DESCRIPTION

PLANE LIGHT SOURCE DEVICE AND DISPLAY DEVICE

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is an application filed under 35 U. S. C. §111 (a) claiming benefit pursuant to 35 U. S. C. §119 (e) (1) of the filing dates of Provisional Application 60/714,858 filed September 8, 2005 pursuant to 35 U. S. C. §111 (b) .

TECHNICAL FIELD

The present invention relates to a plane light source device and a display device. More specifically, the present invention relates to a plane light source device which radiates light emitted from a light source outwardly from a surface of a light guide plate, being used for a liquid crystal display apparatus or the like; and a display device equipped with the plane light source device.

BACKGROUND ART

In recent years, there have been widely used display devices, such as liquid crystal display devices, which use a plane light source device (backlight) for illuminating a panel with light from the back surface, the side surface or the like

thereof. Among such apparatuses, in liquid crystal television sets and liquid crystal monitors, fluorescent tubes such as hot cathode-type and cold cathode-type are conventionally used as backlight devices. In the light source device such as the backlight devices equipped with fluorescent tubes, there is a so-called direct lighting type device wherein fluorescent tubes are arranged on a plane directly below (back surface of) a liquid crystal panel. There is also a so-called edge-light-type device wherein fluorescent tubes are provided only at edge(s) of a light guide plate made of transparent resin, wherein light incident into the light guide plate is reflected in reflection part provided on the back surface of the light guide plate, and the liquid crystal panel surface is illuminated with the reflected light. The direct lighting type device has an advantage of assuring high brightness but has a disadvantage of not providing thinner devices. On the other hand, the edge light-type device has an advantage of providing thinner devices than the direct lighting type but has a disadvantage of not providing larger devices with uniform brightness . Accordingly, as substitutes for such backlight devices equipped with fluorescent tubes, there have been recently investigated backlight devices wherein a light emitting diode (LED) , one of solid-state light emitting elements, is used as a light source.

Fig. 11 is a schematic cross-sectional view showing configuration of a conventional direct lighting type plane light source device 102 equipped with light-emitting diodes placed directly below a liquid crystal display panel 101 disclosed in Non-patent document 1. This plane light source device 102 is provided with an LED substrate (mounted substrate) on the base of a housing 104, wherein the substrate, not shown in the figure, is equipped with a plurality of light-emitting diodes 103 as light sources that are disposed in a planar arrangement. The base surface and side surface of the housing are covered with a reflection sheet 105. A diffusion plate 106 and a prism sheet 107 are placed at a distance of usually 2 to 5 cm from the light-emitting diode 103.

When the light-emitting diode 103 is made to emit light, the emitted light proceeds toward the diffusion plate 106 directly or after reflected on the reflection sheet 105, undergoes diffuse reflection in the diffusion plate 106, and then passes through the prism sheet 107. The light beam is thus bent in the perpendicular direction and enters into the liquid crystal panel 101. Light beams emitted from different light-emitting diodes 103 are mixed in a space between the light-emitting diodes and the diffusion plate 106 and mixing is enhanced through diffused reflection in the diffusion plate 106, making the brightness uniform. In general, since

brightness at positions directly above light-emitting diodes 103 is higher, uniformity in brightness can be enhanced further through increasing diffusivity at these positions in the diffusion plate 106. The conventional plane light source device equipped with light-emitting diodes, however, has a problem that brightness at the position directly above the light-emitting diode becomes higher even though a diffusion plate was installed and the distance between the light-emitting diode and the diffusion plate was set large in order to make brightness uniform as mentioned above. Particularly, when multicolor (RGB) light-emitting diodes are used instead of monochromatic light-emitting diodes and color mixing is performed, there is a problem that color nonuniformity appeared because of insufficient color mixing. In order to reduce nonuniformity in brightness and color, brightness at the position directly above the light-emitting diode has been reduced either by increasing diffusivity at the position directly above the light-emitting diode in the diffusion plate or by providing a so-called lighting curtain at the position directly above the light-emitting diode. These means, however, has brought about reduction of utilization efficiency of light. Although nonuniformity in brightness and color can be also reduced by increasing the distance between the light-emitting diode and

the diffusion plate, this method increases the thickness of the backlight, which is undesired for flat panel displays.

Non-patent document 1: Techno-Frontier Symposium 2005.

Symposium on Techniques of Heat Design and Countermeasure, issued on April 20, 2005 (Japan Management Association) ,

Session G3 Case Study I on the Latest Design of Heat-dissipating

Mounting (pp. G3-3-1 to G-3-3-4) .

DISCLOSURE OF THE INVENTION One of objects of the present invention is to provide a plane light source device wherein brightness nonuniformity due to higher brightness at a position directly above a light source is reduced without increasing the thickness of the plane light source device. Another one of objects of the present invention is to provide a plane light source device wherein nonuniformity in color as well as in brightness is reduced when multicolor (RGB) light-emitting diodes are used.

Furthermore, the present invention has also as one of objects to provide a display device comprising such a plane light source device.

The present inventors found a method for making brightness and chromaticity uniform in a plane light source device

(backlight) equipped with a solid-state light-emitting element as a light source, and thus attained the present invention. Namely, the present invention includes, for example, the following embodiments (1) to (10) . (1) A plane light source device comprising a substrate on which a solid-state light-emitting element is placed, a light guide member disposed above the substrate in a face-to-face manner and a reflection member disposed on the undersurface of the light guide member, wherein a concave part is provided on at least one face of the top surface and the back surface of the light guide member.

(2) The plane light source device according to the embodiment (1), wherein the concave part is provided at a position directly above the solid-state light-emitting element.

(3) The plane light source device according to the embodiment (1) or (2), wherein the shape of the concave part is in the form of cone, pyramid, cylinder, prism or hemisphere.

(4) The plane light source device according to any one of the embodiments (1) to (3) , wherein the concave part is provided on the undersurface of the light guide member.

(5) The plane light source device according to any one of the embodiments (1) to (4), wherein light scattering dots are formed on the undersurface of the light guide member.

(6) The plane light source device according to any one of the embodiments (1) to (5), wherein the solid-state light-emitting element comprises a red solid-state light-emitting element, a green solid-state light-emitting element and a blue solid-state light-emitting element.

(7) The plane light source device according to the embodiment (5) or (6), wherein the distance from the position directly above each solid-state light-emitting element to the nearest-neighboring light scattering dot is larger than the distance from the solid-state light-emitting element to the nearest-neighboring solid-state light-emitting element.

(8) The plane light source device according to any one of the embodiments (1) to (7) , wherein a concave part is provided on the top surface of the substrate and the solid-state light-emitting element is disposed in this concave part so that the top end of the solid-state light-emitting element is positioned lower than the top surface of the substrate.

(9) A display device comprising the plane light source device according to any one of the embodiments (1) to (8) . (10) The display device according to the embodiment (9), wherein the display part is a liquid crystal panel.

EFFECT OF THE INVENTION

The plane light source device of the present invention, although of a thin type, exhibits excellent uniformity in brightness and chromatidty. Particularly, using the plane light source device of the present invention as a backlight of a liquid crystal display, high quality image can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 shows the whole configuration of a liquid crystal display device to which the embodiment of the present invention pertains.

Fig.2 is a schematic cross-sectional view of a plane light ^■ source device according to one embodiment of the present invention.

Fig.3 is a schematic cross-sectional view of a plane light source device according to another embodiment of the present invention.

Fig.4 is a schematic cross-sectional view of a plane light source device according to a still other embodiment of the present invention. Fig.5 is a schematic cross-sectional view of a plane light source device according to another embodiment of the present invention.

Fig. 6 is a schematic cross-sectional view of a plane light source device according to a still other embodiment of the

present invention.

Fig. 7 is a figure illustrating an example of a print pattern formed on the top surface of a light guide member.

Fig. 8 is a figure illustrating arrangement of LED chips on an aluminum base printed substrate (LED light source) in Examples .

Fig. 9 is a figure illustrating arrangement of an aluminum base printed substrate (LED light source) on base of an aluminum-made housing in Examples . Fig. 10 is a schematic cross-sectional view of the LED light source at the section defined by line A-A in Fig. 8.

Fig. 11 is a schematic cross-sectional view of a conventional liquid crystal display panel and a direct lighting type plane light source device disposed directly under the panel .

10 Plane light source (backlight) device

11 Backlight frame (Housing)

12 LED substrate (Mounted substrate)

12a Concave part on the top surface of LED substrate (Mounted substrate)

13 Light-emitting diode (LED)

14 Light guide member (Plate or film)

14a Concave part on the light guide member (Plate or film)

15 Diffusion member (Plate or diffusion film)

16 Prism sheet

17 Prism sheet

18 Reflection member (plate or film (Coated film) ) 20 Heat-dissipating substrate (Aluminum substrate) 21 Circuit board

22 Aluminum base printed substrate

23 Light-emitting diode (LED) 23R Red light-emitting diode (LED) 23G Green light-emitting diode (LED) 23B Blue light-emitting diode (LED)

24 Light guide member (Plate or film)

25 Housing

26 Opening on circuit board

27 Encapsulating resin 28 Reflector

30 Liquid crystal display module

31 Liquid crystal panel

32 Polarizing plate (Polarizing filter)

33 Polarizing plate (Polarizing filter) 101 Liquid crystal panel

102 Plane light source device

103 Light-emitting diode (LED)

104 Housing

105 Reflection sheet

106 Diffusion plate

107 Lens sheet (Prism sheet)

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments for carrying out the present invention will be explained in detail with reference to drawings appended. The present invention is not limited to the following embodiments .

In the description of embodiments for carrying out the present invention, the direction from a solid-state light-emitting element to a light guide member is referred to as "above" for convenience.

In a so-called direct lighting type backlight, using a light guide member is valuable for attaining sufficient uniformity in brightness and chromaticity while avoiding increase in the thickness of a plane light source device. In the direct lighting type backlight, since the solid-state light-emitting element is faced upward, it is desired that light is diffused and transmitted uniformly in the light guide member . Fig. 1 shows the whole configuration of a liquid crystal display device to which the present embodiment pertains. The liquid crystal apparatus to which the present embodiment pertains comprises, as a direct lighting type backlight device

(backlight) 10, a backlight frame (housing) 11 for housing a

light-emitting part, and an LED substrate (mounted substrate) 12, wherein a plurality of light-emitting diodes (LEDs) 13, which are solid-state light-emitting elements serving as light sources, are disposed on the substrate. Further, the backlight device 10 is provided with a light guide member (plate or film) 14, which is characteristic of the present invention, housed in the backlight frame (housing) 11 on the LED substrate (mounted substrate) 12. On the light guide member (plate or film) 14, there is also provided a diffusion member (plate or diffusion film) 15 in a form of a laminate of optical compensating sheets, which is a member (plate or film) for scattering and diffusing light to equalize brightness in the whole plane, and prism sheets 16 and 17, which are diffraction grating films having function of condensing light forward. The liquid crystal apparatus further comprises, as a liquid crystal display module 30, a liquid crystal panel 31 wherein liquid crystal is sandwiched by two glass substrates, and polarizing plates (polarizing filters) 32 and 33, which restrict oscillation of light wave to a given direction and are laminated on each of the glass substrates of this liquid crystal panel 31. The liquid display device is further provided with peripheral members such as an LSI for driving not shown in the figure.

This liquid crystal panel 31 is composed of various kinds of components not shown in the figure. For example, on the two glass substrates, there are provided with a display electrode, active elements such as a thin film transistor (TFT) , liquid crystal, a spacer, a sealing material, an alignment film, a common electrode, a protection film, a color filter and the like, not shown in the figure.

Fig. 2 is a schematic partial cross-sectional view of the light guide member (plate or film) 14, which is a characteristic part of the plane light source device according to one embodiment of the present invention, and the LED Substrate (mounted substrate) 12. Here are provided the LED substrate (mounted substrate) 12 on which a plurality of light-emitting diodes 13 are placed, the light guide member (plate or film) 14 and a reflection member (plate or film (coated film) ) 18 disposed on the undersurface of the light guide member 14. Concave parts 14a are provided on the undersurface of the light guide member 14. The reflection member (plate or film (coated film) ) 18 is exemplified by a white film, a thin film of metal, a coated film painted white and a reflector member (metal, white resin or the like) adhered to at least one of the undersurface of the light guide member (plate or film) 14 and the surface of the LED substrate (mounted substrate)- 12.

By adopting such a configuration, at least a part of light

emitted upward from the light source exits from the top surface of the light guide member after reflected on the top surface and/or undersurface of the light guide member as well as is scattered on the concave part. Thus, nonuniformity in brightness due to higher brightness of the light guide member above the light source can be prevented and distribution of brightness becomes uniform on the light emitting surface of the light guide member.

Here, the concave part is provided at the position directly above the light-emitting diode in the light guide member, where the brightness is the highest, thus nonuniformity in brightness above the light-emitting diode can be efficiently reduced.

In this case, "the position directly above the light-emitting diode" means the position wherein the distance from the light-emitting diode concerned is the shortest in the top surface or the undersurface of the light guide member. Accordingly, in a case where "the concave part is provided at the position directly above the light-emitting diode on the surface of the light guide member", the position directly above the light-emitting diode coincides with the center of the cross-section of the concave part on the surface of the light guide member, as shown in Fig. 3. However, strict coincidence is not necessary and a small error is allowable.

Although the concave part 14a is provided only on the

undersurface of the light guide member 14 in Fig.2, the position of the concave part 14a may be on both top surface and undersurface of the light guide member 14 as shown in Fig. 4, and it may be only on the top surface of the light guide member 14 as shown in Fig. 5.

In the plane light source device of the present invention, the shape of the concave part is preferably in the form of cone, pyramid, cylinder, prism or hemisphere.

When the light-emitting diode 13 has a lens part and this lens part protrudes from the LED substrate 12 on which the light-emitting diode is disposed as shown in Fig. 2, the shape of the concave part 14a on the undersurface of the light guide member 14 is preferably in the form of cylinder, cuboid, pyramid or hemisphere. Pyramid or hemisphere is particularly preferred. In the case of pyramid, the apex angle is preferably 120 degree or less, especially preferably 90 degree or less and 45 degree or more. Here, the apex angle means an apex angle of a triangle formed by a cross-sectional plane created by comprising a line drawn down from the apex of the pyramid perpendicularly to the base plane of the pyramid and at least one side edge which forms the side plane of the pyramid.

When the light source is an LED light source wherein a plurality of light-emitting diodes are disposed on the same substrate without any protruding part as shown in Figs. 3 to

6, the shape of the concave part formed on the undersurface of the light guide member is preferably in the form of cylinder, cuboid, pyramid, cone or hemisphere. Pyramid or cone is particularly preferred and its apex angle is preferably 120 degree or less, especially preferably 90 degree or less and 45 degree or more. As shown in these figures, it is more preferred that a concave part 12a is provided on the top surface of the LED substrate 12 and that the light-emitting diode 13 is placed inside this concave part 12a in such a manner that the top end of the light-emitting diode is positioned below the top surface of the LED substrate, that is, the light-emitting diode does not protrude beyond the top surface of the LED substrate 12. By making the distance between the light emitting diode 13 and the concave part 14a long, a wide range of the concave part 14a can be irradiated, enhancing the light diffusing function of the concave part. The concave part 12a is preferably filled with transparent resin up to the height of the top surface of the light-emitting member 9 after the light emitting diode 13 is placed. Furthermore, by forming a concave part with a shape in the form of cylinder, cuboid, pyramid, cone or hemisphere on the top surface of the light guide member 14, light emitted from the light source can be bent in the transverse direction and thus the brightness at the position directly above the

light-emitting diode can be reduced without loss of light. As the shape of the concave part, pyramid or cone is particularly preferred and its apex angle is preferably 120 degree or less, and especially preferably 90 degree or less and 45 degree or more.

The cross-sectional area at the position at which the cross-sectional area of the light-emitting diode 13 is maximum when viewed from the side of light guide member 14 may be smaller or larger than the cross-sectional area at the position at which the cross-sectional area of the concave part 14a provided on the light guide member is maximum, or they may be equal . Figures 2 to 5 show cases wherein the cross-sectional area of the light-emitting diode 13 is smaller than the cross-sectional area of the concave part 14a, while Fig. 6 shows a case wherein the cross-sectional area of the light-emitting diode 3 is larger than the cross-sectional area of the concave part 14a.

Brightness at the position directly above the light-emitting diode can also be reduced without loss of light by forming a print pattern, instead of forming a concave part, on the top surface of the light guide member. Preferably, the print pattern is a circular pattern whose center is at the position directly above the light-emitting diode, wherein the transmittance of the light guide member is lowest at the center of the pattern and increases with increase in the distance from

the center. Particularly preferred is a pattern wherein, when the distance from the center of the print pattern is a (mm) , the transmittance b (%) is represented by a quadratic function of a (mm) . An example of the pattern is shown in Fig. 7. The hatched part represents printed part for controlling the light transmittance to reduce. The pattern is configured such that, as the value of a increases, the area of the printed pattern decreases and the light transmittance increases. A plurality of concentric circular lines shown in Fig.7 are drawn for aiding explanation, and these circular lines need not be provided in the actual print pattern. Although the unit pattern in Fig. 7 has a shape formed by cutting out a part of an annular ring, the shape of the unit pattern is not limited to this shape. The unit pattern may be formed in any of various shapes such as circle and rectangle.

In order to bring light out efficiently from the light guide member, it is preferred to form light scattering dots on the undersurface of the light guide member. The light scattering dots can be created either by dot-printing with scattering ink or in the one-piece forming process with the light guide member.

Particularly, when a red light-emitting diode, a green light-emitting diode and a blue light-emitting diode are used as light-emitting diodes and color mixing is carried out, the

light scattering dot is preferably formed at a position away from each light-emitting diode. More specifically, on the surface of the light guide member on which the light scattering dot is formed, the light scattering dot is formed preferably at the position at which the distance from the position directly above each light-emitting diode to the nearest neighboring light scattering dot is larger than the distance from the light-emitting diode to the nearest neighboring light-emitting diode. If the light scattering dot is formed at the position closer than the preferred position, mixing of three primary colors (RGB) tends to be insufficient causing nonuniformity in color in the surface emitting light source.

It is preferred to provide a diffusion member (plate or film) above the light guide member like the liquid crystal display panel shown in Fig.11. In the embodiment of the present invention, only the light guide member is disposed between the light emitting diode 103 and the diffusion plate 106, and therefore, the thickness of the backlight dose not increase.

EXAMPLES

The present invention will be explained with examples below. However, the present invention is not limited to these examples . [Example 1]

An LED light source was prepared by mounting IW grade-LED chips of 1-mm square in a linear array in a sequence of green, red, blue, green, green, red, blue, green with an interval of 13 iron between the centers of the LED chips on an aluminum base printed substrate 22 having 110 mm (width) x 40 mm (length) as schematically shown in Fig. 8.

On the inner surface of an aluminum-made housing 21 with a bottom surface of 280 mm (width) x 234 mm (length) and a depth of 30 mm, white reflection film (Toray Industries, Inc., Lumirror (trademark) 60L) was adhered.

Then, on the bottom surface of the aluminum-made housing 21, the six LED light sources mounted with the LED chips were fixed in an arrangement with three rows and two columns, as shown in Fig. 9. Fig. 10 is a schematic cross-sectional view of the LED light source shown in Fig. 8 at the section defined by line A-A. The aluminum base printed substrate 22 is composed by superposing a circuit board 21 on which a circuit pattern is formed (a circuit pattern is formed on a glass epoxy substrate) on a heat-dissipating aluminum-made substrate 20 through an insulating layer. An LED chip 23 is adhered on the heat-dissipating substrate 20 with heat-dissipating grease at an opening 26 provided on the insulating layer, and an electrode

of the LED chip 23 and a terminal provided on the circuit board 21 are connected to each other with a boding wire.

The circumference of each LED chip 23 is surrounded by an aluminum-made reflector 28 which has openings, wherein the reflector 28 has a thickness sufficiently large to make the top surface of the reflector higher than the highest part of the bonding wire. The reflector is fixed on the circuit board with adhesive. The opening of the reflector 28 has a tilted plane wherein the aperture diameter at the top is larger than that at the bottom, where the reflector is adhered to the circuit board. This reflector has function of effectively guiding light, which is emitted from the LED, upward by reflecting the light on the tilted plane of the reflector. The opening of the reflector is filled with encapsulating resin 27 such as silicone resin to seal the LED chip 23, and the surface of the encapsulating resin 27 is made almost the same height as the surface of the reflector.

A light guide plate 24 was placed on the LED light sources arranged as shown in Fig. 9. The light guide plate is a transparent polymethacrylate plate with a width of 270 mm, a length of 230 mm and a thickness of 3 mm, and a cone-shaped concave part with an apex angle of 90 degree and a depth of 2.5 mm is provided thereon at the position directly above the center of each LED chip. This transparent polymethacrylate plate was

fixed by superposing on the top surface of the reflector with adhesive in such a manner that the bottom surface of the cone was facing the LED chip.

A polycarbonate diffusion plate (Teijin Chemicals Ltd. PC9391-50HL) was fixed at the position with a height of 30 mm from the inner bottom surface of the aluminum-made housing to produce a plane light source device.

Next, electric current of 200 mA, 190 mA or 128 itiA was applied to red LEDs, green LEDs or blue LEDs, respectively, so that the chromaticity coordinate became (x = 0.300, y = 0.300) at the position separated by approximately 1 m from the surface of the diffusion plate at the center of the diffusion plate (the point of intersection of the diagonals of the diffusion plate) . Chromaticity coordinates and values of brightness were measured with a spectral radiation luminance meter (spectroscopic type)

Spectroradiometer CS-IOOOS (Konica Minolta Holdings, Inc.).

The brightness at the center of the diffusion plate (the point of intersection of the diagonals of the diffusion plate) was 2536 cd/m ² , measured at the position separated by approximately 1 m from the surface of the diffusion plate. With respect to nonuniformity in color, chromaticity coordinates were measured at the most reddish, greenish and bluish parts and differences from the value at the center were compared for evaluation at the position separated by approximately 1 m from

the surface of the diffusion plate. The chromaticity coordinate at the most reddish part was (x = 0.320, y = 0.290) and the difference from the value at the center was (δx = 0.02, δy = 0.01) . The corresponding values at the most greenish part were (x = 0.310, y = 0.320) and (δx = 0.01, δy = 0.02) while those at the most bluish part were (x = 0.290, y = 0.290) and (δx = 0.01, δy = 0.01) .

[Example 2] A surface emitting light source was produced in the same way as Example 1 except that the apex angle of the cone-shaped concave part on the transparent polymethacrylate plate used in Example 1 was 60 degree. Evaluation was carried out in the same way as Example 1. The brightness at the center of the diffusion plate (the point of intersection of the diagonals of the diffusion plate) was 2486 cd/m ² at the position separated by approximately 1 m from the surface of the diffusion plate. At the position separated by approximately 1 m from the surface, the chromaticity coordinate at the most reddish part was (x = 0.315, y= 0.290) and the difference from the value at the center was (δx = 0.015, δy = 0.01) . The corresponding values at the most greenish part were (x = 0.310, y = 0.315) and (δx = 0.01, δy = 0.015) while those at the most bluish part were (x = 0.285,

y = 0.285) and (δx = 0.015, δy = 0.015).

[Example 3]

A plane light source device was produced in the same way as Example 1 except that the depth of the cone-shaped concave part on the transparent polyiuethacrylate plate used in Example 1 was 1.4 mm and that on the top surface of this transparent plate, at the position directly above each of these cone-shaped concave parts, a cone shaped concave part with an apex angle of 90 degree and a depth of 1.4 mm was formed. Evaluation was carried out in the same way as Example 1. The brightness at the center of the diffusion plate (the point of intersection of the diagonals of the diffusion plate) was 2425 cd/m ² at the position separated by approximately 1 m from the surface of the diffusion plate. At the position separated by approximately 1 m from the surface, the chromaticity coordinate at the most reddish part was (x = 0.308, y = 0.290) and the difference from the value at the center was (δx = 0.008, δy = 0.01) . The corresponding values at the most greenish part were (x = 0.310, y = 0.308) and (δx = 0.01, δy = 0.008) while those at the most bluish part were (x = 0.292, y = 0.290) and (δx = 0.008, δy = 0.01) .

[Example 4]

A plane light source device was produced in the same way as Example 1 except that the cone-shaped concave part on the transparent polymethacrylate plate used in Example 1 was changed to a pyramid-shaped concave part with a square base of 2 mm x 2 mm and a depth of 2.5 mm. Evaluation was carried out in the same way as Example 1. The brightness at the center of the diffusion plate (the point of intersection of the diagonals of the diffusion plate) was 2588 cd/m ² at the position separated by approximately 1 m from the surface of the diffusion plate. At the position separated by approximately 1 m from the surface, the chromaticity coordinate at the most reddish part was (x = 0.310, y= 0.290) and the difference from the value at the center was (δx = 0.01, δy = 0.01) . The corresponding values at the most greenish part were (x = 0.310, y = 0.315) and (δx = 0.01, δy = 0.015) while those at the most bluish part were (x = 0.285, y = 0.290) and (δx = 0.015, δy = 0.01) .

[Example 5]

A plane light source device ^' was produced in the same way as Example 1 except that a pattern with an outer contour of 16 mm in diameter for controlling light transmittance was printed at the position directly above the concave part on the top surface of the transparent polymethacrylate plate used in Example 1. Here, the pattern was formed so that the

transmittance b (%) of the light guide plate at the position with a distance of a (mm) from the center of the print pattern approximately satisfied the relationship b = (5/6) a ² . Evaluation was carried out in the same way as Example 1. The brightness at the center of the diffusion plate (the point of intersection of the diagonals of the diffusion plate) was 2486 cd/m ² at the position separated by approximately 1 m from the surface of the diffusion plate. At the position separated by approximately 1 m from the surface, the chromaticity coordinate at the most reddish part was (x = 0.315, y = 0.290) and the difference from the value at the center was (δx = 0.015, δy =

0.01) . The corresponding values at the most greenish part were

(x = 0.310, y = 0.315) and (δx = 0.01, δy = 0.015) while those at the most bluish part were (x = 0.285, y = 0.285) and (δx = 0.015, δy = 0.015) .

[Comparative example 1]

A plane light source device was produced in the same way as Example 1 except that no transparent polymethacrylate plate was used. Evaluation was carried out in the same way as Example 1. The brightness at the center of the diffusion plate (the point of intersection of the diagonals of the diffusion plate) was 2965 cd/m ² at the position separated by approximately 1 m from the surface of the diffusion plate. At the position

separated by approximately 1 m. from the surface, the chromaticity coordinate at the most reddish part was (x = 0.335, y = 0.28) and the difference from the value at the center was (δx = 0.035, δy = 0.02) . The corresponding values at the most greenish part were (x = 0.330, y = 0.350) and (δx = 0.03, δy = 0.05) while those at the most bluish part were (x = 0.260, y = 0.275) and (δx = 0.04, δy = 0.025) .

[Comparative example 2] A plane light source device was produced in the same way as Example 1 except that a transparent polymethacrylate smooth plate without concave part was used. Evaluation was carried out in the same way as Example 1. The brightness at the center of the diffusion plate (the point of intersection of the diagonals of the diffusion plate) was 2965 cd/m ² at the position separated by approximately 1 m from the surface of the diffusion plate. At the position separated by approximately 1 m from the surface, the chromaticity coordinate at the most reddish part was (x = 0.350, y = 0.280) and the difference from the value at the center was (δx = 0.05, δy = 0.02) . The corresponding values at the most greenish part were (x = 0.330, y = 0.350) and (δx = 0.03, δy = 0.05) while those at the most bluish part were (x = 0.260, y = 0.275) and (δx = 0.04, δy = 0.025) .

The above examples illustrate only a few example cases and various types of modification may be adopted as described below. Although a bare chip type was used as the light-emitting diode (LED) , there may be used a packaged LED chip or a LED chip integrated with a member having a function of a lens or the like . Instead of the aluminum base printed substrate, as the heat-dissipating substrate, there may be used other materials with high heat conductance including metal plate such as copper and stainless steel, ceramic substrate such as aluminum nitride and the like. As means for adhering LED chips to the heat-dissipating substrate, there may be used, instead of the heat-dissipating grease, other means including conductive paste such as silver paste, solder and the like. The configuration of the circuit board is not limited to the configuration shown in Fig.10. As means for mounting LED chips on a substrate, there may be used facedown bonding using bumps or anisotropic conductive materials instead of face-up bonding by wire-bonding. As the reflector, there may be used metal other than aluminum or white resin material with high reflectance. For fixing the reflector on the circuit board and fixing the light guide plate on the reflector, the fixing may be mechanically performed using screws or the like instead of adhesive. As the sealant, epoxy resin may be also used instead of silicone resin.