JP4599988B2 - Backlight unit - Google Patents

Description

  The present invention relates to another backlight unit for a liquid crystal display device including a light guide plate using a diffraction grating as a light emitting means.

  At present, in the cellular phones and portable information terminals that are widely used, a backlight system is mainly used as an illumination device. The backlight of such a liquid crystal display device needs to illuminate the entire plane with high luminance and no luminance unevenness.

  In recent years, a light guide plate used for a backlight has become a mainstream method of processing V-shaped grooves on the order of several tens of μm in place of the conventional white dot printing method.

  Further, in order to reduce luminance unevenness and increase luminance, it is common to use optical sheets such as a scattering diffuser plate and a prism sheet.

  FIG. 5 shows a configuration of a conventional general backlight. The backlight is provided on the back surface of the liquid crystal panel, a white light source 1 such as a light emitting diode (LED) or a cold cathode tube lamp (CCFL), a light guide plate 2 on which an emission pattern 3 such as a V-shaped groove is disposed, The reflection plate 4 provided on the back surface side of the light guide plate 2, the scattering diffuser plate 5 provided on the front surface side of the light guide plate 2, and the front surface side of the scattering diffuser plate for condensing light. It consists of a prism sheet 6.

  Light from the light source 1 enters the light guide plate 2 from the end face of the light guide plate 2. The light guide plate 2 is a transparent plate for spreading the light incident from the end face and emitting it, and acrylic resin or polycarbonate is mainly used. A dot pattern or a V-shaped groove 3 is formed mainly on the lower surface of the light guide plate 2. The light incident on the light guide plate 2 propagates by repeating total internal reflection, and is scattered or reflected by the emission pattern 3 and emitted from the upper surface of the light guide plate 2.

  The reflection plate 4 is provided on the back surface side of the light guide plate 2 and functions to return light leaking from the back surface of the light guide plate 2 toward the light guide plate 2.

  The scattering diffuser plate 5 is provided on the exit surface side of the light guide plate 2 and has a role of making the in-plane distribution of luminance uniform. In some cases, a diffusion pattern is also provided on the upper surface of the light guide plate 2, and brightness unevenness is reduced together with the scattering diffuser plate 5.

  The prism sheet 6 is provided on the upper side of the scattering diffuser plate 5 and functions to increase the peak luminance of the exit surface by imparting directivity to the light emitted from the scattering diffuser plate 5. Further, it plays the role of aligning the traveling direction of the light emitted from the scattering diffuser plate 5 in a direction substantially perpendicular to the surface of the light guide plate.

  Elements required for a backlight of a liquid crystal display device include a reduction in thickness and cost in addition to an increase in luminance. As one of new technologies for reducing the number of members, there is a light guide plate that uses a diffraction grating as a light emitting means, as disclosed in Japanese Patent Application Laid-Open No. H10-228707. By using a diffraction grating, the light emission angle from the light guide plate can be freely controlled compared to conventional dot patterns and V-shaped grooves, so light control by a prism sheet or the like is not required, and the number of members can be reduced. Is possible. Furthermore, since the light utilization efficiency can be controlled, it is possible to reduce luminance unevenness.

However, since the diffraction grating generates a spectrum when diffracting light, when the diffraction grating is used as an emission pattern, there is a problem that it is difficult to obtain white light because the color changes according to the observation angle.

The patent literature is as follows.
Japanese Patent Laid-Open No. 7-248496

  In the backlight unit using the diffraction grating light guide plate, the present invention reduces the coloring caused by the spectrum generated by the diffraction grating and obtains white light, thereby providing a member such as a prism sheet which is an advantage of the original diffraction grating light guide plate The purpose is to realize a backlight with high brightness and low cost.

  In order to achieve the above object, the backlight unit of the present invention includes a light guide plate in which a diffraction grating is disposed as a light emission pattern on a light emission surface or an opposite surface thereof, and a diffraction grating on the light emission surface side. A diffusion plate having a diffusion pattern arranged to diffuse at least the light emitted in the mean light guide direction of the light guide plate.

  As a result, a plurality of wavelength components of the light separated by the diffraction grating are mixed, and the scattering components are also overlapped to realize white light.

  Here, the average light guide direction is a macroscopic traveling direction of light incident on the light guide plate. Since the light incident from the entrance end surface of the light guide plate has a certain extent, the traveling direction of the light is various. In general, the incident light has a light amount distribution centered on the front direction. Therefore, the front direction of incident light, that is, the direction perpendicular to the entrance end surface of the light guide plate can be regarded as a macroscopic traveling direction of light.

  The diffusion structure is a unidirectional diffusion structure in which a restriction is imposed on the scattering direction. In addition, the unidirectional diffusion structure in which the scattering direction is limited may be a lenticular lens.

  In the above diffusion structure, elongated concavo-convex structures having a length, width and depth of about several microns are randomly arranged, and the major axis direction of the concavo-convex structure is approximately parallel to the grating line direction of the diffraction grating. It is assumed that it is installed in.

  In such a diffusion structure, since the diffusion is strongly diffused in the short axis direction of the elongated concavo-convex structure, the long axis direction of the concavo-convex structure is approximately parallel to the lattice line direction of the diffraction grating, that is, the short axis direction of the concavo-convex structure is the average guide of the light guide plate. By installing so as to substantially coincide with the light direction, coloring due to spectroscopy can be effectively reduced.

  In addition, when the diffusion structure is integrally formed with the light guide plate, a lot of scattered light is generated from the diffusion structure and the light loss increases. However, the diffusion structure can be diffused by using an independent unidirectional diffusion plate. Generation of scattered light from the structure can be suppressed and light can be diffused efficiently.

  The diffraction grating may be a blazed grating having a sawtooth cross section. By using a blazed grating, the diffraction efficiency and diffraction angle of diffracted light, such as improving the diffraction efficiency of first-order diffracted light, can be controlled with high efficiency and precision, and the light use efficiency as an illumination device can be further enhanced. it can.

  The backlight unit may further include a scattering diffuser on the exit surface side of the unidirectional diffuser. By using the scattering diffuser together, it is possible to further reduce the luminance unevenness.

  The backlight unit according to the present invention mitigates the coloration caused by the spectrum caused by the diffraction grating by providing a diffusion plate having a unidirectional diffusion structure on the light exit surface side of the light guide plate on which the diffraction grating is arranged as a light emission pattern. In addition, white light can be obtained even in a light guide plate using a diffraction grating. Therefore, it is possible to realize a light guide plate that makes full use of the characteristics of the original diffraction grating, in which the emission angle and light utilization efficiency can be controlled to a high degree, and it is possible to construct a thin and high-brightness backlight unit.

  Hereinafter, the light guide plate of the present invention will be described based on examples.

  FIG. 1 shows a configuration of a backlight using a diffraction grating light guide plate. This backlight is provided on the back surface of the liquid crystal panel, and a white light source 1 such as a light emitting diode (LED) or a cold cathode tube lamp (CCFL) and an emission pattern 31a by a diffraction grating are provided on the light emission surface or the opposite surface. A unidirectional diffusion plate 7 in which a diffusion pattern 31b that diffuses light emitted by the diffraction grating at least in the average light guide direction of the light guide plate is disposed on the light exit surface side of the light guide plate 2; It consists of a reflecting plate 4 provided on the back side of the light guide plate 2 and a scattering diffuser plate 5 provided on the exit surface side of the unidirectional diffusion plate 7.

  As shown in FIG. 2, when light enters the light guide plate 2 from the light source 1 and propagates, the propagating light travels repeatedly with an angle satisfying the total reflection condition. When the light guide plate 2 is made of acrylic resin, if the refractive index is about 1.5, the critical angle is 41.8 ° (≈42 °), and the upper surface 21a and the lower surface 21b of the light guide plate are at an angle of 42 ° or more. The incident light is confined in the light guide plate by total reflection and propagates.

  When light from the light source enters the diffraction grating 31a, the light is diffracted and split. The dispersed light is diffused by the diffusion pattern 31b of the unidirectional diffusion plate provided on the exit surface side, a plurality of wavelength components are mixed, and the scattering components are also overlapped to realize white light.

  At this time, the provided diffusion pattern is a unidirectional diffusion pattern and has a structure in which light is diffused particularly strongly in the average light guide direction of the light guide plate. For this reason, light dispersed by the diffraction grating is efficiently diffused. Normally, the luminance tends to decrease as the diffusion increases. However, in the unidirectional diffusion pattern, since the diffusion direction is limited, the luminance can be reduced less.

  In addition, when the diffusing structure is integrally formed with the light guide plate, a large amount of scattered light is generated because light directly enters the diffusing structure. Such scattered light reduces the utilization efficiency of light as a backlight. However, if the diffusing structure is an independent unidirectional diffusing plate, an air layer exists between the light guide plate and the unidirectional diffusing plate, making it difficult for scattered light derived from the diffusing structure to be generated, and effectively Can be diffused.

  Also, as shown in FIG. 3, by arranging the diffraction grating cells 31c having three types of spatial frequencies in a matrix, white light composed of R, G, and B components can be diffracted at a desired angle. It becomes easier to obtain white light. There may be three or more types of spatial frequencies, and the more types, the closer to white light.

Such a diffraction grating pattern is produced as follows, for example. A diffraction grating cell having an arbitrary spatial frequency can be produced by drawing a diffraction grating with an electron beam on a glass substrate or the like on which a photosensitive material layer is formed. Alternatively, it can be obtained by two-beam interferometry using a photosensitive material, cutting with a metal blade, or the like. A unidirectional diffusion structure can be obtained in the same manner.

  If the substrate on which the diffraction grating and the diffusion structure are formed is made into a metal plate by, for example, electroforming, it can be copied in large quantities by injection molding or UV resin replication.

  Here, the unidirectional diffusion structure may be a lenticular lens, and a similar color correction effect can be obtained.

  In addition, by using a blazed grating with a sawtooth cross section, the diffraction efficiency and angle of diffracted light can be controlled with high efficiency and precision, such as improving the diffraction efficiency of primary diffracted light. The light utilization efficiency can be further increased.

  FIG. 4 shows a state of light diffusion in the backlight unit according to the present invention. Here, it is assumed that a diffraction grating is provided on the lower surface of the light guide plate. The light diffracted by the diffraction grating 31a provided on the lower surface of the light guide plate 2 is strongly dispersed in the average light guide direction of the light guide plate. Since the dispersed light is diffused particularly strongly in the average light guide direction of the light guide plate by the diffusion structure 31b of the unidirectional diffuser plate provided on the light exit surface side, coloring due to the spectrum is alleviated. Further, the light is uniformly diffused in all directions by the scattering diffuser 5 provided on the light exit surface side of the unidirectional diffuser.

  The present invention is not limited to the embodiment described above, and includes various modifications within the scope not departing from the gist of the present invention. For example, a stripe type can be used instead of the matrix type. However, in the case of forming a diffraction grating by EB drawing, the matrix type has a limit of the region size that can be drawn, and the cell unit structure is easier to manufacture.

  The present invention relates to a backlight unit for other full-color display devices for liquid crystal display devices.

It is a conceptual sectional view of the plane illumination device of the present invention. In the flat illuminating device of this invention, it is a conceptual sectional view which shows the incidence of the light in a light guide plate, and the propagation of the light in a light guide plate. It is a top view which shows the example of arrangement | positioning of the diffraction grating in this invention. It is a conceptual perspective view which shows the mode of the spreading | diffusion of light in this invention. It is a conceptual sectional view of the conventional general plane illumination device.

Explanation of symbols

1: Light source 2: Light guide plate 21a: Light guide plate upper surface 21b: Light guide plate lower surface 3: Dot pattern or prism pattern 31a: Diffraction grating pattern 31b: Diffusion pattern 31c: Diffraction grating cell 4: Reflector plate 5: Scattering diffusion plate 6: Prism sheet 7: Unidirectional diffuser

Claims (6)

In a backlight unit composed of at least one light source and a light guide plate, a light guide plate in which a diffraction grating is disposed as a light emission means on the light emission surface or the opposite surface, and at least light is emitted on the light emission surface side. A diffusion plate having a diffusion pattern that diffuses in an average light guide direction of the light guide plate, and the diffusion structure of the diffusion plate is a unidirectional diffusion structure in which a restriction is provided in a scattering direction. Backlight unit characterized by that. The unidirectional diffusion structure having a restriction on the scattering direction is a surface relief type diffusion plate having a structure in which elongated concavo-convex structures having a length, width and depth of about several microns are randomly arranged. The backlight unit according to claim 1. The backlight unit according to claim 2, wherein the unidirectional diffusion structure in which the scattering direction is limited is a lenticular lens. 4. The backlight unit according to claim 1, wherein the diffraction grating is a blazed grating having a sawtooth cross section. The diffraction grating is characterized in that white light composed of R, G, and B components can be diffracted at a target angle by arranging diffraction grating cells having three types of spatial frequencies in a matrix. The backlight unit according to claim 1. The backlight unit according to any one of claims 1 to 5, further comprising a scattering diffuser on the exit surface side of the diffuser.

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