WO2009057844A1 - Light guide panel for lcd back light unit and lcd back light unit thereby - Google Patents

Light guide panel for lcd back light unit and lcd back light unit thereby Download PDF

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Publication number
WO2009057844A1
WO2009057844A1 PCT/KR2007/005976 KR2007005976W WO2009057844A1 WO 2009057844 A1 WO2009057844 A1 WO 2009057844A1 KR 2007005976 W KR2007005976 W KR 2007005976W WO 2009057844 A1 WO2009057844 A1 WO 2009057844A1
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WO
WIPO (PCT)
Prior art keywords
light
guide plate
light guide
plate according
prism
Prior art date
Application number
PCT/KR2007/005976
Other languages
French (fr)
Inventor
O Yong Jeong
Seong Hoon Lee
Man Suk Kim
Original Assignee
Cheil Industries Inc.
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Cheil Industries Inc. filed Critical Cheil Industries Inc.
Priority to CN2007801013897A priority Critical patent/CN101842738B/en
Publication of WO2009057844A1 publication Critical patent/WO2009057844A1/en

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Classifications

    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells
    • G02F1/133—Constructional arrangements; Operation of liquid crystal cells; Circuit arrangements
    • G02F1/1333—Constructional arrangements; Manufacturing methods
    • G02F1/1335—Structural association of cells with optical devices, e.g. polarisers or reflectors
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/0001—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings specially adapted for lighting devices or systems
    • G02B6/0011—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings specially adapted for lighting devices or systems the light guides being planar or of plate-like form
    • G02B6/0033—Means for improving the coupling-out of light from the light guide
    • G02B6/0035—Means for improving the coupling-out of light from the light guide provided on the surface of the light guide or in the bulk of it
    • G02B6/0036—2-D arrangement of prisms, protrusions, indentations or roughened surfaces
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/0001—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings specially adapted for lighting devices or systems
    • G02B6/0011—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings specially adapted for lighting devices or systems the light guides being planar or of plate-like form
    • G02B6/0033—Means for improving the coupling-out of light from the light guide
    • G02B6/0058—Means for improving the coupling-out of light from the light guide varying in density, size, shape or depth along the light guide
    • G02B6/0061—Means for improving the coupling-out of light from the light guide varying in density, size, shape or depth along the light guide to provide homogeneous light output intensity
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/0001—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings specially adapted for lighting devices or systems
    • G02B6/0011—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings specially adapted for lighting devices or systems the light guides being planar or of plate-like form
    • G02B6/0033—Means for improving the coupling-out of light from the light guide
    • G02B6/0035—Means for improving the coupling-out of light from the light guide provided on the surface of the light guide or in the bulk of it
    • G02B6/0038—Linear indentations or grooves, e.g. arc-shaped grooves or meandering grooves, extending over the full length or width of the light guide
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/0001—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings specially adapted for lighting devices or systems
    • G02B6/0011—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings specially adapted for lighting devices or systems the light guides being planar or of plate-like form
    • G02B6/0033—Means for improving the coupling-out of light from the light guide
    • G02B6/0035—Means for improving the coupling-out of light from the light guide provided on the surface of the light guide or in the bulk of it
    • G02B6/0045—Means for improving the coupling-out of light from the light guide provided on the surface of the light guide or in the bulk of it by shaping at least a portion of the light guide
    • G02B6/0046—Tapered light guide, e.g. wedge-shaped light guide

Definitions

  • the present invention relates to a light guide plate for a back light unit of a liquid crystal display device. More specifically, the present invention relates to a wedge-type light guide plate for a back light unit of a liquid crystal display device, which has a rear surface formed with a plurality of prisms, each having an asymmetric triangular cross-section so as to increase a vertical half viewing angle and improve optical properties.
  • a liquid crystal display (hereinafter, referred to simply as a "LCD”) device refers to a device which displays numerals or images through application of an electric field to liquid crystals disposed between two glass substrates, in which the liquid crystals are made of a material having an intermediate phase between a liquid and a solid.
  • the LCD device Since the LCD device is not a self-luminescent device, it must be provided with a back light unit as a light source to generate light. An image is displayed in such a manner that transmittance of light generated from the back light unit is adjusted in a liquid crystal panel, in which liquid crystals are uniformly arranged.
  • a LCD back light unit is classified to a direct-type back light unit in which the light source is directly positioned at a rear side of a liquid crystal panel of an LCD, and an edge-type back light unit in which the light source is positioned at a side of the liquid crystal panel of an LCD.
  • the edge-type back light unit is sub-classified to a wedge-type back light unit and a flat-type back light unit.
  • FIG. 9 is a view illustrating the structure of a conventional back light unit using a wedge-type light guide plate.
  • a light source 2 serves to emit light initially in the LCD device. Although various types of light source can be used, the LCD device generally employs a cold cathode fluorescence lamp (CCFL) which has low power consumption and emits highly bright white light.
  • CCFL cold cathode fluorescence lamp
  • a light guide plate 4 is provided below an LCD panel and at one side of the light source 2 and serves to convert spot light generated from the light source 2 into plane light and thus project the plane light forward to the LCD panel.
  • a reflection plate 3 is provided on a rear side of the light guide plate 4, and serves to reflect light emitted from the light source 2 toward the LCD panel disposed in front of the reflection plate 3.
  • the diffusion sheet 5 is provided on a front side of the light guide plate 4, and serves to uniformize light passing through the light guide plate 4.
  • prism sheets 6 are used to reflect and concentrate the light, thereby providing enhanced brightness.
  • a protective sheet 7 is provided above the prism sheets 6, and serves to prevent scratches on the prism sheet 6, and to prevent Moire effect from occurring when using the prism sheets 6 arranged in two layers in the horizontal and vertical directions.
  • the back light unit further includes a mold frame (housing) that serves as a case to allow respective components of the back light unit to be fixed so that the back light unit can be integrated, and a lamp cover (back cover) to protect the back light unit while serving to maintain the strength of the back light unit and support the back light unit.
  • a mold frame housing
  • back cover lamp cover
  • the light source 2 is generally located at one edge of the back light unit (for an LCD TV, the light source is often located directly at the rear side of a panel).
  • the light guide plate 4 is used.
  • the light guide plate is generally made of a transparent acrylic resin, which is not easily breakable due to its high strength, and has deformation resistance, light weight and high transmittance of visible light.
  • the light guide plate 4 serves to allow light emitted from the light source 2 to be uniformly projected to the overall surface of the light guide plate 4.
  • the back light unit is disassembled and light is allowed to emit from the light source 2 located at one side of the light guide plate 4, the light is not uniformly projected to the overall surface of the light guide plate 4, but is concentrated on both ends of the light guide plate 4. This is because the light guide plate 4 guides the light from the light source 2 to the opposite side of the light guide plate 4.
  • the rear surface of the light guide plate 4 is subjected to a specific treatment to cause scattered reflection of light in the light guide plate 4 such that light can be transmitted through the overall surface of the light guide plate 4.
  • the rear surface of the light guide plate 4 is formed with a prominence/depression pattern which has a predetermined shape designed in consideration of a distance from the light source 2 and the like.
  • the conventional LCD device constructed as above a portion of the panel with the prominence/depression pattern formed therebelow looks bright, and the other portions of the panel without the prominence/depression pattern therebelow looks dark, so that a spot phenomenon occurs on the panel, thereby deteriorating visibility of the LCD device.
  • a region separated by a far distance from the light source 2 lacks in absolute amount of light reaching there, and looks dark.
  • the diffusion sheet and the prism sheets are used for improvement of light uniformity, and cause an increase in manufacturing costs of the back light unit.
  • the present invention has been made to solve the foregoing problems of the prior art and it is one aspect of the present invention to provide a light guide plate for a LCD back light unit which exhibit superior brightness, uniformity of light and visibility over the entire surface of the LCD panel.
  • a light guide plate for a liquid crystal display device comprising: side surfaces on which light is incident; a front surface from which the light is emitted, the front surface connected to the side surfaces; and a rear surface on which the light is reflected wherein the rear surface is formed with a rear-prism part including a plurality of prisms, each having a longitudinal direction perpendicular to a direction of light emitted from a light source and an asymmetric triangular cross-section.
  • the light guide plate for the liquid crystal display device enables a uniform increase in amount of light reaching an overall surface of a panel of the liquid crystal display device, an increase in vertical half- viewing angle and an improvement in brightness.
  • the brightness and visibility of the light guide plate according to the present invention are significantly improved, in comparison to a conventional light guide plate in which prisms are disposed on a front surface thereof.
  • the back light unit can be realized without one or both of a diffusion sheet and a prism sheet, it' is possible to reduce manufacturing costs of the back light unit.
  • FIGs. 1 and 2 are perspective views illustrating a light guide plate for a LCD back light unit according to a first embodiment of the present invention, when viewed from different angles;
  • FIG. 3 is a sectional view taken along the line A-A in FIG. 2;
  • FIG. 4 is a sectional view taken along the line B-B in FIG. 2;
  • FIG. 5 is a perspective view illustrating a front-lens according to Experimental Examples of the present invention;
  • FIG. 6 is a graph showing brightness of front-lenses according to
  • FIG. 7 is a graph showing rear-prisms according to Experimental and Comparative Examples of the present invention.
  • FIG. 8 is a graph showing brightness of rear-prisms according to Experimental and Comparative Examples of the present invention.
  • FIG. 9 is a sectional-view illustrating the structure of a back light unit using a conventional wedge-type light guide plate.
  • the present invention is directed to a light guide plate for a liquid crystal display device, comprising: side surfaces on which light is incident; a front surface from which the light is emitted, the front surface connected to the side surfaces; and a rear surface on which the light is reflected wherein the rear surface is formed with a rear-prism part including a plurality of prisms, each having a longitudinal direction perpendicular to a direction of light emitted from a light source and an asymmetric triangular cross-section.
  • FIGs. 1 and 2 are perspective views illustrating a light guide plate for an LCD back light unit according to a first embodiment of the present invention, when viewed from different angles.
  • the first embodiment of the present invention is directed to a wedge-type light guide plate in which a light source L is located at one side thereof.
  • the light guide plate is generally made of a transparent acrylic resin, which has high strength, break and deformation resistances, light weight and high transmittance of visible light.
  • the light guide plate comprises a body 100 which comprises side surfaces 102, on which light is incident, a front surface 104, from which the light is emitted, wherein the front surface 104 is connected to the side surfaces 102 while facing a panel (not shown) of the LCD device, and a rear surface
  • the side surfaces 102 are defined as surfaces on which light is incident.
  • the side surfaces 102 refer to two surfaces adjacent to the light source L.
  • a rear-prism part 120 is formed on the rear surface 106 and includes a plurality of prisms 122, each having a longitudinal direction perpendicular to a direction of light emitted from the light sources and an asymmetric triangular cross- section.
  • the rear-prism part 120 uniformizes the brightness of the front surface 104 of the light guide plate.
  • the rear-prism part 120 increases in width w along with an increase in distance from the side surface 102 on which light is incident.
  • the reason for the increase in width w is as follows. As a distance from the side surface 102 on which light is incident increases, an amount of light reaching there becomes smaller.
  • the prisms 120 which serve to reflect and refract light, have a larger length, in spite of a small amount of light reaching the prisms 120, an amount of light reflected or refracted by the prisms 120 is increased, so that uniformity of light emitted toward the front surface 104 can be improved.
  • the rear-prism part 120 may have a stripe pattern in which a width is varied along with a distance from the light source.
  • the rear-prism part 120 may have a dot pattern.
  • the front surface 104 is formed with a front-lens part 140 composed of optical members 142, each having a half-oval cross-section.
  • An angle defined between the longitudinal direction of each optical member 142 and the longitudinal direction of each prism is preferably in the range of 80 to 100°, and more preferably
  • the front-lens part 140 serves to improve optical properties and visibility.
  • each of the optical members 142 has a pitch of 10 to 250 [M.
  • a conventional light guide plate has a smooth surface without any optical member, or has prisms with a triangular cross-section.
  • the rear-prism part formed on the rear surface of the light guide plate is exposed when viewed from the front surface thereof, thus disadvantageously causing a deterioration in visibility and
  • the front-lens part 140 composed of optical members 142, which have a half-oval cross-section is provided on the front surface 104 of the light guide plate and thus covers the rear-prism part 120, thereby improving visibility and optical properties.
  • FIG. 3 is a sectional view taken along the line A-A in FIG. 2.
  • FIG. 4 is a sectional view taken along the line B-B in FIG. 2.
  • optical members 142 are formed along the longitudinal direction of the cross-section of the light guide plate on the front surface thereof. Since the optical members 142 herein used are lenticular lenses, the longitudinal direction of each lenticular lens is the same as the longitudinal direction of the cross-section of each optical member. In other words, the longitudinal direction of the lenticular lens is parallel with the direction of light.
  • each prism is parallel with the longitudinal direction of the light source L.
  • a first internal angle ( ⁇ ) an internal angle adjacent to the light source
  • a second internal angle an internal angle opposite to the light source
  • the first internal angle ( ⁇ ) is in the range of 70° to 90° and the second internal angle ( ⁇ ) is in the range of 40° to 50°. Since a total in all internal angles of a triangle is 180°, an intermediate angle ( ⁇ ) is in the range of 40° to 70°.
  • a shape of prisms is defined by the ranges of the internal angles, so that a half viewing angle is increased. The improvement in half viewing angle will be illustrated in the following section with reference to specific Examples.
  • the optical members 142 disposed on the front surface of the light guide plate have an oval cross-section and the prisms 122 are spaced apart from each other on the rear surface of the light guide plate.
  • the prisms 122 increase in length along with an increase in a distance from the light source.
  • FIG. 5 is sectional views illustrating specific dimensions of each optical member in the form of a lenticular lens formed on the front surface of a light guide plate for a back light unit of an LCD device according to the present invention.
  • the respective lenticular lenses vary in a height H, a pitch P and an angle between the adjacent two lenses and have a uniform curvature radius of 25 ⁇ m.
  • the height H and the pitch P were measured with various angles of 0°, 20°, 40°, 60°, 80°, 100°, 120° and 140°.
  • lenticular lenses were disposed perpendicular to the light source on the front surface of the light guide plate, each of which has an oval cross-section with a curvature radius R of 25 ⁇ m, a height of 25 ⁇ m and a pitch of 50 (M and an angle between the adjacent lenses of 0°.
  • Experimental Examples 2 to 8 were performed in the same manner as in Experimental Example 1 except that the angle between the adjacent lenticular lenses formed on the front surface of the light guide plate were varied by 20° from 20° to 140°, and the height and pitch of the lenticular lenses were determined by the angle.
  • the specific dimensions of the lenticular lenses in Experimental Examples 1 to 8 were summarized in Table 1 and FIG. 5.
  • Curvature 25.0 ⁇ m 25.0 (M 25.0 ⁇ m 25.0 ⁇ m 25.0 ⁇ m 25.0 ⁇ m 25.0 ⁇ m 25.0 ⁇ m 25.0 ⁇ m radius
  • the wedge-type light guide plate was provided with a smooth front surface.
  • prisms were disposed perpendicular to the light source on the front surface of the light guide plate, each of which has a cross-section with a pitch of 50 ⁇ m, a height of 25 ⁇ m, and an angle between the adjacent lenses of 90°.
  • FIG. 6 is a graph showing the brightness according to Experimental and Comparative Examples of the present invention. It can be seen from Table 2 and FIG. 6 that Experimental Examples under the condition that a ratio of lens height to lens pitch is lower than 0.23, exhibited high central brightness, and all of Experimental Examples showed excellent pattern visibility, as compared to Comparative Examples. It can be also confirmed that when comparing both pattern visibility and brightness between Experimental and Comparative Examples, Experimental Examples 5 and 6 were the best.
  • a ratio of pitch to height of the respective optical members arranged on the front surface of the light guide plate is in the range of 1 : 0.1 to 1 : 0.35, an angle between the adjacent optical members is in the range of 60° to 120°, and the curvature radius of each optical member is in the range of 0.58- to 1.0 fold of the pitch of each lens.
  • Experimental Examples 10 to 13 were performed in the same manner as in Experimental Example 9 except that the first internal angle ( ⁇ ) was decreased to 70° by 5°, the intermediate angle ( ⁇ ) was increased to 45° by 5° and the second internal angle ( ⁇ ) was maintained at 45°.
  • the light guide plate was provided with a mirrored front surface.
  • FIG. 8 is a graph showing the brightness of the rear-surface prisms according to Experimental and Comparative Examples.
  • the liquid crystal display device of the present invention it is possible for the liquid crystal display device of the present invention to obtain substantially the same effects as that of the conventional liquid crystal display device without employing a prism sheet which are used in the back light unit of the conventional liquid crystal display device. That is to say, the use of the light guide plate according to the present invention eliminates the necessity of one or two prism sheets among a variety of sheets which are applied to the conventional back light unit.

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Optics & Photonics (AREA)
  • Nonlinear Science (AREA)
  • Mathematical Physics (AREA)
  • Chemical & Material Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Planar Illumination Modules (AREA)
  • Optical Elements Other Than Lenses (AREA)

Abstract

Disclosed herein is a light guide plate for an LCD back light unit. More specifically, provided is a wedge-type light guide plate for an LCD back light unit, which has a rear surface formed with a plurality of prisms, each having an asymmetric triangular cross-section so as to increase a vertical half viewing angle and improve optical properties. The light guide plate comprises side surfaces on which light is incident, a front surface from which the light is emitted, the front surface connected to the side surfaces, and a rear surface on which the light is reflected wherein the rear surface is formed with a rear-prism part including a plurality of prisms, each having a longitudinal direction perpendicular to a direction of light emitted from a light source and an asymmetric triangular cross-section.

Description

[DESCRIPTION] [Invention Title]
LIGHT GUIDE PANEL FOR LCD BACK LIGHT UNIT AND LCD BACK LIGHT UNIT THEREBY
[Technical Field]
The present invention relates to a light guide plate for a back light unit of a liquid crystal display device. More specifically, the present invention relates to a wedge-type light guide plate for a back light unit of a liquid crystal display device, which has a rear surface formed with a plurality of prisms, each having an asymmetric triangular cross-section so as to increase a vertical half viewing angle and improve optical properties.
[Background Art]
In general, a liquid crystal display (hereinafter, referred to simply as a "LCD") device refers to a device which displays numerals or images through application of an electric field to liquid crystals disposed between two glass substrates, in which the liquid crystals are made of a material having an intermediate phase between a liquid and a solid.
Since the LCD device is not a self-luminescent device, it must be provided with a back light unit as a light source to generate light. An image is displayed in such a manner that transmittance of light generated from the back light unit is adjusted in a liquid crystal panel, in which liquid crystals are uniformly arranged.
According to a position of a light source that emits light, a LCD back light unit is classified to a direct-type back light unit in which the light source is directly positioned at a rear side of a liquid crystal panel of an LCD, and an edge-type back light unit in which the light source is positioned at a side of the liquid crystal panel of an LCD. According to a shape of a light guide plate, the edge-type back light unit is sub-classified to a wedge-type back light unit and a flat-type back light unit.
FIG. 9 is a view illustrating the structure of a conventional back light unit using a wedge-type light guide plate. A light source 2 serves to emit light initially in the LCD device. Although various types of light source can be used, the LCD device generally employs a cold cathode fluorescence lamp (CCFL) which has low power consumption and emits highly bright white light. A light guide plate 4 is provided below an LCD panel and at one side of the light source 2 and serves to convert spot light generated from the light source 2 into plane light and thus project the plane light forward to the LCD panel.
A reflection plate 3 is provided on a rear side of the light guide plate 4, and serves to reflect light emitted from the light source 2 toward the LCD panel disposed in front of the reflection plate 3.
The diffusion sheet 5 is provided on a front side of the light guide plate 4, and serves to uniformize light passing through the light guide plate 4.
While the light passes through the diffusion sheet 5, diffusion of light occurs in horizontal and vertical directions so that brightness is rapidly deteriorated. In this regard, prism sheets 6 are used to reflect and concentrate the light, thereby providing enhanced brightness.
A protective sheet 7 is provided above the prism sheets 6, and serves to prevent scratches on the prism sheet 6, and to prevent Moire effect from occurring when using the prism sheets 6 arranged in two layers in the horizontal and vertical directions.
The back light unit further includes a mold frame (housing) that serves as a case to allow respective components of the back light unit to be fixed so that the back light unit can be integrated, and a lamp cover (back cover) to protect the back light unit while serving to maintain the strength of the back light unit and support the back light unit.
As shown in FIG. 9, the light source 2 is generally located at one edge of the back light unit (for an LCD TV, the light source is often located directly at the rear side of a panel). As a result, light is not uniformly transmitted through the overall surface of the unit, so that the edge of the back light unit can be brighter than any other portions of the back light unit. In order to prevent this phenomenon, the light guide plate 4 is used. The light guide plate is generally made of a transparent acrylic resin, which is not easily breakable due to its high strength, and has deformation resistance, light weight and high transmittance of visible light. In other words, the light guide plate 4 serves to allow light emitted from the light source 2 to be uniformly projected to the overall surface of the light guide plate 4. In practice, however, in a case where the back light unit is disassembled and light is allowed to emit from the light source 2 located at one side of the light guide plate 4, the light is not uniformly projected to the overall surface of the light guide plate 4, but is concentrated on both ends of the light guide plate 4. This is because the light guide plate 4 guides the light from the light source 2 to the opposite side of the light guide plate 4.
Thus, the rear surface of the light guide plate 4 is subjected to a specific treatment to cause scattered reflection of light in the light guide plate 4 such that light can be transmitted through the overall surface of the light guide plate 4. Specifically, the rear surface of the light guide plate 4 is formed with a prominence/depression pattern which has a predetermined shape designed in consideration of a distance from the light source 2 and the like. When the prominence/depression pattern is formed on the rear surface of the light guide plate
4, plane light having higher brightness and uniformity is emitted through the overall surface of the light guide panel of the LCD device.
However, in the conventional LCD device constructed as above, a portion of the panel with the prominence/depression pattern formed therebelow looks bright, and the other portions of the panel without the prominence/depression pattern therebelow looks dark, so that a spot phenomenon occurs on the panel, thereby deteriorating visibility of the LCD device. In particular, as the LCD panel is increased in size, a region separated by a far distance from the light source 2 lacks in absolute amount of light reaching there, and looks dark. In addition, the diffusion sheet and the prism sheets are used for improvement of light uniformity, and cause an increase in manufacturing costs of the back light unit.
In order to solve the above problems, there is a need for a great deal more research to obtain plane light with excellent visibility, high brightness and good uniformity over the entire surface of the LCD panel without using the diffusion sheet or the prism sheets.
[Disclosure] [Technical Problem] The present invention has been made to solve the foregoing problems of the prior art and it is one aspect of the present invention to provide a light guide plate for a LCD back light unit which exhibit superior brightness, uniformity of light and visibility over the entire surface of the LCD panel.
It is another aspect of the present invention to provide an LCD back light unit using the light guide plate.
It should be noted that the present invention is not limited to the aspects described above, and that other objects and features of the present invention will be apparent to those skilled in the art according to the following description.
[Technical Solution]
In accordance with one aspect of the present invention for achieving the above aspect, there is provided a light guide plate for a liquid crystal display device, comprising: side surfaces on which light is incident; a front surface from which the light is emitted, the front surface connected to the side surfaces; and a rear surface on which the light is reflected wherein the rear surface is formed with a rear-prism part including a plurality of prisms, each having a longitudinal direction perpendicular to a direction of light emitted from a light source and an asymmetric triangular cross-section.
[Advantageous Effects]
The light guide plate for the liquid crystal display device according to the present invention enables a uniform increase in amount of light reaching an overall surface of a panel of the liquid crystal display device, an increase in vertical half- viewing angle and an improvement in brightness. The brightness and visibility of the light guide plate according to the present invention are significantly improved, in comparison to a conventional light guide plate in which prisms are disposed on a front surface thereof.
Thus, according to the present invention, since the back light unit can be realized without one or both of a diffusion sheet and a prism sheet, it' is possible to reduce manufacturing costs of the back light unit.
[Description of Drawings]
The above and other aspects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:
FIGs. 1 and 2 are perspective views illustrating a light guide plate for a LCD back light unit according to a first embodiment of the present invention, when viewed from different angles;
FIG. 3 is a sectional view taken along the line A-A in FIG. 2; FIG. 4 is a sectional view taken along the line B-B in FIG. 2; FIG. 5 is a perspective view illustrating a front-lens according to Experimental Examples of the present invention; FIG. 6 is a graph showing brightness of front-lenses according to
Experimental and Comparative Examples of the present invention;
FIG. 7 is a graph showing rear-prisms according to Experimental and Comparative Examples of the present invention;
FIG. 8 is a graph showing brightness of rear-prisms according to Experimental and Comparative Examples of the present invention; and
FIG. 9 is a sectional-view illustrating the structure of a back light unit using a conventional wedge-type light guide plate.
[Best Mode]
The present invention is directed to a light guide plate for a liquid crystal display device, comprising: side surfaces on which light is incident; a front surface from which the light is emitted, the front surface connected to the side surfaces; and a rear surface on which the light is reflected wherein the rear surface is formed with a rear-prism part including a plurality of prisms, each having a longitudinal direction perpendicular to a direction of light emitted from a light source and an asymmetric triangular cross-section.
[Mode for Invention] The present invention will now be described with reference to the accompanying drawings in greater detail.
FIGs. 1 and 2 are perspective views illustrating a light guide plate for an LCD back light unit according to a first embodiment of the present invention, when viewed from different angles.
The first embodiment of the present invention is directed to a wedge-type light guide plate in which a light source L is located at one side thereof. The light guide plate is generally made of a transparent acrylic resin, which has high strength, break and deformation resistances, light weight and high transmittance of visible light.
Referring to FIG. 1, the light guide plate comprises a body 100 which comprises side surfaces 102, on which light is incident, a front surface 104, from which the light is emitted, wherein the front surface 104 is connected to the side surfaces 102 while facing a panel (not shown) of the LCD device, and a rear surface
106, on which the light is reflected, the rear surface 106 facing the front surface 104.
Unlike a dictionary's definition, the side surfaces 102 are defined as surfaces on which light is incident. Thus, in FIG. 1 , the side surfaces 102 refer to two surfaces adjacent to the light source L. A rear-prism part 120 is formed on the rear surface 106 and includes a plurality of prisms 122, each having a longitudinal direction perpendicular to a direction of light emitted from the light sources and an asymmetric triangular cross- section.
Referred to Fig. 2, the rear-prism part 120 uniformizes the brightness of the front surface 104 of the light guide plate. As shown in FIG. 2, in the case where a wedge -type light guide plate in which a light source L is located at one side thereof, the rear-prism part 120 increases in width w along with an increase in distance from the side surface 102 on which light is incident. The reason for the increase in width w is as follows. As a distance from the side surface 102 on which light is incident increases, an amount of light reaching there becomes smaller. In this regard, since the prisms 120, which serve to reflect and refract light, have a larger length, in spite of a small amount of light reaching the prisms 120, an amount of light reflected or refracted by the prisms 120 is increased, so that uniformity of light emitted toward the front surface 104 can be improved. As shown in FIG. 2, the rear-prism part 120 may have a stripe pattern in which a width is varied along with a distance from the light source. Alternatively, the rear-prism part 120 may have a dot pattern.
The front surface 104 is formed with a front-lens part 140 composed of optical members 142, each having a half-oval cross-section. An angle defined between the longitudinal direction of each optical member 142 and the longitudinal direction of each prism is preferably in the range of 80 to 100°, and more preferably
90°.
The front-lens part 140 serves to improve optical properties and visibility. Preferably, each of the optical members 142 has a pitch of 10 to 250 [M.
A conventional light guide plate has a smooth surface without any optical member, or has prisms with a triangular cross-section.
However, in the conventional light guide plate, the rear-prism part formed on the rear surface of the light guide plate is exposed when viewed from the front surface thereof, thus disadvantageously causing a deterioration in visibility and
Moire effect due to interference fringe between two surfaces of the prisms.
On the other hand, according to the light guide plate of the present invention, the front-lens part 140 composed of optical members 142, which have a half-oval cross-section is provided on the front surface 104 of the light guide plate and thus covers the rear-prism part 120, thereby improving visibility and optical properties.
FIG. 3 is a sectional view taken along the line A-A in FIG. 2. FIG. 4 is a sectional view taken along the line B-B in FIG. 2.
As can be seen from FIG. 3 which shows the cross-section of the light guide plate taken perpendicularly to the longitudinal direction of the light source, optical members 142 are formed along the longitudinal direction of the cross-section of the light guide plate on the front surface thereof. Since the optical members 142 herein used are lenticular lenses, the longitudinal direction of each lenticular lens is the same as the longitudinal direction of the cross-section of each optical member. In other words, the longitudinal direction of the lenticular lens is parallel with the direction of light.
Referring to the prisms 122 having an asymmetric triangular cross-section disposed on the rear surface, the longitudinal direction of each prism is parallel with the longitudinal direction of the light source L. Hereinafter, of internal angles of the asymmetric prism 122, an internal angle adjacent to the light source is referred to as "a first internal angle (α)" and an internal angle opposite to the light source is referred to as "a second internal angle
(β)" The first internal angle (α) is in the range of 70° to 90° and the second internal angle (β) is in the range of 40° to 50°. Since a total in all internal angles of a triangle is 180°, an intermediate angle (θ) is in the range of 40° to 70°. A shape of prisms is defined by the ranges of the internal angles, so that a half viewing angle is increased. The improvement in half viewing angle will be illustrated in the following section with reference to specific Examples.
As can be seen from FIG. 4 which shows the cross-section of the light guide plate parallel to the longitudinal direction of the light source, the optical members 142 disposed on the front surface of the light guide plate have an oval cross-section and the prisms 122 are spaced apart from each other on the rear surface of the light guide plate.
The prisms 122 increase in length along with an increase in a distance from the light source.
Hereinafter, the fact that the use of the light guide plate according to preferred embodiments of the present invention causes improvement in properties e.g., brightness, uniformity of light and visibility will be illustrated with reference to the following specific Examples. Although not mentioned herein, a more detail of Examples will be appreciated by those skilled in the art.
The advantages and effects of the present invention will be demonstrated from Examples as below:
EXAMPLES
1. Experimental and Comparative Examples of the front-optical member
FIG. 5 is sectional views illustrating specific dimensions of each optical member in the form of a lenticular lens formed on the front surface of a light guide plate for a back light unit of an LCD device according to the present invention.
The respective lenticular lenses vary in a height H, a pitch P and an angle between the adjacent two lenses and have a uniform curvature radius of 25 μm. The height H and the pitch P were measured with various angles of 0°, 20°, 40°, 60°, 80°, 100°, 120° and 140°.
Experimental Example 1>
A rear-prism part [width W x length L x thickness T = 288.8 x 217.7 x 2.4 (start) x 0.8 (end) mm] was formed with a stripe pattern (pitch: 430 μm) of prisms (height: 14.25 μm; pitch: 25 μm; and internal angle: 82.5°) on the rear surface of a wedge-type light guide plate. In addition, lenticular lenses were disposed perpendicular to the light source on the front surface of the light guide plate, each of which has an oval cross-section with a curvature radius R of 25 μm, a height of 25 μm and a pitch of 50 (M and an angle between the adjacent lenses of 0°.
Experimental Examples 2 to 8>
Experimental Examples 2 to 8 were performed in the same manner as in Experimental Example 1 except that the angle between the adjacent lenticular lenses formed on the front surface of the light guide plate were varied by 20° from 20° to 140°, and the height and pitch of the lenticular lenses were determined by the angle. The specific dimensions of the lenticular lenses in Experimental Examples 1 to 8 were summarized in Table 1 and FIG. 5.
TABLE 1
Exp. Exp. Exp. Exp. Exp. Exp. Exp. Exp. Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex. 7 Ex. 8
Curvature 25.0 μm 25.0 (M 25.0 μm 25.0 μm 25.0 μm 25.0 βm 25.0 μm 25.0 μm radius
Lens 25.0 μm 20.7 μm 16.5 μm 12.5 μm 8.9 μs] 5.9 μm 3.4 μm 1.5 μm height
Lens 50.0 μm 49.2 μm 47.0 μm 43.3 μm 38.3 μm 32.1 μm 25.0 μm 17.1 μm pitch
Angle 0° 20° 40° 60° 80° 100° 120° 140°
<Comparative Example 1> A rear-prism part [width W x length L x thickness T = 288.8 x 217.7 x 2.4 (start) x 0.8 (end) mm] was formed with a stripe pattern (pitch: 430 μm) of prisms (height: 14.25 μm; pitch: 25 μm; and internal angle: 82.5°) on the rear surface of a wedge-type light guide plate.
The wedge-type light guide plate was provided with a smooth front surface.
<Comparative Example 2>
A rear-prism part [width W x length L x thickness T = 288.8 x 217.7 x 2.4 (start) x 0.8 (end) mm] was formed with a stripe pattern (pitch: 430 μm) of prisms (height: 14.25 μm; pitch: 25 μm; and internal angle: 82.5°) on the rear surface of a wedge-type light guide plate.
In addition, prisms were disposed perpendicular to the light source on the front surface of the light guide plate, each of which has a cross-section with a pitch of 50 μm, a height of 25 μm, and an angle between the adjacent lenses of 90°.
2. Result analysis of the front-optical member
TABLE 2
Figure imgf000011_0001
Grade on Visibility of Pattern: Excellent (5) > Very Good (4) > Good (3) > Not Bad (2) > Bad (1) Table 2 shows results of Experimental Examples 1 to 8 and Comparative Examples 1 and 2. FIG. 6 is a graph showing the brightness according to Experimental and Comparative Examples of the present invention. It can be seen from Table 2 and FIG. 6 that Experimental Examples under the condition that a ratio of lens height to lens pitch is lower than 0.23, exhibited high central brightness, and all of Experimental Examples showed excellent pattern visibility, as compared to Comparative Examples. It can be also confirmed that when comparing both pattern visibility and brightness between Experimental and Comparative Examples, Experimental Examples 5 and 6 were the best.
These results ascertained it is preferable that a ratio of pitch to height of the respective optical members arranged on the front surface of the light guide plate is in the range of 1 : 0.1 to 1 : 0.35, an angle between the adjacent optical members is in the range of 60° to 120°, and the curvature radius of each optical member is in the range of 0.58- to 1.0 fold of the pitch of each lens.
3. Experimental and Comparative Examples of rear-prism part
<Experimental Example 9> A rear-prism part [width W x length L x thickness T = 288.8 x 217.7 x
2.4 (start) x 0.8 (end) mm] was formed with a stripe pattern (pitch: 430 μm) of prisms (height: 25 μm, pitch: 25 μm, first internal angle (α), 90°, intermediate angle (θ): 45° and second internal angle (β): 45°) on the rear surface of a wedge-type light guide plate. In addition, the light guide plate was provided with a mirror front surface.
<Experimental Examples 10 to 13>
Experimental Examples 10 to 13 were performed in the same manner as in Experimental Example 9 except that the first internal angle (α) was decreased to 70° by 5°, the intermediate angle (θ) was increased to 45° by 5° and the second internal angle (β) was maintained at 45°.
The specific dimensions of the prisms in Experimental Examples 9 to 13 were summarized in Table 3 and FIG. 7. TABLE 3
Figure imgf000013_0001
<Comparative Example 3>
A rear-prism part [width W x length L x thickness T = 288.8 x 217.7 x 2.4 (start) x 0.8 (end) mm] was formed with a stripe pattern (pitch: 430 /mi) of rear- prisms (height: 14.25 μm, pitch: 25 μm, internal angle (α): 82.5°) on the rear surface of a wedge-type light guide plate. In addition, the light guide plate was provided with a mirrored front surface.
4. Result analysis of the rear surface-prism
Figure imgf000013_0002
Table 4 shows results of Experimental Examples 9 to 13 and Comparative Example 3. FIG. 8 is a graph showing the brightness of the rear-surface prisms according to Experimental and Comparative Examples.
It can be seen from Table 4 and FIG. 8 that Experimental Examples, under the condition that a first internal angle is lower than 70°, exhibited superior central brightness, and all of Experimental Examples had a high vertical half-viewing angle, as compared to Comparative Example 3. It can be also confirmed that when comparing both the brightness and vertical half-viewing angle between
Experimental and Comparative Examples, Experimental Examples 11 and 12 were the best.
As apparent from the above description, with the light guide plate according to Experimental Examples of the present invention, it is possible for the liquid crystal display device of the present invention to obtain substantially the same effects as that of the conventional liquid crystal display device without employing a prism sheet which are used in the back light unit of the conventional liquid crystal display device. That is to say, the use of the light guide plate according to the present invention eliminates the necessity of one or two prism sheets among a variety of sheets which are applied to the conventional back light unit.

Claims

[CLAIMS]
[Claim 1]
A light guide plate for a liquid crystal display device, comprising: side surfaces on which light is incident; a front surface from which the light is emitted, the front surface connected to the side surfaces; and a rear surface on which the light is reflected wherein the rear surface is formed with a rear prism part including a plurality of prisms, each having a longitudinal direction perpendicular to a direction of light emitted from a light source and an asymmetric triangular cross-section.
[Claim 2]
The light guide plate according to claim 1, wherein the prism has an internal angle adjacent to the light source in the range of 70° to 90°.
[Claim 3]
The light guide plate according to claim 1, wherein the prism has an internal angle opposite to the light source in the range of 40° to 50°.
[Claim 4]
The light guide plate according to claim 1, wherein the front surface is formed with a front-lens part, including a plurality of optical members, each having an oval cross-section.
[Claim 5]
The light guide plate according to claim 4, wherein an angle defined between the longitudinal direction of the front-lens part and the longitudinal direction of each prism is in the range of 80 to 100°.
[Claim 6]
The light guide plate according to claim 4, wherein the oval cross-section of each of the optical members has a ratio of pitch to height in the range of 1 : 0.16 to 1 : 0.29.
[Claim 7]
The light guide plate according to claim 4, wherein an angle between the adjacent two optical members is in the range of 60° to 120°.
[Claim 8]
The light guide plate according to claim 4, wherein the curvature radius of each of the optical members is in the range of 0.58- to 1.0 fold of the pitch of the optical member.
[Claim 9]
The light guide plate according to claim 1 , wherein the rear-prism part is in the form of a stripe-pattern increasing in width along with an increase in a distance from the side surface on which light is incident.
[Claim 10]
The light guide plate according to claim 1 , wherein the rear-prism part is in the form of a dot-pattern increasing in density along with an increase in a distance from the side surface on which light is incident.
[Claim 11]
A back light unit for a liquid crystal display device, comprising: the light guide plate according to claim 1 ; and a light source arranged at one or both sides of the light guide plate.
[Claim 12]
A liquid crystal display device, comprising: the light guide plate according to claim 1 ; and a light source arranged at one side of the light guide plate.
PCT/KR2007/005976 2007-10-30 2007-11-26 Light guide panel for lcd back light unit and lcd back light unit thereby WO2009057844A1 (en)

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