JP2000147498A - Back light - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a back light capable of reducing the number of sheets while maintaining the desirable luminance of a liquid crystal panel. SOLUTION: The back light 1 is disposed directly under the surface of a liquid crystal panel 2. The back light 1 has a light guide plate 4, a cold cathode fluorescent lamp 5 disposed on one side of the light guide plate 4, a reflecting plate 6 disposed on the under surface of the light guide plate 4 and a prism sheet 7 disposed on the upper surface of the light guide plate 4. The prism sheet 7 has a prism surface 7a and a diffusing surface 7b, the prism surface 7a confronting the light guide plate 4.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a backlight for guiding light having high directivity to a liquid crystal panel, and more particularly to a backlight capable of reducing the number of sheets.

[0002]

2. Description of the Related Art A liquid crystal display device has features such as a flat structure and low power consumption which are not present in a display device such as a conventional CRT display as a display device of a television, a word processor, a personal computer and the like. Expected. The liquid crystal panel of the liquid crystal display device itself is non-luminous, and is inconvenient for use in a dark place. For this reason, a backlight for uniformly illuminating a screen is used in combination with a liquid crystal panel for the purpose of enabling use in a dark place and improving visibility. As the backlight, there is known a backlight in which a cold cathode fluorescent lamp is arranged close to the side of a light guide plate for guiding light.

[0003] A backlight of this type is shown in FIG. The backlight includes a light guide plate 11 made of a transparent synthetic resin material, a cold cathode fluorescent lamp 12 provided near one side surface of the light guide plate 11, and a reflection sheet 13 arranged on the lower surface of the light guide plate 11. And a prism sheet 15 disposed on the upper surface of the light guide plate 11 with the diffusion sheet 14 interposed therebetween. In the figure, reference numeral 16 indicates a frame that accommodates each element.

In the above configuration, light from the cold-negative fluorescent lamp 12 is collected by the reflection sheet 13 and transmitted through the light guide plate 11. This light is scattered by the diffusion sheet 14, reaches the prism sheet 15, and the light directed in a certain direction is transmitted therethrough. This light enters a liquid crystal panel (not shown) to obtain image information, which is displayed on the screen as a display image.

[0005]

In the liquid crystal panel combined with the above-mentioned backlight, the direction of light can be directed in a certain direction by the prism sheet 15 provided on the upper surface of the light guide plate 11, thereby uniformly increasing the luminance. It is possible to increase and obtain a satisfactory high brightness.

[0006] However, in recent years, the transmittance of liquid crystal has been greatly improved, and the tendency to place more importance on the viewing angle than on the luminance has increased. This has attracted interest in backlights using a plurality of diffusion sheets. Here, the additional diffusion sheet is disposed and used on the upper surface of the prism sheet 15. However, in the case of using such a plurality of diffusion sheets, 3
Since the sheet structure is a stack of sheets and the structure is complicated, it is difficult to manufacture a backlight. Also, manufacturing costs increase due to the additional diffusion sheet.

Furthermore, there is a type in which the prism sheet 15 used for obtaining high luminance is omitted and only two diffusion sheets are used. However, in this case, the luminance is also greatly reduced, which is not preferable.

It is an object of the present invention to provide a backlight which can reduce the number of sheets while maintaining a desired luminance of a liquid crystal panel.

[0009]

The invention according to claim 1 is
In a backlight including a light guide plate for guiding light from a light source to a light output surface side, a prism sheet is disposed on the light output surface side of the light guide plate, and a prism sheet having a diffusion surface on the opposite side is disposed.

A second aspect of the present invention is characterized in that, in the first aspect of the present invention, a liquid crystal panel is provided on the prism sheet.

The terms defined in the first and second aspects of the present invention are defined as follows. The prism surface of the prism sheet is a surface on which a large number of prisms having a substantially triangular cross section are arranged in parallel. The diffusion surface of the prism sheet is a surface on which a large number of fine irregularities exist. The prism is preferably formed on a polycarbonate film so as to maintain a uniform shape and refract light uniformly in the same direction in a homogeneous transparent material, but other materials may be used.

In such a backlight, the light source light from the cold cathode fluorescent lamp becomes parallel light having a uniform direction on the prism surface of the prism sheet, and further reaches a diffusion surface and passes therethrough. At this time, the light is scattered on the diffusion surface,
When displayed on the screen of the liquid crystal panel, the angle from the optical axis is widened, and a desired viewing angle can be obtained. This eliminates the need to attach a plurality of diffusion sheets, and can simplify the backlight sheet configuration. In addition, the prism surface makes it possible to obtain the same high luminance as before.

[0013]

Embodiments of the present invention will be described below with reference to the drawings. In FIG. 1, the liquid crystal display device includes a backlight 1 and a liquid crystal panel 2. The backlight 1 is disposed adjacent to the lower surface of the liquid crystal panel 2. The liquid crystal panel 2 has a diffusion plate 3 on the upper surface.
In the present embodiment, the liquid crystal panel 2 is composed of a twisted nematic liquid crystal display element.

The backlight 1 further includes a light guide plate 4, a cold cathode fluorescent lamp 5 disposed near one side surface of the light guide plate 4, and a reflection sheet 6 disposed on the lower surface of the light guide plate 4.
And a prism sheet 7 disposed with a gap between the upper surface of the light guide plate 4 and a frame 8 for accommodating these components.

The prism sheet 7 provided on the upper surface of the light guide plate 4 has a prism surface 7 for aligning the direction of incident light.
a, and a diffusing surface 7b that scatters light on the side opposite to the prism surface. The prism sheet 7 is disposed such that the prism surface 7a faces the emission surface of the light guide plate 4, and at this time, the diffusion surface 7b on the opposite side faces the liquid crystal panel 1.

In this embodiment, the prism sheet 7 is made of a polycarbonate film having a thickness of 0.36 mm.
As shown in FIG. 2, a large number of V-shaped prisms 7c are formed on the prism surface 7a of the prism sheet 7 in parallel with each other. The angle θ of each prism 7c is in the range of 63 to 68 degrees and is kept uniform. Further, the other diffusion surface 7b is subjected to a blast treatment as a rough surface so as to maintain the unevenness.

This embodiment has the above configuration.
The light source from the cold cathode fluorescent lamp 5 is incident on the light guide plate 4 and is guided by repeating total reflection. The light transmitted through the light guide plate 4 is then incident on the prism surface 7a of the prism sheet 7, is refracted in accordance with the angle θ of the prism 7c, and becomes parallel light having a uniform direction. The parallel light reaches the diffusion surface 7b on the opposite side and passes therethrough.

At this time, the parallel light is scattered on the diffused surface 7b having a rough surface as a rough surface. When the transmitted light of the prism sheet 7 obtains image information on the liquid crystal panel 2 and is displayed on a screen by this scattering, the angle from the optical axis is widened, and a desired wide viewing angle can be obtained. That is, by disposing the diffusion surface 7b, a function equivalent to that of the conventional diffusion sheet can be obtained. This eliminates the need to attach a plurality of diffusion sheets, and the backlight 1
Can be simplified.

On the other hand, the prism surface 7a of the prism sheet 7
As a result, the same high luminance as that of the liquid crystal panel 1 can be obtained. The angle θ of the prism 7c, which is preferable to obtain high luminance confirmed by an experiment, is in the range of 63 to 68 degrees. However, since the obtained parallel light is also affected by the irradiation angle of the dot pattern of the light guide plate 4, it is desirable that the angle θ of the prism 7c be equal to or close to the irradiation angle. With this configuration, higher luminance can be obtained.

A gap is provided between the prism sheet 7 and the exit surface of the light guide plate 4 in order to prevent transmitted light from passing through.

Thus, in the present embodiment, it is possible to simplify the backlight 1 by reducing the number of sheets while maintaining the same high luminance as before.

Further, the reduction in the number of sheets makes it possible to reduce the manufacturing cost.

[0023]

According to the present invention, since the prism sheet faces the light guide plate and the diffusion sheet faces the liquid crystal panel, the number of sheets can be reduced while securing a desirable high luminance.

Therefore, according to the present invention, the backlight can be simply constructed, and can be easily manufactured without going through complicated steps.

In addition, the manufacturing cost of the backlight can be reduced, and an inexpensive backlight can be provided.

[Brief description of the drawings]

FIG. 1 is a sectional view showing an embodiment of a backlight combined with a liquid crystal panel according to the present invention.

FIG. 2 is an enlarged sectional view showing details of a prism sheet shown in FIG. 1;

FIG. 3 is a cross-sectional view illustrating an example of a conventional backlight for a liquid crystal display device.

[Explanation of symbols]

 Reference Signs List 1 backlight 2 liquid crystal panel 4 light guide plate 5 cold cathode fluorescent lamp 7 prism sheet 7a prism surface 7b diffusion surface 7c prism

Claims (2)

[Claims] 1. A backlight comprising a light guide plate for guiding light from a light source to a light output surface side, wherein a prism sheet having a prism surface facing the light output surface side of the light guide plate and a diffusion surface on the opposite side is disposed. A backlight characterized in that: 2. The backlight according to claim 1, wherein a liquid crystal panel is provided on the prism sheet.