Integrated front light diffuser for reflective displays

A reflective electronic display includes a front light assembly with a diffuser for enlarging the viewing cone of the display. The front light may include a substrate, a plurality of optical turning features, and a diffuser formed therebetween. The haze of the diffuser may be spatially non-uniform and switchable between two or more levels.

BACKGROUND

1. Field of the Invention

The present invention relates to microelectromechanical systems (MEMS).

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, a display device comprises: an optically transmissive substrate; a plurality of display elements rearward of the substrate; a plurality of turning features forward of the substrate; and a diffuser between the plurality of turning features and the substrate, wherein the haze of the diffuser is spatially non-uniform.

In some embodiments, a display device comprises: a plurality of pixels; a light assembly configured to illuminate the plurality of pixels, the light assembly comprising: a light source to output light; a light guide that is configured to distribute light from the light source to the plurality of pixels; and a diffuser that is switchable between a first state wherein the diffuser has a first haze value at a selected point and a second state wherein the diffuser has a second haze value at the selected point, the second haze value being greater than the first; and a controller configured to place the diffuser in the first state when the output of the light source is above a selected threshold, and to place the diffuser in the second state when the output of the light source is below the selected threshold.

In some embodiments, a display device comprises: means for providing structural support to the display device, said support means being optically transmissive; means for displaying an image, said image display means disposed rearward of the support means; means for turning light, said light turning means disposed forward of the support means; and means for diffusing light, said diffusing means disposed between the light turning means and the support means, wherein the haze of the diffusing means is spatially non-uniform.

In some embodiments, a display device comprises: means for displaying an image; means for illuminating the image display means, the illumination means comprising: means for outputting light; means for guiding and distributing light from the light outputting means to the image display means; means for diffusing light, wherein the light diffusing means is switchable between a first state wherein the light diffusing means has a first haze value at a selected point, and a second state wherein the light diffusing means has a second haze value at the selected point, the second haze value being greater than the first; and means for controlling the light diffusing means to be in the first state when the output of the light outputting means is above a selected threshold, and to place the light diffusing means in the second state when the output of the light outputting means is below the selected threshold.

In some embodiments, a method for fabricating a display device comprises: providing an optically transmissive substrate; disposing a plurality of display elements below the substrate; disposing a diffuser above the substrate, wherein the haze of the diffuser is spatially non-uniform; and disposing a plurality of turning features above the diffuser.

In some embodiments, a method for operating a display device comprises: providing a display device comprising, a plurality of pixels, and a light assembly configured to illuminate the plurality of pixels, the light assembly comprising a light source to output light, a light guide that is configured to distribute light from the light source to the plurality of pixels, and a diffuser that is switchable between a first state wherein the diffuser has a first haze value at a selected point and a second state wherein the diffuser has a second haze value at the selected point, the second haze value being greater than the first; and controlling the diffuser to be in the first state when the output of the light source is above a selected threshold, and controlling the diffuser to be in the second state when the output of the light source is below the selected threshold.

DETAILED DESCRIPTION

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.

Displays for electronic devices generally include an array of display elements that form pixels which modulate light to form a viewable image. For example, the display elements may be interferometric modulators, as described herein. In the case of reflective displays, light incident upon the display elements is modulated by varying the reflectivity of the display elements. In relatively bright ambient operating conditions, the light that is incident upon the display elements may come from an external source. A reflective display may also include a front light assembly that includes for example, a built-in light source and a light guide for illuminating the display elements when the display is operated dim ambient light conditions.

Images generated by reflective displays are typically more specular in nature than diffusive. As a result, images formed by the display may be viewable under a limited range of viewing angles. However, a diffuser can be included within the reflective display to increase the range of angles over which the display is viewable. While the diffuser is intended to scatter light that is incident upon the display from an external source when the display is operated under bright ambient conditions, it may also undesirably scatter light propagating within the light guide when the display is being operated in dim ambient conditions and the front light assembly is activated. Thus, in dim ambient conditions when the front light is activated, the diffuser may scatter light out of the front light's light guide before the light has reflected from the display elements (e.g., interferometric modulators), thus reducing viewing contrast of the display. Additionally, the scattering of light by the diffuser as it propagates through the light guide away from the built-in light source may cause the light output of the display to be non-uniform. For example, portions of the display that are located further from the built-in light source may appear dimmer than portions of the display nearer the built-in light source. These complications can be remedied by designing the diffuser to have a non-uniform haze across the surface of the display, as described in more detail herein.

One interferometric modulator display embodiment comprising an interferometric MEMS display element is illustrated inFIG. 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.

The depicted portion of the pixel array inFIG. 1includes two adjacent interferometric modulators12aand12b. In the interferometric modulator12aon the left, a movable reflective layer14ais illustrated in a relaxed position at a predetermined distance from an optical stack16a, which includes a partially reflective layer. In the interferometric modulator12bon the right, the movable reflective layer14bis illustrated in an actuated position adjacent to the optical stack16b.

The optical stacks16aand16b(collectively referred to as optical stack16), 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 stack16is 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 substrate20. 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 stack16are patterned into parallel strips, and may form row electrodes in a display device as described further below. The movable reflective layers14a,14bmay be formed as a series of parallel strips of a deposited metal layer or layers (orthogonal to the row electrodes of16a,16b) to form columns deposited on top of posts18and an intervening sacrificial material deposited between the posts18. When the sacrificial material is etched away, the movable reflective layers14a,14bare separated from the optical stacks16a,16bby a defined gap19. A highly conductive and reflective material such as aluminum may be used for the reflective layers14, and these strips may form column electrodes in a display device. Note thatFIG. 1may not be to scale. In some embodiments, the spacing between posts18may be on the order of 10-100 um, while the gap19may be on the order of <1000 Angstroms.

With no applied voltage, the gap19remains between the movable reflective layer14aand optical stack16a, with the movable reflective layer14ain a mechanically relaxed state, as illustrated by the pixel12ainFIG. 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 layer14is deformed and is forced against the optical stack16. A dielectric layer (not illustrated in this FIG.) within the optical stack16may prevent shorting and control the separation distance between layers14and16, as illustrated by actuated pixel12bon the right inFIG. 1. The behavior is the same regardless of the polarity of the applied potential difference.

FIGS. 2 through 5illustrate one exemplary process and system for using an array of interferometric modulators in a display application.

FIG. 2is a system block diagram illustrating one embodiment of an electronic device that may incorporate interferometric modulators. The electronic device includes a processor21which 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 processor21may 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 processor21is also configured to communicate with an array driver22. In one embodiment, the array driver22includes a row driver circuit24and a column driver circuit26that provide signals to a display array or panel30. The cross section of the array illustrated inFIG. 1is shown by the lines1-1inFIG. 2. Note that althoughFIG. 2illustrates a 3×3 array of interferometric modulators for the sake of clarity, the display array30may 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).

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 5illustrate one possible actuation protocol for creating a display frame on the 3×3 array ofFIG. 2.FIG. 4illustrates a possible set of column and row voltage levels that may be used for pixels exhibiting the hysteresis curves ofFIG. 3. In theFIG. 4embodiment, actuating a pixel involves setting the appropriate column to −Vbias, 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 +Vbias, 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 +Vbias, or −Vbias. As is also illustrated inFIG. 4, voltages of opposite polarity than those described above can be used, e.g., actuating a pixel can involve setting the appropriate column to +Vbias, and the appropriate row to −ΔV. In this embodiment, releasing the pixel is accomplished by setting the appropriate column to −Vbias, and the appropriate row to the same −ΔV, producing a zero volt potential difference across the pixel.

In theFIG. 5Aframe, 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 inFIG. 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 ofFIG. 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 6Bare system block diagrams illustrating an embodiment of a display device40. The display device40can be, for example, a cellular or mobile telephone. However, the same components of display device40or slight variations thereof are also illustrative of various types of display devices such as televisions and portable media players.

The display device40includes a housing41, a display30, an antenna43, a speaker45, an input device48, and a microphone46. The housing41is generally formed from any of a variety of manufacturing processes, including injection molding, and vacuum forming. In addition, the housing41may 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 housing41includes removable portions (not shown) that may be interchanged with other removable portions of different color, or containing different logos, pictures, or symbols.

The display30of exemplary display device40may be any of a variety of displays, including a bi-stable display, as described herein. In other embodiments, the display30includes 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 display30includes an interferometric modulator display, as described herein.

The components of one embodiment of exemplary display device40are schematically illustrated inFIG. 6B. The illustrated exemplary display device40includes a housing41and can include additional components at least partially enclosed therein. For example, in one embodiment, the exemplary display device40includes a network interface27that includes an antenna43which is coupled to a transceiver47. The transceiver47is connected to a processor21, which is connected to conditioning hardware52. The conditioning hardware52may be configured to condition a signal (e.g. filter a signal). The conditioning hardware52is connected to a speaker45and a microphone46. The processor21is also connected to an input device48and a driver controller29. The driver controller29is coupled to a frame buffer28, and to an array driver22, which in turn is coupled to a display array30. A power supply50provides power to all components as required by the particular exemplary display device40design.

The network interface27includes the antenna43and the transceiver47so that the exemplary display device40can communicate with one ore more devices over a network. In one embodiment the network interface27may also have some processing capabilities to relieve requirements of the processor21. The antenna43is 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 transceiver47pre-processes the signals received from the antenna43so that they may be received by and further manipulated by the processor21. The transceiver47also processes signals received from the processor21so that they may be transmitted from the exemplary display device40via the antenna43.

In an alternative embodiment, the transceiver47can be replaced by a receiver. In yet another alternative embodiment, network interface27can be replaced by an image source, which can store or generate image data to be sent to the processor21. 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.

Processor21generally controls the overall operation of the exemplary display device40. The processor21receives data, such as compressed image data from the network interface27or an image source, and processes the data into raw image data or into a format that is readily processed into raw image data. The processor21then sends the processed data to the driver controller29or to frame buffer28for 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 processor21includes a microcontroller, CPU, or logic unit to control operation of the exemplary display device40. Conditioning hardware52generally includes amplifiers and filters for transmitting signals to the speaker45, and for receiving signals from the microphone46. Conditioning hardware52may be discrete components within the exemplary display device40, or may be incorporated within the processor21or other components.

The driver controller29takes the raw image data generated by the processor21either directly from the processor21or from the frame buffer28and reformats the raw image data appropriately for high speed transmission to the array driver22. Specifically, the driver controller29reformats 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 array30. Then the driver controller29sends the formatted information to the array driver22. Although a driver controller29, such as a LCD controller, is often associated with the system processor21as a stand-alone Integrated Circuit (IC), such controllers may be implemented in many ways. They may be embedded in the processor21as hardware, embedded in the processor21as software, or fully integrated in hardware with the array driver22.

Typically, the array driver22receives the formatted information from the driver controller29and 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's x-y matrix of pixels.

In one embodiment, the driver controller29, array driver22, and display array30are appropriate for any of the types of displays described herein. For example, in one embodiment, driver controller29is a conventional display controller or a bi-stable display controller (e.g., an interferometric modulator controller). In another embodiment, array driver22is a conventional driver or a bi-stable display driver (e.g., an interferometric modulator display). In one embodiment, a driver controller29is integrated with the array driver22. Such an embodiment is common in highly integrated systems such as cellular phones, watches, and other small area displays. In yet another embodiment, display array30is a typical display array or a bi-stable display array (e.g., a display including an array of interferometric modulators).

The input device48allows a user to control the operation of the exemplary display device40. In one embodiment, input device48includes 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 microphone46is an input device for the exemplary display device40. When the microphone46is used to input data to the device, voice commands may be provided by a user for controlling operations of the exemplary display device40.

Power supply50can include a variety of energy storage devices as are well known in the art. For example, in one embodiment, power supply50is a rechargeable battery, such as a nickel-cadmium battery or a lithium ion battery. In another embodiment, power supply50is a renewable energy source, a capacitor, or a solar cell, including a plastic solar cell, and solar-cell paint. In another embodiment, power supply50is 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 driver22. 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-7Eillustrate five different embodiments of the movable reflective layer14and its supporting structures.FIG. 7Ais a cross section of the embodiment ofFIG. 1, where a strip of metal material14is deposited on orthogonally extending supports18. InFIG. 7B, the moveable reflective layer14of each interferometric modulator is square or rectangular in shape and attached to supports at the corners only, on tethers32. InFIG. 7C, the moveable reflective layer14is square or rectangular in shape and suspended from a deformable layer34, which may comprise a flexible metal. The deformable layer34connects, directly or indirectly, to the substrate20around the perimeter of the deformable layer34. These connections are herein referred to as support posts. The embodiment illustrated inFIG. 7Dhas support post plugs42upon which the deformable layer34rests. The movable reflective layer14remains suspended over the gap, as inFIGS. 7A-7C, but the deformable layer34does not form the support posts by filling holes between the deformable layer34and the optical stack16. Rather, the support posts are formed of a planarization material, which is used to form support post plugs42. The embodiment illustrated inFIG. 7Eis based on the embodiment shown inFIG. 7D, but may also be adapted to work with any of the embodiments illustrated inFIGS. 7A-7Cas well as additional embodiments not shown. In the embodiment shown inFIG. 7E, an extra layer of metal or other conductive material has been used to form a bus structure44. 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 substrate20.

In embodiments such as those shown inFIG. 7, the interferometric modulators function as direct-view devices, in which images are viewed from the front side of the transparent substrate20, the side opposite to that upon which the modulator is arranged. In these embodiments, the reflective layer14optically shields the portions of the interferometric modulator on the side of the reflective layer opposite the substrate20, including the deformable layer34. This allows the shielded areas to be configured and operated upon without negatively affecting the image quality. For example, such shielding allows the bus structure44inFIG. 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 inFIGS. 7C-7Ehave additional benefits deriving from the decoupling of the optical properties of the reflective layer14from its mechanical properties, which are carried out by the deformable layer34. This allows the structural design and materials used for the reflective layer14to be optimized with respect to the optical properties, and the structural design and materials used for the deformable layer34to be optimized with respect to desired mechanical properties.

As described above, interferometric modulators can be used in reflective display technologies. A reflective display generally relies on a source of light incident upon reflective display elements (e.g., interferometric modulators, as described herein) to produce a viewable image. This light may come from an external source, such as the ambient lighting conditions where the display is being used. Alternatively, a built-in source of illumination may also be provided for illumination of the reflective display in dark ambient environments. The built-in illumination source for reflective displays may, for example, be a front light assembly that uses a light guide to collect light through an input port and redirect it towards reflective display elements that are modulated to form an image.

One drawback of certain reflective displays is that they are undesirably specular in nature. As a result, the displays are satisfactorily viewable only over a relatively small range of viewing angles (i.e., the viewing cone). One solution to enlarge the viewing cone of a reflective display is to incorporate a diffuser within the display at some location along the optical path of light reflected by the display. For example, a diffuser could be situated on the viewer side of a light guide used in a front light assembly for the reflective display. However, this placement of the diffuser may detrimentally affect the optical performance of the reflective display by decreasing the displayable resolution and contrast. Alternatively, the diffuser may be situated on the opposite side of the light guide, for example, between the light guide and an array of interferometric modulator display elements. Yet, this location has certain disadvantages in terms of case of fabrication. For example, in some embodiments the diffuser is made of organic materials with a thickness of many microns (e.g., 1-20 microns) which can be difficult to fabricate with interferometric modulator display elements made up of inorganic materials with sub-micron thicknesses.

In order to avoid the difficulties associated with placement of a diffuser either in front of, or behind, a light guide in a reflective display, in certain embodiments, a diffuser is integrated within the light guide. In some embodiments comprising a light guide with an integrated diffuser, the optical properties of the diffuser can be more easily matched to the other portions of the light guide to reduce Fresnel reflections and reduce some loss in viewing contrast. For example, embedding the diffuser within the light guide allows for the refractive index variation at the interface of the diffuser with the other portions of the light guide to be reduced. In addition, the diffuser is closer to the display or modulator elements, e.g., interferometric modulators, thereby increasing resolution when compared to a diffuser spaced farther from the display elements. In addition, such an integrated diffuser can be easily fabricated.

FIG. 8is a cross section of a light guide802for use in a reflective display800. In the embodiment illustrated inFIG. 8, the light guide802includes an integrated diffuser830. The light guide802also includes an optically transmissive substrate820slab, sheet, or plate that is disposed in front of an array of reflective display elements810(e.g., interferometric modulators). The light guide802also includes a turning layer840to direct light propagating within the light guide802toward the reflective display elements810.

The integrated diffuser830, in this case a diffusive layer, is formed between the optically transmissive substrate820and the turning layer840. The diffuser830scatters light incident upon it in a range of directions to enlarge the viewing cone of the reflective display800and to give a more desirable paper-like appearance. The integrated diffuser830is embedded within the light guide802. In some embodiments, the integrated diffuser830is formed between a substrate820and a turning layer840, and in some cases adjacent the substrate820and/or turning layer840, though this is not required. In some embodiments, the substrate820, the turning layer840, and the diffuser830form a monolithic light guide802. It should be understood that in some embodiments, the turning layer840, the diffusive layer830and the substrate820include one or more sub-layers. In addition, the light guide802may also include additional layers that serve, for example, mechanical functions such as adding strength to the light guide802, or optical functions such as controlling how light is guided through the light guide802.

The degree of haze of the diffuser830can be quantified by a haze value. One way of quantifying a haze value is to measure the extent to which a collimated beam of light that is incident upon the diffuser830is scattered outside a ±2.5° cone centered on the beam. (See also, e.g., ASTM D1003 “Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics”). Diffusers that have a greater level of diffuseness have a correspondingly higher haze value. Although the haze value of the diffuser830varies from embodiment to embodiment, typically a haze value in the range from approximately 10% to approximately 90%, or more particularly from approximately 40% to 80%, is satisfactory. For example, a diffuser830with a haze value of approximately 50% is used in some embodiments.

The light guide802is formed of optically transmissive materials. For example, the substrate820can be formed of various types of glass or plastic chosen for their physical and optical characteristics such as their refractive indexes and durability. The turning layer840may be made of an optically transmissive material such as polycarbonate, acrylics, acrylate polymers and copolymers, and other materials, including but not limited to polymethymethacrylate (PMMA), poly(styrene-methylmethacrylate) polymers (PS-PMMA, sold under the name of Zylar), zenor, COC, and optically transmissive plastics.

In some embodiments, the diffuser830is a thin, solid film that contains microscopic scattering elements that scatter incident light. These microscopic scattering elements may have dimensions that are, for example, on the order of the wavelengths of visible light propagating within the light guide802. Scattering features can, however, also be many times larger (e.g., 10-100 times larger) than the wavelength of light propagating through the diffuser830. For example, the actual physical dimensions of scattering features can range from approximately 0.1-10 μm. The diffuser830may also be a fluidic adhesive that, for example, bonds the turning layer840to the substrate820and that also contains scattering elements such as microscopic scattering spheres. Thus, such a diffuser830would perform both the optical function of enlarging the viewing cone of light outputted by the light guide802as well as the mechanical function of bonding two or more other layers of the light guide802together.

In some embodiments, the refractive indexes of the multiple optical layers of the lightguide802, including the substrate820, the diffuser830, and the turning layer840, are advantageously similar such that light may be transmitted through the multiple optical layers without being substantially reflected or refracted. Matching the refractive indexes6fthe various layers of the light guide802improves optical efficiency of the device, as well as brightness and viewing contrast of the reflective display800. In some embodiments, the refractive indexes of the optical layers of the light guide802, including the substrate820, the diffuser830, and the turning layer840, are in the range of about 1.40 to 1.65. In some embodiments, the substrate820is glass with a refractive index of 1.518 (at 580 nm), while adjacent layers have refractive indexes most preferably equal to or slightly greater than that of the substrate.

The reflective display800, including the light guide802, may be formed using any of a variety of manufacturing processes known to those skilled in the art. In some embodiments, the substrate acts as a support layer upon which an array of interferometric modulator display elements810is formed. In some embodiments, the diffusive layer830is formed on the opposite side of the substrate820from the array of interferometric modulator display elements810. The diffuser830may, for example, be coated over, deposited on, laminated to, spun on, applied to, or adhered to the substrate820. In some embodiments, the diffusive layer830includes scattering features etched into the substrate820. In still other embodiments, the diffuser830is a thin film that is grown on the surface of the substrate820.

In some embodiments, the turning layer840is disposed over the diffuser830. For example, the turning layer840may be deposited on or laminated to the diffusive layer830. As described herein, in some embodiments the diffusive layer is an adhesive containing scattering elements, in which case the turning layer840can be adhered to the substrate820by the diffusive layer830.

Some embodiments of the light guide802do not exceed approximately 500 microns in thickness. More specifically, some embodiments of the light guide do not exceed approximately 200 microns in thickness. The thickness of the light guide802may be outside these ranges as well.

In some embodiments of the light guide802, films, layers, components, and/or elements may be added, removed, or rearranged. Additionally, processing steps may be added, removed, or reordered. Also, each layer or film may include multiple sub-layers. Thus, any one of several arrangements of the several layers of the light guide802can be selected depending upon the particular application.

The operation of the light guide802is illustrated by light rays852,854,856, and858. Light ray852is a ray propagating internally within the light guide802, which is formed from materials whose refractive indexes are greater than the surrounding medium (e.g., air). Light ray852, originating, for example, from a built-in light source (not shown), is incident upon the turning layer840from within the light guide802. Since the refractive index of the light guide802is greater than the surrounding air, light incident on the turning layer-air interface at an angle greater than the critical angle is reflected back into the light guide802.

The turning layer840has a plurality of turning features for redirecting light incident upon the turning layer-air interface towards the array of display elements810. For example, in the embodiment illustrated inFIG. 8, each turning feature is a micro-prism with a long, shallow facet and a short, steep facet. If light strikes the long, shallow facet and then the short, steep facet sequentially, total internal reflection occurs at both facet-air interfaces and the light is turned through a large angle (e.g., between about 70°-90°).

The turning layer840can be designed (e.g., by adjusting the relative angles of the facets) such that light incident upon the turning features from within the light guide802between a predefined range of angles is re-directed toward the array of display elements810. In some embodiments, the turning layer840is designed to direct light toward the display elements810at or near normal incidence. After being turned through a large angle by the turning layer840, light ray852is then transmitted through the thickness of the light guide802toward the array of display elements810where it may be reflected back through the light guide802, this time at an angle less than the critical angle of the turning layer-air interface such that the ray exits the light guide802, towards a viewer disposed in front of the reflective display800. While a turning layer made up of micro-prisms is illustrated inFIG. 8, in other embodiments the turning layer840may be a holographic or diffractive turning film. Other types of turning films will also be apparent to those skilled in the art.

Light ray854is incident upon the light guide802from an external light source. The ray is transmitted through the turning layer840to the diffuser830. At the diffuser, light ray854is scattered in several different directions. Some of the scattered light rays are incident upon the array of display elements810at angles where they can be reflected back through the thickness of the light guide and transmitted towards a viewer. Others of the scattered light rays may propagate at angles within the light guide802, either before or after reflecting from the array of display elements810, such that they exit the light guide802prematurely, for example, without contributing to the useful formation of an image.

Light ray856is incident upon the array of display elements810, whether after having been re-directed by the turning layer840such as light ray852, or after having entered the light guide802from an external light source such as light ray854. Light ray856is reflected by the array of reflective display elements810(e.g., interferometric modulators) and is then incident upon the diffuser830. Once again, the diffuser830scatters the light ray856in several different directions. Some of the scattered light rays are incident upon the turning layer-air interface at an angle less than the critical angle such that they exit the light guide802. The scattered light rays exit the light guide802over a relatively large range of angles, forming an enlarged viewing cone, e.g., between about −45 and +45 degrees. Others of the light rays scattered by the diffuser830are re-directed back into the light guide802.

Light ray858is a ray propagating within the light guide802in a direction generally along the width of the light guide (e.g., from one side of the reflective display800towards another). Light ray858may have originated from, for example, a built-in light source (not shown). As illustrated, light ray858is incident upon the diffuser830and scattered in several different directions. Some of these scattered rays propagate in directions such that they can be utilized and turned toward the array of display elements810and eventually transmitted toward a viewer. However, some of the scattered light rays may exit the light guide802prematurely before having been reflected by the array of display elements810. As such, some light propagating through the light guide802is lost when it could more advantageously have been internally reflected to provide more light to display elements located at more distal positions of the light guide802. This type of loss of light from the light guide802is generally undesirable as it reduces the brightness and/or viewing contrast of the reflective display800. The consequences of this light loss due to scattering by the integrated diffuser830are illustrated in more detail inFIG. 9.

FIG. 9illustrates the operation of a light guide902having an integrated diffuser930whose haze is spatially constant across the width of the light guide902. The reflective display900includes a built-in light source980as well as a light guide902and an array of display elements910(e.g., an array of interferometric modulators). The light guide902includes a substrate920, a diffuser930, and a turning layer940. The light guide902also has an input port optically coupled to the built-in light source980. In the illustrated embodiment, the input port is the left-hand edge of the light guide902. The operation of the light guide902is similar to the light guide802ofFIG. 8. The built-in light source980can be, for example, a linear-type light source that extends along a side (e.g., into the page) of the light guide902. The light source980injects light into an input port of the light guide902. For example, the input port of the light guide902can be the edge of the light guide902along which the light source980extends. The light source980and input port of the light guide902can also be configured in many other arrangements that will be apparent to those of ordinary skill in the art.

After having been inputted by the light source980, several light rays958propagate within the light guide902. Similarly to light ray858ofFIG. 8, when light rays958are incident upon the integrated diffuser930, they are scattered in a number of different directions. The haze profile of the diffuser930is illustrated by plot970. Plot970illustrates the haze value of the diffuser930as a function of position along the width of the light guide902. As illustrated by line972, the haze value of the diffuser930is constant across the width of the light guide902at macroscopic distance scales (it being understood that the uniform haze value over macroscopic distances may in fact result from light scattering processes whose amount and direction of scattering may vary significantly when considered at microscopic scales).

A portion of the scattered light rays that result when light rays958are incident upon the diffuser930, for example, those labeled with reference number960, are incident upon the turning layer-air interface at an angle greater than the critical angle and are re-directed toward the array of display elements910as light rays962. However, a portion of the scattered light rays, for example, those labeled with reference number964, are incident upon the turning layer-air interface at an angle less than the critical angle and exit the light guide902. Thus, light rays964may exit the light guide902prematurely when taken in comparison to a light guide configuration without the integrated diffuser930. For example, light rays964may exit the light guide902without having reflected off of the array of display elements910and without necessarily contributing to the formation of a viewable image. This effect may reduce the viewing contrast of the reflective display900.

In addition, the premature loss of some light that is scattered by the diffuser930can reduce the propagation of light from the built-in light source980toward the distal end of the light guide902. The result is progressively lesser amounts of light flux in the light guide902as the distance from the input port of the light guide902increases. This lessening of light flux within the light guide with increasing distance from the input port is illustrated schematically inFIG. 9by the decreasing density of light rays958towards the side of the light guide902opposite the light source980. The decreased amount of light flux within the light guide902at extremities away from the input port also results in a corresponding decrease in light outputted from the reflective display900in these areas. This decreasing amount of light outputted by the light guide902with increasing distance from the input port of the light source980is illustrated by the decreasing density of output rays966with increasing distance from the light source980. This effect results in the portion of the reflective display900nearer the input port of the light guide902appearing brighter than portions of the display900that are further away from the input port of the light guide902. It is generally preferable for the display900to have a uniform brightness and contrast across its spatial extents rather than to have this type of spatial hot spot in any particular region of the display900.

In contrast withFIG. 9,FIG. 10illustrates the operation of a light guide1002having an integrated diffuser1030whose haze varies macroscopically across at least a portion of the light guide1002(e.g., across the light guide's width and/or height). The reflective display1000includes a built-in light source1080as well as a light guide1002and an array of display elements1010(e.g., an array of interferometric modulators). The light guide1002includes a substrate1020, a diffuser1030, and a turning layer1040. The light guide1002also has an input port optically coupled to the built-in light source1080. In the illustrated embodiment, the input port is the left-hand edge of the light guide1002. The operation of the light guide1002is similar to the light guide902ofFIG. 9in some respects. However, the haze of the diffuser1030is non-uniform. This is illustrated in plot1070where the haze value of the diffuser1030is plotted as a function of distance on the display900from the input port of the light source1080. In this case, line1072represents a linear relationship between haze value of the diffuser1030and distance from the input port. The diffuser1030can also be designed so that its haze varies non-linearly. In some embodiments, the haze of the diffuser1030, for example as measured by its haze value, increases monotonically with distance from the input port of the light guide1002, though this is not required. In addition, the haze of the diffuser1030may be gradated such that it varies smoothly. The haze of the diffuser may also vary in a step-wise fashion. In some embodiments, the haze of the diffuser1030increases over at least 90% of the display1000. In some embodiments, the haze of the diffuser1030increases over at least 75% of the display1000. In some embodiments the haze of the diffuser1030increases over at least 50% of the display1000. In some embodiments, one half of the diffuser1030(e.g., the half more distant from the light source input port) has a haze that is, on average, greater than the other half For example, the half of the diffuser1030most distant from the light source input port may, on average, have a haze value at least twice the average haze value of the half nearest the light source input port.

The haze of the diffuser1030can be varied, for example, by spatially varying the thickness of the diffuser or by spatially varying the density of scattering features within the diffuser. Other methods of varying the haze are also possible. In some embodiments, the haze of the diffuser is measured over a dimension at least as large as the thickness of the diffuser. In other embodiments, the haze of the diffuser is measured over an area of at least about 5×5 mm2or over an area the size of a beam of light used to measure haze values according to accepted standards.

As noted herein, in some embodiments the haze of the diffuser (e.g.,1030) increases with increasing distance from an input port of a light guide. For example, in some embodiments, if the display or the portion of the diffuser overlapping display elements (e.g., pixels) were divided up into a number of segments (e.g., equal segments) located different distances from an input port of a light guide, then a measurement of the average haze of the diffuser or the display at any given section would be greater than a measurement of the average haze of a neighboring section located closer to the light guide input port. In some embodiments, if the display or the portion of the diffuser overlapping display elements were divided into ten equal sections located at different distances from an input port of a light guide, then the average haze of the section nearest the input port would give the lowest haze value, while measurements of the average haze at each of the remaining nine sections would result in haze values larger than the immediately neighboring section closer to the input port (e.g., the average haze of the second section being greater than the average haze of the first, the average haze of the third section being greater than the average haze of the second section . . . the average haze of the tenth section being greater than the average haze of the ninth section). In other embodiments, the same would be true of a diffuser or display divided into, for example, two, three, four, or eight equal sections rather than ten.

Referring to the cross-section of the light guide1002illustrated inFIG. 10, the built-in light source1080of the reflective display1000injects light at the input port of the light guide1002. Light rays1058propagate within the light guide1002and are incident upon the integrated diffuser1030. The diffuser1030scatters the light rays1058into several different directions. The increasing haze of the diffuser1030with distance from the light input port of the light guide1002is schematically represented inFIG. 10by the increasing number of scattered rays1064with distance from the input port. Similarly to what is illustrated inFIG. 9, a portion of the scattered light rays that result when light rays1058are incident upon the diffuser1030, for example, those labeled with reference number1060, are incident upon the turning layer-air interface at an angle greater than the critical angle and are re-directed toward the array of display elements1010as light rays1062. However, a portion of the scattered light rays, for example, those labeled with reference number1064, are incident upon the turning layer-air interface at an angle less than the critical angle and exit the light guide1002. As discussed herein, these light rays1064may exit the light guide1002prematurely without having first reflected from the array of display elements1010and/or before having traveled the intended distance within the light guide1002.

However, notwithstanding the light rays1064that may exit the light guide1002prematurely, the light flux in the light guide1052changes relatively less than illustrated inFIG. 9as a result of the spatially non-uniform haze of the diffuser1030. In fact, in some embodiments, the light flux within the light guide1002remains relatively constant as the distance from the light source1080increases. This relatively constant light flux within the light guide1002with increasing distance from the light source1080is illustrated schematically inFIG. 10by the relatively constant density of light rays1058towards the side of the light guide1002opposite the light source1080. The relatively constant amount of light flux within the light guide1002at extremities away from the optical input port also results in a corresponding relative uniformity in light outputted from the reflective display1000in these areas. This relatively uniform spatial emittance distribution is illustrated by the uniform density of output rays1066with increasing distance from the optical input port1002. Thus, the spatial hotspots in the brightness of the reflective display900inFIG. 9are reduced in the embodiment ofFIG. 10, making for more uniform brightness and contrast across the face of the display1000.

Although the turning features shown inFIG. 10have a substantially constant periodicity, in other embodiments, the periodicity and/or other characteristic, e.g., shape, size, etc., of the turning features may vary. In certain embodiments, for example, the turning features vary across the turning layer1040so as to increase turning efficiency from one end of the light guide1002to another. Such a design may also address the fact that the intensity of light within the light guide1002may be higher closer to the light source1080than further from the light source where much of the light has already been turned out of the light guide. Uniformity in brightness across the display can be increased with such variation in the turning features. Having a diffuser that additionally varies with distance from the light source1080may further enhance uniformity in brightness.

There are several ways of creating a diffuser1030with spatially non-uniform haze. For example, in some embodiments, the diffuser1030is made up of microscopic scattering features. The haze of the diffuser1030can be made to spatially vary by altering the spacing, size, or shape of the scattering features at different locations in the light guide1102, as is understood by those skilled in the art. In some embodiments, the diffuser1030is a diffractive or holographic diffuser. The haze of the holographic diffuser can likewise be made to spatially vary, for example by recording the variation therein.

FIG. 11illustrates yet another way of creating a diffuser with spatially non-uniform haze. The reflective display1100includes a built-in light source1180as well as a light guide1102and an array of display elements1110(e.g., an array of interferometric modulators). The light guide1102includes a substrate1120, a diffuser1130, and a turning layer1140. The light guide1102also has an input port optically coupled to the built-in light source1180. In the illustrated embodiment, the input port is the left-hand edge of the light guide1102. The operation of the light guide1102is similar to the light guide1002ofFIG. 10. The haze of the diffuser1130is non-uniform. This is illustrated in plot1170where the haze value of the diffuser1130is plotted as a function of distance across the width of the display1100from the input port of the light source1180. In this case, line1172represents a non-linear, monotonically increasing relationship between haze value of the diffuser1130and distance from the input port. However, it should be understood that the diffuser1130can also be designed so that its haze varies spatially in other ways.

The diffuser1130includes individual scattering particles1132suspended within a solid or liquid film. As illustrated, the localized density of the scattering particles1132increases with increasing distance from the input port of the light guide1102. The increasing density of the scattering particles corresponds to a greater level of diffuseness and increased haze value. The consequence of the non-uniform haze of the diffuser1130is a relatively uniform spatial emittance distribution, as illustrated by the uniform density of output rays1166with increasing distance from the optical input port of the light guide1102.

While the presence of a diffuser (e.g.,930) within a light guide (e.g.,902) of a reflective display (e.g.,900) can have the desirable effect of enlarging the viewing cone of the display, the presence of the diffuser can also introduce certain complications. For example, as described herein, the diffuser may scatter light out of the light guide prematurely and may also reduce the propagation of light within the light guide downstream from the optical input port of a built-in light source (e.g.,980). WhileFIGS. 10 and 11illustrate embodiments that address these complications with spatially non-uniform diffusers, these effects can also be addressed by causing the haze of the diffuser to be temporally non-uniform. For example, the diffuser can be made switchable between two or more levels of haze. The diffuser may be switched to a relatively high level of haze when the display's built-in light source is off and the display is operating with an external source of light. In this state, the relatively high level of haze reduces the specular behavior of the reflective display and enlarges the viewing cone. When the display is operating with its built-in light source turned on, for example in dark ambient conditions, the diffuser may be switched to have a relatively low level of haze. The reduced haze of the diffuser lessens the types of losses sometimes associated with the integrated diffuser, as described herein.

FIGS. 12 and 13illustrate light guides with diffusers that have controllable levels of haze. The reflective display1200includes a built-in light source1280as well as a light guide1202and an array of display elements1210(e.g., an array of interferometric modulators). The light guide1202includes a substrate1220, a switchable diffuser1230, and a turning layer1240. The switchable diffuser1230can be controlled to have two or more different levels of haze. These may be discrete levels or a continuous range of haze between a maximum level with a maximum haze value and a minimum level with a minimum haze value. In one embodiment, the switchable diffuser1230is a polymer-dispersed liquid crystal (PDLC) film. The haze of a PDLC film changes depending upon the amount of electric voltage applied across it, as is known in the art. The light guide1202also has an input port optically coupled to the built-in light source1280. In the illustrated embodiment, the input port is the left-hand edge of the light guide1202. The operation of the light guide1202is similar to that of other embodiments described herein.

In the illustrated embodiment, the switchable diffuser1230is electrically controllable. As such, the light guide1202includes electrodes1294and1296. The electrodes1294and1296may be, for example, layers of an optically transmissive material such as transparent conducting oxide (TCO) like indium tin oxide (ITO) formed on opposing sides of the switchable diffuser1230. ZnO can also be used as a transparent conductor. The electrodes1294and1296are electrically connected to a source of electrical power1290. The haze state of the switchable diffuser1230is controlled by a switch1292that is used to apply differing voltages to the switchable diffuser1230. Other methods and configurations for electrically switching the diffuser1230will be apparent to those skilled in the art.

The light guide1202inFIG. 12is operating under relatively bright ambient lighting conditions such that an external source provides the light for the image formed by the reflective display1200. The external light source is illustrated by light rays1254that are incident upon the face of the light guide1202from the exterior of the display1200. Under external lighting conditions, the switch1292is positioned such that the switchable diffuser1230is controlled to have a relatively high level of haze. For example, in some embodiments, the reflective display1200includes a controller (not shown) configured to set the haze of the switchable diffuser1230at a maximum value when the display's built-in light source1280is off and the display is operating using an external light source. This relatively high level of haze scatters light reflected by the display1200over a larger range of angles, enhancing the viewing experience for a user.

The light guide1302illustrated inFIG. 13is identical to the light guide1202inFIG. 12. However, in the light guide1302ofFIG. 13, the display's built-in light source1380is on and the switch1392is positioned such that the switchable diffuser1330has a relatively low level of haze. The decreased haze of the diffuser1330reduces the amount of light scattered out of the light guide1302prematurely and increases the amount of light that propagates to the distal side of the light guide1302from the optical input port of the built-in light source1380. Importantly, under dark ambient lighting conditions, the specular nature of the reflective display1300is less problematic than the case where the display is operating with ambient light because the turning features of the turning layer can be designed to turn light from the artificial light source to ensure proper light distribution within the viewing cone for the viewer. The shape of the turning features, for example, may provide such control over the viewing cone. Thus, the switchable diffuser1330can be less diffuse when the light guide1302is operating with the built-in light source1380without unduly sacrificing optical performance of the display1300.

In some embodiments, when the built-in light source1380is switched on, the switchable diffuser1330is controlled to have its minimum level of haze. For example, in some embodiments, the haze of the diffuser1330at a given spatial location is at least about 30% less than the haze of the diffuser1330at the same location while in the maximum haze state. In some embodiments, the haze of the diffuser1330at a given spatial location is at least about 80% less than the haze of the diffuser1330at the same location while in the maximum haze state. As illustrated inFIGS. 12 and 13, the haze of the switchable diffuser1230,1330, besides being controllable, is also spatially non-uniform, though this is not required. For example, in some embodiments, the haze of the switchable diffuser1230,1330is spatially uniform. In particular, if the level of haze can be controlled to become very low, the spatial distribution of the haze may become less important to the point that a uniform spatial haze distribution may be considered in some applications. For example, if the level of haze can be controlled to become effectively zero, then it may be beneficial for the haze to be spatially constant. In practice, however, it may be difficult to control the haze to be come effectively zero, in which case a non-uniform spatial distribution of haze may be particularly useful.

Although only certain preferred embodiments and examples have been explicitly disclosed, other embodiments will be apparent to those skilled in the art. It is also contemplated that some embodiments include various combinations or sub-combinations of the specific features and aspects of the explicitly disclosed embodiments. It should be understood that various features and aspects of the disclosed embodiments can be combined with, or substituted for, one another in order to form additional embodiments. Thus, it is intended that the scope of the claims should not be limited by the particular disclosed embodiments described above.