Abstract:
Magneto-optical displays offer numerous advantages in a multitude of applications, most notably large format signage applications. New designs and architectures such as the use of optical fluids, segmented magneto-optical element densities, frontplane lenses and color filters significantly expand the utility of these displays.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of and priority to U.S. Provisional Application No. 60/847,601, filed Sep. 27, 2006, U.S. Provisional Application No. 60/847,603, filed Sep. 27, 2006 and U.S. Provisional Application No. 60/875,514, filed Dec. 18, 2006, all of which are incorporated herein by reference in their entirety. 
     
     BACKGROUND OF THE INVENTION 
       [0002]    1. The Field of the Invention 
         [0003]    Embodiments of the present invention relate to improvements in the design of reflective bi-stable magneto-optical displays. Such displays are suitable for a multitude of applications especially low-cost, large-format information displays and outdoor signage applications. 
         [0004]    2. The Relevant Technology 
         [0005]    Traditional flat panel display technologies like plasma, liquid crystal and organic light emitting diode displays have proven unsuccessful for use in large format signage applications. These display technologies do not offer sufficient brightness and contrast when exposed to direct sunlight. In addition they are not cost effective or reliable when put in large format, outdoor environments. Today, large panel arrays of discrete light emitting diodes (LEDs) have been developed to fill the need of the digital signage market. These indoor and outdoor digital signage devices are very expensive products with a high number of discrete components and high power requirements. They must use a high number of densely packed high-brightness LEDs to emit enough light to compete directly with sunlight in outdoor and high brightness applications. Direct sunlight can be up to 100,000 lux. A simple one color, 4 line×20 character LED display will typically use approximately 2,800 discrete LEDs. An LED “Jumbotron” large format, color video screen may require more than 1 million discrete components. 
         [0006]    Additional features the sign industry is looking for are reflective technology in place of light emitting. In this scenario, direct sunlight is used during the day for illumination. At night, illumination requirements are far lower so the power used by a reflective display even with night lighting will be much lower. A reflective bistable display (a display that keeps the image without continuous power) can be very low power and effective for signs. This is because many signs will not required information to be changed frequently. A gas price sign may only be updated once a day. A clock only changes one digit every 60 seconds. Bistable displays can be ultra-low power making it possible for them to be powered by small batteries or solar cells. This would eliminate the need for costly wiring especially where the sign may not be near utilities. Finally light weight and thin digital signs are desirable because they reduce the cost of installation and the support structure required. The invention described herein has all of these features: reflective, bistable, low power, thin and light. 
         [0007]    Prior to LED signs, starting in the early 1900s, there were various electronic signage technologies that used electromagnetic actuators (Naylor U.S. Pat. No. 1,191,023, Taylor U.S. Pat. No. 3,140,553 and Browne U.S. Pat. No. 4,577,427). These discrete actuators were generally variations of small electromagnetic coils or motors with a reflective mechanical flap apparatus. They were broadly used and a small number of these device are still in use today for specific outdoor signage applications like score boards and transit signs however, they have become obsolete with the advent of LEDs because they were very expensive to construct, suffer from reliability issues and are limited in resolution due to the size of the discrete mechanical assemblies needed for each pixel. 
         [0008]    Development continued in the use of magnetic actuators for displays by Weiacht (U.S. Pat. No. 6,510,632) where magnets were attached to large flaps or “flags” that could be rotated 180 degrees in forming a signage character. This type of technology was continued with the patents of Fischer et al and others (U.S. Pat. No. 6,603,458, U.S. Pat. No. 3,936,818) again discussing the use of magnetically driven flaps as individual display pixels. These devices are mounted on a mechanical axis and only one magneto-optic “flag” is used in each individual device. 
         [0009]    While magnetics itself and the study of magnetic materials is an old discipline that dates back centuries and is still studied heavily today (Spaldin, N., Magnetic Materials: Fundamentals and Device Applications, Cambridge Univ. Press, 2003; Kittel, C. Introduction to Solid State Physics, Wiley, 1996), few attempts have been made to develop reflective magnetic-based display technology. In 1898 and 1928 Poulsen and Pfleumer, respectively, leveraging magnetic phenomena, developed the use of magnetics in recording media. In the 1960&#39;s magnetic materials were used to store memory, first by Forrester et al and others (U.S. Pat. No. 2,736,880, U.S. Pat. No. 2,667,542, U.S. Pat. No. 2,708,722). Magnetic core memory, pioneered by Olsen and others (U.S. Pat. No. 3,161,861, U.S. Pat. No. 4,161,037, U.S. Pat. No. 4,464,752) has seen significant attention for decades due to its implications on the computing industry but for a number of reasons was replaced by silicon-based memory devices. The only work in the use of magnetics in displays outside of the older “flap” technologies utilize bi-colored magnetic particles instead of mechanical flaps. Here, the use of smaller discrete magnetic spheres has been envisioned as a display design by Magnavox (Lee, L. IEEE Transactions on Electron Devices, ED-22, P758) and more recently by Katsuragawa et al. (Japan Patent Publication 2002-006346) and to a lesser extent by Masatori (Japan Patent Publication 08-197891). This prior art deals primarily with the use of small magnetic particles which respond to external magnetic field by rotating. The rotation of these magnetic particles is in response to the external field and the particles desire to lower their potential energy. Here, the magnetic spheres are magnetized along the center axis. The north and south poles of the spheres are coated with different colors. A magnetic field is then applied that rotates the particles between the two color states. This method of making an electromagnetic display has several problems which will now be discussed. 
         [0010]    A first problem is an external magnetic field needs to be sustained. Without an ongoing electromagnetic field, the particles will align to each other in a low energy, grey state. Two methods to sustain the magnetic field are: 1) Use constant current to the electromagnetic coil—the issue here is constant power consumption and it requires much higher cost control electronics. 2) Add a layer of ferrite/“writable” magnetic material. This layer must sustain a sufficient magnetic field after it has been “written” by an electromagnetic write pulse. One issue here is that the added layer is a complex magnetic material that needs to be developed. Also the strength of the magnetic field and stability of the “writable” magnetic state is complex. There is significant added cost of manufacturing with this more complex structure. Finally, the energy required to create a sufficient B magnetic field inside the material to “write” a sustainable magnetic state can be considerable. 
         [0011]    A second problem is crosstalk between pixels. The flux patterns created by two pixels of different states will create curved flux lines between their poles. The transition from one pixel&#39;s color state (North) to the second pixel&#39;s color state (South) will create flux patterns that are displayed as artifacts between pixels that will be viewed as a “grey” zone. Creating distinct separation between pixels without crosstalk is a major problem. Another form of crosstalk can occur when the magnetic field of one pixel “overpowers” a neighboring pixel and reverse its state during the write process. 
         [0012]    A third problem is low contrast. Spherical particles, if placed in a single layer, will have limited contrast because they do not have full coverage of the display plane. If multiple layers of magnetic spheres are used, this increases their interference with each other, making it more difficult to control their states and thus requiring a stronger external magnetic field. 
         [0013]    A fourth problem is magnetic materials. Development of new materials of construction and methods of fabrication are needed. The materials and methods must create low cost, light weight, environmentally stable, permanent magneto optical elements with accurate alignment of the magnetic field to the color planes. 
         [0014]    A fifth problem is resolution limitations. This is because the domains of permanent magnetic particles lose stability when they are isolated or particles are made too small (for example, below 100 microns a drop in stability in magnetic particles starts to be observed depending on the specific material). In addition, electromagnetic coil assemblies become less efficient when the size is below 1 mm due to heat losses in the wire windings from the increased resistance of the smaller conductors. Also, in order for low cost, industrial manufacturing processes to replace high cost “clean room” processes, the manufacturing tolerances need to be in the range of ±25 microns or larger to enable ease of manufacturing. 
         [0015]    A sixth problem is design of a low cost electronic backplane. The highest cost processes in flat panel displays is the manufacturing of active matrix backplane layers that control the display. Most liquid crystal (LCD), organic light emitting diode (OLED) and even the latest electronic-paper (ePaper) displays utilize an array of thin film transistors (TFTs) that are manufactured over the entire back of the display to control the image. The cost of this backplane can equal 50% of the entire process costs of the display panel. A low cost backplane alternative is needed for large format signage applications. 
       BRIEF SUMMARY 
       [0016]    A first embodiment disclosed herein relates to a magneto-optical display. The display includes a plurality of magneto-optical display elements, a cavity configured to house the plurality of magneto-optical display elements and permit rotation of the plurality of magneto-optical display elements, and an optical carrier fluid disposed within the cavity, wherein the plurality of magneto-optical display elements are placed in the optical carrier fluid. The plurality of magneto-optical display elements include a magnetic component, two colored surfaces that are configured to be rotated into a first and second viewable state, a top portion configured to have a density that is less than the optical carrier fluid, such that the top portion is buoyant; and a bottom portion that has a density greater than the density of the optical carrier fluid, such that the bottom portion is non-buoyant. 
         [0017]    A second embodiment disclosed herein relate to a magneto-optical display. The display includes a plurality of reflective bi-stable magneto-optical display elements configured to change from a first viewable color to a second viewable color, the plurality of magneto-optical display elements defining at least a portion of one or more display portions that changes color, one or more display portions that do not change color or that include a shadow, and a front panel including one or more optical elements configured to redirect light from the one or more display portions that do not change colors or include a shadow to the one or more display portions that do change color. 
         [0018]    A third embodiment disclosed herein relate to a magneto-optical display. The display includes a plurality of reflective bi-stable magneto-optical display elements configured to change from a first viewable color to a second viewable color, a frontplane disposed in front of the plurality of magneto-optical display elements, and one or more light sources placed at one or more edges of the frontplane, wherein the frontplane is configured to direct light generated by the one or more light sources towards the plurality of magneto-optical display elements. 
         [0019]    A fourth embodiment disclosed herein relate to a magneto-optical display. The display includes a plurality of reflective bi-stable magneto-optical display elements configured to change from a first viewable color to a second viewable color, wherein the first color is a color configured to at least partially reflect light and the second color is a color configured to at least partially absorb light, a frontplane disposed in front of the plurality of magneto-optical display elements, and one or more color filters placed between the plurality of magneto-optical display elements and an observer of the display, wherein the one more color filters are of a different color than the first and second colors. 
         [0020]    A fifth embodiment disclosed herein relate to a magneto-optical display. The display includes a plurality of reflective bi-stable magneto-optical display elements configured to change from a first viewable color to a second viewable color, frontplane disposed in front of the plurality of magneto-optical display elements, and one or more graphics placed between the plurality of magneto-optical display elements and an observer of the display, wherein changing from the first viewable color to the second viewable color causes the one or more graphics to change from a third viewable color to a fourth viewable color. 
         [0021]    A sixth embodiment disclosed herein relate to a magneto-optical display. The display includes a plurality of magneto-optical display elements configured as one or more pixels, wherein the one or more pixels are configured in a full Red, Green and Blue (RGB) display, and wherein the one or more pixels are configured to be selectively activated when subjected to a magnetic field such that the activated pixels generate a plurality of color combinations viewable in the magneto-optical display. 
         [0022]    This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. 
         [0023]    Additional features and advantages will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by the practice of the teaching herein. The features and advantages of the teaching herein may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims. These and other features will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    To further clarify the above and other advantages and features of the present invention, a more particular description of the invention will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. It is appreciated that these drawings depict only typical embodiments of the invention and are therefore not to be considered limiting of its scope. The invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which: 
           [0025]      FIG. 1A  is a schematic representation of a MOE with varying or segmented density; 
           [0026]      FIG. 1B  is a cross section of a magneto-optical display containing optical fluid; 
           [0027]      FIG. 2  shows a cross section of a magneto-optical display utilizing lenses and reflective separators within the display structure; 
           [0028]      FIG. 3  is a magneto-optical display arrangement utilizing side-let “light-pipe” faceplate; 
           [0029]      FIG. 4A  depicts a magneto-optical display with the white color side of the MOEs facing the viewer whereby color is generated by color filter and  FIG. 4B  is the same display wherein black side of MOEs is facing viewer; 
           [0030]      FIG. 5A  is a cross-sectional representation of a segmented magneto-optical display and  FIG. 5B  is a front view of a segmented display showing an icon segment; and 
           [0031]      FIG. 6  represents an X-Y array of red (R), green (G) and blue (B) sub-pixels arranged in a multi-RGB pixel array. 
       
    
    
     DETAILED DESCRIPTION 
       [0032]    The embodiments disclosed herein are of a significantly different architecture from prior art systems, namely magneto-optical displays and these inventions. It has been shown that a two dimensional array of MOEs (magneto optical elements) can be arranged into a bi-stable, reflective flat panel display frontplane. This display frontplane when combined with electromagnetic write heads can make a very low power, reflective flat panel digital display. The embodiments disclosed herein discuss methods for improving the optical performance and color rendering of this type of display. 
         [0033]    While a reflective, bi-stable, two-color display can be achieved, for many applications a wider range of colors is desirable. Increasing the number of colors displayed requires new methods of construction. Also it is desirable to achieve the highest possible contrast between the color states. And it is desirable to have night illumination as well as daytime reflectance. However, the architecture of the magneto-optical elements (herein referred to as MOEs) in the display frontplane without improvements has limitations that can reduce color contrast, limit the number of colors and make illumination difficult. Some limiting factors that can be improved are: (1) shadows created between MOE segments, (2) the fixed separation wall or spacing between pixels creates an optical “dead space”, (3) displays tend to be limited to two colors because the pixels are bi-stable and change between only two colors, and (4) the opaque nature of the display materials used requires front lighting (cannot be transparent and therefore cannot be backlit). 
         [0034]    In referring to  FIG. 1A , a MOE can have an architecture such that the average specific gravity or the density of the top portion  2  of the MOE is less then the average specific gravity or density of lower portion  1 . The MOE color layers are represented by reference numeral  3 . MOEs made with this type of architecture can be produced in a wider variety of shapes because the alignment mechanism is based on gravity and not mechanical constraint. Shapes may include variations of spherical, tubular or cylindrical. 
         [0035]    In more detail, in referring again to  FIG. 1A , lower portion  1  represents the higher density area of the MOE and in most cases it is advisable to place the permanent magnetic material, which is generally a “rare earth” magnetic material, in this location since this metallic material is higher density then the plastic and colorant materials used in MOE construction. Of course, it will be appreciated that the rare earth magnetic material may be placed in any portion of the MOE. In addition, the density or specific gravity of the magnetic powders used to make the MOEs is generally six or greater. Therefore, as mentioned above, the bottom (denser) portion  1  of the MOE can contain the magnetic powder material (typically a rare-earth permanent magnetic material such as, but not limited to Nd 2 Fe 14 B or Sm 5 Co) which will provide the contrast in weight needed for alignment. Furthermore, it should be noted that a variety of materials including the optical coatings, various body materials that in part make of the MOE, can also be used to increase the density of one portion of the MOE. 
         [0036]    As an added benefit, the magnetic force of the MOE is concentrated primarily in the bottom area of the MOE, which allows the write head (generally a coil) to concentrate the electromagnetic field in just this area of the pixel to create an actuating force which permits higher efficiency because the field can be constrained to a smaller area to actuate the MOEs. In addition, the weight distribution of the MOEs will cause them to self align parallel to gravity and each other, creating a single constrained axis of rotation perpendicular to gravity. 
         [0037]      FIG. 1B  shows a schematic cross-section of a magneto-optical display wherein an optical fluid  4  is used with the cavities the MOEs, designated at  1 , reside in. In this case, the cavity is formed by a face plate  2  and the backplane  3  of a display. The liquid carrier  4  used can be any optically clear fluid that does not damage the MOEs or other materials of construction. Because rare-earth magnetic materials can oxidize, it may be desirable to use a non-aqueous fluid as the protective carrier fluid. 
         [0038]    The carrier fluid can also be used to protect the display against undo friction, water condensation and frost, and it can assist in heat dissipation. Since MOE displays may be used in outdoor conditions, these protective attributes of the carrier fluid can increase reliability. Moreover, it is most desirable to use a non-oxidizing, low viscosity material with good thermal robustness since this material will be exposed to a wide range of operating temperatures due to the displays&#39; outdoor application areas. It should be noted here that the use of an optically clear liquid within the display surrounding the MOEs is disclosed herein and this specific embodiment need not be limited to the use of MOEs which have a weight distribution but any MOE design. 
         [0039]    In place of constrained mechanical alignment of the axis of rotation, MOE particles with a weight distribution in a carrier liquid can self align parallel to gravity. In one embodiment, by creating a heavier bottom portion  1  ( FIG. 1A ) that is not buoyant and a buoyant top portion  2  ( FIG. 1A ) within the MOEs, they will self-align when placed in the liquid and they will tend to rotate along their vertical axis with relation to gravity. It will be appreciated that having the density or specific gravity of the carrier fluid  4  ( FIG. 1B ) be greater than the density or specific gravity of the top portion  2  ( FIG. 1A ) and less than the density or specific gravity of bottom portion  1  ( FIG. 1A ) will cause the top portion  2  to be buoyant and the bottom portion  1  to be non-buoyant. This embodiment allows for easier assembly and the potential to more easily manufacture displays with smaller MOE particles and less stringent mechanical tolerances. Here, for example, of small MOEs of this general architecture are fabricated and placed in such liquid, they will all self-align due to these differences in densities. This can be taken advantage of in unique ways making handling such small MOEs much easier and less costly. In addition, such MOEs in a display device with such liquid will have same effect which can be taken advantage of in adding in the alignment of such MOEs in the display. This same carrier liquid can also improve optics by leveraging the coefficient of refraction of the fluid, can aid in thermal management of the display, reduce MOE to MOE and MOE to display friction and overall display reliability. 
         [0040]    Another embodiment of the present invention is the use of optical elements such as lenses, reflectors or diffusers in the front panel of a reflective, bi-stable magneto-optical display to redirect the light viewed by the observer to the changing color sections of the display and away from the fixed (non-changing) or shadowed areas of the display in order to improve contrast ratio and observed color quality In reference to  FIG. 2 , there are sections of a magneto-optical display that do not change when the MOEs are activated. These include the dividing walls  2  between pixels and/or in some cases, soft magnetic cores that protrude into the display plane. 
         [0041]    In addition to these unchanging walls  2 , there are areas of stronger shadow between the cylindrical MOEs themselves. While the MOEs themselves, designated at  3 , offer very good reflectivity and visibility, the curved edged where the MOEs make contact afford the opportunity for significant light loss by the creation of a shadow where the MOEs meet. The sizes of these display features are fixed and these fixed features and shadows are generally reasonably large (usually over 0.2 millimeters) so their position in the display can be predicted and aligned to. Optical devices and lenses  1  within the display frontplane (such as lenses, sheet magnifiers (Fresnel Lenses) and the like) can be used that align with these undesirable features and optically redirect the light  4  and the observer to view the center, color changing section of the MOEs while “hiding” these unchanging features. 
         [0042]    One method of creating these optical features for redirecting the observer away from undesirable features is to shape the clear front plate of the display that is made of clear optical material into an array of lenses. These lenses can be designed and aligned to emphasize/enlarge the MOEs center sections that have optimal color quality and at the same time these optical features can optically reduce the undesirable features and shadows so the contrast and color seen by the observer is improved. The lens array implementation would assume the observer is standing at a viewing angle approximately in front of the display. Another approach is the use of rows of light diffusers instead of lenses on the front plate of the display glass that partially diffuse the direct viewing of these undesirable optical feature so they are less pronounced. 
         [0043]    Again, referring to  FIG. 2 , in the case of dividing walls  2 , it is also possible to angle the top of the dividing walls and place a reflective surface on these new angled surfaces as also shown in the figure. In this way, the top of the wall acts as two angled mirrors and the observers would then see the reflection of the MOEs on either side of the wall which will change color rather then seeing the fixed edge of the wall that would be unchanging. 
         [0044]    The use of an integrated lighting system for a reflective display that uses one or more LEDs (Light Emitting Diodes) or CCFL (Cold Cathode Florescent Lamps) or other light sources placed to one or more sides of the display, with a clear face plate that acts as both a protective front plate and as a light pipe to carry the LED illumination to provide lighting to the front of the sign is also an embodiment disclosed herein. 
         [0045]    The architecture of a MOE-based display typically requires a clear front plate made of either plastic or glass that can constrain the MOEs, protect the display components, provide adequate space for easy rotation, and permit viewing by the observer through the front of the display. It is well established that both clear plastics and clear glass can function as light pipes. Light pipe assemblies are commonly used to make backlights for LCD displays and other thin backlit panels. The most common light sources used for these types of applications are LEDs (Light Emitting Diodes) and CCFL (Cold Cathode Florescent Lamps). 
         [0046]    In the present invention, a similar architecture is applied to MOE-based displays for front lighting. In referring to  FIG. 3 , the light source, or sources  4  are placed at one or more edges of the display frontplane  3 . In the figure,  1  represents the backplane of the display and  2  the MOEs. Light sources like a string of LEDs at only one edge of the display is the simplest design but light sources at two or more edges may be desirable if the size of display is large and the length that must be covered by the light pipe is long (over 10 inches). 
         [0047]    One important aspect of this invention is to redirect the light from the light pipe toward the display MOEs. This can be done by manufacturing the front plate glass with an angle such that it reflects the light backward toward the MOEs. The light will then be reflected off the MOEs and pass through the front plate to the observer. A second method of directing light toward the MOEs is by making a series of features in the front plate light pipe that disrupts the light flow and redirects the light toward the MOEs in the display. This has been done in light panels by laser scoring, screen printing white reflectors, embossing and/or texturing and the like. To prevent loss of light through the edges of the front plate it is also desirable to place reflectors on the edges of the front plate light pipe that reflects light that has passed to the edge of the light pipe back into the light pipe rather then being lost out the edges. 
         [0048]    The use of white light for the light source will permit the use of colored MOEs and the white light can also be beneficial for lighting any printed graphics that may be adjacent to the MOEs and under the same front plate light pipe as the MOEs. Colored light or multi-colored light can also be used and controlled so that the light reflected off the MOEs can be adjusted in color by the light source. This may be most effective when the MOEs are white and will reflect whatever color they are illuminated with. 
         [0049]    It is also possible to use UV light combined with phosphor pigments in the MOEs themselves. UV light from 230 to 400 nm can be used. Similar to UV light, high frequency blue light from 400-470 nm may be used. In the UV and high frequency blue light ranges it is possible to activate MOEs that contain phosphor pigments in their outside optical layers. This will make the MOEs color light emitting in the visual spectrum. By selecting different colored phosphors for use in the MOEs optical layers, combined with a UV or high frequency blue light source it is possible to make a display using MOEs that are color light emitting in a variety of colors when illuminated by UV and/or high frequency blue light. Phosphor materials (photoluminescent materials) are well known. These materials can be incorporated into the MOEs themselves by means of there production process or by deposition or painting processes. 
         [0050]    Typical phosphor classes for UV and blue light activation include substituted and doped metal oxides, metal sulfides including garnets (YAG, for example YAG:Ce or YAG:Tb) doped with various elements, Silicates (for example Y 2 SiO 5 :Ce) also doped or substituted with various elements to produce target absorption and emission behaviors as well as doped sulfides such as CaS:Eu and a multitude of others. 
         [0051]    MOEs are bi-stable and in their most common implementation they are two colored with different optical surfaces on opposing sides. However, it is possible to have multiple color sections of MOEs in the same display to create a multi-color display. For example, a “don&#39;t walk/walk” sign may have the word “walk” made of MOEs that are green on one side and red on the other. In this example, the word “walk” has two states, green and red. The word “don&#39;t” would have two color states as well. They would be black (equal to off) and red. Therefore the sign could function as a two state sign with the walk state equals “walk” in green and “don&#39;t” in black with a shared black background. The opposing state is “don&#39;t walk” with “don&#39;t” in red and “walk” in red. 
         [0052]    Another example is a transportation sign used in a train station or airport where there are fixed columns for gate, destination, flight/track or the like. Each column can use different sets of MOE colors. This way the sign is easier to read and has a multi-color appearance. The limitation of this approach is that any individual section will be only two colors. 
         [0053]    It has previously been discussed that MOEs of different colors can be used in different sections of a display in order to highlight certain rows or other information using different colors. This way a display can contain various colored sections, not just two colors for the entire display. However, assembly of displays using custom MOEs of different colors can present a manufacturing challenge and inventory problems. 
         [0054]    In many cases the colors of text in each of the different sections may only need to alternate between a black state and a single color state. A method to produce color information displays of this type is by using white and black MOEs across the entire display. Then transparent color filters can be printed over the sections of the display where different colors are desired. This color filter layer only needs to be between the MOEs and the observer. It is likely to (but not a requirement) be printed on the inside of the face plate so it is close to the MOEs and protected. 
         [0055]    In reference to  FIG. 4A , when the white sides of the MOEs  2  are displayed, the reflected light will pass through the intended color filter  4  and the text or graphics will be observed as the color filter color. As represented in  FIG. 4B , when the black side of the MOEs  2  are displayed toward the viewer (outward toward the frontplane  3 ), then the pixel or segment of the display under the transparent color filter will appear black. In this way a complex array of color filters can be printed over a white and black MOE display and various color sections can displayed. An advantage of this invention is that simple printing can replace complex assembly and manufacturing of a variety of MOEs with each having different colored optical layers. In this Figure,  1  represents the magneto-optical display magnetic write head or backplane. 
         [0056]    Another embodiment disclosed herein is utilizing groups of MOEs controlled together to display complex icons and/or graphics, as can be seen in  FIG. 5A  and  FIG. 5B . The icons or graphics can be printed in color or silhouettes either on the front plane  4  or directly on the MOEs themselves. In this case, there is a complex graphic in the display that changes between two images, through the backplane  5  rotating the two to two different color states  1  and  2 , or between an image and a solid color state or between on and off states. 
         [0057]    One such example would be an alarm clock icon used to indicate the alarm is set. Other examples are “on” and “off” indicators. Icons like this can be implemented by controlling a larger block of MOEs that covers the entire icon area. As stated above, one architecture for creating the icons is to directly print the graphic on one side of the MOEs themselves. Ink Jet printing, spray painting and pad printing are proven methods of conformal printing that have been done on curved surfaces. The two graphics indicating the two states of the icon can be printed on the two sides of the MOEs corresponding to the two bi-stable states. 
         [0058]    Another method is to print color image(s) on both sides of the MOEs themselves. This can be done with several conformal printing methods including ink jet printing, pad printing or the like. The MOEs can then alternate between two complex color images. The above methods permit complex graphics and icons that can be controlled and displayed at much higher resolutions then the MOE pixels themselves. 
         [0059]    An additional architecture useful to accomplishing this is to produce a silhouette of the desired icon  3  on a layer of the display that is between the MOEs and the observer as shown in  FIG. 5A . For example, this can be printed on the inside of the front plate of the display. In this way, the observer sees the change in the underlying MOEs color block through the silhouette of the icon graphic. An example is a solid green/black section of MOEs could be placed beneath a black silhouette of the graphic “on”. In this case the area outlining the text is black and the “on” text is left clear so the MOEs can be viewed through the clear outline. When this section of MOEs is switched it will change between a green “on” graphic to all black where the MOEs and outline are the same color. 
         [0060]    Another method is the use of printed, transparent color images and MOEs with a white side. This icon image can be full color as long as it uses transparent color filter inks (like CMYK color printing). The MOEs underneath this color icon section can switch between white and black. In this way when the MOEs are in the white state the full color, transparent icon image will be viewed. When the MOEs turn to the black state the transparent color icon will turn off (go black). 
         [0061]    As displays grow in size it is possible to start blending colors through juxtaposition of color sub-pixels. A pixel or segment is divided into separately controlled sub-pixels. Each of these sub-pixels has a different set of two colors. The size of the pixels have to be small enough that at the normal viewing distance the eye of the observer will blend the sub-pixels together. This principle is used in most color printing and color displays. A simple example of this embodiment would be a pixel that has two sub-pixels. The colors of the sub-pixels are black and yellow for sub-pixel one, and white and green for sub-pixel two. Four combinations of colors can be created by this pixel example with these two, bi-stable sub-pixels: black and white=grey, yellow and white=light yellow, black and green=dark green, white and green=light green. By increasing the number of sub-pixels greater control and greater numbers of colors can be created. 
         [0062]    Full Color RGB Display using MOE Sub-Pixels 
         [0063]    Using the same principles disclosed above, and in referring to  FIG. 6 , it is possible to create a full RGB display using MOE subpixels. The simplest method is having three sub-pixels per pixel (red and black, green and black, and blue and black). In the figure is shown an array of red (R), green (G) and blue (B) sub-pixels, grouped together as RGB pixels and thus a pixel array. Here, the MOEs themselves are two-color, red-black, green-black and blue-black. This can be accomplished by either using colored MOEs or by using the color filter approach described above wherein the MOEs themselves are all white and black with color filters printed above them, or this can be accomplished for light emitting applications through the use of Red, Green and Blue emitting phosphors similar to what has been disclosed above. Because MOE pixels are bi-stable only solid colors are achieved. To create grey scales and full color images dot patterns, dithering half-tone patterns such as error diffusion or stochastic patterns can be used to create full color images from fixed size digital pixels. This general approach is known to those educated in the state of the art yet has never been applied to such a display as disclosed here. 
         [0064]    The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, 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.