Abstract:
An optical guide plate serves as a surface illuminant in a liquid crystal display, and includes a transparent optical guide body shaped in a trapezoidal prism, a line light source radiating light toward the bottom surface serving as an incident surface and a reflecting sheet opposed to a granulated diffusion surface extending between the incident surface and a top surface and opposed to a light output surface, wherein micro projections are formed on the granulated diffusion surface so as to form an air layer between the granulated diffusion surface and the reflection sheet without any direct contact between the granulated diffusion surface and the reflection sheet, thereby permitting the incident light to be propagated through the air layer.

Description:
FIELD OF THE INVENTION 
     This invention relates to a liquid crystal display and, more particularly, to a liquid crystal display equipped with an optical guide plate and a process for fabrication thereof. 
     DESCRIPTION OF THE RELATED ART 
     An optical guide plate is incorporated in a liquid crystal display together with a light source such as a lighting equipment or a back light. The light source radiates light, and the optical guide plate directs the light toward an array of pixels. The optical guide plate makes the light source serve as a surface illuminant. 
     A typical example of the optical guide plate has a diffusing surface, a light output surface and an incident surface. The diffusing surface is opposed to the light output surface, and the incident surface extends between the diffusing surface and the light output surface. The light source is attached to the incident surface, and the light is incident on the incident surface. Most of the light repeats the reflection at the boundary between the optical guide plate and the air due to the difference of optical density therebetween, and proceeds in the optical guide plate. While the light is proceeding, the light is irregularly reflected on an irregular reflecting means. When a light component is incident on the irregular reflecting means at the incident angle equal to or less than the critical angle, the light component is output from the light output surface. The irregular reflecting means is implemented by a white printed layer of diffusing material such as white ink. Otherwise, the diffusing surface is granulated or roughened so as to make it granulated. 
     User requests the manufacturer to make the surface illuminant thin. The granulated surface is suitable for the user&#39;s request. An example of the prior art optical guide plate with the granulated surface is disclosed in Japanese Patent Publication of Unexamined Application (laid-open) No. 2-176629. 
     FIG. 1 illustrates the prior art optical guide plate disclosed in the Japanese Patent Publication of Unexamined Application. The prior art optical guide plate comprises an optical guide body, a line light source  3 , a curved mirror  4  and a reflecting sheet  5 . The optical guide body  1  has a diffusing surface  2   a , a light output surface  2   b  and a side surface therebetween. The diffusing surface  2   a  is inclined with respect to the light output surface  2   b , and the distance therebetween is decreased from the side surface toward the other side surface. The side surface serves as an incident surface, and the line light source  3  is opposed to the side surface. The curved mirror  5  is attached to the optical guide body  1  so as to encapsulate the line light source  3  inside the space defined by the curved mirror and the side surface. The diffusing surface  2   a  is granulated, and the reflecting sheet  5  is attached to the granulated diffusion surface  2   a . The air is confined between the granulated diffusion surface  2   a  and the reflecting sheet  5 . 
     When the line light source  3  is energized, the line light source  3  radiates light, and the light is incident onto the side surface. The incident light proceeds in the prior art optical guide plate. The light radiated from the optical guide body  1  toward the reflecting sheet  5 , and is reflected thereon. The light is partially reflected on the diffusion surface  2   a . Thus, the light repeats the reflection and refraction at the boundary between the air and the optical guide body  1 , and proceeds toward the side surface opposite to the incident surface. The light is partially reflected on the diffusing surface  2   a , and is radiated from the light output surface  2   b.    
     A problem is encountered in the prior art optical guide plate in that the luminance on the light output surface  2   b  is gradually reduced toward the side surface opposite to the incident surface. Another problem is that the light output surface  2   b  is not uniform in brightness. 
     SUMMARY OF THE INVENTION 
     It is therefore an important object of the present invention to provide an optical guide plate, which has a light output surface constant in the luminance over the distance between an incident surface and the opposite surface and, accordingly, uniform in the brightness thereover. 
     It is also an important object of the present invention to provide a process for fabricating the optical guide plate. 
     The present inventor contemplated the problem inherent in the prior art optical guide plate, and found that the air was locally evacuated from between the optical guide body  1  and the reflecting sheet  5 . The non-uniform brightness and the reduction of luminance were derived from the local evacuation of the air as follows. The surface roughness of the diffusion surface  2   a  was 1 to 5 microns, and the optical guide body  1  tended to locally hold the reflecting sheet  5  in contact with the diffusion surface  2   a . The air was evacuated from between the part of the diffusion surface  2   a  and the reflecting sheet  5 . The total reflection took place on the part of the diffusion surface  2   a , and the part of the diffusion surface interrupted the optical propagation through the air. If there was the air constant over the boundary between the diffusion surface  2   a  and the reflecting sheet  5 , all the light was not taken into the optical guide body  1 , again, due to the difference in density between the optical guide body  1  and the air. Thus, the amount of light was reduced in the direction leaving the incident surface, and the luminance was reduced due to the shortage of the light. On the other hand, the total reflection locally increased the amount of output light, and made the brightness on the light output surface  2   b  non-uniform. 
     To accomplish the object, the present invention proposes to form micro-projections large enough to space the reflecting sheet from the diffusion surface. 
     In accordance with one aspect of the present invention, there is provided an optical guide plate used in a liquid crystal display comprising an optical guide body formed of a transparent material, and including an incident surface, a granulated diffusion surface for scattering light components, a light output surface for radiating the light components therefrom and micro-projections formed on the granulated diffusion surface and having peaks spaced from the granulated diffusion surface wider than the granulation, a reflecting sheet supported by the peaks of the micro-projections so as to form an air layer therebetween, and a light source opposed to the incident surface for supplying the light components to the optical guide body. 
     In accordance with another aspect of the present invention, there is provided a process for fabricating an optical guide plate, comprising the steps of forming a transparent optical guide body having an incident surface, a granulated diffusion surface, a light output surface and micro-projections formed on the granulated diffusion surface and having peaks spaced from the granulated diffusion surface wider than the granulation, and assembling a reflecting sheet and a light source with the transparent optical guide in such a manner that the reflecting sheet is supported by the peaks and that the light source is opposed to the incident surface. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features and advantages of the optical guide plate and the process of fabrication will be more clearly understood from the following description taken in conjunction with the accompanying drawings in which: 
     FIG. 1 is a cross sectional view showing the structure of the prior art optical guide plate disclosed in Japanese Patent Publication of Unexamined Application No. 2-176629; 
     FIG. 2 is a cross sectional view showing the structure of an optical guide plate according to the present invention; 
     FIG. 3 is a schematic view showing reflection in the optical guide plate; 
     FIG. 4 is a cross sectional view showing the structure of another optical guide plate according to the present invention; and 
     FIG. 5 is a cross sectional view showing the structure of yet another optical guide plate according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
     Referring to FIG. 2 of the drawings, an optical guide plate embodying the present invention largely comprises an optical guide body  11 , a line light source  3 , a concave mirror  4  and a reflecting sheet  5 . The optical guide body  11  is formed of acrylic resin, which is transparent and, accordingly, high in optical transmittance. The optical guide body  11  has two major surfaces  2   a / 2   b  and two side surfaces extending between the two major surfaces. The major surface  2   a  is inclined with respect to the other major surface  2   b , and the major surfaces  2   a / 2   b  and the side surfaces define a trapezoidal cross section. The thickness or the distance between the major surfaces  2   a / 2   b  is rightward decreased from 3 millimeters to 1 millimeter. 
     The major surface  2   a  is made granulated, and serves as a diffusing surface. The surface roughness of the major surface  2   a  or the depth from the master flat plane to the deepest valley ranges from 1 micron to 5 microns. The granulation is at low dense on the left side close to the line light source  3 , and gets higher in the direction leaving the line light source  3 . In other words, the granulation is increased from the incident surface toward the other side surface. 
     A plenty of micro semi-spherical projections  16  are formed on the major surface  2   a , and are fifty to a hundred per square centimeter. The micro semi-spherical projections  16  are 10 to 20 microns in height. The other major surface  2   b  serves as a light output surface, and the wide side surface serves as an incident surface. 
     The other major surface  2   b  serves as a light output surface, and is over-lapped with a glass substrate (not shown), which confines liquid crystal together with another glass substrate. 
     The line light source  3  extends along the wide side surface, and is spaced therefrom. A small-sized fluorescent lamp or a filament lamp is available for the optical guide plate. The concave mirror  4  is attached to the optical guide plate  1 , and an inner space is defined between the incident surface and the concave mirror  4 . The line light source  3  radiates light, and the light is confined in the space. The concave mirror  4  directs the light toward the incident surface, and all the light is incident on the incident surface. 
     The reflection sheet  5  is supported by the plenty of micro semi-spherical projections  16 , and is certainly spaced from the granulated surface  2   a  without any direct contact with the granulated surface  2   a  by virtue of the micro semi-spherical projections  16 . Thus, the air layer  7  intervenes between the entire granulated surface  2   a  and the reflection sheet  5 . Light is reflected on the reflection sheet  5 , and is partially taken into the optical guide body  1 , again. 
     The optical guide plate guides light toward the light output surface  2   b . FIG. 3 illustrates light components A, B, and C propagated from the line light source  3  through typical optical paths. The light components A, B and C behave as follows. 
     The light component A directly proceeds to the light output surface  2   b . Only a small amount of light component A is radiated from the light output surface  2   b , and most of the light component A is inwardly reflected on the light output surface  2   b , and is directed toward the diffusion surface  2   a . The light component A is scattered through the irregular reflection on the granulation of the diffusion surface  2   a , and light sub-components proceed toward the light output surface  2   b . However, part of the light component A may pass the diffusion surface  2   a . The part of the light component A proceeds to the reflection sheet  5 , and is reflected thereon. The behavior of the reflected light will be hereinlater described in detail. 
     The light components B and C proceed toward the diffusion surface  2   a , and pass the diffusion surface  2   a . The light components B/C may be partially scattered on the diffusion surface  2   a . The light components B and C proceed to the reflection sheet  5 , and is reflected on the reflection sheet  5 . 
     The reflected light components A/B/C repeat the reflection and the refraction so as to proceed through the air layer  7  in the direction leaving the incident surface. When the light component A/B/C is reflected on the diffusion surface  2   a , part of the light component A/B/C passes through the diffusion surface  2   a , and proceeds toward the light output surface  2   b . If the part of the light component A/B/C is incident on the light output surface  2   b  at an incident angle less than the critical angle, the part of the light component A/B/C is radiated from the light output surface  2   b . All the reflected light components A/B/C are propagated through the air layer  7  without any intervention of direct contact between the reflecting sheet  5  and the diffusion surface  2   a . As a result, the luminance is uniform over the light output surface  2   b , and the light output surface  2   b  achieves uniform brightness. Any reduction of luminance and any locally high brightness do not take place. 
     The optical guide plate is fabricated as follows. First, an injection molding die unit is prepared. A space is defined in the injection molding die unit, and is identical in configuration with the optical guide body  11 . The injection molding die unit has an inner granulated surface formed with micro semi-spherical recesses. The inner surface is corresponding to the major surface  2   a  of the optical guide body  11 . 
     The inner granulated surface with the micro semi-spherical recesses is formed as follows. The inner surface is granulated by using a standard technique. A honing machine (not shown) is opposed to the inner granulated surface of the molding die unit, and 60-mesh glass beads are impinged to the inner surface. The 60-mesh glass beads forms the micro semi-spherical recesses in the inner granulated surface. 
     Subsequently, the melted acrylic resin is injected into the injection molding die unit, and is spread in the space. The melted acrylic resin is solidified so as to form the optical guide body  11 . The granulation is transferred from the inner granulated surface of the injection molding die unit to the major surface  2   a  of the optical guide body  11 , and the micro semi-spherical recesses permit the major surface  2   a  to conformably project so as to form the micro semi-spherical projections  16 . 
     Finally, the reflection sheet S is attached to the optical guide body  1  in such a manner as to form the air layer  7 . Namely, the micro semi-spherical projections  16  support the reflection sheet  5  without any direct contact between the reflection sheet  5  and the granulated diffusion surface  2   a . The line light source  3  and the concave mirror  4  are assembled with the optical guide body  11 . The assembling work between the line light source /concave mirror  3 / 4  and the optical guide body  11  may be carried out prior to the assembling work between the reflection sheet  5  and the optical guide body  11 . 
     As will be understood from the foregoing description, the micro semi-spherical projections project from the granulated diffusion surface  2   a , and prohibits the reflection sheet  5  from any direct contact with the granulated diffusion surface  2   a . The air layer  7  is surely formed between the granulated diffusion surface  2   a  and the reflection sheet  5 , and the air layer  7  extends under the entire light output surface  2   b . The incident light is uniformly diffused over the optical guide body  11 , and the luminance and the brightnes&#39;s become uniform. 
     Second Embodiment 
     Turning to FIG. 4 of the drawing, another optical guide plate embodying the present invention comprises an optical guide body  21 . Other parts of the optical guide plate implementing the second embodiment are similar to those of the first embodiment, and are labeled with the references designating corresponding parts of the first embodiment without detailed- description. 
     The optical guide body  21  is formed of transparent material such as, for example, acrylic resin, and micro-projections  26  are formed on the granulated diffusion surface  2   a . The micro-projections  26  are shaped in a circular cone. The micro-projections  26  may be as high as the micro semi-spherical projections  16 , and are fallen within the same density range. The optical guide plate achieves all the advantages of the first embodiment. 
     Third Embodiment 
     FIG. 5 shows yet another optical guide plate embodying the present invention. The optical guide plate implementing the third embodiment comprises an optical guide body  31 . Other parts of the optical guide plate implementing the third embodiment are similar to those of the first embodiment, and are labeled with the references designating corresponding parts of the first embodiment without detailed description. The optical guide plate achieves all the advantages of the first embodiment. 
     The optical guide body  31  is formed of transparent material such as, for example, acrylic resin, and micro-projections  36  are formed on the granulated diffusion surface  2   a . The micro-projections  36  are shaped in a triangular prism. In this instance, the micro-projections  36  are 10 microns in height, 30 microns in width and 200 microns in length, and are arranged at pitches of 200 microns. 
     As will be appreciated from the foregoing description, the micro projections surely form the air layer  7  between the diffusion surface  2   a  and the reflection sheet  5 , and the air layer  7  permits the light to proceed toward the side surface opposite to the incident surface. This results in the uniform luminance and brightness. 
     Although particular embodiments of the present invention have been shown and described, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the present invention. For example, the micro-projections may have an appropriate configuration different from the semi-spherical configuration, the circular cone and the triangular prism. The optical guide body may be formed of another kind of transparent material.