This application claims priority to Japanese Patent Application No. 2011-105621 filed on May 10, 2011. The entire disclosure of Japanese Patent Application No. 2011-105621 is hereby incorporated herein by reference.
1. Field of the Invention
The present invention generally relates to a backlight unit. More specifically, the present invention relates to a backlight unit having a light guide plate.
2. Background Information
A typical conventional edge-lit backlight unit is built into a liquid crystal module. With the conventional edge-lit backlight unit, a light source is disposed along a light incident face at one end of a light guide plate. The light guide plate is made of a transparent synthetic resin. With the edge-lit backlight unit, light reflecting tapes are affixed to end faces other than the light incident face of the light guide plate such that light incident from the light incident face on the light guide plate will not be emitted from any end faces other than the light incident face of the light guide plate. Thus, most of the incident light is emitted from a light emitting face of the light guide plate, which improves light utilization efficiency.
Meanwhile, another conventional light guide plate for an edge-lit backlight unit has been proposed (see Japanese Laid-Open Patent Application Publication No. 2007-200741, for example). This light guide plate includes a surface relief structure having functions of reflecting and refracting part of the rectilinear component of light from a light source in the front or rear face direction of the light guide plate, and refracting and transmitting the other part. The surface relief structure is provided to a final end face opposite an end face of the light guide plate on which the light source is disposed.
Yet another conventional light guide plate has also been proposed (see Japanese Laid-Open Patent Application Publication No. 2003-315560, for example). With this light guide plate, a light take-off mechanism has numerous recess arrays with smooth, inclined faces with an inclination angle of 15° to 75° relative to the light source on a face of the light guide plate opposite a light emitting face.
Furthermore, yet another light guide plate has been proposed (Japanese Laid-Open Patent Application 2003-156632). With this light guide plate, an opposing side face of the light guide plate that is opposite a light incident face is an inclined face with an inner angle θ of less than 90° such that at least part of the light projected from a direction of the light incident face will be reflected non-parallel to a light emitting face. Furthermore, a reflecting layer made of an aluminum coating is formed on this inclined face.
However, it has been discovered that when light utilization efficiency is increased by affixing the light reflecting tapes to the end faces other than the light incident face at the one end of the light guide plate, it is very hard to affix the light reflecting tapes to the thinner light guide plates in use today. Thus, the cost of the light reflecting tapes is added to the labor cost of affixing the light reflecting tapes, which drives up the cost of the product.
Also, when the surface relief structure is provided to the final end face opposite the end face of the conventional light guide plate on which the light source is disposed (see Japanese Laid-Open Patent Application Publication No. 2007-200741, for example), it is not easy to form the surface relief structure, the mold for forming the light guide plate is more expensive, and this again drives up the cost of the product.
Meanwhile, when the numerous recess arrays with the smooth, inclined faces are disposed on the face opposite the light emitting face (see Japanese Laid-Open Patent Application Publication No. 2003-315560, for example), light cannot be prevented from exiting the other end face opposite the light incident face at one end of the light guide plate. Thus, even when this technology is applied, the above-mentioned problem encountered with the conventional edge-lit backlight unit cannot be solved.
Furthermore, when the opposing side face of the light guide plate that is opposite the light incident face is an inclined face, and the reflecting layer made of the aluminum coating is formed on this inclined face (see Japanese Laid-Open Patent Application 2003-156632), it is not easy to form the reflective layer. Thus, this can lead to higher costs.
An improved backlight unit was conceived in light of the above-mentioned problem. One object of the present disclosure is to provide a backlight unit which allows light that is incident from a light incident face of a light guide plate to be emitted from a light emitting face of the light guide plate without providing a light reflecting tape.
In accordance with one aspect of the present disclosure, a backlight unit includes a light source unit and a light guide plate. The light source unit is configured to emit light. The light guide plate is arranged relative to the light source unit. The light guide plate includes a first end surface and a second end surface that is opposite the first end surface. The first end surface faces with the light source unit. The second end surface has first and second inclined faces. The first inclined face slopes away from the second inclined face as approaching the first end surface of the light guide plate. The second inclined face slopes away from the first inclined face as approaching the first end surface of the light guide plate. The first and second inclined faces are arranged relative to the first end surface of the light guide plate such that a first total internal reflection of the light that is incident from the light source unit onto one of the first and second inclined faces is configured to occur at the one of the first and second inclined faces, and such that a second total internal reflection of the light that is reflected at the one of the first and second inclined faces and is incident onto the other of the first and second inclined faces is configured to occur at the other of the first and second inclined faces.
These and other objects, features, aspects and advantages will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses a preferred embodiment.
Referring now to the attached drawings which form a part of this original disclosure:
A preferred embodiment will now be explained with reference to the drawings. It will be apparent to those skilled in the art from these disclosures that the following descriptions of the preferred embodiment are provided for illustration only and not for the purpose of limiting the invention as defined by the appended claims and their equivalents.
Referring to
The light guide plate 1 includes a pair of recess components 1a on both sides of the light guide plate 1. Furthermore, the rear frame 3 has a pair of positioning-use bent tabs 3a. The positioning-use bent tabs 3a of the rear frame 3 are fitted into these recess components 1a, respectively, thereby positioning the light guide plate 1 in an up and down direction and in a left and right direction of the light guide plate 1. Gaps are provided between the recess components 1a and the bent tabs 3a in order to absorb thermal expansion and contraction of the light guide plate 1.
As shown in
As shown in
As shown in
The above-mentioned effect is particularly pronounced with the edge-lit backlight unit 10 when the inclination angle θ of the first inclined face 1B and the second inclined face 1C of the light guide plate 1 is set to an optimal angle 45°.
The LED bar 2, in which the LEDs 2a are installed on the wiring board 2b, is used as the light source with the edge-lit backlight unit 10. However, a cold cathode tube or another such linear light source can be installed along the light incident face 1A instead.
Also, the light guide plate 1 used in the edge-lit backlight unit 10 is made of an acrylic resin (e.g., polymethyl methacrylate) with excellent transparency. However, a light guide plate can of course be used which is made of some other synthetic resin with good transparency, such as a polycarbonate. When the light guide plate of different material from the polymethyl methacrylate, the inclination angle θ can be different from the angle described above since different materials has different critical angles θc.
As shown in
Although not shown in the drawings, an X-board is connected to the end of the liquid crystal panel 7 via a chip-on-film carrying a source driver IC chip, and a Y-board is connected via a chip-on-film carrying a gate driver IC chip.
With this liquid crystal module, the rear face of the liquid crystal panel 7 is brightly illuminated by the edge-lit backlight unit 10. Thus, the brightness of the display face is increased and a clear image can be viewed.
With this edge-lit backlight unit 10 of the liquid crystal module, most of the light incident from the light incident face 1A at one end of the light guide plate 1 is reflected by the other end face and can be emitted from the light emitting face 1D of the light guide plate 1, without having to affix any light reflecting tapes on the other end face opposite the light incident face 1A at one end of the light guide plate 1.
With this edge-lit backlight unit 10, the other end face opposite the light incident face 1A at one end of the light guide plate 1 is formed as a curved surface with a specific triangular peaked shape, which allows most of the light that is incident from the light incident face 1A of the light guide plate 1 and reaches the other end face to be returned to the light guide plate 1 and emitted from the light emitting face 1D.
With the edge-lit backlight unit 10, the LED bar 3 is disposed along the light incident face 1A at one end of the light guide plate 1. The other end face opposite the light incident face 1A at one end of the light guide plate 1 is formed as a curved surface in the shape of a triangular peak that protrudes to the outside. The other end face has the first inclined face 1B that slopes diagonally upward toward the light incident face 1A at one end of the light guide plate 1, and the second inclined face 1C that slopes diagonally downward toward the light incident face 1A at one end of the light guide plate 1. The inclination angle θ of the first inclined face 1B and the second inclined face 1C with respect to the light incident face 1A at one end of the light guide plate 1 is set to be greater than the critical angle θc of light refraction for the resin material that makes up the light guide plate 1, and to be less than (180°−critical angle θc)/3.
Thus, with the edge-lit backlight unit 10, the light guide plate 1 is made of polymethyl methacrylate. The inclination angle θ of the first inclined face 1B and the second inclined face 1C with respect to the light incident face 1A at one end of the light guide plate 1 is set to be greater than 42°, which is the critical angle of the optical refractive index of polymethyl methacrylate, and to be less than 46°. It is particularly favorable for the inclination angle θ of the first inclined face 1B and the second inclined face 1C to be set to 45°.
With the liquid crystal module, the liquid crystal panel 7 is disposed on the light emitting face 1D of the light guide plate 1 of the edge-lit backlight unit 10, with the optical sheets 6 in between.
As shown in
For all of the incident light L1 to be reflected by the first inclined face 1B, it is essential that the incidence angle a be greater than the critical angle θc of light refraction for the resin material that makes up the light guide plate 1. Furthermore, for all of the reflected light L2 to be reflected by the second inclined face 1C, it is essential that the incidence angle g be greater than the critical angle θc. Since the incidence angle a is equal to the inclination angle θ, and the incidence angle g is 180°−3θ, it is essential that the critical angle θc<inclination angle θ, and that the critical angle θc<180°−3θ. Therefore, this gives the following inequality:
critical angle θc<inclination angle θ<(180°−critical angle θc)/3.
With the edge-lit backlight unit 10, the inclination angle θ of the first inclined face 1B and the second inclined face 1C with respect to the light incident face 1A of the light guide plate 1 is set to be greater than the critical angle θc of light refraction for the resin material that makes up the light guide plate 1, and to be less than (180°−critical angle θc)/3. Since the above inequality is satisfied, the incident light L1 can be reflected by the first inclined face 1B and the second inclined face 1C and returned to the light guide plate 1. Therefore, even though no light reflecting tape is affixed to the other end face opposite the light incident face 1A of the light guide plate 1 as in the past, the majority of the incident light can be emitted from the light emitting face 1D of the light guide plate 1. Thus, the light utilization efficiency becomes higher, and furthermore, there is no cost of light reflecting tape or labor cost of affixing the light reflecting tape. Accordingly, the cost of the product can be reduced.
Part of the light incident from the light incident face 1A of the light guide plate 1 is reflected and scattered by the lower face or the light emitting face 1D of the light guide plate 1, and reaches the first inclined face 1B and the second inclined face 1C. Thus, the majority of this light is emitted to the outside after being incident on the first inclined face 1B and the second inclined face 1C at an angle that is less than the critical angle θc. Therefore, the amount of light that can actually be returned to the light guide plate 1 is about 20 to 25% of the amount of incident light.
Also, when the light guide plate 1 is made of polymethyl methacrylate, the critical angle θc is 42′. Therefore, if the inclination angle θ of the first inclined face 1B and the second inclined face 1C is set to be greater than 42° and less than 46° so as to satisfy the above inequality, then part of the incident light can be reflected by the first inclined face 1B and the second inclined face 1C and returned to the light guide plate 1, which raises the light utilization efficiency. The effect is particularly pronounced when the inclination angle θ is set to 45°.
With the liquid crystal module, the liquid crystal panel 7 is disposed via the optical sheets 6 on the light emitting face 1D of the light guide plate 1 of the edge-lit backlight unit 10. The brightness of the display face of the liquid crystal panel 7 can be enhanced by the edge-lit backlight unit 10.
With the liquid crystal module, the first and second inclined faces 1B and 1C have the same inclination angle θ relative to the light incident face 1A of the light guide plate 1. However, the first and second inclined faces 1B and 1C can have different inclination angles relative to the light incident face 1A of the light guide plate 1 as long as the first and second inclined faces 1B and 1C are configured such that the first and second total internal reflections occur. In this case, the ridge formed by the interface of the first and second inclined faces 1B and 1C can be offset relative to the thickness center of the light guide plate 1.
In understanding the scope of the present invention, the term “comprising” and its derivatives, as used herein, are intended to be open ended terms that specify the presence of the stated features, elements, components and groups, but do not exclude the presence of other unstated features, elements, components and groups. The foregoing also applies to words having similar meanings such as the terms, “including”, “having” and their derivatives. Also, the terms “part,” “section,” “portion,” “member” or “element” when used in the singular can have the dual meaning of a single part or a plurality of parts.
While a preferred embodiment has been chosen to illustrate the present invention, it will be apparent to those skilled in the art from these disclosures that various changes and modifications can be made herein without departing from the scope of the invention as defined in the appended claims. Furthermore, the foregoing descriptions of the preferred embodiment according to the present invention are provided for illustration only, and not for the purpose of limiting the invention as defined by the appended claims and their equivalents.
Number | Date | Country | Kind |
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2011-105621 | May 2011 | JP | national |