Abstract:
A display apparatus projects an image onto a display screen and includes a grating light valve, a focusing arrangement and a scanning device. The grating light valve is located near but not at a focal plane of a line illumination such that in operation the grating light valve produces real or virtual two dimensional images. The grating light valve is configured such that the modular members on the surface of the grating light valve are neither parallel nor perpendicular to the effective pixel are, thereby arranged diagonally across the pixel area in order to eliminate any imperfections or defects in the line illumination. A seal glass is coupled with the grating light valve and forms an air gap between itself and the grating light valve such that the line illumination must pass through the seal glass and the air gap before reaching the grating light valve. Additionally, an absorbing aperture is affixed to the surface of the seal glass at the focal point of the line illumination in order to further filter background light.

Description:
FIELD OF THE INVENTION 
     This invention relates to the field of image projectors. More particularly, this invention relates to the field of modulation of light out of the focal plan in a grating light valve based projection system. 
     BACKGROUND OF THE INVENTION 
     In recent years, light modulators have been developed using MEMS (micro-electro-mechanical systems) technology in which moveable elements are configurable to direct light. An example of such light modulators is a grating light valve type device (GLV type device) taught in U.S. Pat. No. 5,311,360 to Bloom et al., in which the GLV type device is configurable in a reflecting mode and a diffracting mode. The GLV type device taught by Bloom et al. is isometrically illustrated in FIG.  1 . The GLV type device  10  includes moveable elongated elements  12  suspended over a substrate  14 . 
     A first side view of the GLV type device  10  of the prior art is illustrated in FIG. 2A, which shows the GLV type device  10  in the reflecting mode. The moveable elongated elements  12  each include a first reflective coating  16 . Interspersed between the moveable elongated elements  12  are second reflective coatings  18 . In the reflecting mode, upper surfaces of the first and second reflective coatings,  16  and  18 , are separated by a height difference of a half wavelength λ/2 of incident light I. The incident light I reflecting from the second reflecting coatings  18  travels a full wavelength further than the incident light I reflecting form the first reflecting coatings  16 . So the incident light I, reflecting from the first and second reflecting coatings,  16  and  18 , constructively combines to form reflected light R. Thus, in the reflecting mode the GLV type device  10  produces the reflected light R. 
     A second side view of the GLV type device  10  of the prior art is illustrated in FIG. 2B, which shows the GLV type device in the diffracting mode. To transition from the reflecting mode to the diffracting mode, an electrostatic potential between the moveable elongated elements  12  and the substrate  14  moves the moveable elongated elements  12  to contact the substrate  14 . To maintain the diffracting mode, the electrostatic potential holds the moveable elongated elements  12  against the substrate  14 . In the diffracting mode, the upper surfaces of the first and second reflective coatings,  16  and  18 , are separated by a quarter wavelength λ/4 of the incident light I. The incident light I reflecting from the second reflecting surfaces  18  travels a half wavelength further than the incident light I reflecting from the first reflective coatings  16 . So the incident light I, reflecting from the first and second reflecting coatings,  16  and  18 , destructively interferes to produce diffraction. The diffraction includes a plus one diffraction order D +1  and a minus one diffraction order D −1 . Thus, in the diffracting mode, the GLV type device  10  produces the plus one and minus one diffraction orders, D +1  and D −1 . 
     A first alternative GLV type device of the prior art is illustrated in FIGS. 3A and 3B. The first alternative GLV type device  10 A includes first elongated elements  22  interdigitated with second elongated elements  23 . The first elongated elements  22  include third reflective coatings  26 ; the second elongated elements  23  include fourth reflective coating  28 . In the reflecting mode, illustrated in FIG. 3A, the third and fourth reflective coatings,  26  and  28 , are maintained at the same height to produce the reflected light R. In the diffracting mode, illustrated in FIG. 3B, the first and second reflected coatings,  26  and  28 , are separated by the second height difference of the quarter wavelength λ/4 of the incident light Ito produce the diffraction including the plus one and minus one diffraction orders, D +1  and D −1 . 
     A display system utilizing a GLV type device is taught in U.S. Pat. No. 5,982,553 to Bloom et al. The display system includes red, green, and blue lasers, a dichroic filter group, illumination optics, the GLV type device, Schlieren optics, projection optics, a scanning mirror, and display electronics, which project a color image onto a display screen. The red, green, and blue lasers, driven by the display electronics and coupled to the GLV type device (via the dichroic filter group and the illumination optics) sequentially illuminate the GLV type device with red, green, and blue illuminations. The GLV type device, driven by the display electronics, produces a linear array of pixels which changes with time in response to a signal from the display electronics, each pixel configured in the reflecting mode or the diffracting mode at a given instant in time. Thus, the GLV type device produces sequential linear arrays of red, green, and blue pixels with each of the red, green, and blue pixels in the reflecting mode or the diffracting mode. 
     The red, green, and blue pixels are then coupled to the Schlieren optics which blocks the reflecting mode and allows at least the plus one and minus one diffraction order, D +1  and D −1 , to pass the Schlieren optics. Thus, after passing the Schlieren optics, the linear arrays of the red, green, and blue pixels have light pixels corresponding to the pixels at the GLV type device in the diffracting mode and dark pixels corresponding to pixels at the GLV type device in the reflecting mode. The projection optics (via the scanning mirror) project the linear arrays of the red, green, and blue pixels onto the display screen while the scanning mirror, driven by the display electronics, scans the linear arrays of the red, green, and blue pixels across the display screen. Thus, the display system produces a two dimensional color image on the display screen. 
     A graphical top cross sectional view of a GLV type device is illustrated in FIG. 4. A scale in millimeters is provided to show an approximate layout of the system. In a system such as this, the GLV type device  10  is positioned such that the focal point  36  of the incident light I is located on the surface of the GLV type device  10 . A seal glass  34  is positioned over the GLV type device  10  forming an air gap  32  between the GLV type device  10  and the seal glass  34 . In this prior art embodiment, the seal glass  34  permits the incident light I to pass to the air gap  32  and then to the GLV type device  10 . The reflected light R travels through the air gap  32  and the seal glass  34  before a portion of the reflected light R is reflected by an outer surface  38  of the seal glass. As depicted in FIG. 4, this second reflected light R 2  eventually leaves the seal glass at a different point than the reflected light R. This second reflected light R 2 , as well as additional reflected light derivative of R 2 , creates undesirable background light in the final display. 
     An alternative display system utilizing the GLV type device includes the red, green, and blue lasers, red, green, and blue illumination optics; first, second, and third GLV type devices; the dichroic filter group; the projection optics; the scanning mirror; and the display electronics. The red, green, and blue lasers, via the red, green, and blue illumination optics, illuminate the first, second, and third GLV type devices, respectively. The first, second, and third GLV type devices produce the linear arrays of the red, green, and blue pixels, respcetively, in response to signals from the display electronics. The dichroic filter group directs the light from the linear arrays of the red, green, and blue pixels to the Schlieren optics, which allows at least the plus one and minus one diffraction order, D +1  and D −1 , to pass the Schlieren optics. The projection optics, via the scanning mirror, project the linear arrays of the red, green, and blue pixels onto the display screen while the scanning mirror, driven by the display electronics, scans the linear arrays of the red, green, and blue pixels across the display screen. Thus, the alternative display system produces the two dimensional color image on the display screen. 
     Examples of applications for a GLV type device based display system include a home entertainment system, a boardroom application, and a cinema application among others. In the home entertainment system or the boardroom application, the GLV type device based display system projects the two dimensional color image onto the display screen located on a wall. In the cinema application, the GLV type device based display system projects the two dimensional color image from a display booth onto a cinema screen. 
     A GLV type device based display system may also be utilized in printing applications. In such a case, the system would not include a scanning mirror, and the printing media, replacing a screen, would move to effectuate printing from a fixed line of light. 
     What is needed is a method of reducing the problems of space, heat, noise, and vibration in the home entertainment system, the boardroom application, and the cinema application. The problem of heat in various applications affects the operation of the GLV type device and shortens the GLV type device&#39;s life span. Specifically, illumination intensity affects the operation of the GLV type device and ultimately shortens the GLV type device lifespan when a GLV type device is positioned at the focal plane of a line illumination. Typically, the optical focal point in a system such as this is at the GLV type device modulator, thereby exposing the GLV type device to high optical intensities anywhere from 50 Watts to 100 Watts per color. These intensities must be reduced in order to preserve the high quality function of the GLV type device over longer periods of time, thus extending the overall life span of the GLV type device. What is also needed are methods for further correcting imperfections and defects in the line illumination that cause imperfections in the video display, while implementing an additional filtering step to eliminate more background light, thus further clarifying the display. 
     SUMMARY OF THE INVENTION 
     The present invention is a display apparatus and method for modulating light out of the focal plan in a grating light valve type device based projection system. The display apparatus and method includes positioning a grating light valve type device near but not at a focal plane of a line illumination such that the grating light valve type device produces either a two dimensional real image and a two dimensional virtual image. 
     The grating light valve type device in the present invention is also configured such that the modular members of the grating light valve type device are neither parallel nor perpendicular with the boundaries of the effective pixel area to broaden the width of the video line, thus correcting imperfections and striations attributable to ribbon defects. A seal glass is coupled with the grating light valve type device in such a manner that the line illumination must pass through the seal glass and an air gap between the seal glass and the grating light valve type device before reaching the grating light valve type device. 
     The present invention also embodies an absorbing aperture that is affixed to the outer surface of the seal glass at the focal plane of the line illumination in order to filter additional background light to achieve higher contrast, thus providing a much clearer picture. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates an isometric view of a conventional grating light valve type device (GLV type device). 
     FIG. 2 illustrates a side view of the conventional GLV type device. 
     FIG. 3 illustrates a side view of an alternative conventional GLV type device. 
     FIG. 4 illustrates a graphical top cross sectional view of a conventional GLV type device. 
     FIG. 5 schematically illustrates a display apparatus of the present invention. 
     FIG. 6 illustrates a plan view of display optics of the present invention. 
     FIG. 7 illustrates an elevation view of the display optics of the present invention with the display optics unfolded along an optical axis. 
     FIG. 8 illustrates a graphical top cross sectional view of the GLV type device of the preferred embodiment of the present invention. 
     FIG. 9 illustrates a front view of the GLV type device of an alternative embodiment of the present invention. 
     FIG. 10 illustrates a top view of a GLV type device system of an alternative embodiment of the present invention. 
     FIG. 11 illustrates a top view of a GLV type device system of an alternative embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A display system of the present invention is illustrated schematically in FIG.  5 . The display system  40  includes display optics  42  and display electronics  44 . The display optics  42  comprise a laser  46 , illumination optics  48 , a grating light valve type device (GLV type device)  50 , Schlieren optics  52 , projection and scanning optics  56 , and a display screen  58 . The display electronics  44  are coupled to the laser source  46 , the GLV type device  50 , and the projection and scanning optics  56 . 
     The display electronics  44  power the laser  46 . The laser  46  emits a laser illumination. The illumination optics  48  focus the laser illumination onto the GLV type device  50 . The GLV type device  50  is located in a first image plane  60 . The display electronics  44  control the GLV type device  50 . The GLV type device  50  modulates the laser illumination forming reflected light or diffracted light for a linear array of pixels. The Schlieren optics  52  separates the reflected light from the diffracted light allowing at least plus one and minus one diffraction orders to pass the Schlieren optics  52 . 
     The display electronics  44  drive a scanning mirror of the projection and scanning optics  56 . The projection and scanning optics  56  project the line image onto the display screen  58  and scan the line image across the display screen  58  to form a two dimensional image on the display screen  58 . The display screen  58  is located in a third image plane  64 . 
     The display optics  42  of the present invention are further illustrated in FIGS. 6 and 7. FIG. 6 illustrates a plan view of the display optics  42 . FIG. 7 illustrates an elevation view of the display optics  42 , with the display optics  42  unfolded along an optic axis  70 . The laser  46  emits the laser illumination  72 . The illumination optics comprise a divergent lens  74 , a collimation lens  76 , and a cylindrical lens  78 . The illumination optics  48  focus the laser illumination  72  onto the GLV type device  50  in a focus line having a focus width. Note that FIG. 6 illustrates the laser illumination  72  illuminating the GLV type device  50  with an angle of incidence of 45°. Ideally, the angle of incidence is a minimum angle of incidence which allows the laser illumination  72  to illuminate the GLV type device  50  while allowing the reflected and diffracted light to reach the Schlieren optics  52 . It will be readily apparent to one skilled in the art that other optics arrangements can be used to illuminate the GLV type device  50 . It will also be readily apparent to one skilled in the art that depiction of lenses in the present invention is not limited to single component lenses and that any given lens can be replaced with a compound lens or a reflective optical element. 
     The GLV type device  50  modulates the laser illumination  72  as the linear array of pixels along the focus line, forming the reflected light R or the diffracted light, including the plus one and minus one diffraction orders, D +1  and D −1 , for each pixel. Preferably, the GLV type device  50  produces a linear array of 1,080 pixels. Alternatively, the GLV type device  50  produces more or less than 1,080 pixels. Note that FIG. 7 illustrates the reflected light R and the plus one and minus one diffraction orders, D +1  and D −1 , for two pixels for illustration purposes. If a given pixel is modulated to reflect light, the reflected light R will be present and the plus one and minus one diffraction orders, D +1  and D −1 , will not be present. Alternatively, if the given pixel is modulated to diffract light, the plus one and minus one diffraction orders, D +1  and D −1 , will be present and the reflected light R will not be present. In some instances it is desirable to modulate the given pixel to produce the reflected light R and the plus one and minus one diffraction orders, D +1  and D −1 , in order to reduce a brightness of the given pixel in a resulting image, which provides a gray scale effect in the resulting image. 
     FIG. 8 is a graphical top cross sectional view of the GLV type device  50  of the preferred embodiment of the present invention. A scale in millimeters is provided to show an approximate layout of the system. Here, the GLV type device  50  is positioned such that the focal point  54  of the incident light I is near but not on the surface of the GLV type device  50 . A seal glass  62  is positioned over the GLV type device  50  forming an air gap  66  between the GLV type device  50  and the seal glass  62 . The seal glass  62  permits the incident light I to pass to the air gap  66  and to the GLV type device  50 . In the preferred embodiment of the present invention, the GLV type device  50  is positioned such that the focal point  54  occurs beyond the surface of the GLV type device  50  as depicted in FIG.  8 . After the incident light I reaches the GLV type device  50 , the reflected light R travels through the air gap  66  and the seal glass  62  and into the Schlieren optics  52 . The outer surface  68  of the seal glass  62  may further reflect a portion of the reflected light R producing unwanted background light. An additional remedy to this problem will be introduced in the alternative embodiment. 
     In the preferred embodiment of the present invention, the GLV type device  50  is positioned such that the focal point  54  is positioned at the outer surface  68  of the seal glass  62 . It will be readily apparent to one skilled in the art that the arrangement of the GLV type device  50  and the seal glass  62  is not limited to the arrangement depicted in FIG.  8 . 
     Referring back to FIG. 7, the Schlieren optics  52  include a Schlieren stop  80  located between first and second relay lenses,  82  and  84 . The Schlieren stop  80  stops the reflected light R and allows the plus one and minus one diffraction orders, D +1  and D −1 , to pass the Schlieren stop  80 . The Schlieren stop  80  is preferably located in a first transform plane  85 . Alternatively, the Schlieren stop  80  is located near the first transform plane  85 . 
     The projection and scanning optics  56  comprise a projection lens  86  and the scanning mirror  88 . The projection lens  86 , via the scanning mirror  88 , projects the line image  90  onto the display screen  58 . The projection lens  86  also reforms the wavefront having the spatial phase variation across the line image width  92  on the display screen  58 . The scanning mirror  88  is preferably located at about a second transform plane  94 . 
     The scanning mirror  88  moves with a first scan motion A and, thus, scans the line image  90  across the display screen  58  with a second scan motion B. Preferably, the first scan motion A is a sawtooth scan motion where a first part of a scan cycle illuminates the display screen  58  and a second part of the scan cycle returns the scanning mirror  88  back to a beginning of the scan cycle. By repeatedly scanning the line image  90  across the display screen  58 , a two dimensional image is formed on the display screen  58 . It will be readily apparent to one skilled in the art that other scan motions can be used to scan the line image  90  across the display screen  58 . It will also be readily apparent to one skilled in the art that a transmissive scanning device such as an objective scanner having zero optical power can replace the scanning mirror  88 . 
     As the line image  90  scans across the display screen  58 , the GLV type device  50  modulates the linear array of pixels thus producing the two dimensional image made up of a rectangular array of pixels. For a high definition television (HDTV) format, the GLV type device  50  modulates 1,920 times as the line image  90  scans across the display screen  58 . Thus, the GLV type device  50  preferably produces a 1,920 by 1,080 rectangular array forming the two dimensional image for the HDTV format. For other picture formats, the GLV type device  50  modulates more or less than the 1,920 times as the line image  90  scans across the display screen  58  depending upon which of the other picture formats is being displayed. 
     As the line image width  92  scans across the display screen  58 , the wavefront having the spatial phase variation produces the multiple speckle patterns with time. The multiple speckle patterns reduce the speckle that is detected by the eye or the intensity detector of the optical system. 
     The display optics  42  depicted in FIGS. 5,  6 , and  7  produce a monochrome image. Color display optics comprise the display optics  42 , two additional lasers, two additional illumination optics, two additional GLV type device&#39;s, and a dichroic filter group. In the color display optics, red, green, and blue lasers illuminate the three GLV type device&#39;s producing red, green, and blue linear arrays of pixels. The dichroic filter group combines the reflected and diffracted light from the three GLV type device&#39;s and directs the reflected and diffracted light to the Schlieren optics  52 . For the color display optics, the spatial phase variation across the line image width  92  preferably has an optimum amplitude for one of red, green, and blue laser illuminations (e.g., the green laser illumination), or a wavelength that is a specific average of participating wavelengths. The red, green, and blue wavefronts produce the multiple speckle patterns over time as the line image  90  is scanned across the display screen  58  and, thus, reduce the speckle in the color display optics. Alternatively, in the color display optics, the dichroic filter group combines the red, green, and blue laser illuminations to sequentially illuminate a single GLV type device. 
     Alternative embodiments of the present invention may include an angled GLV type device  96  as depicted in FIG.  9 . Referring simultaneously to FIG. 6, FIG. 8, and FIG.9, the Schlieren optics  52  and scanning optics  56  must now use the focal point  54  as the object for projection, not the GLV type device  50  itself, in order to recover a square pixel. The moveable elongated elements  98  of an angled GLV type device  96  are positioned such that they are neither parallel nor perpendicular with the boundaries of the effective pixel area  100 . This configuration of the moveable elongated elements  98  allows striations, artifacts or imperfections within the angled GLV type device  96  to be smeared out within the effective pixel area  100 , rather than be dragged across the entire picture. This is particularly important for defects running the length of the ribbon such as ribbon bow tilt and deflection. By eliminating these problems within the effective pixel area  100 , a softer picture is attained. Additionally, an angled GLV type device  96  reduces scanning defects and functions optimally with a wide illumination beam. 
     Another alternative embodiment of the present invention is depicted in FIGS. 10 &amp; 11. In these two drawings, an absorbing aperture  102  is affixed to the outer surface  68  of the seal glass  62 . The absorbing aperture  102  is fashioned such that it has an opening  104  where the focal point  54  meets the outer surface  68  of the seal glass  62 . This alternative embodiment can be easily implemented in that it can be affixed away from the surface of the GLV type device  50  before the seal glass  62  is put in place above the GLV type device  50 . This absorbing aperture  102  is implemented to block additional background light while allowing the reflected light R to pass to the Schlieren optics  52 , thus improving the picture contrast and overall quality. 
     One modification to the preferred embodiment may include, but is not limited to, implementing a standard GLV type device rather than a blazed type. This modification can be implemented if throughput is not an issue, as in some printing applications. In which case, one of the diffraction orders would simply be ignored. Additionally, the technique in the preferred embodiment is also applicable to monochrome systems, since the single color would still be on-axis for the projection system. 
     It will be readily apparent to one skilled in the art that other various modifications may be made to the preferred embodiment without departing from the spirit and scope of the invention as defined by the appended claims.