Abstract:
A display system comprises a light modulator having at least three linear arrays of light modulating devices on a common substrate, at least one light source producing at least three colors of light for illuminating the at least three linear arrays, a lens for creating line images of the at least three linear arrays on a display surface, and a scanning mirror for scanning the line images to create a two-dimensional image on the display surface.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates to a display system with a linear array of electromechanical grating devices that is scanned in order to generate a two-dimensional image. More particularly, the invention relates to a display system containing a trilinear array of electromechanical grating devices.  
       BACKGROUND OF THE INVENTION  
       [0002]     Spatial light modulators based on electromechanical grating devices are important for a wide range of applications, including display, data storage, spectroscopy and printing. Such systems require large numbers of individually addressable devices in either a linear or area array, with over a million addressable devices desirable for an area modulator array in a high-quality display.  
         [0003]     Linear arrays are particularly advantaged over their area array counterparts by virtue of higher resolution, reduced cost, and simplified illumination optics. Of particular interest: linear arrays are more suitable modulators for laser light than are their two-dimensional counterparts. Grating Light Valve (GLV) linear arrays, as described in U.S. Pat. No. 5,311,360 (Bloom et al.) are one earlier type of linear array that offers a workable solution for high-brightness imaging using laser sources, for example. Another experimental type of linear array just recently disclosed and in early development stages is the flexible micromirror linear array, as described in the article “Flexible micromirror linear array for high resolution projection display” by Francis Picard, et al. in  MOEMS Display and Imaging Systems, Proceedings of SPIE  Vol. 4985 (2003). The prototype flexible micromirror linear array described in the Picard et al. article employs a line of reflective “microbridges” which are individually switched to modulate light to form a linear image.  
         [0004]     Recently, an electromechanical conformal grating device consisting of ribbon elements suspended above a substrate by a periodic sequence of intermediate supports was disclosed by Kowarz in U.S. Pat. No. 6,307,663, issued on Oct. 23, 2001, entitled “SPATIAL LIGHT MODULATOR WITH CONFORMAL GRATING DEVICE.” The electromechanical conformal grating device is operated by electrostatic actuation, which causes the ribbon elements to conform around the support substructure, thereby producing a grating. The device of &#39;663 has more recently become known as the conformal GEMS device or, more simply, GEMS device, with GEMS standing for grating electromechanical system. The GEMS device possesses a number of attractive features. It provides high-speed digital light modulation with high contrast and good efficiency. In addition, in a linear array of GEMS devices, the active region is relatively large and the grating period is oriented perpendicular to the array direction. This orientation of the grating period causes diffracted light beams to separate in close proximity to the linear array and to remain spatially separated throughout most of an optical system and enables a simpler optical system design with smaller optical elements.  
         [0005]     Display systems based on a linear array of GEMS devices were described by Kowarz et al. in U.S. Pat. No. 6,411,425, entitled “ELECTROMECHANICAL GRATING DISPLAY SYSTEM WITH SPATIALLY SEPARATED LIGHT BEAMS,” issued Jun. 25, 2002 and by Kowarz et al. in U.S. Pat. No. 6,476,848, entitled “ELECTROMECHANICAL GRATING DISPLAY SYSTEM WITH SEGMENTED WAVEPLATE,” issued Nov. 5, 2002. Display systems based on GLV devices are disclosed in U.S. Pat. No. 5,982,553, entitled “DISPLAY DEVICE INCORPORATING ONE-DIMENSIONAL GRATING LIGHT-VALVE ARRAY” issued to Bloom et al. on Nov. 9, 1999.  
         [0006]     Current GLV and GEMS color display system architectures employ three separate color paths, Red, Green, and Blue (RGB), each color path provided with a linear array of electromechanical grating devices. Each linear array of electromechanical grating devices modulates its component red, green, or blue laser light. The resulting modulated light beams are then combined onto the same output axis to provide a full-color image that is then scanned to the display screen. Referring to  FIG. 1 , there is shown a prior art display system  10  using three separate optical paths in this way.  
         [0007]     For red color modulation, a red light source  70   r , typically a laser, provides illumination that is conditioned through a spherical lens  72   r  and a cylindrical lens  74   r  and directed towards a turning mirror  82   r . Light reflected from turning mirror  82   r  is modulated by diffraction at an electromechanical grating light modulator  85   r . Modulated diffracted light from electromechanical grating light modulator  85   r  is diffracted past turning mirror  82   r  and to a color combiner  100 , such as an X-cube or other dichroic combiner. The modulated light from color combiner  100  is then directed by a lens  75 , through an optional cross-order filter  110  (not shown), to a scanning mirror  77  for projection onto a display surface  90 . Green color modulation uses a similar set of components for providing light to color combiner  100 , with a green light source  70   g , typically a laser, providing illumination through a spherical lens  72   g  and a cylindrical lens  74   g  and directed towards a turning mirror  82   g . Light reflected from turning mirror  82   g  is modulated by diffraction at an electromechanical grating light modulator  85   g . Modulated diffracted light from electromechanical grating light modulator  85   g  is diffracted past turning mirror  82   g  and to color combiner  100 .  
         [0008]     Similarly, blue light source  70   b , typically a laser, provides illumination through a spherical lens  72   b  and a cylindrical lens  74   b  and directs light towards a turning mirror  82   b . Light reflected from turning mirror  82   b  is modulated by diffraction at an electromechanical grating light modulator  85   b , diffracted past turning mirror  82   b  and to color combiner  100 .  
         [0009]     In order to maintain precise color registration with the system of  FIG. 1 , each of the three linear arrays, electromechanical grating light modulators  85   r ,  85   g , and  85   b , must be aligned to each other, to within sub-pixel tolerances. This precision of alignment is difficult to obtain. Moreover, this alignment must prove stable so that it can be maintained over a range of temperatures and other environmental conditions, such as shock and vibration. Color combiner  100  is a particularly costly component and can be the source of undesirable image aberrations.  
         [0010]     One design solution that eliminates the color channel alignment problem is color-sequential operation, whereby a single electromechanical grating light modulator serves to modulate each color light in sequence. This type of system is described, for example, in the disclosure of U.S. Pat. No. 6,411,425 entitled “Electromechanical Grating Display System with Spatially Separated Light Beams” to Kowarz et al. (see cols. 9-10). However, as noted in the &#39;425 Kowarz et al. disclosure, color-sequential techniques, by modulating only one color at a time, effectively waste two-thirds of the available light. Thus, although color-sequential solutions minimize alignment concerns, these solutions have performance drawbacks.  
         [0011]     Thus, it can be seen that there would be advantages to a projection solution that provides the performance benefits of electromechanical grating light modulators without requiring frequent adjustment of alignment for each color path or requiring complex and costly mounting apparatus that would be able to maintain alignment over a wide range of operating conditions, and that provides a compact, low-cost apparatus, without the requirement for a color-combining prism.  
       SUMMARY OF THE INVENTION  
       [0012]     The present invention is directed to overcoming one or more of the problems set forth above. Briefly summarized, according to one aspect, the present invention provides a display system, comprising:  
         [0013]     a) a light modulator having at least three linear arrays of light modulating devices on a common substrate;  
         [0014]     b) at least one light source producing at least three colors of light for illuminating the at least three linear arrays;  
         [0015]     c) a lens for creating line images of the at least three linear arrays on a screen; and  
         [0016]     d) a scanning mirror for sweeping the line images to create a two-dimensional image.  
         [0017]     It is a feature of the present invention that it uses arrays of electromechanical grating devices that are fabricated on a common substrate.  
         [0018]     It is an advantage of the present invention that it provides a full-color display system using arrays of electromechanical grating devices that does not require alignment of individual color paths for modulated light.  
         [0019]     It is an advantage of the present invention that it provides a full-color display system using arrays of electromechanical grating devices with a reduced number of components, allowing design of a more compact and lower cost apparatus than provided by earlier designs. With its reduced number of components and minimized requirements for adjustment, the apparatus of the present invention provides a robust solution for imaging using electromechanical grating array devices.  
         [0020]     It is a further advantage of the present invention that it eliminates the need for a color combining cube in the path of modulated light.  
         [0021]     These and other objects, features, and advantages of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described an illustrative embodiment of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]     While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter of the present invention, it is believed that the invention will be better understood from the following description when taken in conjunction with the accompanying drawings, wherein:  
         [0023]      FIG. 1  is a schematic block diagram showing a prior art display apparatus employing an electromechanical grating light modulator in each of three color modulation channels;  
         [0024]      FIG. 2  is a schematic block diagram showing the arrangement of color modulation components in one embodiment of the present invention;  
         [0025]      FIG. 3  is a plan view showing three arrays of electromechanical grating light modulators on a single substrate according to one embodiment;  
         [0026]      FIG. 4   a  is plan view showing routing of control traces for three arrays of electromechanical grating light modulators according to another embodiment;  
         [0027]      FIG. 4   b  is a side view showing the arrangement of a control trace used in the embodiment of  FIG. 4   a;    
         [0028]      FIG. 5  is a plan view showing a scheme for routing of control traces from driver circuits for three arrays of electromechanical grating light modulators according to another embodiment;  
         [0029]      FIG. 6  is a schematic block diagram showing an arrangement of color modulation components in an alternate embodiment of the present invention;  
         [0030]      FIG. 7  is a schematic block diagram showing an arrangement of color modulation components in another alternate embodiment of the present invention, employing a cover prism as an alternative to a cover plate; and,  
         [0031]      FIG. 8  is a plan view of a patterned micromirror used in some embodiments of the present invention. 
     
    
       [0032]     To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0033]     The present description is directed in particular to elements forming part of, or cooperating more directly with, apparatus in accordance with the invention. It is to be understood that elements not specifically shown or described may take various forms well known to those skilled in the art.  
         [0034]     For the description that follows, components specific to a single color path may be more particularly identified with a letter appended to the part number. Where used, letters correspond to color paths; for example, “r” is appended for red, “b” for blue, “g” for green.  
         [0035]     Referring to  FIG. 2 , there is shown an embodiment of display system  10  of the present invention, employing a trilinear array of light modulating devices  60  for modulating light from red, green, and blue light sources  70   r ,  70   g , and  70   b . A dichroic combiner  94  is provided in the illumination path, for directing illumination from each of red, green, and blue light sources  70   r ,  70   g , and  70   b  toward a patterned micromirror assembly  62 . Patterned micromirror assembly  62 , in turn, directs the illumination for each color to a corresponding linear array of light modulating devices  185   r ,  185   g , or  185   b  in trilinear array of light modulating devices  60 . Referring to  FIG. 8 , there is shown an arrangement of patterned micromirror assembly  62  for one embodiment. An arrangement of reflective segments  24  is provided, with transmissive regions  26  spaced between reflective segments  24 . Reflective segments  24  may be fabricated using deposition of reflective metallic materials such as aluminum or silver, applied using shadow mask techniques or other suitable fabrication techniques. Transmissive regions  26  may be fabricated in a number of ways, such as applying an anti-reflective coating onto a glass substrate, for example.  
         [0036]     In a preferred embodiment, trilinear array of light modulating devices  60  is formed from three separate electromechanical grating light modulator sections, aligned on the same substrate, each electromechanical grating light modulator section corresponding to a linear array of light modulating devices  185   r ,  185   g , or  185   b . In one embodiment, linear array of light modulating devices  185   r ,  185   g , and  185   b  are GEMS devices. Alternately, any one or all of linear array of light modulating devices  185   r ,  185   g , or  185   b  could be a GLV device or a flexible micromirror linear array, as described in the Picard et al. article noted in the background section hereinabove. As is well-known to those skilled in the art, the specific layout and design of supporting optical elements in display system  10  must be tailored to the specific type of light modulating device that is used.  
         [0037]     Referring again to  FIG. 2 , reflective segments  24  of patterned micromirror assembly  62  are angularly aligned with each linear array of light modulating devices  185   r ,  185   g , and  185   b  in display system  10 . Modulated light passes through the glass substrate of patterned micromirror assembly  62  and is then directed through lens  75  and optional cross-order filter  10 , placed at focal distance f from lens  75 , to scanning mirror  77  for forming the final color image on display surface  90 , as was described with reference to the prior art apparatus of  FIG. 1 . It is worthwhile to observe that dichroic combiner  94  can be an X-cube or X-prism or similar device. However, in contrast to the arrangement of  FIG. 1 , this component is in the illumination path, rather than in the path of modulated light. Thus, there is wider tolerance for fabrication of dichroic combiner  94  of  FIG. 2  than for the conventional X-prism used as color combiner  100  in  FIG. 1 , allowing this component to have significantly lower cost. For example, an X-prism used as color combiner  100  would be smaller than an X-prism used for modulated light. Additional cost benefits relate to the overall optical performance requirements for projection optics. With the arrangement of  FIG. 1 , for example, lens  75  requires a longer back focal length and must provide acceptable imaging through color combiner  100 , since color combiner  100  is in the object path of lens  75 . Moving the color-combining function to the illumination path thus simplifies lens  75  design and cost.  
         [0038]     Other types of combining components could alternately be used in the illumination path; typically, these components employ an arrangement of dichroic surfaces. While some amount of alignment is needed for red, green, and blue light sources  70   r ,  70   g , and  70   b , the arrangement of  FIG. 2  allows some tolerance for imperfect alignment.  
         [0039]     It is instructive to observe that the three linear array of light modulating devices  185   r ,  185   g , or  185   b  provide line images that are spatially separated. This necessitates incorporating a time delay of some duration into the display data stream, so that lines of the output image are written at the appropriate time, as paced by scanning hardware and depending on spacing between the three linear array of light modulating devices  185   r ,  185   g , or  185   b . At any one moment, for example, linear array of light modulating devices  185   r  may be writing a line (line n) of the output image; linear array of light modulating devices  185   g  may be writing an adjacent or nearby line (line (n+1) or line (n+2)) of the output image; and linear array of light modulating devices  185   b  may be writing a further line of the output image. It may not be feasible or desirable to write adjacent lines of different colors at precisely the same time; instead, some amount of timing delay would be required. In any case, some amount of timing offset is needed in order to compensate for spacing between the three linear array of light modulating devices  185   r ,  185   g , or  185   b.    
         [0040]     Referring now to  FIG. 3 , there is shown a plan view representation of linear arrays of light modulating devices  185   r ,  185   g , and  185   b  on trilinear array of light modulating devices  60  in a simple embodiment. In operation, each linear array of light modulating devices  185   r ,  185   g , and  185   b  operates independently, having its own control signals, routed from driver circuitry (not shown in  FIG. 2 ).  
         [0000]     Options for Control Signal Routing  
         [0041]     In the embodiment of  FIG. 3 , there are difficulties in properly routing control signals to green linear array of light modulating devices  185   g , shown in the center, due to its position between red and blue linear arrays of light modulating devices  185   r  and  185   b , respectively.  FIG. 4   a  shows an arrangement of control traces  64  and contacts  66  that could be used to provide this electrical connection in one embodiment. In the embodiments of  FIGS. 4   a  and  4   b , linear arrays of light modulating devices  185   r ,  185   g , and  185   b  are depicted as GEMS devices. Each set of ribbon elements  46  has a corresponding contact  66  used for activation. Interspersed with ribbon elements  46  in the outer red and blue linear array of light modulating devices  185   r  and  185   b  are control traces  64  that provide pass-through control signals for green linear array of light modulating devices  185   g . Ribbon elements  46  are provided with underlying supports  29 , as was described in U.S. Pat. No. 6,307,663, cited earlier. Control traces  64  are provided with additional retainers  30 , as shown in  FIG. 4   a  and in the side view of  FIG. 4   b , so that these structures do not bend and thereby unintentially act as an active part of the optical grating for outer red and blue linear array of light modulating devices  185   r  and  185   b . This arrangement, using both retainers  30  and supports  29  for control traces, helps to minimize cross-talk between color channels.  
         [0042]     The arrangement of  FIGS. 4   a  and  4   b  is workable in particular because the relative resolution of red, green, and blue modulated light need not be the same. As is well known in the imaging arts, it is most important for the green modulated light to have high resolution. Lower resolution of the other colors, particularly of blue, is acceptable and has little impact on image quality. In  FIG. 4   a , for example, the resolution of green linear array of light modulating devices  185   g  is higher than that of red or blue linear array of light modulating devices  185   r  or  185   b.    
         [0043]     Using a lower resolution value for blue, the arrangement shown in  FIG. 5  rearranges the relative positions of linear array of light modulating devices  185   r ,  185   g , and  185   b  from that of  FIG. 4   a . In  FIG. 5 , blue linear array of light modulating devices  185   b  has lower resolution and is positioned between red and green linear array of light modulating devices  185   r  and  185   g . Red, green, and blue driver chips  68   r ,  68   g , and  68   b  can then route signal lines using control traces  64  for providing electrical connection in a simple pattern as shown. Because blue linear array of light modulating devices  185   b  has lower resolution, fewer control traces  64  are needed for this middle section of trilinear array of light modulating devices  60 .  
         [0044]     In a preferred embodiment for use with an HDTV system having 1080×1920 resolution, the following dimensions are illustrative:  
         [0000]     540 blue pixels (half the number of green pixels and of red pixels)  
         [0000]     3 um per line/1 um spacing for control traces  64   
         [0000]     135×4 um=540 um space requirement for control traces  64  to blue pixels, plus additional margin.  
         [0000]     Alternate Embodiments for Display System  10   
         [0045]     Referring to  FIG. 6 , there is shown an alternate embodiment for display system  10  using a combination with a dichroic combiner  92 , an achromatic polarizing bearnsplitter  93 , an achromatic quarter waveplate  96 , and a patterned stop assembly  52 , having a stop  54  for each modulated light path (rather than having reflective segments  24  as does patterned micromirror assembly  62 ). Here, dichroic combiner  92  provides a single surface for reflecting red and green light and transmitting blue light towards polarizing beamsplitter  93 . Achromatic quarter waveplate  96  could alternately be a multiorder quarter wave plate. The embodiment of  FIG. 6  effectively de-couples the input path (illumination) from the output path (modulated light), making the arrangement of  FIG. 6  advantageous over the arrangement of  FIG. 2 , and easing tolerance requirements. However, both polarization beamsplitter  93  and achromatic quarter waveplate  96  are more costly with this arrangement.  
         [0046]     Referring to  FIG. 7 , there is shown another alternate embodiment having advantages similar to the embodiment of  FIG. 6  and eliminating the need for both polarization beamsplitter  93  and achromatic quarter waveplate  96 , relatively expensive components. Here, illumination is incident to trilinear array of light modulating devices  60  at an oblique angle. A cover prism  98 , with the arrangement of surfaces as shown in  FIG. 7 , serves to eliminate the requirement for a separate cover glass near the surface of trilinear array of light modulating devices  60 ; alternately, cover prism  98  could simply be bonded to the cover glass. The cover prism  98  arrangement thereby removes the cause of undesirable cover reflection that would otherwise contribute to reduced contrast with light oblique to the surface of trilinear array of light modulating devices  60 . As shown in  FIG. 7 , light enters and exits from trilinear array of light modulating devices  60  at close to normal angles with respect to surfaces of cover prism  98 .  
         [0047]     The invention has been described with reference to a preferred embodiment. However, it will be appreciated that variations and modifications can be effected by a person of ordinary skill in the art without departing from the scope of the invention. Thus, what is provided is a display apparatus and method using a trilinear electromechanical grating light modulator to provide high-quality imaging at lower cost.  
       PARTS LIST  
       [0000]    
       
           10  Display system  
           24  Reflective segment  
           26  Transmissive region  
           29  Support  
           30  Retainer  
           46  Ribbon elements  
           48  Retaining elements  
           52  Patterned stop assembly  
           54  Stop  
           60  Trilinear array of light modulating devices  
           62  Patterned micromirror assembly  
           64  Control trace  
           66  Contact  
           68 ,  68   r ,  68   g ,  68   b  Driver chip; Driver chip, red; Driver chip, green; Driver chip, blue.  
           70 ,  70   r ,  70   g ,  70   b  Light source; Light source, red; Light source, green; Light source, blue.  
           72 ,  72   r ,  72   g ,  72   b  Spherical lens; Spherical lens, red; Spherical lens, green; Spherical lens, blue  
           74 ,  74   r ,  74   g ,  74   b  Cylindrical lens; Cylindrical lens, red; Cylindrical lens, green; Cylindrical lens, blue  
           75  Lens  
           77  Scanning mirror  
           82 ,  82   r ,  82   g ,  82   b  Turning mirror; Turning mirror, red; Turning mirror, green; Turning mirror, blue  
           85  Electromechanical grating light modulator  
           85   r ,  85   g ,  85   b  Electromechanical grating light modulator, red; Electromechanical grating light modulator, green; Electromechanical grating light modulator, blue  
           90  Display surface  
           92  Dichroic combiner  
           94  Dichroic combiner  
           93  Polarizing beamsplitter  
           96  Quarter-wave plate  
           98  Cover prism  
           100  Color combiner  
           110  Cross-order filter  
           185   r ,  185   g ,  185   b  Linear array of light modulating devices, red; Linear array of light modulating devices, green; Linear array of light modulating devices, blue