JP5097704B2 - Surface light source device - Google Patents

Description

  The present invention relates to a surface light source device.

  As a method of a surface light source device used for a liquid crystal display device or the like, there are an edge light method in which a light source is attached to an edge portion of a light guide plate, and a direct type method in which a light source is disposed directly under a diffusion plate and light is diffused by the diffusion plate. . In the liquid crystal display device having a relatively small screen size, it is a mainstream to adopt the edge light method.

  In the edge light method, light emitted from a light source attached to the edge portion of the light guide plate enters from the edge portion of the light guide plate, and light incident on the light guide plate surface at a critical angle or more is from the exit surface of the light guide plate. Exit. On the other hand, most of the light traveling at an angle less than the critical angle with respect to the surface of the light guide plate is repeatedly reflected on the surface and travels in the light guide plate, and hardly emits from the light exit surface of the light guide plate. In order to emit light traveling in the light guide plate from the light guide plate exit surface, a light scattering agent is added to the light guide plate and a light diffusion layer is applied to the back surface of the light guide plate exit surface. .

  In a liquid crystal display device, the center of the screen is often watched more than the screen edge. For this reason, it is necessary to make the luminance at the center of the screen equal to or higher than the luminance at the edge of the screen.

  Of the light that enters the light guide plate from the light source, the amount of light that enters the light guide plate surface at a critical angle or more, that is, the amount of light that exits from the exit surface, decreases as the distance from the light source increases, and the screen starts from the edge of the screen. There is a problem that the brightness of the central portion is lowered. As a solution, it is known to provide a discontinuous light diffusion layer on the back surface of the emission surface. Specifically, it is common to control the luminance to a desired distribution by using a so-called gradation pattern in which the area ratio per unit area of the light diffusion layer continuously increases as the distance from the light source increases. Yes. As a method for providing this light diffusion layer, various printing methods typified by a screen printing method, an injection molding method using a stamper, a shape transfer method typified by a press molding method, or a sheet provided with a light diffusion layer in advance. For example, a method of pasting the objects. Among these, various printing methods typified by screen printing on the back surface of the exit surface of the light guide plate are often used from the viewpoint of mass productivity and economy.

  Liquid crystal display devices have strong demands for high-definition display of images, such as support for digital high-definition. In order to display an image with high definition, it is necessary to reduce the aperture of the liquid crystal panel. However, when the aperture is lowered, the light transmittance of the liquid crystal panel is lowered and the screen becomes dark. Therefore, a surface light source device with higher brightness is required.

In order to achieve high brightness, a method has been proposed in which a light scattering agent is dispersed in a light guide plate to increase the emission of light incident on the light guide plate as much as possible (see Patent Document 1).

In this method, although the luminance of the surface light source device is improved by dispersing the light scattering agent in the light guide plate, the level is not satisfactory, and further higher luminance is required.
JP 2002-148443 A

  An object of the present invention is to provide a high-luminance surface light source device suitable for a liquid crystal display device in view of the high-luminance requirements as described above.

  As a result of intensive studies to solve the above problems, the present inventors have found that a high-luminance surface light source device suitable for a liquid crystal display device can be provided.

  The present invention includes a light guide plate having a light exit surface and one or more light incident surfaces, a light source for supplying light to the incident surface, and light formed on the back surface of the light exit surface of the light guide plate. A surface light source device comprising a diffusion layer and a reflector disposed on the back surface side, wherein the light diffusion layer has an area of the light diffusion layer that occupies per unit area of the back surface as the distance from the light source increases. The surface light source device is dispersed so that the ratio increases, the ten-point average roughness of the surface of the light diffusion layer is 8 μm to 25 μm, and the average interval of the irregularities on the surface of the light diffusion layer is 20 μm to 150 μm.

  ADVANTAGE OF THE INVENTION According to this invention, the high-intensity surface light source device suitable for liquid crystal display devices can be provided.

It is the schematic which shows an example of a surface light source device.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Light guide plate 2, 2 'Cold cathode tube 3 Reflector 5, 5' Lamp reflector 6, 6 'Diffusion film 7 Prism sheet 8 Light diffusion layer

  Although the preferable aspect of this invention is demonstrated concretely below, this invention is not limited only to these aspects.

  The light guide plate used in the present invention has one or more light incident surfaces.

  Examples of the substrate constituting the light guide plate include transparent organic materials such as methacrylic resin, polycarbonate resin, styrene resin, cyclic olefin resin, and amorphous polyester resin, and transparent inorganic materials such as inorganic glass. Among these, methacrylic resin, polycarbonate resin, and cyclic olefin resin are preferable, and methacrylic resin is more preferable.

  The method for forming the light guide plate is not particularly limited, and a known method can be used. For example, after adding various additives as necessary to the syrup containing the raw material monomer of the organic transparent material as a base material or a partial polymer of the raw material monomer, cast polymerization to obtain a sheet molded body, Thereafter, the sheet is cut into a predetermined size and the cut surface is polished, or a resin composition comprising various additives added as necessary and the resin constituting the substrate is used as a sheet extrusion molding machine or press molding machine. And a method of obtaining a sheet molded body, then cutting it into a predetermined size, and polishing the cut surface. In that case, known additives such as a release agent for improving the releasability between the substrate and the mold and an ultraviolet absorber for delaying the deterioration due to ultraviolet rays can be added.

  As the shape of the light guide plate, there are a flat plate type having a constant plate thickness and a wedge shape in which the plate thickness decreases as the distance from the light source increases, but the flat plate type is preferable.

  Moreover, the light-scattering agent which has a refractive index different from the refractive index of the base material which comprises a light-guide plate can be added. By adding the light scattering agent to the light guide plate used in the present invention, higher brightness can be achieved.

The shape of the light scattering agent may be a true sphere, a sphere, a scaly shape, an indefinite shape, or the like, and is not particularly limited. Examples of the light scattering agent include inorganic particles such as silica, calcium carbonate, barium sulfate, titanium oxide, aluminum oxide, and aluminum hydroxide, and organic materials such as urethane beads, silicone beads, PMMA beads, MS beads, and styrene beads. Fine particles are mentioned. The number average particle diameter of these light scattering agents is preferably 0.1 μm or more and 50 μm or less. In the case of an indefinite shape, the major axis length is preferably 0.1 μm or more and 50 μm or less. If the number average particle size of the light scattering agent (long axis length in the case of an indefinite shape) is too small, the wavelength dependence of light scattering increases and the emitted light is colored. If the number average particle size of the light scattering agent is too large, glare and brightness unevenness due to scattered light may occur. As the light scattering agent in the light guide plate, the refractive index difference x between the light guide plate and the light scattering agent is in the range of 0.01 ≦ x <0.1.
The content y (ppm) of the light scattering agent is in the range of 100 ≦ y ≦ 1000,
In the range of 0.1 ≦ x <0.5, 10 ≦ y ≦ 500 range,
In the range of 0.5 ≦ x 1, the range of 1 ≦ y ≦ 10 is preferable.

  A light scattering agent having a small refractive index difference from the base material of the light guide plate increases the amount of addition, and a light scattering agent having a large refractive index difference from the base material of the light guide plate decreases the amount of addition.

  When the light scattering agent amount y corresponding to the refractive index difference x is too low, the obtained luminance is lowered. If the light scattering agent amount y corresponding to the refractive index difference x is too high, the uniformity of the emitted light and the light emission pattern can be obtained as a gradation pattern in which the area ratio of the light diffusion layer is changed between the light source vicinity and the center. Is difficult to adjust, and there is a problem that the brightness at the center of the screen is lower than the edge of the screen. “Ppm” represents a weight part per million with respect to the light guide plate containing the light scattering agent.

  A light diffusing layer is provided on the back surface of the exit surface of the light guide plate. The light diffusing layer is dispersed such that the proportion of the area of the light diffusing layer occupying per unit area of the back surface of the light exiting surface of the light guide plate increases as the distance from the light source increases. There is no restriction | limiting in particular in the dispersion | distribution shape of the said light-diffusion layer, What kind of shapes, such as dot shape, dot shape, polygonal shape, stripe shape, a grid | lattice shape, may be sufficient. The luminance can be controlled to a desired distribution by using a so-called gradation pattern in which the ratio of the area occupied by the light diffusion layer per unit area of the surface increases as the distance from the light source increases. When the ratio of the area of the light diffusion layer does not increase as the distance from the light source increases, the luminance at the center of the screen tends to be lower than that in the vicinity of the light source, which is not practically used as a surface light source device. As the shape of the surface of the diffusion layer, the ten-point average roughness (Rz) is 8 μm to 25 μm, preferably 8 μm to 15 μm. The average interval (Sm) of irregularities on the surface of the diffusion layer is 20 μm to 150 μm, preferably 40 μm to 140 μm. Here, the ten-point average roughness (Rz) and the average interval (Sm) of the unevenness are values measured in accordance with JIS B0601-1994. In addition, the average interval (Sm) of unevenness | corrugation means "the average length of a contour curve element" prescribed | regulated to JISB0601-1994.

  If the ten-point average roughness (Rz) is less than 8 μm, the angle-changing effect of the light striking the light diffusion layer out of the light traveling inside the light guide plate is reduced. The ratio of the emitted light is reduced and the luminance is lowered. On the other hand, when the ten-point average roughness (Rz) exceeds 25 μm, the surface irregularities are easily damaged, and this causes a so-called glare that locally increases the luminance.

  When the average interval (Sm) of the unevenness is less than 20 μm, the unevenness on the surface is easily damaged and local unevenness in brightness is likely to occur. When the average interval (Sm) of the unevenness exceeds 150 μm, the diffusion effect of the light hitting the light diffusion layer is reduced, so the ratio of the light emitted from the emission surface exceeding the critical angle with respect to the light guide plate surface is The brightness decreases.

  As a method for forming the light diffusion layer, various printing methods represented by a screen printing method, a shape transfer method represented by an injection molding method using a stamper or a press molding method, or a light diffusion layer previously provided. Examples include a method for bonding sheet-like materials. Among these, it is preferable to use a printing method from the viewpoint of high mass productivity and low cost. Among the printing methods, the screen printing method is preferably used from the viewpoint of ease of adjusting the area ratio of the light diffusion layer. In the screen printing method, an ink in which fine particles are dispersed in a binder resin is passed through a plate made of a mesh such as polyester or nylon, and a desired printing pattern is transferred to a light guide plate. When the average roughness (Rz) exceeds 25 μm, it is necessary to add particles having a large particle diameter to the ink. In such a case, the mesh constituting the plate may be clogged. Absent. In order to obtain stable printability without causing clogging of the mesh, it is preferable to further reduce the particle diameter of the fine particles and set the ten-point average roughness (Rz) to 15 μm or less.

In the screen printing method, the mesh constituting the plate for printing the desired light diffusion layer has an unopened portion in which the passage of the ink is restricted by the emulsion, and the ink is allowed to pass therethrough so that the desired printing pattern is transferred to the light guide plate. There is an opening for. The ratio of the area of the light diffusing layer per unit area of the back surface of the light exit surface of the light guide plate is hereinafter referred to as “area ratio”. For example, when one light diffusion layer 1 mm × 1 mm is formed in the surface of the light guide plate surface 2 mm × 2 mm, the area ratio occupied by the light diffusion layer is 25%. In addition, many light diffusion layer groups which are not continuous are collectively referred to as a “light diffusion layer”. As a method for adjusting the area ratio of the light diffusion layer, it is generally performed to change the ratio of the area of the non-opening portion and the opening portion of the mesh constituting the plate. In order to increase the area ratio of the light diffusion layer, for example, the area of the opening occupying per unit area of the mesh constituting the plate is increased and the area of the non-opening is decreased. In order to reduce the area ratio of the light diffusion layer, the area of the opening occupied per unit area of the mesh constituting the plate is reduced and the area of the non-opening is increased. It is preferable to form and disperse one or more light diffusion layers with respect to the area of the light guide plate surface of 3 mm ² , and it is more preferable to form and disperse one or more light diffusion layers with respect to the area of the light guide plate surface of 1 mm ² . In the printing method, various types of ink can be used. In thermosetting inks, it is common to use a binder resin diluted with a solvent to which inorganic fine particles or organic fine particles are added.

  Examples of the method for adjusting the ten-point average roughness (Rz) and the average interval of unevenness (Sm) include changing the ratio of the solvent, the binder resin and the fine particles in the ink, and the particle size of the fine particles. For example, in the case of screen printing, in order to increase the ten-point average roughness (Rz), it is necessary to increase the ratio of the solvent in the ink and increase the particle diameter of the fine particles. Also, increase the ink permeation volume by reducing the wire diameter of the yarns that make up the mesh, or by reducing the number of meshes that represent the mesh openings, and increase the thickness of the ink that adheres to the surface of the light guide plate. However, the ten-point average roughness (Rz) can be increased. Conversely, in order to reduce the ten-point average roughness (Rz), it is necessary to reduce the proportion of the solvent in the ink and to reduce the particle diameter of the fine particles. Also, by increasing the wire diameter of the yarns constituting the mesh and increasing the number of meshes, the ink transmission volume can be reduced, and the thickness of the ink adhering to the surface of the light guide plate can also be reduced. Rz) can be reduced.

  As the inorganic fine particles and organic fine particles added to the diffusion layer, those exemplified as the light scattering agent added to the light guide plate can be used. Moreover, a well-known mat agent etc. can also be used.

  As the solvent, a solvent that sufficiently dissolves the binder resin and has a low evaporation rate is preferable. When a solvent having low solubility of the binder resin is used, the binder resin is precipitated during ink storage or printing, which causes printing failure. Further, when a solvent having a high evaporation rate is used, clogging of the mesh is likely to occur during printing, resulting in a decrease in mass productivity.

  In addition, photocurable ink can also be used. In the photocurable ink, in order to increase the ten-point average roughness (Rz), it is possible to increase the particle diameter of the fine particles in the ink or to increase the shrinkage amount during photocuring. The photocurable ink is cured by polymerizing a monomer component, and shrinkage is caused by this polymerization. Therefore, there is a method of increasing the amount of monomer components in the ink in order to increase the shrinkage during photocuring. In order to reduce the ten-point average roughness (Rz), it is possible to reduce the particle size of the fine particles in the ink or to reduce the amount of shrinkage during photocuring by reducing the monomer component in the ink. .

  In order to increase the average interval (Sm) of the irregularities, it is possible to reduce the ratio of the fine particles in the ink. Conversely, in order to reduce the average interval (Sm) of the irregularities, the ratio of the fine particles in the ink Can be raised.

  When the particle diameter of the fine particles is increased in order to increase the ten-point average roughness (Rz), if the weight or volume ratio of the fine particles in the ink is the same, the number of the fine particles is decreased, so that the average interval of unevenness (Sm) Becomes larger. Accordingly, in order to increase the ten-point average roughness (Rz) and to make the average interval (Sm) of the unevenness equal or smaller, the particle diameter of the fine particles is increased and the weight or volume ratio of the fine particles in the ink is increased. There is a need to. As the weight or volume ratio of the fine particles in the ink is increased, the ratio of the solvent and the binder resin is relatively decreased, so that the fluidity of the ink is lowered and the printability tends to be lowered. When screen printing is performed using an ink in which the proportion of fine particles in the ink is excessively increased, that is, the fluidity is greatly reduced, unevenness occurs in the transmission volume of the ink transferred to the light guide plate, and a uniform light diffusion layer is obtained. It becomes difficult. When the light diffusing layer is not uniform, if the light source is arranged at the edge of the light guide plate and the light source is turned on, the luminance of the light exit surface of the light guide plate becomes local unevenness, which is not suitable for practical use. Therefore, a light diffusion layer having an excessively large ten-point average roughness (Rz) and a small unevenness average interval (Sm) results in a decrease in printing yield.

  In order to obtain a uniform light diffusing layer, the amount of the binder resin in the ink is preferably more than 5 parts by weight and more preferably more than 10 parts by weight in 100 parts by weight of the ink.

  In order to increase the luminance, it is preferable to diffuse a large amount of light at the air interface of the light diffusion layer. For this reason, as the binder resin constituting the light diffusion layer, a resin having a refractive index difference within 0.2 with respect to the light guide plate itself is suitably used. If the refractive index difference between the binder resin and the light guide plate is too large, the amount of light that is regularly reflected at the interface between the light diffusion layer and the light guide plate increases, and the proportion of light diffused in the light diffusion layer decreases, so that the light exits from the exit surface. This is not preferable because the ratio of light to be emitted tends to decrease and the luminance tends to decrease. Further, the difference in refractive index between the binder resin and the fine particles is preferably within 1.0. If the refractive index difference between the binder resin and the fine particles is too large, the transmittance of the light diffusing layer is lowered, the loss of light is increased, and the luminance tends to decrease, which is not preferable.

  The thickness of the light diffusion layer after the ink is solidified is preferably 0.1 to 50 μm, and more preferably 0.5 to 40 μm.

  The plate thickness of the light guide plate is preferably 0.1 to 15 mm, and more preferably 0.2 to 12 mm. The shape of the exit surface may be flat or roughened, and a lens-like object typified by a columnar triangular prism may be formed.

  A diffusion film, a prism sheet, and a lens sheet for adjusting the angle characteristics of the scattered outgoing light intensity may be arranged on the exit surface side of the light guide plate, and a prism sheet and a lens sheet are arranged on the exit side of the diffusion film. May be. In addition, a prism sheet may be disposed on the exit surface side of the diffusion film and a diffusion film may be disposed on the exit surface side. However, the configuration of these films is only an example and is not limited thereto. The diffusion film may be a type in which acrylic or silica beads are applied together with a binder, and has both a diffusion function and a light polarization function. The prism sheet is a transparent sheet in which a large number of columnar triangular prisms are continuously formed in a parallel state on the surface. One prism sheet can be arranged in such a direction that the prism ridges are parallel to the incident surface of the light guide plate, and two prism sheets can be arranged in the direction in which the prism ridges are orthogonal to each other. The lens sheet is formed by forming a plurality of lens-like objects having concave and convex curved surfaces on the surface, and one or a plurality of lens sheets can be arranged.

  As the light source, a linear light source such as a cold cathode tube or a hot cathode tube can be disposed on one or more incident surfaces. In this case, one or a plurality of incident surfaces can be arranged with respect to one incident surface. In addition to the linear light source, a point light source such as an LED or a laser can be used.

  A reflector is disposed on the back side of the light exit surface of the light guide plate. The reflection plate can increase the light utilization efficiency by reflecting the light emitted from the back surface of the emission surface to the light guide plate side. Although there is no restriction | limiting in particular as a reflecting plate, For example, a white reflecting sheet etc. can be used.

  With the configuration as described above, it is possible to obtain a high-luminance surface light source device suitable for various uses, particularly for a liquid crystal display device.

  EXAMPLES Hereinafter, although this invention is further demonstrated using an Example and a comparative example, this invention is not limited at all by these examples.

[Examples 1-7 and Comparative Examples 1-6]
(1) Creation of light guide plate

Titanium oxide (refractive index: 2.52) as a light scattering agent was dispersed in a syrup composed of 20% by weight polymethyl methacrylate and 80% by weight methyl methacrylate at an addition concentration shown in Table 1. Next, 0.03 part by weight of 2,2′-azobis (2,4-dimethylvaleronitrile) was added as a polymerization initiator, and 2- (5-methyl-2-hydroxyphenyl) -benzotriazole was added as an ultraviolet absorber. 0.005 part by weight was added and stirred for 30 minutes to prepare a polymerizable raw material syrup. Polymeric raw material syrup is poured into a mold in which two tempered glass plates of 650 mm long, 450 mm wide and 6 mm thick are arranged through an endless tube made of polyvinyl chloride around the periphery, and adjusted to a predetermined interval And immersed in warm water at 70 ° C. for 2 hours, and then polymerized in an air bath at 130 ° C. for 1 hour. The obtained acrylic plate of 600 mm x 400 mm x 6 mm is cut with a panel saw (trade name SZIVG-4000 manufactured by SHINX), and the four surrounding sides are mirror polished with a grinding and polishing machine (trade name made by Megaro Technica). As a result, a 388 mm × 291 mm × 6 mm light guide plate was obtained.

(2) Formation of light diffusing layer The light diffusing layer was formed on the back surface of the light emitting plate by the screen printing method.

  In order to form a binder resin on the back surface of the exit surface of the light guide plate having a size of 388 mm × 291 mm × 6 mm, Teikoku VAR-000 medium was used to form a binder resin, and urethane beads A, urethane beads B, matting agent K ( As described above, all are manufactured by Seiko Advance), aluminum hydroxide (Showa Denko H320), isophorone is used as a diluent solvent, these are mixed in the addition amounts shown in Table 1, and then manually stirred using a spatula for 10 minutes. And mixed uniformly to obtain a mixed ink. The binder resin content of Teikoku VAR-000 medium was calculated as follows. A pre-weighed VAR-000 medium was applied on a 30 cm × 30 cm reinforced plate glass and dried in a hot air circulating drying oven at 80 ° C. for 48 hours to prepare a binder resin film having a thickness of 50 μm. The percentage of weight was calculated. As a result, the binder resin content in the VAR-000 medium was 37% by weight. In Example 6, instead of the VAR-000 medium, a polycarbonate binder (solution) in which 22 parts by weight of polycarbonate resin (Taflon FN1700A manufactured by Idemitsu Kosan Co., Ltd.) was dissolved in 78 parts by weight of trichloroethane was used. In the same manner as when the VAR-000 medium was used, fine particles and a diluting solvent were added to the polycarbonate binder and stirred to obtain a mixed ink. The amount of the binder resin, fine particles and diluent solvent (trichloroethane) added at this time is shown in Table 1. Using a screen printing machine (LS-560, manufactured by Neurong Seimitsu Kogyo Co., Ltd.) and a nylon screen plate (Mesh Co., Ltd.) having a film thickness of 10 μm and 355 mesh, the mixed ink after stirring was printed on the back surface of the light exiting surface of the light guide plate. . At this time, a 388 mm × 6 mm surface of the light guide plate was used as a light incident surface, and a screen pattern with a gradation pattern in which the luminance in the central portion was higher than the luminance in the vicinity of the light source of the light guide plate was used. For Examples 1 to 3 and 6, 7 and Comparative Examples 1, 2, 4 to 6, the area ratio of the light diffusion layer in the vicinity of the light source was set to 22%, and the light diffusion in the central part farthest from the light source parts at both ends The pattern was such that the area ratio of the light diffusion layer increased linearly toward the center in the direction of the 291 mm length of the light guide plate so that the area ratio of the layer was 68%. In Examples 4 and 5 and Comparative Example 3, the area ratio of the light diffusion layer in the vicinity of the light source is set to 32%, and the area ratio of the light diffusion layer in the central part farthest from the light source parts at both ends is set to 68%. A pattern in which the area ratio of the light diffusion layer increases linearly toward the center in the 291 mm length direction of the light guide plate. At this time, the light diffusion layer was printed with one square dot of various sizes in a size of 1 mm × 1 mm on the surface of the light guide plate. After printing, it was left to stand at room temperature for 12 hours and dried to obtain a light guide plate with a light diffusion layer of 388 mm × 291 mm × 6 mm, which was printed over the entire surface.

(3) Refractive index measurement of binder resin and light guide plate Employed ink VAR-000 medium or polycarbonate binder used to form the binder resin was applied on a 30 cm x 30 cm reinforced plate glass, and a hot air circulating furnace at 80 ° C. After drying for 48 hours, it was peeled from the tempered plate glass to prepare a binder resin film having a thickness of 50 μm. This binder resin film was cut into a width of 8 mm and a length of 20 mm. The cut out binder resin film was measured for refractive index according to JIS K7142 A method. At this time, as Abbe refractometer, Atago Co., Ltd. Abbe refractometer type 1 or 4 was used, and the refractive index was measured at a temperature of 23 ° C. using methylene iodide as an immersion liquid. The binder resin film for medium was 1.518, and the binder resin film for polycarbonate binder was 1.585.

  In addition, the light guide plates used in Examples and Comparative Examples were cut into a width of 8 mm, a length of 20 mm, and a thickness of 5 mm. All cut surfaces were polished to a mirror surface using water-resistant abrasive paper # 2000 manufactured by Sankyo Rikagaku Co., Ltd., to obtain a light guide plate refractive index measurement sample. The refractive index of the light guide plate refractive index measurement sample was measured according to JIS K7142 A method using an Abbe refractometer type 1-4 manufactured by Atago Co., Ltd. at a temperature of 23 ° C. At this time, methylene iodide was used as the immersion liquid. The refractive index of the light guide plate used in Examples 1 to 3 and 6, 7 and Comparative Examples 1, 2, 4 to 6 was 1.486. The refractive index of the light guide plate used in Examples 4 and 5 and Comparative Example 3 was 1.485.

(4) Refractive Index Measurement of Fine Particles Refractive index was measured according to JIS K7142 B method for urethane beads B manufactured by Seiko Advance, which are fine particles. At this time, a mixed liquid of tri-n-butyl citrate and 1-bromonaphthalene was used as an infiltrating liquid. The immersion liquid was placed on a glass preparation, fine particles were dispersed on the immersion liquid, and a cover glass was placed thereon to create a cell set. The fine particles in the cell set were observed with an optical microscope MX61L manufactured by OLYMPUS at a magnification of 200 times, and the immersion liquid was adjusted to a mixing ratio at which the Becke line did not move even when the focal length was shifted. At this time, a wavelength of 589 nm was used with a monochromator TYPE H20VIS manufactured by INSTRUMENTS using a power supply device model number BA-X500 and a light source device model number SX-UID501XAMQ as a light source. Thereafter, the refractive index of the infiltrated liquid with the mixing ratio at which the Becke line does not move was measured using the JIS K7142 A method and found to be 1.525. The refractive index of this immersion liquid was taken as the refractive index of the fine particles.

  When the refractive index was similarly measured for Seiko Advance urethane beads A and matting agent K, and Showa Denko aluminum hydroxide H320, urethane beads A were 1.525, matting agent K was 1.259, and hydroxylation was performed. Aluminum H320 was 1.562.

(5) Measurement of luminance and luminance unevenness The light source plate with a light diffusion layer prepared as described above was used as the surface light source device shown in FIG.
That is, a reflector film with adhesive (SU-119 made by Tsujiden) is provided on two end faces except for the light incident end face (both end faces of 388 mm in length), the exit face, and the face provided with the light diffusion layer. (W)) was pasted.

  The white reflecting plate 3 (RFID 188 manufactured by Tsujiden) is disposed on the surface of the light guide plate 1 to which the light diffusion layer 8 is applied, the diffusion film 6 (D122 manufactured by TSJIDEN) is disposed on the light output surface side of the light guide plate 1, and then the prism. One sheet 7 (Sumitomo 3M BEFII) is placed in a direction in which the prism lens forming surface is opposite to the light guide plate and the ridge of the prism lens array is parallel to the incident end surface, and then a diffusion film 6 ′ (Duji Tsujiden) is placed. Arranged in order.

  As a light source, one cold cathode tube 2 and 2 ′ (Harison Toshiba cold cathode fluorescent lamp) having a diameter of 3 mm and a length of 419 mm is introduced to each incident end face as a light source at the center position in the end face plate thickness direction of the incident face. The distance from the incident end face of the optical plate 1 was set 1 mm apart. In addition, an inverter (HIU-766 52K made by Harrison Toshiba) is used for the cold cathode tube, and “Ko” is used so that the cold cathode tube is covered with a reflector sheet (Reuil mirror 150W05 made by Reiko) as lamp reflectors 5 and 5 ′. The surface light source device was formed by arranging in a letter shape. The overlapping length of the reflector sheet and the light guide plate 1 was 0.5 mm.

  The cold cathode tube was energized with a voltage of 12 V and a tube current of 7 mA and left to stabilize for about 20 minutes, and the luminance of the surface light source device was measured.

  For measurement of luminance, a luminance meter (CA1500W manufactured by Konica Minolta) was installed at a position 670 mm away from the center of the surface of the light guide plate in the normal direction. The luminance measurement areas were 368 mm × 271 mm inside except 10 mm from the four end faces, and the average luminance of this area is shown in Table 1 as the luminance.

  Further, as a measurement of luminance unevenness, the presence or absence of local unevenness of the luminance of the entire surface was visually confirmed. When mixed ink with poor printability is used, the luminance uniformity over the entire surface is impaired and local unevenness occurs, which is not suitable for practical use. On the other hand, when mixed ink with good printability is used, there is no local unevenness and it is suitable for practical use.

(6) Measurement of luminance distribution Each of the luminance measurement areas is 368 mm x 271 mm on the inner side excluding 10 mm from the four end faces, and this area is divided into 19 equal parts each having a long side and a short side, and a size of 19.37 mm x 14.26 mm. Was defined as one section. The section at the center of the long side that is closest to one of the cold cathode tubes is designated as position 1, and numbers are assigned in order of position 2, position 3,... 10. Position 11, Position 12... Position 19 as approaching the other cold cathode tube. The position 19 is a section closest to the other cold cathode tube. The average luminance at 19 locations at 19.37 mm in the direction parallel to the cold cathode tube and 14.26 mm in the direction between the two cold cathode tubes was measured, and the value of the average luminance at the position 10 was taken as the luminance at the center portion. As a result, the lower average luminance value at position 1 or position 19 was taken. Other than that, it was the same as the measurement of luminance. As a result, in Examples 1 to 7 and Comparative Examples 1 to 5, the luminance at the position 10 was higher than the luminance at the position 1 and the position 19, and the luminance distribution was suitable as a surface light source device. In Comparative Example 6, since luminance unevenness was confirmed, the luminance distribution and average luminance were not measured.

(7) Measurement of 10-point average roughness of surface of light diffusing layer and average interval of irregularities Ten-point average roughness (Rz) of surface of light diffusing layer of light guide plate with obtained 388 mm × 291 mm × 6 mm light diffusing layer And the average space | interval (Sm) of an unevenness | corrugation was measured using the surface roughness measuring machine (Tokyo Seimitsu Surfcom 1500DX). The diagonal direction of the square shape was measured 5 times on a dot having a dot occupation area ratio of 68%, and the average value was obtained. The measurement length was 0.5 mm and the measurement was performed according to JIS B0601-1994 except that the cutoff wavelength was 0.25 mm. Table 10 shows the obtained ten-point average roughness (Rz) and the average interval of unevenness (Sm).

  In Examples 1-3, 6, and 7, the luminance is higher than those in Comparative Examples 1-2, 4, and 5. In Examples 4 and 5, the luminance is higher than that in Comparative Example 3. In Comparative Example 6, the printability is poor and the luminance is locally uneven. In both the examples and the comparative examples, the central luminance is higher than the peripheral luminance.

  Therefore, it can be seen that by using the method of the present invention, it is possible to provide an optimum surface light source device suitable for a liquid crystal display device, having high luminance and good luminance distribution and free from local unevenness of luminance.

  The surface light source device of the present invention having a high luminance and an optimal luminance distribution is suitable for a liquid crystal display device or the like.

Claims (5)

  A light guide plate having a light exit surface and at least one light incident surface; a light source for supplying light to the incident surface; a light diffusion layer formed on the back surface of the light exit surface of the light guide plate; The surface light source device includes a reflector disposed on the back surface side, and the light diffusion layer increases in a ratio of the area of the light diffusion layer per unit area of the back surface as the distance from the light source increases. A surface light source device in which the ten-point average roughness of the surface of the light diffusion layer is 8 μm to 25 μm and the average interval of the irregularities on the surface of the light diffusion layer is 20 μm to 150 μm.   The surface light source device according to claim 1, wherein a light diffusion layer is formed by printing.   The surface light source device according to claim 2, wherein the light diffusion layer comprises a binder resin and fine particles.   The surface light source device according to claim 3, wherein a difference in refractive index between the light guide plate and the binder resin is within 0.2.   The surface light source device according to claim 4, further comprising a light diffusion layer having a refractive index difference between the binder resin and the fine particles within 1.0.

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