Patent Application
20090290098
The present invention relates to a reflection sheet that is used with a prismatic light guide plate for the backlight of a liquid crystal display device, and to the backlight of a liquid crystal display device using the reflection sheet.
Liquid crystal display devices are in wide use in mobile phones, digital cameras, personal computers, office automation equipment, and the like. Light weight, reduced thickness, and reduced power consumption are in particular demand for display devices for use in mobile equipment. Further improvement of uniformity and display quality, and improved reliability of backlights are in demand due to improvements in the display quality of liquid crystals. Television picture display has recently come into demand in laptop computers as well, and high brightness and improved display quality and durability of display devices are in particular demand.
White resin reflection sheets compounded with white pigment, and reflections sheets obtained by sputtering, vapor depositing, or otherwise applying a film layer composed of silver, aluminum, or another metal having high specular reflection on such a white resin reflection sheet are coming into wide use in order to address these problems. However, a reflection sheet provided with a film layer composed of silver, aluminum, or another metal having high specular reflection reflects light as a mirror, so that there are such problems as that light interference readily develops and defects in backlight are easily perceived as bright lines or bright spots.
While addressing these problems, reflection sheets having a minutely concavo-convex layer formed on a surface (JP-A 2004-69867), reflection sheets in which a smooth surface is integrally formed on a surface having a layer provided with protrusions (JP-A 2004-252383), and embossed reflection sheets (JP-A 2005-319588), (JP-A 2001-266629) have been proposed in order to improve the brightness and reduce the brightness defects (moiré) of a backlight. However, reflection sheets provided with a minutely concavo-convex layer, reflection sheets in which a smooth surface is integrally formed on a surface having a layer provided with protrusions, and embossed reflection sheets are configured so that a light guide plate is supported by points provided by protrusions, convex portions, or other points that still generate brightness defects (moiré). Therefore, there are problems in that the light guide plate and the reflection sheet are damaged by the vibration and the like of the reflection sheet disposed underneath the light guide plate, accompanied by the generation of bright lines or bright spots.
Patent Document 1: JP-A 2004-69867
Patent Document 2: JP-A 2004-252383
Patent Document 3: JP-A 2005-319588
Patent Document 4: JP-A 2001-266629
An object of the present invention is to provide a reflection sheet that is used together with a prismatic light guide plate for the backlight of a liquid crystal display device, wherein the reflection sheet makes it possible to create a liquid crystal display device that has high brightness and high uniformity of light as well as few bright lines, bright spots, and brightness defects, that possesses improved resistance to damage in the prismatic light guide plate and reflection sheet, and that can preserve the high brightness over a long period of time; and to provide a backlight for a liquid crystal display device in which said reflection sheet is used.
The main point of the present invention is a reflection sheet being used together with a prismatic light guide plate for a backlight of a liquid crystal display device, a prism layer is formed on one surface of the reflection sheet. The prisms of the reflection sheet are configured so that the vertex angle of the prisms is 145° or greater and 168° or less, and the distance between the vertices of the prisms on the reflection sheet (distance between an apex of a cross section of a prism on the reflection sheet and an apex of a cross section of an adjacent prism) is 50 μm or greater and 550 μm or less.
The reflection sheet of the present invention comprises at least a prism layer, a resin sheet layer, and a metal film layer. An anticorrosive layer and an adhesive layer can also be optionally provided, when needed, and an adhesive agent layer and a resin sheet layer can be further provided.
Furthermore, the backlight of the liquid crystal display device is characterized in that the reflection sheet of the present invention is disposed so that the prism array direction of the reflection sheet are arranged at an angle of 5° or greater and 20° or less in relation to the prism array direction of the prismatic light guide plate.
Using the reflection sheet of the present invention provides an effect wherein a liquid crystal display device can be achieved that has high uniformity of light, few bright lines, bright spots, and brightness defects (moiré); possesses improved resistance to damage in the prismatic light guide plate and reflection sheet; and can maintain high brightness over a long period of time while maintaining the high brightness that can be achieved by specular reflection and the like.
The present invention is a reflection sheet 3 that is used together with the prismatic light guide plate 2 for the backlight of the liquid crystal display device, wherein a prism layer is formed on one side of the reflection sheet 3. One example of the shape of the prism layer of the reflection sheet is shown in
A prism layer 11 of a reflection sheet is schematically shown in
The reflection sheet of the present invention may be a reflection sheet in which the shape of the ridgelines made by the vertices of the prisms is a straight line, curved line, or wavy line.
The reflection sheet of the present invention has a resin sheet as a substrate, and a prism layer is mounted thereon, and a metal film layer can be further provided. In other words, a film layer of silver, aluminum, or another metal having high specular reflection can be provided by sputtering, vapor deposition, or another method. The metal film layers in wide use are preferably metal layers primarily composed of silver, aluminum, and another metal having a maximum reflectivity in the visible-light region.
The reflection sheet of the present invention is substantially a reflection sheet in which a prism layer is provided on a resin sheet. In practice, a metal film layer, an optional adhesive layer and anticorrosive layer, and also an adhesive agent layer and an additional resin sheet layer can be provided in addition to the prism layer. A specific example of a configuration of the reflection sheet is shown in
Another example of a specific configuration of the reflection sheet of the present invention is shown in
The specularly reflective metal film layer is closely affected by the degree to which the layer adheres to the resin sheet layer and is sometimes modified by the aggregation, oxidation, sulfidation, chloridation, or another change in silver or other metals, resulting in reduced reflectivity. The anticorrosive layer 23 is provided in order to prevent the reduction in reflectivity based on a modification of the metal film layer. Materials having low oxygen permeability and moisture permeability are preferably used for the anticorrosive layer 23 and the adhesive layer 25.
The resin sheet used as a substrate for the reflection sheet of the present invention can be composed of a variety of resins. Examples of such resins include polyester resin such as polyethylene terephthalate and polyethylene naphthalate, vinylidene resin such as polyvinylidene chloride; polyolefin resin such as polyethylene; polycarbonate resin such as bisphenol A polycarbonate; and polyethersulfone and the like. However, it is not necessarily limited to these resins, and any resin having a high glass transition point and melting point can be appropriately used. A sheet produced by biaxial stretching is more preferred.
The reflection sheet of the present invention is used together with the prismatic light guide plate, but it is preferable to dispose the reflection sheet so that the angle formed by the prism array direction of the reflection sheet in relation to the prism array direction or the light incidence direction of the prismatic light guide plate is 5° or greater and 20° or less. In particular, brightness defects (moiré) can be reduced by disposing the reflection sheet of the present invention in the foregoing manner. The brightness defects (moiré) of the backlight cannot be removed when the angle is less than 5°, and although the brightness defects (moiré) can be reduced when the angle exceeds 20°, such an angle is still not preferred because of reduced brightness.
The angle at which the prism array direction of the reflection sheet is arranged in relation to the prism array direction of the prismatic light guide plate is described with reference to
The present invention is more specifically described below based on examples. However, the present invention is not limited to these examples.
Methods for Evaluating Evaluation Items
1. GPL Friction Properties
Measurement is performed using a measuring apparatus such as the one shown in
A sample (reflection sheet) is cut into pieces, each being in a shape of 2 cm squares and is secured by double-sided tape so that the reflection surface of the reflection sheet is on the side that is rubbed against a test jig 32 (the surface of the reflection sheet provided with the prisms faces toward the prismatic light guide plate). The prism surface of the prismatic light guide plate 35 is arranged to face upward (toward the surface of the reflection sheet), and the prismatic light guide plate 35 is mounted on a flat measurement stand 36. A 700 g load is applied on the test jig 32 so that the prism surface of the prismatic light guide plate 35 and the prism surface of the reflection sheet (sample) are made to rub against each other. The test jig 32 is made to perform a reciprocating motion 2 cm to the left and right ten times, and the prismatic light guide plate and the reflection sheet are observed to see whether they are damaged or not.
2. Bogo Test
Measurement was performed using a measuring apparatus such as the one shown in
A prismatic light guide plate 43 whose prism surface faces upward is placed on a flat board, and a reflection sheet 42 is further mounted thereupon so that the prism surface faces the prism surface of the prismatic light guide plate 43. The reflection sheet 42 is struck 1000 times from above the reflection sheet 42 with a pressure part 41 that is composed of a metal tip 1 cm square and that is moved a distance of 4 cm at a speed of 130 strokes per minute under a load of 2 kg/cm2. This measures the ease with which the reflection sheet of a backlight for a liquid crystal can be damaged (damage properties) by being brought into contact with a prismatic light guide plate.
3. Method for Measuring Relative Brightness
A color brightness meter manufactured by Topcon Corporation (product number: BM-7) was used to measure relative brightness. In other words, the relative brightness of a conventional white reflection sheet and a reflection sheet of a measurement certificate in is measured at a backlight for a liquid crystal, and relative brightness is measured in relation to the white reflection sheet.
4. Moiré Evaluation Method
The back surface (surface without any emitting light) of a backlight for a liquid crystal provided with a reflection sheet is brought into contact, and the resulting interference pattern of light is visually compared. The results are compared with those for a white reflection sheet, the superiority or inferiority of the brightness defects is observed, and a passing grade is given if the result is the same as that for the white reflection sheet.
Method for Measuring the Distance Between Prisms and the Prism Vertex Angle
The distance between the prisms and the prism vertex angle of a sheet having prisms is measured in the following manner. An ultra-deep shape measuring microscope manufactured by Keyence Corporation (VK-8500) is used as the measuring instrument.
1. Method for Measuring Prism Vertex Angle
Measurement is performed using a 50× objective lens at a measuring pitch of 0.1 μm. A profile is selected from the top menu on the measurement screen, and a measurement perpendicular to the prism formation direction of the sheet having the prisms is selected, whereupon the irregularities on the measurement surface are displayed. The cursor is placed on the highest and lowest points of the prism portion displayed. The angle from the lowest point of a prism to the highest point can be measured by a measuring instrument. This measurement is performed to the right and left of the prism; a combined value is calculated; the formula A=(180°−Lens vertex angle) is used, where A is the measurement value; and the lens vertex angle is measured.
2. Method for Measuring the Interval Between Prisms
Measurement is performed using a 50× objective lens at a measuring pitch of 0.1 μm. A profile is selected from the top menu on the measurement screen, and a measurement perpendicular to the prism formation direction of the sheet having the prisms is selected, whereupon the irregularities on the measurement surface are displayed. The interval between prisms can be automatically measured by placing the cursor over an apex of a prism and over the apex of an adjacent prism.
A polyester film (A4300 manufactured by Toyobo, thickness: 125 μm) was used as a substrate (resin sheet layer 26 in
The resulting reflection sheet having variable prism vertex angles and variable distances between prism apices was used to evaluate the performance of the reflection sheet in terms of GPL friction properties, Bogo tests, relative brightness, and brightness defects (moiré). The GPL friction properties and Bogo tests were evaluated using a special measuring apparatus, and brightness and brightness defects were evaluated by incorporating the reflection sheet into a liquid crystal display device. The prism lens sheet in the liquid crystal display device was brought into contact with the prismatic light guide plate such that the prism surface faced downward, the prism array direction was perpendicular to the incidence direction of light, the prismatic light guide plate was brought into contact with the prism surface of the reflection sheet such that the prism surface of the plate faced downward, and the prism array direction of the prismatic light guide plate was aligned with the direction of incidence light (perpendicular to the prism array direction of the prism lens sheet). The reflection sheet was arranged in such a way that the prism array direction of the reflection sheet formed an angle of 5° in relation to the prism array direction of the prismatic light guide plate. The prism lens sheet and prismatic light guide plate used were those having a prism vertex angle of 90°.
The results of the evaluation are shown in Table 1. The prism vertex angles are the angles shown by the symbol α in
A reflection sheet having a transparent resin layer that does not have prisms is best in terms of brightness only, but is worst in terms of brightness defects (moiré). It can be seen that brightness defects (moiré) are remarkably abated by forming prisms on the reflection sheet. The light guide body and the reflection sheet are resistant to damage, have few brightness defects, and possess a satisfactory relative brightness of about 120 when the prism vertex angle ranges from 145° to 168°. It can be seen from the foregoing that keeping the vertex angles of the prisms on the prism layer of the reflection sheet at 145° or greater and 168° or less is preferable from the standpoint of brightness, damage properties, and brightness defects. It can be also seen that when focus is made on the distance between a prism apex and an adjacent apex, keeping the distance in a range of 50 μm or greater and 550 μm or less is preferable from the standpoint of brightness, damage properties, and brightness defects. It can be seen that the GPL friction properties, Bogo tests, and relative brightness are unsatisfactory regardless of the distance between prism vertices when the prism vertex angle is 140° or less, but that the GPL friction properties, Bogo tests, and relative brightness are satisfactory when the prism vertex angle is 145° or greater and 168° or less. In addition, the GPL friction properties and the Bogo tests deteriorate somewhat when the prism vertex angle is 165° and the distance between prism vertices is 800 μm, but the GPL friction properties and the Bogo tests are satisfactory when the prism vertex angle is 168° and the distance between prism vertices is 550 μm. It can be seen from the foregoing that a light-type backlight reflection sheet in which the prism vertex angle is 145° to 168° and the distance between prism vertices is 50 μm to 550 μm is preferred from the standpoint of brightness, damage properties, and brightness defects.
A reflection sheet was subsequently manufactured in the same manner as in example 1. The sheet had a prism vertex angle of 145°, and the distance between the prism bases, indicated by a letter s in
Table 2 shows the relation of the angle to the brightness and brightness defects for a case of varying the angle at which the prisms of the reflection sheet are arranged in relation to the prism array direction of the prismatic light guide plate. It can be seen from Table 2 that brightness is high and brightness defects are minor when the angle ranges from 5° or greater to 20° or less. It can be also seen that brightness is low and brightness defects are major in a reflection sheet having particles of the comparative example.
As a comparative example, a reflection sheet was manufactured in which an undercoat layer (thickness: 2 μm) was formed on a polyester film (A4300 manufactured by Toyobo; thickness: 125 μm), a metal film layer composed of silver (thickness: 100 nm) was diposited by vacuum vapor deposition, and a resin layer containing particles having an mean grain size of 4 μm being mixed therein was further formed by wet coating (thickness: 5 μm) on the film layer as a layer for preventing brightness nonuniformities.
Use of the reflection sheet of the present invention can provide a liquid crystal display device that has high light uniformity as well as few bright lines, bright spots, and brightness defects (moiré); possesses improved resistance to damage on the light guide plate and reflection sheet; and can maintain high brightness over a long period of time while maintaining the high brightness achieved through specular reflection and the like. Liquid crystal display devices are in wide use not only in the industrial world but also as household and personal items, and the effects of the contribution of the present invention to the industrial world are extremely large.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2005-173056 | Jun 2005 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2006/311839 | 6/13/2006 | WO | 00 | 12/13/2007 |
