Panel light source device and back light module for liquid crystal display device

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.

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 toFIG. 1, it depicts the structure of a conventional liquid crystal display. Generally, the liquid crystal display device includes a liquid crystal panel10which has two substrates and a liquid crystal material sealed therebetween, a back light unit20disposed under the liquid crystal panel10, and cases11and12.

The back light unit20is utilized to distribute the light from a light source uniformly over the surface of the liquid crystal panel10. There are several kinds of back light units20such as a direct back light type (or direct type) and an edge light type.

Referring toFIG. 2, which is a cross-sectional view along line2—2ofFIG. 1, it depicts a back light module21of the direct type. The direct type back light module21includes a housing70which has a reflective sheet60disposed on the bottom surface of the housing70and, a lamp50, such as a fluorescence cathode tube, disposed at the bottom portion of the housing70, a diffusing sheet40disposed on the upper surface of the housing70and a prism sheet30disposed on the diffusing sheet40. Since the lamp50is spacedly disposed inside the display area of the liquid crystal panel10, the brightness distribution of the liquid crystal panel may be not uniform. Also, the TFT (Thin Film Transistor) switching elements of the liquid crystal panel10may be damaged by the heat of the back light source.

Therefore, some distance or space between the lamp50and the diffusing sheet40is required. This distance or space is provided typically by means of supports55disposed in the housing70. Because of these required structural elements, it is very difficult to reduce the thickness of the direct type back light module21.

Referring toFIG. 3, which is a cross-sectional view along line2—2ofFIG. 1, it depicts a back light module22of the edge light type. The edge light type back light module22includes a light guide80, a lamp50which is attached to at least one edge of the light guide80, and a U-shaped reflector61which surrounds the lamp50. An open portion of the reflector61is disposed at the edge of the light guide80, a reflecting sheet60is disposed at the bottom of the light guide80, a diffusing sheet40is disposed on the light guide80and a prism sheet30is disposed on the diffusing sheet40. Because the lamp50is disposed at the edge of the light guide80, the thickness of the LCD can be relatively decreased.

The light guide80includes a printed dot pattern or a V-shaped notched pattern on at least one surface for scattering the light in the light guide80and illuminating the liquid crystal panel10. The light guide80is typically made of PMMA by the process of press or ejection molding. The diffusing sheet40is disposed on the light guide80and typically made of half-transparent PET or polycarbonate for further evenly diffusing the light emitted from the light guide80. The prism sheet30is disposed on the diffusing sheet40for gathering the diffused light from the diffusing sheet40in the direction perpendicular thereto.

Since the liquid crystal panel10has 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 film35is typically disposed on the prism sheet30for 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.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring toFIG. 4, it depicts a liquid crystal display100according to a first embodiment of the present invention. The liquid crystal display100includes a liquid crystal panel110and a back light module120. The liquid crystal panel110has two transparent substrates112,114and a liquid crystal material116disposed therebetween. The outer surfaces of transparent substrates112,114of the liquid crystal panel110are covered with polarizing sheet118,119, and the inner surfaces thereof are provided with switching elements for changing the alignments of the molecular of the liquid crystal material116and thus generating images.

The back light module120is served as a panel light device for illuminating the liquid crystal panel110. The back light module120includes a light source150, a U-shaped reflector161surrounding the light source150, a wedge light guide180and a plurality of layers of optical films, such as diffusing sheet140and a prism sheet130. The light source150can be a cold cathode fluorescent lamp; CCFL. The diffusing sheet is used for further evenly diffusing the light emitted from the light guide80. The prism sheet30is 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 guide180is provided with scattering elements182, such as a printed dot pattern or a V-shaped notched pattern on the bottom surface of the light guide180, for scattering the light in the light guides180and transmitting the light out of the upper surface or the outgoing surface of the light guide180so as to serve as a uniform panel light source. The light guide180is typically made of PMMA by the process of press or ejection molding. The light guide180further comprises a reflector165disposed in the distal end of the light guide180for reflecting the light back to the light guide180.

The incoming surface184of the light guide180further comprises metal grid wires190. Now referring toFIG. 5, the metal grid wires190are spaced and formed directly on the incoming surface184such that the light with the polarizing direction perpendicular to the metal grid wires190transmits into the light guide180and the light with the polarizing direction parallel to the metal grid wires190is reflected. As shown inFIG. 4, the light emitted from the light source150is unpolarized light (random polarization), and consists of the component P, of which polarization is parallel to the metal grid wires190, and the component V, of which polarization is perpendicular thereto. The parallel component P is reflected by the metal grid wires190and the perpendicular component V transmits into the light guide180. Therefore, only the light with the polarizing direction perpendicular to the metal grid wires190(the perpendicular component V) transmits through the metal grid wires190into the light guide180. The reflected light (the parallel component P) is then reflected by the reflector161and the polarization thereof is changed, and thereby transmitting through the metal grid wires190into the light guide180. In the light guide180, the polarizing light (the perpendicular component V) is scattered by the scattering elements182and transmits into the liquid crystal display panel110. The polarization of the light which transmits into the liquid crystal display panel110is substantially perpendicular to the metal grid wires190. Therefore, the light absorbed by the polarizing sheet119of the liquid crystal display panel110is decreased so the light efficiency of the liquid crystal display100is increased.

According to the present invention, the metal grid wires190are made of aluminum, silver, copper or alloy, and preferably are made of aluminum. As shown inFIG. 5, preferably, the pitch P of the metal grid wires190is below about 300 nm, the thickness T of the metal grid wires190is 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 wires190is 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 wires190according to the present invention can also be formed on a substrate which is disposed between the light guide180and the light source150.

It will be apparent to those skilled in the art that the metal grid wires190function 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 reflector161for transmitting through the metal grid wires such that the light efficacy of the liquid crystal display100is 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 wires190.

Obviously, it will be apparent to those skilled in the art that the polarizing direction of the polarizing sheet119on the liquid crystal panel110is corresponding to that of the reflecting type polarizer such that the brightness of the liquid crystal display100is increased. Generally speaking, because of the arrangement of the liquid crystal display, the back light module120according 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 toFIG. 6, it depicts a liquid crystal display200according to the second embodiment of the present invention. The liquid crystal display200is generally similar to the liquid crystal display100wherein the similar elements are designated with the similar reference numerals. The liquid crystal display200comprises a back light module220having two light sources250respectively disposed on the opposite sides of the light guide280. The light guide280is substantially cuboid and has two sets of metal grid wires290respectively disposed between the two light sources250and the sides of the light guide280for polarizing the incoming light. The function and principle of the liquid crystal display200is similar to that of the liquid crystal display100of the first embodiment and is not repeatedly described for the sake of simplicity and clarity.

Now referring toFIG. 7, it depicts a back light module320according to the third embodiment of the present invention. The back light module320is generally similar to the back light module220wherein the similar elements are designated with the similar reference numerals. The back light module320has three light sources350respectively disposed on the three sides of the light guide380. Between the three sides of the light guide380and the light sources350are the metal grid wires390disposed. The function and principle of the back light module320is similar to that of the back light module220of the second embodiment and is not repeatedly described for the sake of simplicity and clarity.

Now referring toFIG. 8, it depicts a back light module420according to the fourth embodiment of the present invention. The back-light module420is generally similar to the back light module220wherein the similar elements are designated with the similar reference numerals. The back light module420has four light sources450respectively disposed on the four sides of the light guide480. Between the four sides of the light guide480and the light sources450are the metal grid wires490disposed. The function and principle of the back-light module420is similar to that of the back-light module220of 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.