Abstract:
An electro-optic device includes a light source, a light guide panel configured to output light beams emitted from the light source through a light exit plane; a display panel illuminated by the light beams outputted from the light exit plane of the light guide panel passing therethrough; a prism sheet provided between the light guide panel and the display panel and formed with prisms having a triangular cross-section on a surface on the side of the display panel; and a light diffusing member provided between the light guide panel and the prism sheet. The prisms each having a ridge line extending substantially vertically to the direction of emission of the light beam emitted from the light source, and an apex angle is set to a range from 60° to 70°.

Description:
[0001]    The entire disclosure of Japanese Patent Application Nos. 2006-207520, filed Jul. 31, 2006 and 2007-023806, filed Feb. 2, 2007 are expressly incorporated by reference herein. 
       BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    The present invention relates to an electro-optic device suitable to be used for displaying various information. 
         [0004]    2. Related Art 
         [0005]    In a liquid crystal device, an illumination device is provided on the back side of a liquid crystal display panel for achieving transmissive display. In general, the illumination device includes a light source, a light guide panel that converts light from the light source into a planar light and guides the same onto the back side of the liquid crystal display panel, a sheet that diffuses light outgoing from the light guide panel, and a prism sheet that converges light. 
         [0006]    The light incoming from the light source to the light guide panel repeats reflection between an light exit plane and a reflecting plane of the light guide panel, and goes out through the light exit plane. The light outputted from the light guide panel enters the prism sheet. The prism sheet has prisms each having a triangular cross-section, and refracts incoming light by the prism to change the direction of the light. Accordingly, the prism sheet causes the light to go out toward the liquid crystal panel. 
         [0007]    However, two prism sheets are used in the general illumination device and the two prism sheets are arranged so that respective ridge lines of the prisms extend orthogonally to each other. Therefore, a range that outgoing light from the illumination device concentrates is only about 25° at half angle at half maximum. 
         [0008]    JP-A-2003-35825 proposes a technology to reduce uneven luminance occurring in an illumination device by forming prisms on a light exit plane of a light guide panel having a single point light source at an end surface thereof concentrically about the point light source. 
       SUMMARY 
       [0009]    An advantage of some aspects of the invention is to provide an electro-optic device which achieves a wide visual angle. 
         [0010]    According to an aspect of the invention, an electro-optic device includes a light source, a light guide panel configured to output light beams emitted from the light source through a light exit plane; a display panel illuminated by the light beams outputted from the light exit plane of the light guide panel passing therethrough; a prism sheet provided between the light guide panel and the display panel and formed with prisms having a triangular cross-section on a surface on the side of the display panel; and a light diffusing member provided between the light guide panel and the prism sheet, wherein the prisms are formed in such a manner that the ridge lines thereof extend substantially vertically to the direction of light beams emitted from the light source, and the apex angle is set to the range from 60° to 70°. 
         [0011]    The electro-optic device is, for example, a liquid crystal device, and includes the light source, the light guide panel, the display panel, the prism sheet and the light diffusing member. The light guide panel outputs the light beams emitted from the light source through the light exit plane. The display panel is, for example, a liquid crystal display panel, and is illuminated by the light beams outputted through the light exit plane of the light guide panel passing therethrough. The prism sheet is installed between the light guide panel and the display panel, and is formed with the prisms having a triangular cross-section on the surface on the side of the display panel. The prisms are formed in such a manner that the ridge lines thereof extend substantially vertically to the direction of light beams emitted from the light source, and the apex angle is set to the range from 60° to 70°. The light diffusing member is, for example, a diffusing sheet, and is installed between the light guide panel and the prism sheet. Accordingly, in the electro-optic device according to the embodiment of the invention, a range in which the desirable luminance is achieved is widened in comparison with a general electro-optic device in which two prism sheets having prisms whose apex angle is 90° are laid on one top of another. 
         [0012]    It is preferable that the light guide panel is formed with grooves having a triangular cross-section on a reflecting plane, and the angle of inclination of the each triangular groove on the side of the light source is set to a range from 0.10° to 5°. Accordingly, in the electro-optic device according to the embodiment of the invention, peaks of the luminance are concentrated to an area near the center of a screen. 
         [0013]    It is preferable that the light diffusing member is a diffusing sheet, and the diffusing sheet is further provided between the light guide panel and the prism sheet, and the diffusing sheet is applied with a plurality of beads on a surface thereof. Accordingly, the diffusing sheet is provided with a light converging function. Therefore, according to the electro-optic device in the embodiment of the invention, the luminance is improved. 
         [0014]    It is preferable that the prism sheet is applied with beads on a surface on the side of the light guide panel as the light diffusing member. Accordingly, the prism sheet is provided with a light converging function. Therefore, according to the electro-optic device in the embodiment of the invention, the luminance is improved, and since the diffusing sheet is not provided, the thin profile is achieved correspondingly. 
         [0015]    It is preferable that the display panel is a liquid crystal display panel, wherein when the vertical direction of the liquid crystal display panel is assumed to be 0°, a range in which the contrast ratio is at least 10:1 is at least ±60° in the lateral direction of the display panel. In the liquid crystal device, a characteristic that a contrast characteristic depends on the visual angle caused by birefringence of the liquid crystal is generally known in addition to the above-described lowering of the luminance. That is, when the liquid crystal device is viewed from the oblique direction, the contrast characteristic is deteriorated and hence the liquid crystal device is no longer suitable for being viewed. That is, the liquid crystal device will be further suitable for being viewed by achieving desirable luminance characteristics in a wider angular range and, in addition, by setting the contrast characteristic adequately in this angular range. More specifically, it is achieved by setting the visual angle characteristic to obtain the lateral contrast characteristic of 10:1 in a range wider than the range of the visual angle from 0° to ±60°, in which an attempt is made to improve the luminance characteristics by the invention. In contrast, in the visual angle out of the range of the visual angle from 0° to ±60°, a sufficient luminance characteristic cannot be obtained even though the contrast characteristics are desirably set. Therefore, necessity to maintain the desirable contrast characteristic is relatively low. The object of the invention is almost achieved as long as the contrast characteristic is at least 10:1 at a visual angle from 0° to ±80°. In order to achieve the object, for example, a visual angle compensating layer may be provided at least on one side of the display panel. The visual angle compensating layer such as a visual angle compensating layer in which an expression nx=ny&gt;nz (nx and ny represent refractive indexes in the x and y directions in the film surface, and nz represents an refractive index in the thickness direction) is satisfied, a visual angle compensating panel having a relation nx&gt;ny=nz, a liquid crystal film in which nematic hybrid alignment is fixed so that the liquid crystal is aligned obliquely in the thickness direction, or a liquid crystal film in which discotic hybrid alignment is fixed is arranged independently or in combination on the front or back or the both sides of the display panel according to the type of the display panel. According to another aspect of the invention, an electronic apparatus having the electro-optic device described above as a display unit is provided. 
         [0016]    According to still another aspect of the invention, an illumination device includes a light source, a light guide panel configured to output light beams emitted from the light source through a light exit plane, and a prism sheet installed on the side of the light exit plane of the light guide panel and being formed with prisms having a triangular-cross section on a surface opposite from the side of the light guide panel, and the prisms each have a ridge line extending substantially vertically to the direction of emission of the light beam emitted from the light source, and an apex angle is set to a range from 60 to 70°. With this illumination device as well, a range in which the desirable luminance is achieved is widened. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    The invention will be described with reference to the accompanying drawings, wherein like numbers reference like elements. 
           [0018]      FIG. 1  is a cross-sectional view showing a schematic configuration of a liquid crystal device according to an embodiment. 
           [0019]      FIG. 2  is a plan view showing the positional relation between a light guide panel and a prism sheet. 
           [0020]      FIGS. 3A and 3B  are cross-sectional views showing schematic configurations of the liquid crystal device. 
           [0021]      FIGS. 4A and 4B  are cross-sectional views showing schematic configurations of the liquid crystal device. 
           [0022]      FIGS. 5A and 5B  are cross-sectional views showing schematic configurations of the liquid crystal device. 
           [0023]      FIGS. 6A and 6B  are graphs showing the relation between a visual angle and the luminance in the liquid crystal device. 
           [0024]      FIGS. 7A and 7B  are graphs showing the relation between the visual angle and the luminance in the liquid crystal device. 
           [0025]      FIGS. 8A and 8B  are graphs showing the relation between the visual angle and the luminance in the liquid crystal device. 
           [0026]      FIG. 9A  is a schematic drawing showing an electronic apparatus to which the liquid crystal device according to the embodiment of the invention is applied. 
           [0027]      FIG. 9B  is a schematic drawing showing an electronic apparatus to which the liquid crystal device according to the embodiment of the invention is applied. 
       
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0028]    Referring now to the drawings, preferred embodiments of the invention will be described. In the following embodiments, an illumination device according to an aspect of the invention is applied to a liquid crystal device. 
       Configuration of Liquid Crystal Device 
       [0029]      FIG. 1  is a cross-sectional view of a liquid crystal device  100  according to an embodiment. The liquid crystal device  100  mainly includes an illumination device  10  and a liquid crystal display panel  20 . The liquid crystal display panel  20  is configured in such a manner that substrates  1  and  2  such as glass are bonded together via a sealing material  3  to form a cell structure, and liquid crystal  4  is encapsulated therein. Arranged on the front and back sides of the liquid crystal display panel  20  are polarizing panels, not shown, and a wave panel and a visual angle compensating panel are provided between the liquid crystal display panel  20  and the polarizing panels. 
         [0030]    The illumination device  10  mainly includes a light guide panel  11  and a light source  15 . The liquid crystal display panel  20  is arranged on the upper side of the light guide panel  11  so as to oppose thereto. The illumination device  10  is provided with a reflection sheet  14  on the lower surface side of the light guide panel  11 . 
         [0031]    The light source  15  is arranged on an end surface  11   c  of the light guide panel  11 , and is provided with a plurality of LEDs  16  as a point light source. 
         [0032]    Light beams L emitted from the respective LEDs  16  enter the light guide panel  11  through the end surface  11   c  of the light guide panel  11 . An upper surface functions as a light exit plane  11   a  for outputting the light beams, and the lower surface functions as a reflecting plane  11   b  that reflects the light beams in the light guide panel  11 . The light beams L change the direction by repeating reflection between the light exit plane  11   a  and the reflecting plane  11   b  of the light guide panel  11 , and go out through the light exit plane  11   a  to the outside. The outgoing light beams L proceed toward the liquid crystal display panel  20 . 
         [0033]    The illumination device  10  further includes a diffusing sheet  12  and a prism sheet  13 . The diffusing sheet  12  and the prism sheet  13  are installed between the light guide panel  11  and the liquid crystal display panel  20 , and are installed from the side of the light exit plane  11   a  of the light guide panel  11  in sequence. The light beams L outputting from the light guide panel  11  pass through the diffusing sheet  12  and the prism sheet  13 , and then pass through the liquid crystal display panel  20 , so that illumination is achieved. 
         [0034]    The diffusing sheet  12  basically has a role to diffuse the light beams L outputting from the light guide panel  11  to all directions. The diffusing sheet  12 , being different from a general diffusing sheet, is applied with a plurality of beads formed of ultra-fine particles on a surface thereof, and has a high light converging function in comparison with general diffusing sheet because the beads function as, so-called, microlenses to converge incoming light beams. Through the usage of the diffusing sheet  12  according to the embodiments of the invention, the luminance of the light beams outputting from the diffusing sheet  12  is enhanced, and the luminance of a display screen of the liquid crystal device  100  is enhanced. 
         [0035]    The prism sheet  13  has a role to converge the light beams L onto the liquid crystal display panel  20 . The prism sheet  13  is formed with prisms having a triangular cross-section on the surface on the side of the liquid crystal display panel  20 , that is, on the surface opposite from the light guide panel  11 . The ridge lines of the prisms are indicated as “PL”. 
         [0036]      FIG. 2  is a plan view showing a positional relation between the light guide panel  11  and the prism sheet  13 . As shown in  FIG. 2 , the ridge lines of the prisms PL are substantially vertical to the direction of emission of the light beams L emitted from the LEDs  16  of the light source  15 . In other words, the prism sheet  13  has a shape having prisms whose ridge lines are extending in the direction vertical to the direction of emission of the light beams L. 
         [0037]    In the liquid crystal device  100  according to this embodiment, as will be described in detail later, an apex angle β of the prism on the prism sheet  13  is set within a range from 60° to 70°. In this arrangement, a desirable liquid crystal device having a desired luminance in a wide angular range in comparison with the general liquid crystal device is provided. 
         [0038]    In the liquid crystal device  100  according to the embodiment, as will be described in detail later, the light guide panel  11  is formed with grooves having triangular cross-section on the reflecting plane  11   b  as shown in  FIG. 1 , and an angle of inclination α of each triangular groove on the side of the light source  15  is set to a range from 0.1 to 5°. In this configuration, the peak of luminance may be concentrated on the area around the center of the screen. 
         [0039]    Change of Visual Angle and Luminance according to the Configuration of the Device 
         [0040]    Referring now to  FIG. 3A  to  FIG. 8B , a change of visual angle and the luminance according to the configuration of the liquid crystal device will be described.  FIG. 3A  to  FIG. 5B  show liquid crystal devices in various configurations, and  FIG. 6A  to  FIG. 8B  are graphs showing the relation between the visual angle and the luminance in the liquid crystal devices in various configurations. In the graphs from  FIG. 6A  to  FIG. 8B , solid lines indicate the relation between the visual angle and the luminance in the lateral direction of the screen, and broken lines indicate the relation between the visual angle and the luminance in the vertical direction of the screen. Here, the lateral direction of the screen corresponds to the lateral direction of paper planes of  FIG. 3A  to  FIG. 5B , and the vertical direction of the screen corresponds to the vertical direction of the paper planes of  FIG. 3A  to  FIG. 5B . The term “visual angle” means an angle at which the observer can view the liquid crystal device  100  in the lateral direction of the screen, and in the liquid crystal device, it is highly necessary to widen the angular range with a high luminance in the lateral direction of the screen. Therefore, in the description given below, the relation between the visual angle and the luminance in the lateral direction of the screen will be described. 
         [0041]      FIG. 3A  shows a configuration of a general liquid crystal device  100   a.    
         [0042]    In the general liquid crystal device  100   a , being different from the liquid crystal device  100  according to the embodiment described above, a general diffusing sheet  12   a  is arranged instead of the diffusing sheet having the converging function and two general prism sheets  13   a  having prisms whose apex angle β1 is 90° are arranged instead of the prism sheet  13  having prisms whose apex angle β is in the range from 60° to 70°. There, the two prism sheets  13   a  are laid one on top of another so that the ridge lines of the prisms extend orthogonally to each other. In  FIG. 3A , the prism sheet  13   a  having prisms extending in the lateral direction of the paper plane and the prism sheet  13   a  having prisms extending in the vertical direction of the paper plane are laid one on top of another. 
         [0043]    In the general liquid crystal device  100   a , being different from the liquid crystal device  100  according to the embodiment described above, a light guide panel  11   x  formed with grooves on the reflecting plane, having triangular cross-section and having an angle of inclination α1 on the side of the light source  15  set to 25°, is arranged instead of the light guide panel  11  described above. 
         [0044]      FIG. 6A  shows the relation between the visual angle and the luminance in the general liquid crystal device  100   a . As seen in  FIG. 6A , the luminance of the general liquid crystal device  100   a  abruptly drops at a visual angle of ±25° in the lateral direction of the screen. That is, in the general liquid crystal device  100   a , the visual angle which makes the luminance characteristics effective is only in the range of ±25°. 
         [0045]      FIG. 3B  shows a configuration of a liquid crystal device  100   b  which corresponds to the liquid crystal device  100   a  but there is only one prism sheet  13   a.    
         [0046]    More specifically, the liquid crystal device  100   b  corresponds to the liquid crystal device  100   a  but the prism sheet  13   a  having the prisms extending in the lateral direction of the paper plane is removed. Therefore, the liquid crystal device  100   b  only has the prism sheet  13   a  having the prisms extending in the vertical direction of the paper plane as the prism sheet. 
         [0047]      FIG. 6B  shows the relation between the visual angle and the luminance in the liquid crystal device  100   b . As seen in  FIG. 6B , the luminance of the liquid crystal device  100   b  abruptly drops at a visual angle of ±60° in the lateral direction of the screen. That is, in the liquid crystal device  100   b , the effective visual angle is on the order of ±60°, and hence the range of visual angle which achieves desired luminance is increased in comparison with the liquid crystal device  100   a.    
         [0048]      FIG. 4A  shows a configuration of a liquid crystal device  100   c  which corresponds to the liquid crystal device  100   b  but the diffusing sheet  12   a  is replaced by the diffusing sheet  12 .  FIG. 7A  shows the relation between the visual angle and the luminance in the liquid crystal device  100   c . When  FIG. 7A  and  FIG. 6B  are compared, it is seen that the luminance in the range of the effective visual angle ±60° may be improved by arranging the diffusing sheet  12  having a high converging function. 
         [0049]      FIG. 4B  shows a configuration of a liquid crystal device  100   d  which corresponds to the liquid crystal device  100   c  but the prism sheet  13   a  is replaced by the prism sheet  13 .  FIG. 7B  shows the relation between the visual angle and the luminance in the liquid crystal device  100   d . In this embodiment, the apex angle β of the prism sheet  13  is set to 63°. 
         [0050]    As seen in  FIG. 7B , the luminance of the liquid crystal device  100   d  is different from the luminance of the liquid crystal device  100   c , and does not drop even at a visual angle exceeding the range of ±60° in the lateral direction of the screen. For example, the luminance of the liquid crystal device  100   d  does not drop at a visual angle near ±80° in the lateral direction of the screen. As will be seen from the description above, by changing the apex angle of the prism on the prism sheet from 90° to 63°, the range of the visual angle which can provide a desired luminance in the lateral direction of the screen of the liquid crystal device may be widened in comparison with the general liquid crystal device  100   a . The range of the value of the apex angle β of the prism which can widen the visual angle may be a certain range including 63°, and the range of the visual angle which provides a desirable luminance as described above may be widened when the range is from 60° to 70°. 
         [0051]      FIG. 5A  shows a configuration of the liquid crystal device  100  according to the embodiment. The liquid crystal device  100  has a configuration which corresponds to the liquid crystal device  100   d  but the light guide panel  11   x  is replaced by the light guide panel  11 .  FIG. 8A  shows the relation between the visual angle and the luminance in the liquid crystal device  100 . In the embodiment, the angle of inclination α of the grooves having a triangular cross-section formed on the reflecting plane  11   b  of the light guide panel  11  is set to 3°. 
         [0052]    In  FIG. 7B  described above, the peak of the luminance exists in the area near the center of the screen, that is, the visual angle exists near 0°. In addition, a peak demonstrating a high luminance exists also near the visual angle of 60°. Basically, since the observer views the center of the screen in many cases, it is ideal to concentrate the peaks of the luminance in the area near the center of the screen. As seen in  FIG. 8A , in the liquid crystal device  100 , peaks of the luminance near 60° are restrained, and the peaks of the luminance may be concentrated in the area near the center of the screen, so that the peaks of the luminance may be increased in the area near the center of the screen. In this manner, by setting the angle of inclination of each groove, being formed on the reflecting plane of the light guide panel and having a triangular cross-section, on the side of the light source  15  to a range from 25° to 3°, the peaks of the luminance can be concentrated to the area near the center of the screen. The range of the value of the angle of inclination α in which the peaks of the luminance are concentrated to the area near the center of the screen may be a certain range including 3°, and in a range from 0.1° to 5°, the peaks of the luminance may be concentrated in the area near the center of the screen. 
       Modification 
       [0053]      FIG. 5B  shows a liquid crystal device  100   e  as a modification of the liquid crystal device  100  in the embodiment. The liquid crystal device  100   e  corresponds to the liquid crystal device  100  but a prism sheet  13   b  having a plurality of beads applied on the surface of the prism sheet  13  on the side of the entrance plane, that is, on the surface on the side of the light guide panel  11  is arranged instead of the diffusing sheet  12  and the prism sheet  13 .  FIG. 8B  shows the relation between the visual angle and the luminance in the liquid crystal device  100   e.    
         [0054]    As seen in  FIG. 8B , in the liquid crystal device  100   e  according to the modification as well, the prism sheet  13   b  has the converging function, and hence the luminance in the range of the effective visual angle may be improved as in the case of the liquid crystal device  100  in this embodiment. In the liquid crystal device  100   e  according to the modification, since the diffusing sheet is not provided, the thin profile is achieved correspondingly. 
         [0055]    In the respective embodiments shown above, the light guide panel formed with the grooves having a triangular cross-section is used as the light guide panel on the reflecting plane. However, the light guide panel is not limited thereto, and the light guide panel of a wedge shape may also be employed. In this case, when the angle of inclination of the reflecting plane of the light guide panel is set to a range of 0.1° to 5°, the peaks of the luminance are concentrated to the area near the center of the screen as described above. 
         [0056]    In the embodiments shown above, the liquid crystal display panel is employed as the display panel, the invention is not limited thereto, and other display panels such as electrophoresis display panel may be employed as the display panel. 
       Electronic Apparatus 
       [0057]    Referring now to  FIG. 9 , an embodiment of an electronic apparatus to which the liquid crystal device  100  is according to the embodiment of the invention is applicable. 
         [0058]    An example in which the liquid crystal device  100  according to the embodiments of the invention is applied to a display unit of a portable personal computer (so-called, notebook personal computer) will be described.  FIG. 9A  is a perspective view showing a configuration of the personal computer. As shown in the same drawing, a personal computer  710  includes a main body  712  having a keyboard  711  and a display unit  713  in which the liquid crystal device  100  according to the embodiments of the invention is applied. 
         [0059]    Subsequently, an example in which the liquid crystal device  100  according to the embodiment of the invention is applied to a display unit of a mobile phone.  FIG. 9B  is a perspective view showing a configuration of the mobile phone. As shown in the drawing, a mobile phone  720  includes a plurality of operating buttons  721 , an ear piece  722 , a mouth piece  723  as well as a display unit  724  to which the liquid crystal device  100  according to the embodiments of the invention is applied. 
         [0060]    Examples of the electronic apparatus to which the liquid crystal device  100  according to the embodiments of the invention is applicable includes liquid crystal televisions, view finder type or monitor direct-view video tape recorders, car navigation systems, pagers, electronic data books, calculators, word processors, workstations, TV telephones, POS terminals and digital still cameras in addition to the personal computer shown in  FIG. 9A  and the mobile phone shown in  FIG. 9B .