JP2000348515A - Sheet-form light source device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a sheet-from light source device which generates an appropriate light distribution, high brightness, and high display quality. SOLUTION: A light source device is composed of a rod-shaped light source 1, reflector 3, a photo-guide plate 2, whereby the light incident to its side face extending in the longitudinal direction of the rod-shaped light source is emitted ahead from the emission surface, a reflection sheet 4 extending along the back surface of the photo-guide plate, a prism sheet 5 laid alongside the emission surface and having a prism row, whose recesses and projections extend approx. parallel with the side face, and a diffusion sheet 9 arranged between the emission surface and prism sheet and having a haze value of 70% or lower.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a so-called side edge type planar light source having a light source on a side surface of a light guide plate used as a backlight light source of a liquid crystal display device or a light source of an emergency guide light or an advertising display board. Related to the device.

[0002]

2. Description of the Related Art FIG. 8 is a sectional view of a conventional planar light source device. Referring to FIG. 8, the device includes a cylindrical light source 101,
A light guide plate 102 for emitting light from the side surface 110 and propagating inward and emitting the light forward from the emission surface 111 (upward in the plane of the paper), and a reflection surface for allowing light emitted from the light source to be incident on the side surface 110 without waste. And a reflector 103 comprising: In front of the light guide plate 102, a prism sheet 105 for adjusting light distribution characteristics of light and a diffusion sheet 106 for making the light distribution angle of light emitted from the prism sheet gentle are provided.

[0003] A fluorescent lamp is widely used as the light source 101, and a resin material such as acrylic is used for the light guide plate 102.
The shape is often a flat plate or a wedge. Light guide plate 1
As a means for emitting light uniformly from the emission surface 111, a method of performing white dot printing 107 on a surface 112 (hereinafter referred to as “back surface”) opposite to the emission surface to change the diameter of the dot is provided for 02. Widely used. Reflector 1
For 03, a metal plate having a silver foil as a surface layer, a PET sheet or a white sheet on which a silver foil is stuck is used. In general, a white sheet, a sheet on which silver foil or aluminum foil is pasted, is used as the reflection sheet 104 for returning light emitted from the back surface 112 of the light guide plate to the light guide plate. As the prism sheet 105 for adjusting the light distribution characteristics, a prism sheet having a vertex angle of about 50 ° to 90 ° is often used. As the diffusion sheet 106, generally, a sheet obtained by applying beads to a PET sheet or a sheet obtained by embossing a PC sheet is used.

The operation of each member in this conventional device will be described below. The light emitted from the light source 101 is directly or after being reflected by the reflector 103,
It enters from 10. As shown in FIG. 9, the incident light L propagates inward while repeating total reflection in the light guide plate 102. At this time, a part of the propagating light is reflected by the means for uniformly emitting the light over the emission surface, for example, the printed white dots 107, and the angle formed by the normal to the emission surface 111 is smaller than the critical angle, The light exits from the exit surface. Also, part of the propagating light is emitted from the back surface 112, but this light is reflected by the reflection sheet 104, and is again reflected on the light guide plate 102.
Then, the light is emitted from the emission surface 111. The light emitted from the emission surface 111 during the inward propagation of the light guide plate 102 enters the critical angle at the angle between the normal to the emission surface 111 and the normal to the emission surface 111 in the in-plane direction perpendicular to the rod-shaped light source 101. As a result, much light is emitted. For this reason, the light distribution of the emitted light is a biased light distribution. The prism sheet 105 changes the direction of the light as shown in FIG. 9 and emits the light by reflecting the light incident from an oblique direction on the surface adjacent to the incident surface. The apex angle of the prism of the prism sheet 105 is adjusted according to the direction of the light emitted from the light guide plate so that the maximum luminance is in the front direction in front (the direction perpendicular to the emission surface).

[0005]

In the conventional configuration, in the in-plane direction perpendicular to the light source 101, the viewing angle of the light emitted from the emission surface is narrow and the light distribution is asymmetric about the front direction. ing. Therefore, by slightly changing the line of sight, an extreme difference in luminance is felt, and display quality is impaired.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a planar light source device having an appropriate light distribution, high luminance and high display quality.

[0007]

SUMMARY OF THE INVENTION A planar light source device according to the present invention comprises at least one rod-shaped light source, a side surface extending along the longitudinal direction of the rod-shaped light source, on which light from the rod-shaped light source is incident, and a side surface orthogonal to the side surface. A light guide plate that has an emission surface, propagates light incident from the side surface in an inward direction, and emits light forward from the emission surface, and surrounds the rod-shaped light source with the light guide plate, and a reflector that reflects light on the inner surface, In the vicinity of the back surface facing the emission surface of the light guide plate, a reflection sheet disposed along the back surface, and a concave and convex prism array in which the extending direction of the concave and convex portions is substantially parallel to the side surface is provided on one surface side.
A prism sheet disposed along the light exit surface; and a diffusion sheet having a haze value of about 70% or less disposed between the light exit surface of the light guide plate and the prism sheet.

As described above, by arranging the diffusion sheet in front of the prism sheet, light incident on the prism sheet becomes light with extremely reduced directivity. Further, by setting the haze value of the diffusion sheet to 70% or less,
It is possible to obtain a display with less decrease in luminance. Since the prism sheet emits light by changing the angle of light incident on the prism sheet mainly due to reflection, it is possible to reduce extreme directivity and bias while securing high luminance in combination with the above-described operation of the diffusion sheet. it can. Therefore,
The biased light distribution can be changed to an appropriate light distribution having a luminance center in front of the front in the in-plane direction perpendicular to the longitudinal direction of the rod-shaped light source. As a result, it is possible to realize a light distribution having high luminance and having a luminance center in the front direction symmetrical with respect to the front.

In the above planar light source device, the concave / convex prism array having the extending direction of the concave and convex portions substantially perpendicular to the side surface is provided on the light emitting surface, the rear surface, or both surfaces of the light guide plate. Preferably, it is formed.

[0010] By forming the above-mentioned concavo-convex prism array, the light distribution in the in-plane direction parallel to the rod-shaped light source and perpendicular to the emission surface can be made symmetric with respect to the front surface and has a luminance center at the front surface. it can. As a result, this planar light source device has a light distribution that is two-dimensionally symmetrical with respect to the front and has a luminance center at the front, in addition to having an appropriate light distribution in the in-plane direction perpendicular to the rod-shaped light source. It is possible to have a distribution. The above-mentioned uneven prism row is specifically formed on any one of the above-mentioned surfaces of the light guide plate, thereby realizing an appropriate light distribution in an in-plane direction parallel to the rod-shaped light source and perpendicular to the emission surface. .

In the above planar light source device, the second prism sheet having the concave and convex prism array on one surface side, in which the extending direction of the concave portion and the convex portion is substantially perpendicular to the side surface, is provided on the front side of the emission surface. It is desirable to further arrange.

According to the above configuration, the light distribution distribution in the in-plane direction parallel to the rod-shaped light source and perpendicular to the emission surface can be obtained without forming an uneven prism array in the same direction as the second prism sheet on the surface of the light guide plate. , The light distribution can be made symmetrical with respect to the front and having a luminance center in the front. as a result,
The planar light source device is two-dimensionally combined with an appropriate light distribution in an in-plane direction perpendicular to the rod-shaped light source.
It is possible to have a light distribution that is symmetric with respect to the front and has a luminance center in the front. The function of the second prism sheet is exerted without forming an uneven prism array in the same direction as the second prism sheet on the surface of the light guide plate, but such an uneven prism array is formed on the surface of the light guide plate. Even if it does, it works in a direction to further enhance its action.
Therefore, the second prism sheet can be arranged to obtain the above-described operation regardless of the presence or absence of the formation of the concave and convex prism rows on the light guide plate surface.

In the above planar light source device, it is desirable that the roughness on the surface of the light guide plate increases as the distance from the light source increases.

Since the intensity of the light in the light guide plate decreases with the distance from the light source, the intensity of the light at the exit surface is increased by increasing the roughness according to the distance and increasing the amount of light traveling toward the exit surface. Can be made uniform.

In the above planar light source device, in some cases, it is desirable that the roughness is adjusted on the concave and convex surfaces forming the concave and convex prism array which is the surface of the light guide plate.

In the case where it is preferable to adjust the roughness of the uneven surface of the prism array for processing reasons or other reasons, it is possible to make the light intensity distribution on the emission surface uniform by the above configuration. Becomes

[0017]

Next, an embodiment of the present invention will be described with reference to the drawings.

(Embodiment 1) FIG. 1 is a sectional view showing a planar light source device according to Embodiment 1 of the present invention. FIG.
With reference to FIG. 1, the light source 1 emits light from a light source 1, a light guide plate 2 that causes light to enter from a side surface 10 and propagate inward while emitting the light forward (upward in the paper) from an emission surface 11. A reflector 3 comprising a reflecting surface for allowing light to enter the side surface 10 without waste. In front of the light guide plate 2, a diffusion sheet 9 for further improving the light distribution, which will be described in detail below, and a prism for adjusting light distribution characteristics of light in an in-plane direction orthogonal to the rod-shaped light source. A sheet 5 and a diffusion sheet 6 for reducing the light distribution angle are provided. Further, the rear surface 12 of the light guide plate 2 is a roughened surface 15 whose roughness increases as the distance from the light source increases. Behind the light guide plate, there is provided a reflection sheet 4 that reflects light emitted from the back surface 12 of the light guide plate and sends it back to the light guide plate.

As the light source 1, a columnar fluorescent lamp is widely used, and a resin material such as acrylic is used for the light guide plate 2, and its shape is often a flat plate or a wedge. As the reflector 3, a metal plate having a silver foil as a surface layer, or a PET sheet or a white sheet on which a silver foil is stuck is used. Generally, a white sheet or a sheet on which a silver foil or an aluminum foil is pasted is used as the reflection sheet 4 for returning light emitted from the back surface 12 of the light guide plate to the light guide plate. The prism sheet 5 having a vertex angle of about 50 ° to 90 ° is often used. As the diffusion sheet 6, a sheet obtained by applying beads to a PET sheet or a sheet obtained by embossing a PC sheet is generally used.

Since the back surface 12 of the light guide plate is a roughened surface 15 whose roughness increases as the distance from the light source 1 increases, the degree of reflection increases as the intensity of light in the light guide plate decreases. For this reason, light with substantially uniform intensity is emitted over the emission surface 11.

The diffusion sheet 9 is disposed between the light exit surface 11 of the light guide plate 2 and the prism sheet 5. The light emitted from the light guide plate 2 is diffused by the diffusion sheet 9,
The light enters the prism sheet 5. Light distribution characteristics that greatly affect display quality can be controlled by changing the haze value of the diffusion sheet 9 as described below.

FIG. 2 is a diagram showing a light distribution which varies depending on the haze value of the diffusion sheet 9. FIG. 3 shows how to determine the angle of the light distribution. In a plane perpendicular to the rod-shaped light source, the normal direction of the emission surface is set to 0 °, the direction inclined toward the light source is defined as a plus angle, and the direction inclined toward the side away from the light source is defined as a minus angle. FIGS. 2A to 2D show the diffusion sheet 9 having a haze value of about 30%, 50%, 70%, and 85% in the planar light source device having the configuration of FIG.
It is a figure which shows the light distribution at the time of using. FIG. 2
(E) shows the light distribution when the diffusion sheet 9 having a haze value of 85% is used as a reference and the prism sheet 5 is removed, and FIG. 2 (f) shows the case where the diffusion sheet 9 is not used as a reference. FIG. 9 is a diagram showing a light distribution in the case of the conventional example shown in FIG.

As shown in FIG. 2F, when the diffusion sheet is not used, the light distribution is asymmetric around the angle of 0 °. This means that, when viewing A and B, which are both ends of the screen from the front of the center of the screen, as shown in FIG. 4, the brightness differs between point A and point B, which degrades the display quality. Note that the diffusion sheet 6 disposed on the most front side is not the diffusion sheet 9 between the emission surface 11 and the prism sheet 5.
Is changed to a sheet having a large haze value, the light distribution becomes 0
Although it becomes slightly gentle around °, the asymmetry of the light distribution is not improved, and the luminance is greatly reduced (not shown).

FIG. 2E shows a light distribution in a configuration using a diffusion sheet having a haze value of 85% without using the prism sheet 5. As shown in FIG. 2E, the luminance is greatly reduced and the light distribution is asymmetric.

On the other hand, by disposing the diffusion sheet 9 on these, as shown in FIGS. 2A to 2D, the asymmetry of the light distribution is improved. However, as the haze value increases, the width of the light distribution increases, but the luminance decreases significantly. When a diffusion sheet having a haze value of 85% is used, as can be seen from a comparison between FIGS. 2D and 2E, a change in luminance due to the presence or absence of the prism sheet 5 is slight.
There is no point in arranging the expensive prism sheet 5. Therefore, it is preferable to use a diffusion sheet having a haze value of about 70% or less, and preferably a haze value of about 50% or less. More desirably, one having a haze value of about 30% or less is used.

When a diffusion sheet 9 having a small haze value is used, when a white dot pattern is provided on the back surface of the light guide plate in order to make outgoing light uniform, the white dot pattern becomes easily visible. However, in the present embodiment, since the back surface is subjected to surface roughening processing to control the uniformity of the light intensity on the emission surface by the roughness, a good screen display can be realized. Further, as a result of eliminating wasted light and increasing the light use efficiency, it is possible to obtain a high-brightness screen display.

In the present embodiment, the back surface of the light guide plate 2 is a roughened surface, but the emission surface may be a roughened surface. Further, both the emission surface and the back surface may be roughened surfaces.

(Embodiment 2) FIG. 5 is a perspective view showing a planar light source device according to Embodiment 2. FIG. The same members as those in FIG. 1 are denoted by the corresponding reference numerals, and overlapping description will be omitted.
Also in the present embodiment, as in the first embodiment, in order to uniformly emit light within the emission surface, the emission surface of light guide plate 2 is a roughened surface. On the back surface, an irregular prism array 16 whose cross section extends in a direction perpendicular to the light source 1 and has a triangular wave cross section is formed repeatedly along a direction parallel to the light source 1. The light incident on the light guide plate 2 is controlled to spread in the direction along the rod-shaped light source 1 by the reflection and refraction effects of the prism array 16 during the propagation process into the light guide plate. That is, the light distribution in the in-plane direction parallel to the rod-shaped light source and orthogonal to the emission surface is adjusted. Therefore, the light obtained from the present device has a light distribution characteristic in an in-plane direction parallel to the rod-shaped light source and perpendicular to the emission surface by the prism array 16 formed on the light guide plate 2, and orthogonal to the rod-shaped light source 1 by the prism sheet 5. The light distribution characteristics in the in-plane direction are adjusted. As a result, a two-dimensionally appropriate light distribution is obtained and the light emitted from the light source is used without waste, so that a high-luminance planar light source device can be obtained.

In this embodiment, the light emitting surface 11 of the light guide plate 2 is a roughened surface, and the rear surface 12 has a prism array 16.
However, it is also possible to form a prism array on the emission surface where the direction in which the irregularities extend is perpendicular to the rod-shaped light source, and make the back surface a roughened surface. Further, a roughening process may be performed on the uneven surface of the uneven prism array, and the flat surface may be left untreated.

In this embodiment, the cross-sectional shape of the prism array formed on the light guide plate is triangular. However, the cross-sectional shape of the prism array may be trapezoidal or sine, and is not limited to triangular. Absent.

(Embodiment 3) FIG. 6 is a perspective view of a planar light source device according to Embodiment 3. The same members as those in FIG. 1 are denoted by the corresponding reference numerals, and redundant description will be omitted. In the present embodiment, the unevenness is provided on the back surface of the light guide plate 2 by the light source 1.
A concave / convex prism array 16 having a triangular-wave cross section extending in a direction perpendicular to the light source 1 is repeatedly formed in a direction parallel to the light source 1. Furthermore, to emit light almost uniformly from the exit surface,
The prism surface of the prism array 16 is adjusted as a roughened surface whose roughness increases as the distance from the light source increases, so that the intensity of the emitted light on the emission surface becomes uniform.

The light incident on the light guide plate 2 from the side surface 10 is controlled to spread in the direction along the rod-shaped light source by the effect of the prism array 16 in the process of propagating inward of the light guide plate.
That is, the light distribution in the in-plane direction parallel to the rod-shaped light source and orthogonal to the emission surface is adjusted. Therefore, this planar light source device adjusts the light distribution in an in-plane direction parallel to the rod-shaped light source and perpendicular to the emission surface by the prism array 16 formed on the back surface of the light guide plate 2, and the light source is formed by the effect of the prism sheet 5. The light distribution in the in-plane direction perpendicular to 1 is adjusted. As a result, a planar light source device having an appropriate two-dimensionally adjusted light distribution as a planar light source device used for a liquid crystal display device and the like, and having high brightness because light emitted from the light source is not wasted. It is possible to obtain.

In this embodiment, a prism array is formed on the back surface of the light guide plate 2 so that the roughness of the prism surface increases as the distance from the light source increases. However, a prism surface whose roughness is changed as described above may be formed on the exit surface. Further, the roughening process may be performed on the flat surface without performing the roughening process on the uneven surface of the prism. Further, in the present embodiment, the cross-sectional shape of the prism array is triangular, but is not limited to triangular, and may be trapezoidal or sinusoidal.

(Embodiment 4) FIG. 7 is a perspective view of a planar light source device according to Embodiment 4. FIG. The same members as those in FIG. 1 are denoted by the corresponding reference numerals, and overlapping description will be omitted. In the present embodiment, the back surface 12 of the light guide plate 2 is a roughened surface 15 in order to uniformly emit light on the emission surface. Between the first prism sheet 5 and the diffusion sheet 6,
A second prism sheet 25 having an uneven surface on the side of the diffusion sheet 6 and a flat surface on the side of the first prism sheet 5 and having a prism array extending in a direction orthogonal to the light source 1 is arranged. The uneven surface of the second prism sheet may be arranged forward instead of the prism sheet side as shown in FIG. 7, or conversely, the uneven surface may be arranged on the prism sheet side. The process until the light incident on the light guide plate from the side surface 10 is emitted from the first prism sheet 5 is described in Embodiment 1.
The description is omitted because it is the same as. The light emitted from the first prism sheet 5 enters the second prism sheet 25. In the second prism sheet 25, the light distribution in an in-plane direction parallel to the light source 1 and perpendicular to the emission surface is adjusted mainly by the refraction of the prism. Therefore, the light obtained from the planar light source device in the present embodiment has a light distribution distribution in an in-plane direction orthogonal to the light source by the first prism sheet 5 and is parallel to the light source 1 by the second prism sheet 25. Thus, the light distribution is adjusted in the in-plane direction orthogonal to the emission surface. As a result, it is possible to obtain a high-intensity planar light source device having a suitable light distribution as a planar light source device used for a liquid crystal display device and the like and high light use efficiency.

In the present embodiment, the back surface of the light guide plate 2 is a roughened surface, but only the light emitting surface or both the back surface and the light emitting surface may be roughened surfaces. Also,
In the present embodiment, the prism cross-sectional shape of the second prism sheet 25 is triangular, but the cross-sectional shape may be trapezoidal or sinusoidal, and is not limited to triangular.

(Other Embodiments) Embodiments 1, 2,
In 3 and 4, the prism sheet 5 may have an emission surface side as a flat surface and the diffusion sheet 6 side as an uneven surface. As described above, the prism cross-sectional shape of the prism sheet 5 may be trapezoidal or sinusoidal, and is not limited to triangular. In the surface light source device of the present invention, the diffusion sheet 6 may be omitted. Further, the material of the light guide plate is not limited to acrylic, but may be made of another transparent resin or a resin in which fine particles that scatter light are mixed in a transparent resin. In the embodiment of the present invention, as a method of emitting light substantially uniformly in the emission surface of the light guide plate,
In all cases, a roughening treatment was performed in which the roughness increased as the distance from the light source increased. However, the method of emitting light of uniform intensity on the emission surface is not limited to the above-described surface roughening treatment, but may be white pattern printing on the back surface or the like, or shading of the white printing. Further, a semi-spherical or pyramid-shaped unevenness may be formed on the back surface or the like to make the light intensity uniform. Moreover,
A row of prisms whose direction of extension is parallel to the light source is formed on the back, and the height of the irregularities is increased as the distance from the light source increases, and the intensity of the light reflected from the back side to the emission surface side is adjusted to uniform May be achieved.

The embodiments disclosed this time are to be considered in all respects as illustrative and not restrictive. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

[0038]

According to the present invention, a high-brightness planar light source device having an appropriate light distribution can be provided. Further, to provide a planar light source device having a high display quality with a uniform surface brightness by increasing the roughness of a light-emitting plate, a light-emitting surface, a rear surface, or both surfaces of the light-guiding plate as a roughened surface as the distance from the light source increases. Can be. Further, by forming a prism array on the light guide plate or providing the second prism sheet, it is possible to provide a high-luminance planar light source device in which light distribution characteristics are adjusted in all directions.

[Brief description of the drawings]

FIG. 1 is a sectional view of a planar light source device according to a first embodiment.

FIG. 2 is a diagram showing a light distribution of light emitted from a planar light source device. (a) is for the diffusion sheet 9 having a haze value of 30%, (b) is for the diffusion sheet 9 having a haze value of 50%,
(c) is a diffusion sheet 9 having a haze value of 70%, (d) is a diffusion sheet 9 having a haze value of 85%, (e) is a diffusion sheet 9 having a haze value of 85% without using the prism sheet 5. (F) is a diagram showing a light distribution when the diffusion sheet 9 is not used.

FIG. 3 is a diagram showing how to set an angle of a light distribution of the planar light source device.

FIG. 4 is a diagram for explaining the importance of symmetry in light distribution.

FIG. 5 is a perspective view of a planar light source device according to a second embodiment.

FIG. 6 is a perspective view of a planar light source device according to a third embodiment.

FIG. 7 is a perspective view of a planar light source device according to a fourth embodiment.

FIG. 8 is a sectional view of a conventional planar light source device.

FIG. 9 is a diagram for explaining the operation of a conventional planar light source device.

[Explanation of symbols]

Reference Signs List 1 light source, 2 light guide plate, 3 reflector, 4 reflection sheet, 5 prism sheet or first prism sheet, 2
Reference numeral 5: a second prism sheet, 6: a diffusion sheet disposed at the forefront, 9: a diffusion sheet having an adjusted haze value, 10: a light guide plate side surface, 11: an emission surface, 12: a back surface, 15: a roughened surface having an adjusted roughness; A prism array formed on a light guide plate extending in a direction perpendicular to the light source.

Continued on the front page (72) Inventor Masayuki Kono 997 Miyoshi, Nishigoshi-cho, Kikuchi-gun, Kumamoto Prefecture F-term in the Advanced Display Co., Ltd. (reference) 2H038 AA55 BA06 2H091 FA14Z FA21Z FA23Z FA31Z FA41Z FB08 FC25 FD01 FD01 FD06 LA16 LA17 LA19

Claims (5)

[Claims] 1. At least one rod-shaped light source, a side surface extending along the longitudinal direction of the rod-shaped light source, on which light from the rod-shaped light source is incident, and an emission surface orthogonal to the side surface, and incident from the side surface A light guide plate that propagates the emitted light in an inward direction and emits the light forward from the emission surface, and surrounds the rod-shaped light source around the light guide plate,
A reflector that reflects light on the inner surface; a reflective sheet disposed near and along the back surface of the light guide plate facing the emission surface; and a direction in which the concave portions and the convex portions extend are substantially parallel to the side surfaces. A prism sheet arranged along the emission surface, which has a concave-convex prism array on one surface side; and a haze value of about 70% arranged between the emission surface and the prism sheet of the light guide plate. The following diffusion sheet,
A planar light source device comprising: 2. An uneven prism array having a direction in which a concave portion and a convex portion extend in a direction substantially orthogonal to the side surface is formed on the light emitting surface, the rear surface, or both surfaces, which are surfaces of the light guide plate. The planar light source device according to claim 1. . 3. A second prism sheet having, on one surface side, a row of concave and convex prisms in which the direction in which the concave and convex portions extend is substantially perpendicular to the side surface, is further disposed on the front side of the emission surface. The planar light source device according to claim 1. 4. The light guide plate according to claim 1, wherein the roughness of the surface of the light guide plate increases as the distance from the light source increases.
The planar light source device according to any one of claims 1 to 3. 5. The method according to claim 1, wherein the adjusting of the roughness is performed on an uneven surface forming an uneven prism row, which is a surface of the light guide plate.
The planar light source device according to claim 4.