Patent Publication Number: US-2012026576-A1

Title: Devices and methods for enhancing brightness of displays using angle conversion layers

Description:
PRIORITY 
     This application is a continuation of U.S. patent application Ser. No. 12/369,630, filed Feb. 11, 2009, U.S. Pat. No. 8,040,589 (issue date: Oct. 18, 2011), entitled “DEVICES AND METHODS FOR ENHANCING BRIGHTNESS OF DISPLAYS USING ANGLE CONVERSION LAYERS,” which claims priority under 35 U.S.C. §119(e) to U.S. Provisional Application No. 61/028,145, filed on Feb. 12, 2008, entitled “DEVICES AND METHODS FOR ENHANCING BRIGHTNESS OF DISPLAYS USING ANGLE CONVERSION LAYERS,” both of which are assigned to the assignee hereof. The disclosures of the prior applications are considered part of this disclosure and are incorporated by reference in their entireties. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     Embodiments of the present invention relate to enhancing brightness of reflective displays. In some embodiments, devices include a light-turning features and diffractive microstructure. 
     2. Description of Related Technology 
     Microelectromechanical systems (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 
     In some embodiments, an illumination apparatus is provided, the apparatus comprising a light guide that guides light propagating therein at an angle greater than a critical angle for the light guide and ejects light from the light guide to provide illumination; diffractive microstructure disposed to receive ambient light at a first angle smaller than said critical angle and to diffract said ambient light to produce diffracted light at a second larger angle; and light-turning features configured to turn the diffracted light and direct the turned light out of the light guide. The second angle may be greater than the critical angle of the light guide. 
     In some embodiments, a method of manufacturing an illumination apparatus is provided, the method including providing a light guide that guides light propagating therein at an angle greater than a critical angle for the light guide and ejects light therefrom to provide illumination; disposing diffractive microstructure to receive ambient light at a first angle smaller than said critical angle and to diffract said ambient light to produce diffracted light at a second larger angle; and providing light-turning features configured to turn the diffracted light and direct the turned light out of the light guide. 
     In some embodiments, an illumination apparatus is provided, the illumination apparatus comprising means for guiding light propagating therein at an angle greater than a critical angle for the light guiding means and ejecting light from the light guiding means to provide illumination; means for diffracting ambient light received at a first angle smaller than said critical angle to produce diffracted light at a second larger angle; and means for turning the diffracted light and directing the turned light out of said light guiding means. 
     In some embodiments, an illumination apparatus is provided, the illumination apparatus comprising a light guide that guides light propagating therein at an angle greater than a critical angle for the light guide and ejects light from the light guide to provide illumination; and an angle converting structure disposed to receive ambient light at a first angle greater than said critical angle and to diffract said ambient light to produce diffracted light at a second smaller angle, wherein a refractive index of said angle converting structure is less than a refractive index of said light guide. 
    
    
     
       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 minor 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. 
         FIGS. 5A and 5B  illustrate one exemplary timing diagram for row and column signals that may be used to write a frame of display data to the 3×3 interferometric modulator display of  FIG. 2 . 
         FIGS. 6A and 6B  are system block diagrams illustrating an embodiment of a visual display device comprising a plurality of interferometric modulators. 
         FIG. 7A  is a cross section of the device of  FIG. 1 . 
         FIG. 7B  is a cross section of an alternative embodiment of an interferometric modulator. 
         FIG. 7C  is a cross section of another alternative embodiment of an interferometric modulator. 
         FIG. 7D  is a cross section of yet another alternative embodiment of an interferometric modulator. 
         FIG. 7E  is a cross section of an additional alternative embodiment of an interferometric modulator. 
         FIG. 8A  schematically illustrates light incident on a display device within the field-of-view of the display device such that light is reflected therefrom to a viewer within the field-of-view of the display device. 
         FIG. 8B  schematically illustrates a display device comprising an array of display elements and having a field-of-view that is tilted with respect to the array of display elements. 
         FIG. 8C  schematically illustrates light incident on a display device at an angle outside the field-of-view of the display device such that the light is reflected outside the field-of-view of the display device. 
         FIG. 8D  schematically illustrates a display device having an angular conversion layer disposed forward an array of display elements that redirects light incident on the display device at an angle outside the field-of-view into an angle more normal to the array of display elements and within the field-of-view of the display device. 
         FIG. 8E  schematically illustrates a display device having an angular conversion layer forward a plurality of display elements that redirects light incident on the display device at an angle outside the field-of-view into a larger (more grazing incidence) angle such that the light is guided in a light guide forward the array of display elements. 
         FIG. 9  schematically illustrates an illumination apparatus comprising a light guide forward an array of display elements, diffractive microstructure that couples light incident on the display device at an angle outside the field-of-view into so as to be guided in the light guide, and light turning features that redirect the light guided by the light guide onto the array of display elements at near normal incidence. 
         FIG. 10  schematically illustrates an illumination apparatus further comprising an artificial light source such as an light emitting diode or a light bar for providing supplemental illumination. 
         FIG. 11  schematically illustrates the field-of-view of the display device and the angular range for optical modes guided within the light guide. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The following detailed description is directed to certain specific embodiments. However, the teachings herein can be applied in a multitude of different ways. In this description, reference is made to the drawings wherein like parts are designated with like numerals throughout. 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. 
     The perceived brightness of reflective displays can depend on available lighting. In various embodiments of the present invention, an illumination apparatus for front illuminating reflective display elements is configured to increase the amount of ambient light that is incident on the display elements and reflected therefrom within a usable field-of-view to the viewer. This illumination apparatus may comprise a light guide, light-diffractive microstructure, and turning features. The diffractive microstructure diffracts light incident on the illumination apparatus at an angle outside the field-of-view away from the normal to the array of display elements such that ambient light outside the field-of-view may be coupled into the light guide. The light turning features turn this light guided within the light guide to the display elements at an angle near normal to the array of display elements. Therefore, the amount of ambient light that can be directed at angles near normal to the array of display elements and reflected by the display elements at angles near normal to the array (or otherwise within the desired field-of-view) can be increased. In various embodiments, the display elements comprise reflective display elements and in some embodiments, the display elements comprise reflective interferometric modulators. 
     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 (“relaxed” or “open”) state, the display element reflects a large portion of incident visible light to a user. When in the dark (“actuated” 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 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 ) to form columns 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. Note that  FIG. 1  may not be to scale. In some embodiments, the spacing between posts  18  may be on the order of 10-100 um, while the gap  19  may be on the order of &lt;1000 Angstroms. 
     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 (voltage) 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 actuated pixel  12   b  on the right in  FIG. 1 . The behavior is the same regardless of the polarity of the applied potential difference. 
       FIGS. 2 through 5  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 interferometric modulators. The electronic device includes a processor  21  which may be any general purpose single- or multi-chip microprocessor such as an ARM®, Pentium®, 8051, MIPS®, Power PC®, or 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 . Note that although  FIG. 2  illustrates a 3×3 array of interferometric modulators for the sake of clarity, the display array  30  may contain a very large number of interferometric modulators, and may have a different number of interferometric modulators in rows than in columns (e.g., 300 pixels per row by 190 pixels per column). 
       FIG. 3  is a diagram of movable minor position versus applied voltage for one exemplary embodiment of an interferometric modulator of  FIG. 1 . For MEMS interferometric modulators, the row/column actuation protocol may take advantage of a hysteresis property of these devices as illustrated in  FIG. 3 . An interferometric modulator 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. There is thus a range of voltage, about 3 to 7 V in the example illustrated in  FIG. 3 , where there exists a window of applied voltage 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 or bias 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. 
     As described further below, in typical applications, a frame of an image may be created by sending a set of data signals (each having a certain voltage level) across 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 a first row electrode, actuating the pixels corresponding to the set of data signals. The set of data signals is then changed to correspond to the desired set of actuated pixels in a second row. A pulse is then applied to the second row electrode, actuating the appropriate pixels in the second row in accordance with the data signals. The first row of pixels are unaffected by the second row pulse, and remain in the state they were set to during the first row 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 image 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 image frames may be used. 
       FIGS. 4 and 5  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 , 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 initially 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 . The same procedure can be employed for arrays of dozens or hundreds of rows and columns. 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. 
       FIGS. 6A and 6B  are system block diagrams illustrating an embodiment of a display device  40 . The display device  40  can be, for example, a cellular or mobile telephone. However, the same components of display device  40  or slight variations thereof are also illustrative of various types of display devices such as televisions and portable media players. 
     The display device  40  includes a housing  41 , a display  30 , an antenna  43 , a speaker  45 , an input device  48 , and a microphone  46 . The housing  41  is generally formed from any of a variety of manufacturing processes, including injection molding, and vacuum forming. In addition, the housing  41  may be made from any of a variety of materials, including but not limited to plastic, metal, glass, rubber, and ceramic, or a combination thereof. In one embodiment the housing  41  includes removable portions (not shown) that may be interchanged with other removable portions of different color, or containing different logos, pictures, or symbols. 
     The display  30  of exemplary display device  40  may be any of a variety of displays, including a bi-stable display, as described herein. In other embodiments, the display  30  includes a flat-panel display, such as plasma, EL, OLED, STN LCD, or TFT LCD as described above, or a non-flat-panel display, such as a CRT or other tube device. However, for purposes of describing the present embodiment, the display  30  includes an interferometric modulator display, as described herein. 
     The components of one embodiment of exemplary display device  40  are schematically illustrated in  FIG. 6B . The illustrated exemplary display device  40  includes a housing  41  and can include additional components at least partially enclosed therein. For example, in one embodiment, the exemplary display device  40  includes a network interface  27  that includes an antenna  43  which is coupled to a transceiver  47 . The transceiver  47  is connected to a processor  21 , which is connected to conditioning hardware  52 . The conditioning hardware  52  may be configured to condition a signal (e.g. filter a signal). The conditioning hardware  52  is connected to a speaker  45  and a microphone  46 . The processor  21  is also connected to an input device  48  and a driver controller  29 . The driver controller  29  is coupled to a frame buffer  28 , and to an array driver  22 , which in turn is coupled to a display array  30 . A power supply  50  provides power to all components as required by the particular exemplary display device  40  design. 
     The network interface  27  includes the antenna  43  and the transceiver  47  so that the exemplary display device  40  can communicate with one or more devices over a network. In one embodiment the network interface  27  may also have some processing capabilities to relieve requirements of the processor  21 . The antenna  43  is any antenna for transmitting and receiving signals. In one embodiment, the antenna transmits and receives RF signals according to the IEEE 802.11 standard, including IEEE 802.11(a), (b), or (g). In another embodiment, the antenna transmits and receives RF signals according to the BLUETOOTH standard. In the case of a cellular telephone, the antenna is designed to receive CDMA, GSM, AMPS, W-CDMA, or other known signals that are used to communicate within a wireless cell phone network. The transceiver  47  pre-processes the signals received from the antenna  43  so that they may be received by and further manipulated by the processor  21 . The transceiver  47  also processes signals received from the processor  21  so that they may be transmitted from the exemplary display device  40  via the antenna  43 . 
     In an alternative embodiment, the transceiver  47  can be replaced by a receiver. In yet another alternative embodiment, network interface  27  can be replaced by an image source, which can store or generate image data to be sent to the processor  21 . For example, the image source can be a digital video disc (DVD) or a hard-disc drive that contains image data, or a software module that generates image data. 
     Processor  21  generally controls the overall operation of the exemplary display device  40 . The processor  21  receives data, such as compressed image data from the network interface  27  or an image source, and processes the data into raw image data or into a format that is readily processed into raw image data. The processor  21  then sends the processed data to the driver controller  29  or to frame buffer  28  for storage. Raw data typically refers to the information that identifies the image characteristics at each location within an image. For example, such image characteristics can include color, saturation, and gray-scale level. 
     In one embodiment, the processor  21  includes a microcontroller, CPU, or logic unit to control operation of the exemplary display device  40 . Conditioning hardware  52  generally includes amplifiers and filters for transmitting signals to the speaker  45 , and for receiving signals from the microphone  46 . Conditioning hardware  52  may be discrete components within the exemplary display device  40 , or may be incorporated within the processor  21  or other components. 
     The driver controller  29  takes the raw image data generated by the processor  21  either directly from the processor  21  or from the frame buffer  28  and reformats the raw image data appropriately for high speed transmission to the array driver  22 . Specifically, the driver controller  29  reformats the raw image data into a data flow having a raster-like format, such that it has a time order suitable for scanning across the display array  30 . Then the driver controller  29  sends the formatted information to the array driver  22 . Although a driver controller  29 , such as a LCD controller, is often associated with the system processor  21  as a stand-alone Integrated Circuit (IC), such controllers may be implemented in many ways. They may be embedded in the processor  21  as hardware, embedded in the processor  21  as software, or fully integrated in hardware with the array driver  22 . 
     Typically, the array driver  22  receives the formatted information from the driver controller  29  and reformats the video data into a parallel set of waveforms that are applied many times per second to the hundreds and sometimes thousands of leads coming from the display&#39;s x-y matrix of pixels. 
     In one embodiment, the driver controller  29 , array driver  22 , and display array  30  are appropriate for any of the types of displays described herein. For example, in one embodiment, driver controller  29  is a conventional display controller or a bi-stable display controller (e.g., an interferometric modulator controller). In another embodiment, array driver  22  is a conventional driver or a bi-stable display driver (e.g., an interferometric modulator display). In one embodiment, a driver controller  29  is integrated with the array driver  22 . Such an embodiment is common in highly integrated systems such as cellular phones, watches, and other small area displays. In yet another embodiment, display array  30  is a typical display array or a bi-stable display array (e.g., a display including an array of interferometric modulators). 
     The input device  48  allows a user to control the operation of the exemplary display device  40 . In one embodiment, input device  48  includes a keypad, such as a QWERTY keyboard or a telephone keypad, a button, a switch, a touch-sensitive screen, a pressure- or heat-sensitive membrane. In one embodiment, the microphone  46  is an input device for the exemplary display device  40 . When the microphone  46  is used to input data to the device, voice commands may be provided by a user for controlling operations of the exemplary display device  40 . 
     Power supply  50  can include a variety of energy storage devices as are well known in the art. For example, in one embodiment, power supply  50  is a rechargeable battery, such as a nickel-cadmium battery or a lithium ion battery. In another embodiment, power supply  50  is a renewable energy source, a capacitor, or a solar cell, including a plastic solar cell, and solar-cell paint. In another embodiment, power supply  50  is configured to receive power from a wall outlet. 
     In some implementations control programmability resides, as described above, in a driver controller which can be located in several places in the electronic display system. In some cases control programmability resides in the array driver  22 . The above-described optimization may be implemented in any number of hardware and/or software components and in various configurations. 
     The details of the structure of interferometric modulators that operate in accordance with the principles set forth above may vary widely. For example,  FIGS. 7A-7E  illustrate five different embodiments of the movable reflective layer  14  and its supporting structures.  FIG. 7A  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. 7B , the moveable reflective layer  14  of each interferometric modulator is square or rectangular in shape and attached to supports at the corners only, on tethers  32 . In  FIG. 7C , the moveable reflective layer  14  is square or rectangular in shape and 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. 7D  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. 7E  is based on the embodiment shown in  FIG. 7D , 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. 7E , 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. 7 , 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. For example, such shielding allows the bus structure  44  in  FIG. 7E , 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. 7C-7E  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. 
     Various embodiments of the present invention relate to increasing the amount of light available to display elements of a display device. In certain embodiments, a display device comprises a plurality of reflective display elements having a preferred field-of-view from which a viewer will view image content displayed by the display elements. Improved brightness may be achieved in certain embodiments by increasing the amount of ambient light output by the display in within the field-of-view of the device. 
     In various embodiments described herein, display devices comprise a plurality of reflective display elements such as reflective spatial light modulators. Reflective interferometric modulators are examples of such reflective spatial light modulators. In certain embodiments, only light incident on the display device within the field-of-view of the display device is reflected within the field-of-view of the device. Accordingly, in such embodiments ambient illumination of the display device is generally limited to ambient light incident on the display device within the field-of-view of the device. 
       FIG. 8A  schematically illustrates the situation where light incident on a display device  800  having a field-of-view  830 ′ is within the field-of-view of the display device and is reflected from the display device to a viewer  803  at an angle also within the field-of-view of the device.  FIG. 8A  shows a plurality of display elements  801  having a light guide  802  or other optically transmissive medium disposed forward (on the viewing side) of the display elements. Light is incident on the light guide  802  or optically transmissive medium at an angle within the field-of-view  830 ′ of the display device. 
     Although the optically transmissive medium  802  is shown as a single layer, in other embodiments, the optically transmissive medium may comprises a plurality of layers. For example, one or more films or layers may form part of the light guide  802 . Other embodiments may include additional layers in addition to the light guide  802 . Alternatively, some embodiments may exclude the light guide  802 . In such embodiments, the optically transmissive medium  802  disposed forward the display elements  801  may comprise, for example, one or more other optically transmissive layers such a substrate on which the display elements are formed, a protective glass or plastic plate or sheet, or one or more other optically transmissive films, layers, sheets, plates, etc. In other embodiments, a substrate on which the display elements are formed, a protective glass or plastic plate or sheet, etc. may also form part of the light guide  802 . 
     In general, the optically transmissive medium  802  has a first surface  805  that defines an interface, which may be an interface between, for example, air (above or on the viewing side of the first surface  805 ) and the optically transmissive medium  801  (below or on a spatial modulator side of the first surface  805 ). Alternatively, the interface  805  may be between another medium above first surface  805  and the optically transmissive medium  801  below first surface  805 . In some embodiments, the medium above the first surface  805  is not part of the display device  800 , wherein in other embodiments, it is. 
     An incident light ray  810  can be characterized by a first incident angle  815  measured with respect to the normal  820  to the surface  805  and to the array of display elements  801 . The incident light ray  810  is refracted at the surface  805  to produce a refracted light ray  810   a  characterized by a first transmission angle  815   a . The refracted light ray  810   a  is reflected at a second surface  825  corresponding to the plurality of display elements  801  to produce a reflected light ray  810   b . The reflected light ray  810   b  encounters the first surface  805  of the reflecting device at a second incident angle  815   b . The reflected light ray  810   b  is again refracted and becomes an output light ray  810   c , characterized by a second transmission angle  815   c  with respect to the normal  820 . 
     A first angular range  830  corresponding to the field-of-view  830 ′ of the device  800  is shown in  FIG. 8A . A second angular range  830   a  corresponding to the effective field-of-view  830 ′ within the optically transmissive medium  802  is also shown. The second angular range  830   a  is smaller than the first angular range  830  due to refraction within the optically transmissive medium. A third angular range  830   b  symmetrical or, in some embodiments, identical to the second angular range  830   a  is also shown displaced to where the ray  810   b  is incident on surface  805  and exits from the optically transmissive medium  802 . A fourth angular range  830   c  symmetrical or, in some embodiments, identical to the first angular range  830  is also shown at the location where the ray  810   b  is incident on surface  805  and exits from the optically transmissive medium  802 . This fourth angular range  830   c  corresponds to the field-of-view  830 ′ of the device  800  and shows whether a given ray of light reflected from the display device is within the field-of-view  830 ′ of the device. Similarly, these other angular ranges  830 ,  830   a ,  830   b , correspond to the field-of-view  830 ′ of the device  800  and are replicated at different locations (inside and outside of the optically transmissive medium  802 ) as a reference to show whether a given ray of light incident on, refracted by, or reflected from portions of the display device  800  is within the field-of-view  830 ′ of the display device. In the embodiment shown in  FIG. 8A , these angular ranges  830 ,  830   a ,  830   b ,  830   c  show whether the first incident angles  815 , the first transmitted angles  815   a , the second incident angles  815   b  and the second transmitted angles  815   c  will be viewable upon exiting the device  800 . Thus, if a light ray, such as  810  which is within the first angular range  830 , it can be expected that the transmitted light ray  810   a , the reflected light ray  810   b , and the output light ray  810   c  will be oriented at angles within the first angular range  830   a , the second angular range  830   b  and the third angular range  830   c , respectively. 
     In some instances, the second angular range  830   a  and the third angular range  830   b  include substantially the same range of angles. In some instances, the first angular range  830  and the fourth angular range  830   c  include substantially the same range of angles. In other instances, the second angular range  830   a  and the third angular  830   b  and/or the first angular range  830  and the fourth angular range  830   c  do not include substantially the same range of angles. For example, surface irregularities, tilted fields-of-view, and/or a plurality of display device components may contribute to such differences in the angular regions. 
     The field-of-view  830 ′ and corresponding angular ranges  830 ,  830   a ,  830   b ,  830   c  may vary depending on, for example, the design of the device  800 , materials used in the device, how a design is used, or external device properties. In some embodiments, one or both of the first angular range  830  and the fourth angular range  830   c  include a range of about 0° from the normal to about 60° or about 0° from the normal to about 180° from the normal. In some embodiments, one or both of the first angular range  830  and the fourth angular range  830   c  include a range of about 0° from the normal to about 60° or about 10° to about 60° from the normal (e.g., from about 0° or 10° from the normal to about 30°, to about 45°, or to about 60° depending, for example, on the usage model of the displays). The angular ranges can depend, for example, on factors, such as display size and viewing distance. In some embodiments, one or both of the second angular range  830   a  and the third angular range  830   b  include a range of about 0° from the normal to about 40° from the normal. In some embodiments, one or both of the second angular range  830   a  and the third angular region  830   b  include a range of about 0° from the normal to about 20° from the normal. In certain embodiments, the range of the second angular range  830   a  and/or the third angular range  830   b  may be less than the range of the first angular range  830  and the fourth angular region  830   c , for example, as a result of refraction. In other embodiments, the range of the second angular range  830   a  and/or the third angular range  830   b  may be greater than the range of the first angular range  830  and the fourth angular region  830   c  depending on the index of refraction above and below the interface  805 . The fourth angular range  830   c  may be approximately 1 to approximately 3 times as large as the second angular range  830   a . For example, the fourth angular range  830   c  and the second angular range  830   a  may be about 80° and about 41°, respectively; about 60° and about 35°, respectively; about 40° and about 20°, respectively; about 20° and about 13°, respectively; or about 10° and about 7°, respectively, in some embodiments. 
       FIG. 8B  shows an embodiment wherein the field-of-view  83 W is tilted and not centered or symmetrical about the normal  820 . Similarly, angular ranges  830 ,  830   a ,  830   b  and  830   c  are not centered or symmetrical about the normal  820 . Non-symmetric field-of-views  83 W may be applicable, for example, to display devices  800  for viewing at a tilted angle. It will be understood that embodiments herein are not limited to symmetric viewing cones centered about the normal  820 . The second angular range  830   a  may be mirror images of the third angular range  830   b . (For example, if third angular range  830   b  includes angles between −35° and 45°, second angular range  830   a  could include angles between −45° and 35°.) Similarly, the first angular range  830  may include angles that are substantially mirror images of the fourth angular range  830   d . In other embodiments, however, these angular ranges  830 ,  830   a ,  830   b ,  830   c  need not be mirror images. 
       FIG. 8C  schematically illustrates the situation where light incident on a display device  800  outside the field-of-view  830 ′ of the display device and is reflected from the display device at an angle also outside the field-of-view of the device. Light ray  810 , for example, is shown incident on the light guide  802  or optically transmissive medium at an angle outside the field-of-view  830 ′ of the display device. 
       FIG. 8C  also shows four corresponding angular regions  835 ,  835   a ,  835   b  and  835   c  outside the field-of-view  830 ′. A first angular region  835 , a second angular region  835   a , a third angular region  835   b , and a fourth angular region  835   c  indicate ranges of the first incident angles  815 , the first transmitted angles  815   a , the second incident angles  815   b  and the second transmitted angles  815   c , for which light will not be within the field-of-view  830 ′ upon exiting the device. Thus, if a light ray such as  810  is within the first angular region  835 , it can be expected that the transmitted light ray  810   a , the reflected light ray  810   b , and the output light ray  810   c  will be characterized by angles within the second angular region  835   a , the third angular region  835   b  and the fourth angular region  835   c , respectively and not within the field-of-view  830 ′. 
     Further,  FIG. 8C  shows first and second forbidden angular regions  840   a ,  840   b . Light from above the interface  805  will not be refracted into these forbidden angular regions  840   a ,  840   b  if the index of refraction above the interface is less than the index of refraction below the interface. For example, even if incident light ray  810  encountered the surface  805  at the largest angle possible, refraction would prevent the light from entering the first device angular region  840   a  and therefore from being reflected into the second device angular region  840   b . Typically, the angles within the angular regions  835 ,  835   a ,  835   b  and  835   c  outside the field-of-view will be larger than angles within the angular regions  830 ,  830   a ,  830   b  and  830   c  corresponding to the field-of-view  830 ′ of the device  800 , and the angles within the forbidden angular regions  840   a  and  840   b  will be larger than angles within the angular regions  835   a  and  835   b  outside the field-of-view  830 ′ of the device. 
     In order to, for example, enhance the brightness of the display device  800 , it can be advantageous to redirect light incident on the display device outside the field-of-view (e.g., in first angular region  835 ) into the field-of-view  830 ′ (e.g., into second angular region  830   a , third angular region  830   b , and fourth angular region  830   c ). Therefore, more incident (e.g., ambient) light can directed to the viewer  803  upon reflection from of the plurality of display elements  801 .  FIG. 8D  shows a strategy to increase the amount of ambient light collected using an angle converting device  845 , such as a diffractive layer. The angle converting device  845  re-directs light outside the field-of-view  830 ′ by changing the direction of the transmitted light rays towards the surface normal  820  (e.g., by reflective or transmissive diffraction). In some embodiments, an index of refraction of the angle conversion layer  845  comprises a holographic or diffractive layer. Thus, at least some of the incident light (e.g., light ray  810 ) that would have been within the first angular region  835   a  outside the effective field-of-view is re-directed, such that the transmitted light (e.g., transmitted light ray  810   b ) is within the first angular  830   a  inside the effective field-of-view. This light is reflected from the array of display elements  801  into the third angular region  830   b  and output into the fourth angular region  830   c  within the field-of-view  83 W of the display device  800 . This light is therefore directed to a viewer  803 . In essence, the first angular range  830  is redefined as a larger angular region  831 . More light can be collected and directed into the field-of-view  830 ′,  831  and be used to convey image content to the viewer  803 . The display device is thus brighter. 
     As illustrated in  FIG. 8E , various embodiments of the present invention include an angle converting device  850  that re-directs light by changing the direction of the transmitted light rays away from the surface normal  820 . The angle converting device  850  may therefore increase the angle of the transmitted ray  810   a  as measured with respect to the normal  820 . For example, the angle converting device  850  receives light ray  810  and transmits light ray  810   a  to be within the first forbidden angular region  840   a . Thus, in some instances, the light is re-directed to an angle greater than a critical angle, such as for example the critical angle associated with the boundaries of the light guide  802 . This light is therefore coupled into the light guide  802  so as to be guided therein by total internal reflection. The light is optically guided in the light guide  802  via total internal reflection in a customary manner for waveguides. In certain embodiments, turning features are included to eject light from the light guide  802  at near normal angles. This light then reflects from the array of display elements  801  through the light guide  802  at near normal angles and out of the display device  800  within the field-of-view  83 W to a viewer  803 . 
       FIG. 9  shows an embodiment of a display device  900  comprising an illumination apparatus  900 ′ and a plurality of display elements  901  such as intereferometric modulators. The illumination apparatus  900 ′ is forward of the plurality of display elements  901  and assists in front illumination thereof. The illumination apparatus  900 ′ of the display device  900  may include a light guide or light guide region  902  that guides light propagating therein (e.g., light ray  920 ) at an angle greater than a critical angle for the light guide. The light  920  is ejected from the light guide  902 , for example, to provide illumination of the array of display elements  901  rearward of the light guide. The light guide  902  may comprise one or more layers and/or components. These layers may comprise glass or polymeric material or other substantially optically transparent material. In some embodiments the light guide  902  comprises one or more of glass, polycarbonate, polyether or polyester such as, e.g., PET, acrylic or acrylate and acrylate polymers and copolymers including but not limited to polymethymethacrylate (PMMA), styrene-acrylic copolymer, and poly(styrene-methylmethacrylate) (PS-PMMA), sold under the name of Zylar, and other optically transmissive plastics although other materials may also be used. In some embodiments the light guide region  902  has a thickness in the range of between about 100 μm and about 1 cm, e.g. between 0.1 mm and 0.4 mm, although the thickness may be larger or smaller. In some embodiments, the light guide region  902  has a thickness of less than about 400 μm, such as, for examples, embodiments in which the light guide does not include a substrate. In some embodiments, the substrate is part of the light guide region  902  and thus the thickness of the light guide region  902  may be larger, such as about 100 μm to about 1 cm. 
     The light guide region  902  may include a substrate  915  in certain embodiments. This substrate  915  may comprise substantially optically transmissive material such as for example glass or plastic or other materials. As described above, the material may comprise aluminum silicate or borosilicate glasses although other materials may also be used. For example polycarbonate, polyether and polyesters such as, e.g., PET or PEN, acrylics or acylates and acrylate polymers and copolymers including but not limited to PMMA, poly(styrene-methylmethacrylate) (PS-PMMA) sold under the name of Zylar, and other optically transmissive plastics may be used. The materials that may be employed, however, are not limited to those specifically recited herein. The substrate  915  may have a thickness between about 0.1 mm and about 1 cm, (e.g. between 0.1 mm and 0.4 mm), although the thickness may be larger or smaller. In some embodiments, the substrate  915  may have a thickness sufficient to support other layers or films thereon. 
     The illumination apparatus  900 ′ may also include light-turning features  903 . A light-turning layer  905  may comprise a plurality of light-turning features  903 . The light-turning features  903  may include, for example, prismatic and/or diffractive features. The light-turning features  903  may be shaped and/or oriented to turn light such that light guided within the light guide  902  is directed out of the light guide. Additionally, light-turning features  903  may be shaped and/or oriented such that the angle as measured with respect to the normal  920  to the light guide  902  and/or array of display elements  901  of the turned light is reduced and is therefore more normal, for example, as compared to light prior to interacting with the turning features. In some embodiments, the light-turning features  903  may be shaped and/or oriented to increase the amount of light within the field-of-view of the display device  900  and/or to increase the percentage of incident and/or ambient light that is output into the field-of-view of the display device. Alternatively, the light-turning features  903  may be shaped and/or oriented to reduce the angular size of the field-of-view of the display device  900 . For example, the light-turning features  903  may assist in concentrating light output or reflected from display device  900  into a smaller angular region. 
     In  FIG. 9 , the light-turning features are shown as arranged on a layer. This layer forms an upper portion, and in particular, an upper boundary of the light guide  902 . The light-turning features  903  need not be disposed at an upper portion of the light-guide  902  but may be located elsewhere, for example, in the middle or low portions of the light guide closer to the display elements  901 . In some embodiments, the light-turning features  903  need not be included in a single layer. 
     In some embodiments the light-turning features  903  are reflective. Light guided within the light guide region  902  may be turned upon reflecting from such light-tuning features  903 . 
     In one example, the light-turning features  903  comprise prismatic features. Such prismatic features may reflect light off of multiple facets via total internal reflection.  FIG. 9  shows an example of such facets that form prismatic features. These prismatic features may be disposed in a film. This film may be substantially optically transmissive. In some embodiments, this film comprises a polymeric material such as, e.g., PC, PET, or PMMA, although other materials may also be used. In some embodiments, the film comprises a UV-curable resins molded on a plastic carrier film, such as, e.g., PC, PET or PMMA. Accordingly, the film may comprise polymeric material such as an optically transmissive material including but not limited to polycarbonate, acrylics or acrylates and acrylate polymers and copolymers including but not limited poly(styrene-methylmethacrylate) (PS-PMMA), sold under the name of Zylar, and other optically transmissive plastics. The materials that may be employed, however, are not limited to those specifically recited herein. This film may be between about 50 μm and about 500 μm (e.g. 100 μm and about 500 μm) thick or may have a thickness outside this range. In some embodiments the light turning features are between about 1 μm and about 50 μm deep and in some embodiments may be between about 0.5 and 50 μm wide although the light turning features may have other sizes in other embodiments. These features  903  have been exaggerated in size in  FIG. 9  for illustrative purposes. Likewise the size, shape, arrangement, and other characteristics may be different. Moreover, the light-turning features  903  may comprise different structures in other embodiments. 
     The illumination apparatus  900  may also include diffractive microstructure, which may be included in a diffractive layer  910 . This diffractive layer  910  may comprise one or more diffractive or holographic layers that provide the angle conversion as described above with respect to  FIG. 8  (e.g.  FIG. 8E ). The diffractive microstructures may comprise surface and/or volume features that form, for example, one or more surface and/or volume diffractive optical elements or holograms. Such a diffractive layer  910  may be transmissive in certain embodiments and may operate on light transmitted therethrough. The diffractive layer  910  may operate on light incident thereon from forward of the display device  900  and may be customized to operate on light incident from a particular angle or set of angles such as ambient light incident on the illumination apparatus  900 ′ at large angles with respect to the normal. As described above, this light may be incident on the illumination apparatus  900 ′ and diffractive layer  910  at angles outside the field-of-view of the device  900 . 
     The diffractive layer  910  may comprise, for example, holographic recording films or coatings, such as mixtures of acrylates and vinyl copolymers, or other photopolymers. The diffractive layer may include a holographic material such as, for example, a silver halide material, a dichromated gelatin material, a photoresist material, and/or a photorefractive crystal. Other materials may include those described in, for example, J. E. Boyd et al., Applied Optics. vol 39, iss. 14, p. 2353-2358 (10 May 2000), references cited therein, and/or www.hololight.net/materials.html. In various embodiments wherein the diffractive features  910  are surface features, the diffractive layer  910  may further comprise a planarized layer and/or a coating positioned over or under the diffractive microstructure. The planaraization layer may comprise a wet-coated polymeric coating or a spin-on glass in certain embodiments although the material need not be limited to such material. The diffractive layer  910  may be of any suitable thickness, such as, for example, between about 10 and about 100 microns although values outside this range are possible as well. 
     The diffractive microstructure and/or the diffractive layer  910  may be located below or rearward of the light-turning features  902  and/or light-turning layer  905  with respect to incident light on the display device  900 . Thus, ambient light may be transmitted through the light-turning features  902  prior to being received by the diffractive microstructure. The diffractive microstructure and/or the diffractive layer  910  may be configured to receive light at a first angle smaller than a critical angle for the light guide  902  and to diffract the light to produce diffracted light at a second larger angle. The first and second angles may be measured with respect to the normal. The second larger angle may be greater than the critical angle of the light guide  902  such that the light is coupled into the light guide so as to be propagated therein by total internal reflection. In some embodiments, the refractive index of the light-turning layer  905  is similar to or the same as the index of refraction of the diffractive layer  910 . Reflection of light passing through the interface between the light-turning layer  905  and the diffractive layer  910  can thereby be reduced. In other embodiments the refractive index of the diffractive layer  910  is lower than or higher (which, in some embodiments, is advantageous over “lower”) than that of the light-turning layer  905 . The light-turning features  902  may be configured such that light traveling from the diffractive layer  910  to the light-turning features  902  is turned to be directed out of the light guide  902  and/or to reduce the angle with respect to the normal to the illumination apparatus  900 ′ or display device  900 . 
     As described above, in some embodiments, the illumination apparatus  900  includes a substrate  915 . This substrate  915  may provide support for the diffractive layer  910  and/or the light-turning layer, for example during fabrication or use. The diffractive layer  910  and/or the light-turning layer  905  may be formed over, for example, deposited on or applied (e.g., laminated) to the substrate  915  or one or more layers formed on the substrate. In some embodiments, the diffractive layer  910  may be formed over, for example, deposited on or applied (e.g., laminated) to the substrate  915  or one or more layers formed thereon and the light-turning layer  905  may be formed over, for example, deposited on or applied (e.g., laminated) to the diffractive layer  910  or one or more layers formed thereon. Accordingly, in some embodiments the substrate  915  may be located beneath the diffractive microstructure and/or the diffractive layer  910  with respect to incident light. In other embodiments, the diffractive microstructure and/or the diffractive layer  910  is formed below or rearward of the substrate  915 . In other embodiments, the illumination apparatus  900  does not include a substrate  915 . 
     In some embodiments the substrate  915  forms part of the light guide  902 . In the embodiment shown in  FIG. 9 , the critical angle for the lower or rearward boundary of the light guide  902  is determined by the interface of the substrate  915  and an optical medium rearward of the substrate  915 . In the embodiment shown in  FIG. 9 , an air gap  916  is disposed rearward of the substrate and illumination apparatus  900 ′ and forward of one or more or an array of display elements  901 . The interface between the substrate  915  and the air gap in this embodiment determines the critical angle for reflection from the lower or rearward boundary of the light guide  902 . 
     In other embodiments, this gap  916  may be filled with material Likewise, in certain embodiments, one or more layers may be attached to the substrate  915  rearward of the substrate and form port of the light guide  902 . These layers may or may not be part of the light guide region  902  depending, for example, on the index of refraction of these layers. 
     In the embodiment shown in  FIG. 9 , the critical angle for the upper or forward boundary of the light guide  902  is determined by the interface of the light-turning layer  905  and an optical medium forward of the light-turning  905  or illumination apparatus  90 W. In the embodiment shown in  FIG. 9 , an air layer is disposed rearward of the substrate and illumination apparatus  900 ′ and forward of an array of display elements  901 . The interface between the light-turning film  905  and the air in this embodiment determines the critical angle for reflection from the upper or forward boundary of the light guide  902 . 
     In other embodiments, the light-turning layer  905  is not the uppermost or forwardmost layer. In such embodiments, one or more layers forward the light-turning layer  905  may determine the critical angle for the upper or forward boundary of the light guide  902  depending on index of refraction. Likewise, in certain embodiments, one or more layers may be attached to the light turning layer forward of the light-turning layer  905  and form part of the light guide  902  or define a boundary of the light guide  902 . A planarization layer may be disposed on the light-turning layer  905 . The layer or layers forward the light-turning layer  905  may or may not be part of the light guide region  902  depending, for example, on the respective indices of refraction. 
     More generally, the critical angle for the upper or forward boundary of the light guide  902  may be determined by the interface of the forward most layer of the light guide  900  and the optical medium directly forward of the forwardmost layer. The critical angle for the lower or rearward boundary of the light guide  902  may be determined by the interface of the rearwardmost layer of the light guide  900  and the optical medium directly rearward of the rearwardmost layer. 
     In some embodiments, an isolation layer is disposed between the light guide region  902  and the plurality of display elements  901 . This isolation layer, for example, may comprise a material having an index of refraction lower than the light guide  902 . In the absence of the air gap  916  or isolation layer, the light guide  902  may be disposed directly on the array of display elements  901 . In such a configuration, light guided within the light guide  902  may be incident on the array of display elements  901  may be absorbed. 
       FIG. 9  shows an example trajectory of a ray of light  920  through the illumination apparatus  900 . The light ray  920  enters the illumination apparatus  900  at the top surface of the light-turning layer  905 . Due to a difference in refractive indices, the light beam  920  is refracted as shown by transmitted light ray  920   a . In this example, the light ray  920   a  is transmitted through the light-turning layer  905  into the diffractive layer  910 . The diffractive layer  910  diffracts and re-directs of the light ray  920   a , producing a diffracted light ray  920   b  directed at an angle  930  from the normal to the display apparatus  900 ′ and one or more or an array of display elements  901 . This angle  930  is larger than the angle  925  of an undiffracted ray that would result in the absence of the diffractive layer  925 . 
     The diffracted light beam  920   b  is totally internally reflected at the interface between the substrate  915  and the air gap  916  to produce the reflected light beam  920   c . The reflected light beam  920   c  travels through the diffractive layer  910  into the light-turning layer  905 . The light-turning features  902  then turn the light, such that the turned light beam  920   d  has a reduced angle with respect to the normal as compared to the angle with respect to the normal of the reflected light beam  920   c . The turned light beam  920   d  is then transmitted through the diffractive layer  910  and the substrate  915  to exit the illumination apparatus  900  and is incident on the array of display elements  901 . Although not shown, the turned light beam  920   d  may be reflected from the array of display element  901  depending, for example, on the state of the reflective light modulators. Accordingly, the turned light beam  920   d  may be directed out of the display device toward a viewer in a direction near normal to the array of display element  901  and within the field-of-view of the display device  900 . Thus, the diffractive layer  910  redirects light from a first set of angles into a second set of angles and thereby enables ambient light directed into a light guide region to be redirected into an angle that is guided by the light guide region and otherwise forbidden from being directly accessed by ambient light. 
       FIG. 10  shows a display device  1000  comprising an illumination apparatus  1000 ′ in which the diffractive layer  910  is separated from the light-turning layer  905 . One or more separation layers  1007  may separate the diffractive layer  910  and the light-turning layer  905 . The one or more separation layers  1007  is substantially optically transmissive and may be diffusive in some embodiments. The one or more separation layers  1007  may have a refractive index lower than that of the light-turning layer  905  such that the light-turning layer  905  can guide light therein. The one or more separation layers  1007  may have a refractive index greater than that of the diffraction layer  910 . 
     The one or more separation layers  1007  may material selected from the group of acrylics, polyesters, polyethers, or cycloolefin polymers. In some embodiments, for example, the separation layers  1007  may comprise an optically transmissive material such as, e.g., polycarbonate, acrylics or acrylates and acrylate polymers and copolymers including but not limited polymethymethacrylate (PMMA), poly(styrene-methylmethacrylate) (PS-PMMA), sold under the name of Zylar, and other optically transmissive plastics. In some embodiments, the one or more separation layers  1007  may comprise a pressure sensitive adhesive. The one or more separation layers  1007  may be of any suitable thickness, such as, for example, between about 1 to about 100 microns (e.g., between about 1 and 30 microns) although values outside this range are also possible. 
     The embodiment shown in  FIG. 10  also includes a light source  1002  that provides light to the illumination apparatus  1000 . The light source  1002  may comprise an edge light source, located adjacent to the illumination apparatus  1000  so as to inject light into an edge thereof. The light source  1002  may comprise for example one or more light emitters such as LED and may comprise, for example, a linear array of LEDs. In certain embodiments, the light source  1002  may also comprise a light bar and one or more emitters disposed to inject light into the light bar. 
     The separation layer  1007  forms a light guiding region  1004  for the light emitted from the light source  1002 . This light guiding region  1004  may comprise, for example, the light-turning layer. Light  1035  from the light source  1002  may enter the light-turning layer  905  as represented by a first light ray  1035   a  and may be guided by totally internally reflection within the light-turning layer  905 , until a light-turning feature  902  turns the first light ray  1035   a . An example turned light beam  1035   b  is shown directed to the array of display elements  901 . 
     The separation layer  1007  forms a boundary for the light guiding region  1004  for the light emitted from the light source  1002 . In the embodiment shown in  FIG. 10 , the separation layer  1007  optically decouples the light turning layer  905  from the diffractive layer  910 . The separation layer  1007  may reduce or prevent interactions of the light emitted  1035   a  from the light source  1002  with the diffractive layer  910 . 
     In some embodiments, the separation layer  1007  is excluded and the refractive index of the light-turning layer  905  is higher than that of the diffractive layer  910 . In such embodiments the light-turning layer  905  may guide light therein via in part by total internal reflection from the interface between the light-turning layer  905  and the diffractive layer  910 . 
     The embodiment shown in  FIG. 10  also includes an optical isolation layer  1008  disposed between the substrate  915  and the array of display elements  901 . This optical isolation layer  1008  may have an index of refraction lower than that of the layer forward of the optical isolation layer, which in this case is the substrate  915 . Although the optical isolation layer  1008  is shown as a single layer, in other various embodiments the optical isolation layer comprises a multilayer stack. This optical isolation layer  1008  may comprise for example, acrylic or acrylate and acrylate polymers and copolymers including but not limited to polymethymethacrylate (PMMA) and poly(styrene-methylmethacrylate) (PS-PMMA), sold under the name of Zylar, fluorine containing polymers, and polycarbonate, other optically transmissive plastics or silicon oxide, although other materials may be used. In some embodiments, the optical isolation layer  1008  may comprise pressure sensitive adhesive. The isolation layer  1008  may be of any suitable thickness, such as, for example, between about 1 and about 100 microns or between about 1 and about 30 microns, although the isolation layer may be thicker or thinner. In another embodiment, the isolation layer  1008  may be in close vicinity of the display element  901 , and comprise inorganic material with different index than the substrate  915 . 
     In the absence of the isolation layer  1008 , light diffracted by the diffractive layer  910  such as ray  920   b  may be incident on the array of display elements  901  instead of or in addition to being reflected as ray  920   c  toward the light-turning layer  905  where the light such as ray  920   d  is turned at near normal angles toward the display elements. The light (ray  920   b ) prematurely incident on the plurality of display elements  901  may be absorbed by the display elements or reflected at angles outside the field-of-view of the display device  1000 . In certain embodiments, separation layer  1007  forms the lower boundary for the light from LED, while the isolation layer  1008  forms the lower boundary for the “converted” beam by the diffractive layer  910  from the ambient light  920 . In certain embodiments they may be combined. Accordingly, in various embodiments, the isolation layer  1008  is positioned below the diffractive layer  910 . In some embodiments, the substrate  915  may comprise the isolation layer  1008  or the optical isolation layer may be disposed elsewhere. Additionally, in some embodiments, a second substrate may be provided between the isolation layer  1008  and the display elements  901 . The second substrate may serve to support the display pixels  901 , while the substrate  915  may support films attached to the display. 
       FIG. 11  schematically illustrates how the illumination apparatus  1000 ′ shown in  FIG. 10  can operate.  FIG. 11  includes an angular region  1115  corresponding to the direction of light within the light guiding region  1004  into which ambient light can be coupled in the absence of the angle conversion layer  910 . This light, however, is not guided in the light guiding region  1004  by total internal reflection. The boundaries  1105  of this angular region  1115  are defined by the critical angle established by the interface between the light-turning layer  905  and the air above. Angles greater than this critical angle  1105  as measured from the normal (z-axis) are generally forbidden or not accessible from air without, for example, the angle conversion layer  910 . This critical angle  1105  defining the angular boundary  1105  may be about 20°, about 25°, about 30°, about 35°, about 40°, about 45° or about 50° in certain embodiments although the angle should not be so limited. 
       FIG. 11  also includes an angular region  1120  corresponding to the direction of light within the light guiding region  1004  that is guided by the light guide region  1004 . Thus, light within angular region  1120  totally internally reflects both at the interface between the light-turning layer  905  and the air above and at the interface between the light-turning layer  905  and the separation layer  1007 . The boundaries  1110  of this region  1120  are defined by the critical angle established by an interface between the light-turning layer  905  and the separation layer  1007  and/or by an interface between the light-turning layer  905  and the diffractive layer  910  below. Angles greater than this critical angle  1110  as measured from the normal (z-axis) are guided by the light guide region  1004 . Light incident at angles greater than this critical angle  1110  totally internally reflect at the interface between the light-turning layer  905  and the separation layer  1007 . This critical angle  1110  defining the angular boundary  1110  may be approximately about 40°, about 50°, about 60°, about 65°, about 70°, about 75°, or about 80° although the angle should not be so limited. 
     Arrow  1123  shows the effect of another embodiment of the angle conversion layer  910 . Such an angle conversion layer  910  may redirect light from a first set of angles into a third set of angles and enable ambient light directed into the light guide region  1004  to be redirected into an angle that is guided by a light guide region  1010  comprising the light-turning layer  910 , the angle conversion layer  910  and the substrate  915 . 
     Whether the ambient light turned by the angle conversion layer  910  is directed into either of the light guide regions  1004 ,  1010  may be determined at least in part by the angle conversion layer. Additionally, the selection of materials and corresponding index of refraction of the layers within the illumination apparatus  100 W, such as the index of refraction of the angle conversion layer  910  itself may affect whether the light is guided within the light-turning layer  905  alone or is guided within the light-turning layer, the separation layer  1007 , the angle conversion layer  910  and the substrate  915  or elsewhere. Alternative configurations are also possible. 
     A wide variety of different embodiments of the invention are possible. For example, components (e.g., layers) may be added, removed, or rearranged. Similarly, processing and method steps may be added, removed, or reordered. Also, although the terms film and layer have been used herein, such terms as used herein include film stacks and multilayers. Such film stacks and multilayers may be adhered to other structures using adhesive or may be formed on other structures using deposition or in other manners. 
     In certain embodiments, the light-turning features  903  may comprise different structures and may be diffractive or holographic optical elements, for example. In various embodiments, the light-turning features  903  may turn light transmitted through the light-turning features. The light-turning features  903 , for example, may comprises transmissive diffractive or holographic layers that redirect light as the light is transmitted through the diffractive or holographic layer. 
     In some embodiments the diffractive layer  910  may be disposed forward the light-turning features  903 . In various embodiments, the diffractive layer  910  may be reflective. 
     Still other variations are also possible. 
     Accordingly, 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. The scope of the invention is indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.