Abstract:
A projection system includes a light source, a beam splitter module, a projection lens, and an optical combiner module includes at least one light valve for forming the image. A light beam emitted from the light source is split by the beam splitter module into three color lights and such three color lights are thereafter combined by the optical combiner module. The optical combiner module comprises a combiner and a plurality of prisms disposed adjacent to the combiner. The combiner comprises at least a pared-corner, and one side of at least one prism is leant adjacent to the pared-corner of the combiner so as to minimize the size of the optical combiner module and shorten the back focal length between the light valve and the projection lens. In addition, the projection quality of the digital light projection system is not easy to be influenced by heat temperature.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application claims the priority benefit of Taiwan application serial no. 93101928, filed Jan. 29, 2004. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention generally relates to a combiner and a miniaturized optical combiner module. More particularly, the present invention relates to a combiner and an optical combiner module having small size and a digital light projection system using the optical combiner module to shorten back focal length. 
   2. Description of Related Art 
   Conventionally, the key component of the digital light processing (DLP) projector is a semiconductor component controlled by two-bits pulse tuning, wherein the semiconductor component is generally called a digital micro-mirror device (DMD). The DMD may control a digital optical switch very rapidly and may control the light source very precisely. The image displaying method of DLP projector is different from the conventional liquid crystal projector (LCP). In the conventional LCP, the image is displayed by the transmitted light via a liquid crystal panel. However, in the DLP projector, the image is displayed by reflecting the light from the micro mirrors on the digital micro-mirror device (DMD). Therefore, the weight of the DLP projector can be reduced down to less than 2.5 kg, however, the weight of a conventional projector is generally up to 8 to 15 kg. In addition, the size of the DLP projector is less than that of the conventional projector. Moreover, the optical efficiency and resolution of the DLP projector are much better than that of the conventional projector. Since the digital micro-mirror device (DMD) has high reflectivity and high fill factor, the optical efficiency of the digital micro-mirror device (DMD) is high. Therefore, the DLP projector is very suitable for the application requiring high brightness and high resolution. Moreover, the DLP projector may provides fully digitized color display, and precise and stable image display. 
     FIG. 1  is a plan diagram schematically illustrating the internal structure of a conventional digital processing projection device. Referring to  FIG. 1 , a conventional DLP projector is generally constructed by a light source  111 , a rod integrator  112 , an aspheric lens set  113 , a reflection mirror  115 , a beam splitter module  120  and a projection lens. The light  11  emitted by the light source  111  is propagated through the rod integrator  112  and aspheric lens set  113  and condensed in the reflection mirror  115 . 
   Referring to  FIG. 1 , a conventional beam splitter and optical combiner module  120  includes a total internal reflection (TIR) prism  117  and a Philips prism  121 . The light  11  described above is reflected by the reflection mirror  115  to the total reflection plane  117   a  of the total internal reflection (TIR) prism  117 . It is noted that the incident angle A 1  of the light  11  incident on the total reflection plane  117   a  is larger than the critical angle of total reflection. In addition, there is an air gap between prisms  31  and  32  of the total internal reflection (TIR) prism  117 . Therefore, the light  11  is propagated from optically denser medium to optically less denser medium, and thus the light  11  is totally reflected on the total reflection plane  117   a  and is incident to the Philips prism  121 . 
   There are two layers of coating in the Philips prism. For the coating  118   a , the red light  13  of the light  11  is reflected, and the remainder color lights are transmitted. For the coating  118   b , the blue light  15  of the remained color lights transmitted from the coating  118   a  is reflected, and thus the remainder green light  17  is transmitted. Accordingly, after the light  11  propagated through the Philips prism  121 , the light  11  is split into the red light  13 , the green light  17  and the blue light  15 . Each of the three color lights are incident to digital micro-mirror devices (DMD)  121 ,  122  and  123  by a specific angle respectively. 
   The red light  13 , green light  17  and blue light  15  incident on the digital micro-mirror devices (DMD)  121 ,  122  and  123  are reflected respectively, and the reflected red light  13 , green light  17  and blue light  15  represent the red image, green image and blue image respectively. Thereafter, the red light  13 , green light  17  and blue light  15  reflected by the digital micro-mirror devices (DMD)  121 ,  122  and  123  are incident on the total reflection plane  117   a . In the meanwhile, the incident angle A 2  of the red light  13 , green light  17  and blue light  15  incident on the total reflection plane  117   a  is less than the critical angle of the total reflection of that. Therefore, the red light  13 , the green light  17  and the blue light  15  are transmitted through the total reflection plane  117   a  of the total internal reflection (TIR) prism  117 . Therefore, the red light  13 , green light  17  and blue light  15  are projected via the projection lens  119 . 
   In a conventional DLP projector, before the light is incident on the Philips prism, it is not split into the red light, the green light and the blue light. The color splitting of the light is performed by the Philips prism. The Philips prism at least has the disadvantages of heavy weight and large size. The Philips prism is the heaviest component of the whole DLP projector. In addition, the back focal length of the Philips prism is long. Furthermore, since the color splitting and combining of the light are all performed in the Philips prism, the thermal problem is usually generated and the projection quality is adversely influenced. 
   SUMMARY OF THE INVENTION 
   Accordingly, the present invention is directed to a combiner including at least a paired corner. The combiner has the advantages of light weight, small size, short back focal length, and without aforementioned thermal problem. 
   Accordingly, the present invention is directed to an optical combiner module having the advantages of light weight, small size, short back focal length, and without aforementioned thermal problem. 
   In addition, the present invention is also related to a digital light projection system having the advantages of light weight, small size, short back focal length. Moreover, the projection quality is not easily influenced by temperature. 
   According to one embodiment of the invention, a combiner comprising at least paired corner is provided. 
   According to another embodiment of the invention, an optical combiner module comprising, for example but not limited to, a combiner and a plurality of prisms disposed adjacent to the combiner is provided. The combiner comprises at least a pared-corner, and one side of at least one prism is leaned against the pared-corner of the combiner to minimize the size of the optical combiner module. 
   According to still another embodiment of the invention, an optical combiner module disposed in an intersection of a plurality of color lights split by a beam splitter module is provided. The optical combiner module may comprise a combiner, a plurality of prisms and a plurality of light valves. The combiner may comprise at least a pared-corner. The prisms may be disposed adjacent to the combiner, wherein one side of one of the prisms is leaned adjacent to the pared-corner of the combiner. The light valves may be adopted for forming the image, respectively disposed in a path of the color lights after the prisms, wherein the color lights are reflected to the light valves by a total reflection plane of each of the prisms respectively and said color lights reflected by the light valves are transmitted through the prisms and thereafter are combined by the combiner. 
   In one embodiment of the invention, the combiner comprises, for example but not limited to, X-prism or other optical component may be provided as optical combiner. The combiner comprises four sides, a top surface, a bottom surface and three lean surfaces. The top surface and bottom surface are adjacent to the sides respectively. A portion of the lean surfaces is adjacent to the top surface and one side of the combiner, and another portion of lean surfaces is adjacent to the bottom surface and one side of the combiner. Furthermore, the shape of the lean surface is, for example but not limited to, regular triangle. 
   In one embodiment of the invention, the prisms comprise, for example but not limited to, three total internal reflection (TIR) prisms, wherein each TIR prism comprises a first prism and a second prism. The first prism comprises a first light incident plane, a first junction surface and a first light exit plane, and the second prism comprises a second junction surface and a second light exit plane. There is an air gap between the second junction surface and a portion of the first junction surface, and another portion of the first junction surface is disposed adjacent to one of the lean surfaces, and the second light exit plane is disposed adjacent to one side of the combiner. Furthermore, the first light incident plane of each of the TIR prisms is disposed in a first plane, and the top surface of the combiner is disposed in a second plane, and an angle between the first plane and the second plane is about 45°. 
   In one embodiment of the invention, the TIR prisms comprise, for example but not limited to, a red light TIR prism, a green light TIR prism and a blue light TIR prism. The red light TIR prism and the blue light TIR prism are disposed on opposite side of the combiner. Furthermore, the red light TIR prism further comprises, for example but not limited to, a blue light reflecting coating to filter out stray light. Moreover, the blue light TIR prism further comprises, for example but not limited to, a red light reflecting coating to filter out stray light. 
   In one embodiment of the invention, in each TIR prism, the second light exit plane of the second prism comprises, for example but not limited to, a transparent region and a light shielding region, and the light shielding region is provides for filtering out the light at OFF state. 
   According to another embodiment of the invention, a digital light projection system is provided. The digital light projection system comprises, for example but not limited to, a light source, a beam splitter module, an optical combiner module as described above, a combiner and a projection lens. First of all, a light emitted by the light source is propagated into a beam splitter module via a rod integrator. The light is split into a plurality of color lights by the beam splitter module. The optical combiner module is disposed in an intersection of the color lights after the beam splitter module. The projection lens is disposed in the path of the color lights after the optical combiner module. 
   In one embodiment of the invention, the color lights comprise, for example but not limited to, a first color light, a second color light and a third color light. The beam splitter module comprises, for example but not limited to, a first two-way beam splitter mirror and a second two-way beam splitter mirror. The first two-way beam splitter mirror is disposed in the path of light after the light source and the rod integrator for reflecting the first color light and transmitting the second color light and the third color light. The second two-way beam splitter mirror is disposed in the path of the second color light and third color light after the first two-way beam splitter mirror for reflecting the second color light and transmitting the third color light. 
   In one embodiment of the invention, the beam splitter module further comprises, for example but not limited to, a first condenser lens and a second condenser lens. The first condenser lens is disposed in the path of the light between the light source and the first two-way beam splitter mirror, and the second condenser lens is disposed in the path of the first color light, second color light and third color light between the first two-way beam splitter mirror and the TIR prism. A relay processing may be performed to each color light for compensating the difference between the paths of each color lights before each color lights are incident on the combiner  200 . Therefore, a correct image may be obtained. 
   In one embodiment of the invention, the beam splitter module further comprises, for example but not limited to, at least a reflection mirror disposed in the path of light between the first two-way beam splitter mirror and the combiner. 
   Accordingly, in the digital light projection system of the present invention, the light is split into red, green and blue lights by the beam splitter component before incident on the total internal reflection (TIR) prism. Therefore, the light incident on each TIR prism can be combined by X-prism. 
   In addition, the combination of the X-prism and the TIR prism of the present invention have the advantages of light weight, small size, inexpensive, and shorter back focal length compared to Philips prism. Therefore the total weight of the digital light projection system can be reduced drastically, and the projection quality is also improved. 
   Furthermore, the TIR prism of the present invention has less thermal problem in comparison with the Philips prism. Thus, the influence of temperature on projection quality is minimized. 
   It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The following drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
       FIG. 1  is a plan diagram schematically illustrating the internal of a conventional digital processing projection device. 
       FIG. 2  is a three-dimensional diagram schematically illustrating an optical combiner module according to an embodiment of the present invention. 
       FIG. 3  is a perspective diagram schematically illustrating an optical combiner module of  FIG. 2 . 
       FIG. 4A  is a three-dimensional diagram schematically illustrating the combiner of  FIG. 2 . 
       FIG. 4B  is a perspective diagram schematically illustrating the combiner of  FIG. 2 . 
       FIG. 5A  is a three-dimensional diagram schematically illustrating the total internal reflection (TIR) prism of  FIG. 2 . 
       FIG. 5B  is a perspective diagram schematically illustrating the total internal reflection (TIR) prism of  FIG. 2 . 
       FIG. 5C  is a lateral view schematically illustrating the total internal reflection (TIR) prism of  FIG. 2 . 
       FIG. 6  is a top view schematically illustrating a digital light projection system according to an embodiment of the present invention. 
       FIG. 7  is a drawing schematically illustrating a digital micro-mirror device (DMD) according to an embodiment of the present invention. 
       FIG. 8A  is a drawing schematically illustrating an embodiment having an angle 10° between micro mirror and active surface according to the present invention. 
       FIG. 8B  is a drawing schematically illustrating an embodiment having an angle 12° between micro mirror and active surface according to the present invention. 
   

   DESCRIPTION OF PREFERRED EMBODIMENTS 
   The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout. 
     FIG. 2  is a three-dimensional diagram schematically illustrating an optical combiner module according to an embodiment of the present invention, and  FIG. 3  is a perspective diagram schematically illustrating an optical combiner module of  FIG. 2 . Referring to  FIG. 2  and  FIG. 3 , in one embodiment of the present invention, the optical combiner module  200  includes, for example but not limited to, a combiner  210  and a plurality of prisms  220 ,  230  and  240  disposed adjacent to the combiner  210 . In the embodiment, three prisms  220 ,  230  and  240  are provided, however, the number of prisms is not limited to three. 
     FIG. 4A  and  FIG. 4B  are a three-dimensional diagram and a perspective diagram illustrating the combiner shown in  FIG. 2  respectively. Referring to  FIG. 4A  and  FIG. 4B , in one embodiment of the present invention, the combiner  210  has, for example but not limited to, at least a pared-corner C. In the embodiment, although three pared-corners C are provided, however, the amount of the pared-corner C of the invention are not limited and is decided according to the requirement. 
   Referring to  FIG. 4A  and  FIG. 4B , the combiner  210  of one embodiment of the present invention includes, for example but not limited to, an X-prism or other optical combiner. In an embodiment of the present invention, for example but not limited to, there are two optical coatings R and B in the internal of the X-prism. The optical coating R is an optical coating for reflecting red light, and the optical coating B is an optical coating for reflecting blue light. Furthermore, the above-described X-prism is substantially a regular hexahedron having four sides  212 , a top surface  214 , a bottom surface  216  and three lean surfaces  218 . The top surface  214  and the bottom surface  216  are adjacent to sides  212  respectively. A portion of the lean surface  218  is adjacent to the top surface  214  and one of the sides  212 , and another portion of the lean surface  218  is adjacent to the bottom surface  216  and one another side  212 . Furthermore, the shape of the lean surface  218  is, for example but not limited to, a regular triangle. 
   Referring to  FIG. 2 ,  FIG. 3 ,  FIG. 4A  and  FIG. 4B , at least one side of the prism  220 ,  230  and  240  is leaned against the pared-corner C of the combiner  210 . Therefore, the size of the optical combiner module  200  is effectively reduced. 
     FIG. 5A ,  FIG. 5B  and  FIG. 5C  are a three-dimensional diagram, a perspective diagram and a lateral view schematically illustrating the total internal reflection (TIR) prism of  FIG. 2  respectively. Referring to  FIG. 5A ,  FIG. 5B  and  FIG. 5C , in one embodiment of the present embodiment, the prisms  220 ,  230  and  240  comprises, for example but not limited to, total internal reflection (TIR) prism  220   a ,  230   a  and  240   a . Hereinafter, an embodiment using the total internal reflection (TIR) prism  220   a  will be described in detail. 
   Referring to  FIG. 5A  and  FIG. 5B , the total internal reflection (TIR) prism  220   a  comprises, for example but not limited to, a first prism  222  and a second prism  224 . The first prism  222  comprises, for example, a first light incident plane  222   a , a first junction surface  222   b  and a first light exit plane  222   c . The second prism  224  comprises, for example, a second junction surface  224   a  and a second light exit plane  224   b . In addition, in the total internal reflection (TIR) prism  220   a , the second light exit plane  224   b  of the second prism  224  comprises, for example but not limited to, a transparent region A and a light shielding region M. The light shielding region M is provided for filtering out the stray light. 
   Thereafter, referring to  FIG. 5C , in the total internal reflection (TIR) prism  220   a , there is an air gap  216  between the second junction surface  224   a  and a portion of the first junction surface  222   b . Another portion of the first junction surface  222   b  is disposed adjacent to an lean surface  218 . The second light exit plane  224   b  is disposed adjacent to one of the sides  212 . 
   Referring to  FIG. 3  and  FIG. 5C , in one embodiment of the invention, the total internal reflection (TIR) prism  220 ,  230  and  240  comprises, for example but not limited to, a red light TIR prism  220   a , a green light TIR prism  230   a  and a blue light TIR prism  240   a  respectively. The red light TIR prism  220   a  and the blue light TIR prism  240   a  are disposed, for example, on the opposite side of the combiner  210 . In addition, each first light incident plane  222   a ,  232   a  and  242   a  of TIR prisms  220   a ,  230   a  and  240   a  is disposed on a first plane (not shown) respectively. The top surface  214  of the combiner  210  is disposed on a second plane (not shown), and an angle between the first plane and the second plane is, for example but not limited to, 45°. 
   Accordingly, the red light TIR prism  220   a  further comprises, for example but not limited to, a blue light reflecting coating disposed on the first junction surface  222   b  of the first prism  222 , or on the second junction surface  224   a  of the second prism  224  to filter out the stray light except for the blue light. In addition, the blue light TIR prism  240   a  further comprises, for example but not limited to, a red light reflecting coating to filter out the stray light except for the red light. 
     FIG. 6  is a top view schematically illustrating a digital light projection system according to an embodiment of the present invention. Referring to  FIG. 6 , in one embodiment of the present invention, the digital light projection system comprises the above-described optical combiner module  200 , a light source  300 , a beam splitter module  400 , a plurality of digital micro-mirror devices (DMD)  500 R,  500 G and  500 B, and a projection lens  600 . Hereinafter, each component of the digital light projection system will be described in detail. 
   First of all, a light W is emitted from the light source  300 . The light source  300  comprises, for example but not limited to, metal-halide lamp (MHL) or ultra-high performance (UHL) lamp that may provide a high brightness, high color saturation white light. 
   The beam splitter module  400  is disposed in the path of the light W after the light source  300 , and is provided for separating the light W into a first color light, a second color light and a third color light, such as three primary color lights R, G and B. In one embodiment of the invention, the beam splitter module  400  comprises, for example but not limited to, a first two-way beam splitter mirror  402  and a second two-way beam splitter mirror  404 . The first two-way beam splitter mirror is disposed in the path of the light W after the light source  300  for reflecting the color light B and transmitting the color light R and color light G. The second two-way beam splitter mirror  404  is disposed in the light path of the color light R and the color light G after the first two-way beam splitter mirror  402  for reflecting the color light R and transmitting the color light G. 
   The optical combiner module  200  is disposed in the intersection of color lights R, G and B after the beam splitter module  400 . The size of the optical combiner module  200  is effectively reduced. The back focal length is referred to the vertical distance between the digital micro-mirror devices (DMD)  500 R,  500 G and  500 B and the projection lens  600 . Therefore, the back focal length  550  of the projection device of the present invention is also effectively reduced. 
   In order to enhance the projection quality of the digital light projection system of the present invention, the beam splitter module  400  may comprise, for example but not limited to, a first condenser lens  406  and/or second condenser lenses  408  and  410 . The first condenser lens  406  is disposed in the path of the light W between the light source  300  and the first two-way beam splitter mirror  402 . The second condenser lenses  408  and  410  are disposed in the light path of the color lights R and G between the first two-way beam splitter mirror  402  and the combiner  200 . It is noted that, the second condenser lenses  408  and  410  disposed in the path of the color lights R and G are provided for correct incidence of the image on the combiner  200 . In one embodiment of the invention, a relay processing is performed to the color lights R and G before being incident on the combiner  200 . In addition, in one embodiment of the invention, the digital light projection system further comprises a rod integrator  310  disposed in the path of the light W between the light source  300  and the first two-way beam splitter mirror  402 . 
   In order to reduce the size of the digital light projection system, in one embodiment of the invention, it is optional to dispose reflection mirrors  412 ,  414 ,  416 ,  418 ,  420  and  422 . The reflection mirror  412  is disposed in, for example but not limited to, the light path of the color light B between the first two-way beam splitter mirror  402  and the combiner  200 . The reflection mirror  414  is disposed in, for example but not limited to, the path of the color lights R and G between the first two-way beam splitter mirror  402  and the second two-way beam splitter mirror  404 . The reflection mirrors  416  and  418  are disposed in, for example but not limited to, the path of the color light G between the second two-way beam splitter mirror  404  and the combiner  200 . The reflection mirrors  420  and  422  are disposed in, for example but not limited to, the path of the color light R between the second two-way beam splitter mirror  404  and the combiner  200 . 
   The digital micro-mirror devices (DMD)  500 R,  500 G and  500 B are disposed in the path of the color lights R, G and B after the optical combiner module  200  respectively. It is noted that, each color light R, G and B is reflected to the DMDs  500 R,  500 G and  500 B by the air gap of the TIR prisms  220   a ,  230   a  and  240   a  respectively, and then reflected by the DMDs  500 R,  500 G and  500 B and transmitted the TIR prisms  220   a ,  230   a  and  240   a . In addition, the projection lens  600  is disposed in the light path of each color light R, G and B of the optical combiner module  200 . 
   As shown in  FIG. 6 , the path of each color light R, G and B in the beam splitter module  400  is specially designed, and will be described in detail hereinafter. A path of color light B and a path of color light R and G are generated after the light W passes the first two-way beam splitter mirror  402 . In addition, a path of color light R and a path of color light G are generated after the color light R and G passes the second two-way beam splitter mirror  404 . Hereinafter, the disposing of the reflection mirrors in the paths of color light B, color light R and color light G will be described respectively. 
   In the path of color light B, the reflection surface of the first two-way beam splitter mirror  402  is disposed, for example but not limited to, a little upward to reflect the color light B upward. The reflection surface of the reflection mirror  412  is disposed, for example but not limited to, a little downward between the first two-way beam splitter mirror  402  and the combiner  200  to reflect the color light B to the second light incident plane  242   a . In other words, the position of the reflection mirror  412  is a little higher than the first two-way beam splitter mirror  402  and the combiner  200 . 
   In the path of color light G, the reflection surface of the reflection mirror  416  is disposed, for example but not limited to, a little upward to reflect the color light G upward. The reflection surface of the reflection mirror  418  is disposed, for example but not limited to, a little downward to reflect the color light G to the second light incident plane  232   a . In other words, the position of the reflection mirror  416  is a little lower than the second two-way beam splitter mirror  404  and the combiner  200 . However, the position of the reflection mirror  418  is a little higher than the second two-way beam splitter mirror  404  and the combiner  200 . 
   In the path of color light R, the reflection surface of the reflection mirror  420  is disposed, for example but not limited to, a little downward to reflect the color light R upward. The reflection surface of the reflection mirror  422  is disposed, for example but not limited to, a little upward to reflect the color light R to the second light incident plane  222   a  (not shown). In other words, the position of the reflection mirror  420  is a little higher than the second two-way beam splitter mirror  404  and the combiner  200 . The position of the reflection mirror  422  is a little lower than second two-way beam splitter mirror  404  and the combiner  200 . 
     FIG. 7  is a drawing schematically illustrating a digital micro-mirror device (DMD) according to an embodiment of the present invention.  FIG. 8A  is a drawing schematically illustrating an embodiment having an angle 10° between micro mirror and active surface according to the present invention.  FIG. 8B  is a drawing schematically illustrating an embodiment having an angle 12° between micro mirror and active surface according to the present invention. Referring to  FIG. 7 ,  FIG. 8A  and  FIG. 8B , the DMDs  500 R,  500 G or  500 B of  FIG. 7  comprise, for example but not limited to, semiconductor components controlled by two-bits pulse tuning. The DMDs  500 R,  500 G or  500 B comprises an active surface  510 , and a plurality of array arranged micro mirrors  520  is formed on the active surface  510 . The micro mirrors  520  are generally controlled by active components such as metal oxide semiconductor (MOS) disposed on the active surface  510 . 
   In one embodiment of the invention, the tilt angle between the micro mirrors  520  and the active surface  510  is, for example but not limited to, in a range of about +10° to about −10° (as shown in  FIG. 8A ) or in a range of about +12° to about −12° (as shown in  FIG. 8B ). Hereinafter, some embodiments will be described corresponding to the tilt angle of the micro mirror  520 . 
   When the tilt angle between the micro mirror  520  and the active surface is about +10° or about +12°, the angle between the incident light L 0  and the reflection light L 1  is about 20° or 24°. Thus, the reflection light L 1  may exit from the transparent region A (as shown in  FIG. 5A  and  FIG. 5B ), and the state of the micro mirror  520  is ON. 
   When the tilt angle between the micro mirror  520  and the active surface is about −10° or −12°, the angle between the incident light L 0  and the reflection light L 2  is about 40° or 48°. Thus, the reflection light L 2  cannot exit the transparent region A (as shown in  FIG. 5A  and  FIG. 5B ), and the state of the micro mirror  520  is OFF. 
   It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided that they fall within the scope of the following claims and their equivalents.