JP6368998B2 - Light guide plate - Google Patents

Description

  The present invention relates to a light guide plate, and more particularly to a light guide plate suitable for an edge light type surface light emitting device.

  Conventionally, a liquid crystal display device is used for a liquid crystal television or the like. The liquid crystal display device includes a surface light emitting device and a liquid crystal panel arranged on the light emitting surface side of the surface light emitting device. As the surface light emitting device, for example, a direct type and an edge light type are known.

  In the direct type surface light emitting device, the light source is disposed on the back surface opposite to the light emitting surface. When a point light source such as a light emitting diode (Light Emitting Diode) is used as the light source, a large number of LED chips are required to compensate for the brightness, and the variation in luminance characteristics becomes very large.

  For this reason, at present, edge light type surface light emitting devices have become mainstream. The edge light type surface light emitting device includes a light source such as an LED, a light guide plate, a reflection plate (or reflection film), and the like. A light source is arrange | positioned at the side surface which becomes a orthogonal direction with respect to a light-projection surface. The light guide plate is arranged to propagate light from the light source to the inside by total reflection and to emit the light in a planar shape. A resin plate such as an acrylic resin is generally used as the light guide plate (see Patent Documents 1 to 4). The reflecting plate is disposed on the light reflecting surface opposite to the light emitting surface, and is disposed to reflect light passing through the light reflecting surface to emit light on a display surface such as a liquid crystal panel. In addition, in order to emit light uniformly on a display surface such as a liquid crystal panel, a diffusion plate (diffusion film) may be disposed on the light exit surface side of the light guide plate.

  FIG. 1 is a conceptual cross-sectional view showing an example of an edge light type surface light emitting device 1. The edge light type surface light emitting device 1 includes a light source 2 such as an LED, a light guide plate 3, a reflection plate 4, and a diffusion plate 5. Light from the light source 2 enters from the end face of the light guide plate 3 and propagates into the light guide plate 3. The light that reaches the light reflecting surface 6 is reflected by the reflecting plate 4, travels toward the light emitting surface 7, and is diffused by the diffusion plate 5. As a result, a display surface such as a liquid crystal panel disposed above the diffusion plate 5 can emit light uniformly.

JP 2012-123933 A JP 2012-138345 A JP 2012-216523 A JP 2012-216528 A

In the edge light type surface emitting device, when light is generated from the light source, heat is generated, and accordingly, the temperature of the light guide plate also increases. When a resin plate is used as the light guide plate, the dimensional change due to heat of the light guide plate is larger than the dimensional change of the liquid crystal panel. This is due to the high thermal expansion coefficient of the resin plate. For example, the thermal expansion coefficient of the acrylic resin plate is about 700 × 10 ⁻⁷ / ° C. For this reason, until now, a dimensional change of the light guide plate has been corrected by providing a gap in the frame portion of the liquid crystal display device so that an undue stress is not generated due to a difference in dimensional change.

  However, in recent years, due to the narrow frame of the liquid crystal display device, it is difficult to correct the dimensional change of the light guide plate at the frame portion of the liquid crystal display device.

  The liquid crystal panel uses polarized light. Further, the edge light type surface light emitting device has a different distance from the light source on the light emitting surface. Therefore, in recent years, with an increase in the size of the liquid crystal display device, the polarization state is different within the panel surface, and uneven luminance characteristics are likely to occur.

  Therefore, the present invention has been made in view of the above circumstances, and its technical problem is to create a light guide plate that is less likely to undergo dimensional changes with increasing temperature and that can uniformize the luminance characteristics of a display device. It is.

  As a result of intensive studies, the present inventor can solve the above technical problem by adopting a glass plate having a small dimensional change due to a temperature change as a light guide plate and restricting the retardation of the glass plate to a predetermined range. It is discovered and proposed as the present invention. That is, the light guide plate of the present invention has at least a glass plate, and the retardation of the glass plate at an optical path length of 50 mm is 30 nm or less. The “retardation at an optical path length of 50 mm” can be measured by a commercially available birefringence measuring apparatus, and can be measured by, for example, an optical heterodyne method using PEL-3A-XR manufactured by UNIOPT.

  A display panel such as a liquid crystal panel has a structure in which a display element such as a liquid crystal element is sandwiched between a pair of glass plates. Therefore, when a glass plate is adopted as the light guide plate, a difference in dimensional change between the display panel and the light guide plate is reduced, and it is possible to appropriately cope with a narrow frame of a display device such as a liquid crystal display device.

  The inventor has found that the retardation of the glass plate at an optical path length of 50 mm affects the luminance characteristics of the display device. Therefore, in the present invention, the retardation of the glass plate at an optical path length of 50 mm is regulated to 30 nm or less to achieve uniform luminance characteristics of the display device.

  In the light guide plate of the present invention, the size of at least one side of the glass plate is preferably 1000 mm or more. In this way, it is possible to satisfy the demand for an increase in the size of the display device.

  In the light guide plate of the present invention, the surface roughness Ra of the end surface of the glass plate is preferably 2 μm or less. In this way, light from the light source can be uniformly incident on the light guide plate. Here, “surface roughness Ra” refers to a value measured by a method in accordance with JIS B0601: 2001, with an evaluation length of 8 mm, a cutoff value λc = 0.8 mm, and a cutoff ratio λc / λs = 100. Refers to the value measured under conditions.

In the light guide plate of the present invention, the glass plate preferably has a thermal expansion coefficient of 120 × 10 ⁻⁷ / ° C. or less. Here, “thermal expansion coefficient” refers to a value obtained by measuring an average thermal expansion coefficient at 30 to 380 ° C. based on JIS R3102 using a dilatometer.

  In the light guide plate of the present invention, the strain point of the glass plate is preferably 550 ° C. or higher. If it does in this way, the heat resistance of a light-guide plate will improve. Here, “strain point” refers to a value measured based on JIS R3103.

In the light guide plate of the present invention, the glass plate has a glass composition of mass%, SiO ₂ 40 to 70%, Al ₂ O ₃ 2 to 25%, B ₂ O ₃ 0 to 20%, R ₂ O (R is Li, Na, or one or more of K) 0-25%, MgO 0-10%, CaO 0-15%, SrO 0-10%, BaO 0-15%, ZnO 0-10%, ZrO ₂ 0 It is preferable to contain -10%. In this way, it is possible to achieve both a low thermal expansion coefficient and a high strain point.

  The light guide plate of the present invention is preferably formed by molding a glass plate by an overflow down draw method. Here, the “overflow down-draw method” is a method in which molten glass overflows from both sides of a heat-resistant bowl-shaped molded body, and the molten glass overflows and joins at the lower end of the molded body to be stretched and formed downward. It is a method of manufacturing.

  The light guide plate of the present invention is used for an edge light type surface light emitting device.

  An edge light type surface light emitting device of the present invention includes the light guide plate described above.

  The glass plate of the present invention has a retardation at an optical path length of 50 mm of 20 nm or less, and is used for a light guide plate.

It is a section conceptual diagram showing an example of an edge light type surface emitting device.

  In the light guide plate of the present invention, the retardation of the glass plate at an optical path length of 50 mm is 30 nm or less, preferably 25 nm or less, 20 nm or less, particularly 0.1 to 17.5 nm or less. If the retardation is too large, it is difficult to make the luminance characteristics of the display device uniform.

  In order to reduce the retardation of the glass plate, for example, when molten glass is formed into a glass ribbon in a molding furnace (molded body), the thickness of the end of the glass ribbon is substantially the same as the thickness of the central portion of the glass ribbon. The glass ribbon may be cooled so that the temperature distribution in the width direction of the glass ribbon is as small as possible when the glass ribbon is slowly cooled (cooled) in a slow cooling furnace.

  In the molding process, the reason for forming the glass ribbon so that the thickness of the end of the glass ribbon is substantially the same as the thickness of the center of the glass ribbon is that the thickness of the end of the glass ribbon is different from the thickness of the center of the glass ribbon. In the cooling process after molding, the cooling rate is different between the end and the center of the glass ribbon, and as a result, the retardation increases. For example, when the rotational speed of a forming roll or the like for drawing molten glass into a glass ribbon is adjusted, the thickness of the end portion of the glass ribbon and the thickness of the center portion of the glass ribbon are easily made uniform.

Moreover, in the cooling process in a slow cooling furnace, the following method is mentioned as a method of making temperature distribution in the width direction of a glass ribbon as small as possible.
(1) The number of heaters is increased so that the glass ribbon is uniformly heated, and the temperature difference between the heaters is reduced as much as possible. For example, the temperature difference between the heaters is regulated within ± 1 ° C.
(2) A soaking plate is installed between the heater and the glass ribbon so that the heat from the heater is uniformly transmitted to the glass ribbon.
(3) An enclosure is installed at the end of the glass ribbon or a large number of heaters are arranged at the end so that the difference in the cooling rate between the center and the end of the glass ribbon is reduced.
(4) Lower (slow) the drawing speed.

  Unlike the float method, the overflow downdraw method always increases the low temperature air flow along the surface of the glass ribbon in the direction from the cutting process, which is a low temperature atmosphere, to the slow cooling furnace and the forming furnace, which is a high temperature atmosphere. After the air flow is heated inside a slow cooling furnace or the like, a part of the air leaks into the external atmosphere through a gap in the peripheral wall portion, so that the atmospheric temperature of the slow cooling furnace or the molding furnace is likely to fluctuate. As a result, the glass plate formed by the overflow downdraw method tends to have a large retardation. Therefore, when forming a glass plate by the overflow down draw method, in addition to making the end portions and the center portion of the glass ribbon substantially the same thickness, and reducing the temperature distribution in the width direction of the glass ribbon, a slow cooling furnace It is preferable to suppress an increase in the low-temperature air flow in the molding furnace.

  In order to suppress the rise in the low-temperature air flow in the slow cooling furnace or the forming furnace, for example, a convection prevention plate is provided in the slow cooling furnace or the air pressure in the external atmosphere of the forming furnace or the slow cooling furnace is adjusted using a blower or the like. The air in the molding furnace or the slow cooling furnace may be made difficult to leak into the external atmosphere.

In addition to the above method, increase the content of SiO ₂ , Al ₂ O ₃ , B ₂ O _{3 in} the glass composition to decrease the thermal expansion coefficient or increase the content of alkaline earth metal oxides If the photoelastic constant is lowered, the retardation of the glass plate tends to be lowered.

  In the light guide plate of the present invention, the dimension of at least one side of the glass plate is preferably 1000 mm or more, 1500 mm or more, 2000 mm or more, 2500 mm or more, particularly 3000 mm or more. In this way, it is possible to satisfy the demand for an increase in the size of the display device.

  The surface roughness Ra of the end face of the glass plate is preferably 2 μm or less, 1.5 μm or less, 1 μm or less, 0.7 μm or less, particularly 0.5 μm or less. If it does in this way, it will become easy to scatter the light from a light source on the end surface of a glass plate, and it will become difficult to make the light from a light source inject into a light-guide plate uniformly.

The thermal expansion coefficient of the glass plate is preferably 120 × 10 ⁻⁷ / ° C. or lower, 90 × 10 ⁻⁷ / ° C. or lower, 60 × 10 ⁻⁷ / ° C. or lower, 55 × 10 ⁻⁷ / ° C. or lower, 50 × 10 ^{− 7} / ° C. or lower, 45 × 10 ⁻⁷ / ° C. or lower, particularly 25 × 10 ⁻⁷ to 40 × 10 ⁻⁷ / ° C. or lower. If the thermal expansion coefficient is too high, the difference in dimensional change due to heat between the display panel and the light guide plate becomes large.

  The strain point of the glass plate is preferably 550 ° C. or higher, 580 ° C. or higher, 600 ° C. or higher, 615 ° C. or higher, 630 ° C. or higher, 640 ° C. or higher, particularly 650 ° C. or higher. If the strain point is too low, the heat resistance of the glass plate is likely to be lowered. For example, when a reflective film, a diffusion film, or the like is formed on the surface of the glass plate at a high temperature, the glass plate is likely to be thermally deformed.

Glass plate, as a glass composition, in _{mass%, SiO 2 40~70%, Al} 2 O 3 2~25%, B 2 O 3 0~20%, one _R 2 O (R is Li, Na, K Or 2 or more) 0 to 25%, MgO 0 to 10%, CaO 0 to 15%, SrO 0 to 10%, BaO 0 to 15%, ZnO 0 to 10%, ZrO ₂ 0 to 10% Is preferred. The reason why the content of each component is regulated as described above will be described below. In addition, in description of the containing range of each component,% display means the mass%.

SiO ₂ is a component that becomes a glass network former, and is a component that lowers the thermal expansion coefficient to reduce dimensional change and retardation due to heat. It is a component that increases acid resistance and strain point. The content of SiO ₂ is preferably 40 to 70%, 50 to 67%, particularly 57 to 64%. When the content of SiO ₂ is increased, the high temperature viscosity is increased, the meltability is lowered, and the devitrification blisters of cristobalite are liable to precipitate at the time of molding. On the other hand, when the content of SiO ₂ decreases, the thermal expansion coefficient increases, and the dimensional change and retardation due to heat tend to increase. In addition, acid resistance and strain point are likely to be lowered.

Al ₂ O ₃ is a component that lowers the coefficient of thermal expansion and reduces dimensional changes and retardation due to heat. It also has the effect of increasing the strain point and suppressing the precipitation of devitrified cristobalite during molding. The content of Al ₂ O ₃ is preferably 2 to 25%, 10 to 20%, particularly 14 to 17%. When the content of Al ₂ O ₃ increases, the liquidus temperature rises and it becomes difficult to form a glass plate. On the other hand, when the content of Al ₂ O ₃ decreases, the thermal expansion coefficient increases, and the dimensional change and retardation due to heat tend to increase. In addition, the strain point tends to decrease.

B ₂ O ₃ is a component that acts as a flux, lowers the high temperature viscosity, and improves the meltability. Moreover, it is a component which reduces a thermal expansion coefficient and reduces the dimensional change and retardation by a heat | fever. The content of B ₂ O ₃ is preferably 0 to 20%, 5 to 15%, particularly 7.5 to 12%. When the content of B ₂ O ₃ is increased, the strain point and acid resistance are likely to be lowered. On the other hand, when the content of B ₂ O ₃ decreases, the thermal expansion coefficient increases, and the dimensional change and retardation due to heat tend to increase. In addition, the meltability tends to be lowered.

R ₂ O is a component that lowers the high temperature viscosity and improves the meltability. The content of R ₂ O is preferably 0 to 25%, 0 to 20%, particularly 0 to 15%. As the R ₂ O content increases, the strain point tends to decrease. From the viewpoint of reducing the thermal expansion coefficient, it is preferable to reduce the content of R ₂ O as much as possible, and the content is preferably 5% or less, 1% or less, and particularly preferably 0.5% or less. The contents of Li ₂ O, Na ₂ O, and K ₂ O are also preferably 5% or less, 1% or less, particularly 0.5% or less, respectively.

  MgO is a component that improves the meltability by lowering only the high temperature viscosity without lowering the strain point. It is also a component that lowers the photoelastic constant. The content of MgO is preferably 0 to 10%, 0 to 5%, particularly 0 to 3.5%. When the content of MgO is increased, devitrification beads are likely to precipitate during molding.

  CaO is a component that improves the meltability by lowering only the high temperature viscosity without lowering the strain point. It is also a component that lowers the photoelastic constant. The content of CaO is preferably 0 to 15%, 2 to 12%, particularly 3.5 to 10%. When there is too much content of CaO, devitrification will become easy to precipitate at the time of fabrication.

  SrO is a component that improves chemical resistance and devitrification resistance. It is also a component that lowers the photoelastic constant. The content of SrO is preferably 0 to 10%, more than 0.5 to 8%, particularly 1 to 8%. When the SrO content is increased, the coefficient of thermal expansion is increased, and dimensional change and retardation due to heat are likely to increase.

  BaO is a component that improves chemical resistance and devitrification resistance in the same manner as SrO. It is also a component that lowers the photoelastic constant. The content of BaO is preferably 0 to 15%, 0 to 10%, particularly 0.1 to 8%. When the content of BaO increases, the density increases and the thermal expansion coefficient increases, and the dimensional change and retardation due to heat tend to increase. In addition, the meltability tends to be lowered.

  ZnO is a component that improves meltability. The content of ZnO is preferably 0 to 10%, 0 to 5%, particularly 0 to 1%. When the content of ZnO is increased, the devitrification resistance and the strain point are liable to be lowered.

ZrO ₂ is a component that increases the strain point. The content of ZrO ₂ is preferably 0 to 10%, 0 to 7%, particularly 0 to 5%. When the ZrO ₂ content is increased, the density is remarkably increased, and devitrification spots caused by ZrO ₂ are liable to precipitate during molding.

In addition to the above components, other components may be introduced. For example, in order to lower the liquidus temperature, Y ₂ O ₃ , La ₂ O ₃ , Nb ₂ O ₅ , P ₂ O ₅ are each up to 3%, As ₂ O ₃ , Sb ₂ O ₃ , SnO as refining agents ₂ , SO ₃ , F, Cl or the like may be introduced up to 2% in total. However, As ₂ O ₃ and Sb ₂ O ₃ are environmentally hazardous substances, and when a glass plate is formed by the float process, it is reduced in the float bath to become a metal foreign object, so avoid substantial introduction. More specifically, the content is preferably less than 0.01%.

  In the light guide plate of the present invention, the glass plate is preferably formed by an overflow down draw method. In this way, it is difficult to produce a temperature difference and composition difference between the front and back surfaces of the glass ribbon during molding, and it becomes easy to form a glass plate that is unpolished and has a good surface quality. And uniform brightness characteristics. The reason for this is that, in the case of the overflow downdraw method, the surface to be the surface does not come into contact with the bowl-like refractory and is molded in a free surface state. The structure and material of the bowl-shaped structure are not particularly limited as long as desired dimensions and surface quality can be realized. Moreover, in order to perform the downward extending | stretching shaping | molding, the method of applying force with respect to a glass ribbon will not be specifically limited if a desired dimension and surface quality are realizable. For example, a method may be adopted in which a heat-resistant roll having a sufficiently large width is rotated and stretched in contact with the glass ribbon, or a plurality of pairs of heat-resistant rolls are only near the end face of the glass ribbon. You may employ | adopt the method of making it contact and extending | stretching.

  In addition to the overflow downdraw method, the glass plate can be formed by a slot downdraw method, a float method, a rollout method, a redraw method, or the like. In the float process, a temperature difference and a composition difference between the front and back surfaces of the glass ribbon are likely to occur during molding. However, if the temperature control during the molding is strictly performed, the temperature difference and the composition difference can be reduced.

  The light guide plate of the present invention preferably includes a reflection film on one surface (light reflection surface) side, and preferably includes a diffusion film on the other surface (light emission surface) side. In this way, it becomes easy to make the luminance characteristics of the display device uniform.

  An edge light type surface light emitting device of the present invention includes the light guide plate described above. In addition, the edge light type surface light emitting device of the present invention preferably includes a reflecting plate on one surface (light reflecting surface) side of the light guide plate, and diffuses on the other surface (light emitting surface) side of the light guiding plate. It is preferable to provide a plate. In this way, it becomes easy to make the luminance characteristics of the display device uniform.

  The glass plate of the present invention has a retardation at an optical path length of 50 mm of 30 nm or less, and is used for a light guide plate. Here, the technical features (preferable characteristics, effects, etc.) of the glass plate of the present invention are the same as the technical features of the light guide plate of the present invention. Therefore, detailed description is abbreviate | omitted about the glass plate of this invention.

  The glass plate of the present invention can also be applied to a glass plate used for a display panel to have the function of a light guide plate. In this way, the member configuration of the display device can be simplified.

  Hereinafter, based on an Example, this invention is demonstrated in detail. The following examples are merely illustrative. The present invention is not limited to the following examples.

Example 1
First, the glass composition is glass so that it contains 60% SiO ₂ , 15% Al ₂ O ₃ , 10% B ₂ O ₃ , 1% MgO, 8% CaO, 5% SrO, and 1% BaO. After mixing and mixing the raw materials, the raw glass was melted at a maximum temperature of 1650 ° C. in a continuous melting furnace to obtain molten glass. Next, the obtained molten glass was formed into a plate shape by the overflow down draw method, and then slowly cooled, then cut into dimensions of 2200 mm × 1950 mm × thickness 1.1 mm, and the surface roughness Ra of the end face was 0.5 μm. The glass plate was obtained by polishing. During molding and slow cooling, the temperature distribution between each heater is controlled within ± 1 ° C, and the atmospheric pressure in the external atmosphere of the molding furnace and slow cooling furnace is controlled to suppress the generation of ascending airflow. did.

From the obtained glass plate, measurement samples of thermal expansion coefficient and strain point were prepared, and each measurement was performed. As a result, the strain point was 650 ° C. and the thermal expansion coefficient was 38 × 10 ⁻⁷ / ° C. In addition, a thermal expansion coefficient is the value which measured the average thermal expansion coefficient in 30-380 degreeC based on JISR3102 using the dilatometer. The strain point is a value measured based on JIS R3103.

  The obtained glass plate was further cut into dimensions of 50 mm × 50 mm × thickness 1.1 mm, and the opposite end surfaces were mirror-polished. Next, retardation at an optical path length of 50 mm was measured by an optical heterodyne method using PEL-3A-XR manufactured by UNIOPTO. During the measurement, laser light was irradiated perpendicularly to the optically polished end face. As a result, the retardation was 17.3 nm.

  From the above results, it is considered that the light guide plate having this glass plate is less likely to undergo dimensional changes as the temperature rises and can make the luminance characteristics of the display device uniform.

(Example 2)
First, as a glass composition, it contains SiO ₂ 62%, Al ₂ O ₃ 18%, B ₂ O ₃ 0.5%, MgO 3%, Na ₂ O 14.5%, K ₂ O 2% by mass%. Thus, after glass material was prepared and mixed, it was melted at a maximum temperature of 1650 ° C. in a continuous melting furnace to obtain molten glass. Next, the obtained molten glass is formed into a plate shape by the overflow down draw method, and then slowly cooled, then cut into dimensions of 1800 mm × 1500 mm × thickness 1.1 mm, and the surface roughness Ra of the end face is 0.5 μm. The glass plate was obtained by polishing. During molding and slow cooling, the temperature distribution between each heater is controlled within ± 1 ° C, and the atmospheric pressure in the external atmosphere of the molding furnace and slow cooling furnace is controlled to suppress the generation of ascending airflow. did.

From the obtained glass plate, measurement samples of thermal expansion coefficient and strain point were prepared, and each measurement was performed. As a result, the strain point was 560 ° C. and the thermal expansion coefficient was 91 × 10 ⁻⁷ / ° C. In addition, a thermal expansion coefficient is the value which measured the average thermal expansion coefficient in 30-380 degreeC based on JISR3102 using the dilatometer. The strain point is a value measured based on JIS R3103.

  The obtained glass plate was further cut into dimensions of 50 mm × 50 mm × thickness 1.1 mm, and the opposite end surfaces were mirror-polished. Next, retardation at an optical path length of 50 mm was measured by an optical heterodyne method using PEL-3A-XR manufactured by UNIOPTO. During the measurement, laser light was irradiated perpendicularly to the optically polished end face. As a result, the retardation was 18 nm.

  From the above results, it is considered that the light guide plate having this glass plate is less likely to undergo dimensional changes as the temperature rises and can make the luminance characteristics of the display device uniform.

(Comparative example)
First, the glass composition is glass so that it contains 60% SiO ₂ , 15% Al ₂ O ₃ , 10% B ₂ O ₃ , 1% MgO, 8% CaO, 5% SrO, and 1% BaO. After mixing and mixing the raw materials, the raw glass was melted at a maximum temperature of 1650 ° C. in a continuous melting furnace to obtain molten glass. Next, the obtained molten glass was formed into a plate shape by the overflow down draw method, and then slowly cooled, then cut into dimensions of 2200 mm × 1950 mm × thickness 1.1 mm, and the surface roughness Ra of the end face was 0.5 μm. The glass plate was obtained by polishing. During molding and slow cooling, the temperature distribution between the heaters was not strictly controlled, and the ascending air current was not suppressed.

  The obtained glass plate was further cut into dimensions of 50 mm × 50 mm × thickness 1.1 mm, and the opposite end surfaces were mirror-polished. Next, retardation at an optical path length of 50 mm was measured by an optical heterodyne method using PEL-3A-XR manufactured by UNIOPTO. During the measurement, laser light was irradiated perpendicularly to the optically polished end face. As a result, the retardation was 39.4 nm.

  From the above results, it is considered that the light guide plate having this glass plate is likely to undergo dimensional changes as the temperature rises and the luminance characteristics of the display device are likely to be nonuniform.

DESCRIPTION OF SYMBOLS 1 Edge light type surface emitting device 2 Light source 3 Light guide plate 4 Reflection plate 5 Diffusing plate 6 Light reflection surface 7 Light emission surface

Claims (9)

While having at least a glass plate, the glass plate contains SiO ₂ 40 to 70%, Al ₂ O ₃ 2 to 25%, B ₂ O ₃ 0.5 to 20% by mass% as a glass composition , and the glass plate A light guide plate having a retardation at an optical path length of 50 mm of 25 nm or less and a reflective film or a reflective plate provided on one surface side of the glass plate.   The light guide plate according to claim 1, wherein a dimension of at least one side of the glass plate is 1000 mm or more.   The light guide plate according to claim 1 or 2, wherein the surface roughness Ra of the end face of the glass plate is 2 µm or less. The light guide plate according to claim 1, wherein the glass plate has a thermal expansion coefficient of 120 × 10 ⁻⁷ / ° C. or less.   The light guide plate according to any one of claims 1 to 4, wherein the strain point of the glass plate is 550 ° C or higher. Glass plate, as a glass composition, in _{mass%, SiO 2 40~70%, Al} 2 O 3 2~25%, B 2 O 3 0.5~20%, R 2 O (R is Li, Na, K one or two or more) 0% to 25% of, 0~10% MgO, CaO 0~15% , SrO 0~10%, BaO 0~15%, 0~10% ZnO, containing ZrO 2 0% The light guide plate according to any one of claims 1 to 5, wherein:   The light guide plate according to claim 1, wherein the glass plate is formed by an overflow downdraw method.   The light guide plate according to claim 1, wherein the light guide plate is used for an edge light type surface light emitting device.   An edge light type surface light emitting device comprising the light guide plate according to claim 1.