Abstract:
An auxiliary light source device for a reflective liquid crystal display device includes a light source; a light directing member for directing incident light from the light source toward the reflective liquid crystal display device, the directing member having upper and lower surfaces, the lower surface having a plurality of convex portions protruding from the lower surface of the display device, each of the convex portions having a substantially planar surface at its end, the planar surface of the convex portion being substantially parallel to the lower surface; and a light reflecting member which guides light from the light source into the light directing member.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a reflective liquid crystal display device, more particularly to a reflective liquid crystal display device having an auxiliary light source. 
     2. Description of Related Art 
     In general, liquid crystal displays are divided into transmissive liquid crystal display devices and reflective liquid crystal display devices according to whether the display uses an internal or outer light source. 
     While the transmissive liquid crystal display device uses an internal light source such as a backlight, the reflective liquid crystal display device uses ambient light and thus is affected by its surroundings. For example, the brightness of ambient light in an office differs largely from that outdoors. Also, even in the same location, the brightness of ambient light depends on the time of day (e.g., noon or dusk). An auxiliary light source device for the reflective liquid crystal display device is mainly used when its surroundings get dark. 
       FIG. 1  shows an auxiliary light source device for the reflective liquid crystal display device which has a light-directing member. The auxiliary light source device includes a light source  63 , a lamp reflector  65  and a light-directing member  67 . The auxiliary light source device is mounted on a liquid crystal display panel  61  having a reflector  71 . 
     The light source  63  is located adjacent to an end  67   a  of the light-directing member  67 , and most of the light source  63  is surrounded by the lamp reflector  65 . The end  67   a  constitutes a surface through which the incident light from the light source  63  is emitted. The light-directing member  67  also has upper and lower surfaces  67   b  and  67   c . A wall-reflector  69  is mounted along the side walls of the light-directing member  67  other than the emitting surface  67   a  in order to direct the incident light the incident light from the light source  63  is emitted. The light-directing member  67  also has upper and lower surfaces  67   b  and  67   c . A wall-reflector  69  is mounted along the side walls of the light-directing member  67  other than the emitting surface  67   a  in order to direct the incident light toward the liquid crystal display panel  61  disposed under the light-directing member  67 . 
     The emitted light from the light source  63  is directed toward the surface  67   a  of the light-directing member  67  and to the inside of the light-directing member  67  by the lamp reflector  65 . The light-directing member  67  directs the incident light toward the liquid crystal display panel  61 . The light transmitted to the liquid crystal display panel  61  is reflected by the reflector  71  and is emitted toward the observer  73 . 
     It should be noted that the auxiliary light source device is mounted on the liquid crystal display panel. Thus, there cannot be a diffusion plate and lens sheet between the liquid crystal display panel and the light-directing member, unlike in a transmissive liquid crystal display device. In order to produce a uniform emitted light distribution, the following four conditions are preferred for the auxiliary light source device of the reflective liquid crystal display device. 
     First, an incident light from the light source should be directed only toward the liquid crystal display panel and not the observer. Second, the incident light from the light source should be emitted perpendicular to the liquid crystal display panel. Third, the incident light from the light source should be distributed uniformly toward the liquid crystal display panel by the light-directing member. Fourth, the auxiliary light source device should be designed to minimize undesired effects such as a surface reflection and an angle distortion of the incident light from the auxiliary light source by the light-directing member. 
     Surface light source devices that have a diffusing plate and a lens sheet to increase the brightness are disclosed in U.S. Pat. Nos. 5,584,556 and 5,575,549. But these devices are used for the transmissive liquid crystal display, and the diffusion plate and the lens sheet disposed between the liquid crystal panel and the light-directing member, as explained previously, are not suitable for the reflective liquid crystal display device. 
     SUMMARY OF THE INVENTION 
     An object of this invention is to provide an auxiliary light source device of a reflective liquid crystal display device with a high light utilization efficiency and improved display characteristics. 
     In order to achieve these and other objects, the present invention provides an auxiliary light source device for a reflective liquid crystal display device having a reflector, the auxiliary light source device including a light source extending along a width of the reflector, to emit light along a length of the reflector, and a light directing device located above the reflector and adjacent to the light source to direct light from the light source to the reflector such that a light distribution of the directed light is substantially uniform along the length of the reflector, and such that the directed light is substantially perpendicular to the reflector. 
     According to another aspect of the invention, an auxiliary light source device for a reflective liquid crystal display device having a reflector includes an upper reflective surface to reflect impinging light above a certain incidence angle, a lower reflective surface having a plurality of convex portions extending toward the reflector to direct light from the auxiliary light source device to the reflector, and an entry surface connecting the upper and lower reflective surfaces through which light from a light source enters. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the invention and its advantages, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a sectional view showing a prior art structure of a reflective liquid crystal display device having an auxiliary light source device; 
         FIG. 2  is a schematic view illustrating a principle of light refraction in a light directing member having pyramid shaped convex portions; 
         FIG. 3  is a schematic view illustrating a principle of light refraction in light directing member according to an embodiment of the invention; 
         FIGS. 4A and 4B  are a schematic sectional view illustrating distribution of the convex portions of the light directing member according to the invention; 
         FIG. 5  is an enlarged view of the “V” portion of  FIG. 4A , showing a lower portion of the light directing member; and 
         FIGS. 6   a  to  6   c  are schematic perspective views of the directing member according to the modifications of the invention, where the views are upside down for better understanding. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In order to find an appropriate shape of the light directing member of the auxiliary light source device for the reflective liquid crystal display device, a light directing member having pyramid-shaped convex portions  102  formed on the lower surface thereof and oriented downward is examined, as illustrated in  FIG. 2 . 
     Since the lower surface  101   c  of the light-directing member  101  has a large number of pyramid-shaped convex portions  102  in order to increase the brightness, the direction of the incident light from the light source will be altered by these convex portions. Thus, if the size, shape and position of the pyramid-shaped convex portions  102  are appropriately selected, it is possible to produce a relatively uniform emitting light distribution on the liquid crystal display panel (not shown). Further, an arrangement of these convex portions  102  can be varied according to the amount of the emitting light to produce a uniform emitting light distribution. 
     In order to find the size, shape and position of these convex portions  102  suitable for producing a uniform emitting light distribution, several tests were performed. The lower surface  101   c  of the light-directing member  101  is a boundary between media  1  and  2 . The boundary has boundary surfaces A, B and C, and an angle θa exists between boundary surfaces A and C. An angle θb exists between boundary surfaces B and C and an angle θp exists between the boundary surfaces A and B. 
     In such a structure with pyramid-shaped convex portions  102 , the closer the angle θa or the angle θb become to 90°, the more perpendicularly the incident light is directed to the liquid crystal display panel. Namely, when the surface A or the surface B becomes perpendicular to the surface C, the angle θp between the surfaces A and B becomes zero and the emitting light is directed more perpendicularly toward the liquid crystal display panel. 
     Thus, in the structure shown in  FIG. 2 , the angle θp between the surfaces A and B is preferably smaller than 10°, but such a structure cannot be manufactured easily. 
       FIG. 3  shows a more practical shape of the lower surface of the light directing member, which has a main surface C. Surfaces A, B and D together define a convex portion oriented toward the lower reflector (not shown). The angles θa and θb between the surfaces A and C, and between the surfaces B and C, respectively, approach 90°, and the surface D is substantially parallel to the surface C. This convex portion of the lower surface, which can alter the incident angle of reflected light to an angle close to 90°, is relatively easy to manufacture. 
     Further, to produce a uniform emitting light distribution toward the liquid crystal display panel (not shown), it necessary to control the amount of the emitted light according to a distance from the light source on the lower surface  301   c  of the light directing member  301 . 
     In this structure, the surfaces C and D of the lower surface  301   c  and the upper surface  301   b  effect a total reflection for incidence angles above a certain value, and thus it is necessary to arrange the convex portions of the lower surface  301   c  properly. 
       FIG. 4A  shows paths of the emitting light in the light-directing member according to an embodiment of the present invention. In  FIG. 4A , light generated by a light source  503  is directed by reflector  505  to enter a side of light directing member  501 . The entering light reflects off of upper surface  502  and lower surface  509 , due to the presence of side wall reflector  521 . The lower surface  509  has a plurality of convex portions, a lower surface of which is shown as  511 . When light strikes a side of one of these convex portions, it is directed downward substantially perpendicular to the reflector  507 . 
     In order to distribute, uniformly, the emitting light on the liquid crystal display panel, if it is assumed that all sizes of the convex portions are same, it is required that a distance between adjacent convex portions becomes smaller as the convex portions become more distant from the light source. Alternately, a uniform distribution may be achieved by increasing a size of the convex portions with increasing distance from the light source. As shown  FIG. 4B , light generated by a light source is directed by reflector to enter a side of light directing member  601 . The entering light reflects off of upper surface and lower surface  609 , due to the presence of side wall reflector. The lower surface  609  has a plurality of convex portions  611  wherein a size of the plurality of convex portions  611  increases with increasing distance from the light source. When light strikes a side of one of these convex portions  611 , it is directed downward substantially perpendicular to the reflector. 
       FIG. 5  is an enlarged view of the “V” portion of  FIG. 4A , showing a lower portion of the light directing member. As shown in  FIG. 5 , it is preferable that an angle  523  between boundary surfaces  515  or  517  and a line perpendicular to a planar (which is a portion of the lower surface of  509  shown in  FIG. 4A ) is about between 0° and 10°. That is, it is desirable that an angle between boundary surfaces  515  or  517  and surface  513  is between 90° and 100°. 
     In addition, it is preferred that a width d 2  of the convex portions is less than 100 μm, a height  525  of the convex portions is less than 50 μm and a distance d 1  between two adjacent convex portions is variable between 10 μm and 1000 μm. However, other geometries will become apparent to those skilled in the art with the aid of this specification. 
       FIGS. 6   a  to  6   c  are perspective views showing several applicable shapes fabricated with reference to above-mentioned values, but illustrate the shapes of the convex portions upside down. That is, the “tops” of the surfaces shown in  FIGS. 6   a - 6   c  are oriented toward the lower reflector (e.g.,  507  in  FIG. 4A ). 
     As shown in the figures, the plane surface (corresponding to surface  511  in  FIG. 5 ) of the convex portions can be varied among, e.g., a circular shape  527 , a rectangular shape  529  and a bar shape  530 . Light from the light source enters the portions of the light directing members shown in  FIGS. 6   a - 6   c  at respective sides  540 . 
     As explained above, since the auxiliary light source device according to the invention has a light directing member having a plurality of convex portions on its lower surface, which are arranged closer as they are positioned more distant from the light source, the amount of the light emitted to the liquid crystal display panel is uniformly distributed. Furthermore, since the convex portions have substantially vertical surfaces, the light from the light source can be directed to the panel perpendicularly. 
     Although preferred embodiments and advantages thereof have been described heretofore, variations and changes are possible by the skilled in the art without departing from the spirit and scope of the invention, which will be indicated by the following claims.