Abstract:
An optical noise reduction mechanism for reducing undesired frustration of total internal reflected light. Such optical noise may stem from defects in waveguide construction. Such optical noise may also stem from the difference in refractive index between any cladding layers disposed onto the planar waveguide and the refractive index of the medium (e.g., air) between the light sources and the light insertion surface of the planar waveguide. By interposing a material of appropriate refractive index, either as a thin layer onto the light insertion surface of the waveguide or filling the space between the waveguide and the light source, the planar waveguide becomes more tolerant of geometry errors and cladding layer properties because a safe operating area is established between the unadjusted critical angle of the system and the actual range of ray angles allowed admittance into the waveguide.

Description:
TECHNICAL FIELD 
     The present invention relates in general to the field of flat panel displays, and more particularly to reducing the light leakage and improving contrast ratio performance in frustrated total internal reflection (FTIR) display devices. 
     BACKGROUND INFORMATION 
     Flat panel displays and other devices that exploit the principle of frustrated total internal reflection (FTIR) to induce the emission of light from the system may have to satisfy crucial physical criteria to function properly. The display system disclosed in U.S. Pat. No. 5,319,491, which is incorporated by reference in its entirety herein, as representative of a larger class of FTIR-based devices, illustrates the fundamental principles at play within an FTIR-based device. Such a device is able to selectively frustrate the light undergoing total internal reflection within a (generally) planar waveguide. When such frustration occurs, the region of frustration constitutes a pixel suited to external control. A rectangular array of such regions, which are often controlled by electrical/electronic means, is fabricated upon the top active surface of the planar waveguide. This aggregate structure, when suitably configured, functions as a video display capable of color generation usually by exploiting field sequential color and pulse width modulation techniques. 
     The criteria to be satisfied for FTIR systems to function properly involve two fundamental areas: the preconditions for frustration, and the preconditions for non-frustration. There are many mechanisms available to frustrate total internal reflection (five of which are articulated in U.S. Pat. No. 5,319,491), all of which lead to a pixel being in an “on state” (emitting light through the “window” dynamically created in the planar waveguide). At issue is the physical configuration to secure a suitable “off state” where light is intended to remain within the planar waveguide across a given pixel region. 
     The off state (quiescent, inactive state) of individual pixels on a display, and indeed of the display in general, is of the highest importance. If some light is always leaking (by spurious emission, frustration, or other cause) from the display (at the pixels, between the pixels, or in general), this constitutes system noise that compromises the quality of the signal. The contrast ratio of a display is based on its signal-to-noise ratio, and contrast ratio serves as a primary index of display quality and accuracy. Therefore, if an FTIR display emits noise (light when and where no light is supposed to be emitted), this harms the display&#39;s quality. 
     Noise arises when total internal reflection is frustrated when and where it should not be. Different causes can give rise to such system noise, and in most displays more than one cause is operative to add to the noise level. For example, the optical quality of the material selected for the planar waveguide has a direct bearing on noise. If the material has many scattering domains distributed through it (so that it becomes more translucent than transparent), some of the light scattered off these domains will be scattered at angles that do not conserve total internal reflection. For this reason, the waveguide will glow in proportion to the amount of scattering domains distributed within it, thereby raising the noise floor. The solution to this problem is to fabricate the waveguide from the most optically transmissive materials available, thereby securing a meaningful reduction of the noise floor with respect to this specific source of system noise. 
     Other noise sources within FTIR systems do not have so straightforward a solution route. The first involves errors in waveguide geometry (the limits of parallelism and orthogonality), while the second involves noise at the interface of the waveguide and any superadded cladding layers (which can serve to support various required pixel control mechanisms, protect the display surface from external trauma, and/or other purposes). These are sources of system noise (light leakage) that do not have a straightforward solution route. 
     Therefore, there is a need in the art for a means to reduce light leakage (system noise), and thus improve contrast ratio performance, in FTIR display devices where the leakage is due to geometric imperfections in waveguide fabrication and/or leakage at the interface of the waveguide to superadded cladding structures. 
     SUMMARY 
     The problems outlined above may at least in part be solved by interposing material with a tuned refractive index between the light insertion face of the planar waveguide and the light sources that illuminate the waveguide. The interposition of this layer subtly restricts the admissible range of angles entering the planar waveguide. As a result of this restriction, the system becomes more tolerant of errors in geometry. Furthermore, noise generated at the interface of the planar waveguide with any superadded cladding layers can be reduced or eliminated, to the extent the interposed material approaches, matches, or exceeds the refractive index of the cladding material itself. The tuning of the interposed material&#39;s refractive index is optimized by simultaneously maximizing system noise reduction and maximizing the angular range of light that is allowed to enter the waveguide. 
     In one embodiment of the present invention, an FTIR device comprises a rectangular solid planar waveguide composed of transparent, optically transmissive material, one of the four smaller surfaces of which is designated the insertion surface while one of the two large surfaces is designated the display surface where light emission via FTIR is effected. Further, the waveguide may have mirrored surfaces on up to three of the smaller surfaces other than the insertion surface, whether metallic or dielectric in nature. The FTIR device may further comprise one or more light sources (which may or may not be dynamically controllable as to color and cycle frequency), where such light source(s) is/are spaced apart in relation to the insertion surface of the planar waveguide, such that light is directed into the planar waveguide through the insertion face, typically with an air gap or other light coupling means between waveguide insertion face and light source(s). Further, light from the light sources can only enter the planar waveguide at angles where the Fresnel insertion loss is not total, i.e., at TIR-compliant angles. The FTIR device may further comprise one or more TIR frustration mechanisms distributed upon the display surface, the simplest example of which is an elastic polymer membrane with a refractive index close to that of the waveguide which can be mechanically propelled, via controllable electric switching, into and out of contact with the waveguide, such that contact (or near contact) frustrates TIR and causes light within the waveguide to pass into the membrane and (depending on local geometries optimized for optical emission angle) out to the viewer. The TIR-frustrating mechanism(s) may further comprise a stand-off system that keeps, for example, the afore-mentioned elastic polymer in spaced-apart relation to the waveguide in the off-state, until it is electrically actuated and propelled into contact or near-contact with the waveguide. Further, this stand-off system comprises an optical cladding layer, configured with a lower (perhaps significantly lower) refractive index than either the waveguide or the TIR frustration mechanism (e.g., elastic polymer membrane being dynamically deformed into and out of contact with the waveguide). The waveguide may exhibit errors in geometric construction (errors in parallelism between the three sets of opposing surfaces, and errors in orthogonality at all surfaces at a putative 90 degrees separation one from the other). The interface between the cladding (stand-off system) and the waveguide may itself be subject to mild frustration of TIR because the refractive index of the cladding is greater than that of the refractive index of the air between the light sources and the waveguide insertion face, said mismatch meaning the critical angle for TIR is lower than expected at the display surface. Geometric error and mismatch in refractive index between the cladding and waveguide are sources of system noise, both of which can be reduced by a mechanism for TIR noise reduction. This system for TIR noise reduction may comprise a transparent material of refractive index equal to or slightly greater than the refractive index of the cladding layer. Further, this transparent material is interposed between the insertion face of the waveguide and the light source(s), such that the angular range of light admitted in the waveguide is sufficiently restricted to compensate for geometric errors in waveguide construction as well as for the fact that the cladding&#39;s refractive index is greater than that of air. This interposed, refractive-index-tuned interlayer may either be a simple layer added to the insertion face (with light sources still disposed in the air), or may also completely encapsulate both the insertion face and the light sources so that at no point does light emitted from the light sources travel in air, due to the embedding of the light sources within this interposed material. The addition of this noise reduction mechanism improves the contrast ratio and signal-to-noise ratio of the composite system by reducing noise caused by undesired (parasitic) frustration of TIR. 
     The foregoing has outlined rather broadly the features and technical advantages of one or more embodiments of the present invention in order that the detailed description of embodiments of the present invention that follows may be better understood. Additional features and advantages of embodiments of the present invention will be described hereinafter which form the subject of the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A better understanding of the present invention can be obtained when the following detailed description is considered in conjunction with the following drawings, in which: 
         FIG. 1  illustrates an embodiment of the present invention of an exploded view of a noise reduction mechanism; 
         FIG. 2A  illustrates an embodiment of the present invention of an assembled view of the noise reduction mechanism; 
         FIG. 2B  illustrates another embodiment of the present invention of an assembled view of the noise reduction mechanism; 
         FIG. 3  illustrates a perspective view of a flat panel display in accordance with an embodiment of the present invention; 
         FIG. 4A  illustrates a side view of a pixel in a deactivated state in accordance with an embodiment of the present invention; 
         FIG. 4B  illustrates a side view of a pixel in an activated state in accordance with an embodiment of the present invention; and 
         FIG. 5  illustrates a data processing system in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, numerous specific details are set forth to provide a thorough understanding of the present invention. However, it will be apparent to those skilled in the art that the present invention may be practiced without such specific details. In other instances, components have been shown in generalized form in order not to obscure the present invention in unnecessary detail. For the most part, details considering considerations of controlled selective dynamic frustration of total internal reflection (i.e., actual pixel operation) and the like have been omitted inasmuch as such details are not necessary to obtain a complete understanding of the present invention and, while within the skills of persons of ordinary skill in the relevant art, are not directly relevant to the utility and value provided by the present invention. 
     The principles of operation to be disclosed immediately below assume the presence of at least one of the two deleterious noise sources that can arise within frustrated total internal reflection (FTIR) devices that can be suitably mitigated by deploying the present invention. These noise sources are undesired TIR frustration due to errors in parallelism and orthogonality in the fabrication of the waveguide, and undesired TIR frustration due to the difference in refractive index between any cladding (stand-off mechanisms) disposed directly on the waveguide display surface (usually higher than 1.0) and the refractive index of air (nominally 1.0). 
     Among the technologies (flat panel display or other candidate technologies that exploit the principle of frustrated total internal reflection) that lend themselves to implementation of the present invention is the flat panel display disclosed in U.S. Pat. No. 5,319,491, which is hereby incorporated herein by reference in its entirety. The use of a representative flat panel display example throughout this detailed description shall not be construed to limit the applicability of the present invention to that field of use, but is intended for illustrative purposes as touching the matter of deployment of the present invention. 
     Such a representative flat panel display may comprise a matrix of optical shutters commonly referred to as pixels or picture elements as illustrated in  FIG. 3 .  FIG. 3  illustrates a simplified depiction of a flat panel display  300  comprised of a light guidance substrate  301  which may further comprise a flat panel matrix of pixels  302 . Behind the light guidance substrate  301  and in a parallel relationship with substrate  301  may be a transparent (e.g., glass, plastic, etc.) substrate  303 . It is noted that flat panel display  300  may comprise other elements than illustrated such as a light source, an opaque throat, an opaque backing layer, a reflector, and tubular lamps, as disclosed in U.S. Pat. No. 5,319,491. 
     Each pixel  302 , as illustrated in  FIGS. 4A and 4B , may comprise a light guidance substrate  401 , a ground plane  402 , a deformable elastomer layer  403 , and a transparent electrode  404 . 
     Pixel  302  may further comprise a transparent element shown for convenience of description as disk  405  (but not limited to a disk shape), disposed on the top surface of electrode  404 , and formed of high-refractive index material, preferably the same material as comprises light guidance substrate  401 . 
     In this particular embodiment, it is necessary that the distance between light guidance substrate  401  and disk  405  be controlled very accurately. In particular, it has been found that in the quiescent state, the distance between light guidance substrate  401  and disk  405  should be approximately 1.5 times the wavelength of the guided light, but in any event this distance is greater than one wavelength. Thus the relative thicknesses of ground plane  402 , deformable elastomer layer  403 , and electrode  404  are adjusted accordingly. In the active state, disk  405  is pulled by capacitative action, as discussed below, to a distance of less than one wavelength from the top surface of light guidance substrate  401 . 
     In operation, pixel  302  exploits an evanescent coupling effect, whereby TIR (Total Internal Reflection) is violated at pixel  302  by modifying the geometry of deformable elastomer layer  403  such that, under the capacitative attraction effect, a concavity  406  results (which can be seen in  FIG. 4B ). This resulting concavity  406  brings disk  405  within the limit of the light guidance substrate&#39;s evanescent field (generally extending outward from the light guidance substrate  401  up to one wavelength in distance). The electromagnetic wave nature of light causes the light to “jump” the intervening low-refractive-index cladding, i.e., deformable elastomer layer  403 , across to the coupling disk  405  attached to the electrostatically-actuated dynamic concavity  406 , thus defeating the guidance condition and TIR. Light ray  407  (shown in  FIG. 4A ) indicates the quiescent, light guiding state. Light ray  408  (shown in  FIG. 4B ) indicates the active state wherein light is coupled out of light guidance substrate  401 . 
     The distance between electrode  404  and ground plane  402  may be extremely small, e.g., 1 micrometer, and occupied by deformable layer  403  such as a thin deposition of room temperature vulcanizing silicone. While the voltage is small, the electric field between the parallel plates of the capacitor (in effect, electrode  404  and ground plane  402  form a parallel plate capacitor) is high enough to impose a deforming force on the vulcanizing silicone thereby deforming elastomer layer  403  as illustrated in  FIG. 4B . By compressing the vulcanizing silicone to an appropriate fraction, light that is guided within guided substrate  401  will strike the deformation at an angle of incidence greater than the critical angle for the refractive indices present and will couple light out of the substrate  401  through electrode  404  and disk  405 . 
     The electric field between the parallel plates of the capacitor may be controlled by the charging and discharging of the capacitor which effectively causes the attraction between electrode  404  and ground plane  402 . By charging the capacitor, the strength of the electrostatic forces between the plates increases thereby deforming elastomer layer  403  to couple light out of the substrate  401  through electrode  404  and disk  405  as illustrated in  FIG. 4B . By discharging the capacitor, elastomer layer  403  returns to its original geometric shape thereby ceasing the coupling of light out of light guidance substrate  401  as illustrated in  FIG. 4A . 
     As stated in the Background Information section, certain devices that exploit the principle of frustrated total internal reflection lend themselves to contrast ratio enhancement using the present invention, whereby undesired frustration of total internal reflection (whether pixels are in the activated or quiescent state) is significantly attenuated. A pertinent example that will be used throughout this disclosure to illustrate the operative principles in question is shown in  FIG. 1  in exploded view, and in  FIG. 2  in assembled view. 
     It should be understood that this optical example, proceeding from U.S. Pat. No. 5,319,491, is provided for illustrative purposes as a member of a class of valid candidate applications and implementations, and that any device, comprised of any system exploiting the principle of frustrated total internal reflection, can be enhanced with respect to signal-to-noise ratio, contrast ratio, and parasitic system noise where such noise is due to geometry errors in waveguide fabrication or refractive index mismatch between the cladding layer and air. The present invention governs a mechanism for noise reduction for a large family of devices that meet certain specific operational criteria regarding the implementation of FTIR principles, while the specific reduction to practice of any particular device being so enhanced imposes no restriction on the ability of the present invention to reduce optical noise within the device. 
       FIG. 1  depicts, in exploded view, an embodiment of the present invention of a noise reduction mechanism  100 . Mechanism  100  includes a planar waveguide,  104 , which, being a rectangular solid, necessarily has six sides. Waveguide  104  has two large sides designated as  110  and  107 . Waveguide  104  further includes two opposing pairs of remaining sides,  109  and  105 , and  106  and  108 . Side  108  is referred to herein as the “display surface.” Side  105  is referred to herein as the “light insertion surface.” A light source  112  is situated in spaced-apart relation to the light insertion surface  105 , such that light emitted from light source  112  will be incident upon side  105 . The light incident upon side  105  will obey the Fresnel laws concerning reflection and refraction as a function of incidence angle and the respective refractive indices of the material comprising planar waveguide  104  and the medium (presumably air) between light source  112  and the light insertion face  105 . Planar waveguide  104  corresponds in principle to light guidance substrate  401  in  FIGS. 4A-B , excepting that the level of detail (individual identification of the component&#39;s six surfaces, etc.) is made more explicit in  FIG. 1 . 
     A thin cladding layer  101  is deposited on planar waveguide  104 . The material comprising cladding layer  101  has a significantly lower refractive index than planar waveguide  104 . In one embodiment, cladding layer  101  includes sol-gel. In another embodiment, cladding layer  101  includes aerogel. Cladding layer  101  generally corresponds in principle to cladding layer  403  of  FIGS. 4A-B . Cladding layer  101  may also be configured to include voids, such as represented by cavities  102  and  103 , thereby constituting the cladding layer a standoff region serving to keep other layers superadded on top of cladding  101  at a fixed distance from planar waveguide  104 , but allowing sufficient degrees of freedom for the layers to move freely (via deformation or other mechanical/geometrical alteration) toward planar waveguide  104 . It should be noted that the size, quantity, and distribution of such voids is a function of the intended purpose of the device the present invention is to be deployed upon. For flat panel display applications, such voids would be arranged in a rectangular array centered on each putative pixel region, and the number of such voids can easily exceed one million. Therefore, the voids (holes) in cladding layer  102  and  103  are representative for a far larger class of modifications to the geometry of cladding layer  101  to cause it to satisfy other requirements (e.g., electromechanical standoff functionality, etc.). 
     Although the refractive index of cladding layer  101  may be significantly lower than that of planar waveguide  104 , it is not likely to equal the refractive index of the medium (presumably air) through which light travels from light source  112  toward light insertion face  105 . Consequently, by Snell&#39;s law, the angular range of light rays inserted into planar waveguide  104  exceeds by a small amount the angular range of light rays that satisfy the conditions for containment within the planar waveguide by the laws of total internal reflection. This result is obtained because total internal reflection is a function of the ratio of refractive indices across a boundary interface between two different materials, and the refractive index ratio of air to planar waveguide  104  does not match the refractive index ratio of cladding layer  101  to planar waveguide  104 . Consequently, the contact plane between waveguide  104  and cladding layer  101  will evidence a small but detectible amount of frustrated total internal reflection, excepting in regions where the cladding material is not in actual contact with the planar waveguide by virtue of imposed voids (such as the holes represented at  102  and  103 ). This effect raises the system noise floor. 
     Furthermore, errors in parallelism between the opposing pairs of surfaces comprising planar waveguide  104  can also give rise to unintended and undesired frustration of total internal reflection. Fabrication errors that cause surfaces  105  and  109 , and/or surfaces  107  and  108 , and/or surfaces  110  and  107 , not to be parallel with one another, entails a change in incidence angle that accumulates with each internal reflection of light rays traveling inside planar waveguide  104 . This cumulative error will ultimately place rays beyond the threshold for total internal reflection to occur. At that point, noncompliant rays that fail the Snell&#39;s law criterion for total internal reflection will contribute to the system noise floor. This same issue arises with respect to orthogonality: if the four smaller surfaces  105 ,  109 ,  108  and  106  are not each and severally perpendicular to the plane of the large surfaces  110  and  107 , light rays traveling inside planar waveguide  104  will likewise exhibit accumulating error after each reflection event, which can lead to unintended frustration of total internal reflection and thus raise the system noise floor. 
     The present invention deploys an additional component, the purpose of which is to slightly restrict the angular range of light being inserted into planar waveguide  104 . This angular restriction provides a “safe operating area,” an intrinsic tolerance, to the system. The need for just such a safe operating area is easy to illustrate. For example, if the critical angle (based on Snell&#39;s law) for a given planar waveguide were 42 degrees, light from the light source  112  would enter planar waveguide  104  through light insertion face  105  at a range of angles between 0 and 42 degrees. If errors in parallelism or orthogonality caused some of the rays to stray 2 degrees prior to eventual exploitation (propagation through an opened pixel) or depletion (e.g., by scattering or degradation to thermal energy), then rays between 42 and 44 degrees will arise within planar waveguide  104  and will depart the system as unwanted noise. Furthermore, the critical angle is not 42 degrees at the contact plane between the cladding  101  and planar waveguide  104 : it might be a lower value, like 40 degrees, because the cladding&#39;s refractive index will be higher than that of air. Therefore, light rays between 40 and 42 degrees (or 44 degrees, if parallelism errors be simultaneously present) will leave planar waveguide  104  as noise due to unintended frustration of total internal reflection. 
     On the other hand, if it were possible to restrict incoming light by a small angular amount, the perturbations caused by geometry errors and differential refractive index ratios can be safely accommodated. For instance, if the light entering planar waveguide  104  were restricted to 38 degrees, which is 4 degrees less than the 42 degree critical angle described above, the system could tolerate the light rays straying up to 2 degrees due to geometry errors (the range would rise from 38 degrees to 40 degrees) and would simultaneously tolerate the shifted critical angle at the boundary between planar waveguide  104  and cladding layer  101  (which, in the example above, is a 2 degree shift from 42 degrees to 40 degrees). This adjustment of the angular range of inserted light, then, would be sufficient to remove noise from both of these potential contributing factors. Light inside planar waveguide  104  will not undergo undesired frustrated total internal reflection due to either of these effects, if the insertion angles are properly restricted. Therefore, only desired frustration of total internal reflection (contingent upon pixel actuation, which is beyond the scope of the present invention to explicate) will occur. The present invention, as disclosed earlier, does not attenuate noise due to planar waveguide  104  material falling short of 100% transmissivity. To the extent waveguide  104  scatters light traveling within it, due to intrinsic properties of its constituent material, waveguide  104  will be “noisy.” This particular noise source can only be attenuated using better grade materials, higher refractive index materials, or opaque masking being situated around the pixels in such a way as absorb noise but not otherwise perturb light within guide  104  traveling at TIR-conserving angles. Note that noise can also arise due to excessively high surface roughness of waveguide&#39;s  104  surfaces, the most straightforward remedy for which is improved manufacturing quality control. 
     A component that is important to the present invention is the addition of a material of suitable refractive index between light insertion face  105  and light source  112 . This material makes intimate contact with light insertion face  105 . One embodiment of such a structure can be a thin layer added to light insertion face  105 , as represented by structure  111  in  FIG. 1 . In this embodiment, the gap between light insertion surface  105  and light source  112  is filled with the material of a suitable refractive index. The refractive index of the material that comprises  111  (whether construed as a plane of material bonded or otherwise in intimate contact with surface  105 , or a larger mass of material extending from surface  105  and encapsulating light source  112 ) is selected to optimize noise reduction based on the actual tolerance required in the system to achieve desired results. If geometric errors are not a significant noise source, one can select the refractive index of material  111  to be equal to that of cladding  101 . To provide additional immunity to noise, the refractive index of  111  can be chosen slightly higher than that of cladding  101 . If errors in parallelism and orthogonality are anticipated, noise can be reduced by choosing a correspondingly higher value for the refractive index of material  111 . In one embodiment, material  111  includes sol-gel. In another embodiment, material  111  includes aerogel. 
     While  FIG. 1  illustrates key components in exploded view fashion,  FIG. 2A  illustrates an embodiment of the present invention of a noise reduction mechanism  200  (minus such voids within the cladding, like  102  and  103  in  FIG. 1 ) in an assembled state. Mechanism  200  includes cladding layer  201  which corresponds to cladding layer  101  ( FIG. 1 ). Mechanism  200  further includes planar waveguide  202  which corresponds to the entire planar waveguide  104  ( FIG. 1 ). Mechanism  200  further includes a light source  204  which corresponds to light source  112  ( FIG. 1 ). Mechanism  200  further includes tolerance-enhancing material (noise reduction layer)  203  which corresponds to the tolerance-enhancing material  111  ( FIG. 1 ). Light emitted from lamp  204  passes through noise reduction layer  203  before entering planar waveguide  202 . This trajectory through noise reduction layer  203  restricts the range of admissible angles entering waveguide  202 , where the maximum admitted ray angle is less than the critical angle for total internal reflection within planar waveguide  202 . Where the refractive index of noise reduction layer  203  is properly selected, this angular restriction provides intrinsic system immunity to noise caused by errors in parallelism and orthogonality in waveguide  202 , as well as undesired frustration of total internal reflection at the interface between waveguide  202  and cladding layer  201 . In no case should the refractive index of noise reduction layer  203  be less than that of cladding layer  201 . The extent to which it may exceed the value is dependent on the degree of anticipated geometric error in fabricating a perfect rectangular solid  202 , and the extent to which engineering concerns mandate the introduction of additional safe operating area. Note that there are penalties for choosing the refractive index of noise reduction layer  203  too high, since most planar waveguides benefit from maximizing the angular range of light rays traveling within them. It is therefore appropriate to adjust the refractive index of noise reduction layer  203  so that it is only as large as needed to secure meaningful noise reduction, without otherwise harming system operation.  FIG. 2B  illustrates noise reduction mechanism  210  where a noise reduction layer  211  extends from an edge surface of planar waveguide  202  an amount sufficient to enclose at least the emitter portion of lamp  204 . 
     A representative hardware environment for practicing the present invention is depicted in  FIG. 5 , which illustrates an exemplary hardware configuration of data processing system  513  in accordance with the subject invention having central processing unit (CPU)  510 , such as a conventional microprocessor, and a number of other units interconnected via system bus  512 . Data processing system  513  includes random access memory (RAM)  514 , read only memory (ROM)  516 , and a disk adapter  518  for connecting peripheral devices such as disk unit  520  to bus  512 , user interface adapter  522  for connecting keyboard  524 , mouse  526 , and/or other user interface devices such as a touch screen device (not shown) to bus  512 , communication adapter  534  for connecting data processing system  513  to a data processing network, and display adapter  536  for connecting bus  512  to display device  538 . Display device  538  may implement any of the embodiments described herein. Any of the displays described herein may include pixels such as shown in  FIGS. 4A and 4B . CPU  510  may include other circuitry not shown herein, which will include circuitry commonly found within a microprocessor, e.g., execution unit, bus interface unit, arithmetic logic unit, etc. CPU  510  may also reside on a single integrated circuit.