Lighting unit and liquid crystal device using the same

A lighting unit has a light guiding plate having a plurality of corner portions each of which is formed by intersecting adjacent two side surfaces. The side surface between a pair of the corner portions of the light guiding plate forms a light incident face Non-light-emitting portions of a light source correspond to the pair of the corner portions of the light guiding plate, and at least one of the corner portions has an inclined face in contact with the light incident face, the side surface adjacent to the light incident face, and at least one of the top surface and bottom surface, and inclined by an angle θ (90°<θ<180°) with respect to the top surface or the bottom surface.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a lighting unit for use in a liquid crystal display device or the like and to a liquid crystal display device using the same.

2. Description of the Related Art

FIG. 11is a perspective view schematically showing a constitution of main parts of a conventional lighting unit for use in a liquid crystal display device.FIGS. 12 and 13are explanatory views showing a lighting operation of the lighting unit ofFIG. 11, in whichFIG. 12is a sectional view schematically showing the lighting unit andFIG. 13is a top view showing the lighting unit. As shown inFIGS. 11to13, the lighting unit includes a bar-shaped light source1, a light guiding plate2disposed such that the bar-shaped light source1is located close to a side surface2A of the light guiding plate2, a reflection sheet4disposed so as to cover at least a bottom surface2B of the light guiding plate2where scattering patterns3(FIG. 12) are formed, and a diffusion sheet5disposed along a light emanating surface2C of the light guiding plate2. For the purpose of improving luminance as seen from front, such a constitution that a prism sheet having a light converging function is further disposed on a top face of the diffusion sheet5is also generally practiced.

As shown inFIG. 12, in the lighting unit, light emitted from the bar-shaped light source1is incident on the light guiding plate2from the side surface2A (hereinafter, this face is referred to as a light incident face2A) of the light guiding plate2. While traveling inside the light guiding plate2, the light is scattered by the scattering patterns3of the light guiding plate2and reflected by the reflection sheet4(not shown) appropriately. Then, the light emanates upwardly through the light emanating surface2C of the light guiding plate2. Further, this emanating light is diffused by the diffusing sheet5(not shown).

As the bar-shaped light source1, a cold-cathode fluorescent light tube is generally used in the above-described lighting unit, while a hot-cathode fluorescent light tube is used in some large-size lighting units. As shown inFIG. 12, when either one of the fluorescent light tubes is used as the bar-shaped light source1, electrodes15that do not emit light by themselves are respectively disposed at both ends of the bar-shaped light source1. Therefore, in the bar-shaped light source1, non-light-emitting portions1B are respectively formed at both ends of the bar-shaped light source1, and a light-emitting region (hereinafter, referred to as an effective light-emitting region)1A is formed in a center portion (a portion of a glass tube16) sandwiched between the non-light-emitting portions1B.

In the bar-shaped light source1, because the non-light-emitting portions1B are formed at both ends of the bar-shaped light source1, the effective light-emitting region1A is shorter in length than the overall length of the bar-shaped light source1. Furthermore, owing to the non-light-emitting portions1B formed at both ends of the bar-shaped light source1, the amount of incident light at both ends of the light incident face2A is reduced, thereby causing two corner portions X of the light guiding plate2that are opposite to the non-light-emitting portions1B of the bar-shaped light source1to be darkened. As a result, as shown inFIG. 13, the luminance of light emanating from the light emanating surface2C of the light guiding plate2becomes low at the corner portions X. As used herein, the corner portion X refers to a portion where the light incident face2A and a side surface2D adjacent to the light incident face2A intersect.

In the light guiding plate2, as described above, its luminance is reduced at the corner portions X, thereby resulting in non-uniform luminance over the entire light emanating surface2C of the light guiding plate2. Although boundaries between darkened portions (hereinafter, referred to as dark portions)10of the corner portions X, and the remaining bright portion do not appear clearly, in a plan view, the boundaries appear to be inclined so as to extend outwardly from both ends of the effective light-emitting region1A as a whole (the dark portions10inFIG. 13is schematically shown).

In order to solve the above problem, there is an approach in which the overall length of the bar-shaped light source1is extended so that the effective light-emitting region1A reaches both ends of the light incident face2A of the light guiding plate2. However, in this constitution, since the non-light-emitting portion1B of the bar-shaped light source1protrudes from the light guiding plate2, the overall length of the bar-shaped light source1is lengthened, causing the size of the entire lighting unit to be increased. As a result, narrowing of a display panel frame, which is required in the liquid crystal display device, is not achieved.

SUMMARY OF THE INVENTION

The present invention is made to solve the above problem, and an object of the present invention is to provide a lighting unit in which even when non-light-emitting portions of a light source are disposed respectively at end portions of a light incident face of a light guiding plate, corner portions of the light guiding plate that are located close to the non-light-emitting portions are not darkened, so that uniformity of the luminance over the entire top surface of the light guiding plate can be achieved, and a liquid crystal display device in which narrowing of a display panel frame and uniform luminance can be achieved using the lighting unit.

In order to achieve the above object, according to the present invention, there is provided a lighting unit comprising: a light guiding plate having a top surface and a bottom surface as a pair of principal surfaces, a plurality of side surfaces formed on outer peripheries of the principal surfaces, and a plurality of corner portions each of which is formed by intersecting adjacent two side surfaces of the plurality of side surfaces, the side surface between a pair of the corner portions forming a light incident face; and a light source provided along the light incident face of the light guiding plate, the light source having both end portions forming non-light-emitting portions and a portion between the both end portions forming a light-emitting portion, respectively,in which light emitted from the light source and incident on the light incident face of the light guiding plate emanates from the top surface of the light guiding plate,wherein the light source is disposed such that the non-light-emitting portions correspond to the pair of the corner portions, respectively, and at least one of the pair of corner portions of the light guiding plate has an inclined face in contact with the light incident face, the side surface adjacent to the light incident face, and at least one of the top surface and bottom surface, and inclined by an angle θ with respect to the top surface or the bottom surface, a is larger than 90 degrees and smaller than 180 degrees.

With this constitution, since light incident on the inclined face from the light source is reflected there, the luminance at the corner portion having the inclined face is improved, thereby eliminating the dark portions on the top surface.

The inclined face of the light guiding plate may have a light scattering structure for scattering light incident on the inclined face. In this constitution, light incident on the inclined face from an inside of the light guiding plate in addition to the light source is scattered by the light scattering structure, thereby effectively increasing the luminance at the corner portion having the inclined face.

The inclined face of the light guiding plate may be in contact with the top surface of the light guiding plate. In this constitution, the corner portion having the inclined fac has a shape formed by cutting out a predetermined region from a region on the top surface side of the corner portion.

The inclined face of the light guiding plate may be in contact with the bottom surface of the light guiding plate. In this constitution, the corner portion having the inclined face has a shape formed by cutting out a predetermined region from a region on the bottom surface side of the corner portion.

The inclined face of the light guiding plate may be in contact with the top surface and the bottom surface of the light guiding plate. In this constitution, for example, the square inclined face may be formed, or, a plurality of the inclined faces may be formed at the one corner portion.

The above lighting unit may be used as a lighting unit for a liquid crystal display device comprising a liquid crystal panel disposed on a light emanating side of the lighting unit and having a pair of substrates with liquid crystal interposed therebetween. With this constitution, narrowing of the display panel frame of the liquid crystal display device can be achieved, and non-uniformity of the luminance of the liquid crystal display panel can be inhibited.

The object, as well as other objects, features and advantages of the present invention will become more apparent to those skilled in the art from the following detailed description of the preferred embodiments taken with reference to the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1is a perspective view schematically showing a constitution of main parts in a lighting unit according to an embodiment 1 of the present invention.FIG. 2is a partial perspective view schematically showing a constitution of a corner portion of a light guiding plate of the lighting unit of FIG.1.FIG. 3is a top view schematically showing the lighting unit of FIG.1.FIG. 4is a side view showing the light guiding plate of the lighting unit ofFIG. 1as viewed from a direction of an arrow a.FIG. 5is a side view showing the light guiding plate of the lighting unit ofFIG. 1as viewed from a direction of an arrow b.FIG. 6is a sectional view schematically showing a constitution of a liquid crystal display device provided with the lighting unit of FIG.1.

As shown inFIGS. 1to6, the lighting unit includes a bar-shaped light source1, a light guiding plate2disposed such that the bar-shaped light source1is located close to a side surface (i.e., light incident face2A) of the light guiding plate2, a reflection sheet4disposed so as to cover at least a bottom surface2B of the light guiding plate2, a diffusion sheet5disposed on a top of a light emanating surface2C of the light guiding plate2for adjusting light emanating characteristics, a reflector6covering the bar-shaped light source1so as to reflect light from the bar-shaped light source1toward the light guiding plate2, and frames7a,7bfor supporting these members1,2,4, and5.

A bar-shaped cold-cathode fluorescence tube or a hot-cathode fluorescence tube is used as the bar-shaped light source1. Electrodes15are respectively disposed at both end portions of the bar-shaped light source1. Since the electrodes15do not emit light by themselves, both end portions of the bar-shaped light source1where the electrodes15are respectively disposed become non-light-emitting portions1B. And, a center portion (a portion of a glass tube16) sandwiched between the non-light-emitting portions1B at both end portions of the bar-shaped light source1serves as an effective light-emitting region (a light emitting portion)1A. In such a constitution, in the bar-shaped light source1, the length of the effective light-emitting region1A in a direction of an arrow b is shorter than the overall length of the bar-shaped light source1in the same direction.

The light guiding plate2is made by molding polymethylmethacrylate (PMMA) in the form of a substantially rectangular-shaped plate and has a substantially rectangular top surface (light emanating surface)2C and a bottom surface2B which constitute a pair of principal surfaces and four side surfaces formed on outer peripheries of the principal surfaces. Scattering patterns3for scattering light are formed on the bottom surface2B of the light guiding plate2. And, the reflection sheet4is disposed along the bottom surface2B so as to cover at least the bottom surface2B. The bar-shaped light source1is disposed so as to be close to one side surface2A of the four side surfaces. As a result, this side surface2A becomes the light incident face of the light guiding plate2, and light from the bar-shaped light source1is introduced from the light incident face2A into the light guiding plate2.

Here, to prevent the non-light-emitting portions1B from protruding from the light guiding plate2, the bar-shaped light source1is disposed so that both end portions of the light incident face2A of the light guiding plate2respectively conform to outer end portions of the non-light-emitting portions1B. Thereby, the non-light-emitting portions1B of the bar-shaped light source1are disposed so as to be close to two corner portions X of the light guiding plate2. AS shown inFIG. 2, here, each corner portion X refers to an imaginary triangular-prism shaped portion formed by intersecting of the light incident face2A of the light guiding plate2and a side surface2D adjacent to the light incident face2A and includes a pair of apexes105,105′. At each corner portion X, a predetermined region including the apex105(more specifically, a triangular pyramid portion103to be described later) is cut out for chamfering, thereby forming a chamfered portion100including a cutout inclined face102.

The chamfered portion100formed at each corner portion X is formed by cutting out a region of the corner portion X on the side of the bottom surface2B along a plane intersecting a ridge line101formed by intersecting of the light incident face2A and the side surface2D adjacent thereto and inclined by an angle θ (90°<θ<180°) with respect to the bottom surface2B so that the triangular pyramid portion103including the apex105is cut out from the corner portion X. As a result, this cut surface becomes a new surface of the chamfered portion100(i.e., the cutout inclined face)102(seeFIG. 2taken in the direction of an arrow c; the arrow c indicates a direction parallel to the cutout inclined face102).

The cutout inclined face102thus formed is adjacent to the light incident face2A, the side surface2D, and the bottom surface2B and is a triangular flat face inclined by an angle θ with respect to the bottom surface2B. An inner surface of the cutout inclined face102may be flat or may have a light scattering structure for scattering light. The light scattering structure can be formed by forming a minute concave/convex pattern, for example, on the surface. With the cutout inclined face102having the light scattering structure, light coming from a variety of directions can be scattered on the surface102, thereby enabling the light to be used more efficiently. Consequently, the dark portions can be eliminated as described later.

As shown inFIG. 6, in a liquid crystal display device provided with the lighting unit having the above constitution, a liquid crystal display panel P is disposed on the side of the light emanating face2C of the lighting unit and a frame8is mounted on the liquid crystal display panel P from above, thereby constituting the device. Although its figures and detail descriptions are omitted, the liquid crystal display panel P, used to display characters and video images, includes a pair of transparent substrates having electrodes for display provided on their internal faces, between which liquid crystal material is filled. A plurality of driving circuits for displaying an image and the like on the liquid crystal display panel P and a substrate provided with controlling circuits are disposed around the liquid crystal display panel P.

Hereinbelow, operations of the lighting unit having the above constitution and the liquid crystal display device provided with the lighting unit will be described.

As shown inFIG. 3, light L1emitted from the bar-shaped light source1is reflected by the reflector6(FIG. 6) and incident on the inside of the light guiding plate2through the light incident face2A thereof. While traveling inside the light guiding plate2, the light L1is then scattered and reflected by the scattering patterns3and by the reflection sheet4(FIG. 6) appropriately and emanating through the light emanating surface2C of the light guiding plate2.

Here, the light L1traveling inside the light guiding plate2reaches the cutout inclined face102of the chamfered portion100formed at each corner portion X. Then, the light L1is reflected by the cutout inclined face102, so that the light L1is led to the light emanating surface2C. For example, the light L1emitted from the effective light-emitting region1A of the bar-shaped light source1obliquely travels and reaches the chamfered portion100formed at each corner portion X and reflected by the cutout inclined face102. Thereafter, the light L1reflected by the cutout inclined face102is led to the side of the light emanating surface2C. As a result, in the corner portion X where the non-light-emitting portion1B of the bar-shaped light source1is disposed and in the vicinity thereof, the luminance is increased and thereby the dark portion is eliminated.

As described above, in the lighting unit thus constituted, the dark portions are inhibited from being generated at the corner portions X close to the non-light-emitting portions1B of the bar-shaped light source1, by a simple method in which the triangular pyramid portions103are cut out from the corner portions X to form the chamfered portions100. Consequently, uniform luminance throughout the light emanating surface2C of the light guiding plate2can be achieved. Also, in this case, the dark portions can be eliminated without increasing the overall length of the bar-shaped light source1, and hence it is possible to achieve not only uniform luminance but also narrowing of the display panel frame, which are required in the liquid crystal display devices. Furthermore, since the cutout inclined face102is flat, its fabrication is easy.

In the liquid crystal display device provided with the above-described lighting unit, non-uniformity of the luminance of the liquid crystal display panel P can be inhibited, thereby making it possible to improve its display characteristics. Such a liquid crystal display device is suitable for use as a thin display device used in a notebook personal computer, a word processor, and a liquid crystal television, for example.

FIG. 7is a partial perspective view schematically showing a constitution of a corner portion of a light guiding plate of a lighting unit according to an embodiment 2 of the present invention. The lighting unit of the present embodiment is identical in constitution to the lighting unit of the embodiment 1 except that the constitution of the corner portion X of the light guiding plate2is different from that of the embodiment 1. Although only one corner portion X of the light guiding plate2is shown inFIG. 7, the other corner portion X has the same constitution as this, similarly to the embodiment 1.

In the present embodiment, a chamfered portion100′ of the corner portion X is formed by cutting out a region of the corner portion X of the light guiding plate2on the side of the light emanating surface2C along a plane intersecting the ridge line101and inclined by an angle θ′ (90°<θ′<180°) with respect to the light emanating surface2C so that a triangular pyramid portion103′ including an apex105′ is cut out from the corner portion X. As a result, this cut surface becomes a cutout inclined face102′ of the chamfered portion100′ (seeFIG. 7taken in the direction of an arrow d; the arrow d indicates a direction parallel to the cutout inclined face102).

The cutout inclined face102′ thus formed is adjacent to the light incident face2A, the side surface2D, and the light emanating surface2C and is a triangular flat face inclined by the angle θ′ with respect to the light emanating surface2C. An inner surface of the cutout inclined face102′ may be flat or may have a light scattering structure for scattering light.

In the lighting unit provided with the light guiding plate2thus constituted, light reflected by the cutout inclined face102′ travels toward the bottom surface2B. Since the reflection sheet4is disposed along the bottom surface2B, the light is reflected by the reflection sheet4. As a result, the light reflected is led to the light emanating surface2C. With the cutout inclined face102′ of the chamfered portion100′, the same effects as in the embodiment 1 can be obtained in the present embodiment.

While in the embodiments 1, 2 described above, the cases where the triangular pyramid portions103,103′ including a part of the ridge line101are cut out from the corner portion X of the light guiding plate2are described, a triangular pyramid portion including the entire ridge line101(i.e., including both apexes105,105′ of the corner portion X) may be cut out.

Furthermore, while in the embodiments 1, 2 described above, the cases where the triangular pyramid portions103,103′ are cut out from the corner portion X to form the triangular cutout inclined face102,102′ at the chamfered portions100,100′ are described, the shape of the cutout portions is not limited to be triangular pyramidal and the shape of the cutout inclined faces formed at the chamfered portions is not limited to be triangular. Concretely, for example, as shown inFIG. 8, a cutout inclined face102″, which is adjacent to the light incident face2A, the side surface2D, the light emanating surface2C, and the bottom surface2B, and is square face, may be formed at the corner portion X of the light guiding plate2. The cutout inclined face102″ is inclined by an angle ∅″ (90°<θ″<180°) with respect to the light emanating surface2C.

Furthermore, a plurality of cutout inclined faces may be formed at one corner potion. For example, as shown inFIG. 9, a cutout inclined face102i, inclined by an angle θ1 (90°<θ1<180°) with respect to the bottom surface2B, and a cutout inclined face102ii, inclined by an angle θ2 (90°<θ2<180°) with respect to the light emanating surface2C may be formed at the corner portion X. Alternatively, as shown inFIG. 10, a cutout inclined face102iii, inclined by an angle θ3 (90°<θ3<180°) with respect to the light emanating surface2C, and a cutout inclined face102iv, inclined by an angle θ4 (90°<θ4<180°) with respect to the light emanating surface2C may be formed at the corner portion X. In this case, for example, the cutout inclined face102ivmay be formed by cutting out a predetermined region from a region on the side of the light emanating surface2C of the corner potion X, after that, the cutout inclined face102iiimay be formed by further cutting out a predetermined region including a part of the cutout inclined face102iv.]

Furthermore, while in the embodiments 1, 2 described above, the cutout inclined face is formed by cutting out the predetermined region from the corner portion of the light guiding plate, the cutout inclined face at the corner portion of the light guiding plate-may be formed by molding the light guiding plate with using a mold having cut out corner portion.

Furthermore, while in the embodiments 1, 2 described above, the cases where the chamfered portions are provided respectively at the pair of corner portions that correspond to the pair of non-light-emitting portions of the bar-shaped light source are described, the chamfered portion may be provide only at either one of the pair of corner portions. Furthermore, while in the embodiments 1, 2 described above, the cases where the light sources are bar-shaped are described, the shape of the light source is not limited to be bar shape, but other shapes may be adopted. For example, in the case where an L-shaped light source is used, the L-shaped light source is disposed so as to comply in shape with two adjacent side surfaces of the light guiding plate, and non-light-emitting portions are disposed respectively at two corner portions on a diagonal line of the light guiding plate. In this case, thus, a chamfered portion is formed in at least one of these two corner portions located on the diagonal line. Alternatively, as another example, two bar-shaped light sources may be disposed respectively along two opposing side surfaces of the light guiding plate. In this case, non-light-emitting portions of the bar-shaped light sources are disposed so as to be close to four corner portions of the light guiding plate. In this case, thus, a chamfered portion is formed in at least one of these four corner portions.