Abstract:
A micro-mirror light modulator and associated projection display system. The projection display system includes a light source, a linear light source illumination system that transforms a light emitted from the light source into a thin linear light, a micro-mirror light modulator that selectively diverts the direction of the incident thin linear light to create reflected light, a light transmitting layer that selectively transmits or filters out the reflected light, a condenser lens that focuses the light transmitted by the light transmitting layer, and a scanner that scans the focused light such that the light forms a projected image. The micro-mirror light modulator includes a reflection electrode that deflects toward an electrode when a voltage is applied. The reflection electrode reflects light depending on its deflection state. The micro-mirror light modulator is driven by low voltage, is easily fabricated such that the cost is reduced, and exhibits enhanced image contrast.

Description:
his application claims the benefit of the Korean Patent Application No. 10-2004-0079445, filed on Oct. 6, 2004 which is hereby incorporated herein by reference as if fully set forth herein. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a projection display system having a light modulator display device, and more particularly, to a micro-mirror light modulator and projection display system using the same. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for displaying an image by modulating a luminous intensity of pixels by means of micro-mirrors. 
   2. Discussion of the Related Art 
   Generally, projection display systems display a wide image on a wide screen by enlarging and projecting a small image. An example of a conventional projection display system is an LCD (liquid crystal display) projection display system that uses a lamp and LCD display device. 
     FIG. 1  is a schematic diagram of a conventional LCD projection display system. Referring to  FIG. 1 , in a basic configuration of an LCD projection display system in the prior art, a light emitted from a lamp  11  is collimated in one direction by a reflector. A red light is transmitted through a red filter  12 , whereas green and blue lights are reflected by the red filter  12 . 
   The red light is reflected by a red mirror  13  to be irradiated onto an R-LCD (Red LCD)  17 , the green light is reflected by a blue filter  14  to be irradiated onto a G-LCD (Green LCD)  18 , and the blue light is transmitted through the blue filter  14 . The projected blue light is reflected by a first blue mirror  15  and a second blue mirror  16  to be irradiated onto a B-LCD (Blue-LCD)  19 . 
   Each of the R-, G- and B-LCDs  17 ,  18  and  19  displays an image for each color in response to an electrical signal in which the corresponding image is encoded. The images of the respective colors are combined by a prism  20 . The combined color image is projected to a projection optical system  21  so that the projected image can be viewed on a screen  22 . 
   The LCD display device, which has a relatively slow response speed, exhibits the problem of causing image artifacts when displaying a fast-moving picture. In addition, when the LCD display device operates to render a dark pixel in response to an off value of the electrical signal, the liquid crystal layer of the LCD display device generally is unable to completely block the light of the pixel. This dark state light leakage typically reduces the contrast of LCD projection display systems. 
   Conventional LCD projection display devices typically include optical systems with color separation and combination systems that increase the complexity and the overall size of the projection display devices. 
     FIG. 2  is a perspective diagram of a DMD (digital micro-mirror device) according to a related art. Referring to  FIG. 2 , a DMD is fabricated by covering each memory cell of a CMOS SRAM with a micro-mirror. The DMD is configured to have a pair of micro-mirrors  23  and  23 ′ for one memory cell. One mirror  23  is rotated by +10° while the other  23 ′ is rotated by −10°. A binary state of ‘0’ or ‘1’ is represented in each basic memory cell. 
   In  FIG. 2 , reference numbers  25 ,  26 ,  27 ,  28 ,  29 ,  30  and  31  indicate a yoke landing tip, support post, tension hinge, yoke, mirror landing electrode, yoke address electrode and mirror address electrode, respectively. 
   The DMD having the above configured memory cells is fabricated by regularly arranging 100,000˜10,000,000 micro-mirrors in width and length directions according to a semiconductor process. By controlling the tilt of each of the mirrors by ±10° according to a voltage applied to each of the mirrors, the intensity of the light reflected by the corresponding mirror is adjusted to implement video information of each pixel. 
   Namely, an on-state light is reflected in the direction of the projection lens (not shown in the drawing) with a specific angle by the micro-mirrors  23  and  23 ′ moving in a diagonal direction, whereas an off-state light is reflected with an opposite angle in a direction away from the projection lens. Hence, the DMD can be used as a spatial light modulator. 
   However, as can be seen in  FIG. 2 , the configuration of the DMD is complicated and three-dimensional. Hence, the fabrication throughput of the DMD is low, which makes the DMD relatively expensive. 
   Alternatively, the projection display system may include a GLV (grating light valve), which is a display device employing micro-ribbons.  FIG. 3A  is a schematic perspective diagram of a GLV. Referring to  FIG. 3A , a set of six ribbons  33  and  34  forms one pixel  100 . The ribbons  33  and  34  are alternately arranged. In this case, operational ribbons  33  are moved by an electrode  32 , whereas fixed ribbons  34  are not moved by the electrode  32 . In the above configuration, 100˜10,000 micro-ribbons are arranged by a semiconductor process to form a linear display device, which can be used to render an image on a line of pixels. 
     FIG. 3B  is a diagram of a projection display system employing the above configured GLV. Referring to  FIG. 3B , the projection display system includes a first condenser lens  35  for focusing the R, G and B components of the image, a GLV  36  with three rows of pixel elements (i.e., R, G, B), a second condenser lens  37  for focusing the light from the GLV  36 , and a scanner  38  for scanning the light from the second condenser lens  37  onto a screen  39 . 
   When a voltage is applied to the above configured projection display system, the operational ribbons  33 , to which the voltage is applied, are deflected downward while the fixed ribbons  34  do not move. Thus, this configuration of ribbons forms a grating with a periodic step shape in height. As the R, G and B light components are directed onto the GLV  36  via the first condenser lens  35 , the grating diffracts the light. 
   The light diffracted by the GLV  36  is scanned by the scanner  38  to effectively convert the one-dimensional array of pixels associated with the GLV  36  to an image projected onto the screen  39  as a two-dimensional array of pixels. 
   In the projection display system employing the GLV, the GLV has a hollow solid shape. However, the deflected operational ribbons  33  contact the substrate over a relatively large area, which often results in sticking of the deflected operational ribbons  33  with the substrate. Moreover, a high voltage is needed to move the relatively wide ribbons. 
   SUMMARY OF THE INVENTION 
   Accordingly, the present invention is directed to a micro-mirror light modulator and projection display system using the same that substantially obviate one or more problems due to limitations and disadvantages of the related art. The projection display systems of the present invention include a micro-mirror light modulator that modulates a linear light source by selectively reflecting the linear light in response to an electrical signal that encodes an image. 
   The micro-mirror elements of the micro-mirror light modulator include a reflecting electrode displaced from an electrode. When no voltage is applied, the reflecting electrode is substantially horizontal and parallel with respect to the electrode. When the voltage is applied, the reflecting electrode tilts and is deflected downward toward the electrode. This tilting or deflection of the reflecting electrode permits the direction of reflection of incident light to be selected, which permits the luminous intensity of the light associated with the individual pixels of the projection display system to be modulated. 
   The micro-mirror light modulator includes a linear array of pixel elements that correspond to the pixels in one row or column of the two-dimensional screen of the projection display system. The two-dimensional image is formed on the screen by scanning a sequence of linear images generated by the micro-mirror light modulator. 
   In contrast to conventional GLV projection display devices, the micro-mirrors of the present invention do not exhibit sticking problems. The sticking is eliminated primarily because the electrode contacts a substrate of the reflecting electrode rather than the reflecting electrode itself, and the contact area of the electrode is relatively small. Moreover, the voltage required to activate the micro-mirror elements is relatively small, and switching times are short. In addition, the three-dimensional shape of the micro-mirror elements is relatively simple, which makes fabrication easier and results in a relatively low-cost projection display system. 
   Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objectives and other advantages of the invention may be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this application, illustrate embodiment(s) of the invention and together with the description serve to explain the principle of the invention. In the drawings: 
       FIG. 1  is a schematic diagram of a conventional LCD projection display system; 
       FIG. 2  is a perspective diagram of a prior art DMD structure; 
       FIG. 3A  is a schematic perspective diagram of a GLV structure; 
       FIG. 3B  is a diagram of a projection display system employing the GLV in  FIG. 3A ; 
       FIG. 4  is a schematic diagram of a projection display system according to the present invention; 
       FIG. 5A  is a perspective diagram of a micro-mirror light modulator in  FIG. 4 ; 
       FIG. 5B  is a cross-sectional diagram of a micro-mirror light modulator in  FIG. 4 ; 
       FIG. 5C  is a diagram illustrating various aspects of the operation of the micro-mirror light modulator in  FIG. 4 ; 
       FIG. 5D  is a diagram illustrating various aspects of the operation of the micro-mirror light modulator to which power is applied in  FIG. 4 ; and 
       FIG. 6  is a time chart for gray scale processing of an image associated with a pixel according to the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
     FIG. 4  is a schematic diagram of a projection display system according to the present invention. Referring to  FIG. 4 , the projection display system according to the present invention includes a light source  40 , a linear optical illumination system  41  transforming a light emitted from the light source  40  into a thin linear light, and a micro-mirror light modulator  42  for diverting the direction of incident light. The light emitted from the light source  40  and transformed by the linear optical illumination system is linear in the sense that it is incident upon the micro-mirror light modulator  42  along a one-dimensional line that corresponds to the linear shape of micro-mirror light modulator  42 . The projection display system further includes a light transmitting layer  43  transmitting the light reflected by the light modulator  42 , a projection lens  44  that projects light transmitted through the light transmitting layer  43  and enlarges the corresponding image, and a scanner  45  that scans a linear image to a screen  46 . 
   A detailed configuration of the micro-mirror light modulator  42  is explained with reference to  FIG. 5A  and  FIG. 5B  as follows.  FIG. 5A  is a perspective diagram of the micro-mirror light modulator of  FIG. 4 , while  FIG. 5B  is a cross-sectional diagram of the micro-mirror light modulator of  FIG. 4 . 
   Referring to  FIG. 5A  and  FIG. 5B , a micro-mirror light modulator  42  according to the present invention includes a movable reflecting electrode  421 , an electrode  422 , a support structure  423  that supports the reflecting electrode  421  to act as a fixed base with respect to which the reflecting electrode  421  moves, a reflecting electrode substrate  424  on which the reflecting electrode  421  is formed, an electrode wiring layer  425  enabling an external electrical signal to be applied to the electrode  422 , and a substrate  426 . 
   One end of the reflecting electrode  421  and one end of the reflecting electrode substrate  424  are attached to the support structure  423  and need to be spaced apart from the electrode  422  at a predetermined displacement to remain substantially parallel to the substrate  425  when a voltage is not applied to the electrode  422 . 
   When a predetermined voltage is applied to the electrode  422 , the reflecting electrode  421  and the reflecting electrode substrate  424  are configured to be tilted by an attractive electrostatic force generated from the electrode  422  such that the electrode substrate  424  contacts the electrode  422 . 
   The micro-mirror light modulator  42  of  FIG. 5A  and  FIG. 5B  generally includes a linear array of the micro-mirror elements of  FIG. 5A  and  FIG. 5B . The linear array can be used to form a linear image associated with one row of pixels of the screen of the projection display system in response to an inputted video signal. 
   The reflecting electrode  421  is preferably formed from a material having high light reflectivity and good electrical conductivity, such as Ag, Al, and the like. 
   The light transmitting layer  43 , as shown in  FIG. 4 , includes an aperture  431  on a predetermined part of the light transmitting layer  43 . The light transmitting layer  43  and the associated aperture  431  are configured to transmit only light that is reflected from the micro-mirror light modulator  42  at a predetermined angle and are further configured to filter out other light that does not have the predetermined angle. 
   The operation of the projection display system while a voltage is not applied to the electrode  422  of the micro-mirror light modulator  42  of  FIG. 4  is explained with reference to  FIG. 5C  as follows. Referring to  FIG. 5C , when power is turned off, there exists no electrostatic force since a voltage is not applied between the reflecting electrode  421  and the electrode  422 . Hence, there is no an attractive force between the reflecting electrode  421  and the electrode  422 . 
   In this situation, the reflecting electrode  421  maintains its horizontal state. In this example, an incident light ‘a’ is reflected from the reflecting electrode  421  as reflected light ‘b.’ In view of the geometry illustrated in  FIG. 5D , the reflected light ‘b’ falls upon a portion of the light transmitting layer  43  other than the aperture  431 . Hence, the reflected light ‘b’ does not transmit through light transmitting layer  43 , but is instead blocked, or filtered out, by the light transmitting layer  43 . 
   In contrast, the operation of the projection display system while a voltage is applied to the electrode  422  of the micro-mirror light modulator  42  of  FIG. 4  is explained with reference to  FIG. 5D  as follows. Referring to  FIG. 5D , when power is turned on, a voltage is applied between the reflecting electrode  421  and the electrode  422  to generate an electrostatic force. Hence, an attractive force occurs between the reflecting electrode  421  and the electrode  422  so that the reflecting electrode  421  is tilted downward, which results in the reflecting electrode substrate coming into contact with the electrode  422 . 
   In this example, the incident light ‘a’ is reflected at a specific angle by the tilted reflecting electrode  421  as reflected light ‘b.’ The reflected light ‘b’ as shown in  FIG. 5D  is directed to the aperture  431  of the light transmitting layer  43  and passes therethrough. 
   In view of the foregoing, the reflected light ‘b’ of  FIG. 5C  is a first portion of the reflected light from the micro-mirror light modulator of the present invention and is reflected at a first angle of reflection. In this case, the reflected light ‘b’ of  FIG. 5C  is filtered out by the light transmitting layer  43  and is generated, for example, in response to an electrical signal that indicates that the corresponding pixel is not to be illuminated. The reflected light ‘b’ of  FIG. 5D  is a second portion of the reflected light from the micro-mirror light modulator of the present invention and is reflected at a second angle of reflection. In this case, the reflected light ‘b’ of  FIG. 5D  passes through the aperture  431  of the light transmitting layer  43  and is generated, for example, in response to an electrical signal that indicates that the corresponding pixel is to be illuminated. 
   As mentioned above, the micro-mirror light modulator  42  generally includes a linear array of micro-mirror elements to form a linear image along one row of pixels of the screen  46  of the projection display system in one of the two dimensions of the screen from an inputted video signal. The linear image reflected by the micro-mirror light modulator  42  is directed to the projection lens  44  via the aperture  431 . The projection lens  44  then enlarges and projects the linear image. The linear image is sequentially scanned by the scanner  45  to form a two-dimensional image on the screen  46 . 
   To implement the image using the light modulator  42  and the scanner  45 , brightness of each pixel needs to be adjusted. A gray scale method according to the present invention is explained with reference to  FIG. 6 , which is a time chart for gray scale processing of a portion of an image associated with a pixel according to the present invention. 
     FIG. 6  corresponds to an image rendered at 60 Hz with XGA resolution (1024×768). In this case, the light modulator  42  forms one linear image with 768 pixels of image data, which corresponds to one vertical column of pixels of the projection display system. 
   During this process, the scanner  45  is driven at 60 Hz to scan the linear image received from the micro-mirror light modulator  42  to form a single two-dimensional image on the screen of the projection display system for 1/60 second. One micro-mirror of the light modulator  42  forms one pixel of one linear image for 1/(60×1024) second, which results from dividing 1/60 second by the horizontal resolution of the projection display system. 
   The luminous intensity, or gray scale value, associated with the image rendered in one cycle on a single pixel is obtained by selecting the time ratio of mirror states over the relevant period of time. In case of a 256-value gray scale for adjusting the brightness of a single pixel, one gray scale level generated by the micro-mirror corresponds to 1/(60×1024×256) second. In other words, the micro-mirror is activated for about 63 ns to generate one unit of a 256-value gray scale. 
   Accordingly, the projection display system of the present invention has the following effects or advantages. First of all, the present invention facilitates the fabrication of the micro-mirror light modulator in view of the relatively simple layered configuration of the light modulator. In addition, the gap between the reflecting electrode  421  and the electrode  422  is easily formed because the gap is laterally open. 
   Secondly, the electrode  422  is brought into contact with the reflecting electrode substrate  424  as the reflecting electrode  421  is attracted to the electrode  422 . Thus, the reflecting electrode  421  and the electrode  422  do not actually contact each other during operation of the micro-mirror light modulator  42 . Moreover, the size of the electrode  422  is relatively small and has a small contact area. Thus, the micro-mirror light modulator  42  of the invention does not exhibit the sticking problems experienced in the prior art, in which moveable elements, such as operational ribbons of grating light valves, tend to stick to the electrode. Moreover, the micro-mirror elements of the invention can be switched quickly at a relatively low voltage, since the displacement of the reflecting electrode is small. 
   Therefore, the present invention enables the display device to be driven by a low voltage, facilitates the corresponding fabrication, reduces the product cost, and enhances contrast of the images rendered by the projection display system. 
   It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.