Abstract:
A back light unit having a light guide plate, a light guide bar connecting to the light guide plate, at least a white LED light source, and at least a color LED light source. Each of the LED light sources is positioned at a side of the light guide bar. The color LED light source compensates white light from the white LED light source that cannot produce preferable white light. Accordingly, the back light unit can produce white light that provides preferable color saturation to a display in cooperation of the white LED light source and the color LED light source.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The invention relates to a back light unit, and more particularly, to a back light unit using light emitting diodes (LEDs) as its back light source.  
         [0003]     2. Description of the Prior Art  
         [0004]     A light emitting diode (LED) is a new type of small light sources. It has advantages of long life, small size, high shock resistance, and low power consumption. As a result, LEDs have been widely used as the indicator lights or light sources of electronic appliances and machines. Recently, due to the development of the colorfulness and brightness of LEDs, they had been applied to mobile electronics as the back light sources of the mid- or small-size displays, especially for the small color liquid crystal displays (LCD).  
         [0005]     Because white lights are used as the major back light source for the LCD displays of most information electronic products, the LEDs used in the back light units must be able to produce white lights. However, comparing to the lighting equipments which are currently popular, such as incandescent tungsten lamps and fluorescent lamps, the technology of white LED is not mature yet for that the white LED light is more costly and less efficient. Furthermore, it also has the disadvantage of smaller spectral distribution. Currently two methods are used in the industry to produce white LED lights. The first method is to use a blue LED chip with yellow-green phosphor to produce white light. This method has low cost and low efficiency wherein it is widely used by the industry. However, the problem is that the white light produced with this method may have insufficient intensities of the red light between 600 to 700 nanometers (nm) and of the green light between 480 to 580 nm. The insufficiency of the red light is enormous. This problem affects the color saturation of LCD displays. The manufacturers now use color filters to increase the color saturation, which raises the cost dramatically due to the difficulties in processing and uncontrollable yield rates.  
         [0006]     Another method is to package a red, a blue and a green LED chips as a single package, and adjust the electric currents among the LED chips to produce white light. This method provides the light sources with the wavelengths of red, blue, and green lights. Nevertheless, it has difficulties in controlling the brightness of each individual LED chips and in adjusting the uneven mixture of colors. And the efficiency of emitting light is still low. In addition, since all three LED chips are lightened simultaneously, more voltages are required in this kind of back light units than those with a single LED chip; the power consumption of the displays is increased.  
         [0007]     As a result, it is still a topic which must be researched in the industry to improve the white light emitted by LED light source with low cost.  
       SUMMARY OF THE INVENTION  
       [0008]     It is therefore a primary objective of the claimed invention to provide a back light unit producing white light with preferable color saturation and a method of adjusting spectral distribution of a back light unit to solve the problems of displays which can not produce preferable colors due to the limited technology of white LED light sources.  
         [0009]     According to the claimed invention, the back light unit comprises a light guide plate, a light guide bar connecting to the light guide plate, at least a white LED light source, and at least a color LED light source. Each of the LED light sources is positioned on a side of the light guide bar.  
         [0010]     According to the claimed invention, a method of adjusting spectral distribution of a back light unit is further provided. The back light unit comprises a light guide plate and a white LED light source. First the spectrum diagram of the white light emitted by the white LED light source is compared with that of the natural light. A kind of light with a specific range of wavelength which the white light emitted by the white LED package lacks is determined. Then a color LED light source for the light with the specific wavelength is provided so that the lights from the white LED light source and from the color LED light source can be mixed in the light guide plate to produce white light with preferable color saturation.  
         [0011]     It is an advantage of the technology of color LED light sources with single LED chip is fully developed so that it may be used to effectively adjust the white light having small spectral distribution that is produced by the white LED light source with immature technology. Accordingly, the back light unit may provide a white light source close to the natural light to the display.  
         [0012]     These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]      FIG. 1  is a schematic diagram of a back light unit according to a first embodiment of this invention.  
         [0014]      FIG. 2  is a schematic diagram of a back light unit according to a second embodiment of this invention.  
         [0015]      FIG. 3  is a schematic diagram of a back light unit according to a third embodiment of this invention.  
         [0016]      FIG. 4  is a schematic diagram of a back light unit according to a forth embodiment of this invention.  
         [0017]      FIG. 5  is a schematic diagram of a back light unit according to a fifth embodiment of this invention. 
     
    
     DETAILED DESCRIPTION  
       [0018]     Please refer to  FIG. 1 .  FIG. 1  is a schematic diagram of a back light unit  10  according to a first embodiment of the in this invention. The back light unit  10  comprises a light guide plate  12 , a light guide bar  14  on one side of the light guide plate, three white LED light sources  16  on the side of the light guide bar  14  opposite to the side connecting to the light guide plate  12 , and two color LED light sources  18 . As shown in  FIG. 1 , the light guide plate  12  is a flat or wedge-shaped plate. One side of the light guide plate  12  is an incidence face  13  of the light guide plate  12 . A light guide bar  14  is set on this incidence face  13  and connects to the light guide plate  12 . The light guide bar  14  and the light guide plate  12  may be a monolithic structure or be made of same materials. The first light-incidence face  20  of the light guide bar  14  is positioned at the surface opposite to the incidence face  13  of the light guide plate  12 . Two sides of the light guide bar  14  are two nicks, and the second light-incidence face  22  and the third light-incidence face  24  of the light guide bar  14  are on the two nicks. Three white LED light sources  16  are set in a row on the surface of the first light-incidence face  20 . Two color LED light sources  18  are individually set on the edge of two sides of the light guide bar  14 , the second and the third light-incidence faces  22 ,  24 .  
         [0019]     In this embodiment, each of the package of the white LED light sources  16  is composed by a blue LED chip with yttrium aluminum garnet (YAG) or other similar yellow-green phosphor. The white light produced by this package has small spectral distribution and lacks the color of red light. Therefore, to compensate the shortage of red light in the white light produced by the white LED light sources  16 , a red light LED package with a single red LED chip is used as the color LED light source  18  on each side of the light guide bar  14 . Via the light guide bar  14 , the red light is transmitted from the color LED light sources to the light guide plate  12 . The light from the color LED light sources  18  then may be fully mixed with the white light from the white LED light sources  16  to provide a complete white light source with wavelengths of red, blue, and green lights. In addition, while the color LED light sources  18  are being positioned, the included angles of the nicks between the first and the second light-incidence faces  20 , 22  and between the first and the third light-incidence faces  20 ,  24  of the light guide bar  14  may be modified based on the propagating angles of the light from the package of the color LED light sources  18  (about 110 degrees) to adjust the incidence angle of the color light on the light guide plate  12 , as the arrows shown in  FIG. 1 . A distinguish feature of this embodiment is to set up the color LED light sources  18  on two sides of the plurality of the white LED light sources  16  which have insufficient spectral distribution or on two sides of the light guide plate  12  and the light guide bar  14 . By adjusting or special designs of the face of the nicks between the first light-incidence face  20  and the second or third light-incidence faces  22 ,  24  of the light guide bar  14 , the light of the color LED light sources  18  is uniformly transmitted to the light guide plate  12  to compensate the shortage of the white light produced by the white LED light sources  16 .  
         [0020]     Please refer to  FIG. 2 .  FIG. 2  shows the second embodiment of the back light unit according to the present invention. The symbols of each part here are the same as in the previous embodiment. In this embodiment, the color LED light sources  18  are set between each two nearby white LED light sources  16  and on the surface of the first light-incidence face  20 . Therefore, the light produced by the white LED light sources  16  and the color LED light sources  18  will be mixed completely in the light guide bar  14 , and provide white light with preferable color saturation to the light guide plate  12 . The arrows with solid lines in  FIG. 2  indicate the light produced by the white LED light sources  16 , and the arrows with dashed lines indicate the light produced by the color LED light sources  18 .  
         [0021]     In the above-mentioned first and second embodiments, a white LED light source  16  comprises a single blue chip with phosphor. The light produced by these light sources lacks the light with red wavelength. Therefore, the single-chip red LED light packages are used as the color LED light sources  18  to compensate the insufficiency in colors. Because the brightness or intensity of red LED chips is weaker than the white LED light sources  16 , multiple red LED light sources  18  added onto the back light unit  10  may effectively compensate the lack of the white light from the white LED light sources  16  and improve the color performance of the back light unit  10 .  
         [0022]     Similarly, when the white LED light source  16  is composed of color LED chip other than blue LED chips, different color LED light sources, usually single-chip packages, may be used to compensate the specific range of wavelength which is missing in the white light produced by the white LED light sources and is identified by the comparison of the spectrum diagrams of the white light and the natural light. The position and design of the light guide plate  12  and the light guide bar  14  may be used to adjust and mix the light from the white LED light sources and the color LED light sources to provide preferable light sources to the back light unit. Therefore, other than the single-chip packages with red LED chips, the single-chip packages with green chips or chips in different colors may be used as the color LED light sources to compensate the weak colors in the white light. The single-chip LED packages with well-developed technology hereby may be used to easily adjust the colors of the back light unit to provide the preferable back light source to the market.  
         [0023]     Please refer to  FIG. 3 .  FIG. 3  shows the third embodiment of the back light unit of the present invention. All symbols used for parts here are the same as in previous embodiments. In this embodiment, the first light-incidence face  20  of the light guide bar  14  further comprises a plurality of V-cuts  26  or has been roughened. As a result, the light from the color LED light sources  18  on the second and third light-incidence faces  22 ,  24  may be transmitted to the center of the light guide bar  14 . Also, due to the differences between the refraction in the air and the refraction in the light guide bar  14 , the V-cuts  26  or rough edges of the light guide bar  14  may serve as a prism structure that creates a prism effects and reflect the light to the light guide plate  12  through its incidence face  13  (the light-exit face of the light guide bar  14 ), as the arrows show in  FIG. 3 .  
         [0024]     Please refer to  FIG. 4 .  FIG. 4  is a schematic diagram of the forth embodiment of the present invention. The back light unit  50  comprises a light guide plate  52 , a light guide bar  54 , and at least a white LED light source  56 .  FIG. 4  shows two white LED light sources  56 . Each of the white LED light source  56  is a double-chip LED package comprising a red LED chip, a blue LED chip, and florescent powders. It produces white light comprising the wavelengths of red, blue, and green lights, while it usually has a problem of uneven color mixing. To solve the problem, two double-chip white LED light sources  56  are separately set up on two sides of the light guide bar  54 , and the surface  60  of the light guide bar  54  has been specially processed.  
         [0025]     The surface  60  of the light guide bar  54  may contain V-cuts  66  or may be roughened to create the prism effects. Due to the differences between the refractions in the air and in the light guide bar  54 , the light from the white LED light sources  56  may be transmitted to the center of the light guide bar  54 , and reflected into the light guide bar  54 , completely mixed, and then transmitted into the light guide plate  52 , as the arrows shown in  FIG. 4 . This design effectively solves the problems of color underperformance of displays due to the uneven color mixtures and uneven brightness created by the direct light incident on the light guide plate  52  from the double-chip white LED light sources  56 . It should be noted that adjusting the angles and pitches of V-cuts  66  may create better reflection angles of the light and mixture in both this embodiment and the third embodiment.  
         [0026]     Please refer to  FIG. 5 .  FIG. 5  is a schematic diagram of the fifth embodiment of the present invention. The symbols for parts here are the same as in the forth embodiment. In this embodiment, the surface of the side  60  of the light guide bar  54  opposite to the incidence face  53  of the light guide plate is convex. The double-chip white LED light sources  56  are still set on the two light-incidence faces  58  of the light guide bar  54 . While the light from the white LED light sources  56  is transmitted in the light guide bar  54 , partial light incident to the convex side  60  will be reflected to the light guide bar  54 , and then be uniformly transmitted to the light guide plate  52 . Accordingly, the side  60  serves as a reflection face of the light guide bar  54  for reflecting light passing into the light guide bar  54  through the light-incidence face  58  back to the light guide bar  54 . Therefore, with the special design on the surface of the side  60  of the light guide bar  54 , the light from the white LED light sources  56  in this embodiment is uniformly reflected and fully mixed to create a preferable white light in the light guide plate  52 .  
         [0027]     In the forth and fifth embodiment, when the white light produced by the white LED light sources  56  lacks light with a specific range of wavelengths, color LED light sources may also be used to create better spectral distribution according to the spirit in the first embodiment.  
         [0028]     The back light unit introduced here may be applied to a display as the back light source of a flat display panel. In preferable embodiments, the flat display panel is a LCD panel. In contrast to the prior art, single-chip color LED light sources are used to compensate the white LED light sources producing lights with smaller spectral distribution in the present invention. Three advantages of this design are identified. First, without complicated processes and changes of materials, the improvement of color performance may be reached by optimizing the position of the color LED light sources or the mechanic design in the original back light unit in the display. Second, the single-chip LED packages with well-developed technology are used to provide the missing light with a specific range of wavelength from the white LED light sources. Since this technology is simple and economic, it may directly apply to products in mass manufacturing. It may also avoid the uncertainty in developing new materials and new technology. Third, since the brightness and intensity of the color LED sources is comparatively small, it may not affect the colorfulness of the white light from the original back light unit, but may increase the color saturation of the unit.  
         [0029]     Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.