Patent Publication Number: US-2012037790-A1

Title: Adjustably transmissive mems-based devices

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 12/255,423, filed Oct. 21, 2008, scheduled to issue on Nov. 8, 2011 as U.S. Pat. No. 8,054,527, which claims priority to U.S. Provisional Application No. 60/982,094, filed Oct. 23, 2007, the disclosure of each of which is hereby incorporated by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to microelectromechanical systems (MEMS). 
     DESCRIPTION OF THE RELATED ART 
     MEMS include micro mechanical elements, actuators, and electronics. Micromechanical elements may be created using deposition, etching, and/or other micromachining processes that etch away parts of substrates and/or deposited material layers or that add layers to form electrical and electromechanical devices. One type of MEMS device is called an interferometric modulator. As used herein, the term interferometric modulator or interferometric light modulator refers to a device that selectively absorbs and/or reflects light using the principles of optical interference. In certain embodiments, an interferometric modulator may comprise a pair of conductive plates, one or both of which may be transparent and/or reflective in whole or part and capable of relative motion upon application of an appropriate electrical signal. In a particular embodiment, one plate may comprise a stationary layer deposited on a substrate and the other plate may comprise a metallic membrane separated from the stationary layer by an air gap. As described herein in more detail, the position of one plate in relation to another can change the optical interference of light incident on the interferometric modulator. Such devices have a wide range of applications, and it would be beneficial in the art to utilize and/or modify the characteristics of these types of devices so that their features can be exploited in improving existing products and creating new products that have not yet been developed. 
     SUMMARY OF THE INVENTION 
     In one aspect, a MEMS device includes a first film stack disposed upon a substrate, and a second film stack spaced apart from the first film stack by a gap, where the second film stack is movable between a first position and a second position, where the MEMS device transmits substantially more visible light in the first position than in the second position, and where the MEMS device transmits substantially the same amount of infrared light when the MEMS device is in the first position as in the second position. 
     In another aspect, a MEMS device includes a first film stack disposed upon a substrate, and a second film stack spaced apart from the first film stack by an air gap, where the second film stack is movable between a first position and a second position, where the MEMS device transmits substantially more visible light and infrared light when in the first position than is transmitted in the second position. 
     In another aspect, a glass pane includes a first glass layer, a second glass layer sealed to the first glass layer to define a cavity therebetween, a MEMS device disposed on a surface of the first glass layer facing the second glass layer, the MEMS device including a layer movable between a first position where the amount of infrared radiation transmitted through the MEMS device is dependent upon the position of the movable layer. 
     In another aspect, a MEMS device includes a first composite layer disposed upon a substrate, the first composite layer including: a first conductive layer, and a first optical layer, a second composite layer separated from the first composite layer via an air gap, where the second composite layer is movable toward the first composite layer, the second composite layer including: a second conductive layer, and a second optical layer, where displacement of the second composite layer towards the first composite layer alters the transmission of infrared light through the MEMS device. 
     In another aspect, a MEMS system includes a transparent substrate, a MEMS device disposed on or adjacent the transparent substrate, the MEMS device including a layer movable between a first position where the device is switchable between a first state which is substantially transmissive to incident light, and a second state in which the reflection of incident light is increased, a sensor configured to sense incident light in a location proximate the substrate, and control circuitry in electrical communication with the sensor, where the control circuitry controls the state of the MEMS device based at least in part upon the state of the sensor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an isometric view depicting a portion of one embodiment of an interferometric modulator display in which a movable reflective layer of a first interferometric modulator is in a relaxed position and a movable reflective layer of a second interferometric modulator is in an actuated position. 
         FIG. 2  is a system block diagram illustrating one embodiment of an electronic device incorporating a 3×3 interferometric modulator display. 
         FIG. 3  is a diagram of movable mirror position versus applied voltage for one exemplary embodiment of an interferometric modulator of  FIG. 1 . 
         FIG. 4  is an illustration of a set of row and column voltages that may be used to drive an interferometric modulator display. 
         FIG. 5A  illustrates one exemplary frame of display data in the 3×3 interferometric modulator display of  FIG. 2 . 
         FIG. 5B  illustrates one exemplary timing diagram for row and column signals that may be used to write the frame of  FIG. 5A . 
         FIG. 6A  is a cross section of the device of  FIG. 1 . 
         FIG. 6B  is a cross section of an alternative embodiment of an interferometric modulator. 
         FIG. 6C  is a cross section of another alternative embodiment of an interferometric modulator. 
         FIG. 6D  is a cross section of yet another alternative embodiment of an interferometric modulator. 
         FIG. 6E  is a cross section of an additional alternative embodiment of an interferometric modulator. 
         FIG. 7A  is a schematic cross-section of a modulator device capable of switching between a highly transmissive state and a highly reflective state. 
         FIG. 7B  is a plot of the index of refraction of an ideal theoretical material used in the modulator device of  FIG. 7A  as a function of wavelength. 
         FIG. 7C  is a plot of the reflection of the modulator device of  FIG. 8A  as a function of wavelength and air gap height. 
         FIG. 7D  is a plot of the transmission of the modulator device of  FIG. 8A  as a function of wavelength and air gap height. 
         FIG. 8A  is a schematic cross-section of another embodiment of a modulator device capable of switching between a highly transmissive state and a highly reflective state. 
         FIG. 8B  is a plot of the reflection of the modulator device of  FIG. 8A  as a function of wavelength and air gap height. 
         FIG. 8C  is a plot of the transmission of the modulator device of  FIG. 8A  as a function of wavelength and air gap height. 
         FIG. 8D  is a plot of the reflection of the modulator device of  FIG. 8A  as a function of wavelength for an air gap height of 15 nm. 
         FIG. 8E  is a plot of the transmission of the modulator device of  FIG. 8A  as a function of wavelength for an air gap height of 15 nm. 
         FIG. 8F  is a plot of the reflection of the modulator device of  FIG. 8A  as a function of wavelength for an air gap height of 170 nm. 
         FIG. 8G  is a plot of the transmission of the modulator device of  FIG. 8A  as a function of wavelength for an air gap height of 170 nm. 
         FIG. 9A  is a schematic cross-section of another embodiment of a modulator device comprising SiC optical layers. 
         FIG. 9B  is a plot of the reflection of the modulator device of  FIG. 9A  as a function of wavelength and air gap height. 
         FIG. 9C  is a plot of the transmission of the modulator device of  FIG. 9A  as a function of wavelength and air gap height. 
         FIG. 9D  is a plot of the reflection of the modulator device of  FIG. 9A  as a function of wavelength for an air gap height of 15 nm. 
         FIG. 9E  is a plot of the transmission of the modulator device of  FIG. 9A  as a function of wavelength for an air gap height of 15 nm. 
         FIG. 9F  is a plot of the reflection of the modulator device of  FIG. 9A  as a function of wavelength for an air gap height of 170 nm. 
         FIG. 9G  is a plot of the transmission of the modulator device of  FIG. 9A  as a function of wavelength for an air gap height of 170 nm. 
         FIG. 10A  is a schematic cross-section of an embodiment of an electrostatically actuatable modulator device comprising two or more conductive layers. 
         FIG. 10B  is a plot of the transmission and reflection of the modulator device of  FIG. 10A  as a function of wavelength for two air gap heights. 
         FIG. 11A  is a schematic cross-section of an embodiment of an electrostatically actuatable modulator device comprising multiple air gaps. 
         FIG. 11B  is a plot of the transmission of the modulator device of  FIG. 10A  as a function of wavelength for two gap heights. 
         FIG. 12A  is a schematic cross-section of an embodiment of an electrostatically actuatable modulator device comprising a pair of dielectric mirrors. 
         FIG. 12B  is a plot of the transmission of the modulator device of  FIG. 12A  in first and second states. 
         FIG. 13  is a schematic cross-section of a double pane window comprising a modulator device such as that of  FIG. 10A . 
         FIG. 14  is a schematic cross-section of the partially fabricated modulator device of  FIG. 13 . 
         FIG. 15  is a schematic cross-section of a double pane window comprising a modulator device and an additional device. 
         FIG. 16  is a plot of the transmission of a modulator device having multiple intermediate dielectric layers in first and second states. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description is directed to certain specific embodiments of the invention. However, the invention can be embodied in a multitude of different ways. In this description, reference is made to the drawings wherein like parts are designated with like numerals throughout. As will be apparent from the following description, the embodiments may be implemented in any device that is configured to display an image, whether in motion (e.g., video) or stationary (e.g., still image), and whether textual or pictorial. More particularly, it is contemplated that the embodiments may be implemented in or associated with a variety of electronic devices such as, but not limited to, mobile telephones, wireless devices, personal data assistants (PDAs), hand-held or portable computers, GPS receivers/navigators, cameras, MP3 players, camcorders, game consoles, wrist watches, clocks, calculators, television monitors, flat panel displays, computer monitors, auto displays (e.g., odometer display, etc.), cockpit controls and/or displays, display of camera views (e.g., display of a rear view camera in a vehicle), electronic photographs, electronic billboards or signs, projectors, architectural structures, packaging, and aesthetic structures (e.g., display of images on a piece of jewelry). MEMS devices of similar structure to those described herein can also be used in non-display applications such as in electronic switching devices. 
     While a fixed thin film stack used to modify the transmission of particular wavelengths of light may be suitable for certain applications, a MEMS-based modulator device which comprises thin films spaced apart from each other by an air (or other gas or fluid) gap permits active management of the transmissive or reflective properties by switching between two or more states. For example, a window which is highly reflective to infrared radiation from the sun may be desirable during hot summer months, but less desirable during winter months or at night. By providing a modulator device in place of a fixed thin film stack, the window can be adjusted to be less reflective to incident infrared light when desired. Similarly, the transmission or reflection of visible light may be modified as well, so as to provide, for example, privacy glass which can be easily switched to a highly transmissive state, which may be used in lieu of or in conjunction with drapes or blinds in a dwelling. In some embodiments, multiple such devices can be utilized to provide the desired transmissive and reflective properties, including the use of fixed thin film stacks in conjunction with movable modulator devices. 
     One interferometric modulator display embodiment comprising an interferometric MEMS display element is illustrated in  FIG. 1 . In these devices, the pixels are in either a bright or dark state. In the bright (“on” or “open”) state, the display element reflects a large portion of incident visible light to a user. When in the dark (“off” or “closed”) state, the display element reflects little incident visible light to the user. Depending on the embodiment, the light reflectance properties of the “on” and “off” states may be reversed. MEMS pixels can be configured to reflect predominantly at selected colors, allowing for a color display in addition to black and white. 
       FIG. 1  is an isometric view depicting two adjacent pixels in a series of pixels of a visual display, wherein each pixel comprises a MEMS interferometric modulator. In some embodiments, an interferometric modulator display comprises a row/column array of these interferometric modulators. Each interferometric modulator includes a pair of reflective layers positioned at a variable and controllable distance from each other to form a resonant optical gap (“air gap” or simply “gap”) with at least one variable dimension. In one embodiment, one of the reflective layers may be moved between two positions. In the first position, referred to herein as the relaxed position, the movable reflective layer is positioned at a relatively large distance from a fixed partially reflective layer. In the second position, referred to herein as the actuated position, the movable reflective layer is positioned more closely adjacent to the partially reflective layer. Incident light that reflects from the two layers interferes constructively or destructively depending on the position of the movable reflective layer, producing either an overall reflective or non-reflective state for each pixel. 
     The depicted portion of the pixel array in  FIG. 1  includes two adjacent interferometric modulators  12   a  and  12   b.  In the interferometric modulator  12   a  on the left, a movable reflective layer  14   a  is illustrated in a relaxed position at a predetermined distance from an optical stack  16   a,  which includes a partially reflective layer. In the interferometric modulator  12   b  on the right, the movable reflective layer  14   b  is illustrated in an actuated position adjacent to the optical stack  16   b.    
     The optical stacks  16   a  and  16   b  (collectively referred to as optical stack  16 ), as referenced herein, typically comprise several fused layers, which can include an electrode layer, such as indium tin oxide (ITO), a partially reflective layer, such as chromium, and a transparent dielectric. The optical stack  16  is thus electrically conductive, partially transparent, and partially reflective, and may be fabricated, for example, by depositing one or more of the above layers onto a transparent substrate  20 . The partially reflective layer can be formed from a variety of materials that are partially reflective such as various metals, semiconductors, and dielectrics. The partially reflective layer can be formed of one or more layers of materials, and each of the layers can be formed of a single material or a combination of materials. 
     In some embodiments, the layers of the optical stack  16  are patterned into parallel strips, and may form row electrodes in a display device as described further below. The movable reflective layers  14   a,    14   b  may be formed as a series of parallel strips of a deposited metal layer or layers (orthogonal to the row electrodes of  16   a,    16   b ) deposited on top of posts  18  and an intervening sacrificial material deposited between the posts  18 . When the sacrificial material is etched away, the movable reflective layers  14   a,    14   b  are separated from the optical stacks  16   a,    16   b  by a defined gap  19 . A highly conductive and reflective material such as aluminum may be used for the reflective layers  14 , and these strips may form column electrodes in a display device. 
     With no applied voltage, the gap  19  remains between the movable reflective layer  14   a  and optical stack  16   a,  with the movable reflective layer  14   a  in a mechanically relaxed state, as illustrated by the pixel  12   a  in  FIG. 1 . However, when a potential difference is applied to a selected row and column, the capacitor formed at the intersection of the row and column electrodes at the corresponding pixel becomes charged, and electrostatic forces pull the electrodes together. If the voltage is high enough, the movable reflective layer  14  is deformed and is forced against the optical stack  16 . A dielectric layer (not illustrated in this Figure) within the optical stack  16  may prevent shorting and control the separation distance between layers  14  and  16 , as illustrated by pixel  12   b  on the right in  FIG. 1 . The behavior is the same regardless of the polarity of the applied potential difference. In this way, row/column actuation that can control the reflective vs. non-reflective pixel states is analogous in many ways to that used in conventional LCD and other display technologies. 
       FIGS. 2 through 5B  illustrate one exemplary process and system for using an array of interferometric modulators in a display application. 
       FIG. 2  is a system block diagram illustrating one embodiment of an electronic device that may incorporate aspects of the invention. In the exemplary embodiment, the electronic device includes a processor  21  which may be any general purpose single- or multi-chip microprocessor such as an ARM, Pentium®, Pentium II®, Pentium III®, Pentium IV®, Pentium® Pro, an 8051, a MIPS®, a Power PC®, an ALPHA®, or any special purpose microprocessor such as a digital signal processor, microcontroller, or a programmable gate array. As is conventional in the art, the processor  21  may be configured to execute one or more software modules. In addition to executing an operating system, the processor may be configured to execute one or more software applications, including a web browser, a telephone application, an email program, or any other software application. 
     In one embodiment, the processor  21  is also configured to communicate with an array driver  22 . In one embodiment, the array driver  22  includes a row driver circuit  24  and a column driver circuit  26  that provide signals to a display array or panel  30 . The cross section of the array illustrated in  FIG. 1  is shown by the lines  1 - 1  in  FIG. 2 . For MEMS interferometric modulators, the row/column actuation protocol may take advantage of a hysteresis property of these devices illustrated in  FIG. 3 . It may require, for example, a 10 volt potential difference to cause a movable layer to deform from the relaxed state to the actuated state. However, when the voltage is reduced from that value, the movable layer maintains its state as the voltage drops back below 10 volts. In the exemplary embodiment of  FIG. 3 , the movable layer does not relax completely until the voltage drops below 2 volts. Thus, there exists a window of applied voltage, about 3 to 7 V in the example illustrated in  FIG. 3 , within which the device is stable in either the relaxed or actuated state. This is referred to herein as the “hysteresis window” or “stability window.” For a display array having the hysteresis characteristics of  FIG. 3 , the row/column actuation protocol can be designed such that during row strobing, pixels in the strobed row that are to be actuated are exposed to a voltage difference of about 10 volts, and pixels that are to be relaxed are exposed to a voltage difference of close to zero volts. After the strobe, the pixels are exposed to a steady state voltage difference of about 5 volts such that they remain in whatever state the row strobe put them in. After being written, each pixel sees a potential difference within the “stability window” of 3-7 volts in this example. This feature makes the pixel design illustrated in  FIG. 1  stable under the same applied voltage conditions in either an actuated or relaxed pre-existing state. Since each pixel of the interferometric modulator, whether in the actuated or relaxed state, is essentially a capacitor formed by the fixed and moving reflective layers, this stable state can be held at a voltage within the hysteresis window with almost no power dissipation. Essentially no current flows into the pixel if the applied potential is fixed. 
     In typical applications, a display frame may be created by asserting the set of column electrodes in accordance with the desired set of actuated pixels in the first row. A row pulse is then applied to the row  1  electrode, actuating the pixels corresponding to the asserted column lines. The asserted set of column electrodes is then changed to correspond to the desired set of actuated pixels in the second row. A pulse is then applied to the row  2  electrode, actuating the appropriate pixels in row  2  in accordance with the asserted column electrodes. The row  1  pixels are unaffected by the row  2  pulse, and remain in the state they were set to during the row  1  pulse. This may be repeated for the entire series of rows in a sequential fashion to produce the frame. Generally, the frames are refreshed and/or updated with new display data by continually repeating this process at some desired number of frames per second. A wide variety of protocols for driving row and column electrodes of pixel arrays to produce display frames are also well known and may be used in conjunction with the present invention. 
       FIGS. 4 ,  5 A, and  5 B illustrate one possible actuation protocol for creating a display frame on the 3×3 array of  FIG. 2 .  FIG. 4  illustrates a possible set of column and row voltage levels that may be used for pixels exhibiting the hysteresis curves of  FIG. 3 . In the  FIG. 4  embodiment, actuating a pixel involves setting the appropriate column to −V bias , and the appropriate row to +ΔV, which may correspond to −5 volts and +5 volts, respectively. Relaxing the pixel is accomplished by setting the appropriate column to +V bias , and the appropriate row to the same +ΔV, producing a zero volt potential difference across the pixel. In those rows where the row voltage is held at zero volts, the pixels are stable in whatever state they were originally in, regardless of whether the column is at +V bias , or −V bias . As is also illustrated in  FIG. 4 , it will be appreciated that voltages of opposite polarity than those described above can be used, e.g., actuating a pixel can involve setting the appropriate column to +V bias , and the appropriate row to −ΔV. In this embodiment, releasing the pixel is accomplished by setting the appropriate column to −V bias , and the appropriate row to the same −ΔV, producing a zero volt potential difference across the pixel. 
       FIG. 5B  is a timing diagram showing a series of row and column signals applied to the 3×3 array of  FIG. 2  which will result in the display arrangement illustrated in  FIG. 5A , where actuated pixels are non-reflective. Prior to writing the frame illustrated in  FIG. 5A , the pixels can be in any state, and in this example, all the rows are at 0 volts, and all the columns are at +5 volts. With these applied voltages, all pixels are stable in their existing actuated or relaxed states. 
     In the  FIG. 5A  frame, pixels ( 1 , 1 ), ( 1 , 2 ), ( 2 , 2 ), ( 3 , 2 ) and ( 3 , 3 ) are actuated. To accomplish this, during a “line time” for row  1 , columns  1  and  2  are set to −5 volts, and column  3  is set to +5 volts. This does not change the state of any pixels, because all the pixels remain in the 3-7 volt stability window. Row  1  is then strobed with a pulse that goes from 0, up to 5 volts, and back to zero. This actuates the ( 1 , 1 ) and ( 1 , 2 ) pixels and relaxes the ( 1 , 3 ) pixel. No other pixels in the array are affected. To set row  2  as desired, column  2  is set to −5 volts, and columns  1  and  3  are set to +5 volts. The same strobe applied to row  2  will then actuate pixel ( 2 , 2 ) and relax pixels ( 2 , 1 ) and ( 2 , 3 ). Again, no other pixels of the array are affected. Row  3  is similarly set by setting columns  2  and  3  to −5 volts, and column  1  to +5 volts. The row  3  strobe sets the row  3  pixels as shown in  FIG. 5A . After writing the frame, the row potentials are zero, and the column potentials can remain at either +5 or −5 volts, and the display is then stable in the arrangement of  FIG. 5A . It will be appreciated that the same procedure can be employed for arrays of dozens or hundreds of rows and columns. It will also be appreciated that the timing, sequence, and levels of voltages used to perform row and column actuation can be varied widely within the general principles outlined above, and the above example is exemplary only, and any actuation voltage method can be used with the systems and methods described herein. 
     The details of the structure of interferometric modulators that operate in accordance with the principles set forth above may vary widely. For example,  FIGS. 6A-6E  illustrate five different embodiments of the movable reflective layer  14  and its supporting structures.  FIG. 6A  is a cross section of the embodiment of  FIG. 1 , where a strip of metal material  14  is deposited on orthogonally extending supports  18 . In  FIG. 6B , the moveable reflective layer  14  is attached to supports at the corners only, on tethers  32 . In  FIG. 6C , the moveable reflective layer  14  is suspended from a deformable layer  34 , which may comprise a flexible metal. The deformable layer  34  connects, directly or indirectly, to the substrate  20  around the perimeter of the deformable layer  34 . These connections are herein referred to as support posts. The embodiment illustrated in  FIG. 6D  has support post plugs  42  upon which the deformable layer  34  rests. The movable reflective layer  14  remains suspended over the gap, as in  FIGS. 7A-7C , but the deformable layer  34  does not form the support posts by filling holes between the deformable layer  34  and the optical stack  16 . Rather, the support posts are formed of a planarization material, which is used to form support post plugs  42 . The embodiment illustrated in  FIG. 6E  is based on the embodiment shown in  FIG. 6D , but may also be adapted to work with any of the embodiments illustrated in  FIGS. 7A-7C , as well as additional embodiments not shown. In the embodiment shown in  FIG. 6E , an extra layer of metal or other conductive material has been used to form a bus structure  44 . This allows signal routing along the back of the interferometric modulators, eliminating a number of electrodes that may otherwise have had to be formed on the substrate  20 . 
     In embodiments such as those shown in  FIG. 6 , the interferometric modulators function as direct-view devices, in which images are viewed from the front side of the transparent substrate  20 , the side opposite to that upon which the modulator is arranged. In these embodiments, the reflective layer  14  optically shields the portions of the interferometric modulator on the side of the reflective layer opposite the substrate  20 , including the deformable layer  34 . This allows the shielded areas to be configured and operated upon without negatively affecting the image quality. Such shielding allows the bus structure  44  in  FIG. 6E , which provides the ability to separate the optical properties of the modulator from the electromechanical properties of the modulator, such as addressing and the movements that result from that addressing. This separable modulator architecture allows the structural design and materials used for the electromechanical aspects and the optical aspects of the modulator to be selected and to function independently of each other. Moreover, the embodiments shown in  FIGS. 6C-6E  have additional benefits deriving from the decoupling of the optical properties of the reflective layer  14  from its mechanical properties, which are carried out by the deformable layer  34 . This allows the structural design and materials used for the reflective layer  14  to be optimized with respect to the optical properties, and the structural design and materials used for the deformable layer  34  to be optimized with respect to desired mechanical properties. 
     Conventional double pane windows can be modified to utilize the interferometric properties of fixed thin film stacks to alter the reflective properties of the windows. This may be used, for example, to reflect incident sunlight in warmer climates so as to maintain a cooler environment inside a dwelling. The U.S. Department of Energy estimates that windows contribute to 30% of the heating and cooling energy spent worldwide, in order to maintain buildings at a desired temperatures. In the United States, this amounts to roughly 30 billion dollars annually, and accounts for roughly 30% of the carbon dioxide emissions generated by electrical power plants. In vehicles, the additional fuel consumed in a car when the air conditioning is on can be as much as 30%. 
     Because roughly 50% of the radiant heat from sunlight transmitted through a window is in the form of visible light, and roughly 50% is in the form of infrared light, a reduction in the amount of infrared light transmitted through a window can result in a significant reduction in the amount of heat transfer into the dwelling. Therefore for example, a thin film stack may be provided in double pane windows to interferometrically reflect undesired infrared light without significantly altering the transmission of visible light. In windows for use in other climates, the reflectance of the windows may be optimized for the particular climate. For example in cold climates, radiant heat can be reflected back into the dwelling via low-emissivity coatings (low-e coatings) so as to retain heat during the winter months, and/or the infrared portion of sunlight can be transmitted into the dwelling to add heat to the dwelling. 
     However, the use of only a fixed thin film stack does not permit adjustment of transmissive and reflective properties to optimally address diverse heating and lighting situations. Although high reflectance of solar infrared radiation may be desirable during the summer in warmer climates, such high reflectance may be less desirable during the winter months or even at night. Furthermore, as such windows typically have a high transmissivity to visible light, no reduction in the amount of heat transmitted via visible light is possible. In certain embodiments, windows comprising a movable layer may be utilized to selectively alter the transmissivity and/or reflectivity of the window when desired. Although a window might not have constant high reflection of visible light, control over the amount of light reflected by a window permits the reflection of visible light when desired while permitting transmission of visible light at other times. 
     In addition, particular wavelength ranges within the infrared and visible light ranges may be of interest. For example, thermal infrared having a wavelength of 10-20 microns may be reflected so as to keep heat inside a house or vehicle during the winter months or at night. Near IR having a wavelength of less than 1500 nm may be reflected as discussed above so as to reduce the amount of heat transmission from incident sunlight. Most midrange IR having wavelengths between 1.5 and 10 microns is typically absorbed by carbon dioxide and water vapor in the atmosphere, but in certain embodiments a modulator device may be configured to reflect such wavelengths, as well. Therefore, in some embodiments, the transmissive and/or reflective characteristics can be selected or tuned in a wavelength-dependent manner. For example in some embodiments, while infrared energy is selectively reflected, visible light may be transmitted with little change. 
     The refractive index of a material may vary as a function of wavelength. Thus, for light incident at an angle upon an interferometric modulator, the effective optical path may vary for different wavelengths of light, depending on the materials used in the optical stack and the movable layer.  FIG. 7A  illustrates a simplified modulator device  100  having two layers  102   a  and  102   b  movable relative to one another and separated by an air gap  104 . Note that in  FIG. 7A  and  FIGS. 8A ,  9 A,  10 A,  11 A, and  12 A, features such as posts  18  (shown in  FIG. 6A ) that separate the layers  102   a  and  102   b  are not shown for the sake of clarity. Further, other structures such as shown in  FIGS. 1-6E  are omitted for the sake of clarity, although such features may be included in the device.  FIG. 7B  illustrates the refractive index versus wavelength λ (in nm) of an ideal theoretical material having a refractive index which varies linearly based on wavelength. Such a material can be used to create a simulated modulator device which is highly transmissive for a first air gap height and highly reflective for a second air gap height, due to the variance in the index of refraction as a function of wavelength seen in  FIG. 7B . 
     For a simulated device in which the layers  100   a  and  100   b  are formed from the theoretical material of  FIG. 7B , and have thicknesses of roughly 43 nm, their predicted reflection as a function of wavelength λ (in nm) and the size of the air gap (in nm)  104  is shown in  FIG. 7C . Similarly, the transmission as a function of wavelength λ (in nm) and air gap  104  size (in nm) can be seen in  FIG. 7D . Such a simulated device using the theoretical material could thus move from being highly transmissive to highly reflective across a broad wavelength range. 
     The predicted plots of transmission and reflection in  FIGS. 7C and 7D , as well as the ones shown elsewhere in the application, are based upon optical models of the described system, taking into account the specific materials and thicknesses, as well as the optical properties of those materials, such as the index of refraction. 
     In another simulated device,  FIG. 8A  illustrates a simplified modulator device  110  which comprises layers  112   a  and  112   b  of the theoretical material of  FIG. 7B , supported on two comparatively thick glass substrates  116   a  and  116   b,  and spaced apart from one another by the air gap  114 . If a layer such as the glass substrate  116   a  or  116   b  is thick enough relative to the wavelength of the light in question, it no longer functions as a thin film layer and will have little effect on the optical properties of the simulated modulator device  110 . For example, if the layer is thicker than the coherent length of the incident light, e.g., greater than 10 microns, the layer will no longer act as a thin film and will have little optical effect beyond the reflectivity of the layer. If the layer is comparatively thin, the optical properties of the simulated modulator device will be affected by the layer.  FIG. 8B  illustrates the transmission as a function of wavelength and gap size, and  FIG. 8C  illustrates the reflectance as a function of wavelength and gap size. It can be seen that the inclusion of the glass layers does not have a significant effect on the optical properties of the simulated modulator device  110  when compared with those of the simulated modulator device  100  of  FIG. 7A . 
       FIGS. 8D-8G  illustrate the modeled transmission and reflection of the simulated modulator device  110  as a function of wavelength λ for particular gap sizes.  FIGS. 8D and 8E  illustrate reflection and transmission as a function of wavelength λ, respectively, when the gap size is equal to 15 nm.  FIGS. 8F and 8G  illustrate reflection and transmission as a function of wavelength λ, respectively, when the gap size is equal to 120 nm. It can be seen that the simulated modulator device  110  can be moved from a state which is almost completely transmissive to one which is roughly 80% reflective for most wavelengths of visible light. 
     In another embodiment,  FIG. 9A  illustrates an embodiment of a simplified realizable modulator device  120  similar to that of modulator device  110 . The modulator device includes a layer  122   a  supported by glass substrate  126   a  and spaced apart by an air gap  124  from another layer  122   b  supported by glass substrate  126   b.  The layers  122   a  and  122   b  comprise layers of SiC which are roughly 25 nm in thickness.  FIG. 9B  illustrates the reflection of the modulator device  120  as a function of wavelength λ and air gap thickness, and  FIG. 9C  illustrates the transmission of the modulator device  120  as a function of wavelength λ and air gap thickness. 
       FIG. 9D  illustrates the modeled reflection as a function of wavelength λ when the air gap is at 20 nm, and  FIG. 9E  illustrates the modeled transmission as a function of wavelength λ in the same state.  FIG. 9F  illustrates the modeled reflection as a function of wavelength λ when the air gap is at 180 mm, and  FIG. 9G  illustrates the modeled transmission as a function of wavelength λ in the same state. It can be seen that when the air gap is at 20 nm, the modulator device  120  is substantially transmissive across all visible wavelengths, and thus will transmit light without a significant hue. In practice, due to surface roughness of the layers which come into contact with one another, an air gap on the order of roughly 20 nm may in certain embodiments be the smallest achievable air gap when the layers are brought into contact with one another. Similarly, when the air gap is at 180 nm, roughly 50% of the incident visible light will be reflected, and although there is some wavelength-dependency in the degree of reflection, there will not be a significant hue to either the transmitted or reflected light. 
     In some embodiments, a modulator device may include actuation elements integrated into the thin-film stack which permit displacement of portions of layers relative to one another so as to alter the spacing therebetween.  FIG. 10A  illustrates an exemplary modulator device  130  which is electrostatically actuatable. The device  130  includes a conductive layer  138   a  supported by a substrate  136   a,  and an optical layer  132   a  overlying the conductive layer  138   a.  Another conductive layer  138   b  is supported by substrate  136   b  and an optical layer  132   b  overlies the conductive layer  138   b.  The optical layer  132   a  and  132   b  are separated from one another by an air gap. As discussed above, application of a voltage across conductive layers  138   a  and  138   b  will cause the one of the layers to deform towards the other one. 
     In some embodiments, the conductive layers  138   a  and  138   b  may comprise a transparent or light-transmissive material, such as indium tin oxide (ITO), for example, although other suitable materials may be used. The optical layers  132   a  and  132   b  may comprise a material having a high index of refraction. In some particular embodiments, the optical layers  132   a  and  132   b  may comprise titanium dioxide, although other materials may be used as well, such as lead oxide, zinc oxide, and zirconium dioxide, for example. The substrates may comprise glass, for example, and at least one of the substrates may be sufficiently thin to permit deformation of one of the layers towards the other. 
     In an embodiment in which the conductive layers  138   a  and  138   b  comprise ITO and are 80 nm in thickness, the optical layers  132   a  and  132   b  comprise titanium dioxide and are 40 nm in thickness, and the air gap is initially 170 nm in height,  FIG. 10B  illustrates plots across the visible and a portion of infrared wavelengths of the modeled transmission and reflectivity as a function of wavelength λ of the modulator device  130  both when the device is in an actuated state with an air gap of 15 nm and in an unactuated state with an air gap of 170 nm. As previously discussed, the 15 nm air gap represents a fully actuated state, but surface roughness may in some embodiments prevent a further reduction in air gap size. In particular, line  142  illustrates the transmission as a function of wavelength when the device is in an unactuated position (T(170)), and line  144  illustrates the reflectivity in the same state (R(170)). Similarly, line  146  illustrates the transmission as a function of wavelength when the device is in an actuated position (T(15)), and line  148  illustrates the reflectivity in the actuated position (R(15)). 
     It can be seen from these plots that the modulator device  130  is highly transmissive across visible wavelengths when in an actuated state with a small air gap (15 nm), particularly for those wavelengths of less than about 800 nm. When in an unactuated state with a larger air gap (170 nm), the device becomes roughly 70% reflective to those same wavelengths. In contrast, the reflectivity and transmission of the higher wavelengths, such as infrared wavelengths, does not significantly change with actuation of the device. Thus, the modulator device  130  can be used to selectively alter the transmission/reflection of a wide range of visible wavelengths, without significantly altering the infrared transmission/reflection. As can be seen in  FIG. 10B , the transmissivity of infrared light does not significantly vary when the MEMS device is actuated, as the difference in transmissivity between the unactuated and actuated states remains within roughly 10% of the incident infrared light. By changing the thicknesses or types of materials comprising the layers within the modulator device, it would be possible maintain this small difference in transmittance while increasing or decreasing the overall transmittance of infrared light. 
     In another embodiment, a modulator device may be provided having multiple air gaps.  FIG. 11A  illustrates such a modulator device  150 . The modulator device  150  includes a lower composite layer  160   a,  comprising a base substrate  156   a  on which a first conductive layer  158   a  is formed. An intermediate composite layer  160   b  comprising a support layer  156   b,  a first optical layer  152   a,  and a second conductive layer  158   b  is spaced apart from the lower composite layer  160   a  via a first air gap  154   a.  Above the intermediate composite layer  160   b,  an upper composite layer  160   c  includes a support layer  156   c,  a second optical layer  152   b,  and a third conductive layer  158   c.  The upper composite layer  160   c  is spaced apart from the intermediate composite layer  160   b  via second air gap  154   b.    
     In a particular embodiment, the substrate  156   a  may comprise glass, and the support layers  156   b  and  156   c  may comprise layers of silicon dioxide roughly 10 nm in thickness. The conductive layers  158   a,    158   b,  and  158   c  may comprise layers of ITO roughly 10 nm in thickness. The optical layers  152   a  and  152   b  may comprise layers of titanium dioxide roughly 30 nm in thickness.  FIG. 11B  illustrates the modeled transmissivity of such a modulator device  150  as a function of wavelength λ in two states. Line  164  shows the transmissivity in the collapsed state, where the air gaps  162   a  and  162   b  have a height of roughly 0 nm, and the composite layers are substantially in contact with one another. Line  166  shows the transmissivity in the uncollapsed state, with an air gap of 170 nm. 
     It can be seen that, similar to the particular embodiment of modulator device  140  of  FIG. 10A  discussed above, the particular embodiment of the modulator device  150  of  FIG. 11A  can be switched between a state having high transmissivity to a wide range of visible light and a state having high reflectivity to roughly the same range of visible light. The modulator device  150  will have a somewhat greenish-gold tint, given the particular transmissivity of certain wavelength ranges. This can be done without significantly altering the infrared transmissivity. 
     In another embodiment, a modulator device may be provided wherein dielectric mirrors are movable relative to one another.  FIG. 12A  illustrates an example of such a modulator device  170 . The modulator device  170  includes a lower composite layer  180   a  comprising a substrate  176   a,  a conductive layer  178   a,  and a dielectric optical layer  172   a.  The dielectric layer  172   a  may comprise dielectric sublayers  182   a  and  184   b.  Modulator device  170  also includes an upper composite layer  180   b  separated from lower composite layer  180   a  via an air gap  174 . Upper composite layer  180   b  may comprise a substrate  176   b,  a conductive layer  178   b,  and a dielectric optical layer  172   b,  which itself may comprise dielectric sublayers  184   b  and  186   b.    
     In a particular embodiment, the substrates  176   a  and  176   b  may comprise silicon dioxide, and the conductive layers  178   a  and  178   b  may comprise ITO. The dielectric optical layers  172   a  and  172   b  may comprise sublayers  184   a  and  184   a  of silicon oxide and sublayers  186   a  and  186   a  of titanium dioxide. In other embodiments, alternate materials may be used, and additional sublayers may be included in the dielectric layer, as well. In particular, although only a single sublayer of each type is shown in each dielectric layer for the sake of simplicity, multiple sublayers of each type may be arranged in an alternating manner. In certain embodiments, 20 such sublayers of each type may be included in a dielectric optical layer, but more or less sublayers of each type may be used. In addition, although the illustrated embodiments depicts the sublayers in specific positions relative to one another, the positions of the dielectric sublayers relative to one another may be reversed in one or more of the dielectric mirrors without significantly affecting the overall optical properties of the modulator device. 
       FIG. 12B  illustrates the modeled transmission characteristics of a particular embodiment of modulator device  170  discussed above as a function of wavelength λ wherein the titanium dioxide sublayers have a thickness of 55 nm, the silicon oxide sublayers have a thickness of 20 nm, and the dielectric optical layers  172   a  and  172   b  comprise twenty titanium dioxide sublayers and twenty silicon oxide sublayers arranged in an alternating fashion. In addition, the ITO layers have a thickness of 10 nm, and the air gap is 180 nm in an unactuated state, and 10 nm in an actuated state. In particular,  FIG. 12B  shows the modeled transmission  192  in an actuated state (T(10)) and the modeled transmission  194  in an unactuated state (T(180)). It can be seen that although there is a significant amount of variance over short wavelength ranges characteristic of dielectric mirrors, the overall transmission in the first range is high (roughly 80%) for a wide range of visible and infrared wavelength ranges above roughly 500 nm. In addition, despite the sharpness of the variance within small wavelength ranges, the overall effect will be of a modulator device having only a slight orange-reddish hue. When the modulator device is in the second state, the transmission is reduced to roughly 50%. Not shown in the figure is that for visible wavelengths below about 550 nm, the modeled modulator device gives nearly 100% reflection or transmission. 
     The modulator device  170  thus provides a device which is capable of changing from roughly 80% transmission in visible and infrared to roughly 50% transmissive in visible and infrared. In addition, there is essentially no angle dependence, as the response is substantially constant over wide ranges of wavelengths. The alteration in the length of the optical path which occurs when the angle of view is changed will thus have little effect on the appearance of the device. As will be discussed in greater detail below, if greater reduction in the transmission of a particular range of wavelengths is desired, the modulator device  170  may be provided in conjunction with an additional modulator device so as to obtain the desired level of transmission. In some embodiments, the additional modulator device may be a fixed film having desired properties, and in other embodiments the additional modulator device may be a modulator device such as those discussed above, having a layer which is movable through an air gap. 
     It can also be seen that the lack of significant absorption within the modulator device  170  yields a transmission rate, for example, which is roughly equal to 1 minus the reflection. The lack of absorption may prove useful in certain applications, such as when the modulator device is incorporated into a window. In other embodiments, absorptive material may be used, for example, to provide a modulator device having a desired hue, whether for aesthetic or other reasons. 
     In certain embodiments, modulator devices such as those discussed above may be fabricated using MEMS manufacturing techniques, and may be incorporated into structures such as double pane windows for use in housing, commercial buildings, and the like.  FIG. 13  schematically illustrates an example of a double pane window  200  comprising a modulator device fabricated in such a manner. In particular, it can be seen that the double pane window  200  comprises a modulator device  208  which may in certain embodiments be substantially similar to the modulator device  130  of  FIG. 10A . The modulator device  208  comprises support structures or spacers  202  throughout the modulator device so as to maintain a desired air gap height when the modulator device  208  is in an unactuated state. In one embodiment, the modulator device  208  may be formed via sequential deposition of layers using techniques such as, for example, chemical vapor deposition or physical vapor deposition. The air gap may be formed via deposition and subsequent removal of a sacrificial layer of a desired thickness, or may be formed by lamination techniques with included support structures to form the air gap. The support structures  202  may comprise a dielectric material which has been deposited and patterned. In order to facilitate the fabrication of the modulator device, one of the support layers may comprise a deposited layer of a suitable material, such as silicon oxide, rather than a preformed substrate. 
       FIG. 14  illustrates a partially fabricated modulator device  210  formed by such a process. In the illustrated embodiment, the substrate  204   a  is sufficiently thick that it has a minimal impact on the optical properties of the modulator. A layer  136   b  of a suitable material, such as silicon oxide, is deposited on the substrate  204   a,  followed by a layer  138   b  of conductive material, such as ITO, and an optical layer  132   b,  such as titanium dioxide or another material with a high index of refraction. In some embodiments, materials having an index of refraction greater than about 1.8 may be used, although in other embodiments materials having higher indices of refraction may be used, such as materials with an index of refraction of 1.9 or higher. Above the optical layer  132   b  is a sacrificial layer  212  through which the support structures  202  extend. In some embodiments, the support structures  202  may be formed prior to the deposition of the sacrificial layer  212 , which may be deposited over the support structures  202  and may comprise a planarization material or may be etched back to the desired height. In other embodiments, the sacrificial layer  212  may be formed prior to the support structures, and patterned to form apertures in which the support structures  202  are later formed. The height of the air gap will be affected by the height of the support structures  202  and the sacrificial layer  212 . 
     Subsequently, an optical layer  132   a,  a conductive layer  138   a,  and a support layer  136   a  are formed over the sacrificial layer  212  and the supports  202 . It can be seen that, in the illustrated embodiment, the bottom layers  136   b,    138   b,  and  132   b,  with the exception of the thick substrate  204   a,  are the mirror image of the upper layers  136   a,    138   a,  and  132   a . Electrical connections between the upper conductive layer  128   a  and any necessary driver or actuation circuitry (not shown) can be formed at the periphery of the modulator device, or at the periphery of the pixels, if there are multiple pixels. The sacrificial layer  212  may be subsequently removed, such as by an etching process, so as to release the modulator device  210 . In certain embodiments, small etch holes may be formed in the upper layers  136   a,    138   a , and  132   a  so as to permit access by the etchant to the sacrificial layer  212  throughout the device, rather than only at the exposed periphery. 
     Alternate fabrication techniques may be utilized. In another embodiment, the various layers may be formed by laminating preformed stacks onto the substrate. The spacers may comprise, for example, glass spheres of the desired diameter which are spaced throughout the modulator device. In another embodiment, a combination of thin film deposition and lamination may be utilized to form the modulator device. 
     Once the modulator device  210  of  FIG. 14  has been formed on one of the two glass panes  204   a  and  204   b  of  FIG. 13 , the two panes may be sealed to one another via seal  206  to form the double pane window  200  seen in  FIG. 13 . The air gap between the modulator device  130  and the opposite pane  204   b  may be substantially larger than the height of the modulator device  130 , and the opposite pane will thus serve as a protective backplate, and protect the modulator device  130  from mechanical or other interference. 
     As noted above, multiple transmission-altering or reflection-altering structures may be provided in conjunction with one another, so as to achieve a desired result. In one embodiment, a modulator device may be formed on each of the interior surfaces of the double pane window  200 . In another embodiment, a fixed thin film stack may be provided, either between the modulator device and the supporting substrate, or on an alternate surface of the double pane window. It will also be understood that any suitable modulator device may be formed in place of the modulator device. 
       FIG. 15  illustrates such an embodiment of a window  220 , in which a first modulator device  230  is formed on a first glass pane  204   a,  and a second device  240  is formed on a second glass pane. In one embodiment, the first modulator device  230  comprises a modulator device discussed herein. In a particular embodiment, the first modulator device  230  comprises the modulator device  170  of  FIG. 12A , and is capable of switching between a state which is substantially transmissive to a wide range of visible and IR radiation and another state in which the reflectance across a wide range of visible and IR radiation is increased. 
     The second device  240  may in certain embodiments comprise a device which transmits a certain amount of incident light. In certain embodiments, the device  240  may comprise a device which absorbs a certain amount of incident light. In particular embodiments, the device  240  may be switchable between a first state which is substantially transmissive to incident light, and a second state in which the absorption of at least certain wavelengths is increased. In still other embodiment, the device  240  may comprise a fixed thin film stack having desired transmissive, reflective, or absorptive properties. 
     In certain embodiments, suspended particle devices (SPDs) may be used to change between a transmissive state and an absorptive state. These devices comprise suspended particles which in the absence of an applied electrical field are randomly positioned, so as to absorb and/or diffuse light and appear “hazy.” Upon application of an electrical field, these suspended particles are aligned in a configuration which permits light to pass through. Other devices  240  may have similar functionality. In another embodiment, an interferometric modulator device having similar functionality may be used. 
     Thus, when the device  240  comprises an SPD or a device having similar functionality, the window  220  can be switched between three distinct states: a transmissive state, when both devices  230  and  240  are in a transmissive state, a reflective state, when device  230  is in a reflective state, and an absorptive state, when device  240  is in an absorptive state. Depending on the orientation of the window  220  relative to the incident light, the device  230  may be in a transmissive state when the window  220  is in an absorptive state, and similarly, the device  240  may be in a transmissive state when the window  220  is in an absorptive state. 
     In another embodiment, devices  240  and  230  may comprise multiple movable layers spaced apart from each other by air gaps. In a particular embodiment, the devices may comprise a plurality of dielectric layers spaced apart from one another by a movable air gap. The device may comprise a first conductive layer located on the substrate, and an uppermost movable layer comprising a second conductive layer overlying a dielectric layer. Between the first conductive layer and the second conductive layers are a plurality of movable dielectric layers spaced apart from one another by a uniform distance when the device is in an unactuated state. Upon application of a voltage between the first and second conductive layers, the movable layers may collapse so as to significantly reduce the spaces between the movable layers. 
     In a specific embodiment of a window  220 , the optical response of which is shown as a function of wavelength λ (in nm) in  FIG. 16 , the devices  230  and  240  may comprise first and second conductive layers formed from ITO having a thickness of about 20 nm. The upper movable layer comprises the second conductive layer and an underlying titanium dioxide layer having a thickness of about 52 nm. Nine intermediate titanium dioxide layers having thicknesses of about 52 nm are spaced apart from adjacent layers by air gaps. In an unactuated state, the air gaps of device  230  are about 500 nm, and the air gaps of device  240  are about 330 nm. When voltages are applied so as to pull the uppermost movable layer towards the substrate, the air gaps of both device  230  and  240  collapse to about 20 nm due to surface roughness of the adjacent surfaces. 
     As can be seen in  FIG. 16 , when the devices  230  and  240  are in an actuated state, roughly 50% of incident visible and infrared light is transmitted through the device. The transmittal of light when both devices are in an actuated state is shown by  252 . When the device is released, the device functions as a nearly perfect broad band mirror, reflecting almost all incident visible and infrared light. The transmittal of light when both devices are in an unactuated state is shown by  254 . Although this embodiment has been discussed with respect to a window  220  in which the devices  230  and  240  are on separate panes, a sufficiently thick substrate may be used to support devices  230  and  240  on opposite sides of a single substrate. 
     In an embodiment in which a modulator device is electrostatically actuatable, the size of the air gap may be altered by applying a voltage across the conductive layers, as discussed above, such that one or both of the layers moves through and collapses the air gap. The voltage required to actuate the modulator device depends on a variety of factors, including but not limited to the total area of the modulator device, the height of the air gap, and the effective spring constant of the movable layer. The effective spring constant of the movable layer is dependent upon, among other factors, the residual tensile stress within the layers, the thicknesses and composition of the layers, and the spacing of any support structures throughout the modulator device. The modulator device may be configured such that the voltage necessary to actuate the device is less than the dielectric breakdown strength of the materials located between conductive layers. This number may vary significantly depending on the particular materials used and the thicknesses of the layers, but in certain embodiments, the actuation voltage is less than 100 V. 
     Because the modulator device may in certain embodiments have a uniform reaction across its surface, the modulator device may not need the resolution necessary to function as a display device. In one embodiment, a double pane window may comprise effectively only a single pixel, wherein the conductive layer in the movable layer comprises a contiguous layer, although it may be patterned into a desired shape. In although in other embodiments multiple pixel-like regions may be used, and the movable conductive layer may comprise multiple discrete portions permitting the actuation of one portion without the actuation of all portions. The actuation voltage is independent of the area of the modulator, and thus for given parameters such as residual stress, support spacing, and air gap heights, the actuation voltage will be substantially constant for windows or “pixels” of any size. 
     Because the pixels can be very large, even the entire size of an architectural window or other physically large product, the support structures need not be as tightly packed as in a display device, in which the support structures may help to define the pixels and may be spaced apart from each other on the order of 200 um or less. In one embodiment of the modulator device  130  for example, the spacers may be on the order of several millimeters apart, reducing the effective spring constant and thus the voltage required to actuate the device. 
     In certain embodiments, a modulator device may be used to provide a grayscale-like effect with a level of reflectivity or transmissivity between those provided by the two discrete actuated and unactuated states. Such a grayscale-like effect may be provided in at least two ways. In one embodiment, spatial dithering may be used so that only a portion of the modulator device is actuated at a given time, so as to provide such a grayscale effect. In one embodiment, the modulator device may comprise a plurality of independently controllable areas. In certain embodiments, these areas may have a size small enough that actuation of a plurality of the areas, such as in a uniform or semiregular tiling, e.g., checkerboard, triangular, hexagonal, or other geometric pattern, produces a visual effect that appears that the overall reflectivity and/or transmissivity of the window is being modified without producing a noticeable pattern. In some embodiments, the areas may be actuated to yield a user-noticeable pattern, for example to create “see through” signage, to provide “virtual mullions” (dividers) for a large window, and the like. 
     In another embodiment, temporal dithering may be used, by repeatedly actuating and unactuating the device, such that the modulator device is transmissive and/or reflective for only a particular percentage of the time, yielding a similar grayscale effect. This may also be done in conjunction with the use of small areas, and in certain embodiments may be done in conjunction with spatial dithering, such as by actuating a checkerboard-like pattern of areas and then switching the unactuated areas to actuated and vice versa. 
     Selective actuation of certain areas may be used for other reasons, as well. For example, in sufficiently large windows, it may be desirable to actuate only a portion of the window, such as the portion currently being exposed to incident light, while not actuating another portion, such as a portion shaded by an awning or other structure. 
     In certain embodiments, the actuation of the modulator devices may be user-controlled, such as via a switch. In other embodiments, actuation may be automated, and based upon a time. For example, the modulator may be in an actuated position during the summer months, or during the day. In still other embodiments, sensors connected to the control circuitry may be used to determine when the modulator device is actuated. For example, the amount of incident light and/or the wavelengths of the incident light may determine whether infrared reflectivity is necessary, for example based upon an infrared-sensitive sensor, or whether privacy glass should become reflective or opaque. The sensitivity of a sensor can be controlled, for example, through the use of an overlying filter. In some embodiments, one or more sensors may be placed on both sides of the window, for example to compare visible and/or infrared energy outside a building to that found within the building. Appropriate control logic may be included in a circuit to control the characteristics of the modulator device(s) within the windows, such as wavelength dependence or sensitivity. In certain embodiments, sensors may be used to determine the current lighting conditions of a portion of a window and may independently control the state of those portions accordingly. 
     As previously noted, the embodiments described herein may be included in a wide variety of structures or devices. For example, the devices may be included in windows of any size, including architectural windows such as those in residences or office buildings, as well as vehicle windows and any other windows for which modification of transmittance may be desirable, such as eyeglasses. Because of the thin film nature of certain embodiments described herein, such embodiments may also be included in windows or other structures or devices which have any amount of curvature. 
     While the above detailed description has shown, described and pointed out novel features of the invention as applied to various embodiments, it will be understood that various omissions, substitutions, and changes in the form and details of the device or process illustrated may be made by those skilled in the art without departing from the spirit of the invention. As will be recognized, the present invention may be embodied within a form that does not provide all of the features and benefits set forth herein, as some features may be used or practiced separately from others.