JP2009231018A - Light guide plate, surface light-source device, and liquid crystal display apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide: a light guide plate which substantially improve the efficiency of use of polarized light; a surface light-source device; and a liquid crystal display apparatus. <P>SOLUTION: The light guide plate 320 converts incident light from a light source into planary emitted light, and has optical anisotropy that aligns the direction of a slow axis with a direction perpendicular to or parallel with an X-Z plane parallel with both of the direction of incident of polarized light 410 as incident light and the direction of emission of backlight light 420. The backlight (surface light-source device) has the light guide plate 320 and the light source which emits polarized light 410 whose direction of polarization matches a direction perpendicular to or parallel with the X-Z plane, onto the light guide plate 320. The liquid crystal display includes: the backlight unit; and a liquid crystal unit which is disposed opposite to an emission surface 323 of the light guide plate 320 and changes a state of transmission of emitted light emitted from the emission surface 323 toward an observer. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a light guide plate, a surface light source device using the light guide plate, and a liquid crystal display device using the surface light source device.

  Until now, liquid crystal displays (LCDs) have begun to be installed in desktop personal computers and portable terminals, starting with notebook personal computers. Furthermore, in recent years, high performance and rapid price reduction have led to a large position as one of television displays.

  Since the liquid crystal display device is not a self-luminous display device, it is necessary to include a liquid crystal display element and a planar light source device (hereinafter referred to as “surface light source device” or “backlight”). The backlight is roughly classified into an edge light type and a direct type. The edge light type backlight makes point light or linear light incident on the side surface of a light guide plate made of a transparent material, converts the light into planar light through the light guide plate, and illuminates the liquid crystal display element. The direct type backlight forms a planar light by arranging a number of light emitting elements in a planar shape below the liquid crystal display element, and directly illuminates the liquid crystal display element. In recent years, in particular, demands for lowering power consumption and improving display brightness are increasing, and how to efficiently use light from a backlight for a liquid crystal display element has become a major issue.

  The liquid crystal display element is usually sandwiched between two polarizing plates that transmit only the polarization component in the direction of its own polarization axis. Therefore, of the light emitted from the backlight, the polarization direction is at least in the polarization axis direction (transmission axis direction) of the polarizing plate (hereinafter referred to as “light source side polarizing plate”) disposed on the backlight side of the liquid crystal display element. The light that does not match is not used in the liquid crystal display element.

  Therefore, it has been conventionally proposed to emit linearly polarized light having a polarization direction coincident with the polarization axis direction of the light source side polarizing plate from the backlight. As a result, it is possible to use about half of the light emitted from the backlight, which has been absorbed by the light source side polarizing plate in the past, resulting in a significant increase in light utilization efficiency. Can be improved.

  For example, Patent Document 1 discloses a technique for emitting planar light having a polarization direction that matches the polarization axis direction of the light source side polarizing plate. In the direct type backlight described in Patent Document 1, a large number of surface-emitting laser chips that emit linearly polarized light are arranged in a planar shape. Thereby, planar light whose polarization direction coincides with the polarization axis direction of the light source side polarizing plate can be emitted.

  However, in the technique described in Patent Document 1, in order to obtain a uniform surface light source, the distance from the laser chip to the light source side polarizing plate is increased, a member having a light scattering function is disposed therebetween, or a large number of members are disposed. Measures such as arranging laser chips at a high density are required, and there is a problem that the thickness of the backlight increases. In recent years, demands for thinning liquid crystal display devices have increased, and such an increase in thickness is not desirable.

Therefore, for the edge light type backlight described above, for example, Patent Document 2 discloses a technique for emitting planar light whose polarization direction matches the polarization axis direction of the light source side polarizing plate. The edge light type backlight described in Patent Document 2 includes a light guide plate formed of a material with little birefringence, and a light source that inputs linearly polarized light to the light guide plate. Thereby, planar light whose polarization direction matches the polarization axis direction of the light source side polarizing plate can be emitted in a state in which the deterioration of the polarization characteristics of the original linearly polarized light is suppressed.
Japanese Patent Laying-Open No. 2005-157025 JP 2006-202703 A

  However, with the technique described in Patent Document 2, it is difficult to form a material with sufficiently low birefringence, and depending on the relative relationship between the optical anisotropy of the light guide plate and the polarization direction of polarized light propagating through the light guide plate. A significant decrease in polarization characteristics can occur. Therefore, in the technique described in Patent Document 2, there is a certain limit in improving the utilization efficiency of polarized light.

  This invention is made | formed in view of this point, and it aims at providing the light-guide plate, surface light source device, and liquid crystal display device which can improve the utilization efficiency of polarized light significantly.

  The light guide plate of the present invention is a light guide plate that converts incident light from a light source into planar output light, with respect to a plane parallel to both the incident direction of the incident light and the output direction of the output light, A configuration having optical anisotropy in which the slow axis directions are aligned in a parallel direction or a perpendicular direction is adopted.

  The surface light source device of the present invention employs a configuration having the light guide plate and a light source that causes the polarized light whose polarization direction coincides with a direction parallel to or orthogonal to the plane to be incident on the light guide plate.

  The liquid crystal display device of the present invention includes the above-described surface light source device and a liquid crystal display element that is disposed to face the light exit surface of the light guide plate and switches a transmission state of light emitted from the light exit surface to the viewer side. take.

  According to the present invention, it is possible to propagate incident light in a state where the polarization direction of incident light coincides with either a parallel direction or a direction orthogonal to a plane parallel to the slow axis direction. As a result, it is possible to convert the polarized light into planar light while suppressing the deterioration of the polarization characteristics of the polarized light, and it is possible to greatly improve the use efficiency of the polarized light.

  Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings.

  FIG. 1 is a cross-sectional configuration diagram showing a configuration of a liquid crystal display device according to an embodiment of the present invention. Hereinafter, the side on which the observer of the image of the liquid crystal display device is located is referred to as “observer side”, and the side opposite to the observer side is referred to as “back side”. For convenience of explanation, the normal direction of the exit surface 323 is the Z axis, the direction of the intersection of the exit surface 323 and the entrance surface 321 is the Y axis, and the direction perpendicular to the Y axis and the Z axis is the X axis. To do.

  In FIG. 1, a liquid crystal display device 100 includes a liquid crystal unit 200 that forms an image, and a backlight unit (surface light source device) 300 that is disposed on the back side of the liquid crystal unit 200 and illuminates the liquid crystal unit 200. The backlight unit 300 includes a light source unit 310 and a light guide plate 320.

  First, the configuration of the liquid crystal unit 200 will be described.

  As shown in FIG. 1, the liquid crystal unit 200 includes an observer side polarizing plate 210, a color filter substrate 220, a liquid crystal layer 230, a TFT (thin film transistor) substrate 240, and a light source side polarizing plate (optical film) 250. It is constructed by stacking in order. In the present embodiment, an IPS (in-plane switching) mode liquid crystal unit 200 is employed.

  The observer-side polarizing plate 210 has a polarization axis in a predetermined direction and transmits only a polarization component in the polarization axis direction.

  The color filter substrate 220 includes a pixel layer 223 in which a black matrix 222 and color filters of pixels of each color (red, green, and blue) are arranged on the surface of the transparent substrate 221 on the liquid crystal layer 230 side, an overcoat layer 224, an observation An alignment film 225 whose alignment direction coincides with the polarization axis of the person-side polarizing plate 210 is overlaid in this order.

  The liquid crystal layer 230 is configured by filling a liquid crystal between the color filter substrate 220 and the TFT substrate 240 that are held at intervals by a spacer 231.

  In the TFT substrate 240, an active element 242 such as a TFT element, a common electrode 243, and a signal line 244 are arranged on the surface of the transparent substrate 241 on the liquid crystal layer 230 side. The TFT substrate 240 further includes an insulating film 245, a pixel electrode 246, and an alignment film 247 whose alignment direction is orthogonal to the alignment film 225 of the TFT substrate 240, which are stacked in this order.

  The light source side polarizing plate 250 has a polarization axis that coincides with the alignment direction of the alignment film 247 of the TFT substrate 240 and transmits only the polarization component in the polarization axis direction. That is, the light source side polarizing plate 250 and the observer side polarizing plate 210 have polarization axes orthogonal to each other. Here, it is assumed that the polarization axis direction of the light source side polarizing plate 250 coincides with the X axis direction, and the absorption axis 251 of the light source side polarizing plate 250 coincides with the Y axis direction.

  According to such a liquid crystal unit 200, in a pixel to which a voltage is applied to the liquid crystal layer 230, light incident from the backlight unit 300 side (hereinafter referred to as “backlight light”) reaches the viewer side, and the others In this pixel, it is possible to prevent the backlight from reaching the observer side. However, in the backlight light, the polarization component in the absorption axis direction (here, the Y-axis direction) of the light source side polarizing plate 250 is absorbed regardless of the voltage application state and does not reach the observer side. Therefore, if the backlight light is planar light whose polarization direction coincides with the direction orthogonal to the absorption axis direction of the light source side polarizing plate 250, the loss of the backlight light in the light source side polarizing plate 250 is minimized. It is possible. Hereinafter, for convenience of description, a direction orthogonal to the absorption axis direction of the light source side polarizing plate 250 is defined as a polarization axis direction of the light source side polarizing plate 250.

  When the backlight unit 300 can emit light having a sufficiently high degree of polarization, the light source side polarizing plate 250 can be omitted. In this case, the transmittance can be greatly increased.

  Next, a schematic configuration of the backlight unit 300 will be described.

  The backlight unit 300 is a sidelight type backlight in which linear light enters from the end face of the light guide plate 320.

  The light source unit 310 of the backlight unit 300 includes a light emitting device 311 that synthesizes and emits a linearly polarized point beam. In addition, the light source unit 310 includes a diffractive optical element and various optical lenses in order to expand the outgoing beam of the light emitting device 311 in the Y-axis direction and convert it into linear light. Specifically, the light source unit 310 includes a first aspheric lens 312, a second aspheric lens 313, a prism 314, and a cylindrical Fresnel lens 315. Polarized light 410 emitted from the light emitting device 311 is incident on the light guide plate 320 through the lenses and prisms. In FIG. 1, an arrow 411 indicates the polarization direction of the polarized light 410 immediately after being emitted from the light source unit 310, and an arrow 412 indicates the polarization direction of the polarized light 410 when entering the light guide plate 320. The polarization direction of the polarized light 410 immediately after being emitted from the light source unit 310 and when entering the light guide plate 320 is a direction parallel to the XZ plane (a state having almost no polarization component in the Y-axis direction). . Hereinafter, for convenience of explanation, the polarization direction of the polarized light 410 immediately after being emitted from the light source unit 310 and when entering the light guide plate 320 is referred to as a Z-axis direction. Details of the light source unit 310 will be described later with reference to another drawing.

  The light guide plate 320 of the backlight unit 300 is an optically transparent resin having a rectangular flat plate shape, and the polarized light 410 incident by the light source unit 310 is converted into backlight light 420 that is planar light that illuminates the liquid crystal unit 200. Convert. In FIG. 1, a direction 421 indicates the polarization direction of the backlight light 420. The backlight light 420 is in a state in which the polarization direction coincides with the Z-axis direction (a state in which there is almost no polarization component in the Y-axis direction). Details of the light guide plate 320 will be described later with reference to another drawing.

  According to such a backlight unit 300, the liquid crystal unit 200 can be illuminated with the backlight light 420 generated from the polarized light 410.

  As already described, the planar light whose polarization direction coincides with the polarization axis direction of the light source side polarizing plate 250 is emitted from the backlight unit 300, thereby suppressing the loss of backlight light in the light source side polarizing plate 250. Is possible. Therefore, the backlight unit 300 of the present embodiment allows the polarized light 410 to be incident on the light guide plate 320 while maintaining the linearly polarized light, and has a surface in a desired polarization direction without impairing the polarization characteristics inside the light guide plate 320. The optical anisotropy of the light guide plate 320 and the polarization direction of the polarized light 410 incident on the light guide plate 320 are controlled so that the shaped light is emitted.

  Next, the configuration of the light guide plate 320 of the backlight unit 300 will be described.

  FIG. 2 is a perspective view showing the configuration of the light guide plate 320 according to the present embodiment.

  As shown in FIG. 2, the light guide plate 320 emits the incident surface 321 on which the polarized light 410 is incident, the reflective surface 322 that reflects the polarized light 410 incident from the incident surface 321, and the polarized light 410 that is reflected by the reflective surface 322. It is a rectangular flat plate having a rectangular emission surface 323.

  The light guide plate 320 is a transparent resin, and is optically different so that one of the normal directions of the emission surface 323 or one direction orthogonal to the normal direction is a slow axis direction. It was produced by controlling the direction.

  Examples of the material of the light guide plate 320 include acrylic resins such as polystyrene and polymethyl methacrylate, polyester resins such as polycarbonate, cycloolefin resins, cellulose resins, ABS (acrylonitrile butadiene styrene) resins, and AS (acrylonitrile styrene) resins. Or a copolymer of methacrylate and styrene. When the material has haze, depolarization is likely to occur. Therefore, it is desirable to select a material and a thickness so that the transmission haze value from the reflection surface 322 to the emission surface 323 is 10% or less, preferably 5% or less.

  As a method for producing the light guide plate 320, for example, extrusion molding, injection molding, compression molding, casting method, or the like can be applied. However, the surface accuracy in the flat plate state is high, and the whole is formed in a predetermined direction. Any manufacturing method may be applied as long as the slow axis directions can be aligned with high accuracy. However, in the case of using a material having a large positive intrinsic birefringence, that is, a material in which molecular anisotropy is likely to generate optical anisotropy due to stress or the like, the optical anisotropy is in-plane within the light guide plate 320 by injection molding or the like. Since it is difficult to align the orientation, it is necessary to improve the material as appropriate, for example, by modifying the material so that the fluidity becomes high when the material is melted. The light guide plate 320 may be a film having a thickness of about several hundreds of micrometers. In this case, the slow axis of the film-shaped light guide can be easily controlled by subjecting the polymer film to heat stretching. When a laser is used as the light source as in this embodiment, it is easy to focus the beam at the time of incidence, so even with such a film-shaped light guide, light can be guided with little loss. It is possible to put light into the body.

  In this embodiment, an acrylic flat plate produced by controlling the optical anisotropy by extrusion molding using an acrylic resin as the light guide plate 320 is employed. In addition, it is assumed that the optical anisotropy of the light guide plate 320 is controlled so that the slow axis direction thereof coincides with the normal direction (Z-axis direction) of the emission surface 323. Since acrylic resin has negative intrinsic birefringence, for example, the slow axis direction (optical axis direction) of the light guide plate 320 is changed to the Z axis direction by compression molding in the Z axis direction, that is, the thickness direction of the light guide plate 320. Can match.

  Further, the polarization direction of the light source side polarizing plate 250 of the liquid crystal unit 200 shown in FIG. 1 (the alignment direction of the alignment film 247 of the TFT substrate 240) coincides with the X-axis direction, and the polarization direction of the observer side polarizing plate 210 is It shall be in agreement with the Y-axis direction.

  FIG. 3 is a diagram for explaining the anisotropy of the light guide plate 320. FIG. 3A shows the magnitude of the refractive index in each direction on the XZ plane as an ellipsoid, and FIG. 3B shows the magnitude of the refractive index in each direction on the XY plane. Is represented by an ellipsoid. 3A and 3B, the distance from the center of each part of the ellipsoid indicates the size of the refractive index in the direction from the center to each part.

As shown in FIG. 3A, the refractive index Nz in the Z-axis direction, which is the slow axis direction, is larger than the refractive index Nx in the X-axis direction and is maximum on the XZ plane. On the other hand, as shown in FIG. 3B, the refractive index on the XY plane is the same in all directions, and the refractive index Ny in the Y-axis direction is the same as the refractive index Nx in the X-axis direction. . The relationship between the refractive indexes Nx, Ny, and Nz in the respective axial directions satisfies the following expression (1).
Nx ≒ Ny <Nz (1)

  A reflection structure for emitting light incident on the light guide plate 320 from the emission surface 323 is formed on the reflection surface 322 of the light guide plate 320 shown in FIG. As this reflection structure, there is a structure printed with white ink that scatters light, or a fine prism structure having a shape that reflects incident light as much as possible in the same plane and guides it in the direction of exiting from the exit surface 323. Can be applied. Further, in the case of applying the fine prism structure, the fine prism structure may be provided on the exit surface 323 surface, or may be provided on both the reflection surface 322 and the exit surface 323.

  The fine prisms only need to be arranged so that the planar light emitted from the emission surface 323 is uniform, such as a continuous line shape, an independent random arrangement, and an independent regular order. Various arrangements can be applied, such as those arranged.

  The shape of each prism may be convex or concave, and a triangular shape with various apex angles, a semicircular or elliptical shape, a rectangular shape, or the like is applicable. These are selected on the side where the liquid crystal display element is arranged, and the shape is selected and arranged as appropriate so that the planar illumination light is uniform.

  Note that the prism shape is controlled so that the polarized light 410 incident on the light guide plate 320 propagates straightly in the XY plane without being diffused and is deflected to the exit surface 323 side by the prism portion. Has been. Thereby, when the polarized light 410 is incident in a direction parallel to the XY plane, the polarized light 410 is internally propagated parallel to the XY plane and is emitted from the emission surface 323.

  As a method for forming a fine structure such as a prism on the light guide plate 320, a method of forming by injection molding or compression molding using a mold, or imprinting by heat or UV (ultraviolet) on a flat plate produced by extrusion molding or casting method. A method of transferring by a method, a method of forming by melting and scattering with a carbon dioxide laser, a method of forming by cutting with a diamond tool, and the like can be applied.

  Further, as the shape of the light guide plate 320, a flat plate shape or a wedge shape can be applied. This is advantageous for reducing the thickness and weight of the light guide plate 320. Here, it is assumed that a substantially prism-shaped groove having a substantially V shape is mechanically formed with a diamond tool on the reflecting surface 322 as a fine prism structure (not shown).

  Planar emission light (backlight light 420) from the light guide plate 320 can be arbitrarily adjusted by various optical films. For example, it is possible to make the in-plane distribution more uniform by using a diffusion film, or to improve the light collection efficiency in the front direction by using a prism sheet. The prism sheet and the diffusion sheet are configured, for example, by forming a prism layer and a diffusion layer on a polyester film serving as a base material. Since most polyester films have optical anisotropy, it is preferable that the slow axis direction (maximum refractive index orientation) of the film is parallel or orthogonal to the polarization direction emitted from the light guide plate.

  A reflective sheet (not shown) may be disposed on the reflective surface 322 side of the light guide plate 320. Thereby, the light which comes out on the surface on the opposite side to the liquid crystal display element side can be reused. From the viewpoint of suppressing depolarization as much as possible, this reflective sheet is preferably a mirror-like sheet on which metal is deposited.

  Further, from the viewpoint of suppressing a reduction in utilization efficiency due to light leakage from the end face of the light guide plate 320, for example, a reflective sheet may be attached to each end face via an adhesive.

  According to the light guide plate 320 having such a configuration, the polarized light 410 incident from the incident surface 321 propagates through the light guide plate 320 while reflecting between the reflecting surface 322 and the output surface 323, and is emitted in the fine prism structure. It is deflected in the normal direction of the surface 323. Further, the polarized light 410 propagates parallel to the XZ plane of the light guide plate 320 and is emitted from the emission surface 323. Therefore, if the polarized light 410 has a state in which it does not have a deflection component in the Y-axis direction or a state in which only the polarization component in the Y-axis direction remains in the process of propagating through the light guide plate 320, the polarization characteristics are maintained. Is done. Further, the polarization direction of the backlight light 420 is aligned with the polarization direction of the light source side polarizing plate 250 of the liquid crystal unit 200. The polarization characteristics of the polarized light 410 are retained because the incident direction of the polarized light 410 is parallel to the XZ plane and the polarized light direction of the polarized light 410 is parallel to or orthogonal to the XY plane. This is the case when aligned in the direction.

  Next, the light source unit 310 of the backlight unit 300 will be described.

  FIG. 4 is a cross-sectional configuration diagram showing the configuration of the backlight unit 300.

  As shown in FIG. 4, the light emitting device 311 is disposed at a position on the back side of the light guide plate 320, opposite to the incident surface 321, toward the other end side where the incident surface 321 of the light guide plate 320 is provided. The polarized light 410 that is linearly polarized light is output at an angle parallel to the X-axis direction.

  Specifically, the light emitting device 311 includes three primary color semiconductor laser diodes (LD). As the blue laser diode, for example, a GaN-based or InGaN-based semiconductor material is used. As the green laser diode, for example, a semiconductor material such as InGaN or ZnSe is used. As the red laser diode, for example, an AlGaInP-based semiconductor material is used. The short-wavelength laser light may be generated by using SHG (second harmonic generation) and halving the wavelength of the light. As a result, it is possible to generate laser light having a short wavelength using an infrared laser diode that has a long life and can easily achieve high output. In particular, when an LD pumped fiber laser and SHG are combined, a high-power green laser can be realized, which is more preferable.

  From the viewpoint of color reproducibility of the liquid crystal display device, the three primary color laser diodes preferably have a peak at 430 to 480 nm (nanometer) as blue, 520 to 550 nm as green, and 620 to 660 nm as red. Furthermore, what has a peak between 440-470 nm, 525-545 nm, 625-650 nm is more preferable. Here, a 445 nm blue laser diode, a 535 nm green SHG laser based on an infrared laser diode, and a 635 nm red laser diode are used.

  For example, the light emitting device 311 can be configured by arranging the laser diodes of the respective colors on the same line so that the polarization directions are aligned, and outputs the combined light of the output light of the laser diodes of the respective colors as the polarization 410. Thus, by using a semiconductor laser or SHG, the light emitting device 311 can be easily reduced in size and increased in efficiency, and further can be easily reduced in price by mass production.

  The light emitting device 311 has means for converting the emitted beam from the semiconductor laser diode into a Gaussian shape, and further, for example, mixes three colors of laser light by a cross dichroic prism to form a white light laser. Can be output.

  Here, as described with reference to FIG. 1, the light emitting device 311 emits the polarized light 410 with the polarization direction aligned with the Z-axis direction.

  The first aspherical lens 312 is disposed on the optical path of the polarized light 410 emitted from the light emitting device 311, and the light density in the Y-axis direction with the polarized light 410 aligned in the Z-axis direction. Is spread in the Y-axis direction.

  The second aspherical lens 313 is a parallel light having a width substantially matching the width of the light guide plate 320 with the polarization 410 emitted from the first aspherical lens 312 aligned in the Z-axis direction. To collimate.

  FIG. 5 is a plan view showing how the polarized light 410 is spread. Here, a state when the light source unit 310 is viewed from the light guide plate 320 side is shown.

  As shown in FIG. 5, the polarized light 410 of the parallel light emitted from the light emitting device 311 is expanded to a width substantially equal to the width of the light guide plate 320 by the first aspheric lens, and the second aspheric lens 313 It is converted back to parallel light. However, the first aspherical lens and the second aspherical lens 313 hold the light density of the polarized light 410 emitted from the light emitting device 311 uniformly. As described above, the polarized light 410 emitted from the second aspheric lens 313 becomes parallel light having a width substantially matching the width of the light guide plate 320 and a uniform light density.

  The prism 314 shown in FIG. 4 is disposed so as to face the incident surface 321 of the light guide plate 320, and the polarized light 410 emitted from the second aspheric lens 313 is folded back 180 degrees toward the incident surface 321 of the light guide plate 320. .

  The cylindrical Fresnel lens 315 converges the polarized light 410 emitted from the prism 314 in the Z-axis direction and collects it on the incident surface 321 of the light guide plate 320. Instead of the cylindrical Fresnel lens 315, an optical element such as a rod lens may be used.

  Note that the material of each optical element for transmitting the polarized light 410 from the light emitting device 311 is preferably resin or glass. In this case, since there is almost no birefringence and it is optically isotropic, it becomes easy to suppress the deterioration of the polarization property of the polarized light 410.

  According to the light source unit 310 having such a configuration, the polarized light 410 is enlarged in the width direction of the light guide plate 320 and projected onto the incident surface 321 of the light guide plate 320, so that the light is efficiently converted into a linear shape from point light at a short distance. It can be converted into light and incident on the inside of the light guide plate 320. In addition, the polarized light 410 incident on the light guide plate 320 propagates in parallel with the XZ plane and becomes linearly polarized light whose polarization direction coincides with the Z-axis direction.

  When the incident direction of the polarized light 410 is parallel to the XZ plane and the polarized light direction of the polarized light 410 is aligned in a direction parallel to or orthogonal to the XY plane, the polarized light of the polarized light 410 As described above, the characteristics are maintained. Therefore, since the polarization direction of the polarized light 410 coincides with the Z-axis direction, the polarization direction of the backlight light 420 is aligned with the X-axis direction, that is, the polarization axis of the light source side polarizing plate 250 of the liquid crystal unit 200. . Further, it has already been explained that the loss of the backlight light 420 in the light source side polarizing plate 250 is reduced when the polarization direction of the backlight light 420 coincides with the polarization axis of the light source side polarizing plate 250 of the liquid crystal unit 200. That's right.

  Therefore, according to the liquid crystal display device 100 having the above configuration, the utilization efficiency of the polarized light 410 can be greatly improved.

  Hereinafter, experimental results regarding the utilization efficiency of the polarized light 410 in the liquid crystal display device 100 will be described.

  This inventor observed the optical anisotropy of the acrylic flat plate used as the base of the light-guide plate 320 in this Embodiment under crossed Nicols. That is, a polarizer, an acrylic flat plate, and an analyzer were placed on the table light, and the polarizer and the analyzer were arranged so that the absorption axes thereof were orthogonal to each other. In order to see the birefringence in the XY plane of the acrylic plate, the acrylic plate was arranged so that the XY plane was parallel to the polarizer and the analyzer and rotated on the XY plane, but the transmittance was 0. There was only 0.1%, and light leakage hardly occurred. From this, it was confirmed that the acrylic plate has almost no optical anisotropy in the XY plane.

  Next, in order to see the birefringence in the XZ plane, the acrylic plate was placed so that the XZ plane was parallel to the polarizer and the analyzer and rotated on the XZ plane. As a result, when the angle between the reference axis of the acrylic plate and the reference axis of the polarizer is about 45 degrees, the transmittance is maximum at 30%, and when the angle is near 0 degrees, the transmittance is 1% and minimum. became. That is, the slow axis direction of the acrylic plate is in the Z-axis direction, and the acrylic resin has negative intrinsic birefringence, so it was confirmed that this direction was the slow axis direction of the acrylic plate.

  With the light guide plate 320 made of such an acrylic plate incorporated, the luminance at the center of the screen on the viewer side of each of the backlight unit 300 and the liquid crystal display device 100 was measured using a luminance meter. Then, when the light transmittance of the liquid crystal unit 200 was measured from the measured luminance, a result of 8.5% transmittance was obtained.

  On the other hand, for comparison, a wedge-shaped plate was produced by injection molding under high pressure using polycarbonate as a light guide plate material to be compared. That is, a light guide plate was produced in the same manner as the conventional method without controlling the optical anisotropy. Further, when the optical anisotropy in the XY plane was similarly observed under crossed Nicols with respect to this wedge-shaped plate, there was a region where the transmittance was 15% and light leakage occurred. That is, it was confirmed that the direction of the slow axis direction is not uniform in the XY plane.

  A light guide plate having a prism structure was formed by mechanically forming a substantially V-shaped groove on the bottom surface of such a plate. Further, similarly to the liquid crystal display device 100 of this embodiment, a backlight using a laser as a light source is manufactured using the manufactured light guide plate, and a liquid crystal display device is manufactured using an IPS mode liquid crystal display unit. did. And when the light transmittance of the liquid crystal display unit was measured, a result of 5% transmittance was obtained.

  Thus, the light transmittance of the liquid crystal unit 200 in the liquid crystal display device 100 according to the present embodiment is that of the liquid crystal unit in the liquid crystal display device using the conventional light guide plate in which the optical anisotropy is not controlled. It is higher than the light transmittance.

  Also from the above experimental results, the light guide plate 320 in which the optical anisotropy is controlled and the light source unit 310 that makes the light guide plate 320 incident polarized light whose polarization direction is parallel or orthogonal to the slow axis direction thereof. It can be seen that the use of the liquid crystal display device 100 with very high light utilization efficiency is realized.

  Although it is conceivable that the light guide plate is made of a material having low birefringence, it is possible to suppress a decrease in polarization characteristics of polarized light. Therefore, it is difficult to say that the deterioration of polarization characteristics can be sufficiently suppressed. Furthermore, in order to suppress the birefringence to a small value, it is necessary to select a special material. Therefore, the cost of the light guide plate increases, and it becomes difficult to realize an inexpensive backlight.

  In this respect, the control of the optical anisotropy can be performed relatively easily and inexpensively by pressing the material. Therefore, according to the light guide plate 320 according to the present embodiment, it is possible to significantly improve the light use efficiency while suppressing an increase in cost.

  As described above, according to the present embodiment, the light guide plate 320 has optical anisotropy in which the Z axis, which is the normal direction of the emission surface 323, is the slow axis direction. As a result, when polarized light that propagates parallel to the XZ plane and does not have a polarization component in the Y-axis direction is used, the polarized light is converted into planar light while suppressing deterioration of polarization characteristics. And the use efficiency of polarized light can be greatly improved. In addition, the backlight unit 300 having the light guide plate 320 further includes a light source unit 310 that makes such polarized light incident on the light guide plate 320, and thus the backlight light 420 of planar light whose polarization direction is aligned in the Z-axis direction. Can be output. Furthermore, the liquid crystal display device 100 having the backlight unit 300 further includes a liquid crystal unit 200 that is disposed so as to face the emission surface 323 of the light guide plate 320 and switches the transmission state of the emitted light from the emission surface 323 to the viewer side. Therefore, an image with high contrast and high brightness can be displayed.

  In the above-described embodiment, the case where the slow axis direction coincides with the Z-axis direction that is the normal direction of the emission surface 323 of the light guide plate 320 has been described. However, other embodiments parallel to the XZ plane are described. The optical anisotropy of the light guide plate 320 may be controlled so that the slow axis direction coincides with the direction or the direction orthogonal to the XZ plane. In the case where the delay axis direction coincides with the direction orthogonal to the XZ plane, the polarization direction of the polarized light 420 needs to coincide with the Y axis direction. In this case, it is necessary to make the absorption axis direction of the light source side polarizing plate 250 of the liquid crystal unit 200 coincide with the X axis direction.

  In this embodiment, the example applied to the liquid crystal unit 200 in the IPS mode has been described. However, the present invention is not limited to this, and various liquid crystals such as a TN (twisted nematic) method and a VA (vertically aligned) method are used. It can be applied to a display element. Furthermore, the light incident on the light guide plate 320 is not limited to the laser light, but may be any parallel light having a uniform polarization direction.

  The light guide plate, the surface light source device, and the liquid crystal display device according to the present invention are useful as a light guide plate, a surface light source device, and a liquid crystal display device that can significantly improve the use efficiency of polarized light.

1 is a cross-sectional configuration diagram illustrating a configuration of a liquid crystal display device according to an embodiment of the present invention. The perspective view which shows the structure of the light-guide plate which concerns on this Embodiment. The figure for demonstrating the anisotropy of the light-guide plate which concerns on this Embodiment Cross-sectional configuration diagram showing the configuration of the backlight unit in the present embodiment The top view which shows a mode that the polarization | polarized-light in this Embodiment is spread.

Explanation of symbols

DESCRIPTION OF SYMBOLS 100 Liquid crystal display device 200 Liquid crystal unit 210 Observer side polarizing plate 220 Color filter substrate 221 241 Transparent substrate 222 Black matrix 223 Pixel layer 224 Overcoat layer 225 Alignment film 230 Liquid crystal layer 231 Spacer 240 TFT substrate 242 Active element 243 Common electrode 244 Signal line 245 Insulating film 246 Pixel electrode 247 Alignment film 250 Light source side polarizing plate 300 Backlight unit 310 Light source unit 311 Light emitting device 312 First aspherical lens 313 Second aspherical lens 314 Prism 315 Cylindrical Fresnel lens 320 Light guide plate

Claims (6)

A light guide plate that converts incident light from a light source into planar output light,
With respect to a plane parallel to both the incident direction of the incident light and the outgoing direction of the outgoing light, it has optical anisotropy in which the slow axis direction is aligned in a parallel direction or a direction perpendicular to the plane.
Light guide plate. The incident light is internally propagated parallel to the plane and emitted from the exit surface.
The light guide plate according to claim 1. A light guide plate according to claim 1;
A light source unit that makes polarized light whose polarization direction coincides with a direction parallel to or orthogonal to the plane to be incident on the light guide plate;
A surface light source device. The light source unit is
A semiconductor laser;
An optical system for expanding the width of the laser light emitted from the semiconductor laser and making it incident on the light guide plate with the polarization direction aligned.
The surface light source device according to claim 3. A surface light source device according to claim 3;
A liquid crystal display element disposed opposite to the light exit surface of the light guide plate, and switching a transmission state of the light emitted from the light exit surface to the viewer side;
A liquid crystal display device. A polarizing plate disposed opposite to the light exit surface of the light guide plate and having an absorption axis in a direction orthogonal to the polarization direction of the light emitted from the light exit surface;
The liquid crystal display device according to claim 5.

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