JP2009175597A - Optical sheet and backlight unit using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical sheet capable of making light beams passed through respective Fresnel lens surfaces 1 a of the optical sheet 1 cross one another temporarily by the Fresnel lens surface through convergence, and then diverge to some extent by increasing curvature of a convex shape of each Fresnel lens surface 1a, and to provide a backlight unit using the same. <P>SOLUTION: Angles of inclination of ends of both adjacent Fresnel lens surfaces 1 a, divided in a Fresnel lens shape, on an adjacent border side are so defined as to be inclined all the more in an inward direction because the Fresnel lens surface 1 a is on a side farther from the center of the Fresnel lens. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a convex Fresnel lens-shaped optical sheet and a backlight unit such as a liquid crystal display using the optical sheet.

  For backlights of liquid crystal displays used in television receivers and personal computers, a direct light system in which multiple line light sources are arranged behind a liquid crystal panel via a diffusion sheet, etc., or a diffusion sheet behind a liquid crystal panel, etc. There is an edge light system or the like in which a light guide plate is disposed through the light guide and the light from the line light source on the side of the light guide plate is guided to the back surface of the liquid crystal panel.

  A conventional backlight unit of a direct light type in a liquid crystal display used in a television receiver has a diffusion sheet (diffusion plate) on the front side in which a plurality of straight tube CCFLs (cold cathode tubes) as line light sources are arranged in parallel. In addition, a liquid crystal panel is disposed on the front side of the diffusion sheet or the like. Also, there has conventionally been a backlight unit using a plano-convex linear Fresnel lens-like optical sheet instead of or in addition to a diffusion sheet or the like (see, for example, Patent Document 1). When a plano-convex linear Fresnel lens-shaped optical sheet is used, light emitted radially from a straight tube-shaped CCFL is condensed into a parallel light beam on the liquid crystal panel in the same manner as when a plano-convex cylindrical lens is arranged. In addition, since the lens thickness can be made thinner than that of the plano-convex cylindrical lens, the backlight unit can be significantly reduced in weight and cost and can be reduced in cost.

  A configuration example of a conventional backlight unit using the optical sheet is shown in FIG. In this backlight unit, a plurality of straight tube-type CCFLs 2 with the longitudinal direction oriented in the front-rear direction are arranged below the optical sheet 1 in parallel in the left-right direction at equal intervals. Below the CCFL 2, a reflector 3 for effectively using the light emitted downward is disposed. And the liquid crystal panel 4 is arrange | positioned above this optical sheet 1 (front side). Further, a diffusion sheet or the like may be disposed between the liquid crystal panel 4 above the optical sheet 1. The optical sheet 1 is formed by forming a plurality of plano-convex linear Fresnel lens-like Fresnel lenses on a transparent resin sheet so as to correspond to the respective CCFLs 2 directly below.

  Here, as shown in FIG. 14, the plano-convex linear Fresnel lens 11 divides the convex upper curved surface 12a of the plano-convex cylindrical lens 12 into a plurality of planes D perpendicular to the left-right direction and pushes them downward. This is a lens in which the lens thickness in the vertical direction is reduced. Accordingly, on the upper surface of the plano-convex linear Fresnel lens 11, a plurality of Fresnel lens surfaces 11a having respective curved shapes obtained by dividing the convex upper curved surface 12a of the original plano-convex cylindrical lens 12 in the left-right direction; A Fresnel splitting surface 11b, which is a side plane that connects the ends of the adjacent Fresnel lens surfaces 11a in a stepped shape, is formed.

  Each Fresnel lens surface 11a is often formed so as to have the same height at the upper end as shown in FIG. Further, in the plano-convex linear Fresnel lens 11 shown in FIG. 14, the number of divisions of the Fresnel lens surface 11a is set to 8 in order to make the drawing easy to see. It becomes sufficiently thin (the same applies to FIG. 15 shown below).

  The plano-convex linear Fresnel lens 11 has the same condensing function as the plano-convex cylindrical lens 12, and therefore condenses light that spreads radially in the left-right direction upward from the CCFL 2 disposed below. It can be emitted as upward parallel light. Accordingly, a shape similar to the Fresnel lens surface 11a and Fresnel splitting surface 11b of each of the plurality of plano-convex linear Fresnel lenses 11 is used as a pair of Fresnel lenses, and this Fresnel lens is made to correspond to each CCFL 2 on the upper surface of the resin sheet. If a plurality of optical sheets 1 arranged side by side are used in a backlight unit, the light emitted from the optical sheet 1 and passing through the liquid crystal panel 4 becomes light with many upward components perpendicular to the panel surface. The front brightness of the display can be increased.

  However, since the backlight unit using the conventional optical sheet 1 has a large amount of light having a large upward component perpendicular to the panel surface of the liquid crystal panel 4, even if the front luminance of the liquid crystal display increases, ) Has a problem that the luminance is extremely lowered only by shifting the position slightly from side to side. Moreover, even if the liquid crystal panel 4 has a wide viewing angle, the brightness of the backlight is lowered in the peripheral portion of the wide viewing angle, so that the performance of wide viewing angle can be sufficiently exhibited. There was also a problem that it was not possible.

  In addition, when the CCFL 2 is not accurately disposed directly below the center of the Fresnel lens in the optical sheet 1 and is displaced in the left-right direction, the light passing through the liquid crystal panel 4 is not perpendicular to the panel surface but is slightly inclined light. Therefore, the position with the highest luminance is slightly deviated from the front of the liquid crystal display. For this reason, not only the brightness at the position directly in front of the liquid crystal display is lowered, but also when only a part of the CCFLs 2 is displaced, there is a problem that uneven brightness occurs on the screen of the liquid crystal display.

  Furthermore, when the backlight unit using the conventional optical sheet 1 is used as a surface light source for other uses other than the liquid crystal display, the light distribution characteristics are almost limited to the upward (front) direction. However, there is a problem that an expensive diffusion sheet or the like needs to be additionally used.

  In the plano-convex linear Fresnel lens 11, each Fresnel lens surface 11 a divided into a large number by the Fresnel dividing surface 11 b becomes a convex curved surface such as a cylindrical surface, but actually, as shown in FIG. In many cases, a plano-convex linear Fresnel lens 13 in which each Fresnel lens surface 13a is a flat surface is used. In this plano-convex linear Fresnel lens 13, each Fresnel lens surface 13a divided by the Fresnel dividing surface 13b has an average inclination angle of the curved Fresnel lens surface 11a of the corresponding plano-convex linear Fresnel lens 11, that is, for example, The Fresnel lens surface 11a is constituted by a flat surface having an arithmetic average inclination angle of the maximum value and the minimum value of the inclination angle.

However, hereinafter, unless otherwise specified, each Fresnel lens surface of the conventional convex linear Fresnel lens is like the Fresnel lens surface 11a of the plano-convex linear Fresnel lens 11 shown in FIG. Description will be made assuming that the convex upper curved surface 12a of the plano-convex cylindrical lens 12 has a convex curved surface shape divided in the left-right direction.
JP 2005-338611 A

  The present invention increases the curvature of the convex shape of each Fresnel lens surface of the convex Fresnel lens shape so that the light that has passed through each Fresnel lens surface is crossed by focusing once for each Fresnel lens surface. An optical sheet that can be expanded and a backlight unit using the same are provided.

The optical sheet according to claim 1 is an optical sheet in which at least one sheet surface is divided into a Fresnel lens shape, along the central axis and the radial direction of the Fresnel lens in each Fresnel lens surface divided into the Fresnel lens shape. The curved shape of the longitudinal section has all of the following configurations (A) to (C).
(A) The curve of each Fresnel lens surface has a convex shape in which the inclination angle is monotonously increasing or monotonously decreasing along the radial direction.
(B) The average value of the inclination angle of the curve of each Fresnel lens surface is the value obtained by further rotating the center side of the Fresnel lens in the direction from the inner side of the sheet toward the sheet surface as the distance from the center of the Fresnel lens increases. It becomes the inclination angle of the direction.
(C) The inclination angle of the curve at the end of the adjacent boundary side of both Fresnel lens surfaces concentrically adjacent to each other is such that the Fresnel lens surface away from the center of the Fresnel lens is located on the center side of the Fresnel lens. The inward inclination angle is further rotated in the direction from the surface toward the inside of the seat.

The optical sheet according to claim 2 is an optical sheet in which at least one sheet surface is divided into a linear Fresnel lens shape, and a longitudinal cross-sectional curve orthogonal to the divided surface in each of the Fresnel lens surfaces divided into the linear Fresnel lens shape. The shape is characterized by having all of the following configurations (A) to (C).
(A) The curve of each Fresnel lens surface is a convex shape whose inclination angle increases monotonously or decreases monotonously along a direction orthogonal to the dividing plane.
(B) The average value of the inclination angle of the curve of each Fresnel lens surface is the value obtained by further rotating the center side of the Fresnel lens in the direction from the inner side of the sheet toward the sheet surface as the distance from the center of the Fresnel lens increases. It becomes the inclination angle of the direction.
(C) The inclination angle of the curve at the edge of the adjacent boundary side of both Fresnel lens surfaces adjacent to each other outside the center of the Fresnel lens is such that the Fresnel lens surface far from the center of the Fresnel lens is the center of the Fresnel lens. The inward inclination angle is obtained by further rotating the side in the direction from the seat surface toward the inside of the seat.

  The optical sheet according to claim 3 is characterized in that each Fresnel lens surface is divided into a linear Fresnel lens shape with a pitch of 0.1 mm or more and 1.0 mm or less.

  The optical sheet according to claim 4 is characterized in that each Fresnel lens surface is formed by a cylindrical surface.

  The optical sheet according to claim 5 is characterized in that the optical sheet is made of a translucent material containing a diffusing agent.

  The backlight unit according to claim 6 is characterized in that the optical sheet is arranged in front of one or more light sources.

  According to invention of Claim 1, an optical sheet becomes a convex Fresnel lens shape by the structure (A) and a structure (B), and has a condensing effect | action as a whole. In addition, since the curvature in a broad sense is larger than that in the conventional art due to the configuration (C), the light is once condensed by the condensing action of the convex lens for each of these Fresnel lens surfaces, The collected light diffuses after crossing. For this reason, the optical sheet of the present invention has a condensing function as a whole by a convex Fresnel lens shape, but each Fresnel lens surface diffuses light to some extent. The light is emitted to a certain extent around each Fresnel lens surface rather than the direction in which the light should be emitted. Therefore, for example, when this optical sheet is used in a surface illumination device or the like, the orientation characteristics can be widened to some extent, so that it is not necessary to additionally use an expensive diffusion sheet or the like.

  In addition, the curve here means a line other than a line consisting of only one straight line, not only a smooth curve but also a broken line in which a plurality of straight lines having different inclination angles are connected, or a plurality of smooth curves. It may be a line or the like. In addition, since monotonic increase or monotonic decrease is a broad sense, the same inclination angle may be partially continued, and therefore the curved shape of the longitudinal section of the Fresnel lens surface is not only a smooth curve, It may be a broken line or the like. Further, the average value of the inclination angle of the curve of the Fresnel lens surface refers to the arithmetic average value of the maximum value and the minimum value of the inclination angle of the curve of the Fresnel lens surface.

  According to invention of Claim 2, an optical sheet becomes a convex linear Fresnel lens shape by the structure (A) and a structure (B), and has the condensing effect | action in the width direction as a whole. In addition, because of the configuration (C), each of the Fresnel lens surfaces has a larger curvature in the broader sense than before, so that light is once collected by the condensing action of the convex cylindrical lens on each of these Fresnel lens surfaces. The light is diffused and diffused after the collected light intersects. For this reason, the optical sheet of the present invention as a whole has a light collecting effect in the width direction due to the shape of a convex linear Fresnel lens, but each Fresnel lens surface diffuses light to some extent in the width direction. Due to the convex linear Fresnel lens shape, light is emitted by spreading to the surroundings to some extent for each Fresnel lens surface rather than the direction to be originally emitted. Therefore, for example, when this optical sheet is used in a surface illumination device or the like, the orientation characteristics in the width direction can be widened to some extent, so that it is not necessary to additionally use an expensive diffusion sheet or the like.

  The significance of the curve, the significance of monotonic increase or monotonic decrease, and the significance of the average value of the tilt angle are the same as in the case of claim 1.

  According to the invention of claim 3, since the width of each Fresnel lens surface is not less than 0.1 mm and is not too small, manufacturing is not difficult and the optical sheet is not too expensive. Further, since the width of each Fresnel lens surface is 1.0 mm or less and is not too wide, the optical sheet thickness can be sufficiently reduced, and the unevenness of the diffusion effect by each Fresnel lens surface can be sufficiently reduced. be able to.

  According to the invention of claim 4, since each Fresnel lens surface is a cylindrical surface, manufacture is facilitated. In addition, the curvature radius of the cylindrical surface of this Fresnel lens surface may be common to all Fresnel lens surfaces, or may be common only to Fresnel lens surfaces of the same group divided into two or more groups. The Fresnel lens surface may be different.

  According to the invention of claim 5, since the optical sheet contains a diffusing agent, light can be sufficiently diffused in the sheet, and it is not necessary to additionally use an expensive diffusion sheet or the like.

  According to the invention of claim 6, since the optical sheet has a condensing function as a whole and also has a diffusing action on each Fresnel lens surface, the front brightness does not suddenly decrease in the surroundings, and the backlight unit Even when the arrangement position of the light source is shifted, it is possible to alleviate a decrease in luminance at the front position and to suppress luminance unevenness. When this backlight unit is used for a liquid crystal display, the performance of the liquid crystal panel with a wide viewing angle can be exhibited without waste.

  Hereinafter, the best embodiment of the present invention will be described with reference to FIGS. Also in these drawings, the same reference numerals are given to constituent members having the same functions as those of the conventional example shown in FIGS.

  In the present embodiment, as in the conventional example shown in FIG. 13, a backlight unit of a direct light type in a liquid crystal display using a plano-convex linear Fresnel lens-shaped optical sheet 1 will be described. The backlight unit of the present embodiment is the same as the conventional example shown in FIG. 13 except for the configuration of the optical sheet 1. A straight tube type with the longitudinal direction directed in the front-rear direction is provided below the optical sheet 1. A plurality of CCFLs 2 are arranged in parallel in the left-right direction at equal intervals. A reflector 3 is disposed below the CCFLs 2. And the liquid crystal panel 4 is arrange | positioned above this optical sheet 1 (front side).

  The optical sheet 1 of the present embodiment is made of a transparent resin sheet, and has a flat lower surface and is divided into a plurality of pieces in the left-right direction at a constant pitch in the shape of a plano-convex linear Fresnel lens as shown in FIG. The upper surface is composed of a plurality of Fresnel lens surfaces 1a and a Fresnel dividing surface 1b which is a side plane connecting the ends of adjacent Fresnel lens surfaces in a stepped shape. In addition, it is preferable that the division pitch of the Fresnel lens surface is 0.1 mm or more so as not to be too small to be difficult to manufacture or to make the optical sheet 1 too expensive. The thickness is preferably 1.0 mm or less so that the sheet thickness can be sufficiently reduced.

  Further, as shown in FIG. 2, the optical sheet 1 includes a plurality of Fresnel lenses each having a plurality of Fresnel lens surfaces 1a and a Fresnel splitting surface 1b shown in FIG. They are arranged side by side on the left and right sides. Each CCFL 2 is arranged immediately below the center C of the corresponding set of Fresnel lenses. The center C of the Fresnel lens means the optical axis position of the portion corresponding to this set of Fresnel lenses, that is, one plano-convex linear Fresnel lens, and is actually a plane along the vertical direction and the front-rear direction. It becomes. A plurality of Fresnel lens surfaces 1a and a Fresnel splitting surface 1b constituting a set of Fresnel lenses are usually symmetrical with respect to the center C of the Fresnel lens.

  The center C of the Fresnel lens when the Fresnel lens is a plano-convex type Fresnel lens corresponding to a normal disc-shaped convex lens means the optical axis position of the part corresponding to the plano-convex type Fresnel lens, and the vertical direction Axis along

  As shown in FIG. 1, each Fresnel lens surface 1a of the optical sheet 1 is constituted by a cylindrical surface having a larger curvature than the upper curved surface U of a plano-convex cylindrical lens that has been conventionally supported. That is, each Fresnel lens surface of the optical sheet 1 of the present embodiment is formed by a cylindrical surface having a smaller radius of curvature than the cylindrical surface of the upper curved surface U of the plano-convex cylindrical lens having the same focal length as the optical sheet 1 of the present embodiment. 1a is formed.

  Here, the upper curved surface of the plano-convex cylindrical lens is a cylindrical surface (spherical surface in the case of a plano-convex lens), a conic curved surface such as a paraboloid, an ellipsoid or a hyperboloid, a higher order curve, etc. The curved surface is used. However, regardless of the curved surface, the planoconvex linear Fresnel lens 11 corresponding to the planoconvex cylindrical lens has both Fresnel lenses adjacent to each other via the Fresnel splitting surface 11b, as shown in a longitudinal section in FIG. The inclination angles of the curves approaching the ends of the adjacent boundaries of the surfaces 11a and 11a as much as possible coincide. That is, the cross-sectional curve of the plano-convex cylindrical lens is a differentiable and smooth curve such as an arc, a parabola, an elliptical arc, a hyperbola, or a higher-order curve. This is because the curve of each Fresnel lens surface 11a also becomes a smooth curve when translated in parallel in the vertical direction and connected to the adjacent Fresnel lens surface 11a, and the inclination angles also coincide.

  Further, the plano-convex linear Fresnel lens 13 in which each Fresnel lens surface 11a formed of a curved surface shown in FIG. 3 is a flat surface (conical surface in the case of a plano-convex Fresnel lens) having an average inclination angle of the curved surface. In this case, as shown in a longitudinal section in FIG. 4, the linear inclination angle of both Fresnel lens surfaces 13a, 13a adjacent via the Fresnel dividing surface 13b is the Fresnel lens surface away from the center C of the Fresnel lens. 13a becomes an outward inclination angle. The outward inclination angle means a direction O (in the case of FIG. 4) from the inside of the plano-convex linear Fresnel lens 13 toward the lens surface having the Fresnel lens surface 13a from the center C side of the Fresnel lens surface 13a. The tilt angle when further rotated upward).

  On the other hand, each Fresnel lens surface 1a of the optical sheet 1 of the present embodiment has both Fresnel lens surfaces adjacent to each other through the Fresnel dividing surface 1b outside the center C of the Fresnel lens, as shown in a longitudinal section in FIG. The inclination angle of the curve at the end of the adjacent boundary side of the lens surfaces 1a, 1a becomes an inward inclination angle toward the Fresnel lens surface 1a farther from the center C of the Fresnel lens [Configuration (C)]. The inward inclination angle is a direction I (downward in the case of FIG. 5) from the sheet surface having the Fresnel lens surface 13a toward the inner side of the optical sheet 1 on the Fresnel lens surface 1a. Further, it refers to the angle of inclination when rotated.

  In addition, each Fresnel lens surface 1a of the optical sheet 1 of the present embodiment also has an average inclination angle of a vertical cross-sectional curve orthogonal to the Fresnel splitting surface 1b. It becomes the inclination angle of the direction [Configuration (B)]. The average value of the inclination angle of the curve of the Fresnel lens surface 1a is the arithmetic average value of the maximum value and the minimum value of the inclination angle of the curve of the Fresnel lens surface 1a. In the case of a cylindrical surface like this embodiment, The inclination angle of the tangent of the portion of the curve of the Fresnel lens surface 1a that is as close as possible to both ends in the left-right direction becomes the maximum value and the minimum value. This configuration (B) is common to the plano-convex linear Fresnel lens 11 shown in FIG. 3 and the plano-convex linear Fresnel lens 13 shown in FIG. The Fresnel lens of the sheet 1 also has a light collecting function in the left-right direction. The Fresnel lens of the optical sheet 1 of the present embodiment has the average value of the inclination angle of the curve of each Fresnel lens surface 1a as the curve of each Fresnel lens surface 1a of the plano-convex linear Fresnel lens 11 shown in FIG. A plano-convex linear Fresnel having an equivalent focal length is obtained by matching the average value of the tilt angles or by matching the straight tilt angles of the Fresnel lens surfaces 1a of the plano-convex linear Fresnel lens 13 shown in FIG. It can be associated with the lens 11 and the plano-convex linear Fresnel lens 13.

  Furthermore, each Fresnel lens surface 1a of the optical sheet 1 of the present embodiment has a monotonously increasing or decreasing monotonous in a broad sense along the direction in which the inclination angle of the longitudinal cross section orthogonal to the Fresnel dividing surface 1b is orthogonal to the Fresnel dividing surface 1b. [Configuration (A)]. In the configuration (A), when each Fresnel lens surface 11a of the plano-convex linear Fresnel lens 11 shown in FIG. 3 is a cylindrical surface, a paraboloid, an elliptical surface, a hyperboloid, or the like, this plano-convex type is used. This is also common to the linear Fresnel lens 11. And each Fresnel lens surface 1a of the optical sheet 1 of this embodiment corresponds to each Fresnel lens surface corresponding to the plano-convex linear Fresnel lens 11 having the same focal length from the configuration (C) and the configuration (A). It can be said that it is composed of a curved surface having a larger curvature than 11a. This is because the difference between the maximum value and the minimum value of the inclination angle of the Fresnel lens surface 1 a of the optical sheet 1 of the present embodiment is the maximum value and the minimum value of the inclination angle of the Fresnel lens surface 11 a of the plano-convex linear Fresnel lens 11. Since the difference is larger than the difference, the Fresnel lens surface 1a of the optical sheet 1 of the present embodiment may not locally have a portion with a smaller curvature depending on the change in the curvature of the curve. This is because the curvature of the Fresnel lens surface 1a of the optical sheet 1 of this embodiment is larger.

  As shown in FIGS. 6 and 7, when a chamfer 1c for eliminating an edge is formed at the boundary between the Fresnel lens surface 1a and the Fresnel splitting surface 1b of the optical sheet 1 of the present embodiment, The curved surface (curve) of the Fresnel lens surface 1a does not include the curved surface (curve) of the chamfered portion 1c. This is because the chamfered portion 1c is formed for convenience in manufacturing and use, and is a portion that does not contribute to the refraction of light for performing the original lens function in the Fresnel lens surface 1a. The chamfered portion 1c made of a flat surface as shown in FIG. 6 has a significantly different inclination angle from the Fresnel lens surface 1a, and the chamfered portion 1c made of a curved surface as shown in FIG. 7 also has a curvature compared to the Fresnel lens surface 1a. Is greatly increased, so that the chamfered portion 1c and the Fresnel lens surface 1a can be clearly distinguished.

According to the above configuration, as shown in FIG. 8, the light L emitted from the vicinity of the center in the left-right direction of each Fresnel lens surface 1a of the optical sheet 1 of the present embodiment (the portion where the upper curved surface U and the inclination angle coincide). Similarly to the plano-convex linear Fresnel lenses 11 and 13, _C becomes vertical upward light. However, the light L _S emitted from both sides in the left-right direction of each Fresnel lens surface 1a is a curved surface having a larger curvature than that of the Fresnel lens surfaces 11a and 13a of the plano-convex linear Fresnel lenses 11 and 13. Therefore, the light is once condensed by the condensing action of the convex cylindrical lens, and the condensed light diffuses after intersecting at the upper side. For this reason, the optical sheet 1 of the present embodiment has a converging action in the left-right direction as a whole by the shape of a convex linear Fresnel lens, but each Fresnel lens surface 1a diffuses light to some extent in the left-right direction. As a result, the convex linear Fresnel lens shape allows light to be emitted to a certain extent around each Fresnel lens surface 1a from the direction that should be originally emitted. In addition, if the division pitch of each Fresnel lens surface 1a is set to 1.0 mm or less, light is diffused for each Fresnel lens surface 1a having a sufficiently small width. Can do.

  Moreover, since the light emitted from the optical sheet 1 has a certain extent in the left-right direction rather than vertically upward, the backlight unit of the present embodiment has a sudden drop in luminance when moved from the front of the liquid crystal panel 4 to the left and right. Even when the liquid crystal panel 4 has a wide viewing angle, the wide viewing angle performance can be exhibited without waste. Further, even when the arrangement of CCFL2 is shifted in the left-right direction, it is possible to mitigate a decrease in luminance at the front position of the liquid crystal panel 4, and even when only a part of the arrangement of CCFL2 is shifted, uneven luminance is caused. It is possible to prevent the occurrence.

  In the optical sheet of the above embodiment, each Fresnel lens surface 1a is formed of a cylindrical surface having the same radius of curvature so that the degree of diffusion of light emitted from each Fresnel lens surface 1a is the same. The radius of curvature of the cylindrical surface of each Fresnel lens surface 1a is not necessarily the same. For example, the curvature radius of the cylindrical surface of the Fresnel lens surface 1a is reduced as the distance from the center of the Fresnel lens of the optical sheet 1 decreases, and the light is diffused more widely in the left-right direction toward the Fresnel lens surface 1a closer to the center. You can also

  However, if the radius of curvature of the cylindrical surface of each Fresnel lens surface 1a is made too small, the light undergoes total reflection and effective use of light from the CCFL 2 cannot be achieved. Therefore, this radius of curvature is in a range that does not cause total reflection. It is preferable that In particular, the Fresnel lens surface 1a that is farther from the center of the Fresnel lens of the optical sheet 1 is more likely to cause total reflection when the curvature radius of the cylindrical surface of the Fresnel lens surface 1a is small.

  Moreover, in the optical sheet 1 of the said embodiment, although the case where each Fresnel lens surface 1a consisted of a cylindrical surface was shown, paraboloid other than a cylindrical surface (a longitudinal section is a part of a parabola) and an elliptical surface (a longitudinal section is It may be composed of convex surfaces such as a part of an ellipse), a hyperboloid (longitudinal section is a part of a hyperbola), etc., and a convex surface connecting multiple planes such as a polygonal cylinder surface (vertical section is a polygonal line) It may be constituted by. That is, the Fresnel lens surface 1a may be formed by connecting a plurality of flat surfaces or curved surfaces in addition to a convex surface having a smooth curved surface. And even if it is the convex surface which connected the several plane, the curvature of the curved surface approximated by spline interpolation etc. becomes larger than before, for example, and the curvature in a broad sense becomes large.

  Moreover, in the optical sheet 1 of the said embodiment, although the case where the Fresnel division | segmentation surface 1b was a perpendicular | vertical plane orthogonal to the left-right direction was shown, these Fresnel division | segmentation surfaces 1b are the surfaces along the up-down direction and the left-right direction. As long as they are orthogonal surfaces, they are not necessarily vertical planes, and may be planes inclined in the left-right direction or curved surfaces close to such planes. For example, when the optical sheet 1 is molded using a mold, it is easier to remove the mold if the Fresnel dividing surface 1b is an inclined surface. FIG. 9 shows a case where the Fresnel splitting surface 1b is an inclined surface and the slope is increased as the Fresnel splitting surface 1b is farther from the center of the Fresnel lens. In this case, it is possible to prevent light emitted from the CCFL 2 from being spread radially and blocked by each Fresnel dividing surface 1b, thereby reducing the light use efficiency.

  Moreover, although the case where the optical sheet 1 consists of a transparent resin sheet was shown in the said embodiment, since it should just have the translucency which permeate | transmits light, it does not necessarily need to be transparent. The thickness of the optical sheet 1 is also not particularly limited, and it is preferable that the haze of the original sheet is 67% or more and less than 93%, and generally a thickness of about 0.3 to 5 mm is preferable. Used for.

  As the resin sheet of the optical sheet 1 as described above, polycarbonate, polyester, polyethylene, polypropylene, polyolefin copolymer (for example, poly-4-methylpentene-1), polyvinyl chloride, cyclic polyolefin (for example, norbornene structure). A material made of a light-transmitting thermoplastic resin such as acrylic resin, polystyrene, ionomer, styrene-methyl methacrylate copolymer resin (MS resin) can be used. In particular, among resin sheets made of thermoplastic resin, those made of polycarbonate, polyester (especially polyethylene terephthalate), and cyclic polyolefin have good heat resistance and are deformed by heat dissipation from CCFL2 when used in a backlight unit. It is preferably used because it is less likely to cause wrinkles and wrinkles. Moreover, a resin sheet made of polycarbonate is very preferably used because the polycarbonate itself is a resin having good transparency, has low hygroscopicity, high luminance, and little warpage. Further, the resin sheet may be made of a light-transmitting thermosetting resin such as unsaturated polyester or epoxy resin. And this resin sheet can also use what mixed and alloyed two or more types of resin materials, and was compounded.

  In addition, the optical sheet 1 of the above embodiment can be manufactured using, for example, a press manufacturing method in which a resin sheet having flat surfaces on both sides is pressed and molded, but other press manufacturing methods, casting methods, injection molding methods, and the like. It may be produced by an arbitrary production method such as the above-described molding method, a roll molding method by passing a mold roll, a continuous molding method by an extrusion molding method, or the like.

  Further, the resin sheet of the optical sheet 1 of the above embodiment includes a stabilizer, a lubricant, an impact modifier, an antioxidant, an ultraviolet absorber, a light stabilizer, an antistatic agent, a colorant, a fluorescent whitening necessary for molding. An agent or the like may be appropriately contained. Furthermore, in the optical sheet 1 having a multilayer structure, these additives may be appropriately changed in the kind and mixing ratio of the additive between the base material layer and the surface layer, for example. FIG. 10 shows an example of a two-layer optical sheet 1 in which a Fresnel lens surface 1a is formed on the upper surface layer 102 of the base material layer 101, and FIG. 11 shows a three-layer structure in which a back layer 103 is added to these lower layers. An example of the optical sheet 1 is shown. Since the lower surface of the optical sheet 1 is directly irradiated with ultraviolet rays from the CCFL 2, the upper layer 101 and the surface layer 102 are protected from the ultraviolet rays by making the back surface layer 103 a weather resistant layer containing an ultraviolet absorber. can do.

  Further, the resin sheet of the optical sheet 1 of the above embodiment may contain a light diffusing agent. As the light diffusing agent, inorganic particles, metal oxide particles, organic polymer particles, and the like having a different light refractive index from the resin material of the resin sheet are used alone or in appropriate combination. Inorganic particles include glass [A glass (soda lime glass), C glass (borosilicate glass), E glass (low alkali glass)], silica, mica, synthetic mica, calcium carbonate, magnesium carbonate, barium sulfate, talc, Particles such as montmorillonite, kaolin clay, bentonite, hectorite and silicone are used. As the metal oxide, particles such as titanium oxide, zinc oxide, and alumina are used, and as the organic polymer particles, particles such as acrylic beads, styrene beads, and benzoguanamine are used. If such a light diffusing agent is contained, light can be sufficiently diffused in the optical sheet 1, so that it is not necessary to add an expensive diffusion sheet or the like to the backlight unit.

  The light diffusing agent has an average particle diameter of 0.1 to 100 μm, preferably 0.5 to 50 μm, more preferably 1 to 30 μm. A light diffusing agent having a particle size of less than 0.1 μm is likely to aggregate and thus has poor dispersibility. Even if the light diffusing agent can be uniformly dispersed, the light scattering efficiency is poor because the wavelength of light is large. Therefore, particles having a size of 0.5 μm or more, more preferably 1 μm or more are preferable. On the other hand, a light diffusing agent having a particle size larger than 100 μm causes light scattering to be non-uniform, light transmittance to be reduced, and particles to be visible with the naked eye. For this reason, particles of 50 μm or less, particularly particles of 30 μm or less are preferred.

  Moreover, although the case where the optical sheet 1 was a resin sheet was shown in the said embodiment, since it should just be a translucent sheet | seat (a thin plate is also included), a thin plate glass etc. may be sufficient.

  Moreover, although the case where the lower surface of the optical sheet 1 was flat was shown in the said embodiment, the structure of this lower surface is arbitrary, For example, the light diffusibility was improved by processing into the embossed shape which consists of fine unevenness | corrugations. It may be a thing. Furthermore, although the case where the optical sheet 1 has a plano-convex linear Fresnel lens shape is shown in the above-described embodiment, a biconvex linear Fresnel lens shape in which both sides are convex or a back surface is concave and the entire convex lens is configured. It may be a meniscus convex linear Fresnel lens shape, and these convex linear Fresnel lens shapes can be generally implemented.

  In the above embodiment, the optical sheet 1 has a convex linear Fresnel lens shape that has a light collecting action only in the left-right direction. The present invention can be similarly applied to a convex Fresnel lens-like optical sheet corresponding to a convex lens such as a spherical surface, and other convex Fresnel lens-like optical sheets having a condensing function. In the case of the above embodiment, the Fresnel lens surface 1a is a convex shape made of a cylindrical surface. However, in the case of an optical sheet corresponding to a convex lens such as a normal spherical surface, the Fresnel lens surface is divided concentrically. For example, a convex shape formed of an annular cylindrical surface may be used.

  In the above-described embodiment, the case where a plurality of sets of Fresnel lenses are arranged side by side in the left-right direction of the optical sheet 1 is shown. However, the number of sets of Fresnel lenses is arbitrary, and one set of optical sheets 1 Only a Fresnel lens can be formed. Furthermore, in the case of an optical sheet formed with a convex Fresnel lens-like Fresnel lens corresponding to a convex lens such as a normal spherical surface, the number of Fresnel lenses can be arbitrarily set, and a plurality of Fresnel lenses should be arranged vertically and horizontally. You can also. Furthermore, the shape of the optical sheet is not limited to a square shape or a circular shape, but may be an arbitrary shape such as a polygon or an ellipse.

  Moreover, in the backlight unit of the said embodiment, although the case where the straight tube type CCFL2 was used as a light source was shown, it does not necessarily need to be a straight tube type, for example, a U-shaped tube etc. can also be used. Furthermore, the CCFL 2 is not necessarily required, and a hot cathode tube similar to a fluorescent tube for general illumination can be used, or LEDs (light emitting diodes) can be used side by side, and the type of light source is not limited. In addition, the light source does not need to be a line light source. For example, in the case of an optical sheet or the like corresponding to a normal convex lens, a point light source is often used. Further, the reflector 3 is not essential. For example, in the case of the one that emits light up and down like the CCFL 2 in the above embodiment, another optical sheet 1 may be arranged below to supply light both up and down. Is possible.

  Moreover, although the backlight unit of the said embodiment showed the case where the liquid crystal panel 4 was arrange | positioned directly above the optical sheet 1, a diffusion sheet etc. are arrange | positioned above and / or below this optical sheet 1. FIG. You can also. Furthermore, the backlight unit of the above embodiment can also be used as a backlight other than the liquid crystal panel 4.

  As examples of the optical sheet of the above-described embodiment, two types of Example 1 and Example 2 were produced, and a comparative example of the optical sheet of the conventional example was produced. And in the backlight unit using these optical sheets, the result of measuring the front luminance when the position of the CCFL is shifted in the left-right direction is shown in Table 1, and a graph of the measurement result is shown in FIG. Show.

  Polycarbonate (refractive index: 1.586) was used for the optical sheets of Examples 1 and 2 and the conventional example. The lens width of the Fresnel lens of the optical sheet corresponding to one CCFL was 30 mm. Further, this CCFL is disposed at a position 20 mm below these optical sheets, and when it is disposed directly below the center of the Fresnel lens of these optical sheets, the positional deviation is assumed to be 0.0 mm. The front luminance was measured for each case where the pitch was shifted to 2.0 mm at a pitch of 5 mm.

  The Fresnel lens surface of the optical sheet of the comparative example is a cylindrical surface with a radius of curvature of 20 mm. On the other hand, the Fresnel lens surfaces of the optical sheet of Example 1 were all cylindrical surfaces with a curvature radius of 5 mm. The Fresnel lens surface of the optical sheet of Example 2 was a cylindrical surface with a radius of curvature of 5 mm to 10 mm. That is, the Fresnel lens surface near the center of the Fresnel lens has a radius of curvature of 5 mm, and the radius of curvature gradually increases to 6 mm, 7 mm, 8 mm, and 9 mm as the distance from the center of the Fresnel lens increases. The curvature radius of the surface was 10 mm.

  As is apparent from Table 1 and FIG. 12, in the comparative example, the front luminance decreases significantly as the positional deviation from the center of the CCFL increases, whereas in the first and second embodiments, the position of the CCFL It was found that even when the deviation was 2.0 mm, the decrease in front luminance was slight.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a longitudinal cross-sectional front view illustrating a pair of Fresnel lenses in an optical sheet used in a backlight unit according to an embodiment of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a longitudinal cross-sectional front view showing parts of three sets of Fresnel lenses in an optical sheet used in a backlight unit, showing an embodiment of the present invention. It is a partial enlarged longitudinal cross-sectional front view for demonstrating the inclination angle of the curved Fresnel lens surface which shows a prior art example and is a plano-convex linear Fresnel lens. It is a partial expanded longitudinal cross-sectional front view for demonstrating the example of a prior art, and demonstrating the inclination angle of the planar Fresnel lens surface in a plano-convex linear Fresnel lens. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a partially enlarged longitudinal sectional front view illustrating an embodiment of the present invention and illustrating an inclination angle of a Fresnel lens surface of an optical sheet. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates an embodiment of the present invention, and is a partial enlarged longitudinal sectional front view when a flat chamfered portion is formed on a Fresnel lens surface of an optical sheet. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates an embodiment of the present invention, and is a partial enlarged longitudinal sectional front view when a curved chamfered portion is formed on a Fresnel lens surface of an optical sheet. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a partially enlarged longitudinal sectional front view illustrating a diffusion action by a Fresnel lens surface of an optical sheet according to an embodiment of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates an embodiment of the present invention, and is a longitudinal cross-sectional front view illustrating a part of a set of Fresnel lenses in an optical sheet whose Fresnel division surface is an inclined surface. 1 shows an embodiment of the present invention, and is a longitudinal sectional front view of an optical sheet having two resin sheets. FIG. 1 shows an embodiment of the present invention and is a longitudinal sectional front view of an optical sheet having three resin sheets. FIG. It is a graph which shows the Example of this invention, Comprising: The front luminance of the backlight unit at the time of arrange | positioning CCFL shifted in the left-right direction is shown. It is a disassembled perspective view which shows the structure of the backlight unit using an optical sheet. It is a longitudinal cross-sectional front view which shows a prior art example and shows a plano-convex type linear Fresnel lens with a corresponding plano-convex type cylindrical lens. FIG. 15 is a longitudinal cross-sectional front view illustrating a conventional example and showing a case in which each Fresnel lens surface is a flat surface in the plano-convex linear Fresnel lens shown in FIG. 14.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Optical sheet 1a Fresnel lens surface 1b Fresnel division surface 1c Chamfer 101 Base material layer 102 Surface layer 103 Back surface layer 11 Plano-convex type linear Fresnel lens 11a Fresnel lens surface 11b Fresnel division surface 12 Plano-convex type cylindrical lens 12a Upper curved surface 13 Plane Convex type linear Fresnel lens 13a Fresnel lens surface 13b Fresnel split surface 2 CCFL
3 Reflector 4 Liquid crystal panel

Claims (6)

In an optical sheet in which at least one sheet surface is divided into Fresnel lenses,
The curved shape of the longitudinal section along the central axis and the radial direction of the Fresnel lens on each Fresnel lens surface divided into the Fresnel lens shape has all of the following configurations (A) to (C). An optical sheet characterized by that.
(A) The curve of each Fresnel lens surface has a convex shape in which the inclination angle is monotonously increasing or monotonously decreasing along the radial direction.
(B) The average value of the inclination angle of the curve of each Fresnel lens surface is the value obtained by further rotating the center side of the Fresnel lens in the direction from the inner side of the sheet toward the sheet surface as the distance from the center of the Fresnel lens increases. It becomes the inclination angle of the direction.
(C) The inclination angle of the curve at the end of the adjacent boundary side of both Fresnel lens surfaces concentrically adjacent to each other is such that the Fresnel lens surface away from the center of the Fresnel lens is located on the center side of the Fresnel lens. The inward inclination angle is further rotated in the direction from the surface toward the inside of the seat. In an optical sheet in which at least one sheet surface is divided into a linear Fresnel lens,
The Fresnel lens surface divided into the linear Fresnel lens shape is characterized in that the curved shape of the longitudinal section perpendicular to the divided surface has all of the following configurations (A) to (C). Optical sheet.
(A) The curve of each Fresnel lens surface is a convex shape whose inclination angle increases monotonously or decreases monotonously along a direction orthogonal to the dividing plane.
(B) The average value of the inclination angle of the curve of each Fresnel lens surface is the value obtained by further rotating the center side of the Fresnel lens in the direction from the inner side of the sheet toward the sheet surface as the distance from the center of the Fresnel lens increases. It becomes the inclination angle of the direction.
(C) The inclination angle of the curve at the edge of the adjacent boundary side of both Fresnel lens surfaces adjacent to each other outside the center of the Fresnel lens is such that the Fresnel lens surface far from the center of the Fresnel lens is the center of the Fresnel lens. The inward inclination angle is obtained by further rotating the side in the direction from the seat surface toward the inside of the seat.   The optical sheet according to claim 2, wherein each Fresnel lens surface is divided into a linear Fresnel lens shape with a pitch of 0.1 mm or more and 1.0 mm or less.   The optical sheet according to claim 2, wherein each Fresnel lens surface is formed by a cylindrical surface.   The optical sheet according to claim 1, wherein the optical sheet is made of a translucent material containing a diffusing agent.   A backlight unit using an optical sheet, wherein the optical sheet according to any one of claims 1 to 5 is disposed in front of one or more light sources.

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