Abstract:
A panel light source device comprises a light source, a U-shaped reflector surrounding the light source, a light guide, and a plurality of metal grid wires. The light guide has an incoming surface facing the light source and an outgoing surface. The light is transmitted into the light guide from the incoming surface and then transmitted out of the light guide from the outgoing surface. The metal grid wires are disposed between the incoming surface and the light source for transmitting the light with a predetermined polarization therethrough and reflecting the rest light therefrom.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the priority benefit of Taiwan application serial no. 091117235, filed Jul. 26, 2002, the full disclosure of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to a panel light device, and more particularly, to a back light module for a liquid crystal display (LCD). 
     2. Description of the Related Art 
     Referring to  FIG. 1 , it depicts the structure of a conventional liquid crystal display. Generally, the liquid crystal display device includes a liquid crystal panel  10  which has two substrates and a liquid crystal material sealed therebetween, a back light unit  20  disposed under the liquid crystal panel  10 , and cases  11  and  12 . 
     The back light unit  20  is utilized to distribute the light from a light source uniformly over the surface of the liquid crystal panel  10 . There are several kinds of back light units  20  such as a direct back light type (or direct type) and an edge light type. 
     Referring to  FIG. 2 , which is a cross-sectional view along line  2 — 2  of  FIG. 1 , it depicts a back light module  21  of the direct type. The direct type back light module  21  includes a housing  70  which has a reflective sheet  60  disposed on the bottom surface of the housing  70  and, a lamp  50 , such as a fluorescence cathode tube, disposed at the bottom portion of the housing  70 , a diffusing sheet  40  disposed on the upper surface of the housing  70  and a prism sheet  30  disposed on the diffusing sheet  40 . Since the lamp  50  is spacedly disposed inside the display area of the liquid crystal panel  10 , the brightness distribution of the liquid crystal panel may be not uniform. Also, the TFT (Thin Film Transistor) switching elements of the liquid crystal panel  10  may be damaged by the heat of the back light source. 
     Therefore, some distance or space between the lamp  50  and the diffusing sheet  40  is required. This distance or space is provided typically by means of supports  55  disposed in the housing  70 . Because of these required structural elements, it is very difficult to reduce the thickness of the direct type back light module  21 . 
     Referring to  FIG. 3 , which is a cross-sectional view along line  2 — 2  of  FIG. 1 , it depicts a back light module  22  of the edge light type. The edge light type back light module  22  includes a light guide  80 , a lamp  50  which is attached to at least one edge of the light guide  80 , and a U-shaped reflector  61  which surrounds the lamp  50 . An open portion of the reflector  61  is disposed at the edge of the light guide  80 , a reflecting sheet  60  is disposed at the bottom of the light guide  80 , a diffusing sheet  40  is disposed on the light guide  80  and a prism sheet  30  is disposed on the diffusing sheet  40 . Because the lamp  50  is disposed at the edge of the light guide  80 , the thickness of the LCD can be relatively decreased. 
     The light guide  80  includes a printed dot pattern or a V-shaped notched pattern on at least one surface for scattering the light in the light guide  80  and illuminating the liquid crystal panel  10 . The light guide  80  is typically made of PMMA by the process of press or ejection molding. The diffusing sheet  40  is disposed on the light guide  80  and typically made of half-transparent PET or polycarbonate for further evenly diffusing the light emitted from the light guide  80 . The prism sheet  30  is disposed on the diffusing sheet  40  for gathering the diffused light from the diffusing sheet  40  in the direction perpendicular thereto. 
     Since the liquid crystal panel  10  has a polarizing sheet for transmitting the light with one polarizing direction and absorbing the light with the other polarizing direction, about 50% of the energy of the light is lost when the light passes the polarizing sheet. Therefore, a polarization recycle film  35  is typically disposed on the prism sheet  30  for reflecting the light with the other polarizing direction, and the reflected light then is reflected by the optical element therebeneath and recycled after the polarizing direction thereof is changed, thereby increasing the brightness of the liquid crystal display. However, the above-mentioned polarization recycle film is available typically from 3M™ Company of St. Paul Minn. under the trade name Dual Brightness Enhancement Film (DBEF), which causes the liquid crystal display up to 160-170% brighter, but is significantly expansive. Further, the cost of the liquid crystal display will increase more and more as the dimension of the liquid crystal display increases in recent years. 
     Accordingly, the back light module of the liquid crystal display shall be constructed to meet the requirements of increasing power efficiency and the screen brightness, providing uniform brightness, lowering power consumption and cost, as well as decreasing the dimension. Prior art attempts have been made to meet the requirements and, for example, can be seen in U.S. Pat. No. 6,164,790 issued to Lee on Dec. 26, 2000, U.S. Pat. No. 5,477,422 issued to Hooker et al. on Dec. 19, 1995, and U.S. Pat. No. 5,485,354 issued to Ciupke et al. on Jan. 16, 1996. These patents are all incorporated herein by reference. However, the back light modules in these patents and the other prior art still cannot meet the above-mentioned requirements. 
     Furthermore, U.S. Pat. No. 6,086,212 issued to Onishi et al. on Jul. 11, 2000 discloses a back light unit which comprises an output light modulator provided on the light guide and an input light modulator provided between the light source and the light guide. However, the structure of the back light unit is complex and the brightness of the liquid crystal panel still cannot be significantly increased. 
     Accordingly, there exists a need for a back light module of a liquid crystal display capable of meeting the above-mentioned requirements. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a panel light source device for providing the liquid crystal display with the polarized light so as to increase the energy efficiency of the panel light source and the brightness of the liquid crystal display. 
     It is another object of the present invention to provide a panel light source capable of being manufactured at low cost and increasing the energy efficiency of the panel light source. 
     In order to achieve the above objects, the present invention provides a panel light source device comprises a light source, a reflector surrounding the light source, a light guide, and a plurality of metal grid wires. The light guide has an incoming surface facing the light source and an outgoing surface. The light is transmitted into the light guide from the incoming surface and then transmitted out of the light guide from the outgoing surface. The metal grid wires are disposed between the incoming surface and the light source for transmitting the light with a predetermined polarization therethrough and reflecting the rest light therefrom. 
     According to another aspect of the present invention, the present invention provides a back light module for illuminating a liquid crystal display panel of a liquid crystal display. The back light module comprises a light source, a reflector surrounding the light source, a light guide and metal grid wires. The light guide has an incoming surface facing the light source, scattering elements, and an outgoing surface. The light emitted form the light source is transmitted into the light guide from the incoming surface, scattered by the scattering elements, and then transmitted out of the light guide from the outgoing surface. The metal grid wires is disposed between the incoming surface and the light source for transmitting the light with a predetermined polarization therethrough and reflecting the rest light therefrom. The liquid crystal display further comprises optical films disposed between the outgoing surface of the light guide and the liquid crystal panel. 
     Accordingly, the back light module or the panel light source according to the present invention includes a reflecting type polarizer and thus provides the polarized light without lowering the energy efficiency. Also, the polarized light may transmit through the polarizing film of the liquid crystal display to minimize the light absorbed by the polarizing film. Therefore, the back light module or the panel light source according to the present invention dispenses with the expensive optical film but still meets the requirements of increasing the entire power efficiency, increasing the brightness, and lowering the cost of the liquid crystal display. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects, advantages, and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawing. 
         FIG. 1  is a perspective exploded schematic view of a liquid crystal display according to the prior art. 
         FIG. 2  is a cross sectional schematic view along line  2 — 2  of  FIG. 1  of a direct type back light module according to the prior art. 
         FIG. 3  is a cross sectional schematic view along line  2 — 2  of  FIG. 1  of an edge light type back light module according to the prior art. 
         FIG. 4  is a cross sectional schematic view of a liquid crystal display according to the first embodiment of the present invention. 
         FIG. 5  is a partial enlarged perspective schematic view of an incoming surface of a light guide according to the first embodiment of the present invention. 
         FIG. 6  is a sectional schematic view of a liquid crystal display according to the second embodiment of the present invention. 
         FIG. 7  is a sectional schematic view of a back light module according to the third embodiment of the present invention. 
         FIG. 8  is a sectional schematic view of a back light module according to the fourth embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to  FIG. 4 , it depicts a liquid crystal display  100  according to a first embodiment of the present invention. The liquid crystal display  100  includes a liquid crystal panel  110  and a back light module  120 . The liquid crystal panel  110  has two transparent substrates  112 ,  114  and a liquid crystal material  116  disposed therebetween. The outer surfaces of transparent substrates  112 ,  114  of the liquid crystal panel  110  are covered with polarizing sheet  118 ,  119 , and the inner surfaces thereof are provided with switching elements for changing the alignments of the molecular of the liquid crystal material  116  and thus generating images. 
     The back light module  120  is served as a panel light device for illuminating the liquid crystal panel  110 . The back light module  120  includes a light source  150 , a U-shaped reflector  161  surrounding the light source  150 , a wedge light guide  180  and a plurality of layers of optical films, such as diffusing sheet  140  and a prism sheet  130 . The light source  150  can be a cold cathode fluorescent lamp; CCFL. The diffusing sheet is used for further evenly diffusing the light emitted from the light guide  80 . The prism sheet  30  is commercially available from 3M™ Company of St. Paul Minn. under the trade name Brightness Enhancement Film II (BEF II) for gathering the light in the direction perpendicular thereto. 
     The light guide  180  is provided with scattering elements  182 , such as a printed dot pattern or a V-shaped notched pattern on the bottom surface of the light guide  180 , for scattering the light in the light guides  180  and transmitting the light out of the upper surface or the outgoing surface of the light guide  180  so as to serve as a uniform panel light source. The light guide  180  is typically made of PMMA by the process of press or ejection molding. The light guide  180  further comprises a reflector  165  disposed in the distal end of the light guide  180  for reflecting the light back to the light guide  180 . 
     The incoming surface  184  of the light guide  180  further comprises metal grid wires  190 . Now referring to  FIG. 5 , the metal grid wires  190  are spaced and formed directly on the incoming surface  184  such that the light with the polarizing direction perpendicular to the metal grid wires  190  transmits into the light guide  180  and the light with the polarizing direction parallel to the metal grid wires  190  is reflected. As shown in  FIG. 4 , the light emitted from the light source  150  is unpolarized light (random polarization), and consists of the component P, of which polarization is parallel to the metal grid wires  190 , and the component V, of which polarization is perpendicular thereto. The parallel component P is reflected by the metal grid wires  190  and the perpendicular component V transmits into the light guide  180 . Therefore, only the light with the polarizing direction perpendicular to the metal grid wires  190  (the perpendicular component V) transmits through the metal grid wires  190  into the light guide  180 . The reflected light (the parallel component P) is then reflected by the reflector  161  and the polarization thereof is changed, and thereby transmitting through the metal grid wires  190  into the light guide  180 . In the light guide  180 , the polarizing light (the perpendicular component V) is scattered by the scattering elements  182  and transmits into the liquid crystal display panel  110 . The polarization of the light which transmits into the liquid crystal display panel  110  is substantially perpendicular to the metal grid wires  190 . Therefore, the light absorbed by the polarizing sheet  119  of the liquid crystal display panel  110  is decreased so the light efficiency of the liquid crystal display  100  is increased. 
     According to the present invention, the metal grid wires  190  are made of aluminum, silver, copper or alloy, and preferably are made of aluminum. As shown in  FIG. 5 , preferably, the pitch P of the metal grid wires  190  is below about 300 nm, the thickness T of the metal grid wires  190  is in the range from about 30 nm to about 200 nm, and the ratio of the width W to the pitch P of the metal grid wires  190  is in the range from about 0.1 to about 0.8. It will be apparent to those skilled in the art that the metal grid wires  190  according to the present invention can also be formed on a substrate which is disposed between the light guide  180  and the light source  150 . 
     It will be apparent to those skilled in the art that the metal grid wires  190  function as a polarizer of reflecting type (polarizing beam splitter) which reflects the light with the parallel polarizing direction and then the polarizing direction of the reflected light will be changed due to the reflection of the reflector  161  for transmitting through the metal grid wires such that the light efficacy of the liquid crystal display  100  is increased. Therefore, the other polarizer of reflecting type, such as cholesteric liquid crystalline polymer, cholesteric liquid crystalline polymer with a 1/4λ plate and the above mentioned DBEF manufactured by 3M™ company, can be used in place of the metal grid wires  190 . 
     Obviously, it will be apparent to those skilled in the art that the polarizing direction of the polarizing sheet  119  on the liquid crystal panel  110  is corresponding to that of the reflecting type polarizer such that the brightness of the liquid crystal display  100  is increased. Generally speaking, because of the arrangement of the liquid crystal display, the back light module  120  according to the present invention is particularly adapted to be used with a thin film transistor (TFT) liquid crystal display panel of In-Plane-Switching, Vertical Alignment, and Multi-Domain Vertical Alignment. 
     Now referring to  FIG. 6 , it depicts a liquid crystal display  200  according to the second embodiment of the present invention. The liquid crystal display  200  is generally similar to the liquid crystal display  100  wherein the similar elements are designated with the similar reference numerals. The liquid crystal display  200  comprises a back light module  220  having two light sources  250  respectively disposed on the opposite sides of the light guide  280 . The light guide  280  is substantially cuboid and has two sets of metal grid wires  290  respectively disposed between the two light sources  250  and the sides of the light guide  280  for polarizing the incoming light. The function and principle of the liquid crystal display  200  is similar to that of the liquid crystal display  100  of the first embodiment and is not repeatedly described for the sake of simplicity and clarity. 
     Now referring to  FIG. 7 , it depicts a back light module  320  according to the third embodiment of the present invention. The back light module  320  is generally similar to the back light module  220  wherein the similar elements are designated with the similar reference numerals. The back light module  320  has three light sources  350  respectively disposed on the three sides of the light guide  380 . Between the three sides of the light guide  380  and the light sources  350  are the metal grid wires  390  disposed. The function and principle of the back light module  320  is similar to that of the back light module  220  of the second embodiment and is not repeatedly described for the sake of simplicity and clarity. 
     Now referring to  FIG. 8 , it depicts a back light module  420  according to the fourth embodiment of the present invention. The back-light module  420  is generally similar to the back light module  220  wherein the similar elements are designated with the similar reference numerals. The back light module  420  has four light sources  450  respectively disposed on the four sides of the light guide  480 . Between the four sides of the light guide  480  and the light sources  450  are the metal grid wires  490  disposed. The function and principle of the back-light module  420  is similar to that of the back-light module  220  of the second embodiment and is not repeatedly described for the sake of simplicity and clarity. 
     As the foregoing description, the back light module or the panel light source according to the present invention provides the polarized light which may transmit through the polarizing film of the liquid crystal display to minimize the light absorbed by the polarizing film. Therefore, the back light module or the panel light source according to the present invention dispenses with the expensive optical film but still meets the requirements of increasing the entire power efficiency, increasing the brightness, and lowering the cost of the liquid crystal display. 
     Although the invention has been explained in relation to its preferred embodiment, it is not used to limit the invention. It is to be understood that many other possible modifications and variations can be made by those skilled in the art without departing from the spirit and scope of the invention as hereinafter claimed.