JP2006227347A - Backlight unit for liquid crystal display device and liquid crystal display device - Google Patents

Abstract

A backlight unit with improved uniformity of luminance distribution of light emitted from an emission surface in a backlight unit (surface light source) for a liquid crystal display device.
A plurality of diffraction grating cells each having a diffraction grating formed on an exit surface of a light guide plate and / or an opposing surface of the exit surface, and light on an exit surface of the light guide plate and / or an opposing surface of the exit surface. It has a structure having a plurality of light scattering cells each having a scattering element, and the areas of the diffraction grating cell and the light scattering cell are different from each other. By changing the area of both cells and setting the arrangement period, arrangement position, and number of arrangements independently, the luminance distribution of light emitted from the emission surface is made uniform.
[Selection] Figure 4

Description

  The present invention relates to a backlight unit for a liquid crystal display device, and the luminance distribution is made uniform on the exit surface by the diffraction grating cell and the light scattering cell arranged on the light exit surface and / or the opposing surface. Provided is a backlight unit that can obtain emitted light.

Conventionally, a so-called backlight, which is an illumination light source used on the back surface of a transmissive liquid crystal panel, uses a light guide plate made of a transparent resin in order to uniformly guide light from the light source to the liquid crystal panel.
As shown in FIG. 1, in a backlight unit 14 in which a light source 12 is disposed on this type of light guide plate 10, light incident on the light guide plate 10 from the light incident surface 11 of the light guide plate 10 is transmitted to the light guide plate 10. Progress while totally reflecting inside. As the light source 12, a point light source such as a light emitting diode (LED) or a linear light source such as a cold cathode tube (CCFL) light source is used. V-grooves 13 are provided in the plane portion of the light guide plate 10 in some places, and light hitting the V-grooves 13 is emitted in an oblique direction as indicated by arrows in the drawing.

  The diffusion plate 16 and the prism sheets 17 and 18 are used in order to guide the direction of the liquid crystal panel 19 while making the luminance distribution of the light traveling in the oblique direction uniform. In order to secure the viewing zone, two prism sheets are arranged, and the prisms are orthogonal to each other.

As a known example of a light guide having a V-shaped groove 16 provided on the back surface as shown in FIG.
In addition, as an example of a backlight unit that does not use a V-shaped groove, there is a method in which light is diffused and emitted by printing scattering dots on the bottom surface of the light guide 10.

  On the other hand, a diffraction grating is known as an optical element that efficiently guides light from a light source in a desired direction. A typical configuration of a diffraction grating is an assembly of parallel lines (lattice lines) arranged at equal intervals. When the grating lines are expressed by shading, an amplitude type diffraction grating is obtained, and when expressed by an uneven shape, a phase type diffraction grating is obtained. . Generally, for visible light, a diffraction grating having a grating interval (pitch) of about 0.5 to 2 μm is used. The light guide plate on which the phase type diffraction grating is formed can efficiently diffract light from the light source arranged on the end face and guide it in the direction of the liquid crystal panel.

  Patent Document 2 is a known example of a light guide plate using a diffraction grating. Patent Document 2 describes a light guide in which a diffraction grating whose density or grating shape is changed is formed on the lower surface in order to control the luminance distribution.

JP-A-5-264819 Japanese Patent No. 2865618

When light incident from the side surface of the light guide plate is emitted to the exit surface by the light guide plate on which the diffraction grating is formed, it is difficult to make the brightness uniform on the exit surface.
Therefore, in order to make the luminance uniform on the exit surface, not only the diffraction grating but also a light scattering element is separately provided on the light guide plate. Since the diffraction grating and the light scattering element are different in size and shape, it is difficult to design and process the pattern if they are handled in the same way. Therefore, the diffraction grating and the light scattering element are separately divided, and divided into regions (cells) having respective optical functions and arranged on the light guide plate, thereby facilitating design and processing, and achieving uniform luminance.

  In order to solve the above-mentioned problems and to uniformly emit light from the light source from the exit surface, it is provided with a diffraction grating cell and a light scattering cell, and by changing their size and periodicity of arrangement, Provided is a backlight unit for a liquid crystal display device which can be controlled more strictly.

The backlight unit for a liquid crystal display device according to claim 1,
A light source;
In the illuminating device including a substantially sheet-shaped light guide plate that guides light incident from the light source and emits the guided light from an exit surface, the exit surface and / or the exit surface of the light guide plate A plurality of diffraction grating cells each having a diffraction grating formed on the opposite surface thereof, and a plurality of light scattering cells having a light scattering element formed on the emission surface of the light guide plate and / or the opposite surface of the emission surface. The diffraction grating cell and the light scattering cell have different areas, and each has a characteristic periodicity.

The backlight unit for a liquid crystal display device according to claim 2,
2. The backlight unit for a liquid crystal display device according to claim 1, wherein an area of the light scattering cell is larger than an area of the diffraction grating cell.

The backlight unit for a liquid crystal display device according to claim 3,
3. The backlight unit for a liquid crystal display device according to claim 1, wherein one of the diffraction grating cell and the light scattering cell is formed in a region where the diffraction grating cell and the light scattering cell are arranged to overlap each other. It is characterized by being.

The backlight unit for a liquid crystal display device according to claim 4,
4. The backlight unit for a liquid crystal display device according to claim 1, wherein either the diffraction grating cell or the light scattering cell is formed on substantially the entire surface of the emission surface and / or the facing surface. It is characterized by that.

The backlight unit for a liquid crystal display device according to claim 5,
5. The backlight unit for a liquid crystal display device according to claim 1, wherein the diffraction grating formed in the diffraction grating cell has a direction normal to the exit surface in a direction of light guided through the light guide plate. The light-scattering element formed in the light-scattering cell scatters light guided in the light guide plate in a direction perpendicular to the average light guide direction of light guided in the light guide plate. I try to let them.

The backlight unit for a liquid crystal display device according to claim 6,
6. The backlight unit for a liquid crystal display device according to claim 1, wherein the number of the diffraction grating cells arranged per unit area increases as the distance from the light source increases, and the arrangement of the light scattering cells per unit area. It is characterized by a decreasing number.

The backlight unit for a liquid crystal display device according to claim 7,
The backlight unit for a liquid crystal display device according to any one of claims 1 to 6, wherein the density of light scattering elements formed in the light scattering cell decreases as the distance from the light source increases.

The backlight unit for a liquid crystal display device according to claim 8,
The backlight unit for a liquid crystal display device according to claim 1, wherein the diffraction grating cell and the light scattering cell have a relief structure.

  The backlight for a liquid crystal display device realized by the present invention has the following effects.

  (1) By changing the area of the diffraction grating cell and the light scattering cell formed on the light guide plate and making their arrangement periods different, the diffraction grating and the light scattering element having different sizes can be handled efficiently. Therefore, the design for making the luminance distribution of the light emitted from the exit surface uniform can be performed more accurately.

  (2) When the grating interval of the diffraction grating is small and the light scattering element is large, it is difficult to make the size of the diffraction grating cell and the light scattering cell the same, and the size of the light scattering cell is larger than that of the diffraction grating cell. It is effective to increase.

  (3) In the region where the diffraction grating cell and the light scattering cell are arranged to overlap each other, only one pattern of the diffraction grating cell and the light scattering cell is formed, thereby generating noise light due to the multiple formed pattern. Can be prevented.

  (4) By arranging either the diffraction grating cell or the light scattering cell on almost the entire exit surface and / or the opposing surface of the light guide plate, the light emitted from the light source can be used efficiently, and at the exit surface The brightness non-uniformity can be reduced.

  (5) The diffraction grating bends the direction of the light guided in the light guide plate in a direction so as to approach the normal direction to the exit surface, and the light scattering element in the average light guide direction of the light guided in the light guide plate By making it scatter in the orthogonal direction, it is possible to achieve uniform luminance improvement in both the average light guiding direction of light and the direction orthogonal to the average light guiding direction of light.

  (6) As the distance from the light source is increased, the number of diffraction grating cells is increased and the number of light scattering cells is decreased, whereby the luminance distribution on the exit surface can be made uniform.

  (7) By decreasing the density of the light scattering elements formed in the light scattering cell as it moves away from the light source, the degree of light scattering can be strictly controlled, and the luminance distribution on the exit surface can be made uniform. Can do.

  (8) By making the diffraction grating and the light scattering element have a relief structure, a thin light guide plate can be realized.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
FIG. 2 is an explanatory diagram showing the positional relationship between the light guide plate 10 on which the diffraction grating is formed and the light sources 12 arranged on the side surfaces thereof. As a backlight unit for a liquid crystal display device, a reflection plate is disposed on the lower surface of the light guide plate, or a member having a light control function for a diffusion plate or a prism sheet is appropriately disposed on the upper surface of the light guide plate. They are omitted including the explanation.

FIG. 3 is a cross-sectional view of FIG.
Most of the light emitted from the light source 12 travels in the direction of the end surface 21 while repeating total reflection in the light guide plate, and is divided into diffracted light and regular reflected light on the surface 15 on which the diffraction grating is formed. The light travels in the direction of the exit surface 14 and exits as exit light 20. The regular reflection light further travels in the light guide plate. Here, when the diffraction grating is formed with a uniform density on the light guide plate, a lot of emitted light exits from the exit surface in an area close to the light source, and as the distance from the light source increases, the amount of emitted light from the exit surface increases. It decreases and a uniform luminance distribution cannot be obtained. Also, by reducing the density of the diffraction grating in the region close to the light source and increasing the density of the diffraction grating as the distance from the light source increases, or by changing the grating spacing of the diffraction grating, the average light guiding direction (light incident surface) The luminance distribution in the direction from 11 to the end surface 21 is improved, but the non-uniformity of the luminance distribution in the direction orthogonal to the average light guide direction is not improved.

  As a method for uniformizing the luminance distribution in the direction orthogonal to the average light guide direction, the diffraction grating is curved instead of linear, and the light is bent in both the average light guide direction and the direction orthogonal to the average light guide direction. There are a method of providing a function and a method of separately forming a light scattering element in order to bend light in a direction orthogonal to the average light guide direction.

In order to produce a light guide plate in which a diffraction grating and light scattering elements are formed, the light guide plate is often processed by an injection molding method using a metal mold in which a pattern is engraved or a cutting method using a metal blade. In either case, the diffraction grating and the light scattering element are each made up of a predetermined number and arrangement within an arbitrary minute size, and it is efficient in terms of design and processing to use them repeatedly. Is good.
In this patent, an arbitrary number of diffraction gratings arranged and having a light diffraction function are called diffraction grating cells, and an arbitrary number of light scattering elements arranged and having a light scattering function are called light scattering cells.

  FIG. 4 shows an embodiment of the backlight unit for a liquid crystal display device according to claim 1, and is a top view showing the top surface (exit surface 14) of the light guide plate. A plurality of diffraction grating cells 31 including a diffraction grating 30 and a light scattering cell 33 including a light scattering element 32 are formed on the lower surface (opposing surface 15) of the light guide plate.

The light emitted from the light source 12 with a certain angle range travels while repeating total reflection in the light guide plate, and the light reaching the diffraction grating cell 31 travels mainly by being divided into diffracted light and regular reflected light.
Diffracted light is emitted from the exit surface, and the specularly reflected light further travels in the light guide plate.
On the other hand, the light reaching the light scattering cell 33 is divided into scattered light and regular reflection light.
Scattered light is scattered at various angles toward the exit surface, and specularly reflected light travels through the light guide plate.

The diffraction grating cells 31 in FIG. 4 are arranged with a period as shown by the dotted line in FIG. 5, and the light scattering cells 33 are arranged with a period as shown in the dotted line in FIG.
The diffraction grating 30 and the light scattering element 32 have different shapes, sizes, depths, and the like.
In order to make the luminance distribution uniform on the exit surface, the areas of the diffraction grating cell and the light scattering cell are not the same, and it is easier and more precise to control the light if each has a unique size. it can.

In general, the diffraction grating used for the light guide plate has a fine grating interval of 1 μm or less, and several types of diffraction gratings having different grating intervals may be used.
On the other hand, as the light scattering element used for the light guide plate, the light scattering element is processed with a size of several μm or more in order to increase the light scattering property.
Therefore, for example, when the diffraction grating cell is a rectangular of 5 μm square, if a light scattering element is arranged in the same size cell, only a scattering element having a size of 5 μm or less can be produced, and the light scattering property is improved. I can't.
In other words, the light scattering element is often larger than the diffraction grating. Therefore, it is effective to increase the light controllability by making each cell unique and making the light scattering cell larger than the diffraction grating cell. It is.

FIG. 7 is an explanatory view showing another embodiment of the backlight unit for a liquid crystal display device according to claim 1.
The diffraction grating cell and the light scattering cell may have a rectangular shape as shown in FIG. 4 or a stripe shape as shown in FIG. 7, and the cell shape is not limited to a rectangular shape.

When arranging diffraction grating cells and light scattering cells of different sizes on the light guide plate, the cells may overlap each other as shown in FIG. 8 or one cell may contain the other cell. There may be a region where the two overlap.
In such a case, if the diffraction grating and the light scattering element are processed in the same region, noise light is generated by the multiple pattern formed, and both light control functions are impaired.
Therefore, in a region where the two overlap each other, for example, as shown in FIG. 9, the diffraction grating cell is preferentially arranged, and the light scattering element in the light scattering cell is omitted.
On the contrary, the light scattering cell may be preferentially arranged and the diffraction grating cell may be omitted, or in one light guide plate, the place where the preferred cell is the diffraction grating cell and the light scattering cell. A certain place may be mixed.

In order to efficiently use the light emitted from the light source, it is desirable that a diffraction grating cell or a light scattering cell be formed on almost the entire emission surface and / or the opposing surface of the light guide plate.
If there is a region where neither the diffraction grating cell nor the light scattering cell exists, light is not emitted from the region, which causes a non-uniform luminance distribution of light emitted from the exit surface.

  The diffraction grating formed in the diffraction grating cell is bent in a direction that brings the direction of light guided through the light guide plate closer to the normal direction to the exit surface, and the light scattering element disposed in the light scattering cell is the light guide plate. By making light scatter in a direction perpendicular to the average light guide direction of light guided in the interior, it is possible to achieve uniform luminance in the light average light guide direction depending on the position and number of the diffraction grating cells alone. The luminance in the direction orthogonal to the average light guide direction can be made uniform by the arrangement position and the number of arrangements of only the light scattering cells.

In the light guide plate that emits light from the light source arranged on the side surface from the emission surface, the intensity of light traveling in the average light guide direction is strong in the region close to the light source as shown in FIG. An area 40 where the light from the light does not reach sufficiently is created, and such a place becomes dark and the luminance becomes uneven.
Therefore, it is better to increase the number of light scattering cells than to the diffraction grating cells.
On the other hand, in a region away from the light source, light is used by a diffraction grating cell or the like as it travels through the light guide plate, and the amount of light that reaches it decreases, so it is better to increase the number of diffraction grating cells.

In order to more precisely control the degree of light scattering by the light scattering cell, the number, shape, depth, and the like of the light scattering elements formed in the light scattering cell may be changed for each cell.
An example of the light scattering cell when the degree of scattering is gradually changed is shown in FIG. The cells 51 having high light scattering properties reduce the average size and the number of the light scattering elements 32.
As the cells 52, 53, 54, and 55 gradually increase the average size of the light scattering element 32 and decrease the number of the light scattering elements 32, the light scattering property can be gradually lowered, and the light scattering is reduced. You can control gender. By arranging the light scattering cells having different scattering properties depending on the position of the light guide plate, it is possible to further uniform the luminance.

When the diffraction grating and the light scattering element are produced with a relief structure, a sufficient light control function can be realized when the depth of the diffraction grating is about 1 μm at the maximum and the depth of the light scattering element is about a few hundred μm at the maximum. .
Therefore, the thickness of the light guide plate is hardly affected, so that a thin backlight can be configured.

Explanatory drawing which shows an example of the backlight unit for conventional liquid crystal display devices. Explanatory drawing which shows the positional relationship of a light source and a light-guide plate. Explanatory drawing which shows a mode that the light from a light source advances the inside of a light-guide plate. Explanatory drawing which shows the light-guide plate which has arrange | positioned the diffraction grating cell and light-scattering cell from which a magnitude | size differs in a different period. The conceptual diagram which shows the arrangement period of the diffraction grating cell of FIG. The conceptual diagram which shows the arrangement | positioning period of the light-scattering cell of FIG. Explanatory drawing which shows the light-guide plate by the diffraction grating cell and light-scattering cell of a magnitude | size different from FIG. Explanatory drawing which shows the condition where the diffraction grating and the light-scattering element were covered when the diffraction grating cell and the light-scattering cell overlapped. Explanatory drawing showing the condition which gave priority and formed either cell when a diffraction grating cell and a light-scattering cell overlap. Explanatory drawing showing the nonuniformity of the luminance distribution in the light-incidence surface vicinity. Explanatory drawing which showed a mode that the light-scattering property of a light-scattering cell was controlled by changing the magnitude | size and number of light-scattering elements.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Light guide plate 11 ... Incident surface 12 ... Light source 13 ... V-groove 14 ... Ejection surface 15 ... Opposite surface 16 ... Diffusing plates 17, 18 ... Prism Sheet 19 ... Transmission type liquid crystal panel 20 ... Emission light 21 ... Terminal surface 22 ... Reflecting plate 23 ... Polarizing plate 30 ... Diffraction grating 31 ... Diffraction grating cell 32 ... Light scattering element 33 ... Light scattering cell 40 ... Areas 51 to 55 where light from the light source does not reach sufficiently 51 to 55 ... Light scattering cell with gradually changing scattering property

Claims (9)

A light source;
In a lighting device comprising: a light guide plate that guides light incident from the light source, and a substantially sheet-shaped light guide plate that emits the guided light from an exit surface;
A plurality of diffraction grating cells in which diffraction gratings are formed on the exit surface of the light guide plate or / and the surface opposite to the exit surface;
A plurality of light scattering cells each having a light scattering element formed on the exit surface of the light guide plate or / and on the surface facing the exit surface;
The backlight unit for a liquid crystal display device, wherein the diffraction grating cell and the light scattering cell have different areas, and are arranged with a specific periodicity. The backlight unit for a liquid crystal display device according to claim 1,
A backlight unit for a liquid crystal display device, wherein an area of the light scattering cell is larger than an area of the diffraction grating cell. The backlight unit for a liquid crystal display device according to claim 1 or 2,
The backlight unit for a liquid crystal display device, wherein either the diffraction grating cell or the light scattering cell is formed in a region where the diffraction grating cell and the light scattering cell are arranged to overlap each other. In the backlight unit for liquid crystal display devices in any one of Claims 1-3,
A backlight unit for a liquid crystal display device, wherein either the diffraction grating cell or the light scattering cell is formed on substantially the entire surface of the emission surface and / or the opposing surface. In the backlight unit for liquid crystal display devices in any one of Claims 1-4,
The diffraction grating formed in the diffraction grating cell bends the direction of light guided through the light guide plate so as to approach the normal direction to the exit surface, and the light scattering element formed in the light scattering cell A backlight unit for a liquid crystal display device, wherein light guided through the light guide plate is scattered in a direction orthogonal to an average light guide direction of the light guided through the light guide plate. In the backlight unit for liquid crystal display devices in any one of Claims 1-5,
The backlight unit for a liquid crystal display device, wherein the number of the diffraction grating cells arranged per unit area increases and the number of the light scattering cells arranged per unit area decreases as the distance from the light source increases. In the backlight unit for liquid crystal display devices in any one of Claims 1-6,
A backlight unit for a liquid crystal display device, wherein the density of light scattering elements formed in the light scattering cell decreases as the distance from the light source increases. In the backlight unit for liquid crystal display devices in any one of Claims 1-7,
The backlight unit for a liquid crystal display device, wherein the diffraction grating cell and the light scattering cell have a relief structure. A liquid crystal panel that defines a display image in accordance with translucency / light shielding in pixel units;
A liquid crystal display device comprising at least the backlight unit according to claim 1.

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