JP2013045745A - Light guide plate, planar light source device, and transmission image display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a light guide plate capable of efficiently extracting light from an emission face, and a planar light source device as well as a transmission image display device including the same.SOLUTION: The light guide plate 40 is provided with an emission face 41 for emitting light, a rear face 42 opposed to the emission face, and an incident face 43 crossing the emission face as well as the rear face for light from the light source 50 to be incident into. The incident face takes on a concave shape recessed toward an inside of the plate against a first virtual plane P1 connecting an incident face side end part each of the emission face and the rear face, and provided, an angle made by at least either region in the incident face at an end part 43b side of an emission face side or a rear face side and the first virtual plane is α, a refraction index of a surface layer at an emission face side of the light guide plate is n, and a refraction index at a further rear face side than the surface layer in a thickness direction of the light guide plate is n, α is an angle satisfying formula (1).

Description

  The present invention relates to a light guide plate, a surface light source device, and a transmissive image display device.

  2. Description of the Related Art A transmissive image display device such as a liquid crystal display device generally includes a surface light source device that is disposed on the back side of a transmissive image display unit such as a liquid crystal display panel and supplies a backlight to the transmissive image display unit. As such a surface light source device, an edge light type surface light source device is known.

  The edge light type surface light source device includes a light-transmitting light guide plate and a light source disposed on the side of the light guide plate for supplying light to the light guide plate. The light output from the light source enters the light guide plate from the side surface of the light guide plate facing the light source, and propagates while totally reflecting inside the light guide plate. A part of the light propagating in the light guide plate by this total reflection is emitted from the light exit surface of the light guide plate, so that planar light is generated. In order to emit light propagating in the light guide plate from the exit surface while totally reflecting, the back surface side of the light guide plate has a function of reflecting a part of the light propagating to the back surface under conditions other than the total reflection condition. Is provided. For example, in the technique described in Patent Document 1, a plurality of reflective dots are formed on the back surface side of the light guide plate, thereby causing reflection under conditions other than the total reflection condition.

JP-A-5-100118

  However, in the conventional light guide plate, there are cases where light incident from the incident surface cannot be efficiently extracted from the output surface.

  Accordingly, an object of the present invention is to provide a light guide plate that can efficiently extract light from an emission surface, a surface light source device including the light guide plate, and a transmissive image display device.

A light guide plate according to the present invention is a light guide plate that emits light from one surface while guiding light incident from a light source. The light output plate emits light, a back surface that faces the output surface, an output surface, and a back surface. And an incident surface on which light from the light source is incident. The incident surface has a concave shape that is recessed toward the inside of the plate with respect to the first virtual plane that connects the end portions on the incident surface side of each of the output surface and the back surface, and the exit surface side and the back surface on the incident surface The angle formed by at least one of the end-side regions on the side and the first virtual plane is α, the refractive index of the surface layer portion on the exit surface side of the light guide plate is n _1, and the surface layer in the thickness direction of the light guide plate when the refractive index of the back side was set to n ₂ than section,
α is an angle satisfying Equation (1).

When light that is incident from the incident surface and propagates in the light guide plate is incident on the output surface at an angle smaller than the critical angle, light is emitted from the output surface, whereas when light is incident on the output surface at an angle larger than the critical angle, the light is emitted. Total reflection on the surface. Therefore, in the light propagation process, light incident on the emission surface is generated at an angle smaller than the critical angle on the emission surface, whereby light can be extracted from the emission surface. The light emitted in this way is likely to have a smaller angle difference between the incident angle and the critical angle of the light when totally reflected.

In the light guide plate having the above configuration, the incident surface has a concave shape with respect to the first virtual plane, and therefore α is greater than zero. Therefore, in the light guide plate according to the present invention, when the incident surface coincides with the first imaginary plane, that is, the angle difference between the incident angle of light and the critical angle in the case of total reflection as compared with the case of α = 0. (Left side of equation (1)) becomes smaller. As a result, in the light guide plate having a concave incident surface on the inner side of the plate, the emission efficiency can be improved. Furthermore, since the incident surface is concave, it is easier to reduce light propagating in the normal direction of the first virtual plane than when the incident surface is flat. Therefore, light is not emitted from the side surface of the light guide plate facing the incident surface, and is easily emitted from the emission surface. Also in this respect, the emission efficiency from the emission surface is improved.

In the light guide plate according to the present invention, the n ₁ and the n ₂ may be equal.

  Preferably, the shape of the cross section of the incident surface that is parallel to the plate thickness direction and orthogonal to the first virtual plane is a triangular shape.

  In this case, it is easier to further reduce the light propagating in the normal direction of the first virtual plane. As a result, it is possible to further improve the emission efficiency from the emission surface.

  Preferably, the incident surface is located in the center of the exit surface and the back surface in the thickness direction and is symmetric with respect to a second virtual plane orthogonal to the thickness direction.

  In this case, both of the angle formed between the region on the exit surface side on the exit surface and the first virtual plane, and the angle formed between the region on the back surface side on the incident surface and the first virtual plane are both. Satisfies the equation (1). As a result, on the incident surface, light incident from both the exit surface side and the back surface side is efficiently emitted from the exit surface.

  Preferably, the light guide plate according to the present invention further includes a reflective dot that is provided on the back surface and reflects light propagating to the back surface side.

  Reflection of light by the reflective dots is light reflection under conditions other than total reflection. As a result, it is easy to enter the exit surface at an angle smaller than the critical angle. As a result, the emission efficiency can be improved.

  The surface light source device according to the present invention includes the light guide plate according to the present invention and a light source that supplies light to an incident surface of the light guide plate.

  Since the surface light source device having the above configuration includes the light guide plate according to the present invention, the light incident on the light guide plate from the light source can be efficiently emitted from the emission surface of the light guide plate.

  A transmissive image display device according to the present invention includes a light guide plate according to the present invention, a light source that supplies light to an incident surface of the light guide plate, and a transmissive type that is illuminated by light emitted from an output surface of the light guide plate. An image display unit.

  Since the transmissive image display device having the above configuration includes the light guide plate according to the present invention, light incident on the light guide plate from the light source can be efficiently emitted from the exit surface of the light guide plate. Therefore, the transmissive image display unit can be illuminated with more light.

  According to the present invention, it is possible to provide a light guide plate capable of efficiently emitting light from the emission surface, a surface light source device including the light guide plate, and a transmissive image display device.

1 is a perspective view schematically showing a schematic configuration of a transmissive image display device to which an embodiment of a light guide plate according to the present invention is applied. It is sectional drawing of the light-guide plate shown in FIG. It is a schematic diagram for demonstrating the light emission conditions on an output surface. It is a schematic diagram which shows schematic structure of other embodiment of the light-guide plate which concerns on this invention.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the description of the drawings, the same elements are denoted by the same reference numerals, and redundant description is omitted. Further, the dimensional ratios in the drawings do not necessarily match those described.

  FIG. 1 is a perspective view schematically showing a schematic configuration of a transmissive image display device to which an embodiment of a light guide plate according to the present invention is applied. In FIG. 1, the transmissive image display device 10 is shown in an exploded manner. The transmissive image display device 10 can be suitably used as a display device for a mobile phone or various electronic devices.

  The transmissive image display device 10 includes a transmissive image display unit 20 and a surface light source device 30 that outputs planar light to be supplied to the transmissive image display unit 20.

  The transmissive image display unit 20 displays an image by being illuminated with planar light emitted from the surface light source device 30. An example of the transmissive image display unit 20 is a liquid crystal display panel as a polarizing plate bonding body in which linear polarizing plates 22 and 23 are arranged on both surfaces of a liquid crystal cell 21 as illustrated in FIG. In this case, the transmissive image display device 10 is a liquid crystal display device. As the liquid crystal cell 21 and the polarizing plates 22 and 23, those employed in a transmissive image display device such as a conventional liquid crystal display device can be used. Examples of the liquid crystal cell 21 are a TFT type liquid crystal cell and an STN type liquid crystal cell.

  The surface light source device 30 is an edge light type backlight unit that supplies a backlight to the transmissive image display unit 20. The surface light source device 30 includes a light guide plate 40 and a plurality of light sources 50 arranged on the sides of the light guide plate 40.

  The plurality of light sources 50 are arranged in a line. In the surface light source device 30 illustrated in FIG. 1, the plurality of light sources 50 are arranged in the short side direction of the light guide plate 40. The light source 50 is a point light source, and an example of the point light source is a light emitting diode. A linear light source may be used instead of the plurality of light sources 50. Examples of linear light sources are cylindrical lamps such as fluorescent lamps or cold cathode tubes.

  As shown in FIG. 1, the surface light source device 30 may include a reflection unit 60 located on the opposite side of the transmissive image display unit 20 with respect to the light guide plate 40. The reflection part 60 is for causing the light emitted from the light guide plate 40 to the reflection part 60 side to enter the light guide plate 40 again. The reflection part 60 can be a sheet-like one as shown in FIG. Further, the reflection unit 60 may be a bottom surface of the housing of the surface light source device 30 that houses the light guide plate 40 and that is mirror-finished.

  The light guide plate 40 is a plate-like body having a substantially flat emission surface (emission surface portion) 41 that emits light, a back surface 42 that faces the emission surface 41, and an incident surface 43 on which light from the light source 50 is incident. is there. The incident surface 43 is also a side surface of the light guide plate 40 that connects the output surface 41 and the back surface 42.

  On the back surface 42 of the light guide plate 40, a plurality of reflective dots 44 (see FIG. 2) for emitting light propagating through the light guide plate 40 from the output surface 41 are provided. The reflective dots 44 of the light guide plate 40 reflect the light propagating to the back surface 42 side while diffusing the light to the exit surface 41 side. Therefore, the reflection dot 44 is also a diffusion dot that diffuses and reflects light. The reflective dots 44 together with the back surface 42 constitute a reflective surface portion that reflects the light propagating toward the back surface 42 toward the exit surface 41 side. Examples of the reflective dots 44 include white dots and microlenses. The reflective dots 44 can be formed by, for example, inkjet printing (inkjet method), photopolymer method, laser drawing, or the like. An example of the shape of the reflective dot 44 when the reflective dot 44 is formed by laser drawing is a concave lens shape.

  The light guide plate 40 is a plate-like body made of a translucent material (or a transparent material). Examples of the refractive index of the translucent material are 1.46 to 1.62. Examples of the translucent material are a translucent resin material and a translucent glass material. Examples of the translucent resin material include polycarbonate resin (refractive index: 1.59), MS resin (methyl methacrylate-styrene copolymer resin) (refractive index: 1.56-1.59), polystyrene resin (refractive index). Ratio: 1.59), AS resin (acrylonitrile-styrene copolymer resin) (refractive index: 1.56-1.59), acrylic ultraviolet curable resin (refractive index: 1.46-1.58), poly And methyl methacrylate (PMMA) (refractive index: 1.49). As the translucent resin material, PMMA is more preferable from the viewpoint of transparency.

  Further, when a translucent resin material is used as the translucent material constituting the light guide plate 40, an ultraviolet absorber, an antistatic agent, an antioxidant, a processing stabilizer, a flame retardant, a lubricant, etc. are added to the translucent resin material. Additives can also be added. These additives can be used alone or in combination of two or more. It is preferable that an ultraviolet absorber is added to the light guide plate 40 because deterioration of the light guide plate 40 due to ultraviolet rays can be prevented when the light output from the light source 50 contains a lot of ultraviolet rays.

  Examples of UV absorbers include benzotriazole UV absorbers, benzophenone UV absorbers, cyanoacrylate UV absorbers, malonic ester UV absorbers, oxalic anilide UV absorbers, and triazine UV absorbers. is there. The ultraviolet absorber is preferably a benzotriazole ultraviolet absorber or a triazine ultraviolet absorber.

  The translucent resin material is usually used without adding a light diffusing agent as an additive. However, a light diffusing agent may be added as long as it is a slight amount that does not impair the object of the present invention.

  As the light diffusing agent, normally, a powder having a refractive index different from that of the translucent material that mainly constitutes the light guide plate 40 is used. The light diffusing agent is used by being dispersed in a translucent material. Examples of the light diffusing agent are organic particles such as styrene resin particles and methacrylic resin particles, or inorganic particles such as potassium carbonate particles and silica particles. An example of the particle size of the light diffusing agent is 0.8 μm to 50 μm.

  Moreover, it is preferable that the output surface 41 is flat. However, the emission surface 41 may have a slight surface layer diffusion to reduce moire.

  The light guide plate 40 can be manufactured by inkjet printing (inkjet method), photopolymer method, extrusion molding, injection molding, or the like.

  In the transmissive image display device 10, when the light source 50 emits light, the light output from the light source 50 enters the light guide plate 40 from the incident surface 43 of the light guide plate 40 facing the light source 50. The light incident on the light guide plate 40 propagates while being totally reflected in the light guide plate 40. When the light propagating through the light guide plate 40 is reflected by the reflective dots 44, it is reflected under conditions other than the total reflection condition. Therefore, the light reflected by the reflective dots 44 is emitted from the emission surface 41.

  Therefore, in the light guide plate 40 configured as described above, the light output from the light source 50 and incident on the light guide plate 40 can be emitted from the emission surface 41 while being guided in the light guide plate 40. The plurality of reflection dots 44 are arranged in a predetermined arrangement pattern such that the luminance of the planar light emitted from the emission surface 41 is substantially uniform.

  Next, the shape of the incident surface 43 that is one of the features of the light guide plate 40 will be described.

  FIG. 2 shows a cross-sectional configuration when the light guide plate 40 is cut along a plane orthogonal to both the incident surface 43 and the output surface 41 of the light guide plate 40. In FIG. 2, the light source 50 is also shown for the sake of explanation.

  As shown in FIG. 2, the incident surface 43 is a virtual plane (first virtual plane) that connects an end 41 a on the incident surface 43 side of the output surface 41 and an end 42 b on the incident surface 43 side of the back surface 42. ) It has a concave shape recessed toward the inside of the plate with respect to P1. The cross-sectional shape of the incident surface 43 is a triangle as shown in FIG. In other words, the cross-sectional shape of the incident surface 43 is a prism shape having a top on the inner side of the plate. The cross-sectional shape of the incident surface 43 shown in FIG. 2 corresponds to a cross-sectional shape that is parallel to the plate thickness direction and orthogonal to the first virtual plane P1. The plane parallel to the plate thickness direction is a plane orthogonal to the flat emission surface 41 in this embodiment.

  In the incident surface 43 having a triangular cross section, the top portion 44a on the inner side of the plate is located at the center between the emission surface 41 and the back surface 42 in the plate thickness direction. In one embodiment, the incident surface 43 may be symmetric with respect to a virtual plane (second virtual plane) P2 that passes through the top 44a and is orthogonal to the thickness direction.

式（２）において、ｎは、導光板４０の屈折率である。また、本実施形態では、入射面４３の断面形状は三角形であるため、αは、入射面４３の底角に対応する。入射面４３の断面形状が三角形でない場合、αは、端部４３ｂ，４３ｂ側近傍において直線近似した場合の近似直線と平面Ｐ１との間のなす角度とすることができる。また、直線近似ができない場合には、入射面４３を曲線でフィッティングし、フィティング曲線の端部４３ｂ，４３ｂでの接線と平面Ｐ１とのなす角度をαとすればよい。 An angle α between the region (end region) on the end portions 43b and 43b side of the incident surface 43 having a triangular cross section and the plane P1 satisfies the formula (2).

In Expression (2), n is the refractive index of the light guide plate 40. In this embodiment, since the cross-sectional shape of the incident surface 43 is a triangle, α corresponds to the base angle of the incident surface 43. When the cross-sectional shape of the incident surface 43 is not a triangle, α can be an angle formed between the approximate straight line and the plane P1 when a linear approximation is performed in the vicinity of the end portions 43b and 43b. If linear approximation cannot be performed, the incident surface 43 may be fitted with a curve, and the angle formed between the tangent line at the ends 43b and 43b of the fitting curve and the plane P1 may be α.

  The incident surface 43 having the above-described configuration is formed by carving a groove (in this embodiment, for example, a V-groove) whose inner surface has the shape of the incident surface 43 on the side surface corresponding to the plane P1 of the plate-like body to be the light guide plate 40. Can be formed. It is also possible to directly manufacture the light guide plate 40 having the concave incident surface 43 by injection molding or the like.

  When α as the base angle of the incident surface 43 satisfies the expression (2), the light guide plate 40 can efficiently extract light from the emission surface 41. This point will be described with reference to FIG. FIG. 3 is a schematic diagram for explaining the light emission conditions on the emission surface 41.

このθ_MAXは、本実施形態において、入射面４３に最大入射角で入射した光が出射面（又は出射面と平行な仮想的な面）４１となす角度である。 When the light incident from the incident surface 43 satisfies the total reflection condition, that is, when the light propagates through the light guide plate 40 without being affected by the reflective dots 44, the minimum incident angle of the light to the output surface 41 The value is θ _MIN . This θ _MIN corresponds to the incident angle of light on the exit surface 41 when light incident on the incident surface 43 at the maximum incident angle propagates through the light guide plate 40 while satisfying the total reflection condition. Here, assuming that 90 ° −θ _MIM is θ _MAX , θ _MAX is expressed by Equation (3).

This θ _MAX is an angle formed by the light incident on the incident surface 43 at the maximum incident angle with the output surface (or a virtual surface parallel to the output surface) 41 in the present embodiment.

The critical angle of the light guide plate 40 having a refractive index n is defined as θ _CRITICAL . θ _CRITICAL is expressed by Expression (4).

In order for light to be emitted from the light guide plate 40, it is necessary to enter the emission surface 41 at an incident angle smaller than θ _CRITICAL . In other words, the light incident on the emission surface 41 at an angle within the hatched area of FIG.

When the light incident on the light guide plate 40 is not reflected by the reflective dots 44, the light propagating in the light guide plate 40 enters the output surface 41 at an incident angle larger than θ _MIN . When α satisfies Expression (2), θ _MIN that is the incident angle is larger than θ _CRITICAL that is the critical angle, as shown in FIG. Therefore, the light is totally reflected by the emission surface 41 and is not emitted from the light guide plate 40.

On the other hand, when the light incident on the light guide plate 40 is reflected by the reflective dots 44, the reflected light, which is the light reflected by the reflective dots 44, is incident on the emission surface 41 at an incident angle smaller than θ _CRITICAL. Is emitted from the emission surface 41.

Here, as shown in FIG. 3, it is important that there is a difference between θ _MIN and θ _CRITICAL . An angle difference corresponding to this deviation is denoted by δ. δ is expressed by equation (5) from equations (3) and (4).

When δ represented by Expression (5) is large, the reflected light from the reflective dots 44 is difficult to enter the exit surface 41 at an angle smaller than θ _CRITICAL . Conversely, if δ is small, the reflected light from the reflective dots 44 is likely to be emitted from the emission surface 41.

  If the plane P1 is a real plane and functions as an incident surface (that is, the incident surface 43 is flat), α = 0. On the other hand, in the light guide plate 40 of the present embodiment, since the incident surface 43 is concave, α is larger than 0. Accordingly, in the light guide plate 40 having the concave incident surface 43, δ is small. As a result, the light guide plate 40 can emit light efficiently from the emission surface 41.

However, when δ is equal to or smaller than 0, that is, θ _MIN is equal to or smaller than θ _CRITICAL , the _light is incident on the light exit surface 41 at an angle smaller than the critical angle and is emitted from the light exit surface 41. Therefore, more light is emitted on the incident surface 43 side, and there may be a case where planar light cannot be generated.

  Therefore, in order to generate planar light, it is necessary that δ> 0. Therefore, by setting α to the shape of the incident surface 43 that satisfies Expression (2), it is possible to efficiently extract planar light from the emission surface 41 while propagating light through the light guide plate 40. . In addition, by setting δ> 0, it is possible to generate planar light with more uniform luminance.

  In addition, when δ is small, since it is easy to be emitted from the emission surface 41 even if the number of times the light hits the reflective dots 44, it is easy to control luminance unevenness on the incident surface 43 side.

  Further, when the incident surface is flat (when α = 0) as in the conventional light guide plate, δ shown in Expression (5) also increases, and light is not easily emitted from the emission surface 41. Therefore, for example, when the refractive index n is large, the light is not emitted from the emission surface 41 unless it hits the reflection dot 44 many times. As a result, it is difficult to control luminance unevenness on the incident surface 43 side.

  On the other hand, in the light guide plate 40, even when the refractive index n is large, it is possible to reduce δ by adjusting α, specifically, by forming the incident surface 43 having a large α shape. . As a result, even in the light guide plate 40 having a large refractive index n, light can be efficiently emitted from the emission surface 41. In addition, by reducing δ, light can be extracted from the exit surface 41 even when the number of times the light hits the reflective dots 44. Therefore, even in the light guide plate 40 having a large refractive index n, the light incident plate 43 side can be improved. Brightness unevenness can be easily controlled.

  Furthermore, when the incident surface 43 is concave toward the inside of the plate, the light incident from the incident surface 43 is likely to spread, and therefore easily hits the reflective dots 44. As a result, the emission efficiency of light from the emission surface 41 is improved.

  Further, by making the incident surface 43 concave, the light propagating in the normal direction of the plane P1 is reduced as compared with the case where the incident surface 43 is flat (when it coincides with the plane P1). Therefore, it is possible to suppress light from being emitted from the side surface (side surface 45 in FIG. 1) facing the incident surface 43. As described above, since the light loss from the side surface 45 facing the incident surface 43 can be suppressed, light can be extracted from the emission surface 41 more efficiently.

  Although the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the spirit of the invention.

In the description of the light guide plate 40 illustrated in FIG. 1, it is described that the refractive index of the light guide plate 40 is uniform in the light guide plate 40, but for example, the light guide plate emits light like the light guide plate 60 illustrated in FIG. 4. The refractive index of the surface layer portion 61 that is a region having a certain depth from the surface 41 may be different from the surface layer portion on the back surface 42 side in the thickness direction. Hereinafter, in the light guide plate 60, the portion of the back surface 42 from the surface layer portion 61 is referred to as a main layer portion 62 for the sake of explanation. Further, the refractive index of the surface layer portion 61 and _{n 1,} the refractive index of the main layer 62 and _{n 2.} When the refractive indexes of the surface layer portion 61 and the main layer portion 62 are different, n ₁ is usually larger than n ₂ . FIG. 4 is a schematic diagram showing a schematic configuration of another embodiment of the light guide plate according to the present invention. In FIG. 4, the exit surface 41 side of the light guide plate 60 is shown enlarged. Since the structure of the light guide plate 60 is the same as that of the light guide plate 40 except that the light guide plate 60 has a two-layer structure, the same components are denoted by the same reference numerals, and redundant description is omitted. FIG. 4 schematically shows a light path when it is emitted from the light source 50 and enters the incident surface 43 at the maximum incident angle.

臨界角であるθ_CRITCALは、ｓｉｎ^−１（１／ｎ_１）である。従って、式（５）を導出した場合と同様に、δ＝θ_MIN−θ_CRITICAL＞０という条件から、αの満たす条件は以下の通りである。

As shown in FIG. 4, when the light incident from the incident surface 43 propagates through the light guide plate 60 while satisfying the total reflection condition, θ _MIN which is the minimum value of the incident angle of the light to the output surface 41 Is expressed by the following equation.

The critical angle θ _CRITCAL is sin ⁻¹ (1 / n ₁ ). Therefore, as in the case where the equation (5) is derived, the condition that α satisfies from the condition that δ = θ _MIN −θ _CRITICAL > 0 is as follows.

That is, in the light guide plate 60, the incident surface 43 having a triangular cross section is formed so that the angle α between the region (end region) on the end 43b, 43b side and the plane P1 satisfies the formula (7). It can be a surface. Here, the case where the refractive index of the surface layer portion is different from the refractive index on the back surface side from the surface layer portion in the thickness direction of the light guide plate 60 has been described as an example of the light guide plate 60 having a different form with respect to the light guide plate 40. . However, in formula (7), when n ₁ = n ₂ , formula (7) matches formula (5). Therefore, the configuration shown in FIG. 1 corresponds to the case where the refractive index of the surface layer portion and that of the main layer portion are equal in the configuration shown in FIG.

  The incident surface 43 has a concave shape on the inner side of the plate with respect to the virtual plane P <b> 1, and the first angle α <b> 1 between the end of the incident surface 43 on the exit surface 41 side and the plane P <b> 1 and the incident surface 43. As long as at least one of the second angles α2 between the end portion on the back surface side and the plane P1 satisfies the condition that the angle α satisfies, the shape may be satisfied. Therefore, for example, the vicinity of the top may be curved.

  Moreover, in the description so far, in order to radiate | emit light from the output surface 41, the form in which the some reflective dot 44 was formed in the back surface 42 was illustrated. However, the back surface 42 may be inclined with respect to the exit surface 41 without providing the reflective dots 44. Even in this case, since the emission surface 41 and the back surface 42 are not substantially parallel as shown in FIGS. 1 and 2, reflection under conditions other than total reflection may occur. Furthermore, a back surface 42 on which a predetermined diffusion pattern is formed may be employed.

  DESCRIPTION OF SYMBOLS 10 ... Transmission-type image display apparatus, 20 ... Transmission-type image display part, 30 ... Surface light source device, 40 ... Light guide plate, 41 ... Output surface, 41a ... End part (end part by the side of the entrance surface of an output surface), 42 ... Back surface, 42b ... end portion (end portion on the incident surface side of the back surface), 43 ... incident surface, 43a ... top portion of the incident surface, 43b ... end portion of the incident surface, 44 ... reflective dot, 50 ... light source, 60 ... light guide plate , P1... Plane (first virtual plane), P2... Plane (second virtual plane).

Claims (7)

A light guide plate that emits light from one surface while guiding light incident from a light source,
An exit surface for emitting light;
A back surface facing the exit surface;
An incident surface that intersects the exit surface and the back surface, and on which light from the light source is incident,
With
The incident surface has a concave shape that is dented toward the inside of the plate with respect to a first virtual plane that connects the end portions on the incident surface side of each of the emission surface and the back surface,
The angle formed by at least one of the regions on the exit surface side and the back end side of the entrance surface and the first virtual plane is α, and the refraction of the surface layer portion on the exit surface side of the light guide plate when the rate as n _1, the refractive index of the back side of the surface layer portion in the thickness direction of the light guide plate was n _2,
Α is an angle satisfying the formula (1).
Light guide plate.

The light guide plate according to claim ₁ , wherein the n ₁ and the n ₂ are equal. The shape of the cross section of the incident surface that is parallel to the plate thickness direction and orthogonal to the first imaginary plane is triangular.
The light guide plate according to claim 1 or 2. The incident surface is located at the center of the exit surface and the back surface in the plate thickness direction and is symmetric with respect to a second virtual plane orthogonal to the plate thickness direction.
The light-guide plate as described in any one of Claims 1-3. Provided on the back surface, and having a plurality of reflective dots that reflect the light propagating to the back surface side,
The light-guide plate as described in any one of Claims 1-4. The light guide plate according to any one of claims 1 to 5,
A light source for supplying light to an incident surface of the light guide plate;
A surface light source device comprising: The light guide plate according to any one of claims 1 to 5,
A light source for supplying light to the incident surface of the light guide plate;
A transmissive image display unit illuminated by light emitted from the exit surface of the light guide plate;
Comprising
Transmission type image display device.

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