Abstract:
Several optical mechanism designs for making luminance of the outer portion of a back light module to become higher than luminance of the inner portion of the back light module of the direct lighting type are disclosed in accordance with the present invention. The disclosed optical mechanism designs can be cooperated with each other to enhance the optical effect of the back light module. Additionally, the back light modules of the present invention are feasible for utilization in various scanners or liquid crystal displays.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to back light modules, and more particularly, to back light modules whose outer portion has higher luminance than inner portion thereof.  
         [0003]     2. Description of the Prior Art  
         [0004]     A back light module is one of the key components for the liquid crystal display (LCD) or the scanner. The back light module comprises light sources and other optical devices for reflecting or refracting light to provide uniform light output. The light source of the back light module is typically a cold cathode fluorescent lamp (CCFL) or a light emitted diode (LED). In general, the back light module can be divided into two groups, the edge lighting type and the direct lighting type. The two groups are categorized by the positions of the light sources thereof.  
         [0005]     In the conventional art, the main concept of designing both the edge lighting type and the direct lighting type is to ensure uniformity of the light output from the back light module, i.e., the luminance distribution of the back light module is uniform. Unfortunately, such a design concept may negatively affect the performance of some applications.  
         [0006]     For example, suppose that a conventional back light module having uniform luminance distribution is employed in a scanner as a back light source required for scanning transparencies, and positive or negative films. The brightness of the outer portion of a scanned image received by an optical module of the scanner is usually lower than the brightness of the inner portion of the scanned image due to the optical characteristics or mechanical designs of the optical module. For these reasons the scanning quality of the scanner is thereby reduced.  
       SUMMARY OF THE INVENTION  
       [0007]     An exemplary embodiment of a back light module of direct lighting type is disclosed comprising: a light emitting device for emitting light, wherein an outer portion of the light emitting device with respect to an axis has higher luminous intensity than an inner portion of the light emitting device; a diffuser positioned on a side of the light emitting device for diffusing light; and a reflecting device positioned on another side of the light emitting device for reflecting light emitted from the light emitting device to the diffuser.  
         [0008]     Another exemplary embodiment of a back light module of direct lighting type is disclosed comprising: a light emitting device for emitting light; a diffuser positioned on a side of the light emitting device for diffusing light; and a reflecting device positioned on another side of the light emitting device for reflecting light emitted from the light emitting device to the diffuser; wherein the outer portion of the reflecting device has higher reflectivity or larger reflecting area than the inner portion of the reflecting device.  
         [0009]     Thereto, another exemplary embodiment of a back light module of direct lighting type is disclosed comprising: a light emitting device for emitting light; a diffuser positioned on a side of the light emitting device for diffusing light, wherein the outer portion of the diffuser has higher light transmittance than the inner portion of the diffuser; and a reflecting device positioned on another side of the light emitting device for reflecting light emitted from the light emitting device to the diffuser.  
         [0010]     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  
       [0011]      FIG. 1  is a simplified diagram of a back light module of the direct lighting type according to one embodiment of the present invention.  
         [0012]      FIG. 2  is a schematic diagram illustrating a luminous intensity distribution of a lamp of  FIG. 1  according to one embodiment of the present invention.  
         [0013]      FIG. 3  is a diagram showing different embodiments of the light emitting device of the back light module of  FIG. 1  in accordance with the present invention.  
         [0014]      FIG. 4  is a simplified diagram of a back light module of the direct lighting type according to another embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0015]     Please refer to  FIG. 1 , which shows a simplified diagram of a back light module  100  of the direct lighting type according to one embodiment of the present invention. The back light module  100  comprises a light emitting device for emitting light. In this embodiment, the light emitting device is implemented with a first lamp  112  and a second lamp  114 . In addition, the back light module  100  further comprises a diffuser  120  positioned on a side of the light emitting device for diffusing light, and a reflecting device positioned on another side of the light emitting device for reflecting light emitted from the light emitting device to the diffuser  120 . In this embodiment, the reflecting device comprises a housing  130  and a reflector  140 . As shown in  FIG. 1 , the reflector  140  is positioned between the light emitting device and the housing  130 .  
         [0016]     In one embodiment, the light emitting device is purposefully designed to increase the luminance of the outer portion of the back light module along an axis  10 . For example, each of the first and second lamps  112  and  114  can be implemented with a lamp whose outer portion along the axis  10  has higher luminous intensity than the inner portion thereof.  
         [0017]     For example,  FIG. 2  illustrates a luminous intensity distribution of the first lamp  112  of the light emitting device according to one embodiment of the present invention. As shown, in this embodiment, the outer portion  112 A and  112 B of the first lamp  112  along the axis  10  have higher luminous intensity than the inner portion  112 C of the first lamp  112 . This configuration results in the two ends of the diffuser  120  receiving more light than the middle portion (i.e., the inner portion) of the diffuser  120 . As a result, the luminance of the outer portion of the back light module  100  along the axis  10  is higher than the luminance of the inner portion thereof, i.e., the back light module  100  has a concave-down luminance distribution. In practice, the second lamp  114  of the light emitting device may have the identical luminous intensity distribution as the first lamp  112 . Additionally, each of the first and second lamps  112  and  114  may be a cold cathode fluorescent lamp (CCFL).  
         [0018]     In the foregoing embodiment, the light emitting device is implemented with two lamps. This is merely an example rather than a limitation of the present invention. For example,  FIG. 3  shows three different embodiments of the light emitting device of the back light module  100  in accordance with the present invention. In implementations, the light emitting device of the back light module  100  may be a lamp having a bended shape such as a U-shape lamp  310  shown in  FIG. 3 . The U-shape lamp  310  can be implemented with a cold cathode fluorescent lamp (CCFL). A light emitting device  320  shown in  FIG. 3  is an alternative embodiment. As shown, the light emitting device  320  comprises a straight lamp  322  (e.g., a straight CCFL) and a plurality of luminance units  324  for enhancing the luminous intensity of the outer portion of the light emitting device  320 . The plurality of luminance units  324  can be a plurality of electro luminances (ELs), a plurality of light emitted diodes (LEDs), or a combination of the two. A light emitting device  330  shown in  FIG. 3  is another embodiment. The light emitting device  330  is composed of a plurality of luminance units  332 . In practice, the plurality of luminance units  332  can be a plurality of ELs, a plurality of LEDs, or a combination of the two. It can be appreciated by those of ordinary skill in the art that the luminous intensity of the outer portion of the emitting device  330  along the axis  10  can become higher than the inner portion of the light emitting device  330  by properly adjusting the arrangement (i.e., spacing) or density of the plurality of luminance units  332 .  
         [0019]     Please note that the light emitting device of the back light module  100  can also be implemented with other design choices. In addition, the number of light emitting devices employed in the back light module  100  is not a restriction of the present invention, i.e., two or more than two sets of light emitting devices may be employed as the light source in the back light module  100 .  
         [0020]     In the previous embodiment, the back light module  100  enhances the luminance of the outer portion thereof by utilizing the light emitting device whose outer portion has higher luminous intensity than the inner portion. In practice, the back light module  100  can obtain the same optical characteristic by adopting other optical mechanisms.  
         [0021]     For example, the luminance distribution of the back light module  100  can be adjusted by modifying the design of the diffuser  120 . In another embodiment, the diffuser  120  of the back light module  100  has non-uniform light transmittance distribution. Specifically, the diffuser  120  of this embodiment is purposefully designed such that the light transmittance of the outer portion of the diffuser  120  along the axis  10  is better than that of the inner portion thereof. As a result, the back light module  100  can obtain the same optical characteristic as the foregoing embodiments, i.e., the luminance of the outer portion of the back light module  100  along the axis  10  is higher than the luminance of the inner portion.  
         [0022]     In another embodiment, the reflecting area of the outer portion of the reflector  140  is designed to be larger than the reflecting area of the inner portion of the reflector  140 . Accordingly, the outer portion of the reflector  140  has better reflecting performance than the inner portion. This exposes the outer portion of the diffuser  120  to more light thereby enhancing the luminance of the outer portion of the back light module  100 . Alternatively, the reflectivity of the outer portion of the reflector  140  can be designed to be higher than the reflectivity of the inner portion to reach or enhance the above optical effect.  
         [0023]      FIG. 4  is a simplified diagram of a back light module  400  of the direct lighting type according to another embodiment of the present invention. The back light module  400  is similar to the above-mentioned back light module  100 . Therefore, components that have the same implementations and operations are labeled the same. A difference between the back light module  400  and the back light module  100  is that the back light module  400  utilizes two reflectors  440  and  450  to replace the single reflector  140  of  FIG. 1 . As shown in  FIG. 4 , the two reflectors  440  and  450  are positioned between the light emitting device and the housing  130  with each corresponding to one of two ends of the light emitting device. In this embodiment, the light emitting device is implemented with the lamps  112  and  114 . The two reflectors  440  and  450  are arranged for reflecting light emitted from the two ends of the light emitting device to the diffuser  120 . The function of the two reflectors  440  and  450  is substantially the same as the reflector  140  of  FIG. 1 . In other words, the two reflectors increase light emitted to the outer portion of the diffuser  120  so that the outer portion of the back light module  400  has higher luminance than the inner portion thereof.  
         [0024]     In practice, the housing  130  can be coated with reflective materials on an inside surface (e.g., a surface facing toward the light emitting device) to become a reflecting housing. For causing the luminance of the outer portion of the back light module  100  or  400  to be higher than that of the inner portion thereof, the reflective materials coated on the inside surface of the housing  130  can be designed to be a pattern that has two broad outer portions and a narrow inner portion. In another embodiment, the middle portion of inside surface of the housing  130  in which corresponding to the inner portion of the light emitting device is coated with a low reflective material while the outside portion of the inside surface corresponding to the outer portion of the light emitting device is coated with another material having a relative high reflectivity. Generally, as long as the outer portion of the reflecting housing  130  has higher reflectivity or larger reflecting area than the inner portion of the reflecting housing  130 , the back light module can obtain the same or similar optical characteristic as the previous embodiment. As a result, the reflector  140 ,  440  or  450  can be omitted thereby reducing the cost of the back light module.  
         [0025]     Note that, the different optical mechanism designs mentioned above can function independently or co-operate with each other to enhance the optical effect.  
         [0026]     The disclosed back light module of the direct lighting type can be utilized as the back light source for various scanners and LCDs. For example, the back light module disclosed in accordance with the present invention can be applied in a scanner capable of scanning transparencies, and positive or negative films. The back light module disclosed in accordance with the present invention will significantly improve the fall off problems of the optical module caused by the optical characteristics or mechanical designs of the optical module.  
         [0027]     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.