JP2008304501A - Diffusion plate - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a diffusion plate capable of emitting light brightly, uniformly and evenly over the whole surface independent of arrangement and shape of a light source, and capable of removing lamp irregularity. <P>SOLUTION: A plurality of prism lenses of the same shape are arranged side by side on one side surface, wherein the cross-section of each prism lens takes the form of a shape formed by connecting at least four oblique lines each of which is a straight line having an angle θ formed with the bottom side of 0° <θ<90° and an apex formed of a curve, symmetrically and combinatorially to the center line. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a diffuser plate, a direct type backlight mounted with the diffuser plate, and 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 direct-type backlight type liquid crystal display has a structure in which a reflector, a linear light source, a diffuser, an optical film, and a liquid crystal panel are sequentially arranged. 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.

  As a method for eliminating the lamp unevenness, a method of thickening the light diffusing material in a resin diffusion plate containing a light diffusing material can be considered. However, this method has a problem in that the luminance is lowered.

  In Patent Documents 1 and 2, a high-functional plate is also developed in which a prism lens shape is formed on the surface of the diffusion plate, brightness reduction is prevented by a light condensing function of the lens, and lamp unevenness is eliminated.

  However, Patent Documents 1 and 2 do not completely eliminate the lamp unevenness.

JP 2007-18939 A JP-A-7-230002

  SUMMARY OF THE INVENTION An object of the present invention is to provide a diffusion plate that can emit light uniformly and evenly on the entire surface and eliminate lamp unevenness, and a direct-type backlight and a liquid crystal display equipped with the diffusion plate.

  That is, the diffuser plate of the present invention has a plurality of prism lenses having the same shape arranged in parallel on one surface, and the cross-sectional shape of the prism lens is a straight line having an angle θ with the base of 0 ° <θ <90 °. It is characterized in that it has a shape in which four or more hypotenuses made of and a top portion made of a curve are connected symmetrically.

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

  Hereinafter, the present invention will be described in detail.

<Backlight, liquid crystal display>
FIG. 1 shows an example of a direct backlight type liquid crystal display of the present invention.

  As shown in FIG. 1, in the liquid crystal display, a reflection plate 1, a linear light source 2, a diffusion plate 3, an optical film 4, and a liquid crystal panel 5 are sequentially arranged from the inside. Among these, the direct backlight 6 is composed of the reflector 1 to the optical film 4.

  The reflector 1 is made of a metal plate coated with a reflector, or a white or silver polyethylene terephthalate (PET) -based or polycarbonate (PC) -based reflective film.

  The linear light source 2 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 diffuser plate 3 is an important optical member that scatters the light from the linear light source 2 and converts the linear light source into a planar light source. As shown in FIG. 1, the diffusing plate 3 is disposed such that the lens forming surface is opposite to the linear light source 2 and the longitudinal direction of the lens coincides with the longitudinal direction of the linear light source 4. Details of the diffusion plate 3 will be described later.

  The optical film 4 improves so-called luminance, such as a diffusion film 4-1 that further scatters or collects light transmitted through the diffusion plate 3, a prism film 4-2 that collects scattered light, and a reflective polarizing film 4-3. It is a group of a plurality of highly functional films such as a film to be made.

  As the liquid crystal panel 5, known ones such as VA type and IPS type can be used.

<Diffusion plate>
FIG. 2 shows a schematic perspective view of an example of the diffusion plate of the present invention. FIG. 3 is a cross-sectional view of the diffusing plate of FIG. 2 on a surface orthogonal to the longitudinal direction of the prism lens.

  As shown in FIG. 2, the diffuser plate of the present invention is such that the same shape of the prism lens 7 is on one surface, and the adjacent prism lenses 7 are in contact with each other at the sheet surface 8 so that the generatrix is parallel. A plurality of them are arranged side by side.

  As shown in FIG. 3, the cross-sectional shape of the prism lens 7 is a shape in which the base 9 that forms the sheet surface 8, the four oblique sides 10 to 14 that are straight lines, and the top portion 12 that is a curve are connected symmetrically. It has become. The angle formed by the hypotenuse 10 and the base 9 is 80 °, the angle formed by the hypotenuse 11 and the base 9 is 40 ° (the inner angle formed by the hypotenuses 10 and 11 is 140 °), and the hypotenuse 13 and the base 9 are formed. The angle is 40 ° (the inner angle formed by the hypotenuses 13 and 14 is 140 °), the angle formed by the hypotenuse 14 and the base 9 is 80 °, and the top 12 is formed by an arc.

  The cross-sectional shape of the prism lens 7 is not limited to that shown in FIG. When the hypotenuse is less than four, the dark portion occupies a larger proportion than the bright portion and the contrast of the light and darkness is large. Therefore, when used in a direct type backlight, even if an optical sheet is combined, the contrast of the light and darkness can be erased. It is not possible to produce uniform light emission. In the present invention, in order to increase the luminance of the dark portion and achieve uniform light emission, the light emission that effectively suppresses the occurrence of the dark portion is enabled by using the directivity of the prism.

  Further, the angle θ formed between the hypotenuse and the base must be 0 ° <θ <90 °. If the angle θ between the hypotenuse and the base exceeds 90 °, it is very difficult to produce a mold for molding the diffusion plate, and the transfer rate from the mold to the resin is greatly reduced, which is not practical.

The value of the angle θ formed by the oblique side and the bottom side increases the brightness of the dark part, reduces the contrast as much as possible, and achieves uniform light emission. The position r of the dark part of the diffusion plate (directly above the linear light source of the diffusion plate) It is preferable to determine based on the following formula according to the shortest distance from the position corresponding to the dark part of the diffusion plate).
r = d · tan α + (t + ½ · tan θ) tan {θ−arcsin (1 / n · sin θ)}
d: Distance from the linear light source to the diffuser α: Incident angle from the linear light source to the diffuser (the normal of the diffuser is 0 °)
t: thickness of diffusion plate n: refractive index of diffusion plate resin

  Further, the angle θ formed between the hypotenuse and the base takes two (two sets) or more different angles (θ1, θ2,... Θm,... Θn...), At least one of them ( One set) is preferably 45 ° or more.

  Furthermore, the difference between the different angles θm and θn is preferably 10 ° or more. When the angle is less than 10 °, the effect of increasing the luminance in the dark portion may be reduced.

  Also, the apex must be a curve, preferably an arc. When the top portion is not a curve, the contrast of light and dark on the light emitting surface is large. Therefore, when used in a direct type backlight, even if an optical sheet is combined, the contrast of light and dark cannot be erased.

  The prism lens pitch (base length) is preferably 50 μm to 500 μm. If the pitch is less than 50 μm, there is a possibility that the emitted light from the lens does not become the intended pattern due to the processing accuracy of the mold for molding the diffusion plate. If the pitch exceeds 500 μm, the incident light is generated even within a single lens. There is a possibility that the angle and the density of the light are remarkably different at the light receiving location and the intended emission pattern is not obtained.

  The height of the prism lens is not particularly limited, but is preferably 25 μm to 1000 μm.

  The thickness of the diffusing plate is preferably 0.4 mm to 5 mm, and more preferably 1 mm to 3 mm.

  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., which have high optical characteristics, particularly high transmittance, are preferable. In order to prevent deformation due to water absorption, a resin having a low water absorption rate is more preferable. For example, styrene resin, MS resin, PC resin, and cyclic olefin resin 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 diffuser plate may be a single sheet in which a light diffusing material (light diffusing fine particles) is blended with a translucent resin, but may be a laminated sheet for improving light resistance and improving 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.

  Examples of the method for forming a lens on the surface of the diffusion plate include extrusion molding, UV molding, thermal transfer, compression molding, shaving, etching, and other various methods.

  Extrusion molding and thermal transfer are continuous molding processes in which the surface shape of a die roll engraved with a lens shape by cutting or etching is transferred to the surface of the diffusion plate and cooled and solidified during the extrusion process for producing the diffusion plate. . UV molding may be a continuous extrusion process or a single wafer batch process, but is a method of forming a lens 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 planar mold engraved with a lens shape onto the surface of a diffusion plate.

  As a method for producing a mold roll or a planar mold, any method may be used as long as a predetermined lens shape can be dug, such as cutting, etching, and electrical discharge machining.

<Evaluation method>
(1) Measurement of lens shape The diffuser plate was cleaved or cut and polished to expose the cross section of the prism, and the angle θ formed by the hypotenuse and the base, the length of the base, and the height were measured under an optical microscope.

(2) Average Luminance, Contrast Ratio, and Visibility of Light Source Image The diffusion plate was mounted on a 32-inch direct backlight type liquid crystal display for evaluation, and the luminance and the visibility of the light source image were measured.

  As shown in FIG. 1, the evaluation liquid crystal display device has 12 cold cathode fluorescent tubes having a diameter of 3 mm arranged in parallel at a pitch of 33 mm as a linear light source 2 on a white PET reflecting sheet (reflecting plate 1). The diffusing plate 3 is provided thereon, and the diffusing film 4-1, the prism film 4-2, and the reflective polarizing film 4-3 are sequentially disposed as the optical film 4, and the VA type liquid crystal panel 5 is disposed thereon. It is installed. The diffusion plate 3 was mounted so that the lens forming surface was in contact with the optical film 4 and the longitudinal direction of the lens and the linear light source was matched.

(2-1) Average luminance, contrast ratio The optical film 4 and the liquid crystal panel 5 are removed, and the luminance at the center of the screen is measured from a position 50 cm away using a luminance meter (Topcon BM-7). Asked.

  The contrast ratio was calculated from the following formula.

  Contrast ratio = (maximum luminance value−minimum luminance value) / (maximum luminance value + minimum luminance value)

(2-2) Visibility of light source image The liquid crystal panel 5 was removed, whether or not the light source image was visually observed from the front of the screen was evaluated and evaluated according to the following criteria.
◎: The light source image cannot be seen from the front or from the front ○: The light source image cannot be seen from the front △: The light source image can be seen a little ×: The light source image can be seen clearly

<Example 1>
100 parts by weight of polystyrene resin (PSPS manufactured by PS Japan Co., Ltd.) as a translucent resin and 0.1 part by weight of silicone-based crosslinked fine particles (KMP manufactured by Shin-Etsu Chemical Co., Ltd., average particle size 2 μm) as a diffusing material are added to an extruder. did.

  The resin melt-kneaded by the extruder was widened and discharged by a mold called a sheet T-die, and wound around three cooling rolls to form a sheet having a thickness of 1.5 mm.

  This sheet was heated to about 200 ° C., and a die having grooves engraved with prisms having a cross-sectional shape shown in FIG. 3 arranged in parallel on the surface was thermocompression bonded to obtain a diffusion plate shown in FIG.

  The evaluation results are shown in Table 1. In Table 1, the numerical value in parentheses in the column of the angle θ formed by the hypotenuse and the base is the inner angle formed by the hypotenuses.

<Examples 2-5, Comparative Examples 1-6>
A diffusion plate was produced and evaluated in the same manner as in Example 1 except that the lens shape was changed as shown in Table 1. The results are shown in Table 1.

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

It is a schematic sectional drawing which shows an example of the liquid crystal display of this invention. It is a schematic perspective view of an example of the diffusion plate of the present invention. It is sectional drawing in the surface orthogonal to the longitudinal direction of the prism lens of the diffusion plate of FIG. FIG. 6 is a cross-sectional view of a plane orthogonal to a longitudinal direction of a prism lens of a diffusion plate of Example 2. FIG. 6 is a cross-sectional view of the diffusion plate of Example 3 on a surface orthogonal to the longitudinal direction of the prism lens. FIG. 6 is a cross-sectional view of the diffusion plate of Example 4 on a surface orthogonal to the longitudinal direction of the prism lens. FIG. 10 is a cross-sectional view of a surface orthogonal to the longitudinal direction of a prism lens of a diffusion plate of Example 5. It is sectional drawing in the surface orthogonal to the longitudinal direction of the prism lens of the diffusion plate of the comparative example 1. It is sectional drawing in the surface orthogonal to the longitudinal direction of the prism lens of the diffusion plate of the comparative example 2. It is sectional drawing in the surface orthogonal to the longitudinal direction of the prism lens of the diffusion plate of the comparative example 3. It is sectional drawing in the surface orthogonal to the longitudinal direction of the prism lens of the diffusion plate of the comparative example 4. It is sectional drawing in the surface orthogonal to the longitudinal direction of the prism lens of the diffusion plate of the comparative example 5. It is sectional drawing in the surface orthogonal to the longitudinal direction of the prism lens of the diffusion plate of the comparative example 6.

Explanation of symbols

1 Reflector 2 Linear light source (cold cathode tube)
DESCRIPTION OF SYMBOLS 3 Diffusion plate 4 Optical film 4-1 Diffusion film 4-2 Prism film 4-3 Reflective polarizing film 5 Liquid crystal panel 6 Direct type backlight 7 Prism lens 8 Sheet surface 9 Base 10, 11, 13, 14 Oblique 12 Top

Claims (6)

  A plurality of prism lenses having the same shape are arranged in parallel on one surface, and the cross-sectional shape of the prism lenses is four or more oblique sides made of a straight line having an angle θ with the base of 0 ° <θ <90 °, A diffusion plate characterized by having a shape in which a top portion made of a curve is connected symmetrically.   The diffusion plate according to claim 1, wherein at least one of the angles θ formed with the base is 45 ° or more.   The diffusion plate according to claim 1, wherein the curved line is an arc.   The diffusion plate according to claim 1, wherein a pitch of the prism lenses is 50 μm to 500 μm.   It is a direct type backlight arrange | positioned in order of a reflecting plate, a linear light source, a diffusion plate, and an optical film, Comprising: The said diffusion hill is a diffusion plate in any one of Claims 1-4, The said prism lens The direct-type backlight is characterized in that is disposed on the opposite side of the linear light source and the longitudinal direction of the prism lens coincides with the longitudinal direction of the linear light source.   6. A liquid crystal display, wherein a liquid crystal panel is disposed on the direct type backlight according to claim 5.

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