Patent Document

FIELD OF THE INVENTION 
     This invention relates to an image-forming system containing an array of electromechanical grating devices. More particularly, the invention relates to the formation of gray levels in a projection display system containing a linear array of electromechanical grating devices. 
     BACKGROUND OF THE INVENTION 
     Many projection display systems employ a spatial light modulator to convert electronic image information into an output image. At present in such display systems, the light source is typically a white light lamp and the spatial light modulator is typically an area array containing liquid crystal devices or micromirror devices. Alternative display system architectures with one or more laser sources and spatial light modulators that are linear arrays of electromechanical grating devices show much promise for the future. To display high quality motion images, the individual devices of these different spatial light modulators must be capable of rapidly producing a large number of gray levels in the image. The limit on the number of possible gray levels is usually dictated either by the device dynamics or by the speed of electronic components within the display system. 
     Prior inventions have disclosed schemes for increasing the number of gray levels in the image without increasing the speed of the modulating elements or of the associated electronics. These schemes vary the illumination that is incident on the spatial light modulator during a frame. Specifically, according to U.S. Pat. No. 5,812,303, issued to Hewlett et al. on Sep. 22, 1998, entitled, “LIGHT AMPLITUDE MODULATION WITH NEUTRAL DENSITY FILTERS,” additional gray levels can be obtained with a micromirror device by using a variable neutral density filter to generate bright and dark gray levels. The dark gray scale is obtained by attenuating the illumination for some time during the display of a frame. The bright gray scale has no attenuation. In practice, the invention is implemented by rotating a filter wheel with a multi-segment neutral density filter in synchronization with the data stream. 
     An alternative approach employs a pulsating light source, such as a pulsed laser to reduce speed requirements on the electronic components, as described in U.S. Pat. No. 5,668,611, issued to Ernstoff et al. on Sep. 16, 1997, entitled “FULL COLOR SEQUENTIAL IMAGE PROJECTION SYSTEM INCORPORATING PULSE RATE MODULATED ILLUMINATION.” The illumination on the spatial light modulator is adjusted by varying the pulse rate or pulse count. Moreover, the average brightness of the light source is determined by the number of pulses. A complementary method uses direct intensity modulation of the light source to obtain multiple levels of brightness, as disclosed in U.S. Pat. No. 5,903,323, issued to Ernstoff al. on May 11, 1999, entitled “FULL COLOR SEQUENTIAL IMAGE PROJECTION SYSTEM INCORPORATING TIME MODULATED ILLUMINATION.” Both U.S. Pat. No. 5,668,611 and U.S. Pat. No. 5,903,323 address the specific problem of having a large enough time window for the electronic components to load new image data bits into the spatial modulator. 
     Each of the above described inventions trade light source efficiency for improved gray levels or reduced electronic component speed requirements. However, efficient use of the light source is needed for high-quality projection displays in order to maximize brightness and color saturation of the projected image. 
     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 to Kowarz 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, with GEMS standing for grating electromechanical system. The conformal 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 conformal 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. Display systems based on a linear array of conformal 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. 
     The conformal Grating Electromechanical System (GEMS) devices disclosed in U.S. Pat. No. 6,307,663 are illustrated in FIGS. 1-3. FIG. 1 shows two side-by-side conformal GEMS devices  5   a  and  5   b  in an unactuated state. The conformal GEMS devices  5   a  and  5   b  are formed on top of a substrate  10  covered by a bottom conductive layer  12 , which acts as an electrode to actuate the devices  5   a ,  5   b . The bottom conductive layer  12  is covered by a dielectric protective layer  14  followed by a standoff layer  16  and a spacer layer  18 . On top of the spacer layer  18 , a ribbon layer  20  is formed which is covered by a reflective layer and conductive layer  22 . The reflective and conductive layer  22  provides electrodes for the actuation of the conformal GEMS devices  5   a  and  5   b . Accordingly, the reflective and conductive layer  22  is patterned to provide electrodes for the two conformal GEMS devices  5   a  and  5   b . The ribbon layer  20 , preferably, comprises a material with a sufficient tensile stress to provide a large restoring force. Each of the two conformal GEMS devices  5   a  and  5   b  has an associated elongated ribbon element  23   a  and  23   b , respectively, patterned from the reflective and conductive layer  22  and the ribbon layer  20 . The elongated ribbon elements  23   a  and  23   b  are supported by end supports  24   a  and  24   b , formed from the spacer layer  18 , and by one or more intermediate supports  27  that are uniformly separated in order to form equal-width channels  25 . The elongated ribbon elements  23   a  and  23   b  are secured to the end supports  24   a  and  24   b  and to the intermediate supports  27 . A plurality of square standoffs  29  is patterned at the bottom of the channels  25  from the standoff layer  16 . These standoffs  29  reduce the possibility of the elongated ribbon elements  23   a  and  23   b  sticking when actuated. 
     A top view of a four-device linear array of conformal GEMS devices  5   a ,  5   b ,  5   c  and  5   d  is shown in FIG.  2 . The elongated ribbon elements  23   a ,  23   b ,  23   c , and  23   d  are depicted partially removed over the portion of the diagram below the line A—A in order to show the underlying structure in an active region  8 . For best optical performance and maximum contrast, the intermediate supports  27  should preferably be completely hidden below the elongated ribbon elements  23   a ,  23   b ,  23   c , and  23   d . Therefore, when viewed from the top, the intermediate supports  27  should not be visible in the gaps  28  between the conformal GEMS devices  5   a - 5   d . Here, each of the conformal GEMS devices  5   a - 5   d  has three intermediate supports  27  with four equal-width channels  25 . The center-to-center separation A of the intermediate supports  27  defines the period of the conformal GEMS devices in the actuated state. The elongated ribbon elements  23   a - 23   d  are mechanically and electrically isolated from one another, allowing independent operation of the four conformal GEMS devices  5   a - 5   d . The bottom conductive layer  12  of FIG. 1 can be common to all of the conformal GEMS devices  5   a - 5   d.    
     FIG. 3 a  is a side view, through line  3 , 7 - 3 , 7  of FIG. 2, of two channels  25  of the conformal GEMS device  5   b  (as shown and described in FIGS. 1 and 2) in an unactuated state. FIG. 3 b  shows the same view for an actuated state. For operation of the device, an attractive electrostatic force is produced by applying a voltage difference between the bottom conductive layer  12  and the reflective and conductive layer  22  of the elongated ribbon element  23   b . In, the unactuated state (see FIG. 3 a ), with no voltage difference (V=0), the ribbon element  23   b  is suspended flat between the supports. In this state, an incident light beam  30  is primarily reflected into a 0th order light beam  32 , as in a simple planar mirror. To obtain the actuated state, a voltage is applied to the conformal GEMS device  5   b , which deforms the elongated ribbon element  23   b  and produces a partially conformal grating with period A. FIG. 3 b  shows the device  5   b  (as shown and described in FIGS. 1 and 2) in the fully actuated state with the applied voltage at V=V HIGH  and the elongated ribbon element  23   b  in contact with standoffs  29 . The height difference between the bottom of element  23   b  and the top of the standoffs  29  is chosen to be approximately ¼ of the wavelength λ of the incident light. The optimum height depends on the specific conformal shape of the actuated device. In the actuated state, the incident light beam  30  is primarily diffracted into the +1st order light beam  35   a  and −1st order light beam  35   b , with additional light diffracted into the +2nd order  36   a  and −2nd order  36   b . A small amount of light is diffracted into even higher orders and some light remains in the 0th order. In general, one or more of the various beams can be collected and used by an optical system, depending on the application. When the applied voltage is removed, the forces due to tensile stress and bending restores the ribbon element  23   b  to its original unactuated state, as shown in FIG. 3 a.    
     FIG. 4 illustrates a display system containing a linear array  85  of conformal GEMS devices, as disclosed in U.S. Pat. No. 6,411,425. Light emitted from a source  70 , preferably a laser, is conditioned by a pair of lenses  72  and  74 , before hitting a turning mirror  82  and illuminating the linear array  85 . The display system forms an entire two-dimensional scene from a scan of a one-dimensional line image of the linear array  85  across the screen  90 . The conformal GEMS devices of the linear array  85  are capable of rapidly modulating incident light to produce multiple lines of pixels with gray levels. The controller  80  selectively activates the linear array  85  to obtain the desired pixel pattern for a given line of a two-dimensional scene. If a particular conformal GEMS device is not actuated, it reflects the incident light beam primarily into the 0th order light beam, which is directed back towards the source  70  by the turning mirror  82 . If a particular conformal GEMS device is actuated, it diffracts the incident light beams primarily into +2 nd , +1 st , −1 st  and −2 nd  order light beams. These diffracted light beams pass around the turning mirror  82  and are projected on the screen  90  by the projection lens system  75 . The scanning mirror  77  sweeps the line image across the screen  90  to form the two-dimensional image. Synchronization between the sweep of the scanning mirror  77  and the image data stream is provided by the controller  80 . 
     In order to form gray levels in an image, pulse-width-modulated (PWM) waveforms are applied to the conformal GEMS devices of the linear array  85 , as described by Kowarz et al., “Conformal Grating Electromechanical System (GEMS) for High-Speed Digital Light Modulation,” IEEE 15 th  International MEMS Conference Technical Digest, pgs. 568-573 (2002). FIG. 5 shows a conventional single-level PWM waveform  45  with voltage V HIGH , together with the corresponding device&#39;s output (e.g., diffracted light intensity). To obtain a desired gray level, the controller  80  selects the voltage pulse width in each modulation window  54 , according to a data stream. When the single-level PWM waveform  45  transitions from 0 V to V HIGH , the device (for example,  5   a  and  5   b  shown in FIG. 1) actuates and begins diffracting light. When the waveform transitions back to 0 V, the device stops diffracting light. This process is applied to each conformal GEMS device of the linear array  85 . In the display system of FIG. 4, the modulation window  54  corresponds to the time used to form a single line of the two-dimensional image. The gray level perceived in a pixel of the image is, therefore, the integrated light intensity  52  within the modulation window  54 . To minimize charging effects within the device, the applied voltage can be periodically switched between V HIGH  and −V HIGH  (see U. S. Pat. No. 6,144,481). Since the force on the ribbon elements  23   a - 23   d  is independent of polarity, the diffracted light intensity is polarity independent. 
     FIG. 6 shows an example of a gray scale  55  generated using the conventional single-level PWM approach of FIG.  5 . In this plot, relative gray levels are shown as a function of pulse width (±V HIGH ) for a modulation window  54  of 20 μsec. When the pulse width is between 0.3 μsec and 20 μsec, the relationship between gray level and pulse width is approximately linear. A non-monotonic (and nonlinear) region  50  is present, however, for pulse widths between approximately 0.3 μsec and 0.1 μsec. This non-monotonic behavior occurs because of the resonant dynamics of the conformal GEMS device The particular shape of the non-monotonic region  50  depends on a number of factors, including device geometry and driver slew rate. For pulse widths shorter than approximately 0.1 μsec, there is a monotonic, non-linear correspondence between pulse width and gray level. The dark (gray) levels in the non-monotonic region  50  can be difficult to use in practice because of the difficulty in determining the exact correspondence between a desired gray level and the pulse width required to generate that particular gray level. 
     The techniques described in the prior art for improving gray levels all lower the average optical power incident on the spatial light modulator for some period of time, thus generating multiple illumination levels corresponding to decreased intensity. Multiple illumination levels reduce the speed requirements of the spatial light modulator and its associated electronic components. However, a serious technical drawback to this approach is that it wastes optical power that is available from the light source during lower illumination intervals. Furthermore, for certain types of light sources, reducing the illumination level increases system complexity. The same drawbacks apply when these techniques are used in a display system based on electromechanical grating devices. There is a need, therefore, for a method of generating enhanced gray levels in an electromechanical grating display system that makes efficient use of available optical power and does not significantly increase system complexity. 
     SUMMARY OF THE INVENTION 
     The aforementioned need is met according to the present invention by providing a method for actuating an electromechanical grating device that has ribbon elements. The method includes the steps of: providing a data stream; 
     generating a pulse width modulated waveform including at least two different non-zero voltages from the data stream; and applying the pulse width modulated waveform to the ribbon elements such that the ribbon elements transition through at least three different states of actuation that correspond to the pulse width modulated waveform. 
     Another aspect of the present invention provides a system for actuating an electromechanical grating device that has ribbon elements. The system includes: a data source that provides a data stream; means for generating a pulse width modulated waveform including at least two different non-zero voltages from the data stream; and means for applying the pulse width modulated waveform to the ribbon elements such that the ribbon elements transition through at least three different states of actuation corresponding to the pulse width modulated waveform. 
     A third aspect of the present invention provides an electromechanical grating device that includes a substrate and a set of elongated ribbon elements suspended above the substrate. Additionally, the electromechanical grating device includes means for generating a pulse width modulated waveform that has at least two different non-zero voltages from a data stream and means for applying the pulse width modulated waveform to the elongated ribbon elements such that the ribbon elements transition through at least three different states of actuation that correspond to the pulse width modulated waveform. 
     As described below, in addition to reducing the clock rate requirements, the present invention has the added benefit of allowing better generation of the dark levels corresponding to the non-monotonic region  50  of FIG.  6 . 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a prior art perspective view of two conformal GEMS devices in a linear array; 
     FIG. 2 is a prior art, partially cut-away top view of four conformal GEMS devices in a linear array; 
     FIGS. 3 a  and  3   b  are prior art cross-sectional views through line  3 , 7 - 3 , 7  in FIG. 2, showing the operation of a conformal GEMS device in an unactuated state with V=0 and a fully actuated state with V=V HIGH , respectively; 
     FIG. 4 is a prior art schematic illustrating a line-scanned display system incorporating a linear array of conformal GEMS devices; 
     FIG. 5 illustrates prior art formation of gray levels through single-level pulse width modulation; 
     FIG. 6 is a plot showing a gray scale for a conformal GEMS device with gray levels obtained through single-level pulse width modulation; 
     FIG. 7 is a cross-sectional views through line  3 , 5 - 3 , 5  in FIG. 2, showing the operation of a conformal GEMS device in a partially actuated state with V=V LOW ; 
     FIG. 8 is a plot of the 0 th  order and 1 st  order light intensity as a function of voltage, illustrating the selection of V LOW  and V HIGH ; 
     FIG. 9 illustrates the formation of gray levels through dual-level pulse width modulation; 
     FIG. 10 is a plot showing the gray scale of a conformal GEMS device with gray levels obtained through dual-level pulse width modulated; 
     FIG. 11 is a plot showing the gray scale of a conformal GEMS device with gray levels obtained by multi-level pulse width modulation; and 
     FIG. 12 shows a block diagram of the electronic architecture used to generate dual-level pulse width modulation; 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the present invention, an additional voltage level V LOW  is used to improve the generation of dark levels. Although best suited for conformal GEMS devices, the invention may also be applied to other electromechanical grating devices such as the Grating Light Valve (GLV) made by Silicon Light Machines. When V LOW  is applied to a conformal GEMS device, a partially actuated state is generated, as illustrated in FIG.  7 . The electrostatic force generated by V LOW  produces a slight deformation in the elongated ribbon element  23   b  and generates a weak grating with period Λ. The elongated ribbon element  23   b  therefore stays suspended well above the underlying structure, i.e., above the standoffs  29 . In this partially actuated state, the majority of the incident beam  30  is reflected into the 0th order light beam  32 , with a small portion of the incident beam diffracted into the various non-zero diffracted orders (+1st order  35   a , −1st order  35   b , +2nd order  36   a  and −2nd order  36   b ). Typically, V LOW  is selected to be a few volts less than the pull-down voltage V PD , where the ribbon element  23   b  snaps into contact with the standoffs  29 . 
     FIG. 8, which is a plot of normalized intensity versus applied voltage for a digital input signal, illustrates the selection of V LOW  and V HIGH  for a particular conformal GEMS device. Here, V HIGH ˜22V, V PD ˜20V and V LOW ˜16V. When the voltage is at V HIGH , the reflected 0 th  order is almost completely extinguished and most of the incident light is diffracted into the non-zero orders. On the other hand, at V LOW , most of the incident light is reflected with only a small percentage present in the non-zero orders. This small percentage can be used to produce finer and more predictable dark levels. 
     FIG. 9 illustrates a dual-level PWM waveform  51  for the present invention also showing the corresponding diffracted light intensity. The two voltage amplitudes V HIGH  and V LOW  generate associated intensity levels I HIGH  and I LOW , respectively. To obtain a desired gray level within each modulation window  54 , the controller  80  selects the voltage amplitude (either V HIGH  or V LOW ), the voltage pulse width, and (optionally) the polarity. At transitions in the dual-level PWM waveform  51  from 0 V to ±V HIGH , the elongated ribbon elements of a conformal GEMS device actuate into contact with the standoffs, thereby causing diffraction of most of the incident light into the non-zero orders. At transitions from 0 V to ±V LOW , the device partially actuates and diffracts a small percentage of light into the non-zero orders. As before, gray levels are obtained from the integrated light intensity  52  within a modulation window  54 . Since the stress on the ribbon element is less in the partially actuated state than in the fully actuated state, and since the majority of image content is relatively dark, the use of a dual-level PWM waveform has the added benefit of reducing device aging. 
     A gray scale  61  produced by the dual-level PWM waveform  51 , of FIG. 9, is shown in FIG.  10 . The two voltage amplitudes V HIGH  and V LOW  each have associated curves that relate gray level to pulse width. For bright levels in an image, the controller  80  selects the voltage amplitude selected to be V HIGH , whereas for dark levels it is V LOW . At a given pulse width, for pulses wider than approximately 0.1 μsec, the gray level corresponding to V=V LOW  is approximately one tenth of the level for V=V HIGH . The non-monotonic region  50  of FIG. 6 can be bypassed by using V=V LOW  with the appropriate pulse width to generate the gray levels of interest. A transition region, where the gray levels can be formed with either V HIGH  or V LOW , can be used to provide headroom for calibration between the two gray level curves so as to produce a smooth continuous gray scale. 
     It is instructive to compare the clock rate of the dual-level PWM waveform  51  of the present invention with conventional (single-level) PWM waveform  45  for a high-quality projection display based on linear arrays of conformal GEMS devices. In this example, the system is chosen to have HDTV resolution with 1,920 scanned lines (1,080 by 1,920 pixels), a frame rate of 60 Hz and a gray scale capability of 13 linear bits per color per frame (8,192 gray levels). For the case of ordinary PWM, the pulse width increment must be somewhat less than 1/(1,920*60*8,192) seconds ˜1nanoseconds to allow for some overhead for scanning. The digital electronics in the controller, therefore, need to generate an effective PWM clock of approximately 1 GHz. This effective clock frequency can be reduced substantially by implementing dual-level PWM, while maintaining the same system specifications. For example, by choosing V HIGH  and V LOW  so that the ratio of the intensity levels is I LOW /I HIGH ˜10, the effective PWM clock frequency can be reduced to approximately 100 MHz. 
     A multi-level PWM waveform (not shown) with more than two voltage amplitudes can be used to further improve the gray scale  63  of an image, as illustrated in FIG. 11 for the case of tri-level pulse width modulation. In this example, the application of V HIGH  causes the elongated ribbon elements to fully actuate into contact with the standoffs, whereas V LOW1  and V LOW2 &lt;V LOW1  produce two different states of partial ribbon actuation. The gray scale  63  then consists of bright levels formed using V HIGH , dark levels (labeled dark levels  1  in FIG. 11) formed using V LOW1  and very dark levels (labeled dark levels  2  in FIG. 11) formed using V LOW2 . Transition regions can again be used at the intersections between the usable ranges of the different gray level curves. The arrows in FIG. 11, which point towards increasing gray levels, illustrate one example of a continuous gray scale  63  formed from three segments on the three curves. 
     FIG. 12 shows a block diagram of an electronic architecture for implementing dual-level PWM in an image-forming system, which could be a projection display or a printing system. A data source  100  provides a (serial) data stream  105  that has been appropriately preprocessed for generating an image from a linear array of electromechanical grating devices, preferably an array of conformal GEMS devices. Since each electromechanical grating device  120  of the linear array has its own device driver  108 , a serial-to-parallel converter  102  is needed to demultiplex the data stream  105  into the appropriate parallel data channels  115 . For example, in the HDTV system mentioned above, there would be  1080  parallel data channels  115  in order to address  1080  electromechanical grating devices  120 , although only one of these parallel data channels  115  is shown in FIG.  12 . The pulse-width-modulated output from each device driver  108  is determined by a pulse width generator  106  which controls the width of each voltage pulse via a clock  104 , and a voltage selector  110  which selects the amplitude and polarity of each voltage pulse. The result is a dual-level PWM waveform similar to the waveform shown in FIG.  9 . 
     In the embodiments described above, the selection of voltage amplitude is done independently for every pixel image. The voltage amplitude applied to each electromechanical grating device of the linear array can be therefore selected independently. This approach provides the most flexible and is most light efficient. If there is sufficient illumination available from the light source, the voltage amplitude may be varied in a periodic fashion. For example, the waveform could transition from V HIGH  to V LOW  to −V HIGH  to −V LOW  on a line-by-line basis or frame-by-frame basis. Although simpler to implement, this approach wastes nearly half of the incident light. 
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. 
     PARTS LIST 
       5   a  conformal grating device 
       5   b  conformal grating device 
       5   c  conformal grating device 
       5   d  conformal grating device 
       8  active region 
       10  substrate 
       12  bottom conductive layer 
       14  protective layer 
       16  standoff layer 
       18  spacer layer 
       20  ribbon layer 
       22  reflective and conductive layer 
       23   a  elongated ribbon element 
       23   b  elongated ribbon element 
       23   c  elongated ribbon element 
       23   d  elongated ribbon element 
       24   a  end support 
       24   b  end support 
       25  channel 
       27  intermediate support 
       28  gap 
       29  standoff 
       30  light beam 
       32  0 th  order light beam 
       35   a  +1 st  order beam 
       35   b  −1 st  order beam 
       36   a  +2 nd  order beam 
       36   b  −2 nd  order beam 
       45  single-level PWM waveform 
       50  non-monotonic region 
       51  dual-level PWM waveform 
       52  integrated light intensity 
       54  modulation window 
       55  gray scale for single-level PWM 
       61  gray scale for dual-level PWM 
       63  gray scale for multi-level PWM 
       70  source 
       72  spherical lens 
       74  cylindrical lens 
       75  projection lens system 
       77  scanning mirror 
       80  controller 
       82  turning mirror 
       85  linear array 
       90  screen 
       100  data source 
       102  serial to parallel converter 
       104  clock 
       105  data stream 
       106  pulse width generator 
       108  device driver 
       110  voltage selector 
       115  parallel data channels 
       120  electromechanical grating device

Technology Category: 3