BACKGROUND OF THE INVENTION 
   a. Field of the Invention 
   The invention relates to an optical system for a projection display and, more particularly, to an optical system for a projection display capable of providing high image contrast and a wide viewing angle. 
   b. Description of the Related Art 
   A projection display typically consists of an illumination system and a projection system. The illumination system incorporates a light path switching device that consists of a plurality of relatively small elements each being used to switch light path individually. After being modulated by the switching elements, light beams emitted from a light source are projected on a projection surface through the projection system. 
   A digital micromirror device (DMD) manufactured by Texas Instruments (TI), as an example of a light path switching device, is composed of thousands of micromirrors. The DMD panel&#39;s micromirrors are mounted on tiny hinges that enable them to tilt either toward the light source (ON mode) or away from it (OFF mode), thus creating a light or dark pixel on the projection surface. 
     FIG. 1  is a schematic view showing a conventional optical system  100  for a projection display. Referring to  FIG. 1 , the tiltable micromirrors on a digital micromirror device  102  may either direct the incoming light l onto a projection lens  104  along path  108  under the “On mode” or direct it away from the projection lens  104  along path  110  under the “Off mode”, thereby creating a light or dark pixel on the projection surface. 
   A total internal reflection (TIR) prism set  106 , composed of two prisms  106   a  and  106   b  adhered to each other with an air gap  112  interposed therebetween, is disposed in a light path between the digital micromirror device  102  and the projection lens  104 . The TIR prism set  106 , inside which total internal reflection occurs at the boundary between the prism  106   b  and the air gap  112 , guides the incoming light l to the digital micromirror device  102  along the light path shown in FIG.  1 . 
   However, through such design, since the tilting range of the micromirror is limited, the light path of the incoming light l between the digital micromirror device  102  and the projection lens  104  under the On mode is almost the same as that under the Off mode; hence, an edge portion of the spread-out incoming light l enters the projection lens  104  under the Off mode to result in a deterioration in the image contrast. Though this problem may be solved by increasing the distance between the projection lens  104  and the TIR prism set  106  to prevent stray light from entering the projection lens under the Off mode, the back focal length, however, is increased accordingly, and thus it is difficult to design a projection lens with a wide viewing angle. 
     FIGS. 2A and 2B  are schematic views showing another optical system  200  for a projection display. The TIR prism set  206  of the optical system  200  includes three prisms, and air gaps  208  and  210  are formed between each two adjacent prisms. Under the On mode as shown in  FIG. 2A , the incoming light l enters the digital micromirror device  202  due to the total internal reflection occurring at the boundary between the air gap  208  and the prism. Then, the light l reflected by the micromirror on the digital micromirror device  202  passes through the TIR prism set  206  and enters a projection lens  204  along a non-reflected optical axis. On the other hand, as shown in  FIG. 2B , under the Off mode the light reflected by the micromirror on the digital micromirror device  202  is reflected outside the optical system  200  due to the total internal reflection occurring at the boundary between the air gap  210  and the prism. Such TIR prism set  206  may render the light paths under the On mode and the Off mode more distinguishable to prevent stray light from entering the projection lens. However, the width W along the non-reflected optical axis of the assembled TIR prism set  206  becomes larger and the back focal length is increased, thus it is also difficult to design a projection lens with a wide viewing angle. 
   BRIEF SUMMARY OF THE INVENTION 
   An object of the invention is to provide an optical system for a projection display capable of eliminating the stray light to enhance the image contrast and shortening the back focal length to provide a projection display with a wide viewing angle. 
   According to the invention, the optical system includes a light source, a light path switching device, and a total internal reflection (TIR) prism set disposed between the light path switching device and the projection lens. The light path switching device has a first mode of operation for directing the light towards a projection lens and a second mode of operation for directing the light away from the projection lens. The TIR prism set includes a first prism, a second prism and a third prism; a first gap is formed between the first prism and the second prism, and a second gap is formed between the first prism and the third prism. The light emitted from the light source enters the light path switching device by means of total internal reflection. Then, under the first mode, the light reflected by the light path switching device passes through the first and the second gaps and enters the projection lens, whereas under the second mode, the light reflected by the light path switching device is totally reflected at the boundary between the first gap and the second prism and away from the projection lens. 
   Through the design of the invention, the light reflected by the light path switching device under the “Off” mode is totally reflected at the boundary between the air gap and the prism that the light reflected by the light path switching device first meets when entering the TIR prism set. Thus, since the incoming light l that is to be removed under the “Off” mode is quickly and completely directed away from the projection lens, the overall thickness along the non-reflected optical axis of the assembled TIR prism set can be greatly reduced, and the back focal length is decreased as a result. Consequently, a wide viewing angle for the projection lens can be achieved easily. 
   Also, according to the invention, the TIR prism set can be shaped to provide the total internal reflection for restricting the light path through which the light reflected by the micromirror array travels under the Off mode. Hence, it can be further ensured that the stray light no longer enters the projection lens under the Off mode. 


   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic view showing a conventional optical system for a projection display 
       FIGS. 2A and 2B  are schematic views showing another optical system for a projection display. 
       FIG. 3  is a schematic view showing an optical system for a projection display according to an embodiment of the invention. 
       FIGS. 4A and 4B  exhibit the light paths through which the incoming light l travels after entering the TIR prism set according to an embodiment of the invention, where  FIG. 4A  shows the light path under the On mode while  FIG. 4B  shows that under the Off mode. 
       FIG. 5  shows an actual dimension of an assembled TIR prism set of the invention compared to that of the prior art. 
       FIG. 6  is a schematic view showing a modification of the TIR prism set according to another embodiment of the invention. 
   


   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIG. 3 , an optical system  10  for a projection display includes a light source  12 , a light guide  14 , a relay lens  16 , a total internal reflection (TIR) prism set  18 , a light path switching device  20 , and a projection lens  22 . A light collector such as an ellipsoid mirror  30  may be arranged to partially surround the light source  12  to focus the light beams, emitted from the light source  12 , onto the light guide  14 . The light guide  14 , which is hollow with interior reflecting walls where total internal reflections successively occur, receives the light beams from the light source  12  and outputs them as evenly distributed light beams. The evenly distributed light beams are then projected on the TIR prism set  18  after passing through the relay lens  16 . 
   A micromirror array  20   a  consists of a plurality of tiltable micromirrors is disposed on the light path switching device  20 . The tiltable micromirrors may either direct the incoming light onto a projection lens  22  under the “On mode” or direct it away from the projection lens  22  under the “Off mode”, thereby creating a light or dark pixel on a projection surface. It should be noted that the modes of operation of the tiltable micromirrors include, but are not limited to, the aforesaid “On mode” and “Off mode”, and can be adapted to the actual demand of the light modulation. 
     FIGS. 4A and 4B  exhibit the light paths through which the incoming light l travels after entering the TIR prism set  18  according to an embodiment of the invention, where  FIG. 4A  shows the light path under the On mode while  FIG. 4B  shows that under the Off mode. In this embodiment, the TIR prism set  18  is composed of a prism  18   a  adjacent to the light guide  14 , a prism  18   b  adjacent to the light path switching device  20 , and a prism  18   c  adjacent to the projection lens  22 . An air gap  24  is formed between the prism  18   a  and the prism  18   c , and another air gap  26  is formed between the prisms  18   a  and  18   b.    
   The incoming light l first strikes the boundary between the prism  18   a  and the air gap  24  at a predetermined incident angle, which is greater than the critical angle calculated from the Snell&#39;s law, so that the incoming light l is totally reflected to the light path switching device  20 . 
   Referring to  FIG. 4A , when micromirror array  20   a  is tilted under the On mode, the light reflected by the micromirror array  20   a  sequentially passes through the air gap  26  and air gap  24  along a non-reflected optical axis and then enters the projection lens  22 . On the other hand, as shown in  FIG. 4B , under the Off mode the light reflected by the micromirror array  20   a  strikes the boundary between the prism  18   b  and the air gap  26  at an incident angle, which is also designed to be greater than the critical angle calculated from the Snell&#39;s law. 
   Hence, through the design of the invention, the incoming light  1  that is to be removed under the Off mode is quickly and completely directed away from the projection lens  22 , for it is totally reflected at the boundary between the prism  18   b  and the air gap  26 . In other words, the light reflected by the micromirror array  20   a  under the Off mode is further totally reflected at the boundary between the air gap  26  and the prism  18   b , and the prism  18   b  is the prism that the light reflected by the micromirror array  20   a  first meets when entering the TIR prism set  18 . 
   Further, the TIR prism set  18  of the invention is designed such that the light reflected at the boundary between the prism  18   b  and the air gap  26  is totally reflected again on the surface S 1  of the prism  18   b  closest to the micromirror array  20   a.    
   In addition, the prism  18   b  can be shaped such that the light reflected by the surface S 1  may further strike the prism surface opposed to the surface S 1  at an incident angle greater than the critical angle. Therefore, the traveling path of the incoming light  1  under the Off mode is restricted within the prism  18   b  as shown in  FIG. 4B , and, finally, the light leaves the optical system via the side surface S 2  of the prism  18   b  to further ensure that the stray light no longer enters the projection lens  22  under the Off mode. 
   Furthermore, since the light under the Off mode leaves the optical system via the side surface S 2  of the prism  18   b , a light-absorbing substance may be applied to the side surface S 2  so as to absorb the light. For example, a light absorbing layer  28  may be coated on the side surface S 2  of the prism  18   b.    
   Through the TIR prism set design of the invention, the light reflected by the micromirror array  20   a  under the “Off” mode is totally reflected at the boundary between the air gap and the prism that the light reflected by the micromirror array  20   a  first meets when entering the TIR prism set. Thus, since the incoming light  1  that is to be removed under the “Off” mode is quickly and completely directed away from the projection lens  22 , the overall thickness along the non-reflected optical axis of the assembled TIR prism set can be greatly reduced, and the back focal length is decreased as a result. This makes it easy to design a projection lens having a wide viewing angle. 
   Also, according to the invention, the TIR prism set can be shaped to provide the total internal reflection for restricting the light path through which the light reflected by the micromirror array travels under the Off mode. As a result, it can be further ensured that the stray light no longer enters the projection lens under the Off mode. 
     FIG. 5  shows the actual dimension along the non-reflected optical axis of an assembled TIR prism set of the invention (on the right) compared to the prior art (on the left). It should be noted that the dimension shown in the figure are determined under the same conditions for providing total internal reflection; for instance, the material of the TIR prism set and the adopted micromirror array are the same. As for the width W along the non-reflected optical axis of the assembled TIR prism set, the conventional three-piece TIR prism set  206  has a thickness W equal to 77.2 mm while the TIR prism set  18  of the invention has a thickness W′ equal to only 23.96 mm. 
     FIG. 6  is a schematic view showing a modification of the TIR prism set  18  according to another embodiment of the invention. According to the invention, the TIR prism set design is required only to maintain an air gap between adjacent prisms to provide the total internal reflection, and the shape or arrangement of the prisms can be adapted to conform to the actual light path. 
   For instance, if the light path of the optical system needs to be shortened, the conventional method is to dispose an additional reflection mirror to change the light path of the incoming light  1 . However, in this embodiment, the shape of the prism  18   a  may be modified so that it becomes a quadrangular prism where an additional reflection surface R is provided, as shown in FIG.  6 . Thereby, the incoming light l is first reflected by the reflection surface R and then incident at the boundary between the prism  18   a  and the air gap  24 , where the total internal reflection occurs. Hence, the light path in the optical system can be changed without the need of any additional element such as a reflection mirror, and thus it is possible to reduce the manufacturing cost and facilitate the assembly. 
   While the invention has been described by way of examples and in terms of preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications. Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications.
An optical system for enhancing the image contrast and shortening the back focal length of a projection display is disclosed, which includes a light source, a light path switching device, and a total internal reflection (TIR) prism set disposed between the light path switching device and the projection lens. The light path switching device has a first mode of operation for directing the light towards a projection lens and a second mode of operation for directing the light away from the projection lens. The TIR prism set includes a first prism, a second prism and a third prism; a first gap is formed between the first prism and the second prism, and a second gap is formed between the first prism and the third prism.