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

Abstract

This light guide plate (30) comprises: a light incident surface (33); an opposing surface (34) opposite to the light incident surface (33); a light emission surface (31) positioned between the light incident surface (33) and the opposing surface (34); a rear surface (32) opposite to the light emission surface (31); and side surfaces (35, 36) connecting the light incident surface (33) and the opposing surface (34) and positioned between the light emission surface (31) and the rear surface (32). The surface roughness (Ra) of the light incident surface (33) is greater than the surface roughness (Ra) of the side surfaces (35, 36).

Description

Light Guide Plate, Surface Light Source Device and Display Device

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

[0002] For example, as a backlight that illuminates a liquid crystal display panel incorporated in a liquid crystal display device from the back side, surface light source devices having a light emitting surface that emits light in a planar shape are widely used (see, for example, Patent Document 1). Patent Document 1 discloses an edge-lit type surface light source device. Further, in the surface light source device disclosed in Patent Document 1, a plurality of columnar unit optical elements that are convex or concave toward the outside of the light guide plate are arranged and provided on the light incident surface of the light guide plate.

[0003] In such an edge-lit type surface light source device, a light source such as a light emitting diode faces the light incident surface formed by a part of the side surface of the light guide plate. Therefore, the edge-lit type surface light source device has an advantage that it can be reduced in thickness. Edge-lit type surface light source devices that can be reduced in thickness are used in mobile devices such as smartphones, tablets, and personal computers.

[0004] Japanese Unexamined Patent Application Publication No. 2013-214378

[0005] In the light guide plate described above, there is a demand for improving the light emission quality of light emitted from the light guide plate.

[0006] The present disclosure has been made in consideration of the above points, and an object thereof is to provide a light guide plate, a surface light source device, and a display device that can improve the light emission quality of light emitted from the light guide plate.

[0007] The present disclosure relates to the following [1] to [9].

[0008] [1] A light guide plate, comprising: a light incident surface; an opposing surface opposite to the light incident surface; a light exit surface located between the light incident surface and the opposing surface; a back surface opposite to the light exit surface; and a side surface that connects the light incident surface and the opposing surface and is located between the light exit surface and the back surface, wherein a surface roughness Ra of the light incident surface is larger than a surface roughness Ra of the side surface.

[0009] [2] The light guide plate according to [1], wherein the surface roughness Ra of the light-receiving surface in the longitudinal direction of the light-receiving surface is 6.0 μm or more and 9.0 μm or less.

[0010] [3] The light guide plate according to [1] or [2], wherein the surface roughness Ra of the light-receiving surface in the width direction of the light-receiving surface is 4.0 μm or more and 8.0 μm or less.

[0011] [4] The light guide plate according to any one of [1] to [3], wherein the surface roughness Ra of the side surface in the longitudinal direction of the side surface is 1.0 μm or more and 5.5 μm or less.

[0012] [5] The light guide plate according to any one of [1] to [4], wherein the surface roughness Ra of the side surface in the width direction of the side surface is 0.5 μm or more and 5.0 μm or less.

[0013] [6] The light guide plate according to any one of [1] to [5], wherein in the longitudinal direction of the light-receiving surface, the surface roughness Ra of the light-receiving surface is 6.0 μm or more and 9.0 μm or less, and in the longitudinal direction of the side surface, the surface roughness Ra of the side surface is 1.0 μm or more and 5.5 μm or less.

[0014] [7] The light guide plate according to any one of [1] to [6], wherein in the width direction of the light-receiving surface, the surface roughness Ra of the light-receiving surface is 4.0 μm or more and 8.0 μm or less, and in the width direction of the side surface, the surface roughness Ra of the side surface is 0.5 μm or more and 5.0 μm or less.

[0015] [8] A surface light source device comprising a light guide plate according to any one of [1] to [7], and a light-emitting element positioned facing the light surface described above of the light guide plate.

[0016] A display device comprising the surface light source device described in [9] and a liquid crystal display panel superimposed on the surface light source device.

[0017] According to this disclosure, the light emission quality of the light emitted from the light guide plate can be improved.

[0018] Figure 1 is a diagram illustrating one embodiment, and is a longitudinal cross-sectional view showing the schematic configuration of a display device and a surface light source device. Figure 2 is a top view showing the surface light source device of Figure 1. Figure 3 is a longitudinal cross-sectional view showing the surface light source device of Figure 1, and is a longitudinal cross-sectional view showing the shape of the light guide plate of the surface light source device in detail.

[0019] The embodiments of this disclosure will be described below with reference to the drawings. Note that, for the sake of illustration and ease of understanding, the scale and aspect ratios of the drawings attached to this specification have been appropriately altered and exaggerated from those of the actual objects.

[0020] As shown in Figure 1, the liquid crystal display device 10 (hereinafter also referred to as the display device 10) comprises a liquid crystal display panel 15 and a surface light source device 20 positioned on the back side of the liquid crystal display panel 15, which illuminates the liquid crystal display panel 15 from the back side. The display device 10 has a display surface 11 for displaying images. The display surface 11 has a rectangular shape in plan view (shape viewed from above).

[0021] The area that will become the display surface 11 of the display device 10 is demarcated by a frame member 18. The frame member 18 is made of a material that is opaque to visible light. The frame member 18 is provided on the portion surrounding the liquid crystal display panel 15. The frame member 18 hides the area where the wiring and other components of the liquid crystal display panel 15 are located. In this display device 10, the area that overlaps with the display surface 11 and does not overlap with the frame member 18 when projected in the forward direction nd becomes the active area Aa. On the other hand, the area that overlaps with the frame member 18 when projected in the forward direction nd becomes the inactive area Ab. In the illustrated example, the inactive area Ab is adjacent to the active area Aa and surrounds the active area Aa circumferentially.

[0022] The liquid crystal display panel 15 is configured to display an image on the display surface 11. This liquid crystal display panel 15 functions as a shutter that controls the transmission or blocking of light from the surface light source device 20 for each pixel. The liquid crystal display panel 15 has an upper polarizing plate 13 located on the light-emitting side, a lower polarizing plate 14 located on the light-receiving side, and a liquid crystal layer 12 located between the upper polarizing plate 13 and the lower polarizing plate 14. The polarizing plates 14 and 13 decompose the incident light into two orthogonal polarization components (P-waves and S-waves). The polarizing plates 14 and 13 also have the function of transmitting linearly polarized components (e.g., P-waves) that vibrate in one direction (parallel to the transmission axis) and absorbing linearly polarized components (e.g., S-waves) that vibrate in the other direction orthogonal to the first direction (parallel to the absorption axis).

[0023] In the liquid crystal layer 12, an electric field can be applied to each region forming a single pixel. The orientation direction of the liquid crystal molecules in the liquid crystal layer 12 changes depending on whether or not an electric field is applied. For example, a polarization component in a specific direction that has passed through the lower polarizing plate 14, which is positioned on the light-receiving side, rotates its polarization direction by 90° when it passes through the liquid crystal layer 12, which is not subjected to an electric field. On the other hand, the deflection component maintains its polarization direction when it passes through the liquid crystal layer 12, which is subjected to an electric field. In this case, by controlling whether or not an electric field is applied to the liquid crystal layer 12, it is possible to control whether the polarization component that has passed through the lower polarizing plate 14 further passes through the upper polarizing plate 13, which is positioned on the light-emitting side of the lower polarizing plate 14, or is absorbed and blocked by the upper polarizing plate 13.

[0024] In this way, the liquid crystal display panel (liquid crystal display unit) 15 is capable of controlling the transmission or blocking of light from the surface light source device 20 for each pixel. Details of the liquid crystal display panel 15 are described in various publicly available documents (for example, "Flat Panel Display Encyclopedia (supervised by Tatsuo Uchida and Hiraki Uchiike)," published by Kogyo Chosakai in 2001), and further detailed explanation is omitted here.

[0025] Next, the surface light source device 20 will be described. The surface light source device 20 has a light-emitting surface 21 that emits light in a planar manner. The light-emitting surface 21 is formed in a rectangular shape in plan view (shape viewed from above). In this embodiment, the surface light source device 20 is used as a device to illuminate the liquid crystal display panel 15 from the back side.

[0026] As shown in Figure 1, the surface light source device 20 is configured as an edge-lit type surface light source device. The surface light source device 20 includes a light guide plate 30, a light source 24 positioned on one side of the light guide plate 30 (the left side in Figure 1), and an optical sheet (prism sheet) 50 and a reflective sheet 28 positioned facing the light guide plate 30, respectively. In the illustrated example, the optical sheet 50 is positioned facing the liquid crystal display panel 15. The light-emitting surface 51 of the optical sheet 50 forms the light-emitting surface 21 of the surface light source device 20. The surface light source device 20 also further includes a frame 55 that protects the light guide plate 30 and the optical sheet 50, and a main frame 60 that houses the light source 24, the light guide plate 30, the optical sheet 50, the reflective sheet 28, and the frame 55.

[0027] As shown in Figure 2, in this embodiment, the light-emitting surface 31 of the light guide plate 30, which will be described later, is formed in a rectangular shape in plan view (shape viewed from above), similar to the display surface 11 of the display device 10 and the light-emitting surface 21 of the surface light source device 20. As a result, the light guide plate 30 is generally configured as a flattened rectangular parallelepiped member having a pair of main surfaces (light-emitting surface 31 and back surface 32) with sides in the thickness direction being smaller than the other sides, and four surfaces are provided between the pair of main surfaces. As shown in Figure 2, two of the four surfaces extend parallel to each other in a first direction d1. The remaining two surfaces extend parallel to each other in a second direction d2 that is perpendicular to the first direction d1. The optical sheet 50 and the reflective sheet 28 are also generally configured as flattened rectangular parallelepiped members with sides in the thickness direction being smaller than the other sides. Note that the optical sheet 50 and the main frame 60 are not shown in the plan view shown in Figure 2. Furthermore, the first direction d1, the second direction d2, and the third direction d3, which is perpendicular to both the first direction d1 and the second direction d2, are shown in appropriate places in the figures used to describe the embodiment. The third direction d3 indicates the thickness direction of the display device 10 and the surface light source device 20.

[0028] The light guide plate 30 has a light-emitting surface 31 formed by one main surface on the liquid crystal display panel 15 side, a back surface 32 formed by the other main surface facing the light-emitting surface 31, and four surfaces extending between the light-emitting surface 31 and the back surface 32. As shown in Figures 1 and 3, the optical sheet 50 described above is positioned facing the light-emitting surface 31 of the light guide plate 30, and the reflective sheet 28 described above is positioned facing the back surface 32 of the light guide plate 30.

[0029] Here, one of the four surfaces forms the light-receiving surface 33. Another of the four surfaces forms the opposing surface 34, opposite to the light-receiving surface 33. Furthermore, the other two of the four surfaces connect the light-receiving surface 33 and the opposing surface 34, and form the first side surface 35 and the second side surface 36, located between the light-emitting surface 31 and the back surface 32. The light-receiving surface 33 is one of the two surfaces that face the first direction d1. As shown in Figures 1 and 3, the light source 24 is provided facing the light-receiving surface 33. Light incident into the light guide plate 30 from the light-receiving surface 33 is guided through the light guide plate 30 generally along the first direction (light-guiding direction) d1 toward the opposing surface 34 opposite to the light-receiving surface 33.

[0030] The light source 24 can be composed of various forms, such as fluorescent lamps like linear cold cathode tubes, or point-shaped LEDs (light-emitting diodes) or incandescent light bulbs. In this embodiment, the light source 24 is composed of a large number of point-shaped light-emitting elements 25. Specifically, the light source 24 is composed of a large number of light-emitting diodes (LEDs). This light source 24 is arranged along a second direction d2 which is the longitudinal direction of the light-receiving surface 33.

[0031] The reflective sheet 28 is a component that reflects light leaking from the back surface 32 of the light guide plate 30 and causes it to be re-incidentated into the light guide plate 30. The reflective sheet 28 can be made of a white scattering reflective sheet, a sheet made of a material with high reflectivity such as metal, or a sheet that includes a thin film (e.g., a thin metal film) made of a material with high reflectivity as a surface layer. The reflection at the reflective sheet 28 may be specular reflection or diffuse reflection. If the reflection at the reflective sheet 28 is diffuse reflection, the diffuse reflection may be isotropic diffuse reflection or anisotropic diffuse reflection.

[0032] In this specification, "light-emitting side" refers to the downstream side in the direction of light propagation (the observer side, for example, the upper side of the paper in Figure 1). In this case, the direction of light propagation refers to the direction in which light travels through the components of the display device 10 without going back, in the order of light source 24, light guide plate 30, optical sheet 50, and liquid crystal display panel 15, and is emitted from the display device 10 toward the observer. "Light-receiving side" refers to the upstream side in the direction of light propagation.

[0033] Furthermore, in this specification, terms such as "sheet," "film," and "plate" are not distinguished from each other solely based on differences in name. Therefore, for example, "sheet" is a concept that includes components that could also be called films or plates.

[0034] Furthermore, in this specification, "sheet surface (plate surface, film surface)" refers to the surface that coincides with the planar direction of the sheet-like member in question when viewed holistically and broadly. In this embodiment, the plate surface of the light guide plate 30, the plate surface of the light guide plate body 40 (described later), the sheet surface of the optical sheet 50, the sheet surface of the reflective sheet 28, the panel surface of the liquid crystal display panel, the display surface 11 of the display device 10, and the light-emitting surface 21 of the surface light source device 20 are parallel to each other. Furthermore, in this specification, the normal direction of a sheet-like member refers to the normal direction of the sheet surface of the member in question. Furthermore, in this specification, "front direction" refers to the normal direction of the light-emitting surface 21 of the surface light source device 20, and in this embodiment, it coincides with the normal direction of the light-emitting surface 21 of the surface light source device 20, the normal direction of the plate surface of the light guide plate 30, the normal direction of the sheet surface of the optical sheet 50, the normal direction of the display surface 11 of the display device 10, etc.

[0035] Next, the light guide plate 30 will be described in more detail with reference to Figures 1 to 3. As described above, the light guide plate 30 includes a light-receiving surface 33, an opposing surface 34 opposite to the light-receiving surface 33, a light-emitting surface 31 located between the light-receiving surface 33 and the opposing surface 34, a back surface 32 opposite to the light-emitting surface 31, and side surfaces 35 and 36 that connect the light-receiving surface 33 and the opposing surface 34 and are located between the light-emitting surface 31 and the back surface 32. The light guide plate 30 is a plate-shaped member that includes the light-emitting surface 31 and the back surface 32 as a pair of main surfaces. One of the four surfaces connecting the light-emitting surface 31 and the back surface 32 forms the light-receiving surface 33 that faces the light source 24. The light-receiving surface 33 faces the opposing surface 34 in the first direction d1.

[0036] As shown in Figures 1 to 3, in this embodiment, the light guide plate 30 includes a plate-shaped light guide plate body 40, a reflective layer 38 provided on the light guide plate body 40, a first coating layer 401, and a second coating layer 402 (see Figure 2).

[0037] The light guide plate body 40 occupies most of the light guide plate 30 and substantially constitutes the outer contour of the light guide plate 30. The light guide plate body 40 has a pair of main surfaces, a light-emitting surface 41 and a back surface 42, and four surfaces connecting the light-emitting surface 41 and the back surface 42. The light-emitting surface 41 of the light guide plate body 40 forms most of the light-emitting surface 31 of the light guide plate 30. The back surface 42 of the light guide plate body 40 forms most of the back surface 32 of the light guide plate 30. One of the four surfaces, the light-receiving surface 43, forms the light-receiving surface 33 of the light guide plate 30. The light guide plate body 40 also includes an opposing surface 44 as a surface facing the light-receiving surface 43 in the first direction d1. Furthermore, as shown in Figure 2, the light guide plate body 40 includes a first side surface 45 and a second side surface 46 that connect the light-receiving surface 43 and the opposing surface 44. The first side surface 45 and the second side surface 46 extend parallel to each other in a first direction d1. The first side surface 45 and the second side surface 46 of the light guide plate body 40 form the first side surface 35 and the second side surface 36 of the light guide plate 30. The first side surface 45 and the second side surface 46, together with the light-receiving surface 43 and the opposing surface 44, constitute the contour of the light-emitting surface 41 of the light guide plate body 40.

[0038] The reflective layer 38 is provided on the opposing surface 44. In particular, in the illustrated example, the reflective layer 38 is formed to cover the entire surface of the opposing surface 44. The first coating layer 401 is provided on the reflective layer 38. In particular, in the illustrated example, the first coating layer 401 is formed to cover the entire surface of the reflective layer 38. In this embodiment, the first coating layer 401 forms the opposing surface 34 of the light guide plate 30.

[0039] Furthermore, as shown in Figure 2, in this embodiment, the second coating layer 402 is provided on the first side surface 45 and the second side surface 46 of the light guide plate body 40. In particular, in the illustrated example, one of the two second coating layers 402 is formed to cover the entire first side surface 45, and the other of the two second coating layers 402 is formed to cover the entire second side surface 46. In this embodiment, the second coating layer 402 forms most of the pair of side surfaces of the light guide plate 30 that extend in the first direction d1 between the light-receiving surface 33 and the opposing surface 34.

[0040] Next, the light guide plate body 40, the reflective layer 38, the first coating layer 401, and the second coating layer 402 will be described in detail.

[0041] The light guide plate body 40 is the part that primarily guides the light incident from the light-receiving surface 43 in the first direction d1. Therefore, the light guide plate body 40 can be formed from a material that transmits visible light, specifically a transparent resin material.

[0042] As shown in Figure 3, the back surface 42 of the light guide plate body 40, which forms the back surface 32 of the light guide plate 30, is formed as an uneven surface. Specifically, due to the unevenness of the back surface 42 of the light guide plate body 40, a plurality of inclined surfaces 47 are formed on the back surface 32. Specifically, the back surface 42 (32) has inclined surfaces 47, a stepped surface 48 extending in the direction normal to the light guide plate 30 (front direction) nd, and a connecting surface 49 extending in the direction of the plate surface of the light guide plate 30. Light guidance within the light guide plate body 40 is performed by total internal reflection at the pair of main surfaces 41 and 42 of the light guide plate body 40. On the other hand, the inclined surfaces 47 are inclined with respect to the plate surface of the light guide plate body 40 so that they approach the light-emitting surface 41 as they move from the light-receiving surface 43 to the opposing surface 44. Therefore, for light reflected by the inclined surfaces 47, the angle of incidence when it enters the pair of main surfaces 41 and 42 becomes small. Then, when the angle of incidence to the pair of main surfaces 41 and 42 becomes less than the critical angle of total internal reflection due to reflection by the inclined surface 47, the light begins to exit the light guide plate 30. In other words, the inclined surface 47 functions as a light extraction element for extracting light from the light guide plate 30.

[0043] The inclined surface 47 is positioned relative to the light guide plate 30 such that its longitudinal direction intersects with the light guidance direction of the light guide plate 30 (the first direction d1 connecting the light-receiving surface 33 and the opposing surface 34 of the light guide plate 30). More precisely, the longitudinal direction of the inclined surface 47 is perpendicular to the light guidance direction of the light guide plate 30 (i.e., the first direction d1), and the arrangement direction of the inclined surfaces 47 is parallel to the light guidance direction of the light guide plate 30 (i.e., the first direction d1). In this embodiment, the longitudinal direction of the inclined surface 47 is parallel to the second direction d2.

[0044] Here, the dimensions of the light guide plate 30 and the light guide plate main body 40 can be set as follows, as an example. The thickness H (see FIG. 1) of the light guide plate 30 and the light guide plate main body 40 can be, for example, not less than 300 µm and not more than 800 µm.

[0045] The light guide plate main body 40 configured as described above can be manufactured by extrusion molding, as an example. Various materials can be used as the material forming the light guide plate main body 40. However, materials that are widely used as materials for optical sheets incorporated in display devices, have excellent mechanical properties, optical properties, stability, processability, etc., and are available at low cost can be preferably used. As the material forming the light guide plate main body 40, for example, a transparent resin containing one or more of acrylic resin, polystyrene resin, polycarbonate resin, polyethylene terephthalate resin, polyacrylonitrile resin, etc. as a main component can be preferably used. Note that, if necessary, a diffusive component having a function of diffusing light may be added into the light guide plate main body 40. As an example, the diffusive component may be particles made of a transparent substance such as silica (silicon dioxide), alumina (aluminum oxide), acrylic resin, polycarbonate resin, or silicone resin having an average particle diameter of about 0.5 µm to 100 µm inclusive.

[0046] Next, the reflective layer 38 will be described. The reflective layer 38 is a layer that reflects light guided in the first direction d1 inside the light guide plate main body 40 from the light incident surface 43 to the opposing surface 44 toward the light incident surface 43 side in the first direction d1. In other words, the reflective layer 38 is a layer that reflects light incident on the reflective layer 38 from the light guide plate main body 40 (such as light transmitted through the opposing surface 44 from the inside of the light guide plate main body 40) toward the light incident surface 43 side in the first direction d1. The light reflected by the reflective layer 38 can exit from the light guide plate 30 through reflection or transmission at the back surface 42. That is, providing the reflective layer 38 prevents leakage of light source light from the opposing surface 34 of the light guide plate 30. This enables efficient use of the light source light, and makes it possible to improve the luminance on the light exit surface 31 of the light guide plate 30 without increasing the output of the light source 24.

[0047] The reflective layer 38 is produced by forming a film of a light-reflective material on the opposing surface 44 of the light guide plate body 40 by coating, printing or the like. More specifically, the reflective layer 38 includes countless fillers having light reflectivity and a base resin that holds the countless fillers. In this case, as a material for forming the reflective layer 38, a material including a light-reflective filler and a resin composition in which the filler is dispersed is used. Then, the reflective layer 38 is formed by providing such a forming material on the opposing surface 44 and curing it.

[0048] The light-reflective filler is light-reflective particles or granules, and may be an inorganic material, an organic material, or a composite material of an inorganic material and an organic material. The filler made of an inorganic material may be metal particles such as aluminum or silver. In the present embodiment, particles made of silver are used as the filler.

[0049] The resin composition in which the filler is dispersed is a material that forms the base resin by curing. The resin composition may be a thermoplastic resin such as polyvinyl chloride. When the resin composition is a thermoplastic resin, the resin composition is provided on the opposing surface 44 in a molten state and then cured by drying to form the reflective layer 38. Further, the resin composition may be a thermosetting resin, a photocurable resin, or the like. Further, the resin composition may be transparent. The term "transparent" means that the visible light transmittance is 50% or more, and may be 80% or more. The visible light transmittance is specified as the average value of transmittance at each wavelength measured at an incident angle of 0° for each wavelength from 380 nm to 780 nm at 1 nm intervals using an ultraviolet-visible-near-infrared spectrophotometer ("V-730" manufactured by JASCO Corporation, compliant with JIS K 0115).

[0050] The first coating layer 401 is provided on the surface opposite to the surface 44 of the reflective layer 38 as described above. The first coating layer 401 is bonded to the reflective layer 38 to prevent the filler from falling off the reflective layer 38. The first coating layer 401 is made by forming a film on the reflective layer 38 by coating or printing a resin composition. The resin composition is applied to the reflective layer 38 and then cured to form the first coating layer 401. The resin composition may be a thermoplastic resin such as polyvinyl chloride. If the resin composition is a thermoplastic resin, the resin composition is applied to the reflective layer 38 in a molten state and then cured by drying to form the first coating layer 401. However, the resin composition forming the first coating layer 401 may be a thermosetting resin or a photocuring resin.

[0051] Furthermore, as described above, the second coating layer 402 is provided on the first side surface 45 and the second side surface 46 of the light guide plate body 40. By bonding the second coating layer 402 to the first side surface 45 and the second side surface 46, it suppresses the deterioration of the light emission quality of the light irradiated from the light guide plate 30. The second coating layer 402 is manufactured by forming a film of a resin composition on the first side surface 45 and the second side surface 46 by coating or printing. The resin composition is applied to the first side surface 45 and the second side surface 46 and then cured to form the second coating layer 402. The resin composition may be a thermoplastic resin such as polyvinyl chloride. If the resin composition is a thermoplastic resin, the resin composition is applied to the first side surface 45 and the second side surface 46 in a molten state and then cured by drying to form the second coating layer 402. However, the resin composition forming the second coating layer 402 may be a thermosetting resin or a photocuring resin.

[0052] Furthermore, the surface roughness Ra of the second coating layer 402 (the side of the second coating layer 402 opposite to the side facing the light guide plate body 40) is, in this embodiment, 300 nm or less as an example. This makes it easier for light entering the second coating layer 402 from the light guide plate body 40 to undergo total internal reflection at the interface between the second coating layer 402 and the air. The surface roughness Ra is a value measured according to JIS B 0601:2013.

[0053] In such a light guide plate 30, the surface roughness Ra of the light-receiving surface 33 is greater than the surface roughness Ra of the first side surface 35 and the surface roughness Ra of the second side surface 36. As a result, the light irradiated from the light source 24 is more easily diffused on the light-receiving surface 33. Therefore, the light is more likely to mix near the light-receiving surface 33, and unevenness in brightness near the light-receiving surface 33 can be suppressed. In addition, because the light is more likely to mix near the light-receiving surface 33, the occurrence of so-called hot spots (unevenness in brightness near the light source 24) can be suppressed.

[0054] In the longitudinal direction (second direction d2) of the light-receiving surface 33, the surface roughness Ra of the light-receiving surface 33 may be 6.0 μm or more and 9.0 μm or less. By having a surface roughness Ra of 6.0 μm or more in the longitudinal direction of the light-receiving surface 33, unevenness in brightness and the occurrence of hot spots in the longitudinal direction of the light-receiving surface 33 can be effectively suppressed. Furthermore, by having a surface roughness Ra of 9.0 μm or less in the longitudinal direction of the light-receiving surface 33, excessive diffusion of light on the light-receiving surface 33 can be suppressed.

[0055] Furthermore, in the width direction (third direction d3) of the light-receiving surface 33, the surface roughness Ra of the light-receiving surface 33 may be 4.0 μm or more and 8.0 μm or less. By having a surface roughness Ra of 4.0 μm or more in the width direction of the light-receiving surface 33, unevenness in brightness and the occurrence of hot spots in the width direction of the light-receiving surface 33 can be effectively suppressed. Also, by having a surface roughness Ra of 8.0 μm or less in the width direction of the light-receiving surface 33, excessive diffusion of light on the light-receiving surface 33 can be suppressed.

[0056] Furthermore, in the longitudinal direction (first direction d1) of the first side surface 35 (second side surface 36), the surface roughness Ra of the first side surface 35 and the surface roughness Ra of the second side surface 36 may each be between 1.0 μm and 5.5 μm. By having the surface roughness Ra of the first side surface 35 and the surface roughness Ra of the second side surface 36 each be 1.0 μm or more in the longitudinal direction of the first side surface 35, the first side surface 35 and the second side surface 36 can be easily processed. Also, by having the surface roughness Ra of the first side surface 35 and the surface roughness Ra of the second side surface 36 each be 5.5 μm or less in the longitudinal direction of the first side surface 35, the first side surface 35 and the second side surface 36 and the second coating layer 402 can be more easily totally reflected.

[0057] In the width direction (third direction d3) of the first side surface 35 (second side surface 36), the surface roughness Ra of the first side surface 35 and the surface roughness Ra of the second side surface 36 may each be 0.5 μm or more and 5.0 μm or less. By having the surface roughness Ra of the first side surface 35 and the surface roughness Ra of the second side surface 36 each be 0.5 μm or more in the width direction of the first side surface 35, the first side surface 35 and the second side surface 36 can be easily processed. Furthermore, by having the surface roughness Ra of the first side surface 35 and the surface roughness Ra of the second side surface 36 each be 5.0 μm or less in the width direction of the first side surface 35, the first side surface 35 and the second side surface 36 and the second coating layer 402 interface can be made more likely to totally reflect light.

[0058] Next, the optical sheet (prism sheet) 50 will be described in more detail, mainly with reference to Figure 3. The optical sheet 50 is a component that has the function of changing the direction of propagation of transmitted light.

[0059] As shown in Figure 3, the optical sheet 50 has a plate-shaped main body 53 and a plurality of unit prisms (unit shape elements, unit optical elements, unit lenses) 54 provided on the main body 53. The main body 53 is configured as a flat plate-shaped member and has a pair of parallel main surfaces: a light-emitting surface 53a and a light-receiving surface 53b. The plurality of unit prisms 54 are formed on the light-receiving surface 53b of the main body 53. The light-emitting surface 53a of the main body 53, which is located on the side not facing the light guide plate 30, constitutes the light-emitting surface 51 of the optical sheet 50.

[0060] In this specification, "unit prism," "unit shape element," "unit optical element," and "unit lens" refer to elements that have the function of changing the direction of light propagation by exerting optical effects such as refraction and reflection, and are not distinguished from each other solely based on differences in name.

[0061] Multiple unit prisms 54 are arranged in a line on the light-receiving side surface 53b of the main body 53. Each unit prism 54 is formed in a columnar shape and extends in a direction intersecting the direction of its arrangement.

[0062] In the illustrated example, each unit prism 54 extends in a straight line. Each unit prism 54 is formed in a columnar shape and has the same cross-sectional shape along its longitudinal direction. Multiple unit prisms 54 included in the optical sheet 50 are identically configured to one another. Furthermore, the multiple unit prisms 54 are arranged without gaps on the light-receiving side surface 53b of the main body 53 along a direction perpendicular to their longitudinal direction. Therefore, the light-receiving surface 52 of the optical sheet 50 is formed by the surfaces (prism surfaces) 54a and 54b of the unit prisms 54 arranged without gaps on the main body 53.

[0063] As described above, the optical sheet 50 is positioned so as to overlap the light guide plate 30, with the unit prisms 54 of the optical sheet 50 facing the light-emitting surface 31 of the light guide plate 30. Also, as shown in Figure 1, the optical sheet 50 is positioned relative to the light guide plate 30 such that the longitudinal direction of the unit prisms 54 intersects with the light-guiding direction (first direction d1) by the light guide plate 30. More precisely, the optical sheet 50 is positioned relative to the light guide plate 30 such that the longitudinal direction of the unit prisms 54 is perpendicular to the light-guiding direction (i.e., the first direction d1) by the light guide plate 30, and the arrangement direction of the unit prisms 54 is parallel to the light-guiding direction d1 by the light guide plate 30. Therefore, each unit prism 54 extends in a second direction d2 parallel to the longitudinal direction of the inclined surface 47.

[0064] As shown in Figure 3, each unit prism 54 has a first prism surface 54a and a second prism surface 54b that are arranged opposite to each other along the arrangement direction of the unit prisms 54 (first direction d1). The first prism surface 54a of each unit prism 54 is located on one side in the first direction d1 (the left side in Figures 1 and 3), and the second prism surface 54b is located on the other side in the first direction d1 (the right side in Figures 1 and 3). More specifically, the first prism surface 54a of each unit prism 54 is located on the side of the light source 24 in the first direction d1 and faces one side in the first direction d1. The second prism surface 54b of each unit prism 54 is located on the side away from the light source 24 in the first direction d1 and faces the other side in the first direction d1. As will be described later, the first prism surface 54a primarily functions as the incident surface when light emitted from the light guide plate 30 enters the optical sheet 50. On the other hand, the second prism surface 54b has the function of reflecting the light that enters the optical sheet 50 and correcting the optical path of said light.

[0065] As shown in Figure 3, the first prism surface 54a and the second prism surface 54b each extend from the main body 53 and are connected to each other. At the position where the first prism surface 54a and the second prism surface 54b are connected to each other, the tip (top) 54c of the unit prism 54, which protrudes furthest towards the light-receiving side from the main body 53, is formed.

[0066] As described above, the cross-sectional shape of each unit prism 54 in a cross section parallel to both the normal direction (front direction) nd and the first direction d1 of the sheet surface of the main body 53 is constant along the longitudinal direction (direction extending in a straight line) of the unit prism 54. Furthermore, in this cross section, each unit prism 54 has a shape that tapers toward the light-receiving side (the side of the light guide plate 30 (the lower side in Figure 3)). In other words, in this cross section, the width of the unit prism 54 parallel to the sheet surface of the main body 53 decreases as it moves away from the main body 53 along the normal direction nd of the main body 53.

[0067] The dimensions of the optical sheet 50 can be set as follows, for example. First, as a specific example of a unit prism 54 having the above configuration, the array pitch of the unit prism 54 along the first direction d1 (corresponding to the width of the unit prism 54 in the illustrated example) can be 10 μm or more and 200 μm or less. However, in recent years, the resolution of the array of unit prisms 54 has been rapidly increasing, so it is preferable to set the array pitch of the unit prism 54 along the first direction d1 to 10 μm or more and 50 μm or less. In addition, the protrusion height of the unit prism 54 from the main body 53 along the normal direction (front direction) nd of the sheet surface of the optical sheet 50 can be 5.5 μm or more and 180 μm or less.

[0068] Next, the frame 55 and the main frame 60 will be described with reference to Figures 1 and 2. First, the main frame 60 supports the components of the surface light source device 20, such as the light source 24, the reflective sheet 28, the light guide plate 30, and the optical sheet 50. The main frame 60 also protects the supported components of the surface light source device 20 from the outside. The light source 24, the reflective sheet 28, the light guide plate 30, and the optical sheet 50 are positioned relative to the main frame 60. The liquid crystal display panel 15 described above is also positioned relative to the main frame 60. Note that the reflective sheet 28, the light guide plate 30, and the optical sheet 50 heat up and expand during use of the surface light source device 20. For this reason, the reflective sheet 28, the light guide plate 30, and the optical sheet 50 are supported by the main frame 60 so that they can expand and contract. Typically, the reflective sheet 28, the light guide plate 30, and the optical sheet 50 are fixed to the main frame 60 on one side in the first direction d1. Furthermore, the reflective sheet 28, the light guide plate 30, and the optical sheet 50 are stretchable to the other side in the first direction d1 when thermal expansion occurs. One side in the first direction d1 is the side close to the light source 24, and the other side in the first direction d1 is the side away from the light source 24. Such a main frame 60 is formed using a metal such as aluminum, stainless steel, or iron.

[0069] The frame 55 is provided between the reflective sheet 28, the light guide plate 30, the optical sheet 50, and the main frame 60. The frame 55 prevents the reflective sheet 28, the light guide plate 30, and the optical sheet 50 from coming into contact with the main frame 60. The frame 55 has the function of protecting the reflective sheet 28, the light guide plate 30, and the optical sheet 50 from the main frame 60. In addition, as shown in Figure 1, the frame 55 supports the liquid crystal display panel 15 in the front direction nd. The frame 55 forms a gap between the liquid crystal display panel 15 and the optical sheet 50. This prevents the optical sheet 50 from coming into contact with the liquid crystal display panel 15.

[0070] In the illustrated example, the frame 55 is supported so as to be movable relative to the main frame 60. The frame 55 is fixed to the opposing surface 34 of the light guide plate 30 via a bonding layer 58. That is, the first coating layer 401 of the light guide plate 30 and the frame 55 are connected by the bonding layer 58 along the first direction d1. The frame 55 moves relative to the main frame 60 as the light guide plate 30 expands due to thermal expansion. Various adhesive or bonding layers used in display devices can be used as the bonding layer 58.

[0071] In the example shown in Figure 1, a gap S is formed between the frame 55 and the main frame 60 in the first direction d1, allowing for thermal expansion of the light guide plate 30. However, at least a portion of the thermal expansion of the light guide plate 30 may be absorbed by the bonding layer 58. In this case, the width Ws [μm] of the gap S between the frame 55 and the main frame 60 along the first direction d1 can be made shorter than the expected thermal expansion length of the light guide plate 30. This makes it possible to narrow the width of the inactive area Ab of the light guide plate 30 and achieve a narrower bezel. Furthermore, in order to absorb the expansion of the light guide plate 30 with the bonding layer 58, it is preferable that the thickness Wj [μm] of the bonding layer 58 along the first direction d1 be set to be large. For example, considering display devices for notebook computers, tablet devices, and even smartphone devices, it is preferable that the thickness be 100 μm or more and 700 μm or less, more preferably 200 μm or more and 600 μm or less, and even more preferably 300 μm or more and 500 μm or less. Moreover, making the thickness Wj [μm] of the bonding layer 58 along the first direction d1 longer than the expected thermal expansion length of the light guide plate 30 is also effective in absorbing the expansion of the light guide plate 30 with the bonding layer 58.

[0072] Next, the operation of the display device 10, which has the above configuration, will be explained.

[0073] First, as shown in Figure 3, the light emitted by the light-emitting element 25, which constitutes the light source 24, enters the light guide plate body 40 of the light guide plate 30 via the light-receiving surface 33. In this embodiment, the surface roughness Ra of the light-receiving surface 33 is greater than the surface roughness Ra of the first side surface 35 and the surface roughness Ra of the second side surface 36. As a result, the light irradiated from the light source 24 diffuses easily on the light-receiving surface 33. Therefore, the light mixes more easily in the vicinity of the light-receiving surface 33, and the occurrence of uneven brightness and hot spots in the vicinity of the light-receiving surface 33 is suppressed.

[0074] As shown in Figure 3, the light L31 and L32 incident on the light guide plate body 40 are reflected at the light-emitting surface 41 and back surface 42 of the light guide plate body 40, and undergo repeated total internal reflection, particularly due to the difference in refractive index between the material making up the light guide plate body 40 and air, and proceed in the first direction (light-guiding direction) d1 connecting the light-receiving surface 43 and the opposing surface 44 of the light guide plate body 40.

[0075] Here, the back surface 42 of the light guide plate body 40 has an inclined surface 47 that slopes closer to the light-emitting surface 41 as it moves from the light-receiving surface 43 towards the opposing surface 44. The inclined surface 47, together with the stepped surface 48 and the connecting surface 49, forms the back surface 42. Of these, the stepped surface 48 extends in the direction normal to the plate surface of the light guide plate body 40 (front direction) nd. Therefore, most of the light traveling through the light guide plate body 40 from the light-receiving surface 43 to the opposing surface 44 is reflected by the inclined surface 47 or the connecting surface 49 of the back surface 42 without being incident on the stepped surface 48. When light is reflected by the inclined surface 47, in the cross-section shown in Figure 3, the direction of propagation of the light changes in a direction in which the inclination angle of the light guide plate body 40 with respect to the plate surface increases. That is, when light is reflected by the inclined surface 47 of the back surface 42, the angle of incidence of the light to the light-emitting surface 41 and the back surface 42 from that point onward decreases. Therefore, the angle of incidence of light traveling through the light guide plate body 40 to the light-emitting surface 41 and the back surface 42 gradually decreases due to one or more reflections on the inclined surface 47 of the back surface 42, and becomes less than the critical angle of total internal reflection. In this case, the light passes through the light-emitting surface 41 or the back surface 42 of the light guide plate body 40 (the light-emitting surface 31 or the back surface 32 of the light guide plate 30) and becomes able to be emitted from the light guide plate 30. The light L31 and L32 emitted from the light-emitting surface 31 are directed toward the optical sheet 50 located on the light-emitting side of the light guide plate 30. On the other hand, the light emitted from the back surface 32 is reflected by the reflective sheet 28 located on the back surface of the light guide plate 30 and is re-entered into the light guide plate 30, traveling through the light guide plate 30.

[0076] Subsequently, the light emitted from the light guide plate 30 enters the optical sheet 50. As described above, the optical sheet 50 has a unit prism 54 whose tip 54c protrudes toward the light guide plate 30. As described above, the longitudinal direction of the unit prism 54 is parallel to the direction that intersects the light guidance direction (first direction d1) by the light guide plate 30, and in particular to the second direction d2 which is perpendicular to the light guidance direction in the illustrated example.

[0077] As a result, as shown in Figure 3, the light rays L31 and L32 heading toward the optical sheet 50 enter the unit prism 54 via the first prism surface 54a, one of the two interconnected prism surfaces 54a and 54b. As shown in Figure 3, these light rays L31 and L32 then undergo total internal reflection at the second prism surface 54b, changing their direction of propagation.

[0078] In the cross-section shown in Figure 3, the direction of propagation of light L31 and L32, which travel in a direction within an angular range that is greatly inclined from the front direction nd, is precisely bent by total internal reflection at the second prism surface 54b of the unit prism 54 so that the angle with respect to the front direction nd becomes small. Through this action, the unit prism 54 narrows the direction of propagation of transmitted light toward the front direction nd for the component of light along the first direction (light guide direction) d1. In other words, the optical sheet 50 exerts a focusing effect on the component of light along the first direction d1.

[0079] Then, the light of one polarized component emitted from the optical sheet 50 forming the light-emitting surface 21 of the surface light source device 20 is incident on the liquid crystal display panel 15 and passes through the lower polarizer 14. The light that has passed through the lower polarizer 14 is selectively transmitted through the upper polarizer 13 depending on the state of the electric field applied to each pixel. In this way, the liquid crystal display panel 15 selectively transmits light from the surface light source device 20 to each pixel, allowing an observer of the display device 10 to observe the image.

[0080] In this embodiment, a reflective layer 38 is provided on the opposing surface 44 of the light guide plate body 40. As shown in Figure 3, the reflective layer 38 reflects the light L61 that reaches the opposing surface 44. The light L61 reflected by the reflective layer 38 travels through the light guide plate body 40 toward the light-receiving surface 43 in the first direction d1. This light L61 is then bent in the direction of travel by, for example, the inclined surface 47, and is emitted from the light guide plate 30 via the light-emitting surface 41 or the back surface 42. This enables efficient use of light from the light source 24 and improves the brightness on the light-emitting surface 31 of the light guide plate 30 without increasing the output of the light source 24.

[0081] As described above, according to this embodiment, the light guide plate 30 comprises a light-receiving surface 33, an opposing surface 34 opposite to the light-receiving surface 33, a light-emitting surface 31 located between the light-receiving surface 33 and the opposing surface 34, a back surface 32 opposite to the light-emitting surface 31, and side surfaces 35 and 36 that connect the light-receiving surface 33 and the opposing surface 34 and are located between the light-emitting surface 31 and the back surface 32. The surface roughness Ra of the light-receiving surface 33 is greater than the surface roughness Ra of the first side surface (side surface) 35 and the surface roughness Ra of the second side surface (side surface) 36. As a result, the light irradiated from the light source 24 is more easily diffused on the light-receiving surface 33. Therefore, the light is more likely to mix in the vicinity of the light-receiving surface 33, and unevenness in brightness in the vicinity of the light-receiving surface 33 can be suppressed. Furthermore, because light is more easily mixed near the light-receiving surface 33, the occurrence of so-called hot spots (uneven brightness near the light source 24) can be suppressed. The ability to suppress uneven brightness and hot spots will be explained in the embodiments described later.

[0082] As described above, one embodiment has been explained with specific examples, but the specific example relating to the above-described embodiment can be implemented with various other specific examples, and various omissions, substitutions, and modifications can be made without departing from the gist of it.

[0083] The embodiment will be described in more detail below using examples, but this embodiment is not limited to these examples.

[0084] First, surface light source devices according to Examples 1-A to 3-C and Comparative Examples 1 to 4 were prepared. Each surface light source device was configured such that a light guide plate, a light source, a reflective sheet, and an optical sheet were arranged in the same positional relationship as described above. As will be explained below, among the surface light source devices according to Examples 1-A to 3-C and Comparative Examples 1 to 4, only the shape of the light guide plate differed from one another; the light source, reflective sheet, and optical sheet were identical. The surface roughness Ra was adjusted by adjusting the radius of curvature of the tip of the cutting tool (diamond bit) used to cut the light-receiving surface and the sides. The surface roughness Ra was measured using a laser microscope (Keyence Corporation, VK8700 (product name)). The magnification was set to 10x.

[0085] <Evaluation Method> A surface light source device, while emitting light, was visually observed at a position 0.5 m away from the light-emitting surface of the surface light source device along the direction normal to the light-emitting surface. At this time, it was checked whether hot spots (uneven brightness near the light source) and uneven brightness consisting of alternating bright and dark areas were observed on the light-emitting surface of each surface light source device. In addition, it was checked whether light leakage from the first side or the second side was observed.

[0086] The results are shown in Table 1. For surface light sources where no hot spots were observed, an "A" is marked in the "Hot Spot" column of Table 1. For surface light sources where hot spots were observed, a "B" is marked in the "Hot Spot" column of Table 1. For surface light sources where no brightness unevenness was observed, an "A" is marked in the "Brightness Unevenness" column of Table 1. For surface light sources where brightness unevenness was observed, a "B" is marked in the "Brightness Unevenness" column of Table 1. For surface light sources where no light leakage from the first and second sides was observed, an "A" is marked in the "Light Leakage" column of Table 1. Furthermore, for surface light sources where light leakage from the first or second side was observed, a "B" is marked in the "Light Leakage" column of Table 1.

[0087]

[0088] As a result, as shown in Table 1, no hot spots or uneven brightness were observed in the surface light source devices according to Examples 1-A to 3-C. Therefore, it was found that the light guide plate according to this embodiment can suppress the occurrence of hot spots and uneven brightness, and improve the light emission quality of the light emitted from the light guide plate. In addition, light leakage from the first and second sides was also suppressed in the surface light source devices according to Examples 1-A to 3-C. Therefore, it was found that the light guide plate according to this embodiment enables efficient utilization of light from the light source.

[0089] Furthermore, the results of Example 1-C show that the surface roughness Ra of the light-receiving surface in the longitudinal direction is greater than the surface roughness Ra of the side surface in the longitudinal direction. On the other hand, the surface roughness Ra of the light-receiving surface in the width direction is smaller than the surface roughness Ra of the side surface in the width direction. Thus, even when the surface roughness Ra of the light-receiving surface in the width direction is smaller than the surface roughness Ra of the side surface in the width direction, no hot spots or brightness unevenness were observed, as shown in Table 1. For this reason, it is considered that the surface roughness Ra of the light-receiving surface has a more positive effect on the optical properties in the longitudinal direction of the light-receiving surface than in the width direction. That is, when the light guide plate is observed from the front direction nd (third direction d3), the longitudinal direction of the light-receiving surface (second direction d2) is perpendicular to the direction of observation. As a result, brightness unevenness occurring in the longitudinal direction of the light-receiving surface becomes more visible to the observer, and it is thought that this brightness unevenness is observed as a hot spot or brightness unevenness. On the other hand, when observing the light guide plate from the front direction nd (third direction d3), the width direction of the light-receiving surface (third direction d3) becomes the direction of observation (depth direction). As a result, even if brightness unevenness occurs in the width direction of the light-receiving surface, the brightness unevenness becomes less visible to the observer. Therefore, considering the observation direction of hot spots and brightness unevenness that occur on the light guide plate, it is considered more important to diffuse the light in the longitudinal direction of the light-receiving surface.

[0090] In addition, while several modifications of the above-described embodiments have been explained, it is naturally possible to combine and apply multiple modifications as appropriate.

Claims

1. A light guide plate comprising: a light-receiving surface; an opposing surface opposite to the light-receiving surface; a light-emitting surface located between the light-receiving surface and the opposing surface; a back surface opposite to the light-emitting surface; and a side surface connecting the light-receiving surface and the opposing surface, and located between the light-emitting surface and the back surface, wherein the surface roughness Ra of the light-receiving surface is greater than the surface roughness Ra of the side surface.

2. The light guide plate according to claim 1, wherein the surface roughness Ra of the light-receiving surface is 6.0 μm or more and 9.0 μm or less in the longitudinal direction of the light-receiving surface.

3. The light guide plate according to claim 1, wherein the surface roughness Ra of the light-receiving surface is 4.0 μm or more and 8.0 μm or less in the width direction of the light-receiving surface.

4. The light guide plate according to claim 1, wherein the surface roughness Ra of the side surface in the longitudinal direction of the side surface is 1.0 μm or more and 5.5 μm or less.

5. The light guide plate according to claim 1, wherein the surface roughness Ra of the side surface is 0.5 μm or more and 5.0 μm or less in the width direction of the side surface.

6. The light guide plate according to claim 1, wherein in the longitudinal direction of the light-receiving surface, the surface roughness Ra of the light-receiving surface is 6.0 μm or more and 9.0 μm or less, and in the longitudinal direction of the side surface, the surface roughness Ra of the side surface is 1.0 μm or more and 5.5 μm or less.

7. The light guide plate according to claim 1, wherein in the width direction of the light-receiving surface, the surface roughness Ra of the light-receiving surface is 4.0 μm or more and 8.0 μm or less, and in the width direction of the side surface, the surface roughness Ra of the side surface is 0.5 μm or more and 5.0 μm or less.

8. A surface light source device comprising a light guide plate according to any one of claims 1 to 7, and a light-emitting element disposed facing the light surface in front of the light guide plate.

9. A display device comprising a surface light source device according to claim 8, and a liquid crystal display panel superimposed on the surface light source device.

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