Abstract:
Provided are an illumination system capable of eliminating laser speckle and a projection system employing the illumination system. The illumination system includes a laser light source having at least one laser, and a first diffractive optical element to divide a laser beam emitted from the laser light source into a plurality of partial beams and to time-average the partial beams through a periodical movement to eliminate speckle. Accordingly, the illumination system can eliminate speckle of the beams emitted from the laser light source and the projection system employing the illumination system can achieve a high quality image.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the priority under 35 U.S.C. § 119 of Korean Patent Application No. 2004-61791, filed on Aug. 5, 2004, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present general inventive concept relates to an illumination system capable of eliminating speckle of laser beams emitted from a laser light source and a projection system employing the illumination system, and more particularly, to an illumination system and a projection system which can effectively reduce or eliminate laser speckle by moving a diffractive optical element and time-averaging partial beams. 
     2. Description of the Related Art 
     Since projection systems including a laser light source have a wide color gamut, they have excellent collimation and high light efficiency. However, such projection systems have speckle due to coherent illumination. Speckle results from an interferential interaction of coherent waves that are scattered from a diffusely reflecting surface. Such speckle constitutes a significant cause of deterioration in image resolution and quality. 
     A conventional projector for eliminating speckle disclosed in U.S. Pat. No. 6,606,173 B2 is illustrated in  FIGS. 1 and 2 . 
     Referring to  FIG. 1 , the conventional projector includes a laser light source  1 , a beam expander  2 , a holographic diffuser  3 , a first field lens  4 , and a second field lens  5 , all of which are aligned on the same axis. A Gaussian laser beam emitted from the laser light source  1  is divided by the beam expander  2 , the holographic diffuser  3 , the first field lens  4 , and the second field lens  5  into beams with uniform rectangular sections. 
     Referring to  FIGS. 1 and 2 , the beam emitted from the laser light source  1  is expanded into a collimated beam by the beam expander  2 . The collimated laser beam is divided by the holographic diffuser  3  into a plurality of rectangular beams. Each of the rectangular beams is diffused with a horizontal angle θ horiz  and a vertical angle θ vert . The horizontal angle θ horiz  and the vertical angle θ vert  can be adjusted according to a diffraction pattern of the holographic diffuser  3 . The rectangular beams are converged by the first field lens  4  such that the beams with a Gaussian distribution are transformed into uniform beams. The rectangular beams are adjusted to have shapes corresponding to a micro display system. Beams having passed through the first field lens  4  are refracted into telecentric beams by the second field lens  5 . 
     Since the collimated beam is transformed into the rectangular beams having a uniform intensity distribution by the holographic diffuser  3  and the rectangular beams are converged, laser speckle is reduced. However, when the laser speckle is reduced using only the holographic diffuser  3 , the laser speckle is not completely eliminated and image quality deteriorates. 
     SUMMARY OF THE INVENTION 
     The present general inventive concept provides a compact illumination system which can effectively reduce or eliminate laser speckle by rotating or periodically moving a diffractive optical element, and a projection system employing the illumination system. 
     Additional aspects of the present general inventive concept will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the general inventive concept. 
     The foregoing and/or other aspects of the present general inventive concept may be achieved by providing an illumination system including a laser light source including at least one laser to emit a beam, and a first diffractive optical element to divide the beam emitted from the laser light source into a plurality of partial beams and to time-average the partial beams through a periodical movement to eliminate speckle from the beam. 
     The foregoing and/or other aspects of the present general inventive concept may also be achieved by providing an illumination system including a laser light source including at least one laser to emit a beam, a first diffractive optical element to divide the beam emitted from the laser light source into a plurality of partial beams, and a wedge unit disposed ahead of or behind the first diffractive optical element to receive one of the beam emitted from the to laser light source and the partial beams and to change paths of the received one of the emitted beam and the partial beams by rotating and time-averaging the received one of the emitted beam and the partial beams to eliminate speckle. 
     The foregoing and/or other aspects of the present general inventive concept may also be achieved by providing a projection system including a laser light source including at least one laser to emit a beam, a first diffractive optical element to divided the beam emitted from the laser light source into a plurality of partial beams and to time-average the partial beams through a periodical movement to eliminate speckle from the beam, a second diffractive optical element to diffract the partial beams emitted from the first diffractive optical element such that the partial beams have a shape corresponding to a display element to form an image, the display element to form the image from beams that pass through the second diffractive optical element, and a projection lens unit to enlarge the image and to project the enlarged image onto a screen. 
     The first diffractive optical element may be rotatable or linearly reciprocable. 
     The foregoing and/or other aspects of the present general inventive concept may also be achieved by providing a projection system including a laser light source including at least one laser to emit a beam, a first diffractive optical element to divide the beam emitted from the laser light source into a plurality of partial beams, a wedge unit disposed ahead of or behind the first diffractive optical element to receive one of the beam emitted from the laser light source and the partial beams to change paths of the partial beams by rotating and time-averaging the partial beams to eliminate speckle, a second diffractive optical element to diffract the partial beams emitted from the first diffractive optical element such that the partial beams have a predetermined shape, a display element having the predetermined shape to form the image from the partial beams diffracted by the second diffractive optical element, and a projection lens unit to enlarge the image and to project the enlarged image onto a screen. 
     The second diffractive optical element may adjust a divergence angle of the partial beams to increase an F/number of the partial beams. 
     The partial beams may include a plurality of beamlets, and the plurality of beamlets may overlap and converge with each other. 
     The display element may include one of a transmissive liquid crystal display, a liquid crystal on silicon, a deformable micro device, and a grating light valve. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and/or other aspects of the present general inventive concept will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which: 
         FIG. 1  is a diagram illustrating a conventional illumination system for a laser projector; 
         FIG. 2  is a diagram illustrating profiles of rectangular beams that are obtained after passing through a beam diffuser included in the conventional illumination system of  FIG. 1 ; 
         FIG. 3  is a schematic diagram illustrating a projection system according to an embodiment of the present general inventive concept; 
         FIG. 4  is a diagram illustrating profiles of partial beams that are obtained after passing through a first diffractive optical element used in the projection system of  FIG. 3 ; 
         FIG. 5A  is a photograph illustrating a beam before laser speckle is eliminated; 
         FIG. 5B  is a photograph illustrating a beam after laser speckle is eliminated by an illumination system according to an embodiment of the present general inventive concept; 
         FIG. 6  is a diagram illustrating profiles of beams that are shaped by a second diffractive optical element included in the projection system of  FIG. 3 ; 
         FIG. 7  is a diagram illustrating a modified example of the projection system of  FIG. 3 ; 
         FIG. 8  is a schematic diagram illustrating a projection system according to another embodiment of the present general inventive concept; and 
         FIG. 9  is a diagram illustrating paths of beams periodically changing as a wedge unit used in the projection system of  FIG. 8  rotates. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to the embodiments of the present general inventive concept, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below in order to explain the present general inventive concept while referring to the figures. 
       FIG. 3  illustrates a projection system capable of eliminating speckle according to an embodiment of the present general inventive concept. 
     Referring to  FIG. 3 , the projection system includes a laser light source  20 , a rotatable first diffractive optical element  24  which eliminates speckle of laser beams emitted from the laser light source  20 , and a display element  28  which forms an image using beams passing through the first diffractive optical element  24 . 
     A second diffractive optical element  26  to shape the beams passing through the first diffractive optical element  24  is disposed on a light path between the display element  28  and the first diffractive optical element  24 . A projection lens unit  30  enlarges and projects the image formed by the display element  28  onto a screen  32 . 
     The first diffractive optical element  24  is rotatable such that the beams from the laser light source  20  can be time-averaged and thus the speckle can be effectively eliminated.  FIG. 4  illustrates operations of the first diffractive optical element  24 . Referring to  FIGS. 3 and 4 , the first diffractive optical element  24  includes a plurality of unit cells  24   a , which can be circular but can alternatively have various other shapes. Each beam from the laser light source  20  passes through the respective unit cells  24   a  of the first diffractive optical element  24  and is transformed into partial beams  27  having different phases and paths with respect to each other. The partial beams  27  can converge and overlap with each other. Each of the partial beams  27  includes a plurality of beamlets  27   a.    
     As the first diffractive optical element  24  is rotated by a motor  22 , the beamlets  27   a  are time-averaged such that the speckle is eliminated.  FIG. 5A  illustrates a laser beam before the speckle is eliminated, and  FIG. 5B  illustrates the laser beam after the speckle is eliminated by rotating the first diffractive optical element  24  of the projection system as illustrated in  FIGS. 3 and 4 . 
     A contrast value C of a laser speckle pattern is given by the following equation, and the laser speckle pattern is invisible to naked eyes if the contrast value C is less than 4%. 
     
       
         
           
             
               
                 
                   C 
                   = 
                   
                     
                       
                         
                           
                             〈 
                             
                               I 
                               i 
                               2 
                             
                             〉 
                           
                           - 
                           
                             
                               〈 
                               
                                 I 
                                 i 
                               
                               〉 
                             
                             2 
                           
                         
                       
                       
                         〈 
                         
                           I 
                           i 
                         
                         〉 
                       
                     
                     = 
                     
                       
                         σ 
                         μ 
                       
                       × 
                       100 
                       ⁢ 
                       
                         ( 
                         % 
                         ) 
                       
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   1 
                 
               
             
           
         
       
     
     where &lt;I i &gt; denotes an average intensity of light passing through an i th  unit cell of the first diffractive optical element  24 , σ denotes a standard deviation, and μ denotes a mean value. 
     Each beam passing through the first diffractive optical element  24  is divided into the partial beams  27  having a uniform intensity distribution. That is, a laser beam having a Gaussian distribution, which is emitted from the laser light source  20 , is divided into the plurality of partial beams  27  having a uniform intensity distribution, which converge with each other after passing through the first diffractive optical element  24 , thereby improving image quality. 
       FIG. 6  illustrates operations of the second diffractive optical element  26 . Referring to  FIG. 6 , the second diffractive optical element  26  diffracts the partial beams into sections corresponding to sections of the display element  28 . The display element  28  may be a transmissive liquid crystal display (LCD), a liquid crystal on silicon (LCoS), a deformable micromirror device (DMD), a grating light valve, or the like. The display element  28  can be rectangular in shape with an aspect ratio of 4:3 or 16:9. Since the beams emitted from the laser light source  20  can have a different shape than that of the display element  28 , it is necessary to enable the beams emitted from the laser light source  20  to have a shape corresponding to that of the display element  28 . For example, as illustrated in  FIG. 6 , the second diffractive optical element  26  diffracts a circular beam so that the beam becomes a rectangular beam to correspond to the shape of the display element  28 . 
     Further, a divergence angle of the beams can be controlled by adjusting a diffraction pattern of the second diffractive optical element  26 . The projection system can be miniaturized by adjusting the divergence angle of the beams and increasing an F/Number (F/No) of the illumination system. 
     The beams shaped by the second diffractive optical element  26  are used by the display element  28  to form the image, and the image formed by the display element  28  is enlarged and projected onto the screen  32  by the projection lens unit  30 . 
     The laser light source  20  may include a plurality of lasers that emit a plurality of laser beams having different wavelengths sequentially by color. For example, the laser light source  20  may include first, second, and third lasers that emit red (R), green (G), and blue (B) laser beams, respectively. The laser light source  20  sequentially emits the R, G, and B laser beams to sequentially form images with the R, G, B beams and the images are projected onto the screen  32  to realize a color image. 
     An illumination system according to an embodiment of the present general inventive concept includes the laser light source  20  including the plurality of lasers to emit the beams of the different wavelengths, the first diffractive optical element  24  to eliminate the speckle of the beams emitted from the laser light source  20 , and the second diffractive optical element  26  to shape the beams corresponding to the display element  28 . The first diffractive optical element  24  divides the each beam emitted from the laser light source  20  into the plurality of partial beams  27 , and the partial beams are time averaged according to the rotation of the first diffractive optical element  24  to eliminate the speckle. 
     The speckle is eliminated by rotating the first diffractive optical element  24  to time-average the partial beams in the projection system of  FIG. 3 .  FIG. 7  illustrates a projection system according to another embodiment of the present general inventive concept. Referring to  FIG. 7 , the speckle may be eliminated by linearly reciprocating a first diffractive optical element  24 ′ using a moving unit  25  to time-average beams that pass through the first diffractive optical element  24 ′. Elements having the same reference numerals in  FIGS. 3 and 7  operate the same, and therefore, a detailed explanation will be omitted. 
       FIG. 8  illustrates an illumination system and a projection system employing the illumination system according to another embodiment of the present general inventive concept. 
     Referring to  FIG. 8 , the illumination system includes a laser light source  40 , a first diffractive optical element  44  that eliminates speckle of beams emitted from the laser light source  40 , and a second diffractive optical element  46  that shapes the beams. A rotatable wedge unit  42  is disposed between the laser light source  40  and the first diffractive optical element  44 , or between the first diffractive optical element  44  and the second diffractive optical element  46 . The wedge unit  42  may be manufactured in the form of a plate (e.g., a glass plate) having a wedge-shaped section. 
     The laser light source  40  includes a plurality of lasers that emit laser beams having different wavelengths, and the wedge unit  42  can be rotated by a motor  41 . 
       FIG. 9  illustrates operations of the wedge unit  42 . Referring to  FIG. 9 , the beams emitted from the laser light source  40  are refracted as they pass through the wedge unit  42  toward the first diffractive optical element  44 . Paths of the beams are periodically changed as the wedge unit  42  rotates. The first diffractive optical element  44  then divides each refracted beam into a plurality of partial beams. Each partial beam includes a plurality of beamlets. 
     Since the paths of the beams are periodically changed according to the rotation of the wedge unit  42  before the beams are incident on the first diffractive optical element  44 , an effect similar to that of the projection system of  FIG. 3  is obtained. Accordingly, the beams are time-averaged and laser speckle is eliminated. Furthermore, the partial beams converge with respect to each other such that beams having a Gaussian distribution can be transformed into beams having a uniform intensity distribution. 
     The beams that are emitted from the laser light source  40  can have a circular section, and the second diffractive optical element  46  transforms the beams into beams having a shape corresponding to the shape of a display element  48 . The display element  48  forms an image from the beams transformed by the second diffractive optical element  46 , and a projection lens unit  50  enlarges the image and projects the enlarged image onto a screen  52 . 
     The laser light source  40  includes the plurality of lasers that emit the beams having the different wavelengths. The plurality of lasers sequentially emit the beams having the different wavelengths to produce a color image. 
     In the embodiment illustrated in  FIG. 8 , the speckle is eliminated by rotating the wedge unit  42  instead of the first diffractive optical element  44 . The embodiment of  FIG. 3 , in which the first diffractive optical element  24  rotates, may require a larger area for the first diffractive optical element  24  and higher costs to design a diffraction pattern of the first diffractive optical element  24  as compared to the embodiment of  FIG. 8 . 
     According to the embodiments of the present general inventive concept, an illumination system can drastically reduce or eliminate speckle of beams emitted from a laser light source, and a projection system can employ the illumination system to achieve improved image quality. 
     As described above, a projection system according to the embodiments of the present general inventive concept can effectively eliminate speckle by rotating a diffractive optical element or a wedge unit to time-average partial beams. Moreover, since a divergence angle of the partial beams can be reduced using the diffractive optical element, the projection system can be compact. 
     Although a few embodiments of the present general inventive concept have been shown and described, it will be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the general inventive concept, the scope of which is defined in the appended claims and their equivalents.