Patent Publication Number: US-8116019-B2

Title: Illumination system

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional application of and claims the priority benefit of U.S. application Ser. No. 11/849,720, filed on Sep. 4, 2007, now allowed, which claims the priority benefit of Taiwan application serial no. 96206078, filed on Apr. 16, 2007. The entirety of each of the above-mentioned patent applications is hereby incorporated by reference herein and made a part of this specification. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an illumination system. More particularly, the present invention relates to an illumination system using a coherent light source. 
     2. Description of Related Art 
     Referring to  FIG. 1 , a conventional laser projection apparatus  100  including an illumination system  110 , a digital micro-mirror device (DMD)  120 , and a projection lens  130 . The illumination system  110  includes a laser light source  112 , a plurality of lenses  114 , and a light integration rod  116 . A laser beam  113  emitted from the laser light source  112  is expanded by the lens  114  and uniformed by the light integration rod  116  to become an illumination beam  113   a . After being projected onto the DMD, the illumination beam  113   a  is converted by the DMD into an image beam  113   b . The image beam  113   b  is projected onto a screen (not shown) through the projection lens  130 , thus forming a display image. 
       FIG. 2  shows a speckle pattern projected on the screen by the laser projection apparatus in  FIG. 1 . Referring to  FIGS. 1 and 2 , due to the high coherence of the laser beam  113 , after the laser beam  113  passes through an optical component (such as a lens, a reflector) having a slightly uneven surface in the laser projection apparatus  100 , a speckle pattern formed by the laser beam  113  is generated on the screen due to the interference phenomenon of the laser beam  113 . The speckle pattern is an irregular noise pattern. The speckle phenomenon causes a non-uniform brightness of the display image, thus degrading the optical quality of the laser projection apparatus  100 . 
     SUMMARY OF THE INVENTION 
     The present invention provides an illumination system, which effectively reduces the degree of the speckle phenomenon. 
     Other advantages of the present invention can be further understood from the technical features disclosed by the present invention. 
     In order to achieve one or a part of or all of the above-mentioned advantages or other advantages, an illumination system comprising at least one coherent light source, a light uniforming element, and a prism is provided. The coherent light source is capable of emitting a coherent beam. The light uniforming element is disposed on a transmission path of the coherent beam. The prism is disposed on the transmission path of the coherent beam and between the coherent light source and the light uniforming element. The prism has a light incident end and a light exit end. The light incident end has at least one incident polygonal pyramid portion protruding away from the light exit end. Moreover, the prism is capable of rotating about an axis, and the axis extends from the light incident end to the light exit end. 
     In the illumination system of the present invention, the prism rotates, and the incident polygonal pyramid portion splits the coherent beam into a plurality of beams incident on the light uniforming element from different angles, and thus the spots formed on the light incident section of the light uniforming element by the beams rotate with the rotation of the prism, and the incident angles of the beams on the light uniforming element vary with the rotation of the prism. Therefore, the speckle pattern output by the illumination system changes with time. Due to visual persistence effect, human eyes observe the superposition of the speckle patterns of different time points in a visual persistence time interval. The speckle distributions in the speckle patterns at different time points are different, such that a pattern having a more uniform brightness is generated after the superposition. Therefore, the illumination system of the invention effectively reduces the degree of the speckle phenomenon, and the projection apparatus using the illumination system of the present invention provides display images having uniform brightness. 
     Other objectives, features and advantages of the present invention will be further understood from the further technological features disclosed by the embodiments of the present invention wherein there are shown and described preferred embodiments of this invention, simply by way of illustration of modes best suited to carry out the invention. 
    
    
     
       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 drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
         FIG. 1  is a schematic structural view of a conventional laser projection apparatus. 
         FIG. 2  shows a speckle pattern projected on a screen by the laser projection apparatus in  FIG. 1 . 
         FIG. 3A  is a schematic structural view of an illumination system according to an embodiment of the present invention. 
         FIG. 3B  is a perspective view of a prism in  FIG. 3A . 
         FIG. 3C  is a schematic structural view of an illumination system according to another embodiment of the invention. 
         FIGS. 4A ,  4 B, and  4 C respectively show positions of spots projected on a light incident section of a light uniforming element when the prism in  FIG. 3A  rotates to three different angles. 
         FIG. 5A  is a perspective view of a prism in an illumination system according to another embodiment of the present invention. 
         FIG. 5B  is a sectional view of the prism in  FIG. 5A  passing through a axis. 
         FIG. 6A  is a perspective view of a prism in an illumination system according to yet another embodiment of the present invention. 
         FIG. 6B  is a sectional view of the prism in  FIG. 6A  passing through a axis. 
         FIGS. 7A and 7B  show a prism and an actuator in an illumination system according to another embodiment of the present invention. 
         FIGS. 8A and 8B  show a prism in an illumination system according to still another embodiment of the present invention. 
         FIG. 9  is a schematic structural view of an illumination system according to still another embodiment of the present invention. 
         FIG. 10  is a schematic structural view of a projection apparatus according to an embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     In the following detailed description of the preferred embodiments, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. In this regard, directional terminology, such as “top,” “bottom,” “front,” “back,” etc., is used with reference to the orientation of the Figure(s) being described. The components of the present invention can be positioned in a number of different orientations. As such, the directional terminology is used for purposes of illustration and is in no way limiting. On the other hand, the drawings are only schematic and the sizes of components may be exaggerated for clarity. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention. Also, it is to be understood that the phraseology and terminology used herein are for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Unless limited otherwise, the terms “connected,” “coupled,” and “mounted” and variations thereof herein are used broadly and encompass direct and indirect connections, couplings, and mountings. Similarly, the terms “facing,” “faces” and variations thereof herein are used broadly and encompass direct and indirect facing, and “adjacent to” and variations thereof herein are used broadly and encompass directly and indirectly “adjacent to”. Therefore, the description of “A” component facing “B” component herein may contain the situations that “A” component facing “B” component directly or one or more additional components is between “A” component and “B” component. Also, the description of “A” component “adjacent to” “B” component herein may contain the situations that “A” component is directly “adjacent to” “B” component or one or more additional components is between “A” component and “B” component. Accordingly, the drawings and descriptions will be regarded as illustrative in nature and not as restrictive. 
     Referring to  FIGS. 3A and 3B , an illumination system  200  according to an embodiment of the present invention is applied in a projection apparatus. The illumination system  200  comprises a coherent light source  210 , a light uniforming element  220 , and a prism  230 . The coherent light source  210  is capable of emitting a coherent beam  212 . In this embodiment, the coherent light source  210  is, for example, a laser light source. The light uniforming element  220  is disposed on a transmission path of the coherent beam  210 , such that the illumination system  200  outputs a uniform beam. In this embodiment, the light uniforming element  220  is, for example, a light integration rod. However, in other embodiments, the light uniforming element  220  is also a lens array or other elements capable of uniforming the beam. The prism  230  is disposed on the transmission path of the coherent beam  212  and between the coherent light source  210  and the light uniforming element  220 . 
     The prism  230  has a light incident end  232  and a light exit end  234 . The light incident end  232  has an incident polygonal pyramid portion  233  protruding away from the light exit end  234 . In this embodiment, the incident polygonal pyramid portion  233  is composed of a plurality of triangular facets  233   a , and the number of the triangular facets  233   a  is greater than or equal to 3. In other words, the polygonal pyramid is, for example, trigonal pyramid, tetragonal pyramid, pentagonal pyramid . . . or N-sided pyramid, where N is an integer greater than or equal to 3. Moreover, in this embodiment, the light exit end  234  of the prism  230  has a curved light exit surface  235  denting towards the light incident end  232  or protruding away from the light incident end  232 . The curved light exit surface  235  shown in  FIG. 3A  is a curved light exit surface denting towards the light incident end  232  as an example. In another embodiment, a curved light exit surface  235   c  of a prism  230   c  of an illumination system  200   c  shown in  FIG. 3C  is a curved light exit surface protruding away from the light incident end  232 . However, in other embodiments, the light exit end  234  is also a light exit plane instead of the curved light exit surface  235 , or the number of the curved light exit surfaces  235  of the light exit end  234  is also more than one. 
     The prism  230  is capable of rotating about an axis A, and the axis A extends from the light incident end  232  to the light exit end  234 . In this embodiment, the axis A is substantially parallel to the chief ray of the coherent beam  212 . Besides, the axis A passes through an apex P of the incident polygonal pyramid portion  233 . Moreover, the illumination system  200  further comprises, for example, an actuator  240  connected with the prism  230  for driving the prism  230  to rotate. The actuator  240  is, for example, a motor or other mechanisms used for driving the prism  230  to rotate. In addition, the illumination system  200  further comprises a beam expander  250  disposed on the transmission path of the coherent beam  212  and between the coherent light source  210  and the prism  230 , so as to expand the diameter of the coherent beam  212 . The beam expander  250  comprises a lens or other optical components capable of expanding the diameter of the coherent beam  212 . 
     In the illumination system  200  of this embodiment, after passing through the incident polygonal pyramid portion  233 , the coherent beam  212  is split into beams  212 ′ by the incident polygonal pyramid portion  233 , and the beams  212 ′ are incident on the light uniforming element  220  from different angles and form a plurality of spots S (shown in  FIG. 4A ) on the light incident section  222  of the light uniforming element  220 . The prism  230  rotates, so the position and angle of the beams  212 ′ incident on the light incident section  222  change with time, such that the spots S (shown in  FIG. 4A ) rotate with the rotation of the prism  230 .  FIGS. 4A ,  4 B, and  4 C show the positions of the spots S projected on the light incident section  222  when the prism  230  rotates to three different angles, respectively. 
     The spots S located on the light incident section rotates with the rotation of the prism  230 , and the incident angle of the beams  212 ′ on the light uniforming element  220  changes with the rotation of the prism  230 , and thus the speckle pattern output by the illumination system  200  changes with time. Due to visual persistence effect, the human eyes observe the superposition of the speckle patterns at different time points in a visual persistence time interval. The speckle distributions in the speckle patterns at different time points are different, such that a pattern having a more uniform brightness is generated after the superposition. Therefore, the illumination system  200  of this embodiment effectively reduces the degree of the speckle phenomenon. 
     Moreover, each of the beams  212 ′ split by the incident polygonal pyramid portion  233  forms a corresponding speckle pattern, so the speckle pattern generated at a time point in this embodiment is a superposition of a plurality of speckle patterns, such that the illumination system  200  of this embodiment remarkably reduces the degree of the speckle phenomenon. In order to further reduce the degree of the speckle phenomenon, in this embodiment, the chief ray of the coherent beam  212  passes through the apex P of the incident polygonal pyramid portion  233 , so as to make each of the beams  212 ′ split by the incident polygonal pyramid portion  233  having substantially the same intensity, and to further make the brightness of the speckle pattern more uniform after the superposition. 
     Experimental results show that the speckle ratio of the speckle pattern generated by the illumination system  200  in this embodiment is 6%, and the speckle ratio of the illumination system without using the prism  230  is 18%, in which the speckle ratio is defined as the ratio of a brightness standard deviation of the points in the speckle pattern to an average brightness of the points. Compared with the illumination system without using the prism  230 , the illumination system  200  of this embodiment has the speckle ratio reduced by ⅔, thereby the illumination system  200  indeed effectively reduces the degree of the speckle phenomenon. However, the degree of the speckle phenomenon reduced in this embodiment is not limited in the scope of the present invention. 
     It should be noted that the present invention does not limit the number of the incident polygonal pyramid portion  233  of the light incident end  232  to be one. In other embodiments, the number of the incident polygonal pyramid portions  233  of the light incident end is also more than one, and the incident polygonal pyramid portions  233  are arranged in an array or other manners. 
     Referring to  FIGS. 5A and 5B , in an illumination system according to another embodiment, a prism  230 ′ is used to replace the prism  230  (referring to FIG.  3 A). The prism  230 ′ is similar to the prism  230 , and the difference between the two is that the light exit end  234 ′ of the prism  230 ′ has an exit polygonal pyramid portion  236  protruding away from the light incident end  232 . In this embodiment, the shape of the exit polygonal pyramid portion  236  is the same as that of the incident polygonal pyramid portion  233  (referring to  FIG. 3B ), i.e., the exit polygonal pyramid portion  236  is also composed of a plurality of triangular facets  236   a , and the number of the triangular facets  236   a  is greater than or equal to 3. The prism  230 ′ also achieves the effect of the prism  230  (referring to  FIG. 3A ) for reducing the degree of the speckle phenomenon, and thus the details will not be repeated herein again. 
     Referring to  FIGS. 6A and 6B , in an illumination system according to another embodiment of the present invention, a prism  230 ″ is similar to the prism  230 ′ (as shown in  FIG. 5 ), and the difference between the two is that an exit polygonal pyramid portion  236 ″ of a light exit end  234 ″ of the prism  230 ″ dents towards the light incident end  232 . The exit polygonal pyramid portion  236 ″ is also composed of a plurality of triangular facets  236   a″.    
     It should be noted that the number of the exit polygonal pyramid portion (such as the exit polygonal pyramid portion  236  or  236 ″) of the light exit end of the prism is not limited to be one in the present invention. In other embodiments, the light exit end of the prism also has a plurality of exit polygonal pyramid portions. 
     Referring to  FIGS. 7A and 7B , in an illumination system according to another embodiment of the present invention, a prism  230   a  is, for example, annular-shaped, and a light incident end  232   a  of the prism  230   a  has a plurality of incident polygonal pyramid portions  233  disposed around the axis A. Moreover, in this embodiment, the light exit end  234   a  of the prism  230   a  has a plurality of curved light exit surfaces  235  disposed around the axis A. Besides, each incident polygonal pyramid portion  233  and the corresponding curved light exit surface  235  are fall on a reference line L parallel to the axis A. In addition, an actuator  240 ′ is, for example, a motor having a rotor  242 , and the prism  230   a  surrounds and is connected with the rotor  242  for making the prism  230   a  to rotate about the axis A. 
     In this embodiment, when the prism  230   a  rotates, the coherent beam passes through the incident polygonal pyramid portions  233  sequentially. For example, the range of the coherent beam projected on the prism  230   a  at a time point is a light acceptance range R as shown in the figures. Therefore, the angle and position of the coherent beam incident on the light uniforming element have more variations, so as to further reduce the degree of the speckle phenomenon. Moreover, the annular-shaped prism  230   a  is capable of connecting the rotor  242  of the actuator  240 ′ with the prism  230   a  in the absence of a gear, thereby simplifying the mechanism. 
     In the illumination system according to other embodiments, a plurality of exit polygonal pyramid portions (such as the exit polygonal pyramid portions  236  and  236 ″) is also used to replace the light exit curved surfaces  235  in the prism  230   a.    
     Refereeing to  FIGS. 8A and 8B , in an illumination system according to another embodiment of the present invention, incident polygonal pyramid portions  233  of a light incident end  232   b  of a prism  230   b  are also disposed around the axis A layer by layer, and the coherent beam passes through the plurality of incident polygonal pyramid portions  233  at a time point, such that the angle and position of the coherent beam incident on the light uniforming element have more variations. Moreover, the light exit end  234   b  of the prism  230   b  has a light exit plane  238 . 
     It should be noted that the number of the coherent light source in the illumination system is not limited to be one in the invention. In other embodiments, the number of the coherent light source in the illumination system is also more than one, and one embodiment is illustrated as follows. 
     Referring to  FIG. 9 , an illumination system  200   a  according to yet another embodiment of the invention has a plurality of coherent light sources  210 , e.g., coherent light sources  210   a ,  210   b , and  210   c , and the coherent beams  212  (e.g., coherent light beams  212   a ,  212   b , and  212   c ) emitted by the coherent light sources  210  have the same or different color(s). Moreover, the illumination system  200   a  further comprises a light combining unit  260  disposed on the transmission paths of the coherent beams  212  and between the coherent light sources  210  and the prism  230 , so as to combine the coherent beams  212  as a whole. 
     In particular, in an embodiment, the coherent beams  212   a ,  212   b , and  212   c  have different colors, and the light combining unit  260  comprises two dichroic mirrors  262   a  and  262   b . The dichroic mirror  262   a  combines the coherent beams  212   a  and  212   b  into a coherent beam  212   d , and the dichroic mirror  262   b  further combines the coherent beams  212   d  and  212   c  as a whole. When the colors of coherent beams  212   a ,  212   b , and  212   c  are red, green, and blue, the light combining unit  260  combines the coherent beams  212  into a white beam. As such, the illumination system  200   a  outputs a white light. In other embodiment, the light combining unit is also an X-cube. 
     Referring to  FIG. 10 , a projection apparatus  300  according to an embodiment of the invention includes the above illumination system  200 , a light valve  310 , and a projection lens  320 . The illumination system  200  is capable of providing an illumination beam  212 ′, and the light uniforming element  220  is used to uniform the coherent beams  212  to form the illumination beam  212 ′. The light valve  310  is disposed on the transmission path of the illumination beam  212 ′, so as to convert the illumination beam  212 ′ into an image beam  212 ″, and the light valve  310  is, for example, a digital micro-mirror device (DMD), a liquid crystal on silicon panel (LCOS panel), a transmissible liquid crystal panel, or other elements having a function of light modulation. The projection lens  320  is disposed on the transmission path of the image beam  212 ″, so as to project the image beam  212 ″ on the screen to form a display image. Since the illumination system  200  effectively reduces the degree of the speckle phenomenon, the projection apparatus  300  of this embodiment provides a display image having a uniform brightness and has a preferred optical quality. 
     It should be noted that, in other embodiments, the illumination system  200  in the projection apparatus  300  is also replaced by the illumination system in other embodiments mentioned above. Moreover, the projection apparatus also includes an illumination system having a plurality of the coherent light sources, and use the light combining unit to combine the plurality of coherent beams emitted by the coherent light sources as a whole. 
     Based on the above, in the illumination system of the invention, the prism rotates, and the incident polygonal pyramid portion splits the coherent beam into a plurality of beams incident on the light uniforming element from different angles, so the spots formed on the light incident section of the light uniforming element by the beams rotate with the rotation of the prism, and the incident angles of the beams on the light uniforming element change with the rotation of the prism. Thereby, the speckle pattern output by the illumination system changes with time. Due to the visual persistence effect, the human eyes observe the superposition of the speckle patterns at different time points in a visual persistence time interval. The speckle distributions in the speckle patterns at different time points are different, such that a pattern having a more uniform brightness is generated after the superposition. Therefore, the illumination system of the present invention effectively reduces the degree of the speckle phenomenon, and the projection apparatus using the illumination system of the present invention provides display images having a uniform brightness. 
     Moreover, each beam split by the incident polygonal pyramid portion forms a corresponding speckle pattern, so the speckle pattern generated at a time point in the present invention is a superposition of a plurality of speckle patterns, such that the illumination system of the present invention remarkably reduces the degree of the speckle phenomenon. 
     The foregoing description of the preferred embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form or to exemplary embodiments disclosed. Accordingly, the foregoing description should be regarded as illustrative rather than restrictive. Obviously, many modifications and variations will be apparent to practitioners skilled in this art. The embodiments are chosen and described in order to best explain the principles of the invention and its best mode practical application, thereby to enable persons skilled in the art to understand the invention for various embodiments and with various modifications as are suited to the particular use or implementation contemplated. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents in which all terms are meant in their broadest reasonable sense unless otherwise indicated. Therefore, the term “the invention”, “the present invention” or the like is not necessary limited the claim scope to a specific embodiment, and the reference to particularly preferred exemplary embodiments of the invention does not imply a limitation on the invention, and no such limitation is to be inferred. The invention is limited only by the spirit and scope of the appended claims. The abstract of the disclosure is provided to comply with the rules requiring an abstract, which will allow a searcher to quickly ascertain the subject matter of the technical disclosure of any patent issued from this disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. Any advantages and benefits described may not apply to all embodiments of the invention. It should be appreciated that variations may be made in the embodiments described by persons skilled in the art without departing from the scope of the present invention as defined by the following claims. Moreover, no element and component in the present disclosure is intended to be dedicated to the public regardless of whether the element or component is explicitly recited in the following claims.