Abstract:
An improved liquid crystal display device of the type having a liquid crystal display panel, a light guide plate below a surface of the display panel and located between a reflecting portion and a diffusing portion, and a light collecting lens located at one end of the light guide plate for collecting and transmitting light to the light guide plate for providing backlight to the display panel. The display device includes a light transmitter located between the light collecting lens and the light guide plate. The light transmitter optically connects the light collecting lens and the light guide plate and separates the light collecting lens from the display panel by a predetermined difference. The light transmitter allows light to be more evenly distributed or reflected onto the display panel so that images displayed on the panel have a more uniform brightness.

Description:
The present invention relates to liquid crystal displays, and more particularly, to liquid crystal displays having mechanisms that collect ambient light to illuminate the display. 
     A liquid crystal display (LCD) includes pairs of opposing transparent electrodes, and liquid crystal, which is contained between the electrodes. The liquid crystal is electro-optically anisotropic. The application of a predetermined voltage between the electrodes forms an electric field with the liquid crystal. This causes the liquid crystal to exhibit optical properties that correspond to the field density. The LCD has a plurality of pixels. An image is formed by applying a different voltage to each pixel so that the pixel obtains the desirable brightness. A typical LCD employs a backlight to illuminate the display from behind. 
     In addition to a compact and thin profile, an LCD has low power consumption. Thus, LCDs are often used in office automation equipment and audio visual equipment. Portable equipment having LCDs are often used outdoors, where ambient light is abundant. In such cases, the ambient light may be used in lieu of the backlight to further decrease power consumption by a significant amount. 
     A prior art LCD will now be described with reference to FIG. 1, which is a cross-sectional view showing an LCD  100 . 
     The LCD  100  has a display panel  10 , a backlight  80  arranged behind the display panel  10 , and a case  70  housing the display panel  10  and the backlight  80 . The backlight  80  includes a light guide plate  20 , a light source  50  arranged on one end of the light guide plate  20 , a collecting lens  60  arranged on the other end of the light guide plate  20 , a diffusing portion  30  arranged on a front side of the light guide plate  20 , and a reflecting portion  40  arranged below or on the rear side of the light guide plate  20 . 
     The light source  50 , which may be a fluorescent lamp, is encompassed by a reflector  51 . The light guide plate  20  may be made of acrylic resin. A convex lens, which is formed separately from the light guide plate  20  or integrally with the light guide plate  20 , may be employed as the collecting lens  60 . The case  70  has an opening  71 , which is located at a position corresponding to the collecting lens  60  which allows the collecting lens  60  to project outward through the opening  71  to allow the collecting lens  60  to collect light from outside of the case  70 . 
     Light, which is emitted by the light source  50  or collected by the collecting lens  60 , is transmitted to the light guide plate  20 . The light is then refracted by the light guide plate  20  toward the diffusing plate  30  or the reflecting portion  40 . The reflecting portion  40  may be a plate, a film or a coating including a reflecting surface. The diffusing portion  30  may also be a plate, a film or a coating including a diffusing surface. The reflecting portion  40  reflects the light toward the diffusing portion  30  through the light guide plate  20 . The diffusing portion  40  diffuses the light so that some of the light forms an even light plane that is irradiated toward the display panel  10 , while the remaining light is returned toward the light guide plate  20  to be reflected again by the reflecting portion  40 . Accordingly, the intensity of the light attenuates as it bounces back and forth horizontally between the diffusing portion  30  and the reflecting portion  40 . 
     The display panel  10  does not emit light by itself. Thus, the display panel  10  is illuminated from behind. The light permeability of the display panel  10  is controlled in order to distribute the light passing through the display panel  10  in a desired pattern so that images can be visualized on the display panel  10 . 
     The light source  50  is lit to illuminate a bright image on the display panel  10  when ambient light is insufficient, such as during indoor usage. The light source  50  is turned off when ambient light is abundant, such as during outdoor usage on a sunny day. Under such conditions, the light collected by the collecting lens  60  is used to illuminate the display panel  10 . Thus, although power for driving the display panel  10  is still necessary, power for driving the light source  50  becomes unnecessary. This reduces the total power consumption of the LCD  100 . 
     The relationship between brightness relative to positions on the display panel  10  during sole usage of the collecting lens  60 , is shown in the graph of FIG.  2 . The horizontal axis represents the lateral position on the display panel  10 , while the vertical axis represents the brightness. Position A indicates the illuminated right end on the display panel  10  that is closest to the collecting lens  60  and position B indicates the illuminated left end of the display panel  10 . The light collected by the collecting lens  60  travels from position A to position B, as viewed in the graph of FIG.  2 . As shown in the graph, there are several brightness peaks. The value of the peak closest to position A is maximum, and the values of the other peaks decrease as the distance from position A increases. 
     The distance between adjacent peaks is determined by the size of the illuminated portion on the display panel  10 . For example, in a two inch LCD, the width of which is 45 millimeters, the distance between adjacent peaks is approximately 15 millimeters. In a fifteen inch LCD, the width of which is 190 millimeters, the distance between adjacent peaks is approximately 60 millimeters. Such peaks, or brightness fluctuation, results in the illumination of visual brightness stripes and lowers the quality of the En image illuminated on the display panel  10 . 
     Accordingly, it is an objective of the present invention to provide an LCD having uniform brightness. 
     SUMMARY OF THE INVENTION 
     To achieve the above objective, the present invention it provides a liquid crystal display comprising a liquid crystal display panel, a light guide plate arranged adjacent to the display panel, a light transmitter optically connected to the light guide plate, and a light collector optically connected to the light transmitter. 
     Other aspects and advantages of the present invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention, together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which: 
     FIG. 1 is a cross-sectional view showing a prior art liquid crystal display; 
     FIG. 2 is graph showing the relationship between brightness relative to positions on the display panel of FIG. 1; 
     FIG. 3 is a cross-sectional view showing a liquid crystal display according to a first embodiment of the present invention; 
     FIG. 4 is a graph showing the relationship between brightness relative to positions on the display panel of FIG. 3; 
     FIG. 5 is a cross-sectional view showing a liquid crystal display,according to a second embodiment of the present invention; and 
     FIG. 6 is a cross-sectional view showing a liquid crystal display according to a third embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the drawings, like numerals are used for like elements throughout. 
     A liquid crystal display (LCD)  110  according to a first embodiment of the present invention will now be described with reference to FIGS. 3 and 4. The following description will mainly center on parts of the LCD  110  differing from the description of the prior art LCD  100 . The LCD  110  includes a collecting lens  61  and a display panel  10  that are separated from each other by a predetermined distance “l”. As shown in FIG. 3, the LCD  110  has a liquid crystal display panel  10 , a backlight  80 , and a case  72 , which houses the display panel  10  and the backlight  80 . The backlight  80  includes a light guide plate  20 , a light source  50 , the collecting lens  61 , a diffusing portion  30 , and a reflecting portion  40 . The display panel  10 , the backlight  80 , the light guide plate  20 , the light source  50 , the diffusing portion  30 , and the reflecting portion  40  are all conventional components, which were described in the description of the prior art. The case  72  and the collecting lens  61 , although similar to the prior art case  70  and lens  60 , have been modified as described below. A light transmitter  21  is arranged between the collecting lens  61  and the display panel  10 . The light transmitter  21  has reflecting portions  41  and  42  on its top and bottom surfaces, respectively. 
     The display panel  10  faces upwards, as viewed in FIG. 3, and includes pairs of transparent electrodes (not shown), and liquid crystal, which is contained between the electrodes. However, the display method of the display panel  10  is not limited and may employ other methods. For example, thin-film transistors using a polycrystal semiconductor such as polysilicon may be employed by the display panel  10 . In this case, it is desirable that a driver, which includes display pixels formed integrally on the same substrate and a peripheral driver integrated circuit (IC), be incorporated in the display panel  10 . This eliminates the necessity for attaching the driver IC to the exterior of the display panel  10  and decreases the size of the frame of the display panel  10 . As a result, a more compact and light LCD  110 , which is optimal for portable equipment, can be manufactured. 
     The diffusing portion  30  is located below the display panel  10 , and the light guide plate  20  is located below the diffusing portion  30 , as viewed in FIG.  3 . The light guide plate  20  is preferably made of a material having high transparency, such as acrylic resin, polycarbonate, or glass. It is preferable that the length of the light guide plate  20  be substantially the same as the length of the display panel  10 , and that the light guide plate  20  extend no more than five millimeters from the display panel  10 . The reflecting portion  40  is located at the lower side of the light guide plate  20 , as viewed in FIG.  3 . Small recesses (not shown) are formed in the surface of the reflecting portion  40  so that light is reflected evenly by the reflecting portion surface. The reflectance and reflecting angle of the reflecting portion surface depends on the size and quantity of the recesses and differs between locations. More specifically, the recesses are formed such that the reflectance of the reflecting portion  40  is increased at positions where the brightness is low, and such that the reflectance is decreased at positions where the brightness is high. Furthermore, the reflecting angle is adjusted such that light is reflected toward positions on the display panel  10  where the brightness is low. 
     The light transmitter  21  separates the light collecting lens  61  from the display panel  10  by a predetermined distance and optically connects the light collecting lens  61  with the light guide plate  20 . The light transmitter  21  can be formed integrally with or separately from the light guide plate  20 . 
     The case  72  has an opening  71 , through which the light collecting lens  61  projects. The light collecting lens  61  is formed integrally with or separately from the right end of the light transmitter  21 . Furthermore, the light collecting lens  61 , which preferably extends generally perpendicular to the plane of FIG. 3, has a convex shape to allow ambient light to converge toward the light transmitter  21 . The light collecting lens  61  has a generally arcurate surface to form a convex shape. The light collecting lens  61  preferably has a diameter, or height that is greater than the thickness of the light guide plate  20  to increase the amount of light it collects. Furthermore, the light collecting lens  61  has an inclined surface that extends diagonally upward from the light transmitter  21 , as viewed in FIG.  3 . 
     The reflecting portions  41 ,  42  extend longitudinally from one end of the light transmitter  21 , which is connected with the light guide plate  20 , to the other end of the light transmitter  21 , which is connected with the light collecting; lens  61 . Thus, except for the end faces that are connected with the light guide plate  40  and the light collecting lens  61 , the light transmitter  21  is preferably completely encompassed by the reflecting portions  41 ,  42 . The reflecting portions  41 ,  42  preferably have even surfaces and do not have small recesses like the reflecting portion  40 . The reflecting portion  41  further extends along and encompasses the display panel side, or the inclined surface, of the light collecting lens  61 . A light source  50  is arranged at the left side of the light guide plate  40 . The light source  50  is lit when ambient light is insufficient, such as during nighttime or indoor usage. The light source  50  is encompassed by a reflector  51  to efficiently direct light toward the light guide plate  20 . The case  71 , although similar to the prior art case  70 , has been modified to enclose the light transmitter  21 , more specifically, the case  71  is longer than the case  70 . 
     The ambient light collected by the light collecting lens  61  is transmitted toward the light guide plate  20  by the light transmitter  21 . The incident light is reflected toward the diffusing portion  30  by the reflecting portion  40  to illuminate the display panel  10  from behind. The reflecting portions  41 ,  42  guide the light in the light transmitter  21  toward the light guide plate  20  without substantially diffusing and attenuating the light. The if light guide plate  20  then reflects the light upward, as viewed in FIG. 3, with the reflecting portion  40  and diffuses the light with the diffusing portion  30 . Some of the light is used to illuminate the display panel  10 , while the remaining light is reflected back toward the reflecting portion  40 . As the light bounces back and force, the light proceeds toward the left end of the display panel  10 . This illuminates a substantially even plane of light on the entire surface of the display panel  10 . As a result, an image is illuminated with uniform brightness on the display panel  10 . In FIG. 3, the bold arrows represent the direction in which light travels. 
     As shown in the graph of FIG. 4, the brightness of the image illuminated on the display panel  10  is more uniform than the images illuminated by the prior art LCD  100 . Position A indicates the right end of an image illuminated on the display panel  10  that is closest to the collecting lens  61  and position B indicates the left end of an image illuminated on the display panel  10 . The light collected by the collecting lens  61  travels toward the left from position A, as viewed in the graph of FIG.  4 . As shown in the graph, there are several brightness peaks. However, in comparison to the peaks of the prior art LCD  100  shown in FIG. 2, the peaks are included within a more narrow range and the average brightness is amplified. This is because the light collected by the light collecting lens  61  is made to be uniform when passing through the light transmitter  21 . More specifically, ambient light is collected from all directions by the light collecting lens  61  and directed toward the light guide plate  20 , while being reflected back and forth by the reflecting portions  41 ,  42 . The light is made substantially uniform in the light transmitter  20  by the repetitive reflection between the reflecting portions  41 ,  42 . Therefore, the brightness at position A, which is closest to the light collecting lens  61 , is prevented from becoming significantly high in comparison to other positions. This increases the overall brightness of the illuminated image. 
     The uniform brightness characteristic, which is apparent from the graph of FIG. 4, differs in accordance with the length of the light transmitter  21 . Experiments have been conducted to confirm the optimal length of the light transmitter  21  that obtains the desirable brightness characteristic. The experiments have been conducted to confirm the optimal length of the light transmitter  21  in relation to the LCD dimensions. For example, a two inch LCD (the display of which has a length L of about 45 millimeters and a height of about 50 millimeters) requires the light transmitter  21  to have a length l of at least approximately five millimeters to obtain the desirable brightness characteristic. The length l of the light transmitter  21  is preferably 10 to 20 millimeters and optimally 15 millimeters. Further, the length l of the light transmitter  21  depends more on the length of the display than on the width (width the direction extending perpendicular to the plane of FIG. 3) of the display. Some of the light passing through the light transmitter  21  is absorbed by the light transmitter  21 . Thus, it is preferable that the light transmitter  21  be shorter than 25 millimeters to prevent a decrease in the brightness of the image illuminated on the display. The ratio between the length l of the light transmitter  21  and the length L of the display panel  10  (l:L) is preferably 1:10 to 5:9, more preferably 2:4 to 4:9, and optimally 1:3. However, in a large LCD, the optimal ratio of 1:3 may result in an excessively long light transmitter  21 . In such cases, the length of the light transmitter  21  may be impacted by other design considerations, within a range that obtains the desirable brightness characteristic. 
     A second embodiment of the present invention will now be described with reference to FIG.  5 . In the second embodiment, an LCD  120  employs a light transmitter  22 , which shape differs from that employed in the first embodiment, and a larger light collecting lens  62 . 
     The light transmitter  22  has an oblique front surface  24 , which is inclined toward the liquid crystal display panel  10 . In addition, the light transmitter  22  does not have a uniform thickness, and is more thick at positions closer to the light collecting lens  62 . The enlarged light collecting lens  62  collects a larger amount of light than that employed in the first embodiment. Except for the end faces that are connected with the light guide plate  20  and the light collecting lens  62 , the light transmitter  22  is completely encompassed by reflecting portions  43 ,  44 . 
     Light is collected by the light collecting lens  62  and transmitted to the light transmitter  22  and reflected back and forth between the reflecting portions  43 ,  44  as the light advances toward the light guide plate  20 . The inclination angle of the front surface  24  of the light transmitter  22  may be changed for different devices, in which the LCD  120  is to be incorporated. 
     The light collecting lens  62  may be replaced by a light collector  63  having a substantially flat outer surface  64 , such as that shown in FIG.  6 . FIG. 6 is a cross-sectional view of a third embodiment of an LCD  130  in accordance with the present invention. In the LCD  130 , the light collector  63  is arranged in the opening  71  of the case  74  and is generally in the shape of a right triangle and the hypotenuse is exposed to ambient light. The light collector  63  has a flat inner surface  65 , which is connected with the light transmitter  23 , and an inclined outer surface  64 , which is inclined relative to the inner surface  65  by a predetermined angle θ. The light collector  63  does not necessarily have to be formed separately from the light transmitter  23 . In other words, the light collector  63  may be formed integrally with the light transmitter  23 . In such case, the surface  65  is a hypothetical plane extending perpendicular to the light guide plate  20 . Horizontal, incident light is refracted by the outer surface  64  of the light collector  63 . Thus, horizontal light is also reflected between the reflecting portions  43 ,  44 . The inclination angle θ guarantees refraction of ambient light toward the light collector  63 . Thus, light is transmitted toward the light transmitter  23 . This generates an image that is illuminated with uniform brightness. The preferable inclination angle θ of the outer surface  64  is about 43 degrees. However, the inclination angle θ may be more or less than 43 degrees. 
     The thickness of the light transmitter  23  may be decreased to obtain the desirable uniform bright properties when the light transmitter  23  is short. However, as shown in FIG. 6, it is desirable that the light transmitter  23  have a length that permits the light entering through a middle portion of the light collector  63  to be reflected by the reflecting portion  44  at least once. Furthermore, it is preferable that the opening  71  be completely filled by the light collector  63  without any gaps. This prevents the leakage of light and decreases the dimensions of the case  74 . 
     It should be apparent to those skilled in the art that the present invention may be embodied in many other specific: forms without departing from the spirit or scope of the invention. Therefore, the present examples and embodiments are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalence of the appended claims.