JP4294306B2 - Optical sheet and backlight unit using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical sheet which has remarkably high optical functions in convergence, diffusion, normal refraction, etc., and to provide a back light unit which is obtained by using the optical sheet and has improved quality including high frontal luminance, uniform brightness, optimized angle of view, thin profile, etc. <P>SOLUTION: The optical sheet 1 is equipped, on the surface, with a micro lens array 3 composed of a plurality of micro lenses 4. These micro lenses 4 are shaped partly in an ellipsoid 5 with the major axis 6 facing the normal line. It is desirable that the micro lenses 4 have a height ratio (H/R<SB>L</SB>) &ge;3/4 and &le;1, flat ratio (R<SB>L</SB>/R<SB>S</SB>) &ge;1.05 and &le;1.7, major axis radius (R<SB>L</SB>) &ge;10 &mu;m and &le;1,000 &mu;m, filling factor &ge;40%, and surface roughness (Ra) &le;10 &mu;m, and that the material constituting the micro lens array 3 has a refractive index &ge;1.3 and &le;1.8. <P>COPYRIGHT: (C)2004,JPO&amp;NCIPI

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. This includes a bead coating sheet 24 having (2) a prism sheet 25 disposed on the surface side of the bead coating sheet 24 and having a function of refraction in the normal direction side.
[0003]
The function of the backlight unit 20 will be described. First, a light beam incident on the light guide plate 22 from the lamp 21 is reflected by a reflective dot or a reflection sheet (not shown) on the back surface of the light guide plate 22 and each side surface. Emitted from the surface. The light beam emitted from the light guide plate 22 enters the bead coating sheet 24, is diffused, and is emitted from the surface. Thereafter, the light beam emitted from the bead coating sheet 24 is incident on 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, a backlight unit in which more optical sheets 23 such as bead coating sheets and prism sheets are arranged in consideration of the light guide characteristics of the light guide plate 22 and the optical functions of the optical sheet 23 described above. There is also.
[0005]
As the conventional bead coated sheet 24, generally, as shown in FIG. 4 (b), a transparent synthetic resin base material layer 26 is laminated on the surface of the base material layer 26 and is light diffusing. (See, for example, JP 2000-89007 A). This optical 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 bead-coated sheet 24. The bead-coated sheet 24 exhibits optical functions such as diffusion, condensing, refraction in the normal direction side (angle change) due to refraction at the lens-shaped fine irregularities and the interface of the resin beads 29.
[0006]
[Patent Document 1]
Japanese Patent Laid-Open No. 2000-89007
[Problems to be solved by the invention]
In the conventional bead coating 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 bead coated sheet 24 has a certain limit in improving optical functions such as condensing, diffusing, and bending. Therefore, the conventional backlight unit 20 needs to include one or two prism sheets 25 despite being expensive and difficult to handle.
[0008]
The present invention has been made in view of these inconveniences, an optical sheet having a remarkably high optical function such as condensing, diffusing, and deflection, and using the same to increase the brightness in the front direction, uniform the brightness, An object of the present invention is to provide a backlight unit in which improvement of quality such as optimization of viewing angle and thinning is promoted.
[0009]
The invention made to solve the above problems is
An optical sheet used in a backlight unit of a liquid crystal display device and having at least a diffusion function,
Consists of a transparent substrate layer and a microlens array composed of a plurality of microlenses formed on the surface of the substrate layer,
This microlens has a partial shape of an ellipsoid with the major axis oriented in the normal 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 flatness ratio (R _L / R _S ) of the major axis radius (R _L ) to the minor axis radius (R _S ) on the elliptical surface of the microlens is 1.05 or more and 1.7 or less,
An optical sheet configured to reduce spherical aberration of the microlens.
[0010]
The optical sheet includes a microlens array on the surface, and the microlens constituting the microlens array has an elliptical partial shape whose major axis is in the normal direction. Spherical aberration is reduced. Therefore, the optical sheet has reduced light loss due to spherical aberration of the microlens and enhanced optical functions such as a condensing function to the front side, a diffusion function, and a function of changing the angle to the normal direction side with respect to the transmitted light. It is done.
[0011]
In the optical sheet, 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 and the major axis radius (R _L ). The flatness ratio (R _L / R _S ) with respect to the minor axis radius (R _S ) is set to 1.05 or more and 1.7 or less. Thus, by setting the height ratio (H / R _L ) and the flatness ratio (R _L / R _S ) within the above ranges, the spherical aberration of the microlens is remarkably reduced, and optical functions such as light collection are remarkably improved. To improve.
[0012]
The major axis radius (R _L ) of the ellipsoidal surface of the microlens is 10 μm or more and 1000 μm or less, the filling factor of the microlens is 40% or more, and the surface roughness (Ra) of the microlens is 10 μm or less. The refractive index of the material constituting the microlens array is preferably from 1.3 to 1.8. By setting the major axis radius (R _L ), microlens filling factor, microlens surface roughness (Ra), and microlens material refractive index within the above ranges, optical functions such as light collection and diffusion are further enhanced. It is done.
[0013]
The two minor axes on the ellipsoidal surface of the microlens are preferably substantially the same length. In this way, by setting the two minor axes of the microlenses to substantially the same length, the optical sheet has an optical function such as light collection isotropic.
[0014]
The arrangement pattern of the microlenses in the microlens array is preferably a regular triangular lattice pattern or a random pattern. Since this equilateral triangular lattice pattern can arrange microlenses more densely, the lens filling rate of the optical sheet can be easily increased, and optical functions such as light collection and diffusion are remarkably improved. In addition, by arranging the microlenses in a random pattern, the occurrence of moire is reduced when the optical sheet is overlapped with another optical member.
[0015]
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.
[0016]
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.
[0017]
Therefore, in a backlight unit for a liquid crystal display device that disperses the light emitted from the lamp and guides it to the surface side, energy efficiency is improved by including the optical sheet having high optical functions such as condensing, diffusing, and changing angle. In addition, the quality is improved by increasing the brightness in the front direction, making the brightness uniform, optimizing the viewing angle, etc., and further promoting the thinning by reducing the number of optical sheets.
[0018]
Here, the “micro lens” is a concept including a convex lens and a concave lens. “Microlens height” refers to 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 “regular triangle lattice pattern” means a pattern in which the surface is divided into equilateral triangles having the same shape and a microlens is arranged at each vertex of the equilateral triangle.
[0019]
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 partial plan view and a schematic partial sectional view showing an optical sheet according to an embodiment of the present invention, and FIG. 2 is a schematic view showing a backlight unit including the optical sheet of FIG. FIG. 3 is a schematic partial sectional view showing an optical sheet according to a different form from the optical sheet of FIG.
[0020]
The optical sheet 1 in FIG. 1 includes a base layer 2 and a microlens array 3 on the surface of the base layer 2.
[0021]
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.
[0022]
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.
[0023]
The microlens array 3 is composed of a number of 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.
[0024]
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.
[0025]
The microlens 4 has a partial shape of an elliptical surface 5. In this elliptical surface 5, the major axis 6 is positioned parallel to the normal direction of the optical sheet 1, and the minor axis 7 and the minor axis 8 are positioned parallel to the optical sheet 1. Thus, by making the microlens 4 the upper partial shape of the elliptical surface 5 whose major axis 6 is parallel to the normal direction, the spherical aberration of the microlens 4 is reduced, and the light collecting function and diffusion of the optical sheet 1 are reduced. Optical functions such as function and angle change function are enhanced.
[0026]
The microlenses 4 are disposed relatively densely and geometrically on the surface of the base material layer 2. The microlenses 4 are arranged in a regular triangular lattice pattern on the surface of the base material layer 2. Accordingly, the pitch of the microlenses 4 and the inter-lens distance (S) are all constant. With this arrangement pattern, the microlenses 4 can be arranged most densely, and the optical functions of the optical sheet 1 such as the light collecting function, the diffusing function, and the angle changing function can be improved.
[0027]
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 achieved, and coupled with the reduction of the spherical aberration, the collection of the optical sheet 1. Optical functions such as light, diffusion and deflection are greatly improved.
[0028]
The lower limit of the flatness ratio (R _L / R _S ) of the major axis radius (R _L ) to the minor axis radius (R _S ) 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 _S ) is preferably 1.7, and particularly preferably 1.5. Here, the “short axis radius (R _S )” means an average value of the radius (R _S1 ) and the radius (R _S2 ) of the two short axes 7 and 8 perpendicular to the long axis 6. By configuring the microlens 4 from the ellipsoidal surface 5 in which the aspect ratio (R _L / R _S ) is in the above range, spherical aberration is remarkably reduced, and the optical sheet 1 has optical properties such as condensing, diffusing, and variable angle. Function is greatly improved.
[0029]
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.
[0030]
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.
[0031]
The upper limit of the surface roughness (Ra) of the microlens 4 is preferably 10 μm, and particularly preferably 2 μm. By setting the surface roughness (Ra) of the microlens 4 to be equal to or less than the above upper limit, a decrease in lens-like optical action due to irregular reflection or irregular refraction on the surface of the microlens 4 is reduced. Optical functions such as bending are effectively exhibited.
[0032]
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.
[0033]
The radius of the minor axis 7 in ellipsoidal 5 of the microlens 4 and (R _S1) of the short axis 8 radii and (R _S2) may be substantially the same length. In this way, by forming the microlens 4 from the spheroid 5 having the two short axes substantially the same length, the optical sheet is optically isotropic, such as condensing light.
[0034]
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 with less spherical aberration as described above. In addition, the optical sheet 1 has a height ratio (H / R _L ), a flatness ratio (R _L / R _S ), 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. 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.
[0035]
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) There is a method of using an electron beam curable resin instead of an ultraviolet curable resin.
[0036]
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.
[0037]
According to the manufacturing method, the microlens array 3 having an arbitrary shape is easily and reliably formed. Therefore, the size of the microlens 4 constituting the microlens array 3 (specifically, the height ratio (H / R _L ), the flatness ratio (R _L / R _S ), and the long axis radius (R _L )), micro The filling rate, arrangement pattern, and the like of the lens 4 are easily and reliably adjusted, and as a result, the optical function of the optical sheet 1 is easily and reliably controlled.
[0038]
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. A light beam emitted from the lamp 10 and emitted from the surface of the light guide plate 9 has a relatively strong peak inclined by a predetermined angle with respect to the normal direction, but the backlight unit has a condensing function to the front side. The optical sheet 1 having extremely high optical functions such as a light diffusing function and a function of changing the angle toward the normal direction side has high energy efficiency, high front luminance and uniform luminance, and an appropriate visual field. Has horns. Therefore, the backlight unit can reduce the number of optical sheets (bead coated sheets and the like) required in the past, and promote the reduction in thickness, quality, and cost. The edge light type backlight unit may be equipped with four, six, etc. lamps 10.
[0039]
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.
[0040]
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.
[0041]
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.
[0042]
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.
[0043]
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.
[0044]
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.
[0045]
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.
[0046]
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.
[0047]
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.
[0048]
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. Further, the arrangement pattern of the microlens is not limited to the regular triangular lattice 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.
[0049]
【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 the control of the optical functions is easy and reliable, and other members that are superposed on each other. Scratching 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 and 1B are a schematic partial plan view and a schematic partial cross-sectional view showing an optical sheet according to an embodiment of the present invention.
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 bead-coated 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 Linear lamp 11 Optical sheet 12 Sticking prevention layer 13 Binder 14 Bead

Claims (10)

An optical sheet used in a backlight unit of a liquid crystal display device and having at least a diffusion function,
Consists of a transparent substrate layer and a microlens array composed of a plurality of microlenses formed on the surface of the substrate layer ,
This microlens has a partial shape of an ellipsoid with the major axis oriented in the normal 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 flatness ratio (R _L / R _S ) of the major axis radius (R _L ) to the minor axis radius (R _S ) on the elliptical surface of the microlens is 1.05 or more and 1.7 or less,
An optical sheet configured to reduce spherical aberration of the microlens. 2. The optical sheet according to claim 1, wherein a major axis radius (R _L ) on the ellipsoidal surface 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 10 μm or less.   The optical sheet according to any one of claims 1 to 4, wherein a refractive index of a material constituting the microlens array is 1.3 or more and 1.8 or less.   The optical sheet according to any one of claims 1 to 5, wherein the arrangement pattern of the microlenses in the microlens array is an equilateral triangular lattice pattern or a random pattern.   The optical sheet according to any one of claims 1 to 6, wherein two minor axes of the ellipsoidal surface of the microlens have substantially the same length.   The optical sheet according to any one of claims 1 to 7, 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 8, 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 9.

Images