Method and system for generating enhanced gray levels in an electromechanical grating display

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.

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 '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'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.

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 .

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;

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 <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.

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