JP2008242002A - Diffuser with prism - Google Patents

Abstract

Disclosed is a diffusion plate for a direct-type backlight type liquid crystal display, which is capable of preventing a decrease in luminance and completely eliminating visible lamp unevenness.
A diffusing plate 3 is mounted on a direct-type backlight type liquid crystal display arranged in the order of a reflecting plate 5, a linear light source 4, a diffusing plate 3, an optical film 2, and a liquid crystal panel 1. The surface of the plate 3 in contact with the optical film 2 has a linear prism shape with an apex angle of 50 to 150 degrees, and the surface of the prism has a surface roughness (Ra) of 0.2 to 20 μm. Irregularities are provided.
[Selection] Figure 1

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 it is important to improve the uniformity by blurring the linear light and darkness (so-called lamp unevenness) formed 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 capable of preventing a decrease in luminance and completely eliminating visible lamp unevenness.

As a result of diligent research to solve the above-mentioned problems, the inventors of the present invention have surprisingly achieved high brightness and visual lamp unevenness by forming a prism shape on the diffusion plate surface and roughening the prism surface. The inventors have found that it can be erased and have reached the present invention.
That is, the present invention
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 having an apex angle of 50 to 150 degrees is formed, and the prism surface is provided with fine irregularities having a surface roughness (Ra) of 0.2 to 20 μm. Diffusion plate,
2. Further, the surface of the diffusion plate facing the linear light source is provided with fine irregularities having a surface roughness (Ra) of 0.2 to 20 μm. It is a diffusion plate as described in.

  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 visual lamp unevenness.

The present invention will be described in detail below.
A direct-type backlight type liquid crystal display often used for liquid crystal televisions has a structure in which a liquid crystal panel is combined with a back light source device called a direct-type backlight, and as shown in FIG. A plate, 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 referred to as a direct type backlight, and the liquid crystal panel combined and integrated is referred to as a liquid crystal display.
As the reflection plate, a metal plate coated with a reflection material or a white or silver polyethylene terephthalate (PET) or polycarbonate (PC) reflection 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 or less with respect to 100 parts 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 mounted on an 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 backlight to increase the brightness as the liquid crystal display, that is, the brightness. .

  One way to increase the brightness of the direct type backlight is to increase the number of linear light sources. At present, in a liquid crystal display for a 32-inch liquid crystal television, 16 to 20 linear light sources are installed, although there are differences 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 in the liquid crystal display screen is seen through like a nonuniformity.
It is a general phenomenon called lamp unevenness that a bright light is directly above a linear light source, a gap between adjacent linear light sources, that is, a so-called lamp becomes 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, amount, etc. of the light diffusing material to be blended in the diffusion plate have been redesigned and efforts have been made to eliminate 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 diffuser plate of the present invention forms a linear prism shape on the surface on the optical film side, and further provides light incident from a linear light source by providing fine irregularities having a specific surface roughness on the prism surface. The optical path is greatly changed using the catadioptric function of 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 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 other various methods.
Extrusion molding or thermal transfer is a continuous roll in which the surface shape of a so-called prism roll, which is carved into a prism by cutting or etching, is transferred to the surface of the diffusion plate and cooled and solidified. It is a mold 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. Further, it is possible to reduce the lamp unevenness and increase the transmittance, that is, to 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 further surprisingly found that the visual lamp unevenness can be completely eliminated by providing fine irregularities on the surface of the prism-shaped surface.
As for the size of the fine unevenness, when the arithmetic average roughness measured by JIS B0601, that is, the so-called surface roughness (Ra) is 0.2-20 μm fine unevenness, the visual lamp unevenness can be completely eliminated. In order to enhance the effect of eliminating lamp unevenness, Ra is 0.2 μm or more, preferably 0.3 μm or more, more preferably 0.5 μm or more.
On the other hand, even if Ra is large, the visual lamp unevenness cannot be completely eliminated, so that it is 20 μm or less, preferably 15 μm or less, more preferably 12 μm or less.

  There are several methods for forming fine irregularities on the prism surface, but during the diffusion plate extrusion process, the surface shape of the prism roll engraved with the fine irregularities described above is transferred to the diffusion plate surface and solidified by cooling. A continuous molding process or a method of press molding on the surface of the diffusion plate using a prism mold in which the above-described fine irregularities are engraved with a single wafer is preferable. The method of making the surface of the prism roll or prism mold, that is, the prism surface fine, can be processed by generally known methods such as sand blasting, etching, chipping, and electric discharge machining.

  In addition, there is a method in which a diffusing material blended in the diffusing plate is raised on the surface of the diffusing plate, and fine irregularities are formed on the prism surface by forming fine irregularities. This method uses a molding temperature difference and utilizes the cooling shrinkage phenomenon of the light diffusing material and the translucent resin as a matrix. The translucent resin heated to 200 ° C. or higher is cooled to 100 ° C. or lower. In this method, the resin is rapidly brought into contact with the mold and the resin is rapidly cooled, whereby the translucent resin around the light diffusing material is rapidly contracted, and the light diffusing material that cannot be contracted is raised to form a granular surface. When the mold temperature is low, rapid cooling is strong and the surface roughness increases, and as the mold temperature increases, the surface roughness decreases. Further, this method is preferable because the surface roughness of fine irregularities can be controlled by the amount of the light diffusing material. When the amount of the light diffusing agent is large, the surface roughness increases, and when the amount of the light diffusing material is small, the surface roughness decreases.

  In the present invention, it is essential that the fine unevenness is on the prism surface in contact with the optical film, but there may be fine unevenness on the opposite surface, that is, the surface facing the linear light source. The size of the fine irregularities at this time is preferably 0.2 to 20 μm in order to eliminate lamp unevenness as in the prism surface. The size of the fine unevenness formed on the surface opposite to the fine unevenness of the prism surface may be the same or different.

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 vertical angle The cross-sectional shape of the diffuser plate was projected by a reflective projector, and the vertical angle of the prism shape of the projected cross section was measured using a protractor.
2. Measurement of surface roughness (Ra) In accordance with JIS B0601, irregularities on the surface of the diffusion plate were measured using a surface roughness meter (SURFCOM manufactured by Tokyo Seimitsu Co., Ltd.), and quantified by the surface roughness (Ra).
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 luminance, the difference from the waviness component was taken, and the value obtained by calculating the standard deviation was quantified 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]
100 parts by weight of polystyrene resin (GPPS manufactured by PS Japan Co., Ltd.) as a translucent resin and 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 were added to an extruder. .
The resin melted and kneaded by the extruder was widened and discharged by a mold called a sheet T-die, wound around three cooling rolls, and contacted to form 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 by a cutting process (prism roll), and is obtained by transferring the prism shape of this mold. A prism shape with an apex angle of 120 degrees is formed on one side of the diffuser plate, as with the mold.
Also, by setting the die mold temperature to 240 ° C., the resin containing the diffusing material discharged from the die is controlled to 220 ° C. at the temperature immediately before contacting the prism roll, and suddenly applied to the cooling roll set to 80 ° C. By contact and rapid cooling, the polystyrene resin around the diffusing material was cooled and contracted, and fine irregularities were raised on the surface of the prism surface.

It was 5 micrometers when the surface roughness (Ra) was measured for the fine unevenness | corrugation using the surface roughness meter (SURFCOM by Tokyo Seimitsu Co., Ltd.) by the method based on JISB0601.
The diffusion plate with the linear prism shape thus produced was mounted on a commercially available 32-inch liquid crystal television, and the luminance 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 white PET reflection sheet, on which 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.

A PS diffuser plate with a linear prism with apex angle of 120 degrees on one side and a surface roughness (Ra) of 5 μm on one side is mounted so that the linear prism surface is in contact with the optical film. The luminance at the center was measured with a luminance meter (BM7 manufactured by Topcon Corporation) and found to be 510 cd. Further, when the lamp unevenness value was measured, it was 0.001, and no lamp unevenness was seen.
The evaluation results are shown in Table 1.

[Comparative Example 1]
In the extruding step, the same procedure as in Example 1 was performed except that the temperature of the cooling roll was set to 120 ° C., to obtain a diffuser plate having a prism shape with a vertex angle of 120 degrees.
Since the roll temperature was high, the rapid cooling was weak and the surface of the linear prism surface was almost smooth. When the surface roughness was measured as in Example 1, Ra = 0.08 μm.
This PS diffuser plate with a linear prism having a vertex angle of 120 degrees was mounted on a commercially available 32-inch liquid crystal television in the same manner as in Example 1 and measured for luminance and lamp unevenness value. As a result, the luminance was 520 cd, the lamp unevenness value was 0.05, and the lamp unevenness value was. The result was visible. The results are shown in Table 1.

[Examples 2 to 4, Comparative Examples 2 and 3]
As shown in Table 1, in the extrusion process, the temperature of the cooling roll was changed to 130 to 90 ° C. in the same manner as in Example 1 to obtain a diffuser plate with a prism shape. Table 1 shows the surface roughness (Ra) of the diffusion plate surface, the luminance and the lamp unevenness value evaluated by mounting on a 32-inch liquid crystal television.
As the surface roughness becomes smooth, that is, the Ra value decreases, the lamp unevenness value increases and the lamp unevenness becomes more visible.

[Examples 5 to 7, Comparative Example 4]
By further etching the surface of the prism roll, Ra = 10, 15, 20, and 30 μm irregularities were dug to form a mat roll with a prism.
It carried out like Example 1 except having installed the mat roll with a prism in an extrusion process. The obtained diffuser plate became a PS diffuser plate having a linear prism shape with an apex angle of 120 degrees transferred on one side, and further, irregularities having a surface roughness (Ra) of 10 to 30 μm transferred to the prism surface. . The obtained diffusion plate was evaluated in the same manner as in Example 1 and shown in Table 1.

[Examples 8 to 10, Comparative Examples 5 and 6]
The same procedure as in Example 1 was performed except that the apex angle of the prism roll was changed to 60 to 160 degrees as shown in Table 1. The evaluation results are shown in Table 1.

[Example 11]
The same procedure as in Example 1 was performed except that a prism roll having an apex angle of 120 degrees and a mat roll in which unevenness of Ra = 10 μm was dug by electric discharge machining, and both roll temperatures were set to 80 ° C. .
The obtained diffusion plate was a PS diffusion plate having a prism surface with Ra = 5 μm fine unevenness on the surface and a mat surface with Ra = 10 μm fine unevenness on the opposite side.
When this diffuser plate was mounted on a 32-inch liquid crystal television in the same manner as in Example 1 and the brightness and lamp unevenness were measured, the brightness was 500 cd, the lamp unevenness value was 0.005, and the lamp unevenness was not visible.

[Example 12]
A diffusion plate with a prism shape was obtained in the same manner as in Example 5 except that the temperature of the cooling roll was 130 ° C. in the extrusion step.
Although unevenness of Ra = 10 μm is transferred to the prism surface, the linear light source side has a substantially smooth surface due to the high cooling roll temperature. The results are shown in Table 1.

[Comparative Example 7]
Evaluation was carried out in the same manner as in Example 1 using a styrene-based diffusion plate having a prism shape on the surface and Ra = 0.1 unevenness on the surface. The luminance of the mounted 32-inch liquid crystal television was 450 cd, the lamp unevenness value was 0.02, and almost no 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

Claims (2)

  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 having an apex angle of 50 to 150 degrees is formed, and the prism surface is provided with fine irregularities having a surface roughness (Ra) of 0.2 to 20 μm. Diffusion plate.   Furthermore, the fine unevenness | corrugation whose surface roughness (Ra) is 0.2-20 micrometers is provided in the surface at the side which opposes the linear light source of a diffusion plate, The diffusion of Claim 1 characterized by the above-mentioned. Board.

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