Light guide plate, surface light source device, and display device

Abstract

To provide a light guide plate, a surface light source device, a display device, and a manufacturing method of the light guide plate capable of suppressing uneven luminance.SOLUTION: A light guide plate 12 for front light includes: a light incident surface 12a on which light is made incident; a light emission surface 12c that intersects with the light incident surface 12a, and from which light is emitted; a back surface 12d opposed to the light emission surface 12c; and an opposing surface 12b facing the light incident surface 12a. The light made incident from the light incident surface 12a is emitted from the light emission surface 12c while guiding the light made incident to the opposing surface 12b side. The light emission surface 12c is provided with a convexoconcave part 13 in which a convex part 13a and a concave part 13b are alternately formed in a direction perpendicular to the light incident surface 12a. The depth of the concave part 13b is 0.1-3.0 μm, and the bottom width of the concave part 13b and the apex width of the convex part 13a are 1-30 μm. The distance from the light emission surface 12c to the back surface 12d is 0.1-0.4 mm. The light guide plate 12 is formed of polystyrene resin.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a light guide plate, a surface light source device, and a display device. [Background technology]

[0002] 2. Description of the Related Art Display devices that display images by illuminating a display unit such as an LCD (Liquid Crystal Display) panel with a surface light source device are known. Surface light source devices are roughly divided into direct-type devices in which the light source is disposed directly below optical members such as various optical sheets, and edge-light types in which the light source is disposed on the side of the optical member. Of these, edge-light type surface light source devices have the advantage that the surface light source device can be made thinner than the direct-type devices because the light source is disposed on the side of the optical member such as a light guide plate, and have been widely used in recent years.

[0003] Generally, in an edge-light type surface light source device, a light source is positioned facing the light entrance surface, which is the side surface of a light guide plate, and the light emitted by the light source enters the light guide plate from the light entrance surface and travels in a direction perpendicular to the light entrance surface (light guide direction) from the light entrance surface to the opposing surface, while being repeatedly reflected by the light exit surface and the back surface opposite it. By changing the direction of light travel by using uneven surfaces on the light guide plate, the light is gradually emitted toward the LCD panel from each position along the light guide direction on the light exit surface (for example, Patent Document 1). Some of these light guide plates are manufactured by extrusion molding, in which resin is extruded to form a shape. In this case, however, light leakage from the light entering through the light entrance surface can occur, causing bright lines to be visible on the light exit surface, resulting in so-called brightness unevenness. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2009-217283 A Summary of the Invention [Problem to be solved by the invention]

[0005] An object of the present invention is to provide a light guide plate, a surface light source device, and a display device that can suppress the occurrence of uneven luminance. [Means for solving the problem]

[0006] The present invention solves the problems by the following solving means. Note that, for ease of understanding, the following description will be given with reference numerals corresponding to the embodiments of the present invention, but the present invention is not limited to these. Furthermore, the configurations described with reference numerals may be appropriately improved, and at least a part of the configurations may be replaced with other configurations. A first invention is a light guide plate (12) for a front light having a light entrance surface (12a) where light enters, a light exit surface (12c) where light exits by intersecting the light entrance surface, a back surface (12d) facing the light exit surface, and an opposing surface (12b) facing the light entrance surface, and causes light incident from the light entrance surface to exit from the light exit surface while guiding it toward the opposing surface, wherein the light exit surface has an uneven portion (13) in which convex portions (13a) and concave portions (13b) are alternately formed in a direction perpendicular to the light entrance surface, the depth of the concave portions is 0.1 μm or more and 3.0 μm or less, the bottom width of the concave portions and the top width of the convex portions are 1 μm or more and 30 μm or less, the distance from the light exit surface to the back surface is 0.1 mm or more and 0.4 mm or less, and the light guide plate is made of polystyrene resin. A second invention is a surface light source device (10) including the light guide plate (12) of the first invention, and a light source section (11) disposed opposite the front light input surface (12a) of the light guide plate and projecting light onto the light input surface. A third invention is a display device (1) including the surface light source device (10) of the second invention, and a display unit (2) arranged on the light output surface (12c) side of the light guide plate (12) provided in the surface light source device. A fourth aspect of the present invention is the display device (1) of the third aspect of the present invention, characterized in that the display section (2) is a reflective display section. A fifth invention is a display device characterized in that, in the display device (1) of the third or fourth invention, a joining member is provided between the display unit (2) and the light guide plate (12) to join the display unit and the light guide plate, and the joining member is formed of a material having a lower refractive index than a material used for the light guide plate. Effect of the Invention

[0007] According to the present invention, it is possible to suppress the occurrence of uneven brightness. [Brief description of the drawings]

[0008] [Figure 1] FIG. 1 is a schematic diagram illustrating a display device 1. [Diagram 2] FIG. 2 is a diagram illustrating a light guide plate 12. [Diagram 3] 2A to 2C are diagrams illustrating the details of a shaping sheet used in the manufacture of the light guide plate 12. [Figure 4] 3A to 3C are diagrams illustrating a method for manufacturing the light guide plate 12. [Diagram 5] 6 is a photograph showing the state of light exiting from the light exit surface of light guide plates of an example and a comparative example. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. However, the technical scope of the present invention is not limited to this embodiment. In the drawings used to explain the embodiment, the size, ratio, etc. of the members are changed or exaggerated as necessary. Hereinafter, an embodiment of the present invention will be described with reference to the drawings, etc. Note that each of the drawings shown below, including Fig. 1, is a schematic diagram, and the size and shape of each part are appropriately exaggerated to facilitate understanding. In this specification, the terms plate, sheet, etc. are used, and in general usage, they are used in the order of thickness, plate, sheet, and film, and this specification follows that order. However, since such distinction in usage has no technical meaning, these terms can be used interchangeably as appropriate.

[0010] The numerical values ​​such as dimensions of each component and the names of materials described in this specification are merely examples of an embodiment, and are not intended to be limiting, and may be appropriately selected and used. In this specification, terms specifying shapes or geometric conditions, such as parallel and orthogonal, are intended to include not only their strict meanings but also states that have a similar optical function and have an error that can be regarded as parallel or orthogonal. In this specification, the sheet surface (plate surface, film surface) refers to the surface of each sheet (plate, film) that is in the planar direction of the sheet (plate, film) when viewed as a whole.

[0011] (Embodiment) FIG. 1 is a diagram illustrating a display device 1 according to the present embodiment. Fig. 2 is a diagram illustrating the light guide plate 12 of this embodiment. Fig. 2(a) is a plan view of the light guide plate 12 as viewed from the light output surface 12c side, and Fig. 2(b) is a partial cross-sectional view of part b in Fig. 2(a). For ease of understanding, in the following drawings including Fig. 1 and the following description, when the display device 1 is in use, of two directions that are parallel to the screen of the display device 1 and perpendicular to each other, the direction perpendicular to the light entrance surface 12a of the light guide plate 12 is defined as the X direction, and the direction perpendicular to the X direction is defined as the Y direction. Also, the direction perpendicular to the screen of the display device 1 (thickness direction) is defined as the Z direction. In the thickness direction (Z direction), the +Z side is defined as the viewer side, and the -Z side is defined as the back side.

[0012] The display device 1 of this embodiment includes an LCD panel 2 and a surface light source device 10. The display device 1 illuminates the LCD panel 2 from the viewer side with the surface light source device 10, and displays image information formed on the LCD panel 2. The screen of the display device 1 in this embodiment corresponds to the surface (hereinafter referred to as the display surface) 10a closest to the +Z side (observer side) of the surface light source device 10, and the "front direction" of the display device 1 is the normal direction of this display surface 10a, which is parallel to the Z direction and coincides with the normal direction of the plate surface of the light guide plate 12 described later.

[0013] The LCD panel 2 is a reflective display unit formed of liquid crystal display elements and forms video information on its display surface. The LCD panel 2 is formed in a substantially flat plate shape. The LCD panel 2 has a rectangular shape when viewed from the Z direction, and has two opposing sides parallel to the X direction and two opposing sides parallel to the Y direction. The surface light source device 10 is a device that illuminates the LCD panel 2 from the viewer side (+Z side), and includes a light source section 11, a light guide plate 12, etc. This surface light source device 10 is an edge-light type surface light source device (front light).

[0014] The light source unit 11 is a part that emits light to illuminate the LCD panel 2. The light source unit 11 is disposed along the Y direction at a position facing a light entrance surface 12a (described later), which is an end surface on one side in the X direction (the +X side) of the light guide plate 12. The light source unit 11 is formed by arranging a plurality of point light sources at a predetermined interval in the Y direction. The point light sources are LED (Light Emitting Diode) light sources. The light source unit 11 may be, for example, a line light source such as a cold cathode fluorescent lamp, or may be a form in which a light source is disposed on an end face of a light guide extending in the Y direction. In order to improve the efficiency of using the light emitted by the light source unit 11, a reflector (not shown) may be provided so as to cover the outside of the light source unit 11.

[0015] Light guide plate 12 constituting surface light source device 10 has a rectangular shape when viewed from the front direction (Z direction), and has two opposing sides parallel to the X direction and two opposing sides parallel to the Y direction. The light guide plate 12 is a substantially flat plate-shaped member that guides light. The light emitted from the light source unit 11 is incident on a light entrance surface 12a, guided within the light guide plate 12, and then emitted from a light exit surface 12c. In this embodiment, the light entrance surface 12a and the opposing surface 12b, which are opposed to each other, are located at both ends (the +X side end and the -X side end) of the light guide plate 12 in the X direction, and are two sides extending parallel to the Y direction when viewed from the normal direction (the Z direction) of the plate surface. The light exit surface 12c and the back surface 12d, which face each other, are located at both ends (the -Z side end and the +Z side end) in the Z direction of the light guide plate 12. The plate surface of the light guide plate 12 is parallel to the XY plane, and the back surface 12d, which becomes the display surface 10a of the display device 1, is a surface parallel to this plate surface. The first side 12e and the second side 12f, which face each other, are located at both ends (-Y side end and +Y side end) of the light guide plate 12 in the Y direction, and are two sides that extend parallel to the X direction when viewed from the normal direction (Z direction) of the plate surface.

[0016] This light guide plate 12 receives light emitted from the light source unit 11 through a light entrance surface 12a, and while totally reflecting the light at a light exit surface 12c and a back surface 12d, guides the light mainly in the X direction toward an opposing surface 12b (X2 side) opposite the light entrance surface 12a, and then emits the light appropriately from the light exit surface 12c toward the LCD panel 2 (-Z side). The light guide plate 12 of this embodiment is formed in a rectangular shape when viewed from the thickness direction (Z direction), and from the viewpoint of guiding more light, the surfaces parallel to the thickness direction that are parallel to the longitudinal direction (Y direction) are designated as the light entrance surface 12a and the opposing surface 12b.

[0017] 2, the light exit surface 12c of the light guide plate 12 has an uneven portion 13 made of minute unevenness alternately formed in a direction (X direction) perpendicular to the light entrance surface 12a. This allows the light guided into the light guide plate 12 to be appropriately emitted from the light exit surface 12c. As shown in Fig. 2(b), the uneven portion 13 extends in the Y direction of the light guide plate 12 and is composed of convex portions 13a with rectangular cross sections and concave portions 13b with rectangular cross sections that are alternately formed in the X direction. The concaves and convex portions that compose the uneven portion 13 are finely formed, and for example, the depth of the concave portions 13b is about 0.1 µm or more and 3.0 µm or less. The bottom width of the concave portions 13b and the top width of the convex portions 13a are about 1 µm or more and 30 µm or less.

[0018] The shape of the uneven portion 13 is not limited to the above-mentioned example, and other shapes may be applied, or a combination of a plurality of shapes may be used. Other forms of the uneven portion include, for example, a form having a predetermined cross-section and extending obliquely with respect to the width direction (Y direction) of the light guide plate 12, a form having a two-dimensional shape in which multiple cone-shaped protrusions are arranged vertically and horizontally in a planar view, and a form in which multiple protrusions are arranged in a dot pattern in a planar view.

[0019] The light guide plate 12 can be made of various materials. Examples include thermoplastic resins such as polymer resins having an alicyclic structure, methacrylic resins, polycarbonate resins, polystyrene resins, acrylonitrile-styrene copolymers, methyl methacrylate-styrene copolymers, ABS resins, and polyethersulfone, as well as epoxy acrylate and urethane acrylate reactive resins. These are widely used as materials for optical components such as light guide plates, and have excellent mechanical properties, optical properties, stability, and processability, and are available at low cost.

[0020] Here, the light guide plate 12 of this embodiment is formed so that the refractive index Nx in a direction perpendicular to the light entrance surface 12a (X direction, direction from the light entrance surface 12a toward the opposing surface 12b) is larger than the refractive index Ny (Nx>Ny) in a width direction (Y direction, direction from the first side surface 12e toward the second side surface 12f) parallel to the light exit surface 12c and parallel to the light entrance surface 12a. This allows the light guide plate 12 of this embodiment to uniformly emit light that enters from the light entrance surface 12a and is guided into the light guide plate 12 from the light exit surface. If the refractive index Nx in the direction perpendicular to the light incident surface 12a is smaller than the refractive index Ny in the width direction (Nx < Ny), light leakage of the light incident from the light incident surface may occur. In that case, since bright lines are visually recognized on the light emitting surface and luminance unevenness occurs, it is not desirable.

[0021] This is because when the refractive index (Nx) in the X direction (light guiding direction) perpendicular to the light incident surface is lower than the refractive index (Ny) in the Y direction (Nx < Ny), the light incident from the incident surface tends to easily advance in the light guiding direction (X direction), and it is considered that the light gathers too much in the X direction and is easily visually recognized as bright lines. On the other hand, when the refractive index in the X direction (light guiding direction) is higher than the refractive index in the Y direction (Nx > Ny), part of the light incident from the incident surface disperses and advances in a direction intersecting the X direction, and thus the excessive gathering of light in the X direction is suppressed. Therefore, it is considered that the generation of bright lines is suppressed. The light guide plate 12 of the present embodiment is formed of a polycarbonate resin having optical anisotropy. The refractive index Nx in the direction perpendicular to the light incident surface 12a is 1.59, and the refractive index Ny in the direction perpendicular to the first side surface 12e is 1.58.

[0022] Here, the light guide plate 12 is particularly effective when used as a front light as in the display device 1 of the present embodiment. For a light guide plate used as a backlight, since not only an LCD panel but also a diffusion plate, a prism sheet, etc. are arranged on the light emitting surface of the light guide plate, even if the above-described bright lines are generated on the light emitting surface of the light guide plate, they may be less noticeable due to the diffusion plate or the like. On the other hand, for the light guide plate 12 used as a front light, since the back surface 12d of the light guide plate directly becomes the display surface 10a of the display device 1, when bright lines are generated, the possibility of being visually recognized becomes very high. Therefore, by providing the light guide plate 12 satisfying Nx > Ny in the surface light source device 10 as in the display device 1 of the present embodiment, it is possible to suppress the above-described bright lines from being visually recognized.

[0023] In addition, as described above, the display device 1 of the present embodiment includes the reflective LCD panel 2. Therefore, when the amount of incident light such as external light on the LCD panel 2 is sufficient, the light source unit 11 provided in the surface light source device 10 is used in a turned-off state. On the other hand, when the amount of incident light on the LCD panel 2 is insufficient and the image displayed on the display surface 10a becomes dark, the display device 1 is used with the light source unit 11 emitting light. Therefore, when the display device 1 is expected to be placed in a relatively bright place, the lighting time of the light source unit 11 becomes very short. Therefore, from the viewpoints of cost reduction and power consumption reduction of the display device 1, etc., the number of point light sources used in the light source unit 11 may be reduced. In such a case, when the refractive index in the light guide direction (X direction) is lower than that in the Y direction (Nx < Ny), the interval between the point light sources becomes wider, and thus there is a risk that the bright lines will be more prominently visible. However, as in the present embodiment, by making the refractive index in the light guide direction (X direction) higher than that in the Y direction (Nx > Ny), it is possible to suppress the bright lines from being visible even in such a case.

[0024] The thickness of the light guide plate 12 (the distance from the light emitting surface 12c (the top of the convex portion) to the back surface 12d in the thickness direction of the light guide plate) is preferably 0.1 mm to 1.0 mm. If the thickness of the light guide plate 12 is 0.1 mm or less, the number of reflections of the light guiding through the light guide plate becomes too many. If the thickness is 1.0 mm or more, the number of reflections of the light guiding through the light guide plate becomes too few. In either case, it becomes difficult to uniformly emit light from the light emitting surface of the light guide plate, which is not desirable. Note that the thickness of the light guide plate 12 of the present embodiment is, for example, 0.4 mm. Moreover, from the viewpoint of more effectively suppressing the above-mentioned bright lines from being visible, the refractive index difference (Nx-Ny) between the refractive index Nx and the refractive index Ny of the light guide plate 12 is preferably 0.002 or more, more preferably 0.005 or more, and even more preferably 0.01 or more. Here, this refractive index difference varies depending on the thickness of the light guide plate, and more specifically, the refractive index difference tends to become smaller as the thickness of the light guide plate becomes thinner, so when the thickness of the light guide plate 12 is 0.1 mm, which is the lower limit of the above-mentioned preferable range, the refractive index difference is preferably 0.002 or more.

[0025] (Regarding the manufacturing method of the light guide plate 12) Before describing the method for manufacturing the light guide plate 12, the shaping sheet 70 used to form the light guide plate 12 will be described. FIG. 3 is a diagram illustrating a shaping sheet 70 used to form the light guide plate 12. As shown in FIG. As shown in FIG. 3, the shaped sheet 70 is a long, flexible molded plate made of a resin in which a base material portion 71 and a molded layer 72 are laminated, and is rolled up before the light guide plate 12 is manufactured. The base material portion 71 is a base material that serves as the foundation of the shaped sheet 70, and is made of, for example, polyethylene terephthalate (PET) resin. In addition to PET resin, it is also possible to use transparent resins such as polycarbonate (PC) resin, methyl methacrylate-butadiene-styrene (MBS) resin, methyl methacrylate-styrene (MS) resin, acrylic-styrene (AS) resin, and acrylonitrile-butadiene-styrene (ABS) resin. The molded layer 72 is a layer in which an uneven shape portion 73 corresponding to the uneven portion 13 of the light guide plate 12 is formed on the surface (front surface) opposite to the substrate portion 71 side. The molded layer 72 is formed of, for example, a urethane acrylate-based ultraviolet curing resin. In addition to the urethane acrylate-based ultraviolet curing resin, for example, a polyester acrylate-based, epoxy acrylate-based, polyether acrylate-based, polythiol-based, butadiene acrylate-based ultraviolet curing resin can also be used. In addition to the above-mentioned ultraviolet curing resin, an electron beam curing resin can also be used as the energy beam curing resin. The uneven portion 73 provided in the molding layer 72 is arranged in a plurality of rows on the surface of the molding layer 72 in order to simultaneously mold a plurality of light guide plates 12. In this embodiment, the uneven portion 73 is formed in three rows in a direction parallel to the short sides of the long shaped sheet 70 and in a plurality of stages in a direction parallel to the long sides.

[0026] Next, a method for producing the light guide plate 12 by extrusion molding will be described. FIG. 4 is a diagram illustrating a method for manufacturing the light guide plate 12. As shown in FIG. 4, the wound shaped sheet 70 is unwound and transported between a first roll 81 and a second roll 82 arranged with a predetermined gap from the first roll 81. Here, the transport direction S of the shaped sheet 70 is a direction parallel to a direction perpendicular to the short side of the shaped sheet 70. Then, a molten thermoplastic resin composition 85 (resin that forms the light guide plate) is extruded from a nozzle 86 between the surface of the shaped sheet 70 on which the uneven portion 73 is formed and the second roll 82 (resin extrusion process).

[0027] Next, the extruded thermoplastic resin composition 85 is sandwiched and pressed between a first roll 81 and a second roll 82 while being transported together with the shaping sheet 70 to form the light guide plate 12 (light guide plate forming process). As a result, the thermoplastic resin composition 85 is filled into the uneven shape portion 73 provided on the surface of the shaping layer 72 (see FIG. 3) of the shaping sheet 70, and hardened by being cooled by the second roll 82 and the atmosphere (open air, the working environment of the manufacturing equipment, etc.), and assumes a shape that conforms to the uneven shape portion 73 of the shaping sheet 70. The thermoplastic resin composition 85 is then further cooled by passing through a third roll 83 and a fourth roll 84, and the shape is finally fixed. In this way, a multi-sided light guide plate sheet 10' in which the light guide plates 12 are attached lengthwise and crosswise on multiple surfaces can be obtained. Next, the light guide plate multi-sided sheet 10 ′ is released from the shaping sheet 70 by a peeling roll 87 . Finally, the multiple light guide plate mounting sheet 10' is subjected to processing such as punching and cutting to obtain a plurality of individual light guide plates 12 (see FIG. 2(a)).

[0028] Here, as a result of extensive research, the inventors of the present invention have discovered that since the thermoplastic resin composition 85 extruded from the nozzle 86 is stretched in the conveying direction S, the refractive index in the conveying direction S tends to be higher than the refractive index in a direction perpendicular to the conveying direction S. 3, each uneven portion 73 provided on the shaping sheet 70 is formed so that a surface 73a corresponding to the light entrance surface 12a of the light guide plate 12 (a surface parallel to the longitudinal direction of the light guide plate 12) is parallel to the short side w1 of the shaping sheet 70. That is, each uneven portion 73 provided on the shaping sheet 70 is disposed on the molding layer 72 so that a surface 73a corresponding to the light entrance surface 12a of the light guide plate 12 is perpendicular to the conveying direction S of the shaping sheet 70.

[0029] Therefore, the thermoplastic resin composition 85 extruded from the nozzle 86 is stretched in a direction perpendicular to the surface 73a of the uneven portion 73, which will be the light entrance surface 12a of the light guide plate 12. As a result, the produced multi-sided light guide plate sheet 10' (light guide plate 12) is formed such that the refractive index Nx in the direction perpendicular to the light entrance surface 12a of the light guide plate 12 (light guide direction) is higher than the refractive index Ny in the direction perpendicular to the first side surface 12e. In this manner, the light guide plate 12 that satisfies the condition of refractive index Nx>refractive index Ny is manufactured. The difference between the refractive indexes Nx and Ny can be adjusted by appropriately adjusting the resin temperature and cooling temperature of the thermoplastic resin composition 85 extruded from the nozzle 86, the conveying speed of the shaped sheet 70, the tension in the conveying direction S, and the like.

[0030] Conventionally, as described in Japanese Patent Publication No. 10-142601, for example, a light guide plate has been manufactured by injection molding or the like, so that it has not been possible to manufacture a light guide plate satisfying Nx>Ny, such as the light guide plate 12 of this embodiment. In addition, it is possible to manufacture a light guide plate 12 satisfying Nx>Ny by processing the uneven portion 13 on one side of a resin film having optical anisotropy, but in this case, it may be very difficult to process the desired uneven portion with high accuracy, and the manufacturing cost may increase accordingly. In contrast, by using the shaped sheet 70 as described above and using the extrusion molding method, the light guide plate 12 satisfying Nx>Ny can be manufactured more easily and more inexpensively.

[0031] (Evaluation of brightness unevenness) Next, the results of evaluation of the relationship between the refractive index in a direction perpendicular to the light entrance surface of the light guide plate and the refractive index in a direction perpendicular to each side surface, and the occurrence of luminance unevenness on the light exit surface of the light guide plate will be described. FIG. 5 is a photograph showing the light emission state on the light emission surface of the light guide plates of the examples and the comparative examples. FIG. 5(a) is a photograph taken from a direction perpendicular to the light emission surface of the light guide plate of the example, and FIG. 5(b) is a photograph taken from a direction inclined 45 degrees with respect to the light emission surface of the light guide plate of the example. Further, FIG. 5(c) is a photograph taken from a direction perpendicular to the light emission surface of the light guide plate of the comparative example, and FIG. 5(d) is a photograph taken from a direction inclined 45 degrees with respect to the light emission surface of the light guide plate of the comparative example.

[0032] The light guide plate of the example used for the evaluation is formed in the same manner as the light guide plate 12 of the above-described embodiment, and is formed of a polycarbonate resin. The light guide plate of the example has a refractive index Nx of 1.59 in the direction perpendicular to the light incident surface (light guiding direction), and a refractive index Ny of 1.58 in the direction from the first side surface to the second side surface (direction perpendicular to each side surface), and is configured such that the refractive index Nx is larger than the refractive index Ny (Nx>Ny). The light guide plate of the comparative example is formed of a polycarbonate resin, has a refractive index Nx of 1.58 in the direction perpendicular to the light incident surface (light guiding direction), and a refractive index Ny of 1.59 in the direction from the first side surface to the second side surface (direction perpendicular to each side surface), and is configured such that the refractive index Nx is smaller than the refractive index Ny (Nx<Ny).

[0033] Here, the refractive indices Nx and Ny of each light guide plate were measured using a measuring device (KOBRA-WR) of the KOBRA series manufactured by Oji Scientific Instruments Co., Ltd. For the light guide plates of the examples and the comparative examples used for the measurement of the refractive index, flat samples having no uneven portions provided on the light emission surface were used. The above-described measuring device has a light source unit, a polarizer (polarizing element), an analyzer (analyzing element), and a light receiving unit arranged in this order, and a sample placement stage is provided between the polarizer and the analyzer. Each of the above samples was placed on the placement stage, irradiated with a single-wavelength light beam from the light source unit, and the phase difference and the orientation angle of the sample were obtained from the angular dependence of the transmitted light intensity when the sample was rotated once around the optical axis while keeping the polarizer and the analyzer in parallel Nicol, and the refractive indices in the X direction and the Y direction were obtained. Here, a light beam having a wavelength of 590 nm was used for the light beam irradiated from the light source unit.

[0034] As shown in FIGS. 5(c) and 5(d), it was confirmed that in the light guide plate of the comparative example, light leakage of light incident from the light entrance surface was visually recognized as bright lines on the light exit surface. In contrast, as shown in Figures 5(a) and 5(b), the light guide plate of the embodiment was confirmed to minimize light leakage of light incident from the light entrance surface, significantly reduce the visibility of bright lines on the light exit surface, and emit light uniformly.

[0035] As described above, the light guide plate 12 of this embodiment is formed so that the refractive index Nx in the direction from the light entrance surface 12a to the opposing surface 12b (direction perpendicular to the light entrance surface 12a) is larger than the refractive index Ny in the direction from the first side surface 12e to the second side surface 12f (direction perpendicular to each side surface) (Nx>Ny). This makes it possible for the light guide plate 12 to suppress the occurrence of uneven brightness in the light emitted from the light exit surface 12c, and to emit the guided light uniformly from the light exit surface 12c.

[0036] Furthermore, in the manufacturing method of the light guide plate 12 of this embodiment, the surface 73a of the uneven portion 73 of the shaped sheet 70, which corresponds to the light entrance surface 12a of the light guide plate 12, is arranged so as to be perpendicular to the resin conveying direction S, so that the light guide plate 12 can be easily manufactured such that the refractive index Nx is greater than the refractive index Ny.

[0037] Although the embodiment of the present invention has been described above, the present invention is not limited to the above-mentioned embodiment, and various modifications and changes are possible, such as the modified forms described below, which are also within the technical scope of the present invention. Furthermore, the effects described in the embodiment are merely a list of the most preferable effects resulting from the present invention, and the effects of the present invention are not limited to those described in the embodiment. The above-mentioned embodiment and the modified forms described below can be used in appropriate combination, but detailed description will be omitted.

[0038] (Variations) (1) In the above embodiment, the light guide plate 12 is used as a front light of a display device, but the present invention is not limited to this and may be used as a back light of a transmissive display device. In this case, the LCD panel needs to be a transmissive type that transmits light from the back side. In addition, a light control sheet such as a prism sheet or a light diffusion sheet may be provided between the LCD panel and the light guide plate.

[0039] (2) In the above embodiment, the light guide plate 12 is formed by the unrolled shaping sheet 70, but the present invention is not limited to this. For example, instead of using a shaping sheet, the light guide plate 12 may be manufactured by using a roll plate having a cylindrical peripheral side surface on which a concave-convex portion is formed.

[0040] (3) In the above-described embodiment, an example was shown in which the display surface 10a of the display device 1 is the rear surface 12d of the light guide plate 12, but this is not limited to this. A transparent protective sheet for preventing scratches on the light guide plate 12 may be attached to the rear surface 12d of the light guide plate 12, and the surface of the protective sheet may be used as the display surface. In this case, it is desirable to use a material having a lower refractive index than the material used for the light guide plate 12 for the bonding layer that bonds the back surface 12d of the light guide plate 12 to the protective sheet, from the viewpoint of efficiently performing total reflection of light within the light guide plate 12. When the light guide plate 12 is formed of polycarbonate resin as in the above-described embodiment, it is desirable to use, for example, an acrylic resin having a lower refractive index than polycarbonate resin for the bonding layer.

[0041] (4) The light output surface 12c of the light guide plate 12 and the LCD panel 2 may be joined by a transparent joining member. In this case, the joining member may be provided so as to fill in the uneven shape formed on the light output surface 12c of the light guide plate 12. In this case, too, from the viewpoint of efficiently performing total reflection of light within the light guide plate 12, it is desirable to use a material for the joining member that has a lower refractive index than the material used for the light guide plate 12. [Explanation of symbols]

[0042] 1 Display device 10 Surface light source device 11 Light source 12 Light guide plate 12a Light incident surface 13 Concavoconvex part 70 Shaping シート 71 Base Material 72 Forming layer 73 Concavoconvex shape part

Claims

1. A light guide plate for a front light, comprising: a light entrance surface into which light is incident; a light exit surface that intersects with the light entrance surface and from which light exits; a back surface that faces the light exit surface; and an opposing surface that faces the light entrance surface, the light guide plate directing light incident from the light entrance surface toward the opposing surface and emitting the light from the light exit surface, The light exit surface is provided with an uneven portion in which convex portions and concave portions are alternately formed in a direction perpendicular to the light entrance surface, The depth of the recess is 0.1 μm or more and 3.0 μm or less, a bottom width of the recess and a top width of the protrusion are 1 μm or more and 30 μm or less; The distance from the light exit surface to the back surface is 0.1 mm or more and 0.4 mm or less, The light guide plate is made of polystyrene resin; The light guide plate is characterized by the above.

2. The light guide plate according to claim 1 ; a light source unit provided at a position facing a front light input surface of the light guide plate and configured to project light onto the light input surface; A surface light source device comprising:

3. The surface light source device according to claim 2 , a display unit disposed on the light output surface side of the light guide plate provided in the surface light source device; A display device comprising:

4. 4. The display device according to claim 3, The display unit is a reflective display unit; A display device comprising:

5. 5. The display device according to claim 3, a joining member provided between the display unit and the light guide plate and joining the display unit and the light guide plate; the bonding member is made of a material having a lower refractive index than a material used for the light guide plate; A display device comprising:

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