LIQUID CRYSTAL DISPLAY DEVICE
CROSS-REFERENCE TO RELATED
APPLICATIONS
[0001] The disclosure of Japanese Patent Application No. 2004-113311 filed on Apr. 7, 2004 including the specification, drawings and abstract is incorporated herein by reference in its entirety.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device having a light guide plate which diffuses a light-source light of a solid light emitting element such as a light emitting diode or the like as a light source in a developed manner on a back surface of a liquid crystal display panel in a planer shape.
[0003] Recently, with respect to a miniaturized portable digital assistant such as a mobile phone, a portable information terminal or the like, a liquid crystal display device which is small-sized and exhibits the low power consumption has been popularly used as a display device. As this type of liquid crystal display device, a liquid crystal display device having the structure which uses an external light as illumination means for visualizing an electronic latent image formed on a liquid crystal display panel or the structure which installs an auxiliary illumination device on a frontsurface side or a back-surface side of the liquid crystal display panel has been used. The auxiliary illumination device which is installed on the back-surface side of the liquid crystal display panel is referred to as a back light device, while the auxiliary illumination device which is installed on the front-surface side of the liquid crystal display panel is referred to as a front light device.
[0004] As such a light source for the auxiliary illumination device of the miniaturized portable digital assistant, there has been known a light source which has the following structure. That is, the light source has a light guide plate which arranges a cold cathode fluorescent lamp on a side wall (side edge) side as in the case of a notebook type personal computer or the like which has a relatively large display screen size. However, in the mobile telephone or the miniaturized portable digital assistant (so-called, PDA or the like), in place of the above-mentioned cold cathode fluorescent lamp, a solid light emitting element as represented by a light emitting diode (LED) which exhibits the small power consumption is popularly used.
[0005] In the backlight device which uses the light emitting diode as a light source, with respect to a light reflection pattern which is applied to the light guide plate for efficiently converting light emitted from the light emitting diode which constitutes a spot light source into a face light source, various shapes have been proposed. Concentric light reflection patterns which are arranged around a spot light source exhibit the highest efficiency. A drawback that the concentric light reflection pattern possesses lies in that when a plurality of light emitting diodes are used as the light source, there arise portions where the concentric light reflection patterns are overlapped to each other and respective enterprises concerned have been carrying out researches and developments on counter measures to cope with the brightness irregularities of these portions.
[0006] FIG. 10 is a cross-sectional view of an essential part for schematically explaining a constitutional example of a liquid crystal display device having a backlight device which uses a light emitting diode as a light emitting element on a light guide plate. This type of backlight device BL is configured such that a plurality of light emitting diodes LED are arranged to face in an opposed manner a side wall (side edge) of the light guide plate GLB made of a light-transmitting resin material or the like which is installed on a back-surface side of the liquid crystal display panel PNL. Further, a light diffusion sheet DIS is adhered to and arranged on a front surface of the light guide plate GLB, while a light reflection sheet RES is adhered to and arranged on a back surface of a light guide plate GLB. Here, the light emitting diodes LED are mounted on a printed circuit board PCB in an erected manner.
[0007] A hard printed circuit board, a flexible printed circuit board or the like is used as a printed circuit board PCB. Although not shown in the drawing, on a back-surface side (a surface opposite to the light emitting diodes LED) of the printed circuit board PCB, a drive IC, other electronic parts EP and the like are mounted. The light emitting diodes LED are arranged in a state that light emitting portions E are brought into contact with a side edge of the light guide plate GLB. Light which is emitted from the light emitting portions E of the light emitting diodes LED is introduced into the inside of the light guide plate GLB which uses the side edge as a light incident surface.
[0008] The light guide plate GLB is, as shown in FIG. 11 which is a plan view of an essential part, configured such that a plurality of radial diffusion patterns DIP are formed on a light emitting surface which is formed on the front surface of the light guide plate GLB thus forming a light diffusion surface DIF on the light emitting surface. The diffusion patterns DIP are constituted of a plurality of diffusion regions DIA, wherein each diffusion region DIA is formed as a semi-columnar recessed portion on the light diffusion surface of the light guide plate GLB. Further, the diffusion patterns DIP are constituted of a first region Rl and a second region R2 which are respectively formed on an approximately lower half and an approximately upper half of the light diffusion surface DIF and a transitional region RT which is formed between the first region Rl and the second region R2 with respect to a longitudinal center line C of the light guide plate GLB.
[0009] A direction of an axis AX of the whole diffusion regions DIA in the first region Rl (hereinafter referred to as an axial direction of the diffusion regions) is aligned with a tangential direction of a circle which has the center on the first light emitting diode LED1 in a plan view, while a direction of an axis of whole diffusion regions in the second region R2 is aligned with the tangential direction of a circle which has the center on the first light emitting diode LED2 in a plan view.
[0010] That is, the light guide plate GLB is configured such that light incident from the plurality of light emitting diodes LED1, LED2 propagates in the inside of the light guide plate, and, at the same time, the light is diffused by a large number of diffusion regions DIA formed on the light diffusion surface DIF and, thereafter, is irradiated from a light irradiation surface. The diffusion patterns DIP which constitute a layout pattern of a large number of diffusion
