JP4671342B2 - Backlight - Google Patents

Description

  The present invention relates to a backlight of a light-receiving display device such as a liquid crystal display device, and more particularly to a backlight with improved light utilization efficiency and directivity of illumination light used for a small liquid crystal display such as a cellular phone PDA.

In recent years, backlights using light-emitting diodes (hereinafter abbreviated as LEDs) as light sources have begun to be used because they have advantages such as longer lifetime and easier chromaticity adjustment of backlight light than fluorescent display tubes.
4A and 4B are diagrams showing a conventional example of a sidelight type backlight using a light emitting diode (hereinafter abbreviated as LED), where FIG. 4A is a plan view and FIG. 4B is a side view.
The backlight in FIG. 4 has a plurality of LED light sources 40 arranged on the side surface of a light guide plate 42 having a substantially rectangular parallelepiped shape. Light emitted from the LED light source 40 enters the light guide plate 42 and is refracted and propagated in the light guide plate. The As the propagation is repeated, the angle formed between the traveling direction of light and the surface of the light guide plate 42 becomes smaller than the critical angle, and light is emitted from the light guide plate. The light emitted from the light guide plate 42 is generally bent by a prism sheet placed between the display device and the backlight and guided to the display device side. In addition, the dotted line in Fig.4 (a) has shown the groove.
As described above, in order to reduce the thickness of the backlight, the LED light source is generally placed on the side surface of the light guide plate.
In the following drawings, the same members are denoted by the same numbers.

However, there are problems inherent in the sidelight type backlight, and the situation will be described with reference to FIG.
FIG. 5 is a diagram for explaining the first problem of the conventional sidelight type backlight. The emitted light 41 of the LED light source 40 is incident on the light guide plate 42, and most of the incident light is on the upper surface of the light guide plate 42 and the groove. The light is propagated while being refracted on the lower surface of the light guide plate 42 provided with 44. However, there is light that passes through the light guide plate 42 as it is without reaching the upper and lower surfaces of the light guide plate 42, as light 46 indicated by hatching in FIG. 5. This light has the strongest intensity among the emitted light 41 of the LED, and as a result, the light use efficiency of the backlight with respect to the light quantity of the LED 40 is deteriorated.
Further, even if a reflection layer is provided on the right end face 43 of the light guide plate 42 and the light 46 is reflected and returned into the light guide plate 42, light loss at the time of reflection on the reflection layer becomes a problem, and the light utilization efficiency of the backlight There is no change in the direction of worsening.

There are not many proposals to use LEDs as a light source for direct type backlights, but in a backlight that illuminates a liquid crystal display device placed above, a reflector that reflects light so that light from the light source does not leak outside is provided. Is provided directly below or on the side of the backlight and a half mirror is placed between the light source and the liquid crystal display device, and the transmittance and reflectance of the half mirror are partially different from each other, so that the brightness of the entire backlight is There is a proposal to keep the brightness constant, that is, to suppress uneven brightness (see, for example, Patent Document 1).
Such a technique has a certain effect in suppressing the luminance unevenness of the backlight, but the half mirror absorbs a large amount of light, and the reflector also absorbs some light each time it reflects light. There is a problem. In the present application, even the reflection at the end face of the light guide plate as described with reference to FIG. 4 is problematic in that the light utilization efficiency is lowered, and further, the light utilization efficiency is greatly reduced by using a half mirror. The proposal is far from improving the light utilization efficiency aimed at by the present invention.

FIG. 6 is a diagram for explaining a second problem of the conventional sidelight type backlight.
6A is a graph showing the optical characteristics of the conventional backlight as shown in FIG. 4, and FIG. 6B is a diagram for explaining the coordinate axes.
As shown in FIG. 6B, the coordinate axes are the X axis in the vertical axis direction, the Z axis in the horizontal axis direction, and the direction perpendicular to the upper surface of the light guide plate 42 with respect to the plane of the light guide plate 42. The orthogonal axis is the Y axis. An angle formed between the direction Y ′ in which the illumination light is emitted from the light guide plate 42 and the Y axis is defined as a light emission angle θ.
FIG. 6A is a graph showing the optical characteristics of the conventional backlight with the coordinate axes determined in this way.
The horizontal axis in FIG. 6A is the light emission angle θ, the vertical axis is the relative value of the light intensity, and the left side of θ = 0 in the figure is the relationship between the direction of the illumination light and the intensity of the illumination light on the XY plane. The right side of θ = 0 indicates the relationship between the direction of illumination light and the intensity of illumination light in the ZY plane. Therefore, the point with the horizontal axis θ = 0 indicates the illumination light intensity in the direction orthogonal to the light guide plate 42, and the point θ = 90 indicates the illumination light intensity in the direction parallel to the surface of the light guide plate 42.
As shown in the graph of FIG. 6A, in the conventional backlight, the light in the direction perpendicular to the surface of the light guide plate 42 is the most in both the characteristic 46 in the XY plane and the characteristic 48 in the ZY plane. Although strong, for example, light at an angle of 30 ° from the vertical direction also has a light intensity of about 60% of that in the vertical direction, and the directivity of light was insufficient.

FIG. 7 is a reference diagram provided for explaining the characteristics of the backlight, and uses the same coordinate axes as in FIG.
FIG. 7A shows the characteristics in the case of having ideal directivity. As indicated by 50 in both the characteristics on the XY plane and the characteristics on the ZY plane, all light is θ. The direction is perpendicular to the surface of the light guide plate where = 0.
FIG. 7B shows characteristics when there is no directivity. The light intensity at each angle with respect to the surface of the light guide plate is constant for both the characteristic 52 on the XY plane and the characteristic 54 on the ZY plane. It has been.
The object of the present invention is to realize a backlight having a characteristic close to the characteristic 50 shown in FIG.

Also, there are applications in which display devices need to narrow the viewing angle in the left-right direction as much as possible in order to prevent information leakage.
8A and 8B are diagrams showing one means for narrowing the viewing angle in the left-right direction of the display device. FIG. 8A is a perspective view of the display substrate, and FIG. 8B is a side view.
In FIG. 8, the display device is a liquid crystal display device. The liquid crystal display device is illuminated from below by a backlight 61, and a light-shielding member 62 having a light absorbing member property is provided for each column on the substrate 60 of the liquid crystal panel. Yes.
The illumination light 63 of the backlight 61 is transmitted through the substrate 60 of the liquid crystal panel, and the light 66 that is perpendicular to the surface of the backlight 61 and the light 68 that is close to the perpendicular pass without being blocked by the light shielding member 62 and are visually recognized. Although it becomes light, the light 64 and 66 inclined from the vertical direction is absorbed by the light shielding member 62. In this way, there is a means in which the viewing angle in the left-right direction of the display device is narrowed, and the width of the viewing angle is set by the height h of the light shielding member 62.

  In such a display device, only light that is perpendicular to the surface of the backlight 61 or light that is nearly perpendicular becomes visible light, and other light is absorbed and lost. If the performance is not high, there is a problem that the light use efficiency of the display device including the backlight is greatly reduced.

JP-A-6-265885

  The problem to be solved is to increase the light utilization efficiency of the backlight and to increase the directivity in the vertical direction.

The backlight of the present invention includes a light emitting diode, a backlight and a light guide plate having a bottom light emitting diode is arranged, the light emitting diode is approximately in the center of the bottom of the backlight a light emitting direction and emitting direction of the light emitting diodes are arranged to coincide, the light guide plate opposite to the bottom surface in the light emitting direction of the backlight of the light emitting direction and the light emitting diode The surface of the light emitting direction is provided with a secondary curved surface at the center, and the secondary curved surface is provided by providing a concave portion with a secondary curved surface, and the bottom portion Is closer to the surface of the light emitting direction as it proceeds from the center toward the outer periphery of the light guide plate, and the light emitted from the light emitting diode and reflected by the concave portion is reflected in the light emitting direction. Wherein the prismatic grooves for reflecting the surface is provided.

Further, in the backlight of the present invention, the prism-shaped groove has a surface extending in parallel to the surface in the light emitting direction in the outer peripheral direction of the light guide plate, and the outer surface in the outer peripheral direction with respect to the extending surface. It is characterized by a staircase shape in which the inclined surfaces that are inclined while approaching the surface in the light emitting direction are alternately and continuously connected .

  According to the present invention, it is possible to realize a backlight having high light utilization efficiency and therefore high luminance and narrow directivity.

  In addition, since the light utilization efficiency of the backlight is increased, if the same color filter as the conventional one is used for the liquid crystal display device, the screen can be brightened with the same power consumption as the conventional one, and if a darker color filter than the conventional one is used, Color reproducibility can be improved while maintaining the conventional brightness with the same power consumption as before.

  In the backlight according to the present invention, the light emitting diode is arranged at substantially the center of the light guide plate so that the light emitting direction of the backlight and the light emitting diode coincide with each other, and the secondary curved surface is provided at the center of the light guide plate. . Further, the secondary curved surface is realized by providing a concave portion having a secondary curved surface on the surface in the light emitting direction of the light guide plate.

1A and 1B are diagrams illustrating an embodiment of a backlight according to the present invention, in which FIG. 1A is a plan view and FIG. 1B is a cross-sectional view taken along line AA ′ of FIG.
As shown in FIGS. 1A and 1B, in the backlight according to the present invention, the light emission direction (upper) of the backlight coincides with the light emission direction (upper) of the LED substantially at the center of the light guide plate 10. Thus, the LEDs 14 are arranged, a prism-shaped groove 12 is provided at the bottom of the light guide plate 10, and a secondary curved surface 16 is provided at the center. The groove 12 is provided concentrically as shown by the dotted line in FIG.
In addition, the secondary curved surface 16 on the light guide plate 10 is realized by providing a concave portion 17 having a secondary curved surface 16 on the surface in the light emitting direction of the light guide plate 10.
The concave portion 17 is conical, and therefore the BB ′ cross section has a concave portion having a shape substantially similar to that shown in FIG.

The locus of the light source light in the backlight according to the present invention will be described with reference to FIG.
In FIG. 2, the 0th-order light 18, which has been refracted and emitted upward from the LED light source 14, is incident on the light guide plate 10 to become light 20 having a slightly changed angle, and then the secondary curved surface 16. To reach. Here, it is divided into light 24 directly emitted from the secondary curved surface 16 of the light guide plate 10 and light refracted on the secondary curved surface, and the refracted light travels to the left and is refracted and emitted from the bottom surface of the light guide plate 10. It becomes. The other light shown proceeds similarly. Here, in the secondary curved surface 16, an angle is set so that a part of the light is emitted and a part of the light is refracted. Since the 0th-order light from the LED 14 is very strong light, such a setting is also possible.
Further, since the LED 14 is a point light source and the 0th order light is very strong light, in order to make the entire light guide plate have a constant luminance, it is necessary to weaken this and distribute the light in all directions of the light guide plate. For this reason, a quadric surface is provided. How to set the shape of the secondary curved surface and the groove shape of the bottom surface of the light guide plate may be determined in design.

In the backlight of the present invention, since the directions of the 0th-order light emitted from the LED and the emitted light of the backlight coincide with each other, very strong 0th-order light can be effectively used as illumination light. Use efficiency can be greatly increased. This is a remarkable effect on the sidelight type backlight.
In the case of a fluorescent display tube that is a line light source, luminance uniformity can be obtained relatively easily by dispersing and arranging the fluorescent display tube. However, in the case of an LED that is a point light source, Thus, by providing the secondary curved surface in the light guide plate, it was possible to obtain luminance uniformity without disperse the LEDs.
Furthermore, since the light emission angle can be set by the prismatic groove on the bottom surface of the light guide plate, the directivity of the emitted light from the backlight can be improved.

FIG. 3 is a diagram showing optical characteristics obtained by the backlight according to the present invention.
FIG. 3 shows optical characteristics drawn on the same coordinate axes as in FIGS. 6A and 7, but both the characteristics 26 on the XY plane and the characteristics 28 on the ZY plane are the same as those in FIG. In comparison, the emitted light is concentrated in the vicinity of the light emission angle θ of 0 °. As described above, the characteristics of the backlight according to the present invention have high directivity in the vertical direction, which approximates the ideal characteristic of FIG.
Thus, since the directivity of the backlight of the present invention can be narrowed, the luminance of the backlight can be increased with the same power consumption as in the prior art.
If a darker color filter is used than before, the color reproducibility can be improved while maintaining the conventional brightness with the same power consumption as before.
Furthermore, when it is used as a backlight of a display device using a means for narrowing the viewing angle in the left-right direction as described in FIG. 8, the light utilization efficiency can be greatly improved.

FIG. 3 is an embodiment diagram of a backlight according to the present invention. It is a figure explaining the locus | trajectory of the light source light in the backlight by this invention. It is the figure which showed the optical characteristic obtained by the backlight by this invention. It is a figure which shows the prior art example of the sidelight type backlight using a light emitting diode. It is a figure explaining the 1st problem of the conventional sidelight type backlight. It is a figure explaining the 2nd problem of the conventional sidelight type backlight. It is a reference figure provided in order to demonstrate the characteristic of a backlight. It is the figure which showed one means for raising the directivity of a display apparatus.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Light guide plate 14 Light emitting diode 16 Quadratic curved surface 17 Concave surface 24 whose surface is a quadratic curved surface Outgoing light 22 on a quadric curved surface 22 Refracted light on a quadric curved surface

Claims (2)

A backlight comprising a light emitting diode and a light guide plate having a bottom portion on which the light emitting diode is disposed,
The light emitting diode, the approximate center of the bottom, are arranged to match the light emitting direction of the light emitting diode and the light emitting direction of the backlight,
The light guide plate faces the bottom and has a surface in the light emitting direction as a light emitting direction of the backlight and a light emitting direction of the light emitting diode,
The surface in the light emitting direction is provided with a quadric surface at the center, and the quadric surface is provided by providing a concave portion with a quadric surface on the surface,
The bottom portion approaches a surface in the light emitting direction as it proceeds from the center toward the outer periphery of the light guide plate, and reflects the light emitted from the light emitting diode and reflected by the concave portion to the surface in the light emitting direction. A backlight characterized in that a groove is provided . The prism-shaped groove is inclined in the outer peripheral direction of the light guide plate, extending in parallel to the light emitting direction surface, and approaching the light emitting direction surface as it proceeds in the outer peripheral direction with respect to the extending surface. The backlight according to claim 1, wherein the inclined surface is in a staircase shape in which the inclined surfaces are connected alternately and continuously .

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