WO2007108579A1

WO2007108579A1 - Light guide panel for lcd and back light unit using the same

Info

Publication number: WO2007108579A1
Application number: PCT/KR2006/002762
Authority: WO
Inventors: Jae-Han Kim; Chul-Goo Chi; Man Suk Kim; O-Yong Jung
Original assignee: Cheil Industries Inc
Current assignee: Cheil Industries Inc
Priority date: 2006-03-17
Filing date: 2006-07-13
Publication date: 2007-09-27
Anticipated expiration: 2008-09-17
Also published as: CN101038392A; CN100561309C; TWI324277B; TW200736756A; KR100662540B1

Abstract

Disclosed herein is a light guide plate for a backlight unit of a liquid crystal display device, which has excellent brightness, uniformity and visibility of light. The light guide plate comprises side surfaces on which light is incident, a front surface from which the light is emitted, and a rear surface on which the light is reflected, wherein the front surface is formed with front-prisms, each having a predetermined cross-section, and the rear surface is formed with stripe-prisms, each increasing in width along with an increase in distance between the stripe- prism and the side surfaces on which the light is incident.

Description

[DESCRIPTION]

[Invention Title]

LIGHT GUIDE PANEL FOR LCD AND BACK LIGHT UNIT USING THE SAME

[Technical Field]

The present invention relates to a light guide plate for a backlight unit of a liquid crystal display device, which comprises a stripe pattern, and, more particularly, to a light guide plate for a liquid crystal display device, which is formed on a rear surface of the light guide panel with stripe-prisms, providing remarkable improvement in visibility and brightness, and to a backlight unit comprising the same.

[Background Art]

In general, a liquid crystal display 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 liquid crystal display device is not an emissive device, it must be provided with a backlight unit as a light source to generate light such that an image is displayed on a liquid crystal panel by adjusting a transmittance amount of light generated from the backlight unit.

Fig. 1 is an exploded perspective view illustrating a backlight unit of a conventional liquid crystal display device.

According to a location of a light source to emit light, backlight units of the liquid crystal display device can be classified to a direct type backlight unit in which the light source is located directly at rear side of a liquid crystal panel 100 of the liquid crystal display device, and an edge type backlight unit in which the light source is located at a side of the panel 100 thereof. In Fig. 1, an edge type backlight unit 10 is shown.

Referring to Fig. 1, a conventional LCD backlight unit comprises a light source 105, a light guide plate 110, a reflection plate 115, a diffusion sheet 120, prism sheets 125, and a protective sheet 130.

The light source 105 serves to emanate light initially in the liquid crystal display device. Although various types of light source can be used, the liquid crystal display device generally employs a cold cathode fluorescence lamp (CCFL) which is characterized by very low power consumption and emission of very bright white light.

The light guide plate 110 is positioned at the rear side of the LCD panel 100 and at one side of the light source 105, and serves to convert spot light generated from the light source 105 to plane light, and then project the plane light in front of the light guide plate. The reflection plate 115 is positioned at the rear of the light guide plate 110, and serves to reflect light emitted from the light source 105 towards the LCD panel 100 in front of the reflection plate.

The diffusion sheet 120 is positioned at the front side of the light guide plate 110, and serves to uniformize light passing through the light guide plate 110. While the light passes through the diffusion sheet 120, diffusion of light occurs in horizontal and vertical directions so that brightness is rapidly deteriorated. In this regard, the prism sheets 125 are used to reflect and focus the light, thereby providing enhanced brightness.

The protective sheet 130 is positioned above the prism sheets 125, and serves to prevent scratches on the prism sheet 125, and to prevent Moire effect from occurring when using the prism sheets 125 in the horizontal and vertical directions.

Although not shown in Fig. 1, the backlight unit 10 further comprises a mold frame to secure respective components, and a lamp cover to protect the backlight unit 10 while serving to maintain strength of the backlight unit and to support the backlight unit.

Figs. 2 and 3 are enlarged cross-sectional views illustrating a path of light after being generated from the light source 105 in the light guide plate 110.

Referring to Fig. 2, the light source 105 is generally located at one edge of the backlight unit 10 (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 backlight unit 10 can be brighter than any other portions of the backlight unit 10. In order to prevent this phenomenon, the light guide plate 110 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 110 serves to allow light emitted from the light source 105 to be uniformly projected to the overall surface of the light guide plate 110. In practice, however, the light is not uniformly projected to the overall surface of the light guide plate 110, but concentrated on both ends of the light guide plate 110. This is because the light guide plate 110 guides the light from the light source 105 to the ends of the light guide plate.

Thus, as shown in Fig. 3, the rear surface of the light guide plate 110 is subjected to a specific treatment to cause scattered reflection of light in the light guide plate 110 such that light can be transmitted through the overall surface of the light guide plate 110. Specifically, the rear surface of the light guide plate 110 is formed with a roughness pattern 113 which has a predetermined shape designed in consideration of a distance from the light source 105 and the like. When the roughness pattern 113 is formed on the rear surface of the light guide plate 110, plane light having higher brightness and uniformity is emitted through the overall surface of the light guide panel of the liquid crystal display device.

However, in the conventional liquid crystal display device constructed as above, a portion of the panel with the roughness pattern 113 formed therebelow looks bright, and other portions of the panel without having the roughness pattern therebelow looks dark, so that there occurs a spot phenomenon on the panel, thereby deteriorating the visibility of the liquid crystal display device. In particular, as the LCD panel is increased in size, a region separated a far distance from the light source 105 lacks in absolute amount of light reaching there, and looks dark.

In addition, the diffusion sheet 120 and the prism sheets 125 are used for improvement of light uniformity, and cause an increase in manufacturing costs of the backlight unit. In order to solve the above problems, it has been investigated to provide a backlight unit which can provide plane light having high brightness and uniformity, and can ensure excellent visibility without using the diffusion sheet 120 or the prism sheets 125.

[Disclosure] [ Technical Problem ]

Therefore, the present invention has been made in view of the above problems, and it is an object of the present invention to provide a light guide plate for a backlight unit of a liquid crystal display device, which comprises stripe- prisms. It is another object of the present invention to provide a backlight unit for a liquid crystal display device, which comprises the light guide plate according to the present invention.

It should be noted that the present invention is not limited to the objects 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 an aspect of the present invention, the above and other objects can be accomplished by the provision of 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, and a rear surface on which the light is reflected, wherein the front surface is formed with front prisms, each having a predetermined cross-section, and the rear surface is formed with stripe-prisms, each increasing in width along with an increase in distance between each stripe- prism and the side surfaces on which the light is incident.

In accordance with another aspect of the present invention, there is provided a backlight unit for a liquid crystal display device, comprising a light source, and a light guide plate of the present invention.

Details of other aspects are included in the detailed description and accompanying drawings.

Embodiments of the present invention will be described in conjunction with the accompanying drawings. It should be noted that the present invention is not limited to embodiments of the present invention, and that various modifications and variations can be made without departing from the scope of the present invention. The embodiments are provided for clear understanding of those skilled in the related art without limiting the scope of the present invention, which is defined only by the accompanying claims, through which like components are denoted by like reference numerals.

For clear description of the present invention, dimensions of components constituting the present invention are exaggerated in the drawings. In the following description, if one component is expressed as being "positioned within" or "connected with" another component, it means that the component may be positioned in contact with the other component or spaced a predetermined distance from the other component. In the case where the component is spaced the predetermined distance from the other component, it should be noted that description of a third component used to fix or couple the component to the other component can be omitted. Fig. 4 is a perspective view illustrating a light guide plate for a backlight unit of a liquid crystal display device according to one embodiment of the present invention.

Referring to Fig. 4, a light guide plate 30 for a backlight unit of a liquid crystal display device according to one embodiment of the present invention is generally made of a transparent acrylic resin, which has high strength, deformation resistance, light weight, and high transmittance of visible light. The light guide plate 30 has a body 300, which comprises side surfaces

301 on which light is incident, a front surface 303 connected to the side surfaces

301 while facing a panel (not shown) of the liquid crystal display device, and a rear surface 305 connected to the side surfaces 301 while facing the front surface 303.

The side surfaces 301 are defined as surfaces on which light is incident. Thus, in Fig. 4, the side surfaces 301 refer to two surfaces adjacent light sources 306, respectively.

The front surface 303 and the rear surface 305 are surfaces to which light is emitted after being generated from the light sources and incident on the side surfaces 306, and are connected to the side surfaces 301.

The front surface 303 is formed with front-prisms 310, each of which has a predetermined cross-section, and serves to uniformly diffract, refract and diffuse light emitted through the body 300 of the light guide plate. The front-prisms 310 may be formed over the entire surface of the front surface 303, and alternatively, may be spaced at a predetermined distance from each other with a separation plane disposed between the front-prisms 310. The front-prisms 310 are disposed in perpendicular to stripe-prisms on the rear surface of the light guide plate as shown in Fig. 4. However, it should be noted that the present invention is not limited to this perpendicular structure, and thus, an angle between the front-prism and the prisms of the stripes can be changed.

Although each of the front prisms is shown as having a triangular cross- section in a longitudinal direction in Fig. 4, the present invention is not limited this structure, and the front prisms may have various cross-sections. Figs. 5 to 7 are perspective views illustrating various embodiments of the cross-section of the front-prism 310.

As shown in Fig. 5, the front-prisms 310 may be spaced a predetermined distance from each other with a separation plane disposed between the front- prisms 310. The reason of disposing the front-prisms 310 to be spaced the predetermined distance "d" from each other instead of densely disposing the front- prisms 310 is to enhance uniformity and visibility of light.

In other words, by disposing the front-prisms 310 to be spaced the predetermined distance "d" from each other, light is not refracted in various angles, but directed to be as perpendicular to a panel (not shown) of the liquid crystal display device as possible, thereby improving the uniformity of light reaching the panel (not shown) of the liquid crystal display device.

Each of the front-prisms 310 may have a trapezoidal cross-section in the longitudinal direction as shown in Fig. 6, or may have a reverse groove-shaped cross-section in the longitudinal direction, in which each side surface of the reverse groove-shaped cross-section has a predetermined curvature, as shown in Fig. 7.

In the case where each of the front-prisms 310 has the trapezoidal cross- section in the longitudinal direction as shown in Fig. 6, the body 300 of the light guide plate may be adapted such that light is directed in perpendicular to the panel (not shown) of the liquid crystal display device through a plane A defined as an upper surface of each trapezoidal front-prism 310.

Furthermore, in the case where each of the front-prisms 310 has the reverse groove-shaped cross-section in the longitudinal direction with side surfaces of the reverse groove-shaped cross-section having the curvature as shown in Fig. 7, the curvature of the side surface thereof is preferably in the range of 0.01 ~ 1.0 mm. A ratio of a total area of the front-prisms 310 to a total area of the separation planes between the front-prisms 310 on the front surface 303 of the body 300 is preferable in the range of 1 :0.5 - 1 :10.

Moreover, in Figs. 4 to 7, each of the front-prisms 310 preferably has a ratio of height h₂ to width W₂ in the range of 0.3-0.6:1. With this range, it is possible to maintain a proper horizontal viewing angle and to prevent reduction in brightness.

Each of the front-prisms has an interior angle of 90 degrees or more, and preferably 90 ~ 120 degrees. With this range of angle, it is possible to maintain a proper brightness at the central region of the panel, and the proper horizontal viewing angle.

Referring to Fig. 4 again, the rear surface 305 of the body 300 is formed with stripes spaced a predetermined distance from each other side by side, which will be referred to as stripe-prisms 320.

Fig. 8 is an enlarged cross-sectional view of each stripe-prism 320 shown in Fig. 4. The stripe-prisms 320 may be formed on the rear surface of the light guide plate via embossed carving or depressed engraving.

Each of the stripe-prisms 320 comprises rear-prisms 322 formed on a surface of the stripe-prism 320. Preferably, the rear-prisms 322 are formed such that each rear-prism 322 has a longitudinal direction perpendicular to a direction (Q direction) of light emitted from the light sources. The reason is that, when each of the rear-prisms 322 is formed perpendicular to the direction (Q direction) of light emitted from the light sources 306, diffraction, refraction and diffusion of light can be efficiently performed.

In addition, as described above, it is preferable that the rear-prisms 322 formed in each stripe-prism 320 are disposed perpendicular to the longitudinal direction (Q direction) of the front-prisms 310, since such orthogonal arrangement is advantageous in uniform refraction and diffusion of the light via the overall surface of the light guide plate.

When each of the rear-prisms 322 formed on the stripe-prisms 320 has a triangular cross-section in the longitudinal direction as shown in Fig. 8, it preferably has a vertex angle θ i of 60 ~ 100 degrees.

With this range of the vertex angle θ _l5 most of emitted light is directed perpendicular to the front surface of the light guide plate, increasing the brightness at the central region of the panel.

In addition, preferably, each stripe-prism 320 has a ratio of width to length in the range of 1:1,000 ~ 1:50,000, and each rear-prism 322 has a ratio of height Ji₁ to width W₁In the range of 0.3 ~ 0.9:1. An angle defined between the longitudinal direction of the stripe-prism and the longitudinal direction of the front-prism is in the range of 30 ~ 90 degrees.

The stripe-prisms 320 formed on the rear surface of the light guide plate, and the rear-prisms 322 formed on the surface of each stripe-prism will be described in detail hereinafter. The body 300 of the light guide plate has a plurality of stripe-prisms 320 form on the rear surface 305 such that each stripe-prism 320 increases in width as it is further from any of the side surfaces on which light is incident. Specifically, in the case where the light is incident on opposite side surfaces, each stripe-prism 320 gradually increases in width from each side surface to the center of the stripe-prism 320, and in the case where the light is incident on a single side surface, each stripe- prism 320 gradually increases in width from the side to the opposite side (see Figs. 9 and 6b).

Meanwhile, it is desirable that the stripe-prism 320 is not formed immediately adjacent the side surface on which the light is incident. The reason is that, when the stripe-prism 320 is formed immediately adjacent the side surface on which the light is incident, light is scattered near the stripe-prism 320, and generates a bright line.

In addition, the rear-prisms 322 are formed on the surface of each stripe- prism 320 so as to be perpendicular to the direction of light incident through the side surfaces.

The rear-prisms 322 can be formed on the surface of each stripe-prism after forming stripe-prisms in a predetermined pattern. In practice, however, several arrays of the rear-prisms 322 having different widths may be formed to produce the stripe-prism type light guide plate of the present invention.

Figs. 9 and 10 are schematic rear views of the body 300 (see Fig. 4) of the light guide plate, showing embodiments of arrangement of the stripe-prisms 320.

Fig. 9 is a plan view illustrating arrangement of the stripe-prisms 320 in the case where light generated from the light sources 305 (see Fig. 4) is incident on the opposite sides of the body 300 of the light guide plate.

As shown in Fig. 9, in the case where the light is incident on both sides 301 of the body 300 from the light sources, the rear surface of the body is adapted to have the stripes 320 gradually increased in width from both sides 301 of the body to the center of each stripe 320. Meanwhile, an increase in distance between each stripe-prism 320 and the sides 301 causes an amount of light reaching thereto to diminish. In this regard, as described above, when gradually increasing the width of the stripe-prism 322 along with an increase in distance from the sides 301 on which the light is incident, the stripe-prism 320 is increase in size, thereby enabling an increase in refraction and reflection amounts of light. Fig. 10 is a plan view illustrating arrangement of the stripe-prisms 320 in the case where the light source 306 (see Fig. 4) is disposed only at one side of the light guide plate.

As shown in Fig. 10, in the case where the light is incident only on one side 301 from the light source, the rear surface of the body is adapted to have the stripe- prisms 320, each of which is gradually increased in width from the one side to the opposite side via the central region of the stripe.

Meanwhile, as described above, it is desirable that the stripe-prisms 320 be not formed immediately adjacent the side surfaces on which the light is incident.

Fig. 11 is a plan view illustrating an example of the rear surface of the light guide plate according to one embodiment of the present invention.

As shown in Fig. 11, the rear surface 305 of the light guide plate may further comprise dot-prisms 325 in each space between the stripe-prisms 320.

The reason of forming the dot-prisms 325 is to improve a refractive index and a reflection rate of light in the space between the stripe-prisms 320. In particular, for BLU employing CCFL light sources, since an emitted amount of light is reduced at a distal end of the light guide plate (i.e., a distal end of the CCFL) due to characteristics of BLD, an edge of the backlight unit becomes dark. Thus, in order to enhance the visibility, the dot-prisms 325 are inserted.

Here, although each of the dot-prisms 325 is shown as having a circular shape in Fig. 11, the present invention is not limited to this shape, and the dot-prism 325 may have various shapes including a triangular shape, a rectangular shape, a pentagonal shape, a hexagonal shape, an elliptical shape, a diamond shape, etc. Furthermore, each of the dot-prisms 325 may be formed as a second shapeless rear- prism without having a particular shape. Figs. 12 and 13 are plan views illustrating the light guide plates according to other embodiments of the present invention. Referring to Fig. 12, a body 300 of a light guide plate has stripe-prisms 320 formed on a rear surface 305 thereof, and each stripe gradually increases in width along with an increase in distance from side surfaces 301 on which light is incident. Meanwhile, according to this embodiment where the light is incident on both side surfaces 301 as shown in Fig. 12, rear-prisms 322 on the stripe-prisms 320 are formed such that the rear-prisms 322 have widths increased near four edges of the rear surface 305 of the light guide plate, thereby preventing the visibility or the brightness from being reduced near the edges of the rear surface 305 of the light guide plate. Such a design of stripe pattern prevents generation of a dark area near the edges of the rear surface of the light guide plate.

Although it is desirable that the rear-prisms 322 be formed at predetermined intervals without a large separation therebetween on the surface of each stripe-prism 320, it is possible to adjust a separation between the rear-prisms 322 on the stripe- prisms, as shown in Fig. 13, in order to allow precise adjustment of the brightness and the visibility.

The subject matter of the present invention is in that the rear surface of the light guide plate body is formed with the stripe-prisms 320, each of which increases in width according to an increase in distance from one or more side surfaces on which light is incident, and comprises prisms formed on the surface thereof. Thus, it can be understood that any of inventions obtained through modification of the stripe-prisms 320 in terms of shape, separation between prisms, etc. are included in the scope of the present invention.

Fig. 14 is a graph comparing brightness of a stripe-prism type light guide plate according to the present invention with that of a conventional dot-prism type light guide plate.

In order to measure the brightness of the stripe-prism type light guide plate and the conventional dot-prism type light guide plate in Fig. 14, a light guide plate for a 21.2-inch type backlight unit was equally divided into 5 parts in the transverse direction and the longitudinal direction to form 25 areas, followed by measuring a brightness of each area in units of cd/m\ From results obtained by comparing the two light guide plates, each of which has an aperture ratio (that is, a ratio of pattern per unit area) as shown in Fig. 14, it can be understood that the light guide plate comprising the stripe-prisms according to the present invention has an average brightness 10 % or more than that of the conventional light guide plate comprising the dot-prisms.

As apparent from the above description, with the light guide plate according to the 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 diffusion sheet and a prism sheet which are used in the backlight unit of the conventional liquid crystal display device. In other words, the light guide plate according to the present invention makes it possible to obtain plane light having excellent brightness, uniformity and visibility with only a light source and the light guide plate.

Although the embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that the present invention is not limited to these embodiments, and that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.

[Advantageous Effects]

The light guide plate for the liquid crystal display device according to the present invention enables an uniform increase in amount of light reaching an overall surface of a panel of the liquid crystal display device, and an increase in amount of light directed in the perpendicular direction, thereby ensuring excellent brightness, uniformity and visibility of light in comparison to the conventional light guide plate. The brightness of the light guide plate according to the invention is improved in comparison to the dot-prism light guide plate.

Thus, since the backlight unit can be realized without one or both of a diffusion sheet and a prism sheet, it is possible to reduce manufacturing costs thereof. [Description of Drawings]

Fig. 1 is an exploded perspective view illustrating a backlight unit of a conventional liquid crystal display device;

Figs. 2 and 3 are enlarged cross-sectional views illustrating a path of light in a light guide plate after being generated from a light source;

Fig. 4 is a perspective view illustrating a light guide plate for a backlight unit of a liquid crystal display device according to one embodiment of the present invention;

Figs. 5 to 7 are perspective views illustrating various embodiments of the cross-section of the front prism;

Fig. 8 is an enlarged cross-sectional view of a stripe-prism on the rear surface of the light guide plate;

Figs. 9 and 10 are schematic plan views illustrating the rear surface of the light guide plate having the stripe-prisms disposed thereon; Fig. 11 is a plan view illustrating the rear surface of the light guide plate having dot-prisms additionally disposed thereon;

Figs. 12 and 13 are plan views illustrating the rear surface of the light guide plate having modified stripe-prisms disposed thereon to prevent dark portion due to reduction in brightness at an edge of the rear surface; and Fig. 14 is a graph comparing brightness of a stripe-prism type light guide plate according to the present invention with that of a conventional dot-prism type light guide plate.

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, and a rear surface on which the light is reflected, wherein the front surface is formed with front-prisms, each having a predetermined cross-section, and the rear surface is formed with stripe-prisms, each comprising rear-prisms formed thereon and increasing in width along with an increase in distance between the stripe-prism and the side surfaces on which the light is incident.

[Claim 2] The light guide plate according to claim 1, wherein each stripe- prism has a longitudinal direction perpendicular to a direction of light emitted from a light source.

[Claim 3] The light guide plate according to claim 1, wherein an angle defined between the longitudinal direction of each stripe-prism and the longitudinal direction of each front-prism is in the range of 30 ~ 90 degrees.

[Claim 4] The light guide plate according to claim 3, wherein the longitudinal direction of the stripe-prisms is disposed perpendicular to the longitudinal direction of the front-prisms.

[Claim 5] The light guide plate according to claim 1, wherein each of the stripe-prisms positioned near the edge of the rear surface of the light guide plate has an increased width.

[Claim 6] The light guide plate according to claim 1, wherein each of the stripe prisms has a triangular cross section with a vertex angle of 60 ~ 100 degrees.

[Claim 7) The light guide plate according to claim 1, wherein each of the front-prisms has a triangular cross section with a vertex angle of 90 ~ 120 degrees.

[Claim 8] The light guide plate according to claim 1, wherein the pitches of the stripe-prisms have variable dimensions.

[Claim 9] The light guide plate according to claim 1, wherein each of the stripe-prisms has a ratio of width to length in the range of 1 :1,000 ~ 1:50,000.

[Claim 10] The light guide plate according to claim 1, wherein the cross section of the front-prisms has a shape selected among a triangular shape, a trapezoidal shape and a reverse groove shape having a predetermined curvature.

[Claim 11 ] The light guide plate according to claim 1, wherein the front- prisms are disposed with a predetermined separation plane provided therebetween.

[Claim 12] The light guide plate according to claim 11, wherein a ratio of a total area of the front-prisms to a total area of the separation planes therebetween is in the range of 1 :0.5 - 1 :10.

[Claim 13] The light guide plate according to claim 1, wherein each front- prism has a ratio of height (h₂) to width (w₂) in the range of 0.3 ~ 0.6.

[Claim 14] The light guide plate according to claim 1, further comprising: additional prisms in the space between the stripe-prisms.

[ Claim 15] A backlight unit, comprising: a light source; and a light guide plate according to claim 1.