JP2005108676A - Backlight device using light emitting diode - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a backlight device which can suppress color irregularity and bright unevenness by using light emitting diodes. <P>SOLUTION: The backlight device comprises a light guide plate 1 which includes a light emitting face 1a, a light entering face 1b, and a reflecting face 1c, and gradually increases in its thickness from the light entering face 1b to a face opposed to the light entering face 1b; a plurality of light emitting diodes 3 arranged so as to supply emitting light to the light entering face 1b of the light guide plate 1; and a prism sheet 2 arranged on the light emitting face 1a of the light guide plate 1. The prism sheet 2 includes a prism surface 2a which continuously arranges thereon a number of prism columns, and a flat surface 2b opposed to the prism surface; and the internal angle of the prism columns opposed to the light emitting face 1a is 80-120°. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a backlight using a light emitting diode as a light source.

  Backlights are used in liquid crystal display devices such as liquid crystal televisions and notebook computers.

  In a backlight device using a light emitting diode, a white light emitting diode in which a yellow phosphor is coated on a blue light emitting diode is used, and a plurality of light emitting diodes of blue, green, and red (hereinafter, RGB) are used. is there. In the former case, if the light from the light-emitting diode is incident on the light guide plate, white is obtained on the light-emitting surface, but there is a difficulty in color reproducibility (particularly red) when liquid crystal is displayed. The latter has good color reproducibility and can display red clearly, but has the disadvantage that it must be mixed and whitened before the RGB light is extracted from the light exit surface of the backlight device.

  FIG. 7 is an enlarged cross-sectional view showing a structure of a sidelight type backlight device that obtains white light by mixing RGB signals announced by LUMILEDS of the United States. A plurality of light emitting diodes 101 are mounted on the substrate 102, and the lenses of all the light emitting diodes 101 are projected from the bottom 103 a of the case 103. Side walls 103b and 103c extend from the edge of the bottom 103a of the case 103, respectively. The reverse wedge-shaped light guide plate 104, whose thickness increases toward the back, sandwiches the edges of the light exit surface 104b and the first reflection surface 104c adjacent to the light entrance surface 104a between the side walls 103b and 103c of the case 103. It is fixed. Further, the first reflecting surface 104c and the second reflecting surface 104d of the light guide plate 104 are provided with reflecting means 105 for allowing light emitted outside the light guide plate 104 to enter again. On the light exit surface 104b of the light guide plate 104, a prism sheet 106a having a plurality of prism columns formed in a triangular prism shape and a prism sheet 106 having a flat surface 106b opposite to the prism surface 106a is arranged with the prism surface 106a facing the prism surface 106a. Has been established.

Similarly, there is a technique such as Japanese Patent Laid-Open No. 2000-147498 as a well-known document in which the prism surface of the prism sheet is arranged facing the light exit surface of the light guide plate.
See Gerard Harbers, "LED Backlighting for LCD displays", published November 2002 [searched August 25, 2003], Lumileds (USA). Figure 21 Page Wedge Shape, Internet <URL http://www.lumileds.com/pdfs/techpaperspres/SID-BA.pdf>

  Regarding the prism sheet 106 arranged with the prism surface 106a facing the light exit surface 104b, there was no explanation in the material published by LUMILEDS. Therefore, it was considered that the prism surface 106a of the prism sheet 106 acts so that the light incident from the prism surface 106a becomes like the emitted light 107 in FIG. Therefore, it was determined that the prism sheet 106 acts to cause the incident light to be directed in front of the light exit surface 104b and to be emitted from the flat surface 106b on the prism surface 106a. Therefore, in this prism sheet 106, the front luminance is high, but depending on the shape of the reflection surface of the light guide plate, the outgoing light is biased and emitted from the light outgoing surface 104b, and luminance unevenness occurs.

  Here, when the backlight shown in FIG. 7 was manufactured and a follow-up test was performed, the result was that the amount of light obtained from the light-emitting surface was small, there was color unevenness, and the uniformity was low. When the cause was investigated, it was found that the reflection at the second reflecting surface 104d was not performed well.

  From this, it can be said that the prism sheet 106 emits incident light from the plane 106b with the angle of the light being in the front direction to increase the front luminance, but does not reduce the luminance unevenness.

  Therefore, an object of the present invention is to provide a backlight that suppresses color unevenness and brightness unevenness.

  In order to achieve the above object, a backlight using the light emitting diode of the present invention has a light emitting surface, a light incident surface, and a reflective surface, and the light guide plate has a thickness that increases from the light incident surface toward the opposite surface. And a plurality of light emitting diodes arranged to supply outgoing light to the light incident surface of the light guide plate and a prism sheet arranged on the output surface of the light guide plate, the prism sheet having a large number of prism columns. The prism surface is arranged continuously and has a flat surface facing the prism surface, and the interior angle of the prism column facing the light exit surface is 80 to 120 degrees.

  Therefore, in this invention, the emitted light from the light-emitting diode that has entered from the light incident surface of the light guide plate is emitted from the light output surface of the light guide plate while being repeatedly reflected in the light guide plate. Further, the light reflected or emitted by the reflecting means is incident on the light exit surface of the light guide plate, is reflected again in the light guide plate, and the light emitted from the light exit surface is reflected or emitted again by the reflector. By repeating these steps, luminance unevenness can be prevented because light can be prevented from being emitted in a biased manner, and the RGB light in the light guide plate can be lengthened since the optical path length from the light exit surface of the light guide plate is increased. Is promoted, and the occurrence of color unevenness can be suppressed.

(First embodiment)
In the following, a first embodiment of the present invention will be described with reference to FIGS. FIG. 1 is a side view of a backlight according to the first embodiment of the present invention.

  The backlight of the present invention includes a light guide plate 1, a first prism sheet 2, a light emitting diode 3, a diffusion sheet 4, a second prism sheet 5, a third prism sheet 6, a first reflecting sheet 7, and a second reflecting sheet. The reflection sheet 8 is used.

  The light guide plate 1 is made of a transparent acrylic resin or the like, and has a light exit surface 1a, a light incident surface 1b, a first reflection surface 1c, a second reflection surface 1d, and side surfaces connecting these surfaces (not shown). ). The total reflection at the interface of each surface of the light guide plate is a perfect reflection with no energy loss, and this is an important factor in the present invention.

  The light exit surface 1a is a surface from which light incident on the light guide plate 1 is emitted. When the light guide plate 1 is disposed in a liquid crystal display device or the like, this surface is the display screen side. The light exit surface 1a is not only a surface from which light is emitted, but also a surface that reflects on the light exit surface when the incident angle with respect to the light exit surface 1a is incident at an angle larger than the critical angle that differs depending on the material of the light guide plate 1. Furthermore, in the present invention, the light reflected by the first prism sheet 2 may enter.

  The light incident surface 1b is a surface that is perpendicular to the light output surface 1a and on which light emitted from the light emitting diode 3 enters. Further, the light incident on the light guide plate 1 may be reflected by the light incident surface 1b.

  The first reflecting surface 1c is a surface that is connected to the light incident surface 1b at an angle larger than perpendicular, and is a surface that reflects light. That is, it is a surface disposed so as to be a reverse wedge-shaped light guide plate whose thickness increases from the light incident surface 1a to the second reflecting surface 1d.

  The second reflecting surface 1d is connected to the light exit surface 1a and the first reflecting surface 1c and becomes a reflecting surface. At this time, the angle formed with the first reflecting surface 1c is desirably an angle larger than 90 degrees, and is preferably in the range of more than 90 to 120 degrees. Further, in order to obtain good luminance on the light exit surface 1a, the range of 95 to 115 degrees is good, and the better range is 101 to 111 degrees.

  The first prism sheet 2 is a sheet having a prism surface 2a in which a large number of prism columns are continuously arranged and a flat surface 2b facing the prism surface 2a. It arrange | positions toward the surface 1a. Here, the prism column of the present invention includes not only a triangular column having a regular triangle, an isosceles triangle, etc. in the cross section, but also a prism column whose corner is rounded. In the present invention, the internal angle of the prism column of the first prism sheet 2 is set to 80 to 120 degrees, and more preferably 90 to 110 degrees. By the first prism sheet 2, the light emitted from the light exit surface 1a of the light guide plate 1 enters from the prism surface 2a and is emitted from the plane 2b or reflected. This will be described later.

  The light emitting diode 3 uses a plurality of monochromatic light emitting diodes such as RGB and is disposed on a substrate (not shown). The substrate is disposed so as to be parallel to the light incident surface 1b of the light guide plate 1, that is, the distance between the plurality of light emitting diodes 3 and the light incident surface 1b of the light guide plate 1 is also substantially the same.

  The diffusion sheet 4 is disposed on the plane 2b side of the first prism sheet 2 and functions to increase the uniformity by diffusing the light emitted from the plane 2b.

  Unlike the first prism sheet 2, the second and third prism sheets 5 and 6 are arranged such that the prism surface faces the front direction of the light exit surface 1 a of the light guide plate 1. The second and third prism sheets 5 and 6 have an effect of increasing the front luminance in front of the light exit surface 1a of the light guide plate 1 so that the prism column angle is not as large as that of the first prism sheet 2. I use it.

  The first and second reflection sheets 7 and 8 are made of a sheet having a specular reflection property or a high reflection property, and are disposed on the first reflection surface, the second reflection surface, and the side surface of the light guide plate 1. This sheet serves to reduce the loss of light by allowing the light emitted from the light guide plate 1 to reenter the light guide plate 1.

  Next, an optical path until light is emitted from the light exit surface in the first embodiment will be described. FIG. 2 is an explanatory diagram showing an example of an optical path in the backlight shown in FIG.

In FIG. 2A, since the light guide plate 1 has a reverse wedge shape, the light emitted from the light emitting diode 3 incident on the light incident surface 1b of the light guide plate 1 as shown in FIG. The incident angle θ ₂ incident on the light incident surface _{1 a is} larger than the incident angle θ ₁ incident on the first light exit surface 1 a. That is, since the incident angle increases as it approaches the second reflecting surface 1d, when the light is incident on the light emitting surface 1a at an angle larger than the critical angle for the first time, the critical angle is increased before the light is reflected by the second reflecting surface 1d. Never exceed. Next, the light is reflected by the second reflecting surface 1d. If the incident angle of the reflected light with respect to the light exiting surface is larger than the critical angle, the incident angles θ ₃ and θ ₄ are increased as the light incident surface 1a is approached. As shown, the incident angle becomes smaller. That is, after the light is reflected by the second reflecting surface 1d, the incident angle becomes smaller as it approaches the light incident surface 1a. Therefore, the incident angle gradually approaches the critical angle, and when the incident angle becomes smaller than the critical angle, the light exits from the light emitting surface. Become.

In FIG. 2 (b), when the optical path as shown, after being reflected by the second reflecting surface 1d, the light becomes smaller incident angle alpha ₁ than the critical angle, is emitted from the light emitting surface 1a, the first After being refracted by the prism surface 2a of the prism sheet 2, the light enters the plane 2b. In this case, when the incident angle β ₁ is larger than the critical angle of the first prism sheet 2, the light is totally reflected by the plane 2 b, emitted from the prism surface 2 a, entered from the light exit surface 1 a, and is reflected by the first reflecting surface 1 c. reflect. Then, at an angle of incidence alpha ₂ again light emitting surface 1a. When this angle is smaller than the critical angle, the light exits from the light exit surface 1a, enters the plane 2b through refraction at the prism surface 2a. In the optical path here, the re-incident light is reflected by the first reflecting surface 1c, and the angle is raised and incident on the light exiting surface. Therefore, the incident angle α ₂ <incident angle α ₁ and the incident angle β ₂ <incident angle. Since it was β ₁ and the incident angle β ₂ was smaller than the critical angle, the light was emitted from the plane 2b.

  Therefore, in order to obtain such an optical path in the present invention, it is important that the light guide plate has a shape that is inclined as it goes from the light incident surface to the back. 3 may be a light guide plate that is optically inclined like a Fresnel lens shape in which a plurality of inclined surfaces and steps are arranged.

Next, the difference in the optical path depending on the angle of the prism column of the prism sheet will be described. FIG. 4 is an explanatory diagram illustrating an example of an optical path of incident light in the vicinity of the first prism sheet in the backlight illustrated in FIG. Then, FIG. 4 (a) interior angle phi ₁ of the prism pillar 110 degrees, (b) is phi ₂ is 65 degrees, both with incident light 9a~9j the same angle of incidence light emitting surface 1a of the light guide plate 1 The optical path after entering and exiting from the light guide plate is shown.

  In FIG. 4A, incident light 9a to 9c, 9e is incident and refracted from the prism surface 2a1, totally reflected by the plane 2b, and emitted and refracted from the prism surface 2a2. After the emission, the incident light 9a is directly incident on the light exit surface 1a, and the incident light 9b, 9c, 9e is incident or reflected on the prism surface 2a1 (only the reflected light is shown here), and the incident angle with the light exit surface 1a is incident. The light is incident on the light exit surface 1a at an angle smaller than that of the light 9a. The incident light 9d is incident and refracted from the prism surface 2a1, is totally reflected by the prism surface 2a2, and is incident on the plane 2b. Therefore, since the incident angle is smaller than the critical angle, the light is emitted from the plane 2b.

  In FIG. 4B, incident lights 9f to 9j are incident and refracted on the prism surface 2a1, totally reflected by the prism surface 2a2, and emitted from the plane 2b.

  As can be seen from FIGS. 4 (a) and 4 (b), when the inner angle of the prism column is 120 degrees, even the light incident at the same angle is totally reflected or emitted from the plane 2b. Even if the light is totally reflected, the angle of incidence on the light exit surface 1a is also changed when entering the light guide plate again. That is, it can be said that the effect of the prism surface 2a acting on the light varies depending on the internal angle of the prism column. Therefore, in the present invention, in order to appropriately reflect or emit light on the plane 2b, the prism column has an inner angle that is not so wide that it is not easily emitted from the plane 2b, and the prism column has an inner angle that is too narrow to reflect on the plane 2b. It is preferable that the inner angle of the prism column of the prism sheet is set to 80 to 120 degrees, and more preferably 90 to 110 degrees.

  In the first embodiment, the light emitted from the light guide plate is emitted or reflected by the first prism sheet. The reflected light reenters the light guide plate, reenters the first prism sheet, and exits or reflects. By repeating these, the optical path length becomes long, so that the mixing of RGB light is promoted and color unevenness can be suppressed. Further, by repeating the emission or reflection with the first prism sheet, various light emission can be achieved. Since light is emitted from the light exit surface, the uniformity is increased and uneven brightness can be suppressed.

  Here, the prism surface of the first prism sheet is subjected to diffusion processing, the corners are rounded as shown in FIG. 5A, the prism pillars are made into random shapes as shown in FIG. 5B, By changing the size of the prism column as shown in FIG. 5C, interference fringes generated due to the regular arrangement of the prisms can be prevented, and the uniformity can be further increased. Can do.

(Second Embodiment)
FIG. 6 is a side view of a backlight according to the second embodiment of the present invention. About each part of this 2nd Embodiment, the same part as each part of the backlight of 1st Embodiment of FIG. 1 is shown with the same code | symbol, and the description is abbreviate | omitted.

  In the second embodiment, the second reflecting surface 1d is formed on the same reflecting surface as the second reflecting surface 104d in FIG.

  6 and FIG. 7 is different in structure in that the first prism sheet 2 whose prism column inner angle is 80 to 120 degrees is disposed on the light exit surface of the light guide plate. The structure until the light emitted from the light emitting diode enters the light guide plate is also different, but is not considered because it is not relevant in the present invention. The difference in effect is that, in FIG. 7, the light exits from the light exit surface 104 b of the light guide plate 104, enters the prism surface 106 a of the prism sheet 106, and exits from the flat surface 106 b. In the same manner, in FIG. 6, the light is incident on the prism surface 2 a of the first prism sheet 2, totally reflected by the plane 2 b, re-entered the light guide plate 1, reflected by the second reflecting surface 1 c, and then again the first The light is incident on the prism surface 2a of the prism sheet 2, and this time, the incident angle to the flat surface 2b is smaller than the critical angle, so that the light is emitted from the flat surface 2b.

  In FIG. 7, as described in the problem, the light amount obtained from the light exit surface 104b is small, the color unevenness is further reduced, and the uniformity is low. In FIG. 6, the first prism which is the reflecting means is obtained. The sheet 2 generates light that repeatedly enters and exits the light exit surface 1 a of the light guide plate 1, so that mixing of RGB light is promoted and color unevenness is improved.

  In FIG. 7, when light emitted from the light exit surface 104 b of the light guide plate 104 enters the prism sheet 106, most of the light exits from the flat surface 106 b of the prism sheet 106. That is, when the light is deflected and reflected by the second reflecting surface 104d of the light guide plate 104 and is also emitted by the light exiting surface 104b, the light emitted from the flat surface 106b of the prism sheet 106 also varies. Brightness unevenness also occurred. In this regard, in FIG. 6, as seen in FIG. 4A, even light incident on the prism sheet is emitted and reflected. In addition, since the reflected light also has the effect of becoming light such as 9a and 9b having different incident angles incident on the light guide plate, the light is emitted from various light exit surfaces of the light guide plate, and brightness unevenness is improved. The

The side view of the backlight which is the 1st Embodiment of this invention. Explanatory drawing which shows an example of the optical path in the backlight shown in FIG. The side view which shows the shape of a Fresnel lens. FIG. 2 is an explanatory diagram illustrating an example of an optical path of incident light in the vicinity of a first prism sheet in the backlight illustrated in FIG. 1. The enlarged side view which shows the other shape of a 1st prism sheet. The side view of the backlight which is the 2nd Embodiment of this invention. The side view of the conventional backlight.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Light guide plate 1a Light emission surface 1b Light incident surface 1c 1st reflective surface 1d 2nd reflective surface 2 1st prism sheet 2a Prism surface 2b Plane 3 Light emitting diode 4 Diffusion sheet 5 2nd prism sheet 6 3rd prism Sheet 7 First reflection sheet 8 Second reflection sheets 9a to 9j Incident light

Claims (2)

A light guide plate having a light exit surface, a light entrance surface, and a reflection surface, the thickness of which increases from the light entrance surface toward the opposite surface, and is arranged to supply outgoing light to the light entrance surface of the light guide plate A plurality of light emitting diodes, a prism surface, and a light exit surface facing the prism surface, the prism surface being disposed toward the light exit surface of the light guide plate, and an inner angle of the prism column facing the light exit surface being 80 to A backlight using a light-emitting diode, characterized by comprising a prism sheet of 120 degrees. 2. The light emitting diode according to claim 1, wherein the prism sheet mainly reflects incident light from a light exit surface, enters the light exit surface of the light guide plate, and emits remaining incident light from the light exit surface. Backlight using.

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