Light concentrating sheet, backlight unit including the light concentrating sheet and liquid crystal display module including the backlight unit

A light concentrating sheet for a liquid crystal display module includes a first base film having flat inner and outer surfaces; a first light concentrating film on the first base film and having a first thermal expansion coefficient; a second light concentrating film on the first light concentrating film and having a second thermal expansion coefficient; and a second base film on the second light concentrating film and having flat inner and outer surfaces.

The present invention claims the benefit of Korean Patent Application No. 10-2007-0115396 filed in Korea on Nov. 13, 2007, which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a backlight unit and a liquid crystal display module (LCDM), and more particularly, to a light concentrating sheet, a backlight unit and a LCDM where a deformation problem in a configuration is prevented.

2. Discussion of the Related Art

Generally, the cathode ray tube (CRT) has been widely used for TV, measuring machines, information terminals and so on. Unfortunately, the CRT has disadvantages on weight, size and so on. To resolve these problems in the CRT; many kinds of flat panel display devices (FPDs), such as liquid crystal display (LCD) devices, plasma display panels (PDPs), electroluminescence display (ELD) devices, vacuum fluorescent display (VFD) device, and so on, have been introduced.

Among these devices, LCD devices are widely used for notebook computers, monitors, TV, and so on, because they have excellent capabilities of a small size, a thin profile, light weight and low power consumption. Moreover, they can display high quality and color images and have a large displaying area.

The liquid crystal molecules have a definite alignment direction as a result of their thin and long shapes. The alignment direction of the liquid crystal molecules can be controlled by applying an electric field across the liquid crystal molecules. In other words, as the intensity or direction of the electric field is changed, the alignment of the liquid crystal molecules also changes. Since incident light is refracted based on the orientation of the liquid crystal molecules due to the optical anisotropy of the liquid crystal molecules, images can be displayed by controlling light transmissivity. Generally, an additional light source is required because the LCD panel is a non-emissive-type display device. Accordingly, a backlight unit is disposed under the LCD panel. The LCD device displays images using light produced by the backlight unit and supplied to the LCD panel.

The backlight unit may be divided into an edge type and a direct type depending on arrangement of light source: In the direct type backlight unit, at least one light source is positioned under an optical sheet. In the edge type backlight unit, one light source is positioned at a side of the backlight unit. Particularly, the light source is positioned at a side of a light guide plate of the backlight unit. In the edge type backlight unit, light emitted from the light source is projected on a light crystal panel using a light guide plate. The light is changed into a plane light source. The backlight unit includes an optical sheet, such as a diffusion sheet and a prism sheet as a light concentrating sheet, to improve optical properties, for example, brightness uniformity.

FIG. 1is a cross-sectional view of a portion of a related art backlight unit. Referring toFIG. 1, the light concentrating sheet10includes a base film17and a prism film15. The prism film15has a convex portion12and a concave portion14. The light guide plate20is disposed under the light concentrating sheet10. The prism film15is disposed between the base film17and the light guide plate20. The light from the light source16at a side of the light guide plate20is guided into the light concentrating sheet10by the light guide plate20.

There are some problems. Because the convex portion12of the prism film15has a peaked shape, there are damages on the prism film15and the light guide plate20at a contacting portion18of the prism film15and the light guide plate20. For example, brightness uniformity may be degraded by particles generated from frictions between the prism film15and the light guide plate20. Moreover, deformation in the prism film15and the light guide plate20causes a light loss such that light efficiency is reduced. When the base film17is disposed between the prism film15and the light guide plate20, there are also problems. For example, since the concave portion12of the prism film15contacts a diffusion sheet (not shown), there are problems at a contacting portion between the diffusion sheet (not shown) and the prism film15.

SUMMARY OF THE INVENTION

Accordingly, embodiments of the invention are directed to a light concentrating sheet, a backlight unit and an LCDM that substantially obviate one or more of the problems due to limitations and disadvantages of the related art.

To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described, a light concentrating sheet for a liquid crystal display module includes a first base film having flat inner and outer surfaces; a first light concentrating film on the first base film and having a first thermal expansion coefficient; a second light concentrating film on the first light concentrating film and having a second thermal expansion coefficient; and a second base film on the second light concentrating film and having flat inner and outer surfaces.

In another aspect, a backlight unit for a liquid crystal display module includes a lamp on a bottom frame; a light concentrating sheet disposed over the lamp, the light concentrating sheet including: a first base film having flat inner and outer surfaces; a first light concentrating film on the first base film and having a first thermal expansion coefficient; a second light concentrating film on the first base film and having a second thermal expansion coefficient; and a second base film on the second light concentrating film and having flat inner and outer surfaces; and a diffusion sheet on the light concentrating sheet.

In another aspect, a liquid crystal display module includes a liquid crystal panel; a backlight unit for projecting light on the liquid crystal panel, the backlight unit including: a lamp on a bottom frame; a light concentrating sheet disposed over the lamp, the light concentrating sheet including: a first base film having flat inner and outer surfaces; a first light concentrating film on the first base film and having a first thermal expansion coefficient; a second light concentrating film on the first base film and having a second thermal expansion coefficient; and a second base film on the second light concentrating film and having flat inner and outer surfaces; and a diffusion sheet on the light concentrating sheet.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Reference will now be made in detail to exemplary embodiments of the invention, which are illustrated in the accompanying drawings.

FIG. 2is an exploded perspective view of a liquid crystal display module (LCDM) according to the present invention. Referring toFIG. 2, the LCDM100includes a liquid crystal panel102and a backlight unit114disposed under the liquid crystal panel102. The liquid crystal panel102includes first and second substrates (not shown) and facing each other and a liquid crystal layer (not shown) therebetween. Moreover, a gate line (not shown) and a data line (not shown), which cross each other to define a pixel region (not shown), are formed on the first substrate (not shown). The first substrate (not shown) may be referred to as an array substrate. A thin film transistor (TFT) (not shown) is disposed at a crossing portion of the gate and data lines (not shown). A pixel electrode (not shown) in each pixel region (not shown) is connected to the TFT (not shown). A black matrix (not shown) having a lattice shape is formed on the second substrate (not shown). The black matrix (not shown) corresponds to a non-display region, such as the gate line (not shown), the data line (not shown) and the TFT (not shown). A color filter layer (not shown), which includes red, green and blue sub-color filters and corresponds to each pixel region (not shown), is formed on the second substrate (not shown). Moreover, a common electrode (not shown) is formed on the black matrix (not shown) and the color filter layer (not shown). The second substrate (not shown) may be referred to as a color filter substrate.

The liquid crystal panel102is connected to a liquid crystal panel driving circuit unit (not shown). The liquid crystal panel driving circuit unit (not shown) provide signals to the gate line (not shown) and the data line (not shown) to drive the liquid crystal panel (not shown). When a common voltage is applied into the common electrode (not shown) and a data signal, which is applied to the pixel electrode (not shown) is controlled, an electric field is induced between the pixel electrode (not shown) and the common electrode (not shown). As a result, as the intensity or direction of the electric field is changed, the alignment of the liquid crystal molecules (not shown) in the liquid crystal layer (not shown) also changes such that light transmissivity is controlled. The LCDM100can display images.

The backlight unit114includes a lamp104as a light source, a lamp housing106, a light guide plate108, an optical sheet110including a light concentrating sheet118and a diffusion sheet120, a reflective sheet112and a bottom frame170. The lamp housing106surrounds the lamp104. The light guide plate108is disposed directly under the liquid crystal panel102or the light concentrating sheet118and changes light emitted from the lamp104into a plane light source. The optical sheet110is disposed between the liquid crystal panel102and the light guide plate108. The reflective sheet112reflects light from the light guide plate108such that the light is projected on the liquid crystal panel102. The light guide plate108includes polymethyl methacrylate (PMMA). The lamp104includes one of a cold cathode fluorescent lamp (CCFL), an external electrode fluorescent lamp (EEFL) and a light emitting diode (LED).

The lamp housing106surrounds and protects the lamp104. A light reflecting material is coated inside of the lamp housing106such that loss of light from the lamp104is minimized. Light from the lamp104is incident to the optical sheet110through the light guide plate108. The light is uniformly diffused by the light guide plate108. A part of the light is incident onto the reflective sheet112through the light guide plate108. However, the light is reflected on the reflective sheet112such that the light is projected again the optical sheet110through the light guide plate108. Although the edge type backlight unit is shown inFIG. 2, the LCDM100may include the direct type backlight unit where the lamp is disposed directly under the liquid crystal panel or the light concentrating sheet118and the light guide plate and the lamp housing are omitted.

FIG. 3is a perspective view of a light concentrating sheet according to a first embodiment of the present invention. Referring toFIG. 3, the light concentrating sheet118includes a base film122, a first light concentrating film124and a second light concentrating film126. The first light concentrating film124is disposed on the base film122and has a first refractive index. The second light concentrating film126is disposed on the first light concentrating film124and has a second refractive index. As mentioned above, the optical sheet110(ofFIG. 2) disposed on the light guide plate108(ofFIG. 2) includes the light concentrating sheet118and the diffusion sheet120(ofFIG. 2). The light concentrating sheet118concentrates light incident from the light guide plate108(ofFIG. 2), and the diffusion sheet120diffuses light from the light concentrating sheet118.

Each of the base film122, the first light concentrating film124and the second light concentrating film126includes one of polyethylene terephthalate (PET), ultraviolet (UV) curable resin and acrylic resin to from a pattern. Due to UV curable resin or acrylic resin, it is possible to be harden by an ultraviolet (UV) light. The first refractive index of the first light concentrating film124is different from the second refractive index of the second light concentrating film126. For example, the first refractive index is smaller than the second refractive index. Different additives are doped into acrylic resin of the first and second light concentrating films124and126such that the first refractive index is different from the second refractive index. For example, the first light concentrating film124has the first refractive index with a range of about 1.3 to about 1.9, while the second light concentrating film126has the second refractive index with a range of about 1.4 to about 2.0.

The base film122has a thickness of one of about 100 micrometers, about 125 micrometers, about 188 micrometers and 250 micrometers. A resin is coated on the base film122. The resin on the base film122is patterned using a mold, and then is hardened by the UV light. As a result, the first light concentrating film124having a convex portion and a concave portion is formed on the base film122. Moreover, the concave portion between the convex portions of the first light concentrating film124is filled up by a resin. The resin is hardened by the UV light such that the second light concentrating film126having a convex portion and a concave portion is formed on the first light concentrating film124. The base film122, the first light concentrating film124and the second light concentrating film126constitute the light concentrating sheet118.

In the light concentrating sheet118, a first refraction index of the first light concentrating film124is smaller than a second refraction index of the second light concentrating film126and greater a refraction index of than that of air. For example, the first refraction index is smaller than about 1.5, and the second refraction index is greater than about 1.5. Light from the light guide plate108(ofFIG. 2) passes an air layer having a refraction index of about 1. The light from the air layer is sequentially incident on the first light concentrating film124and the second light concentrating film126. Namely, light from the light guide plate108(ofFIG. 2) passes material layers having gradually increasing refraction indexes to be concentrated. Moreover, due to the convex and concave portions of the first and second light concentrating films124and126, a light path is changed such that light concentrating efficiency is further improved.

FIG. 4is a schematic view showing a light path in a light concentrating sheet according to the first embodiment of the present invention. Referring toFIG. 4, the convex portions of the first and second light concentrating films124and126has a triangle shape with first and second inner angles of α and β. The light from the light guide plate108(ofFIG. 2) is incident on the first light concentrating film124having the first refraction index with an angle θ through the air layer under the first concentrating film124. The light from the first light concentrating film124is incident on the second light concentrating film126having the second refraction index, and then light is outputted from the light concentrating sheet118(ofFIG. 3) with an angle τ. The angle τ of light outputted from the light concentrating sheet118(ofFIG. 3) is controlled on basis of an angle of the first and second inner angles α and β of the first and second light concentrating films124and126. InFIG. 4, refraction of the light at an interface of the air layer and the first concentrating film124and an interface of the first and second concentrating films124and126.

The concave portion of the first concentrating film124corresponds to the convex portion of the second concentrating film126, and the convex portion of the first concentrating film124corresponds to the concave portion of the second concentrating film126. As a result, the light concentrating sheet118including the laminated first and second light concentrating films124and126has a flat top surface and a flat bottom surface. Accordingly, when the light guide plate108, the light concentrating sheet118and the diffusion sheet120are stacked, problems of abrasion, which is generated in the related art LCDM, can be prevented. In addition, since the light concentrating sheet118has a flat top surface, other optical sheets can be laminated thereon.

Unfortunately, since the base film122, the first light concentrating film124and the second light concentrating film126of the light concentrating sheet118have difference in a thermal expansion coefficient, the light concentrating sheet118according to the first embodiment of the present invention may have a wrinkle appearance or a curl appearance. When the base film122is formed of a material having a thermal expansion coefficient to a material of the first light concentrating film124, there is a wrinkle appearance or a curl appearance because the second light concentrating film126has a thermal expansion coefficient different from the base film122and the first light concentrating film124. When the base film122is formed of a material having a thermal expansion coefficient to a material of the second light concentrating film126, there is also a wrinkle appearance or a curl appearance because the first light concentrating film124has a thermal expansion coefficient different from the base film122and the second light concentrating film126. On the other hand, the first and second light concentrating films124and126may have the same thermal expansion coefficient.

FIGS. 5A and 5Bare perspective views showing a cup curl appearance and a cap curl appearance, respectively. When the first light concentrating film124has a thermal expansion coefficient greater than the second light concentrating film126, there is a cup curl appearance as shown inFIG. 5A. On the other hand, when the second light concentrating film126has a thermal expansion coefficient greater than the first light concentrating film124, there is a cap curl appearance as shown inFIG. 5A.

The above deformation in the light concentrating sheet118resulted from difference in a thermal expansion coefficient is not restored. The wrinkle appearance causes an exterior deterioration in the liquid crystal panel102. In addition, the curl appearance causes decrease of brightness and mechanical deformation. Moreover, since the second light concentrating film126as a top surface of the light concentrating sheet118is exposed, there are damages. For example, when the LCDM is modulated, there are worker's finger marks on the second light concentrating film126.

FIG. 6is a perspective view of a light concentrating sheet according to a second embodiment of the present invention. In the light concentrating sheet shown inFIG. 6, two base films are formed on outer sides of first and second light concentrating films, respectively, to resolve a wrinkle appearance and a curl appearance.

InFIG. 6, a light concentrating sheet128includes a first base film132, a first light concentrating film124, a second light concentrating film126and a second base film134. Each of the first and second base film132and134has a flat top surface and a flat bottom surface. The first light concentrating film124is formed on the first base film132and has a convex portion and a concave portion. The second light concentrating film126is formed on the first light concentrating film124and has a convex portion and a concave portion. The convex portion of the first concentrating film124corresponds to the concave portion of the second concentrating film126, and the concave portion of the first concentrating film124corresponds to the convex portion of the second concentrating film126. Moreover, the second base film134is disposed on the second light concentrating film126. As a result, the light concentrating sheet118including the laminated first and second light concentrating films124and126between the first and second base films132and134has a flat top surface and a flat bottom surface. The first and second light concentrating films124and126have difference in a thermal expansion coefficient, and a refracting index of the first light concentrating film124is smaller than that of the second light concentrating film126.

Each of the first and second base films132and134may be include a synthetic resin, for example, polyethylene terephthalate, polyethylene naphthalate, acrylic resin, polycarbonate, polystyrene, polyolefin, cellulose acetate, or waterproof vinyl chloride. Each of the first and second base films132and134includes a colorless transparent material for transmission of incident light from the light guide plate108. Each of the first light concentrating film124and the second light concentrating film126includes one of polyethylene terephthalate (PET) and acrylic resin to from a pattern.

Referring toFIG. 12showing a fabricating process of a light concentrating sheet according to the second embodiment of the present invention, a fabricating apparatus180for the light concentrating sheet includes a film introducing part182, a first film coating part184, a pattern forming part186, a first hardening part188, a second film coating part190and a second hardening part192. They are serially coupled with each other.

A first base film132is introduced in the film introducing part182. The first base film132is transferred from the film introducing part182to the second hardening part192. After the first base film132is transferred into the first film coating part184, a first layer198is formed on the first base film132by coating a first resin material196. After the first base film132including the first layer198is transferred into the pattern forming part186, a roller194having a convex pattern and a concave pattern contacts the first layer198to form a first light concentrating film124having a convex portion and a concave portion. Then, the first light concentrating film124having the convex and concave portions is hardened by an UV light in the first hardening part188.

After the first base film132including the hardened first light concentrating film124is transferred into the second film coating part190, a second light concentrating film126of a second resin material202is formed on the first light concentrating film124. The second light concentrating film126is hardened by an UV light in the second hardening part192. Next, a second base film134is laminated on the second light concentrating film126such that a light concentrating sheet according to the second embodiment of the present invention is fabricated.

In the light concentrating sheet according to the second embodiment of the present invention, each of the first and second base films132and134has an average thermal expansion coefficient of the first and second light concentrating films124and126. The average thermal expansion coefficient of the first and second light concentrating films124and126is calculated by considering a volume of the first and second light concentrating films124and126and a pitch between the convex and concave portions of the first and second light concentrating films124and126. Namely, difference in a thermal expansion coefficient at an interface of the first base film132and the first light concentrating film124is substantially the same as difference in a thermal expansion coefficient at an interface of the second base film134and the second light concentrating film126. By these features, the first and second base films132and134respectively support the first and second light concentrating films124and126such that a wrinkle appearance and a curl appearance is prevented. Moreover, when the first and second base films132and134have a thickness greater than the first and second light concentrating films124and126, respectively, a wrinkle appearance and a curl appearance is further prevented.

FIG. 10is a perspective view showing a curl appearance in a light concentrating sheet according to the first embodiment of the present invention, andFIG. 11is a perspective view showing a curl appearance in a light concentrating sheet according to the second embodiment of the present invention. The light concentrating sheets according to the first and second embodiments of the present invention is processed under a temperature of about 60 Celsius degrees and a relative humidity of about 90 percentages during about 72 hours. There is a cap curl appearance of about 25.5 mm at both ends of the light concentrating sheet according to the first embodiment of the present invention inFIG. 10, while there is a cup curl appearance of about 1.5 mm at both ends of the light concentrating sheet according to the second embodiment of the present invention inFIG. 11. Namely, a cap or cup curl appearance is prevented in the light concentrating sheet according to the second embodiment of the present invention as compared with the light concentrating sheet according to the first embodiment of the present invention.

In the light concentrating sheet according to the second embodiment of the present invention, the light concentrating sheet includes two base films. Thicker light concentrating sheet causes thicker LCDM. These problems are resolved by third and fourth embodiments of the present invention inFIGS. 7 and 8.

InFIG. 7, a light concentrating sheet128includes a first base film132, a first light concentrating film124, a second light concentrating film126and a second base film134. Different from the light concentrating sheet128inFIG. 6, the second base film134is thinner than the first base film132. When the second base film134has a thermal expansion coefficient greater than the first base film132, there is a cap curl appearance. However, due to the second base film134thinner than the first base film132, a cap curl appearance is prevented.

InFIG. 8, a light concentrating sheet128includes a first base film132, a first light concentrating film124, a second light concentrating film126and a second base film134. Different from the light concentrating sheet128inFIG. 7, the second base film134is thicker than the first base film132. When the first base film132has a thermal expansion coefficient greater than the second base film134, there is a cup curl appearance. However, due to the second base film134thicker than the first base film132, a cup curl appearance is prevented.

FIG. 9is a perspective view of a light concentrating sheet according to a fifth embodiment of the present invention. If the cup or curl appearance still generates, a light concentrating sheet inFIG. 9is introduced to resolve the problem.

InFIG. 9, a binder material layer140including beads142is formed on an outer surface of a second base film134. On the other hand, the binder material layer may140include an UV curable resin. Moreover, the binder material layer140may be formed not only the outer surface of the second base film134but also an outer surface of a first base film132. Accordingly, a light concentrating sheet includes a first base film132, a first light concentrating film124on the first base film132, a second light concentrating film126on the first light concentrating film124, the second base film134on the second light concentrating film126and the binder material layer140. The beads142are attached onto the second base film134by the binder material layer140. On the other hand, the binder material layer140may be formed on an outer surface of the second base film132. Namely, the binder material layer140is formed on one of the first and second base films132and134. When one of the first and second base films132and134has a thermal expansion coefficient greater than the other one of the first and second base films132and134and there is still a curl appearance with different thickness in the first and second base films132and134, a problem of the curl appearance can be overcome due to a binding between the beads140. The beads142prevent a bad effect of particles for light from the light concentrating sheet. Namely, the beads142may cover up the particles. In addition, the beads142make the light from the light concentrating sheet being tender in appearance.

The binder material layer140includes at least one of acrylic resin, polyurethane, polyester, fluorine-base resin, silicon-base resin, polyimide and epoxy resin. The binder material layer140includes a colorless transparent material for transmission of incident light to the first base film132or from the second base film134. The beads142has a spherical shape and includes at least one of acrylic resin, polyurethane, polyvinyl chloride, polystyrene, polyacrylonitrile and polyamide. The beads142also include a colorless transparent material for transmission of incident light to the first base film132or from the second base film134.

It will be apparent to those skilled in the art that various modifications and variations can be made in the exemplary embodiments of the light concentrating sheet, the backlight unit and the LCDM without departing from the spirit or scope of the invention. Thus, it is intended that the invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.