JPH08304607A - Backlight - Google Patents

Abstract

PURPOSE: To obtain a backlight having good directivity, nearly perpendicularly irradiating a liquid crystal display element without the irregularity of luminance and having the high utilization efficiency of light as a backlight constituted so that light emitted from a light source may irradiate the liquid crystal display element via a light transmission plate and a light condensing sheet. CONSTITUTION: This backlight is constituted of the linear light source 2, the light transmission plate 1 opposed to the transmission surface of the light source 2 in the longitudinal direction of the light source 2 and provided with many prism projections 5 whose projection area per unit area nearly becomes larger as they go away from the light source 2, and the light condenser sheet 10 having one surface whose cross section perpendicular to the liner light source 2 is a plane surface and the other surface whose cross section perpendicular thereto is a prism-shaped surface so that they may be arranged adjacently to the transmission surface of the light transmission plate 1. Then, the light condensing sheet 10 can be arranged so that any surface may be turned toward the liquid crystal display element side whether it is the plane surface or the prism-shaped surface.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a backlight of a liquid crystal display device, and more particularly to a backlight having a strong directivity, which illuminates a liquid crystal display device substantially vertically without uneven brightness and has high light utilization efficiency.

[0002]

2. Description of the Related Art With respect to a non-luminous liquid crystal backlight, many proposals have been made for an optical member constituting the backlight or for a constitution of the backlight in order to improve light utilization efficiency.

As an example of a backlight having a strong directivity, a light-condensing sheet is arranged adjacent to a light-transmitting surface of the light guide plate (a surface of the surface facing the liquid crystal display element on the liquid crystal display element side). No. 17 publication is cited. In the backlight disclosed in Japanese Patent Laid-Open No. 2-17, the light guide plate and the reflection surface of the light guide plate are disclosed.
The reflective sheet is disposed to face (the lower surface of the surfaces facing the liquid crystal display element), and the light-condensing sheet is adjacent to the transmissive surface of the light guide plate.

As a method of emitting light rays to the outside of the light guide plate,
A large number of lens units are provided on the transparent surface of the light guide plate. A light ray that hits the lens unit and is emitted from the light guide plate is refracted by a light condensing sheet having a prism-shaped surface on the light guide plate side to form a backlight with a strong directivity.

That is, by utilizing a light guide plate provided with a lens unit for emitting a light beam having a strong directivity, a light-condensing sheet having its prism-shaped surface facing the light guide plate side adjacent to the transmission surface of the light guide plate is used. A backlight has been proposed in which emitted light is focused in a desired direction.

[0006]

However, there are the following problems in the lens unit of the light guide plate in the backlight described in JP-A No. 2-17.

I. When the lens unit is a convex shape having a smooth curved surface In a lens unit having a shape as shown in FIG. 14, when the height of the lens unit is H and the pitch is P, the degree of curved surface (H /
If P) is small, there are many light rays that are not totally reflected by the convex surface and are totally reflected and re-guided inside the light guide plate, and there is little function of emitting the light rays from the light guide plate to the outside.

When the degree of curved surface (H / P) is large,
On the convex surface, the direction of the normal line at the point where the light beam guided inside the light guide plate may strike greatly differs depending on the location.

Therefore, even if the light rays are guided through the inside of the light guide plate at the same angle, the incident angle at the boundary surface of the light rays is significantly different due to the difference in the place where the light rays strike the convex surface. Therefore, even if the light is emitted from the light guide plate, the angle after the light is emitted from the light guide plate is different, and the half-value angle emitted from the transmission surface of the light guide plate is large, and the directivity is not strong.

Even if the lens unit is a concave surface, it is also not preferable. Even if the curved surface has an aspherical shape, the direction of the normal line of the boundary surface is different as described above, which is not preferable. II. In the case of the lens unit of the triangular prismatic lenticular, the vertical angle of the triangular prismatic lenticular is 25 degrees. However, since the apex angle is small, it becomes difficult to mold when the pitch is about several tens of microns. Also, looking at the angle characteristics of the light rays emitted from this light guide plate, there is an emission peak at ± 80 degrees, but
There is also a large amount of distribution of light rays that exit with an angle of around 40 degrees, and it is said that the directivity of light rays that exit from this light guide plate transmission surface is strong when a lens unit of a triangular prism lenticular with an apex angle of 25 degrees is used. can not cut. (Definition of the angle of the light emitted from the transmission surface of the light guide plate is shown in Fig. 2.) Therefore, even when the light condensing sheet is adjacent to this light guide plate, the half-value angle of the light emitted from the light condensing sheet is 50 degrees. , The directivity is not so strong.

Further, since the lens units of the triangular prismatic lenticular lens are continuously formed on the transmitting surface, the light rays guided inside the light guide plate decrease as the distance from the light source increases, so that the uneven brightness of the liquid crystal screen occurs. To do.

The present invention has been made in view of the above problems, and an object of the present invention is to make a light beam emitted from a linear light source,
In a backlight configured to irradiate a liquid crystal display element via a light guide plate and a light-condensing sheet, a backlight having high directivity, irradiating the liquid crystal display element substantially vertically with uniform brightness and high light utilization efficiency is provided. It is intended to be provided.

[0013]

As shown in FIG. 1, a backlight of the present invention comprises a linear light source, a light guide plate and a light-condensing sheet. , The projection area per unit area that increases in the direction of the length of the light source increases as it moves away from the light source, and a small prism-shaped protrusion that refracts the light ray and guides the light ray to the outside of the light guide plate is provided. The light-condensing sheet is arranged adjacent to.

In the light-condensing sheet, it is preferable that one of the sectional shapes perpendicular to the linear light source (the shape shown on the paper surface of FIG. 1) is a flat surface and the other is a prism-shaped surface. Further, in the prism-shaped surface of the light-condensing sheet, the prism hypotenuse length closer to the linear light source is preferably longer than the other prism hypotenuse length.

In the case where the plane of the light collecting sheet faces the light guide plate, the prism shape of the light collecting sheet more preferably has an apex angle on the side opposite to the light source of 10 degrees or more and 40 degrees or less. On the contrary, when the light collecting sheet is arranged such that the flat surface thereof faces the liquid crystal display element, it is more preferable that the light collecting sheet has an apex angle on the side opposite to the light source of 20 degrees or more and 50 degrees or less.

More preferably, it is preferable that the optical member between the light guide plate and the liquid crystal display element constitutes the backlight only with the light collecting sheet. The term "substantially prismatic" in the specification of the present application means that, as shown in FIG. 3, a protrusion is mainly composed of two protrusion slopes, and a space between adjacent protrusions is a plane portion substantially parallel to the reflection surface. Exists, and
It is distinguished from the triangular prismatic lenticular shown in Japanese Patent No. 2-17 and the prism-shaped surface of the light-condensing sheet.

The "protrusion apex angle" is defined as the angle formed by two slopes of the protrusion.

[0018]

In the backlight of the present invention, the light-transmitting surface of the light guide plate is provided with a substantially prism-shaped protrusion facing the longitudinal direction of the light source and having a projected area per unit area that increases substantially as the distance from the light source increases. The light rays are emitted to the outside of the light guide plate by the refraction of the light rays, so that light rays having a substantially uniform amount and strong directivity are emitted from the transmission surface of the light guide plate regardless of the distance from the light source.

When a light-condensing sheet is provided adjacent to the light-transmitting surface of the light guide plate, it is possible to supply a light beam having a stronger directivity (smaller half-value angle) from the light-condensing sheet.
By setting the shape of the light-condensing sheet to have a flat surface on one side and a prism-shaped surface on the other side, it is possible to control the peak angle direction of the light beam having strong directivity.

By making the shape of the light-condensing sheet such that the length of the prism hypotenuse near the light source is longer than the length of the prism hypotenuse farther from the light source, light rays are emitted from the light-condensing sheet in a direction perpendicular to the sheet. Emit.

In the case where the flat surface of the light condensing sheet faces the liquid crystal display element, it is desirable that the apex angle on the side opposite to the light source is 20 degrees or more and 50 degrees or less. Both when the value is below the lower limit and when the value is above the upper limit, it is not possible to emit light rays from the light-condensing sheet perpendicularly to this sheet. Similarly, in the case where the flat surface of the light-condensing sheet faces the light guide plate, the apex angle on the side opposite to the light source is 10
It is desirable to set the angle above 40 degrees. Below the lower limit, it will be difficult to make, and above the upper limit, it will not be possible to emit light rays from the light-condensing sheet perpendicularly to this sheet.

Furthermore, the protrusion apex angle of the protrusion of the light guide plate is 40 degrees or more.
It is desirable to set it to 110 degrees or less. Below the lower limit, it becomes difficult to make, and above the upper limit, the function of emitting light rays from the projections deteriorates, and the light utilization efficiency of the light guide plate decreases, which is not preferable.

Further, when the optical member between the light guide plate and the liquid crystal display element constitutes the backlight only with the light-condensing sheet, the half-value angle of the light beam illuminating the liquid crystal display element becomes small, so that the backlight of the backlight is reduced. Brightness increases almost vertically to the light.

[0024]

EXAMPLES Examples of the backlight of the present invention will be shown below. 1 is a configuration diagram of a backlight of the first embodiment, FIG. 2 is an enlarged view of FIG. 1, and FIG. 3 is an enlarged view of a light guide plate in FIG. In FIGS. 1 and 2, reference numeral 1 denotes a light guide plate that directs light from a light source 2 to a liquid crystal display element 9, and a surface facing the liquid crystal display element and close to the liquid crystal display element is a transmissive surface 4 and a far surface is a reflective surface 3. Has become. The material of the light guide plate is preferably a material having a high internal transmittance, a plastic such as acryl / polycarbonate is preferable in view of cost, and acryl is more preferable because of ease of molding. A substantially prism-shaped projection 5 is provided on the transmissive surface 4 of the acrylic light guide plate 1.

As shown in FIG. 3, the projection 5 in this embodiment is mainly formed by two projection slopes 7. There is also a ray of light traveling inside the light guide plate 1 from the left side to the right side on the paper surface of FIG. 3, which strikes the projection slope 7 on the side closer to the light source, but this is substantially on the incident surface 8 of the light guide plate 1. The light rays enter the inside of the light guide plate 1 at parallel angles, and the amount of distribution is small and can be ignored. Therefore, the portion of the projection 5 where the light ray is likely to hit (projection slope 7 distant from the light source 2) has a linear basic tone in which the directions of the normals are the same. In consideration of easiness of making, forming, and a shape capable of effectively emitting light to the outside of the light guide plate, the projection apex angle, which is the angle formed by the two projection slopes, is 90 degrees, and the projection slope 7 is The angle with the transparent surface 4 is 13
The protrusions 5 were distributed and formed on the transmitting surface 4 at 5 ° and 135 ° and the heights of the protrusions 5 were several microns to several tens of microns.

The pitch of the projections 5 is on the order of several microns to several tens of microns, and the pitch is random. Further, as the distance from the light source 2 is increased, the pitch is narrowed or the width of the protrusion 5 is increased (the height of the protrusion 5 is increased) to increase the ratio of the protrusion to the transmissive surface 4 and to transmit the light regardless of the distance from the light source 2. The amount of light transmitted through the surface 4 was constant.

FIG. 4 and FIG. 5 show the relationship (angle distribution characteristic) between the emission angle and the brightness of the light beam emitted from the light guide plate 1 of this embodiment.
Shown in Note that FIG. 4 shows the angular distribution characteristics of the light rays emitted from the portion close to the light source 2, and FIG. 5 shows the angle distribution characteristics of the light rays emitted from the portion far from the light source 2.

The light beam emitted from the transmissive surface 4 has a sharp emission peak at about −55 degrees at a position near the light source 2 and at about −80 degrees at a position far from the light source 2. That is, although the light rays that hit the projections 5 are efficiently emitted to the outside of the light guide plate, since the light rays are emitted at an angle close to the transmission surface 4 side, the liquid crystal screen is presented to the liquid crystal observer with the backlight configuration as it is. I can't feel bright.

Next, the light collecting sheet 10 will be described with reference to FIG. The light condensing sheet 10 of this embodiment is made of a plastic material like the light guide plate 1. When the refractive index of the plastic material is 1.54, the apex angle 13 on the side opposite to the light source (the apex angle on the side opposite to the light source is defined as the angle formed by the hypotenuse of the prism farther from the light source and the vertical direction of the light collecting sheet) in this specification. 32 degrees and the prism apex angle 16 is 72 degrees. The pitch is 50 microns, and in FIG. 6, the light source is arranged on the left side of the drawing, and the prism hypotenuse length 17 of the left prism hypotenuse A14 is longer than the right prism hypotenuse B15.

As shown in FIG. 1, the light-condensing sheet 10 is arranged such that the prism-shaped surface 11 faces the light guide plate 1. In the backlight of the first embodiment, the light rays guided through the inside of the light guide plate 1 are guided by the refraction action on the projection slope 7 that is farther from the light source 2 among the projection slopes 7 forming the projection 5 of the light guide plate 1. The light is emitted from the transmission surface 4 of the light plate 1 to the outside of the light guide plate 1. The light ray transmitted through the transmission surface 4 of the light guide plate 1 is incident from the prism hypotenuse A14, is totally reflected by the prism hypotenuse B15 adjacent thereto, and is directed in a direction substantially perpendicular to the flat surface 12 of the light collecting sheet. Therefore, since total reflection is performed at the prism hypotenuse B15,
The apex angle 13 on the side opposite to the light source has a great influence on the distribution of the emitted light rays, particularly on the angle at which the emitted light rays become the peak. Further, the flatness of the prism hypotenuse B15 also affects the half-value angle of the emitted light beam.

FIG. 7 and FIG. 8 show the angular distribution characteristics of the light rays emitted from the light collecting sheet 10. As shown in the figure, the peak of the emitted light beam is in the direction of about -5 degrees, the half-value angle is about 20 to 30 degrees, and the directivity is strong, and the liquid crystal display element is irradiated almost vertically without uneven brightness. I got a small backlight.

Next, the backlight of the second embodiment is shown in FIG.
This will be described with reference to FIG. In this embodiment, the same parts as those in the first embodiment are designated by the same reference numerals, and their description will be omitted.

First, as shown in FIG. 12, the light collecting sheet 2
0 has a refractive index of 1.54, the vertical angle 23 on the side opposite to the light source is 20 degrees, and the vertical angle 26 of the prism is 60 degrees. The pitch is 50 microns, the light source 2 is arranged on the left side of the drawing, and the prism-shaped surface 21 of the light-condensing sheet 20 has the prism oblique side length 27 of the left prism oblique side A24 longer than the right prism oblique side B25. Has become.

FIG. 12 and FIG. 13 show the angular distribution characteristics of light rays emitted from the light collecting sheet 20 when the light guide plate 1 and the light collecting sheet 20 are arranged as shown in FIG. Note that FIG. 12 shows the angular distribution characteristics of the light rays emitted from the portion close to the light source 2, and FIG. 13 shows the angle distribution characteristics of the light rays emitted from the portion far from the light source 2.

That is, the light beam emitted from the projection slope 7 distant from the light source 2 to the outside of the light guide plate is refracted and incident on the flat surface 22 of the light condensing sheet 20 and then refracted by the prism hypotenuse B25 in a direction substantially perpendicular to the liquid crystal display.

At this time, the apex angle 23 on the side opposite to the light source is determined by how the emission distribution from the light guide plate 1 and the emission distribution of the condensing sheet 22 are oriented. Further, the prism apex angle 26 is within a range in which light rays do not enter the prism hypotenuse A24 inside the light condensing sheet 20, and if the prism apex angle 26 is large, the light condensing sheet 20 will be described.
Can be made easier. Here, when the light is incident on the prism hypotenuse A24, the light totally reflected by the prism hypotenuse A24 is refracted by the prism hypotenuse B25, and then the light is mainly parallel to the plane 22 of the light condensing sheet 20. Since the light is emitted, the half-value angle of the emitted light becomes large.

As can be seen from FIGS. 12 and 13, the emission peak is in the direction of about -13 to -20 degrees, and the half-value angle is about 12 degrees.
Since the liquid crystal display device has a strong directivity, it is possible to illuminate the liquid crystal display element almost vertically without unevenness in brightness, and obtain a backlight with a small half-value angle.

The present invention is not limited to the above embodiment. The detailed shape of the protrusion of the light guide plate is not particularly limited as long as the half-value angle of the light beam emitted from the transmission surface of the light guide plate is small. Further, the shape of the prism-shaped surface of the light collecting sheet is not limited to the above description.

The pitch may be random to prevent moire. Further, the light collecting sheet may be formed by hybridizing a prism on a transparent substrate, or the apex of the prism may be a convex surface in a minute range.

Further, the vector component of the light ray on the paper surface of FIG. 1 does not change so that the ridgeline of the prism becomes parallel to the paper surface of FIG. Absent.

[0041]

As described above, in the backlight of the present invention, the light-transmitting surface of the light guide plate is opposed in the longitudinal direction of the light source, and the projected area per unit area increases substantially as the distance from the light source increases to refract light rays. By providing a minute prism-shaped protrusion that guides the light beam to the outside of the light guide plate, it is possible to obtain a substantially uniform amount of the emitted light beam with a small half-value angle regardless of the distance from the light source to the light transmitting plate. Adjacent to this light guide plate, by arranging a light collecting sheet having a flat surface on one side and a prism-shaped surface on the other side, it is possible to spread the light distribution from the light guide plate (half-value angle) without increasing the light collecting sheet. It was possible to reduce the half-value angle of the light beam emitted from.

In order to refract a light beam transmitted through the transparent surface of the light guide plate substantially perpendicularly to the liquid crystal display element, the refracting power of the light collecting sheet must be controlled, but the selection width is limited, so that The apex angle on the side opposite to the light source is the most important factor that determines the angular distribution of the light rays emitted from the light collecting sheet.

20 degrees or more and 50 degrees or less when the plane is opposed to the liquid crystal display element, and 10 degrees or more when the plane is opposed to the light guide plate.
By setting the angle to 40 degrees or less, the angle peak of the light beam emitted from the light-condensing sheet could be set to a desired angle.

Further, by setting the projection apex angle of the light guide plate to be 40 degrees or more and 110 degrees or less, it was possible to obtain an outgoing light beam that was easy to make and had a smaller half-value angle from the transmission surface of the light guide plate. Since the optical element between the light guide plate and the liquid crystal display element is composed of only the light-condensing sheet, the number of parts of the optical system can be reduced.
Moreover, since the half-value angle of the light beam emitted from the light-condensing sheet was small, the brightness value in the 0 degree direction could be increased. Therefore, the light utilization efficiency of the backlight is improved. Furthermore, since the optical path lengths of the light rays passing through the liquid crystal layer are substantially the same for all the light rays, it is possible to reproduce an image with an ideal color balance and prevent a decrease in contrast.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a backlight according to a first embodiment.

FIG. 2 is an enlarged view of FIG.

FIG. 3 is an enlarged view of the light guide plate in FIG.

FIG. 4 is a diagram showing an angular distribution characteristic of a light beam emitted from a light guide plate at a portion near a light source in FIG.

5 is a diagram showing an angular distribution characteristic of a light beam emitted from a light guide plate located far from the light source in FIG.

FIG. 6 is an enlarged view of the light collecting sheet in FIG.

FIG. 7 is a diagram showing an angular distribution characteristic of light rays emitted from a light-condensing sheet at a position near the light source in FIG.

FIG. 8 is a diagram showing an angular distribution characteristic of a light beam emitted from a light collecting sheet at a position far from the light source in FIG.

FIG. 9 is a configuration diagram of a backlight according to a second embodiment.

FIG. 10 is an enlarged view of FIG.

FIG. 11 is an enlarged view of the light collecting sheet in FIG.

FIG. 12 is a diagram showing an angular distribution characteristic of light rays emitted from a light-condensing sheet at a position near the light source in FIG.

FIG. 13 is a diagram showing an angular distribution characteristic of a light beam emitted from a light collecting sheet at a position far from the light source in FIG. 9.

FIG. 14 is a diagram illustrating a problem.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Light guide plate 2 Light source 3 Reflection surface 4 Transmission surface 5 Protrusion 9 Liquid crystal display element 10 Condensing sheet

Claims (7)

[Claims] 1. A backlight for a liquid crystal display device, comprising a linear light source, a light guide plate, and a prism-shaped light-condensing sheet disposed between the light guide plate and the liquid crystal display device. Facing the surface in the longitudinal direction of the light source,
The projected area per unit area increases substantially as the distance from the light source increases, and a minute prism-shaped projection that refracts the light beam and guides the light beam to the outside of the light guide plate is provided, and a condensing sheet is provided adjacent to the transmission surface of the light guide plate. A backlight characterized by being placed. 2. The backlight according to claim 1, wherein the light-condensing sheet has a cross-sectional shape perpendicular to the linear light source, one of which is a flat surface and the other of which is a prism-shaped surface. 3. The prism-shaped surface of the light-condensing sheet is characterized in that the prism hypotenuse length closer to the linear light source is longer than the other prism hypotenuse length. Backlight. 4. The flat surface of the light-condensing sheet is arranged so as to face the liquid crystal display element, and the prism-shaped apex angle on the side opposite to the light source is 20 degrees or more and 50 degrees or less. Or the backlight described in 3. 5. The flat surface of the light condensing sheet is arranged so as to face the light guide plate, and the prism-shaped apex angle on the side opposite to the light source is:
The backlight according to claim 2 or 3, wherein the backlight is 10 degrees or more and 40 degrees or less. 6. The protrusion of the light guide plate has a protrusion apex angle of 40.
The backlight according to any one of claims 1 to 5, wherein the backlight has a temperature of not less than 110 degrees and not more than 110 degrees. 7. The backlight according to claim 1, wherein the optical member between the light guide plate and the liquid crystal display element is only the light collecting sheet.