JPH0682634A - Surface light source device - Google Patents

Abstract

PURPOSE:To reduce the reflection on the surface side of a light transmitting plate, to eliminate interference fringes, to make the irregular-reflection layer invisible and to improve the diffusivity by providing a rugged line on both sides of a transparent-prism film or sheet furnished on the surface of a light transmitting plate. CONSTITUTION:A dot-printed irregular-reflection layer is formed on the rear of a light transmitting plate 1 of polymethyl methacrylate. A reflector 2 is furnished on the rear of the plate 1 and a linear light source 5 on the end face of the plat 1. A rugged line consisting of flat slopes is provided on one side of a polycarbonate thin sheet and a rugged line on the opposite side to constitute the transparent prism sheet 3 provided on the surface of the plate 1. The direction and cycle of the rugged line consisting of flat slopes are made almost the same as those of the rugged line on the opposite side, and the average height of the rugged line on the opposite side is controlled to 1/2 or lower of that of the rugged line consisting of flat slopes.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface light source device. In particular, the present invention relates to a surface light source device suitable for a backlight of a display device using liquid crystal.

[0002]

2. Description of the Related Art A thin display device using liquid crystal such as a word processor, a personal computer and a liquid crystal television usually uses a backlight for illuminating the display device using the liquid crystal because the liquid crystal itself does not emit light. ing. This backlight needs to be thin and to illuminate the surface of the display device almost uniformly. A surface light source device is usually used for such a backlight.

The display device using the above liquid crystal is required to be upscaled and downsized, and therefore, the surface light source device is required to be thin as in the display device. Although there are various types of surface light source devices, the most widely used structure of the surface light source device is to provide a light source on the end face of the light guide plate and attach the light emitted from this light source to the surface of the light guide plate or the light guide plate. The light is emitted almost uniformly from the surface of the plate.

In recent years, in order to support thinning and colorization,
There is an increasing demand for improving the brightness of the backlight of a display device. Therefore, in order to improve the brightness with the same light source, it is necessary to regulate the light emitting direction of the surface light source device so that the light is efficiently emitted in the direction viewed by the user to improve the brightness.

More specifically, in general, when a light source is provided on the end face of a thin light guide plate and light is incident from this end face, the light is totally reflected due to the difference in refractive index between air and the light guide plate. Repeatedly, almost never go out of the light guide plate. Therefore, in the surface light source device, a diffuse reflection layer is provided on the back surface of the light guide plate by screen printing of dots such as beads, and a reflection plate is attached to the back surface of the light guide plate to diffusely reflect the light that strikes the back surface of the light guide plate by the diffuse reflection layer, and to perform the diffuse reflection. The reflected light is reflected directly or by a reflection plate and emitted to the outside of the light guide plate.

However, the light emitted from the light source provided on the end surface of the light guide plate normally exits at a very small angle with respect to the surface of the light guide plate, as shown in FIG. That is, the directivity is extremely strong.

This will be described in more detail with reference to FIG. 10. In FIG. 10, 101 is a transparent light guide plate with dot printing on the back surface, and a reflection plate 102 is provided on the back surface of this light guide plate 101. A light source 103 having a linear shape is provided on the end surface of the light guide plate 101.
The light emitted from the light guide plate 101 passes through the light guide plate 101.
The light is diffusely reflected by the diffuse reflection layer provided on the back surface of 01 and directly goes out from the light guide plate 101, or is reflected by the reflection plate 102 and goes out from the surface of the light guide plate 101. The light at point A at that time is shown in FIG.
As indicated by B, the light is extremely directional with a small angle with the light guide plate 101.

However, since it is rare for the user to see in such a direction that the angle with the light guide plate is small, it is necessary to change the angle at which this light is emitted. If a diffusing plate 104 is provided on the surface of the light guide plate 101 as shown in FIG. 11 in order to improve this directionality, the directivity becomes gentle as shown in C of FIG. 11, but it is still insufficient. Is.

As a means for improving such a drawback, as described in JP-A-62-144102, a structure surface in which a large number of right-angled isosceles triangular prisms are regularly arranged on one surface and the surface on the opposite side is provided. There is known a surface light source device in which a film made of a transparent polymer material having a smooth flat surface is provided on a light guide plate with the flat surface side of the film facing. That is,
In this surface light source device, when light having a strong directivity is emitted from the film, the angle at which the light is emitted is changed by the surface of the isosceles right triangle prism to the direction viewed by the user.

Further, as described in Japanese Patent Application Laid-Open No. 2-17, a transparent prism film or sheet in which irregularities made of flat inclined surfaces are regularly provided on the surface of a light guide plate is used. A surface light source device provided in contact with a light guide plate is known. That is, this surface light source device changes the light having a strong directivity in the direction in which the user sees it by the flat inclined surface of the concavo-convex strip of the transparent prism film or sheet, and emits the light from the surface of the transparent prism film or sheet.

[0011]

In order to make the surface light source device thin, it is necessary to make the light guide plate thin. However, if the light guide plate is made thin, the irregular reflection layer screen-printed on the back surface of the light guide plate can be clearly seen. If a display device using liquid crystal is placed in front of this surface light source device, there is a problem that the screen of the display device becomes difficult to see.

Further, when the inventor has conducted an experiment on the surface light source device described in Japanese Patent Laid-Open No. 62-144102, the above-mentioned film is provided on the light guide plate so that the direction of light having a strong directivity can be seen by the user. However, it was found that the light incident on the film from the light guide plate could not be used very efficiently. This is because, when light having a large incident angle is incident on the plane opposite to the structure surface, the light is reflected in the light guide plate direction and a light amount loss occurs.

If the surface of the film in contact with the light guide plate is a flat mirror surface, and the light exit surface of the light guide plate is also a flat mirror surface, the Newton's ring is formed due to a minute gap between the film and the light guide plate. There is also a problem that the interference fringes are formed and the image quality is significantly deteriorated.

Further, in the surface light source device described in Japanese Patent Application Laid-Open No. 2-17, the light passing through the transparent prism film or sheet has a brightness and darkness corresponding to the uneven lines, and a display device using a liquid crystal thereon. The provision of “1” is likely to cause moire interference fringes due to the light and darkness, which is not preferable for a display device.

The object of the present invention is to improve the direction of light having a strong directivity from the light guide plate, reduce reflection on the surface of the layer provided on the surface side of the light guide plate, and eliminate interference fringes.
It is an object of the present invention to provide a surface light source device excellent in diffusivity in which the irregular reflection layer is invisible.

[0016]

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems and achieves the above objects, and is provided on an end surface of a transparent light guide plate having a diffused reflection layer such as dot printing on the back surface. A surface on which a linear light source is provided, and a transparent prism film or sheet is provided on the surface of the light guide plate, which is provided with uneven ridges made of flat inclined surfaces, and a reflection plate is provided on the back surface of this light guide plate. In the light source device, a concavo-convex strip is also provided on the surface of the transparent prism film or sheet opposite to the concavo-convex strip, and the direction and period of the concavo-convex strip made of a flat inclined surface and the surface on the opposite side are provided. The surface light source device is characterized in that the direction and the cycle of the uneven lines are substantially the same.

In the present invention, the transparent prism film or sheet is a film or sheet formed by molding a transparent substance such as glass or transparent synthetic resin. There are various molding methods. For example, there are a pressing method of sandwiching between template plates having uneven surfaces, a method of extruding while molding with an embossing roll, and a method of cutting the surface.

As the transparent prism film or sheet used in the present invention, a synthetic resin transparent prism film or sheet is suitable. A preferred method for producing the transparent prism film or sheet made of synthetic resin is a method of extruding a transparent synthetic resin such as polycarbonate, polymethylmethacrylate, polyester or the like with an embossing roll on the surface of the transparent synthetic resin while providing uneven ridges.

In the present invention, the direction of the uneven strip on the side of the transparent prism film or sheet facing the light guide plate is
Any direction that is not perpendicular to the linear light source may be used. However, it is preferable to provide the uneven strip on the side facing the light guide plate so as to be substantially parallel to the linear light source.

In the present invention, an uneven strip having a flat inclined surface is provided on one surface of the transparent prism film or sheet, and an uneven strip is provided on the opposite surface.
The direction and the period of the uneven line formed of the flat inclined surface are substantially the same as the direction and the period of the uneven line provided on the opposite surface. Further, by stacking two or more transparent prism films or sheets on the light guide plate, the light diffusion effect can be enhanced.

When the ridges and ridges of the ridges and valleys of the flat prism of the transparent prism film or sheet used in the present invention and the ridges and ridges of the opposite surface are curved, light diffuses appropriately. , The brightness of light is not clear and is preferable.

Further, as a result of repeating various experiments by changing the height of the projections of the uneven strips provided on the surface opposite to the uneven strips having the flat inclined surface, the uneven strips provided on the opposite surface. If the height of the projection of is set to 1/2 or less of the average height of the uneven strip formed by the flat inclined surface, whichever surface is facing the light guide plate,
It was found that the light diffused appropriately, and the light and dark were not clear, which was preferable.

[0023]

In the surface light source device of the present invention, since the linear light source is provided on the end surface of the light guide plate, the light emitted from this light source enters the light guide plate from the end surface. Then, the light entering the light guide plate is diffusely reflected by the diffuse reflection layer provided on the back surface of the light guide plate and directly exits from the front surface, or is reflected by the reflector plate provided on the back surface of the light guide plate and goes out to the front surface.

The light emitted from this light guide plate is extremely directional as shown in FIG. Light with strong directionality emitted from the light guide plate is changed in angle by the sloped surface of the uneven strip on the back surface of the transparent prism film or sheet, passes through the transparent prism film or sheet, and passes through the surface of the transparent prism film or sheet. After the angle is further changed by the inclined surface of the uneven strip, the light is emitted in the direction viewed by the user.

At this time, since the transparent prism film or sheet is provided with uneven strips on both sides, and therefore the surface that contacts the light guide plate is also provided with uneven strips, reflection on this surface is reduced, The light from the light guide plate can be efficiently directed to the user. In addition, since the uneven prisms are provided on both sides of the transparent prism film or sheet, even if this transparent prism film or sheet is installed on the light guide plate, there is very little contact between the surface and the light guide plate. Therefore, the Newton ring-shaped interference fringes do not occur.

Further, the direction and the period of the uneven strip formed by the flat inclined surface of the transparent prism film or sheet,
By making the direction and the period of the uneven lines provided on the opposite surface the same, it becomes easier to manufacture the transparent prism film or sheet, the diffusion of light becomes substantially uniform, and it becomes easier to see.

If the ridges of the ridges and ridges formed by the flat inclined surface and the ridges and ridges provided on the opposite surface are curved surfaces, the light and darkness due to the ridges and stripes will be blurred, making it easier to see. Also, the average height of the uneven strips provided on the surface opposite to the uneven strips having the flat inclined surface is 1 of the average height of the uneven strip having the flat inclined surface.
When it is set to / 2 or less, the diffusion of light becomes appropriate and it becomes easier to see.

[0028]

EXAMPLES Next, examples of the present invention will be described. 1 shows a first embodiment of the present invention and is a sectional view of a surface light source device. FIG. 2 shows a second embodiment of the present invention and is a sectional view of a surface light source device. FIG. 3 shows a third embodiment of the present invention and is a sectional view of a surface light source device. FIG. 4 shows a fourth embodiment of the present invention and is a sectional view of a surface light source device.

FIG. 5 shows a first comparative example and is a sectional view of a surface light source device. FIG. 6 shows a second comparative example and is a cross-sectional view of a surface light source device. FIG. 7 shows a third comparative example and is a cross-sectional view of a surface light source device. FIG. 8 shows a fourth comparative example and is a sectional view of a surface light source device. Figure 9
FIG. 9 is a diagram showing the relationship between the incident angle and the light transmittance of the transparent prism sheet used in the example shown in FIG. 2 and the transparent prism sheet used in the comparative example shown in FIG. 6.

In FIGS. 1 and 2, reference numeral 1 is a light guide plate made of polymethylmethacrylate having a thickness of 3 mm, and a dot reflection irregular reflection layer is provided on the back surface of the light guide plate 1. The dots are circular and have a diameter of 0.5 m.
It is m to 1.5 mm. Reference numeral 2 denotes a reflection plate, which is provided on the back surface of the light guide plate 1 to reflect the light emitted to the back surface of the light guide plate 1 and return it to the light guide plate 1.

Reference numeral 3 is a transparent prism sheet, and this transparent prism sheet 3 is provided with an uneven line formed of a flat inclined surface on one surface of a polycarbonate thin plate and an uneven line on the opposite surface. . The apex angle of the uneven strip formed of the flat inclined surface is 90 °, the apex angle of the opposite uneven strip is 150 °, and the pitch of the uneven strip is 200 μm in both cases. In FIG. 1, the surface of the light guide plate 1 is directed to the light guide plate 1 with the surface of the uneven strip having an apex angle of 90 °, and in FIG. Set up in.

Reference numeral 5 is a linear light source provided on the end surface of the light guide plate 1. Reference numeral 6 denotes a reflector, which is attached to the back of the light source 5 so that the light emitted from the light source 5 is efficiently incident on the light guide plate 1.

Next, a method of using the surface light source device shown in FIGS. 1 and 2 will be described. The light source 5 is turned on. Then, the light emitted from the light source 5 is transmitted directly or to the reflector plate 6.
The light is reflected by and enters the light guide plate 1.

The light entering the light guide plate 1 is reflected directly or by the reflection plate 2, and a strong directional light (angle with the light guide plate 1 is about 20 °) is emitted from the surface as shown in FIG. The light emitted from this surface is changed in angle by the inclined surface of the uneven strip on the back surface of the transparent prism sheet 3, passes through the transparent prism film or sheet, and the inclined surface of the uneven strip on the surface of the transparent prism film or sheet. After further changing the angle, the light is emitted from the transparent prism sheet 3 to the liquid crystal unit side.

When the light passing through the transparent prism sheet 3 leaves the surface, less light is reflected on the surface,
Bright light is emitted by being diffused by the uneven strips on the surface.
When the transparent prism sheet 3 was looked into, the diffused reflection layer and the Newton ring-shaped interference fringes were not visible. Also, when a display device using liquid crystal is placed in front of this surface light source device, no moire interference fringes appear on the screen of the display device and there is no difficulty in viewing due to the irregular reflection layer, resulting in an extremely beautiful screen. It was

Next, the embodiment shown in FIGS. 3 and 4 will be described. The embodiment shown in FIGS. 3 and 4 is almost the same as the embodiment shown in FIGS. 1 and 2, respectively. The difference is that the peaks and valleys of the ridges and valleys formed by the flat inclined surface of the transparent prism sheet 3a are curved surfaces with a radius of curvature of 50 μm, and the ridges and valleys of the ridges and grooves on the opposite surface are different. Is that each of the ridges has a curved surface with a radius of curvature of 100 μm. When a display device using liquid crystal was placed in front of the surface light source device of the embodiment shown in FIGS. 3 and 4, the screen was more beautiful than that of the surface light source device of the embodiment shown in FIGS.

Next, a comparative example shown in FIGS. 5 and 6 will be described. In the comparative example shown in FIG. 5, the same concavo-convex stripes (pitch 200 μm, apex angle 90 °) as in the example shown in FIG. 1 are provided on one surface of the transparent prism sheet 3b of the example shown in FIG. The surface is a plane, which is different from the embodiment shown in FIG. Others are the same as the embodiment shown in FIG.
In FIG. 6, the surface of the transparent prism sheet 3b of FIG. 5 facing the light guide plate 1 is changed.

When a display device using liquid crystal was placed in front of the surface light source device of the comparative example shown in FIG. 5, a large number of moire interference fringes were seen on the display device and the screen was difficult to see.

Further, the light emitted from the transparent prism sheet 3b of the surface light source device of the comparative example of FIG. 6 is darker than that of the embodiment shown in FIG. 1, and the surface of the transparent prism sheet 3b is slightly distorted. A clear diffuse reflection layer was confirmed. In addition, Newton-ring-shaped interference fringes were observed in some places. Further, when a display device using liquid crystal was placed in front of this surface light source device, the screen of the display device was very difficult to see because the light and darkness caused by the dots was visible.

Next, a comparative example shown in FIGS. 7 and 8 will be described. The comparative example shown in FIG. 7 is different from the comparative example shown in FIG. 5 in that the ridges of the ridges and valleys of the flat slanted surface are curved surfaces with a curvature of 50 μm. Others are the same as the comparative example shown in FIG. In FIG. 8, the surface of the transparent prism sheet 3c of FIG. 7 facing the light guide plate 1 is changed.

When a display device using a liquid crystal is placed in front of the surface light source device of the comparative example shown in FIG. 7, a lot of moire interference fringes can be seen on the display device, although the display device is more damaged than the comparative example shown in FIG. The screen became difficult to see.

Light emitted from the transparent prism sheet 3c of the surface light source device of the comparative example of FIG. 8 is darker than that of the embodiment shown in FIG. 4, and when the surface of the transparent prism sheet 3c is seen, the light of the comparative example shown in FIG. Although it was scorching, a diffuse reflection layer was confirmed. In addition, Newton-ring-shaped interference fringes were observed in some places. Further, when a display device using liquid crystal was placed in front of this surface light source device, the screen of the display device was difficult to see because light and darkness due to the irregular reflection layer was visible.

As can be seen from the examples shown in FIGS. 1 to 4 and the comparative examples shown in FIGS. 5 to 8, in the surface light source device having the concavo-convex stripes on both sides of the transparent prism sheet, bright light is appropriately diffused and guided. The diffuse reflection layer and the Newton ring-shaped interference fringes provided on the back surface of the light plate cannot be seen. Further, if a display device using liquid crystal is placed in front of this surface light source device, there is no difficulty in viewing due to the irregular reflection layer, and a bright and easy-to-see screen without moire interference fringes.

On the other hand, the transparent prism sheet having a flat uneven strip on one surface of the transparent prism sheet is dark, and the interference fringes of Newton's ring strips can be seen, or the irregular reflection layer can be seen, and the liquid crystal is used in front of the surface light source device. When the display device was placed, it was difficult to see due to the diffused reflection layer, and it was difficult to see the screen such as the appearance of moire interference fringes. It should be noted that when the ridge portion of this uneven strip is formed into a curved surface as in the comparative example shown in FIGS. 7 and 8, it is slightly improved as compared with the comparative example shown in FIGS. 5 and 6, but it is still insufficient.

Next, in order to compare the screen brightness between the embodiment shown in FIG. 2 and the comparison example shown in FIG. 6, the transparent prism sheet 3 used in the embodiment shown in FIG. 2 and the comparison example shown in FIG. FIG. 9 shows a diagram comparing the incident angle and the light transmittance of the transparent prism sheet 3b used in FIG. In FIG. 9, the black dots of X indicate the light transmittance of the embodiment shown in FIG. 2, and the white dots of Y indicate the light transmittance of FIG.

As is clear from FIG. 9, the embodiment shown in FIG. 2 has a better transmittance than the comparative example shown in FIG. 6 at an incident angle of 60 ° or more. Especially as the angle increases,
The comparative example is worse. It is considered that this is because the light is reflected by the surface of the transparent prism sheet 3b of the comparative example shown in FIG. 6 which is a flat surface, and thus the transmittance becomes lower as the angle becomes larger. In the light guide plate as shown in FIG. 10, most of the light has an incident angle larger than 60 °, so that the embodiment is more efficient.

[0047]

In the surface light source device of the present invention, since the uneven prisms are provided on both sides of the transparent prism film or sheet,
The light reflected on the surface is small, Newton-ring-shaped interference fringes do not occur, bright light is appropriately diffused, and the irregular reflection layer provided on the back surface of the light guide plate becomes invisible. When a display device using liquid crystal was placed in front of this surface light source device, the difficulty of viewing due to the irregular reflection layer was eliminated, no moire interference fringes appeared, and a bright screen was obtained, which was very easy to view.

Further, the direction and the cycle of the ridges and valleys formed by the flat inclined surface of the transparent prism film or the sheet and the direction and the cycle of the ridges and grooves provided on the opposite surface are made substantially the same, or the flat inclined surface is formed. The ridge of the ridge and the ridge of the ridge and the ridge of the ridge provided on the opposite surface are curved, or the average height of the ridge provided on the opposite side is the average height of the ridge and flat sloping surface. If it is less than ½ of the above, the light is appropriately diffused and light that is easy to see is obtained.

Further, when a transparent prism film or sheet is provided on the surface of the light guide plate by making the uneven line substantially parallel to the light source of the line and the surface of the uneven line is brought into contact with the surface of the light guide plate, light is emitted. Since the light is inserted into the uneven strip at a substantially right angle, light can efficiently enter the transparent prism or sheet. Thus, the surface light source device of the present invention is extremely excellent and valuable.

[Brief description of drawings]

FIG. 1 shows a first embodiment of the present invention and is a sectional view of a surface light source device.

FIG. 2 shows a second embodiment of the present invention and is a sectional view of a surface light source device.

FIG. 3 shows a third embodiment of the present invention and is a sectional view of a surface light source device.

FIG. 4 shows a fourth embodiment of the present invention and is a sectional view of a surface light source device.

FIG. 5 shows a first comparative example and is a cross-sectional view of a surface light source device.

FIG. 6 is a sectional view of a surface light source device, showing a second comparative example.

FIG. 7 shows a third comparative example and is a cross-sectional view of a surface light source device.

FIG. 8 is a cross-sectional view of a surface light source device, showing a fourth comparative example.

9 is a diagram showing the relationship between the incident angle and the light transmittance of the transparent prism sheet used in the example shown in FIG. 2 and the transparent prism sheet used in the comparative example shown in FIG.

FIG. 10 is an explanatory diagram showing a light distribution characteristic of a conventional surface light source device.

FIG. 11 is an explanatory diagram showing a light distribution characteristic of a conventional surface light source device.

[Explanation of symbols]

1 Light guide plate 2 Reflector plate 3, 3a, 3b, 3c Transparent prism film or sheet 5 Light source 6 Reflector plate

Claims (4)

[Claims] 1. A transparent prism film in which a linear light source is provided on an end surface of a transparent light guide plate having a diffused reflection layer provided on a back surface thereof, and an uneven line formed of a flat inclined surface is provided on the surface of the light guide plate. Alternatively, in a surface light source device in which a sheet is provided and a reflection plate is provided on the back surface of the light guide plate, a concavo-convex strip is also provided on the surface of the transparent prism film or sheet opposite to the concavo-convex strip, and a flat surface is provided. 1. A surface light source device characterized in that the direction and the cycle of an uneven line formed of an inclined surface and the direction and the cycle of an uneven line provided on the opposite surface are substantially the same. 2. The surface light source device according to claim 1, wherein the ridge portion of the uneven strip formed of a flat inclined surface and the ridge of the uneven strip on the opposite surface are curved surfaces. 3. The average height of the uneven strip formed on the surface opposite to the uneven strip having the flat inclined surface is 1/2 or less of the average height of the uneven strip having the flat inclined surface. The surface light source device according to claim 1 or 2. 4. The surface light source device according to claim 1, wherein a plurality of transparent prism films or sheets are stacked.