Abstract:
The present invention provides a light guide plate having reduced hot spots comprising an input surface for receiving light from a plurality of discrete light sources, an output surface for emitting light, a bottom surface opposing to the output surface, and an end surface opposing to the input surface.

Description:
FIELD OF THE INVENTION 
     This invention generally relates to a light guide plate, and more particularly, to a light guide plate having a reversed one- or two-dimensional micro-pattern in its mixing zone to reduce undesirable hot spot defects caused by discrete light sources. 
     BACKGROUND OF THE INVENTION 
     Liquid crystal displays (LCDs) continue to improve in cost and performance, becoming a preferred display type for many computer, instrumentation, and entertainment applications. Typical LCD-based mobile phones, notebooks, and monitors include a light guide plate (LGP) for receiving light from a light source and redistributing the light uniformly across the light output surface of the LGP. The light source, conventionally being a long, linear cold-cathode fluorescent lamp, has evolved to a plurality of discrete light sources such as light emitting diodes (LEDs). For a given size LCD, the number of LEDs has been steadily decreasing to reduce cost. Subsequently, the pitch of the LEDs has become larger, which results in a more noticeable hot spot problem, that is, more light is distributed near each LED than between LEDs in the first few millimeters of the viewing area of the LCD. The hot spot problem occurs because light from the discrete LEDs enters the LGP non-uniformly, that is, more light is distributed near the LEDs than between the LEDs. 
     Many LGPs have been proposed to suppress the hot spot problem. Some LGPs have continuous grooves near their edge such as the ones disclosed in U.S. Pat. No. 7,097,341 (Tsai). Some LGPs have two sets of linear grooves of different pitches on their light output surface, some LGPs have two or more sets of dots of different sizes, and others may have both grooves and dots of different sizes. 
     While the prior art LGPs are capable of suppressing the hot spot problem to a certain degree, they are still not satisfactory due to the complexity in the mass production of those LGPs. Thus, there remains a need for a light guide plate that can be easily made and is capable of suppressing the hot spot problem. 
     SUMMARY OF THE INVENTION 
     The present invention provides a light guide plate having reduced hot spots comprising an input surface for receiving light from a plurality of discrete light sources, an output surface for emitting light, a bottom surface opposing to the output surface, and an end surface opposing to the input surface, wherein the direction from the input surface to the end surface is defined as Y-axis, the direction that is perpendicular to the Y-axis and parallel to the discrete light sources is defined as X-axis, the output surface has a plurality of elongated grooves running parallel to the Y-axis and extending from the input surface corresponding to Y=0 to the end surface, the bottom surface has a core zone extending from a predetermined line corresponding to Y=Y 1  to the end surface and a mixing zone extending from Y=0 to Y=Y 1 ; and a set of lenses are distributed in the core zone and a set of micro-lenses distributed in the mixing zone between Y=Y 0  and Y=Y 1 , wherein the density of the set of micro-lenses varies in the X-axis, having a maximum value at a first location that has a same X value as the center of one of the discrete light sources, and having a minimum value at a second location that has a same X value as the center of two adjacent discrete light sources, and the selected size and the density of the micro-lenses redirect the light from the discrete light sources toward the Y-axis and the ratio L 1 /L 0  is between 0.9 and 1.1 for any Y≧Y 1 . 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  shows a side view of an LCD comprising a plurality of optical components including a prior light guide plate; 
         FIG. 1B  shows a top view of the prior light guide plate; 
         FIG. 1C  shows that the prior light guide plate has prismatic grooves on its light output surface; 
         FIG. 1D  shows that the prior light guide plate has trapezoidal grooves on its light output surface; 
         FIG. 1E  shows that the prior light guide plate has lenticular lenses on its light output surface; 
         FIG. 1F  shows an image of a reverse hot spot problem resulted from the prior light guide plate; 
         FIG. 1G  shows an image of a normal hot spot problem resulted from another prior light guide plate; 
         FIG. 1H-1  to  1 H- 3  compares hot spot contrast between the reverse and normal hot spot problems; 
         FIG. 2A  shows a side view of an LCD comprising a plurality of optical components including a light guide plate of the present invention; 
         FIG. 2B  shows a bottom view of the light guide plate of the present invention; a reversed micro-lens pattern is distributed in part of the mixing zone; 
         FIG. 2C  shows a bottom view of a light guide plate according to a comparative example; a normal micro-lens pattern is distributed in part of the mixing zone; 
         FIG. 2D  shows a bottom view of the light guide plate of the present invention; a reversed and a normal micro-lens patterns are distributed in the mixing zone; 
         FIG. 3A  shows a comparison of the simulated hot spot ratio among an inventive example and two comparative examples; and 
         FIG. 3B  shows a comparison of the average of the simulated light flux among an inventive example and two comparative examples. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1A  shows schematically a side view of an LCD display apparatus  30  comprising an LCD panel  25  and a backlight unit  28 . Backlight unit  28  comprises a plurality of optical components including one or two prismatic films  20 ,  20   a , one or two diffusive films  24 ,  24   a , a bottom reflective film  22 , a top reflective component  26 , and a light guide plate (LGP)  10 . LGP  10  is different from the other optical components in that it receives the light emitted from one or more light sources  12  through its input surface  18 , redirects the light emitted through its bottom surface  17 , end surface  14 , output surface  16 , side surfaces  15   a ,  15   b  (not shown) and reflective film  22 , and eventually provides light relatively uniform to the other optical components. Output surface  16  has a plurality of elongated grooves  36 . Target luminance uniformity is achieved by controlling the density, size, and/or orientation of the lenses  100  (sometimes referred to as discrete elements, or light extractors) on the bottom surface  17 . The top reflective component  26  typically covers the LGP  10  for about 2 to 5 millimeters from the light input surface to allow improved mixing of light. The top reflective component  26  has a highly reflective inner surface  26   a . Top reflective component  26  may have a black outer surface  26   b , and is therefore referred to as “black tape”. Top reflective component  26  may also be any known reflector rather than a black tape. Typically the luminance of a backlight is evaluated from point A, which is at the end of top reflective component  26 , and proceeds through the viewing area to the opposite end of the LGP. LGP  10  has a first direction Y that is parallel to its length direction, and a second direction X (shown in  FIG. 1B ) that is parallel to its width direction. On both output surface  16  and bottom surface  17 , the area between the input surface (Y=0) of LGP  10  and line Y=Y 1  (passing through Point A) is often referred to as the mixing zone. The mixing zone consists of a top mixing zone  38   a  and a bottom mixing zone  38   b . The length between Y=0 and Y=Y 1  is referred to as the length of the mixing zone. The viewing area between line Y=Y 1  and end surface  14  is referred to as the core zone. In mixing zone  38   b  on bottom surface  17 , prior LGPs typically do not have any micro-lenses. When prior LGPs do have micro-lenses on (bumps) or in (holes) bottom mixing zone  38   b  to reduce the hot spot problem, the micro-lenses typically have a two-dimensional density distribution and the density of the two-dimensional micro-lenses is higher at the center distance between two adjacent light sources than at the center of each light source. 
       FIG. 1B  shows a top view of elongated grooves  36  on output surface  16 . Elongated grooves  36  extend from the beginning (Y=0) of LGP  10  to the end (Y=L) of LGP  10 , where L is the length of LGP  10 . As such, elongated grooves  36  extend through mixing zone  38   a  which is on the top or output surface. Elongated grooves  36  have a pitch P and are parallel within ±5° to the length direction of LGP  10 . However, elongated grooves  36  need not have a regular pitch. Also shown in  FIG. 1B  are three exemplary light sources  12   a ,  12   b ,  12   c , corresponding to the light source  12  shown in  FIG. 1A . Light sources  12   a ,  12   b  and  12   c  have a pitch of P 0 . 
     Elongated grooves  36  can be prismatic grooves  36   a  as shown in  FIG. 1C , trapezoidal grooves  36   b  as shown in  FIG. 1D , or lenticular lenses  36   c  as shown in  FIG. 1E . Each of the features has a height H, a width D, a pitch P, and a gap G, where the pitch P=D+G. The gap G varies from 0 to 2D. When gap G=0, the elongated grooves are closely packed. Elongated grooves may take other known shapes such as rounded prisms, prisms that vary in height along their length and the like. 
     Prior art LGP  10  has some advantages in having elongated grooves  36  on its output surface  16 . For example, elongated grooves  36  may hide cosmetic defects from lenses  100  on bottom surface  17 . However, prior art LGP  10  suffers from a hot spot problem. For example, when the pitch P of light sources  12  is 6.6 millimeters (mm), the mixing zone length is 4 millimeters, and elongated grooves  36  are lenticular lenses  36   c  having a height, H=11 microns, a width, D=50 microns, and a gap, G=0, the hot spot extends well into the viewing area. The hot spot is still visible at Y=7 millimeters. Thus prior art LGP  10  having elongated grooves on its output surface does not provide uniform luminance in the viewing area. 
       FIG. 1F  shows an image of a reverse hot spot problem resulting from prior art light guide plate  10  having elongated grooves  36  on its output surface  16  and having no micro-lenses in the bottom mixing zone.  FIG. 1G  shows an image of a normal hot spot problem resulting from another prior art light guide plate that is the same as light guide plate  10  without elongated grooves  36  on its output surface  16 . 
     A comparison between  FIG. 1F  and  FIG. 1G  reveals that the hot spot problems are clearly different for light guide plates with (see  FIG. 1F ) and without (see  FIG. 1G ) elongated grooves on their output surface. When the light guide plate does not have elongated grooves on its output surface (see  FIG. 1G ), the light flux L 0  along a line that passes through the center of a light source and extends along the Y-axis such as LINE 0 is always higher than the light flux L 1  along a line that passes midway between the center of two adjacent light sources and extends along the Y-axis such as LINE 1, for Y between Y 0  and Y 1 . This first type of hot spot will be referred to as “normal” hot spot hereinafter. The normal hot spot has been the target of prior hot spot reduction methods. 
     In comparison, when the light guide plate has elongated grooves on its output surface (see  FIG. 1F ), the light flux L 0  along LINE 0 is lower than the light flux L 1  along LINE 1 in at least an area defined between line Y=Y 0  and line Y=Y 1 . This second type of hot spot will be referred to as “reverse” hot spot hereinafter. 
       FIG. 1H-1  further explains why the reverse hot spot problem occurs when lenticular lenses are added to the output surface of a light guide plate. In this study, the light guide plates all have a mixing zone of 4 mm; the same size micro-lenses, 66 micrometers (μm) in width, are distributed in the core zone. The core zone extends from the end of the mixing zone, Y=4 mm, to the end surface. The light guide plates accept light from discrete light sources. The discrete light sources have a pitch of 7.5 mm, and an emission width of about 2.5 mm. No micro-lenses are located in the mixing zone. The lenticular lenses  36   c  in top mixing zone  38   a  on output surface  16  all have the same radius R=43.0625 μm and gap G=0 (See  FIG. 1E  for definitions). The light guide plates differ by the height H of lenticular lenses  36   c  on its output surface  16 . 
       FIG. 1H-1  shows plots of the hot spot ratio L 1 /L 0  for various H/R ratios, where H and R are the height and radius of lenticular lenses  36   c . L 0  and L 1  are the emitted light flux measured at the output surface  16  along the centerline of the discrete light source  12 , LINE 0, and the centerline between each light source  12 , LINE 1, respectively. A normal hot spot is evident when the ratio L 1 /L 0 &lt;1. The ratio L 1 /L 0 &gt;1 indicates a reverse hot spot, and the ratio L 1 /L 0 =1 indicates equal flux along LINE 0 and LINE 1. In practice, when the ratio L 1 /L 0  is between approximately 0.9 and 1.1, the hot spot may be acceptable depending upon the haze of diffusive films  24  and  24   a . In other words, the normal hot spot is noticeable when the ratio L 1 /L 0 &lt;0.9, while the reverse hot spot is noticeable when the ratio L 1 /L 0 &gt;1.1. In the following, the reverse hot spot is considered to exist when the ratio L 1 /L 0 &gt;1.1 for at least some Y between Y 0  and 2Y 1 , while the normal hot spot is considered to exist when L 1 /L 0 &lt;0.9 for at least some Y between Y 0  and 2Y 1 . 
       FIG. 1H-1  further shows that when the ratio of the height of the lenticular lens to the radius of the lenticular lens equals zero, H/R=0, that is, there is no lenticular lens, the normal hot spot extends to about Y=7.5 mm into the light guide plate. When the H/R ratio increases to 0.0012 (or H=0.05 μm, H/D=0.0120), some portion of L 1 /L 0  starts to exceed 1 for at least some Y between Y 0  and 2Y 1 . Note that 
                 H   D     =     1           2   ⁢   R     H     -   1     2         ,         
and D is the size of the lenticular lens as shown in  FIGS. 1C through 1E . When the H/R ratio increases to 0.1858 (or H=8 μm, H/D=0.1600), L 1 /L 0  exceeds 1 for Y between Y 0  and Y 1 , where Y 0  is determined from L 1 /L 0 =1. As the H/R ratio increases further, the ratio L 1 /L 0  becomes smaller. When the H/R ratio increases to 0.5806 (or H=25 μm, H/D=0.3298), the maximum of L 1 /L 0  just exceeds 1 for at least some Y between Y 0  and 2Y 1 . When the H/R ratio further increases to 0.8128 (or H=35 μm, H/D=0.4137), L 1 /L 0  is smaller than 0.6 for Y between 0 and 4 mm, and beyond. The curve for H/R=0 and the curve for HR=0.8128 are both examples of normal hot spot, where L 1 /L 0 &lt;0.9 for some Y between Y 1  and 2Y 1  and L 1 /L 0 &lt;1.1 for any Y between 0 and 2Y 1 . The curve for H/R=0.0012 and the curve of HR=0.1858 are also examples of normal hot spot, where L 1 /L 0 &lt;0.9 for some Y between Y 1  and 2Y 1  and L 1 /L 0 &lt;1.1 for any Y between 0 and 2Y 1 . The curve for H/R=0.1858 is an example of reverse hot spot because L 1 /L 0 &gt;1.1 for some Y between 0 and 2Y 1 . More specifically, the curve for H/R=0.1858 shows normal hot spot for Y between 0 and Y 0 , and for Y between about 5 mm and about 8 mm, and shows reverse hot spot for at least Y between Y 0  and Y 1 .
 
       FIG. 1H-2  and  FIG. 1H-3  are identical to  FIG. 1H-1  except that the pitch P 0  of the discrete light sources changes from 7.5 mm (in  FIG. 1H-1 ), to 6.6 mm (in  FIG. 1H-2 ), and to 5.5 mm (in  FIG. 1H-3 ). The general conclusions for  FIGS. 1H-2  and  1 H- 3  are the same as those for  FIG. 1H-1 . A comparison of  FIGS. 1H-1  through  1 H- 3  shows the curves for the H/R ratio change with the pitch P 0  of the discrete light sources. For example, for the same H/R=0.1858, Y 0  varies from about 2.2 mm in  FIG. 1H-1  to about 2.8 mm in  FIG. 1H-2 , and to about 1.6 mm in  FIG. 1H-3 .  FIGS. 1H-1  through  1 H- 3  show that the reverse hot spot exists when a light guide plate has certain elongated grooves on its output surface extending from the input surface to the end surface. Even though the examples of reverse hot spot are given for lenticular lenses having a H/R ratio between about 0.0012 and 0.5806, it is conceivable that other types of elongated grooves, as shown in  FIGS. 1C-1D , are also likely to cause reverse hot spot when their geometry, as defined by ratios such as H/R or H/D, is in a certain range. 
       FIG. 2A  shows schematically a side view of an LCD display apparatus  30   a  comprising an LCD panel  25  and a backlight unit  28   a . Backlight unit  28   a  is the same as backlight unit  28  shown in  FIG. 1A  except that backlight unit  28   a  includes an LGP  10   a  which has micro-lenses  110  in the mixing zone  38   b  on its bottom surface  17 , while backlight unit  28  includes LGP  10  which has no micro-lenses in the mixing zone  38   b  on its bottom surface  17 . 
     Referring to  FIG. 2B , lenses  100  are distributed in the core zone for Y between Y 1  and L. For the purpose of illustration, only lenses  100  that are distributed in the core zone for Y between Y 1  and 2Y 1  are shown. Lenses  100  shown have a size S1 and an area density D1 near Y 1 . In comparison, micro-lenses  110  distributed in the bottom mixing zone  38   b  for Y between Y 0  and Y 1  have a size S2 and an area density D2. The area density D2 is either one-dimensional that varies with X or two-dimensional that varies with both X and Y. Additionally, the area density D2 is reversed relative to the position of the discrete light sources  12   a ,  12   b , and  12   c . More specifically, at a given Y, the density D2 has a maximum value at LINE 0 and a minimum value at LINE 1. Note again that LINE 0 passes through the center of a light source  12  and extends along the Y-axis, and LINE 1 passes through the center distance of two adjacent light sources and extends along the Y-axis. 
     In a normal hot spot situation, more light occurs along LINE 0 than along LINE 1; therefore a normal one- or two-dimensional area density is needed. The normal area density has a maximum value at LINE 1 and a minimum value at LINE 0.  FIG. 2C  shows micro-lenses  110   a  with a normal area density placed in the mixing zone in a prior light guide plate. The density of the micro-lenses  110   a  is the same as that of the micro-lenses  110  shown in  FIG. 2B , except that the density of the micro-lenses  110   a  has a maximum value at LINE 1 and a minimum value at LINE 0 for a given Y whereas the density of micro-lenses  110  shown in  FIG. 2B  have a minimum value at LINE 1 and a maximum value at LINE 0 for a given Y. 
       FIG. 3A  shows the comparison of the simulated hot spot ratio L 1 /L 0  vs. Y for different light guide plates.  FIG. 3B  shows the comparison of the simulated the average of emitted flux &lt;L&gt; vs. Y for different light guide plates. The average of emitted light flux &lt;L&gt; is emitted light flux averaged over the pitch P 0  along the X-axis. All of the light guide plates have lenticular lenses on the output surface with a height H=11 μm and radius R=39.9 μm. The lenses  100  in the core zone have a size S1 of 66 μm and a density D1=4%. The mixing zone length is Y 1 =4 mm. The pitch P 0  of the light sources is 6.6 mm. 
     In  FIG. 3A  the curve labeled as “Reference” corresponds to a comparative light guide plate I that has no micro-lenses in the mixing zone. Referring to  FIG. 3A  and the “Reference” curve, the ratio L 1 /L 0 &lt;0.9 for Y&lt;2 mm, indicates a normal hot spot. The ratio L 1 /L 0 &gt;1.1 for Y in the range of about 2 mm and 4 mm, indicates a reverse hot spot. For Y between 4.2 mm and 6.5 mm, L 1 /L 0 &lt;0.9, indicates a normal hot spot. 
     In  FIG. 3A  the curve labeled as “Reverse” corresponds to an inventive light guide plate that has a reverse one-dimensional density distribution of micro-lenses  110  in the bottom mixing zone as shown in  FIG. 2B . The density of the micro-lenses has a maximum value of 15% at LINE 0, and a minimal value of 0% at LINE 1. 
     In  FIG. 3A  the curve labeled as “Normal” corresponds to a comparative light guide plate II that has micro-lenses with a normal one-dimensional distribution in the mixing zone as shown in  FIG. 2C . The density of the micro-lenses has a maximum value of 15% at LINE 1, and a minimal value of 0% at LINE 0. 
       FIG. 2D  shows another embodiment of the micro-lenses in the bottom mixing zone according to the present invention. In addition to the reverse distribution of the micro-lenses placed between Y 0  and Y 1 , as described in  FIG. 2B , additional micro-lenses with a normal distribution are distributed between 0 and Y 0 . 
       FIG. 3A  shows the simulated hot spot ratio L 1 /L 0  vs. Y for three light guide plates. The curve labeled as “Reverse density” corresponds to the inventive light guide plate having a reversed one-dimensional distribution of micro-lenses in the bottom mixing zone, shown in  FIG. 2B . The curve labeled as “normal density” corresponds to the comparative light guide plate II having a normal one-dimensional distribution of micro-lenses in the mixing zone, shown in  FIG. 2C . The curve labeled as “Reference” corresponds to the comparative light guide plate I having no micro-lenses in the mixing zone. All three light guide plates have the same linear lenticular lenses having a height H=11 μm and radius R=39.9 μm on the output surface, have the same mixing zone of 4 mm, and have the same micro-lenses in the core zone where size S1=66 μm and density D1=4%. Both the inventive light guide plate and the comparative light guide plate II have the same micro-lenses in the mixing zone where size S2=40 μm, and an area density where the maximum density is 15% and the minimum density is 0%. 
       FIG. 3B  shows the average of the simulated emitted light flux &lt;L&gt; vs. Y for the same three light guide plates shown in  FIG. 3A . The average &lt;L&gt; of the emitted light flux is averaged over a pitch along the X-axis for a given Y, with an arbitrary unit, referred to as “a.u.” in  FIG. 3B . 
       FIG. 3A  shows that that the reversed one-dimensional micro-lenses in the mixing zone help reduce the hot spot ratio L 1 /L 0  compared to the reference light guide plate. The normal one-dimensional micro-lenses in the mixing zone also helps reduce the hot spot ratio L 1 /L 0  compared to the reference light guide plate to some degree. However, the reversed one-dimensional micro-lenses in the mixing zone is preferred over the normal one-dimensional micro-lenses primarily because the average emitted light flux &lt;L&gt; for the reversed micro-lenses is much lower than that for the normal micro-lenses for Y near Y 1 =4 mm, as shown in  FIG. 3B . A higher emitted light flux &lt;L&gt; near Y 1 =4 mm means higher than unwanted brightness occurs. The higher brightness in the region near Y 1 =4 mm results in a bright band just inside the viewing area which is undesirable from the customer perspective. 
     In summary, the density and the size of the micro-lenses  110  in the mixing zone can be selected to suppress the reverse and normal hot spot, though the actual density and the size of the micro-lenses may vary depending on the pitch P 0  of the light sources and the geometry of the elongated grooves.