows the microlenses having an elliptical planar shape to be arranged more densely, so that the filling ratio of the microlenses of the optical sheet can be easily increased, and optical functions such as light collection and diffusion can be achieved. Greatly improved. Further, by arranging the microlenses in a random pattern, the occurrence of moire is reduced when the optical sheet is overlapped with another optical member.
[0017]
A radiation curable resin or a thermoplastic resin may be used as a material constituting the microlens array. According to such radiation curable resin or thermoplastic resin, a microlens array composed of the above-described microlenses can be easily and reliably formed.
[0018]
The optical sheet may include a sticking prevention layer in which beads are dispersed in a binder on the back surface. By providing the anti-sticking layer on the back surface in this way, sticking between the optical sheet and the light guide plate, prism sheet or the like disposed on the back surface side is prevented.
[0019]
Therefore, the optical sheet may be provided in a backlight unit for a liquid crystal display device that disperses light emitted from the lamp and guides it to the surface side. The backlight unit has improved energy efficiency due to the optical sheet with high optical functions such as condensing, diffusing, and changing angle, and by increasing the brightness in the front direction, uniforming the brightness, optimizing the viewing angle, etc. The quality is improved, and further thinning is promoted by reducing the number of optical sheets. In addition, the backlight unit can efficiently direct the emitted light from the surface of the light guide plate having anisotropy in the normal direction by the optical sheet having anisotropy in the optical function.
[0020]
Here, the “micro lens” is a concept including a convex lens and a concave lens. “Height (H)” is the vertical distance from the base surface of the microlens to the top when the microlens is a convex lens, and the vertical distance from the opening surface of the microlens to the bottom when the microlens is a concave lens. Means distance. “Filling rate” means the occupation ratio of microlenses per unit area in the surface projection shape of the optical sheet. The “rhombic lattice pattern” means a pattern in which a rhombus is assumed on the surface and microlenses are arranged at intersections of the rhombus.
[0021]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings as appropriate. FIG. 1A is a schematic partial plan view showing an optical sheet according to an embodiment of the present invention, and FIGS. 1B and 1C are schematic AA sectional views of the optical sheet of FIG. FIG. 2 is a schematic cross-sectional view showing a backlight unit including the optical sheet of FIG. 1, and FIG. 3 is a schematic partial cross-section showing an optical sheet having a form different from the optical sheet of FIG. FIG.
[0022]
The optical sheet 1 shown in FIGS. 1A, 1B, and 1C includes a base material layer 2 and a microlens array 3 provided on the surface of the base material layer 2.
[0023]
Since the base material layer 2 needs 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. For example, polyethylene terephthalate, polyethylene naphthalate, acrylic resin, polycarbonate, polystyrene, polyolefin, cellulose acetate, weather resistant vinyl chloride, radiation curable resin. Etc. Among these, radiation curable resins such as ultraviolet curable resins and electron beam curable resins excellent in moldability of the microlens array 3 and thermoplastic resins such as polycarbonate and polyolefin are preferable. Further, a polyethylene terephthalate film, a polyethylene naphthalate film, or a polycarbonate film can be used as the base material layer 2, and the microlens 4 can be formed thereon using an ultraviolet curable resin or the like.
[0024]
The thickness (average thickness) of the base material layer 2 is not particularly limited, but is, for example, 10 μm or more and 500 μm or less, preferably 35 μm or more and 250 μm or less, and particularly preferably 50 μm or more and 188 μm or less. When the thickness of the base material layer 2 is less than the above range, inconveniences such as curling tends to occur when exposed to heat in a backlight unit or the like, and handling becomes difficult. On the contrary, if the thickness of the base material layer 2 exceeds the above range, the luminance 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 of the liquid crystal display device. It will also be.
[0025]
The microlens array 3 is composed of a number of convex microlenses 4 protruding from the surface of the base material layer 2. The microlens array 3 may be formed integrally with the base material layer 2 or may be formed separately from the base material layer 2. Since the microlens array 3 is required to transmit light, it is formed of a transparent, particularly colorless and transparent synthetic resin. Specifically, the same synthetic resin as that of the base material layer 2 is used.
[0026]
In addition to the above synthetic resin, for example, a filler, a plasticizer, a stabilizer, a deterioration inhibitor, a dispersant, and the like may be blended in the base material layer 2 and the microlens 4.
[0027]
The microlens 4 has a partial shape of the elliptical surface 5, specifically, a shape obtained by dividing the elliptical surface 5 by a surface parallel to the major axis 6. The major axis 6 of the elliptical surface 5 is located substantially parallel to the predetermined plane direction of the optical sheet 1. The minor axis 7 of the elliptical surface 5 is positioned perpendicular to the major axis 6 and substantially parallel to the plane direction, and the minor axis 8 is positioned perpendicular to the major axis 6 and substantially parallel to the normal direction. The major axis radius (R _L ) is the radius of the major axis 6, the minor axis radius (R _S1 ) is the radius of the minor axis 7, and the minor axis radius (R _S2 ) is the radius of the minor axis 8.
[0028]
The microlenses 4 are disposed relatively densely and geometrically on the surface of the base material layer 2. Specifically, the microlenses 4 are arranged in a rhombus pattern on the surface of the base material layer 2, and the pitch and the distance between the lenses are constant. In this arrangement pattern, the microlenses 4 having an elliptical planar shape can be arranged most densely, and the optical functions of the optical sheet 1 such as the light condensing function, the diffusing function, and the angle changing function can be improved. it can.
[0029]
The height ratio (H / R _L ) of the height (H) of the microlens 4 to the major axis radius (R _L ) of the ellipsoidal surface 5 is preferably 3/4 or more and 1 or less. By setting the height ratio (H / R _L ) of the microlens 4 within the above range, the lens-like refraction action of the microlens 4 is effectively exerted, and the optical sheet 1 is focused, diffused, bent, etc. The optical function is greatly improved.
[0030]
The lower limit of the flatness ratio (R _L / R _S1 ) in the plane direction with respect to the short axis radius (R _S1 ) of the major axis radius (R _L ) on the elliptical surface 5 of the microlens 4 is preferably 1.05. On the other hand, the upper limit of the flatness ratio (R _L / R _S1 ) is preferably 1.7, particularly preferably 1.5. By configuring the microlens 4 from the elliptical surface 5 in which the flatness ratio (R _L / R _S1 ) in the plane direction is in the above range, the length of the microlens 4 can be obtained while having the excellent optical function of the optical sheet 1. A difference in optical function (anisotropy) can be effectively expressed in the direction parallel to and perpendicular to the axis 6.
[0031]
The lower limit of the major axis radius (R _L ) on the ellipsoidal surface 5 of the microlens 4 is preferably 10 μm, and particularly preferably 30 μm. On the other hand, the upper limit of the long axis radius (R _L ) is preferably 1000 μm, and particularly preferably 400 μm. If the major axis radius (R _L ) of the ellipsoid 5 constituting the microlens 4 is smaller than the above range, the influence of diffraction becomes large, optical performance and color separation are likely to occur, and quality is deteriorated. . On the other hand, if the major axis radius (R _L ) of the ellipsoid 5 exceeds the above range, luminance unevenness is likely to occur, and similarly the quality is deteriorated.
[0032]
The lower limit of the filling rate of the microlens 4 is preferably 40%, particularly 60%, and more particularly 70%. Thus, by setting the filling rate of the microlenses 4 to the above lower limit or more, the area occupied by the microlenses 4 on the surface of the optical sheet 1 is increased, and the optical functions such as light collection and diffusion of the optical sheet 1 are remarkably increased. Be improved.
[0033]
The upper limit of the surface roughness (Ra) of the microlens 4 is preferably 2 μm, particularly 0.5 μm, and more particularly 0.1 μm. On the other hand, the lower limit of the surface roughness (Ra) of the microlens 4 is preferably 0.005 μm, particularly preferably 0.01 μm. By setting the surface roughness (Ra) of the microlens 4 within the above range, the reduction in lens-like optical action due to irregular reflection or irregular refraction on the surface of the microlens 4 is reduced, and the optical sheet 1 is condensed, diffused, Optical functions such as bending are effectively exhibited. When the surface roughness (Ra) of the microlens 4 is relatively small within the above range, the optical functions of the optical sheet 1 such as light collection and deflection are increased. On the other hand, when the surface roughness (Ra) of the microlens 4 is increased within the above range, the light diffusion function of the optical sheet 1 is increased.
[0034]
The lower limit of the refractive index of the material constituting the microlens array 3 is preferably 1.3, and particularly preferably 1.45. On the other hand, the upper limit of the refractive index of this material is preferably 1.8, and particularly preferably 1.6. Among these ranges, the refractive index of the material constituting the microlens array 3 is most preferably 1.5. Thus, by setting the refractive index of the material constituting the microlens array 3 within the above range, the lens-like refraction action in the microlens 4 is effectively achieved, and the optical sheet 1 is optically condensed and diffused. Function is further enhanced.
[0035]
The minor axis radius (R _S2 ) in the normal direction on the ellipsoidal surface 5 of the microlens 4 may be substantially equal to or slightly smaller than the major axis radius (R _L ). The flatness ratio (R _S2 / R _S1 ) of the short axis radius (R _S2 ) to the short axis radius (R _S1 ), that is, the flat ratio in the direction perpendicular to the long axis 6 is the flat ratio (R _L / R) in the plane direction. _The same degree as _S1 ) is preferred. By setting the flatness ratio (R _S2 / R _S1 ) of the minor axis radius within the above range, the spherical aberration of the component perpendicular to the major axis 6 of the microlens 4 is reduced, and the condensing function to the front side with respect to the transmitted light beam, Optical functions such as a function of changing the angle toward the normal direction are enhanced.
[0036]
The optical sheet 1 has excellent optical functions such as condensing, diffusing, and changing angle due to the microlens array 3 including the microlenses 4. In addition, the optical sheet 1 has a height ratio (H / R _L ), a flatness ratio (R _L / R _S1 ), a major axis radius (R _L ), a filling rate, and the like of the microlenses 4 constituting the microlens array 3. By adjusting the optical function, the optical function is easily and reliably controlled. The optical sheet 1 has an optical function anisotropy due to the microlens 4 having a partial shape of the ellipsoidal surface 5, and the major axis 6 is longer than the optical function parallel to the major axis 6 of the microlens 4. The optical function in the vertical direction increases. Furthermore, in the optical sheet 1, by adjusting the height (H) of the microlens 4 to be constant, the stress concentration is suppressed, and damage to other stacked members is prevented.
[0037]
The method for manufacturing the optical sheet 1 is not particularly limited as long as the optical sheet 1 having the above structure can be formed, and various methods are employed. As a manufacturing method of the optical sheet 1, a method of separately forming the microlens array 3 after creating the base material layer 2 and a method of integrally forming the base material layer 2 and the microlens array 3 are possible. In particular,
(A) A method of forming the optical sheet 1 by laminating a synthetic resin on a sheet mold having an inverted shape of the surface of the microlens array 3 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 microlens array 3;
(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) An extruded sheet molding method in which a molten resin is passed through a nip between a roll mold having a reverse shape of the surface of the microlens array 3 on the peripheral surface and another roll, and the shape is transferred.
(E) An ultraviolet curable resin is applied to the base material layer, and the shape is transferred to an uncured ultraviolet curable resin by pressing against a sheet mold, mold or roll mold having the same inverted shape as above, A method of curing an ultraviolet curable resin by application,
(F) An uncured ultraviolet curable resin is filled and applied to a mold or roll mold having the same inverted shape as described above, pressed by a base material layer, leveled, and irradiated with ultraviolet rays to cure the ultraviolet curable resin. Method,
(G) A method of injecting or discharging an uncured (liquid) ultraviolet curable resin or the like from a fine nozzle onto a sheet base material, and curing it.
(H) There is a method of using an electron beam curable resin instead of an ultraviolet curable resin.
[0038]
As a method of manufacturing a mold (mold) having an inverted shape of the microlens array 3, for example, a spot-like three-dimensional pattern is formed on a base material using a photoresist material, and the three-dimensional pattern is curved by heating and fluidizing. Thus, a microlens array model can be manufactured, and a metal layer can be laminated on the surface of the microlens array model by electroforming, and the metal layer can be peeled off. In addition, as a method for manufacturing the microlens array model, the method described in (g) above can be adopted.
[0039]
According to the manufacturing method, the microlens array 3 having the above shape is easily and reliably formed. Therefore, the height ratio (H / R _L ), flatness ratio (R _L / R _S1 ), short axis radius (R _S1 , R _S2 ), and long axis radius (R _L ) of the microlens 4 constituting the microlens array 3. ), The filling rate, the arrangement pattern, and the like are easily and reliably adjusted, and as a result, the optical function of the optical sheet 1 is easily and reliably controlled.
[0040]
The edge light type backlight unit shown in FIG. 2 is disposed so as to overlap the light guide plate 9, a pair of linear lamps 10 disposed on the opposite side of the light guide plate 9, and the surface side of the light guide plate 9. An optical sheet 1 is provided. The optical sheet 1 is arranged so that the long axis 6 of the microlens 4 is parallel to the lamp 10. In the edge light type backlight unit, the luminance distribution of light emitted from the lamp 10 and emitted from the surface of the light guide plate 9 has a relatively strong peak inclined at a predetermined angle with respect to the normal direction in the direction perpendicular to the lamp 10. However, there is no significant peak in the direction parallel to the lamp 10. Therefore, the backlight unit has remarkably high optical functions such as a condensing function on the front side, a light diffusing function, and a function of changing the angle toward the normal direction, and is perpendicular to the long axis 6 of the microlens 4. The optical sheet 1 is more effectively directed to the normal direction side by the optical sheet 1 which is larger than the optical function in the direction parallel to the long axis 6, and has high front luminance and an appropriate viewing angle. Therefore, in the backlight unit, the number of optical sheets (light diffusion sheets and the like) required conventionally can be reduced, and the reduction in thickness, quality, and cost can be promoted.
[0041]
The optical sheet 11 of FIG. 3 includes a base material layer 2, a microlens array 3 provided on the surface of the base material layer 2, and an anti-sticking layer 12 laminated on the back surface of the base material layer 2. The base material layer 2 and the microlens array 3 are the same as the optical sheet 1 in FIG. Therefore, the optical sheet 11 also has optical functions such as condensing, diffusing, and changing angle, as in the optical sheet 1, having anisotropy and high controllability in the optical functions, Scratching to other members that are superimposed is suppressed.
[0042]
The anti-sticking layer 12 includes a binder 13 and beads 14 dispersed in the binder 13. The binder 13 is formed by curing a polymer composition containing a base polymer. With this binder 13, beads 14 are arranged and fixed on the back surface of the base material layer 2 at substantially equal density. The thickness of the anti-sticking layer 12 (the thickness of the binder 13 portion excluding the beads 14) is not particularly limited, but is, for example, about 1 μm or more and 10 μm or less.
[0043]
The base polymer is not particularly limited, and examples thereof include acrylic resins, polyurethanes, polyesters, fluorine resins, silicone resins, polyamideimides, epoxy resins, ultraviolet curable resins, and the like. Can be used singly or in combination of two or more. In particular, the base polymer is preferably a polyol that has high processability and can easily form the anti-sticking layer 12 by means such as coating. The base polymer used for the binder 13 is transparent because it is necessary to transmit light, and colorless and transparent is particularly preferable.
[0044]
Examples of the polyol include (a) a polyester polyol obtained under conditions of excess hydroxyl group and (b) a monomer component containing a hydroxyl group-containing unsaturated monomer, and a (meth) acryl unit. Etc. are preferred. The binder 13 having such a polyester polyol or acrylic polyol as a base polymer has high weather resistance and can suppress yellowing of the anti-sticking layer 12 or the like. In addition, any one of this polyester polyol and acrylic polyol may be used, and both may be used.
[0045]
In addition to the base polymer, the polymer composition for forming the binder 13 is, for example, a fine inorganic filler, a curing agent, a plasticizer, a dispersant, various leveling agents, an antistatic agent, an ultraviolet absorber, and an antioxidant. , Viscosity modifiers, lubricants, light stabilizers and the like may be appropriately blended.
[0046]
The material of the beads 14 is roughly classified into an inorganic filler and an organic filler. Specifically, silica, aluminum hydroxide, aluminum oxide, zinc oxide, barium sulfide, magnesium silicate, or a mixture thereof can be used as the inorganic filler. Specific examples of the organic filler include acrylic resin, acrylonitrile resin, polyurethane, polyvinyl chloride, polystyrene, polyacrylonitrile, polyamide, and the like. Among them, an acrylic resin that has high transparency and does not inhibit light transmission is preferable, and polymethyl methacrylate (PMMA) is particularly preferable.
[0047]
The lower limit of the average particle diameter of the beads 14 is preferably 1 μm, particularly 2 μm, more preferably 5 μm, and the upper limit of the average particle diameter is preferably 50 μm, particularly 20 μm, more particularly 15 μm. If the average particle diameter of the beads 14 is smaller than the above lower limit, the convex portion on the back surface of the anti-sticking layer 12 formed by the beads 14 may be small, and a sufficient anti-sticking effect may not be obtained. Conversely, if the average particle diameter of the beads 14 exceeds the above upper limit, the thickness of the optical sheet 11 increases, and there is a risk of scratching other optical members superimposed on the back side.
[0048]
The amount of the beads 14 is relatively small, the beads 14 are separated from each other and dispersed in the binder 13, and many of the beads 14 protrude from the binder 13 by a very small amount. Therefore, when this optical sheet 11 is laminated with the light guide plate, the lower end of the protruding beads 14 contacts the surface of the light guide plate or the like, and the entire back surface of the optical sheet 11 does not contact the light guide plate or the like. As a result, sticking between the optical sheet 11 and the light guide plate or the like is prevented, and uneven brightness on the screen of the liquid crystal display device is suppressed.
[0049]
Examples of the method for forming the anti-sticking layer 12 include: (a) a step of producing a coating solution for anti-sticking layer by mixing beads 14 with a polymer composition constituting the binder 13, and (b) anti-sticking. And a step of laminating the anti-sticking layer 12 by applying a layer coating liquid on the back surface of the base material layer 2.
[0050]
The optical sheet of the present invention is not limited to the above-described embodiment, and for example, a microlens array including concave microlenses is also possible. The concave lens microlens also has an excellent optical function in the same manner as the convex lens microlens. The arrangement pattern of the microlens is not limited to the rhombus pattern that can be densely packed, and a square lattice pattern or a random pattern is also possible. According to the random pattern, the occurrence of moire is reduced when the optical sheet is overlapped with another optical member.
[0051]
【The invention's effect】
As described above, according to the optical sheet of the present invention, the optical functions such as condensing, diffusing, and bending are remarkably high and anisotropic, and the control of the optical functions is easy and reliable. Yes, damage to other members that are superimposed is suppressed. In addition, in the backlight unit using the optical sheet, improvement in quality and cost reduction such as high luminance in the front direction, uniform luminance, proper viewing angle, and thinning are promoted.
[Brief description of the drawings]
1A, 1B, and 1C are a schematic partial plan view, a schematic AA sectional view, and a schematic BB sectional view showing an optical sheet according to an embodiment of the present invention. is there.
FIG. 2 is a schematic cross-sectional view showing an edge light type backlight unit including the optical sheet of FIG.
FIG. 3 is a schematic partial cross-sectional view showing an optical sheet according to a form different from the optical sheet of FIG.
4A is a schematic perspective view showing a conventional general edge light type backlight unit, and FIG. 4B is a schematic cross-sectional view showing a conventional light diffusion sheet.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Optical sheet 2 Base material layer 3 Micro lens array 4 Micro lens 5 Ellipsoidal surface 6 Long axis 7 Short axis 8 Short axis 9 Light guide plate 10 Lamp 11 Optical sheet 12 Sticking prevention layer 13 Binder 14 Bead

Claims (8)

An optical sheet for a backlight unit of a liquid crystal display device having at least a condensing function for transmitted light and a refraction function toward the normal direction side,
A microlens array consisting of multiple microlenses is provided on the surface,
This microlens has an elliptical partial shape,
The major axis of this ellipsoid is located substantially parallel to the predetermined plane direction,
The height ratio (H / R _L ) of the height (H) of the microlens to the major axis radius (R _L ) of the ellipsoid is 3/4 or more and 1 or less,
The oblong ratio (R _L / R _S1 ) of the major axis radius (R _L ) of the ellipsoid to the minor axis radius (R _S1 ) substantially parallel to the plane direction is 1.05 or more and 1.7 or less,
An optical sheet, wherein a refractive index of a material constituting the microlens array is 1.3 or more and 1.8 or less . 2. The optical sheet according to claim 1, wherein a major axis radius (R _L ) of the microlens is 10 μm or more and 1000 μm or less. The optical sheet according to claim 1, wherein a filling rate of the microlens is 40% or more. The optical sheet according to claim 1, wherein the microlens has a surface roughness (Ra) of 2 μm or less. The optical sheet according to any one of claims 1 to 4 , wherein the arrangement pattern of the microlenses in the microlens array is a rhombus pattern or a random pattern. The optical sheet according to any one of claims 1 to 5, wherein a radiation curable resin or a thermoplastic resin is used as a material constituting the microlens array. The optical sheet according to any one of claims 1 to 6, further comprising an anti-sticking layer in which beads are dispersed in a binder on the back surface. In a backlight unit for a liquid crystal display device that guides light emitted from a lamp to the surface side by dispersing,
A backlight unit for a liquid crystal display device comprising the optical sheet according to any one of claims 1 to 7 .

Images