JP2008233708A - Diffusion plate with double-sided configuration - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a diffusion plate for a direct backlight type liquid crystal display, which can completely remove lamp variations while preventing luminance drops. <P>SOLUTION: The diffusion plate is loaded on the direct backlight type liquid crystal display configured by successively arranging a reflector, linear light sources, the diffusion plate, an optical film, and a liquid crystal panel. In the diffusion plate, a linear prism shape with an apex angle of 50° to 150° is formed on the surface of the diffusion plate, which is the contact side with the optical film, and linear lens shapes whose height is 0.2 to 20 μm are formed on the surface of the diffusion plate, which is the side opposed to the linear light sources. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a diffusion plate mounted on a liquid crystal display.

In recent years, flat-panel televisions such as liquid crystal televisions and plasma televisions have been expanded instead of cathode-ray tubes, and particularly liquid crystal televisions are growing rapidly. Since the liquid crystal is not self-luminous, a backlight (also referred to as a back light source device) is necessary, and two types of backlight, an edge light type and a direct type, are generally used.
The edge light type is a method in which a linear light source is placed on the edge of a liquid crystal panel and surface light is emitted by a light guide plate. It is suitable for a thin and light personal computer monitor, but it is said that it is difficult to increase the screen size and brightness. On the other hand, the direct type is a method in which a large number of linear light sources are arranged directly under a liquid crystal panel and surface emission is performed with a diffusion plate. It is easily used for large screens and high brightness and is preferred for liquid crystal televisions.

The structure of the direct type backlight type liquid crystal display has a structure in which a reflector, a linear light source, a diffusion plate, an optical film, and a liquid crystal panel are sequentially arranged as shown in FIG. Among the components, the diffuser diffuses the light from the linear light source and blurs the linear light and darkness (so-called lamp unevenness) that forms in the linear light source gap adjacent to the bright line directly above the linear light source. Have an optical role.
In recent years, there has been a movement to reduce the number of linear light sources used in backlights for cost reduction, and there is a tendency to increase the spacing between adjacent linear light sources (so-called lamp pitch). It was a problem in terms of quality. Therefore, in Patent Document 1, a resin diffusion plate in which a diffusion material is blended is generally widely used. However, as a method for eliminating lamp unevenness, a method of thickening a light diffusing agent has been performed. However, this method has a problem that the luminance is lowered.

In Patent Document 2, a high-functional plate is also developed in which a prism shape is formed on the surface of the diffusion plate, luminance reduction is prevented by a light condensing function of the prism, and lamp unevenness is eliminated. However, the lamp unevenness has not yet been completely eliminated.
Japanese Patent Laid-Open No. 1-172801 JP 2007-18939 A

  An object of the present invention is to provide a diffusing plate for a direct-type backlight type liquid crystal display that can prevent a decrease in luminance and completely eliminate lamp unevenness.

As a result of diligent research to solve the above problems, the present inventors have formed a prism shape on the surface of the diffusion plate and a fine linear lens shape on the surface opposite to the prism surface, thereby achieving high brightness and lamp unevenness. Has been found to be able to be completely eliminated, and the present invention has been made.
That is, the present invention is a diffusion plate mounted on a direct-type backlight type liquid crystal display arranged in the order of a reflector, a linear light source, a diffusion plate, an optical film, and a liquid crystal panel, and the diffusion plate is an optical film. A linear prism shape with an apex angle of 50 to 150 degrees is formed on the surface in contact with the surface, and a linear lens shape with a peak height of 0.2 to 20 μm on the surface of the diffusion plate facing the linear light source Is a diffusion plate characterized by being shaped.

  When the diffusion plate of the present invention is used, it is possible to prevent a decrease in luminance in a direct backlight type liquid crystal display and to completely eliminate lamp unevenness.

The present invention will be described in detail below.
A direct-type backlight type liquid crystal display often used in a liquid crystal television has a structure in which a liquid crystal panel is combined with a back light source device called a direct-type backlight. As shown in FIG. A linear light source, a diffusion plate, an optical film, and a liquid crystal panel are arranged in this order. Of these, the reflector to the optical film are generally called a direct backlight, and a liquid crystal display combined with a liquid crystal panel is called a liquid crystal display.
As the reflector, a metal plate coated with a reflector or a white or silver polyethylene terephthalate (PET) or polycarbonate (PC) reflector film is used.
A linear light source is a light source having a linear shape, and the most common linear light source used in a liquid crystal display is a fluorescent tube having a diameter of 2 to 4 mm called a cold cathode tube (abbreviated as CCFL). Cold cathode tubes include straight lines, U-shaped tubes, W-shaped tubes, etc., and the longer the linear portion, the more preferred for large screens.
The diffusion plate is an important optical member that scatters the light from the linear light source and converts the linear light source into a planar light source. The diffusing plate is preferably one in which light diffusing fine particles are blended as a light diffusing material in a translucent resin.

As the translucent resin, an acrylic resin, a styrene-methyl methacrylate copolymer resin (MS resin), a styrene resin, a PC resin, a cyclic olefin resin, etc. with high optical characteristics, particularly high transmittance, are preferable. From the viewpoint of preventing deformation due to water absorption, styrene resins, MS resins, PC resins, and cyclic olefin resins having a low water absorption are more preferable.
The light diffusing fine particles preferably blended in the translucent resin may be either organic or inorganic fine particles, such as acrylic crosslinked fine particles, MS crosslinked fine particles, styrene crosslinked fine particles, silicone crosslinked fine particles, calcium carbonate, Examples thereof include titanium oxide, barium sulfate, talc, and mica.

The light diffusing fine particles are preferably spherical, spherical, elliptical, flat, scaly, polygonal, cubic, and rectangular parallelepiped, and the particle diameter is preferably 1 to 30 μm with good light scattering performance. The blending amount of the light diffusing fine particles is preferably 3 parts by weight or less with respect to 100 parts by weight of the translucent resin in order to prevent a decrease in luminance.
The diffusion plate may be a single sheet in which the light diffusing material (light diffusing fine particles) is blended with the translucent resin, but may be a laminated sheet for improving light resistance and surface hardness. In particular, it is more preferable to laminate a 10 to 100 μm surface layer containing an ultraviolet absorber for improving light resistance. The thickness of the diffusion plate is preferably 1.0 to 3.0 mm, and more preferably 1.0 to 2.0 mm.

The optical film disposed between the diffusion plate and the liquid crystal panel has a so-called brightness such as a diffusion film that further scatters or collects light transmitted through the diffusion plate, a prism film that collects scattered light, and a reflective polarizing film. A plurality of highly functional film groups such as a film to be improved. In order to prevent the transmission of a linear light source called lamp unevenness, various combinations of optical films have been studied. However, there is a strong demand for reducing optical films in TV applications where there is a strong demand for cost reduction. Eliminating lamp unevenness with film is becoming stricter.
Furthermore, a liquid crystal panel is placed on top of the optical film to become a liquid crystal display. However, since the transmittance of the liquid crystal panel is low, it is necessary to increase the brightness of the direct type backlight in order to increase the brightness as the liquid crystal display, that is, the brightness. is there.

  One way to increase the brightness of the direct type backlight is to increase the number of linear light sources. At present, in liquid crystal displays for 32-inch liquid crystal televisions, as many as 16 to 20 linear light sources are installed depending on the transmittance of the liquid crystal panels of each company. The luminance generally required for a liquid crystal television is said to be 500 cd or more, and for that purpose, the luminance required for a direct type backlight is also said to be 5000 to 15000 cd. A mold backlight is required. Naturally, this required brightness can be secured by increasing the number of linear light sources installed, but considering the recent market price of LCD TVs, there is a strong demand to reduce the number of linear light sources and reduce costs. .

The lamp unevenness mentioned as a problem to be solved by the present invention refers to a phenomenon in which a linear light source, that is, a so-called lamp disposed on the screen of a liquid crystal display can be seen transparently like unevenness.
It is a general phenomenon called lamp unevenness that the light directly above the linear light source is bright, the gap between adjacent linear light sources, that is, the so-called lamps become dark, and light and dark appear on the screen like a stripe pattern. Depending on the combination of the optical films, the distance between the lamps may be brighter, the line light source may be darkened, and striped lamp unevenness may be seen.
It is said that the cause of any lamp unevenness is mainly the diffusion plate. That is, it is said that one of the major causes of lamp unevenness is that the light scattering function of the diffusion plate is weak and the linear light source cannot be made uniform surface emission. Therefore, conventionally, the refractive index, particle size, and amount of the light diffusing material blended in the diffusion plate have been redesigned to eliminate the lamp unevenness.

  However, when the number of linear light sources is reduced, the interval between adjacent linear light sources, so-called lamp intervals, is nearly doubled, and in order to achieve the conventional brightness with a small number of linear light sources, one by one. The book needs to be brightened. As the lamp pitch increases, the dark part of the lamp gap naturally increases, so that the lamp unevenness increases and the light source becomes brighter, so the lamp unevenness becomes clearer. Therefore, it is more important than ever to solve the lamp unevenness.

  In the method of solving the lamp unevenness by increasing the amount of the light diffusing material, even if the lamp unevenness can be solved, the brightness cannot reach the target at all, and the conventional diffuser design has been difficult to apply to this lamp reduction model. In the present invention, in order to meet the demand for lamp reduction, it is proposed not only to design a diffusion plate using a conventional light diffusing material but also to control light. That is, the diffusion plate of the present invention forms a fine linear lens shape on the surface of the linear light source side corresponding to the light incident surface, and forms a linear prism shape on the surface of the optical film side corresponding to the light output surface. The optical path of light incident from the light source is greatly changed using the catadioptric function of both the lens and the prism.

The prism shape needs to be formed on the surface of the diffusion plate that contacts the optical film, that is, the surface opposite to the linear light source. The direction of the linear prism shape is preferably formed so as to be parallel to the linear light source.
In order to achieve uniform surface emission, the apex angle of the prism shape needs to be 50 to 150 degrees. If the apex angle of the prism shape is 50 degrees or more, light can be condensed in the lamp gap. If the angle is 150 degrees or less, the light is not concentrated right above the lamp. Preferably it is 55 to 130 degrees, more preferably 60 to 120 degrees.

The linear prisms formed on the surface of the diffusion plate may be all linear prisms having the same apex angle, or may be a combination of prism shapes having a plurality of apex angles to disperse the light collecting function. . In the case of a combination of prism shapes, prism shapes having different shapes one by one may be arranged, a plurality of rows may be combined, or shapes may be combined in order.
The linear prism shape may be on the entire surface of the diffusion plate, but may be formed only directly above the lamp or only in the gap of the lamp in order to minimize lamp unevenness.
The pitch of the linear prism shape is preferably 50 to 400 μm in view of moldability.

Examples of the method for forming the prism shape on the surface of the diffusion plate include extrusion molding, UV molding, thermal transfer, compression molding, shaving, etching and various other methods.
Extrusion molding or thermal transfer is a continuous molding in which the surface shape of a so-called prism roll, which is engraved with a prism shape by cutting or etching, is transferred to the surface of the diffusion plate and cooled and solidified in the extrusion process for producing the diffusion plate. Is a process.
UV molding may be a continuous extrusion process or a single wafer batch process, but is a method of forming a prism shape on the surface of the diffusion plate by ultraviolet curing rather than cooling and solidification.
Compression molding is a method of forming a shape by thermally compressing a flat die engraved with a prism shape, a so-called prism die, on the surface of the diffusion plate.
When the prism shape is formed on the surface of the diffusion plate, the light diffusion material can be reduced because the prism shape also has a light scattering effect. In other words, it is possible to simultaneously reduce the lamp unevenness and increase the transmittance, that is, increase the luminance.
As a method of manufacturing the prism roll and the prism mold, any method may be used as long as a predetermined prism shape can be dug, such as cutting, etching, and electric discharge machining.

The present inventors have surprisingly found that the lamp unevenness can be completely eliminated by forming a fine linear lens shape on the opposite surface of the linear prism shape, that is, the surface on the linear light source side. It was.
The fine linear lens shape is formed on the surface of the diffuser plate on the side of the linear light source opposite to the linear prism shape surface. If the peak height of the linear lens shape is 0.2 to 20 μm, lamp unevenness is caused. It can be completely eliminated. Preferably it is 0.3-15 micrometers, More preferably, it is 0.5-12 micrometers. The pitch of the linear lens shape is preferably 10 to 400 μm from the viewpoint of moldability, and the length of the line is preferably 100 μm or more.

The linear lens shape is preferably formed in parallel with the linear light source. However, in order to prevent moire, it is more preferable that the linear lens shape intersect at an angle of 0.3 degrees or less, more preferably 0.2 degrees or less.
There are several methods for forming the linear lens shape, but the surface shape of the so-called lens roll, which is a mold roll engraved with the fine linear lens shape described above, is diffused during the diffusion plate extrusion process as with the prism surface. A continuous molding process of transferring to the surface of the plate and cooling and solidification, or a method of press molding on the surface of the diffusion plate using a lens mold in which the above-mentioned fine linear lens shape is engraved with a single wafer is preferable.
As a method of digging the surface of the mold roll into a fine linear lens shape, there are methods such as cutting and etching. The shape of the digging into the mold is preferably a lens shape, but since the resin shrinks when cooled and solidified to form a free-form surface, even if the digging shape is a fine triangular prism shape, quadrangular prism shape or polygonal prism shape, The molded shape becomes a lens shape made of a free-form surface.

  The present invention has a fine linear lens shape on the surface facing the linear light source, first scatters the light incident from the linear light source, and further scatters and condenses the transmitted light on the prism surface to be emitted from the diffusion plate. This is the point to completely eliminate the lamp unevenness.

The present invention will be described based on examples.
[Measuring method]
The measurement method of each measurement item used in Examples and Comparative Examples is as follows.
1. Measurement of prism apex angle The cross-sectional shape of the diffuser plate was projected by a reflection type projector, and the apex angle of the prism shape of the projected cross section was measured using a protractor.
2. Measurement of lens crest height Similar to the prism apex angle, the cross-sectional shape of the diffuser is projected by a reflective projector, and the distance between the line connecting the peak and peak of the projected fine lens shape and the line connecting the trough and valley is determined by the lens. It measured with the micrometer which can be measured to 0.01 micrometer as a peak height of.
3. Measurement of luminance Using a luminance meter (BM-7 manufactured by Topcon Corporation), the luminance of the central portion of the television was measured from a position 50 cm away.
4). Measurement of lamp unevenness Evaluate visually with ○ (not visible), △ (visible), and × (visiblely visible) and measure the surface luminance value of the entire screen using a two-dimensional surface luminance meter (Cybernet Prometric). . For the measured surface brightness, the difference between the waviness component and the calculated standard deviation was digitized as the lamp unevenness value. Naturally, the smaller the lamp unevenness value is, the smaller the lamp unevenness is, and the relationship with the visually determined lamp unevenness indicates that if the lamp unevenness value is 0.02 or less, almost no lamp unevenness is seen, and further 0.01 or less. Then you will not see any lamp unevenness. Conversely, when it exceeded 0.03, lamp unevenness gradually appeared, and when it exceeded 0.1, lamp unevenness was obvious to anyone's eyes.

[Example 1]
1 part by weight of silicone-based crosslinked fine particles (KMP manufactured by Shin-Etsu Chemical Co., Ltd., average particle diameter: 2 μm) as a diffusing agent was blended in 100 parts by weight of a polystyrene resin (PSPS manufactured by PS Japan Co., Ltd.) as a translucent resin, and charged into an extruder.
The resin melt-kneaded by the extruder was widened and discharged by a mold called a sheet T-die, wound around three cooling rolls, and formed into a sheet-like diffusion plate having a thickness of 1.5 mm.
At this time, a linear prism shape having an apex angle of 120 degrees and a pitch of 100 μm is dug in advance on one surface of the cooling roll (referred to as a prism roll), and one cooling roll facing the prism roll is provided. In the same way, a linear lens shape having a peak height of 5 μm and a pitch of 30 μm is engraved on the entire circumference by a cutting process (called a lens roll), and a sheet is extruded and molded so as to pass between the prism roll and the lens roll. Was prepared as a double-sided diffuser plate in which a linear prism shape with an apex angle of 120 degrees and a linear lens shape with a peak height of 5 μm were formed on the other side.

The double-sided diffuser plate thus produced was mounted on a commercially available 32-inch liquid crystal television and the brightness and lamp unevenness were measured.
In this 32-inch liquid crystal television, 12 cold cathode fluorescent lamps having a diameter of 3 mm are arranged in parallel at a pitch of 33 mm as a linear light source on a reflection sheet made of white PET, and a diffusion plate, a diffusion film as an optical film, A prism film and a reflective polarizing film were sequentially disposed, and a VA type liquid crystal panel mounted thereon was used.
The diffusion plate with a double-sided shape manufactured as described above is mounted so that the linear lens-shaped surface faces the linear light source so that the linear prism-shaped surface is in contact with the optical film, and the brightness at the center of the screen is increased. It was 510 cd when measured with a luminance meter (BM7 manufactured by Topcon). The lamp unevenness value was 0.001, and no lamp unevenness was seen.
The evaluation results are shown in Table 1.

[Comparative Example 1]
Except for using a mirror surface smooth cooling roll instead of a lens roll in the extrusion process, the same procedure as in Example 1 was performed to obtain a diffusion plate having a linear prism shape on one side. Of course, the opposite surface had no linear lens shape and was a flat surface with a slight undulation of 0.08 μm in height.
The PS diffuser plate having a flat prism apex angle of 120 degrees on the back was mounted on a commercially available 32-inch liquid crystal television in the same manner as in Example 1, and the luminance and lamp unevenness value were measured. 1. The lamp unevenness was a result visually visible.
The results are shown in Table 1.

[Examples 2 to 7], [Comparative Example 2]
The same operation as in Example 1 was performed except that the height of the digging into the lens roll was changed.
The evaluation results are shown in Table 1.

[Examples 8 to 10], [Comparative Examples 3 to 4]
The same operation as in Example 1 was performed except that the prism apex angle of the prism roll was changed.
The evaluation results are shown in Table 1.

[Comparative Example 5]
Evaluation was made in the same manner using a commercially available styrene-based diffuser plate having no prism shape on the surface and having irregularities of Ra = 0.1 on both surfaces. The luminance of the 32-inch liquid crystal television equipped with the diffusion plate was 450 cd, the lamp unevenness value was 0.02, and a little lamp unevenness was seen.

  The diffusion plate of the present invention can be suitably used as a diffusion plate for a direct type backlight type liquid crystal display.

FIG. 3 is a schematic plan view of a direct-type backlight type liquid crystal display equipped with the diffusion plate of the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Liquid crystal panel 2 Optical film 3 Diffusion plate of this invention 4 Linear light source (cold-cathode tube)
5 Reflector 6 Case 7 Linear prism shape 8 Linear lens shape

Claims (1)

  A diffusing plate mounted on a direct-type backlight type liquid crystal display arranged in the order of a reflector, a linear light source, a diffusing plate, an optical film, and a liquid crystal panel, the diffusing plate on the surface in contact with the optical film A linear prism shape with an apex angle of 50 to 150 degrees is molded, and a linear lens shape with a peak height of 0.2 to 20 μm is molded on the surface of the diffusion plate facing the linear light source. A diffusion plate characterized by having

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