Abstract:
A method and apparatus which uses the edge emitting characteristics of the LED bare chip to form a backlight with uniform illumination. This allows a reduced separation between the diffuser plate and the LED array. In one embodiment, the LED chip(s) is mounted over a prism structure, which redirects the side emitted light outwards.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    NOT APPLICABLE 
       BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to backlight light sources for a Liquid Crystal Display (LCD), and in particular a Light Emitting Diode (LED) light source. 
         [0003]    Most modern color LCDs must be viewed with a light source behind the LCD panel. This light source is usually called a Backlight. At present, most applications use a Cold Cathode Fluorescent Lamp (CCFL) as light source for the backlight. This lamp emits light inside a tubular structure. 
         [0004]      FIG. 1  shows a side, cut-away view of a typical backlight unit  100  using a CCFL  101  which is housed in a reflective box  102 . There is an opening on one side for the lamp to project light into a light guide  104 . A prism film  105  is used to redirect the light towards the surface of the LCD. A reflective surface  103  is used behind the light guide to reflect stray light toward the LCD. 
         [0005]    As shown in  FIG. 2 , in order to provide a more intense illumination, some LCDs have two CCFLs  201  and  202  in opposite ends in the backlight unit. A light guide  203  causes the light to propagate upwards towards a diffuser  204  and a prism film  205 . 
         [0006]    As shown in  FIG. 3 , since the backlight unit used for LCD televisions is not restricted by the thickness of the display and more luminance is often needed for television viewing, multiple units of CCFL  301  are used in the backlight unit housed in a reflective box  302 . The box  302  is covered by a diffuser  303 . Again, a prism film  304  is placed on top of the diffuser to constrain the light propagation to a narrower diverging cone. Although a CCFL is a good light source for a LCD, its power consumption is relatively high and its life time is limited (to approximately 1000 hours in some cases). Light emitting diodes (LEDs) on the other hand have low power consumption and much longer life. With the demand of LCDs in notebook computers and televisions increasing, there has been a continuing effort to develop the LED as an alternative light source to a CCFL. So far, two methods of using LEDs to replace CCFLs have been investigated. 
         [0007]    As shown in  FIG. 4 , one technique is to use a linear array of LEDs  401  to form a light bar  400 . In this case, it is simply a matter of replacing the CCFL in the back light unit by a LED light bar. Typically, LEDs are made with a chip that emits light from its four sides. The chip is then molded inside a high index plastic which serves as light guide to cause the LED to project light upwards. 
         [0008]    As shown in  FIGS. 5(   a ) and  5 ( b ), a second method is to place an array of LEDs  501  behind the LCD.  FIG. 5(   b ) shows the side view of  FIG. 5(   a ). LEDs  501  are bonded to a reflective printed circuit board (PCB)  503 . Because of the diverging cone  505  of each LED, the diffuser  504  must be placed at some distance away from the LEDs so that the cone of light from each LED can overlap with its neighbors. This increases the thickness of the backlight unit. However, the light efficiency of direct illumination is higher than having the illumination coupled into the display from one side of the display. 
       BRIEF SUMMARY OF THE INVENTION 
       [0009]    The present invention provides a method and apparatus which uses the edge emitting characteristics of the LED bare chip to form a backlight with uniform illumination. This allows a reduced separation between the diffuser plate and the LED array. In one embodiment, the LED chip(s) is mounted over the prism structure, which redirects the side emitted light upwards. 
         [0010]    In one embodiment, a plurality of LED chips are used. The LED chips may be different colors, so that the combination produces white light. The LED bare chip is first mounted on a heat sink, which is in turn mounted on a printed circuit board containing a prism film. 
         [0011]    In one embodiment, a second prism film is mounted above the LED, in addition to the first prism film mounted below. Additionally, a diffuser may be mounted above the second prism film. 
         [0012]    In one embodiment, the LED and prism structure are covered with a polymer containing particles having a refractive index different from that of the polymer, to diffract the light. In particular, the particles may be air bubbles. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0013]      FIG. 1  is a diagram of a prior art backlight using a CCFL light source. 
           [0014]      FIG. 2  is a diagram of a prior art backlight using two CCFLs. 
           [0015]      FIG. 3  is a diagram of a prior art backlight unit using multiple CCFLs. 
           [0016]      FIG. 4  is a diagram of a prior art LED light bar. 
           [0017]      FIG. 5  is a diagram of a prior art LED array. 
           [0018]      FIGS. 6(   a ) and ( b ) are diagrams of a prior art ( a ) edge emitting characteristics of an LED bare chip (side view); and ( b ) a top view of light emitted from an LED bare chip 
           [0019]      FIGS. 7(   a ) and ( b ) are diagrams of one embodiment of ( a ) a side view of an LED back light unit of this present invention; and ( b ) a top view of a light pattern projected by a single LED bare chip backlight unit of this present invention. 
           [0020]      FIG. 8  is a diagram of an array of LED bare chips on a diffusing printed circuit board according to an embodiment of this invention. 
           [0021]      FIG. 9  is a diagram of an embodiment of the reflective structure on the printed circuit board of this invention. 
           [0022]      FIG. 10  shows a linear prism array according to an embodiment of the invention. 
           [0023]      FIG. 11  is a diagram of an embodiment of the invention adding a polymer with particles to aid diffusion. 
           [0024]      FIGS. 12(   a )-( c ) show another embodiment of this present invention with LEDs bonded on a rectangular PCB and then encapsulated by a transparent trapezoidal structure. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]      FIG. 6  shows a side view of a prior art LED bare chip  600 . A junction layer  602   25  is grown on top of a substrate  601 . When an electric current passes through the junction layer, light will be emitted from the junction as shown in  FIG. 6(   a ).  FIG. 6(   b ) shows the top view of LED chip  600 . It can be seen that light is emitting from the four sides of the chip. This is the most common technique in fabricating a LED. An LED can be fabricated to emit light from the top surface also. However, the process is more complicated and thus has a higher manufacturing cost. Yet the surface emitting type LED has no advantage in the scheme of this present invention. In a commercially available LED package, the bare chip is often molded inside a high index plastic which serves as light guide to cause the LED to project light upwards. 
         [0026]      FIG. 7  shows a backlight unit with a single LED bare chip according to an embodiment of the invention. It illustrates how to convert the light emitting from the edge of the LED bare chip into light propagating upwards as required by a backlight unit. As can been seen, LED bare chip  701  is first mounted on a heat sink  702  which is in turn mounted on printed circuit board  703 . The heat sink serves to dissipate heat generated by the LED chip and at the same time elevate the LED chip so that the edge emitted light can be projected to a farther distance. The heat sink structure  702  is not necessary if the substrate of the LED is made thicker. A reflective structure  704  (such as a prism film) is attached to the printed circuit board in the area surrounding the LED chip as shown. This reflective structure, or prism structure, has structures (shown as triangles) which reflect the light upward. The printed circuit board is enclosed by reflective walls  707  and  708 . Light emitted by the LED perpendicular to walls  707  and  708  will be scattered by these walls until all the light emitted from the LED propagates vertically.  FIG. 7  shows light rays incident on structure  704  and being reflected upwards. In a similar fashion light rays incident on phase structure  705  are refracted upwards. There is a diffuser  706  on top of the refractive film  705  to make the light from the LED more uniform. 
         [0027]      FIG. 7(   b ) shows the light pattern (circle  708 ) emerging from the diffuser. Depending on the diverging cone of the light emitting from the LED chip, the diameter of the light pattern is often more than twice the separation between the LED chip  701  and the diffuser  706 . 
         [0028]      FIG. 8  is a diagram of a backlight array using an array of bare chip LEDs according to an embodiment of the invention. In using this concept to construct a back light unit  800 , an array of LEDs  801  as shown in  FIG. 8  will be laid down on a printed circuit board (PCB)  802 . Each of the LED units  801  will contain LEDs separately emitting in red light, green light and blue light. Because green LEDs are often weaker than red and blue LEDs, two green LED are used for every red and blue LED. These LEDs are mounted on printed circuit board  802  which has a reflective structure in area surrounding the LEDs. This PCB with the LED will be surrounded by  4  reflective walls and covered with a refractive structure and a diffuser as shown in  FIG. 7 . 
         [0029]    In one embodiment, the refractive structure is a prismatic film such as the one shown in  FIG. 9 . In this top view of the prismatic film, each prism has 4 sides  901 ,  902 ,  903  and  904 . The apex of the prism is  905 . Prisms with more sides can also be utilized in this present invention. In one embodiment, the prism structure is irregular, random or pseudorandom, with a variation in the size, apex, thickness or other aspect of the prisms for at least a plurality of the prisms. 
         [0030]      FIG. 10  shows a linear prism array according to an embodiment of the invention. Two of such linear prisms can be stacked together, with one being rotated to be perpendicular to the first one. The reflective structure surrounding the LEDs can also be made of a prismatic structure or some other manufactured surface to reflect the light upwards. 
         [0031]      FIG. 11  is a diagram of an embodiment of the invention adding a polymer with particles to aid diffusion. In this embodiment the space  1109  inside the enclosure is filled with a polymer containing particles, such as particle  1110 , having a refractive index different from that of the polymer. In one embodiment, the particles are air bubbles. A polymer containing air bubbles is poured into the cavity  1109  and then hardened by heat curing or by ultra violet light. These particles will scatter the light emitted by the LEDs to make the projected light more uniform. 
         [0032]      FIGS. 12(   a )-( c ) show another embodiment of this present invention. Instead of bonding the LED chips directly on a large PCB , the LEDs  1203  are first bonded on a rectangular PCB  1201 . The top view of this LED light bar is shown in  FIG. 12(   a ). The LEDs  1203  are encapsulated by a transparent trapezoidal structure  1202  as shown in  FIG. 12(   b ). This is a preferred structure to couple the light to the side of the light bar  1200  instead of towards the top of the light bar. 
         [0033]      FIG. 12  ( c ) shows how LED light bars are used in the back light of this present invention. LED light bars  1200  are bonded to a large PCB  1204  embedded in between reflective diffusing structure  1205 . As shown, the reflective diffusing structure is a prismatic film. Other reflective structures can be used in this present invention. The LED light bars are covered by a diffuser  1208  and a prismatic film  1206 . The four sides of the back light unit are surrounded by a reflective walls, such as reflective wall  1207 . 
         [0034]    As will be understood by those of skill in the art, the present invention could be embodied in other specific forms without departing from the essential characteristics thereof. For example, only one of the prismatic films of  FIG. 7(   a ) might be used, or a flat reflective surface may be employed rather than a prismatic surface. Alternately, the LED could be in a package, and placed on its side, such as at the end of the back light. Accordingly the foregoing description is intended to be illustrative, but not limiting, of the scope of the invention which is set forth in the following claims.