Abstract:
A gyricon or twisting-particle display based on nonspheroidal (e.g., substantially cylindrical) optically anisotropic particles disposed in a substrate. The particles can be either bichromal or polychromal cylinders, preferably aligned parallel to one another and packed close together in a monolayer. A rotatable disposition of each particle is achievable while the particle is thus disposed in the substrate; for example, the particles can already be rotatable in the substrate, or can be rendered rotatable in the substrate by a nondestructive operation performed on the substrate. In particular, the substrate can be made up of an elastomer that is expanded by application of a fluid thereto so as to render the particles rotatable therein. A particle, when in its rotatable disposition, is not attached to the substrate. The close-packed monolayer configuration of particles provides excellent brightness characteristics and relative ease of manufacture as compared with certain other high-brightness gyricon displays. The substrate containing the cylinders can be fabricated with the swelled-elastomer techniques known from spherical-particle gyricon displays, with a simple agitation process step being used to align the cylinders within the sheeting material. Techniques for fabricating the cylinders are also disclosed.

Description:
RELATED APPLICATION AND PRIORITY 
     THIS APPLICATION is a continuation in part of and therefore entitled to the filing date of copending U.S. patent application Ser. No. 08/716,672 titled “Twisting Cylinder Display” filed Sep. 13, 1996 now U.S. Pat. No. 6,055,091 which claimed priority from the following U.S. provisional applications having the same assignee and at least one common inventor: No. 60/020,651, filed Jun. 27, 1996; and No. 60/020,522, also filed Jun. 27, 1996. 
     INCORPORATION BY REFERENCE 
     The following U.S. patents are fully incorporated herein by reference: U.S. Pat. No. 4,126,854, (Sheridon, “TWISTING BALL PANEL DISPLAY”); U.S. Pat. No. 4,143,103 (Sheridon, “METHOD OF MAKING A TWISTING BALL PANEL DISPLAY”); U.S. Pat. No. 5,262,098 (Crowley et al., “METHOD AND APPARATUS FOR FABRICATING BICHROMAL BALLS FOR A TWISTING BALL DISPLAY”); U.S. Pat. No. 5,344,594 (Sheridon, “METHOD FOR THE FABRICATION OF MULTICOLORED BALLS FOR A TWISTING BALL DISPLAY”); and U.S. Pat. No. 5,389,945 (Sheridon, “WRITING SYSTEM INCLUDING PAPER-LIKE DIGITALLY ADDRESSED MEDIA AND ADDRESSING DEVICE THEREFOR”), U.S. patent application Ser. No. 08/572,779, entitled “POLYCHROMAL SEGMENTED BALLS FOR A TWISTING BALL DISPLAY”; U.S. patent application Ser. No. 08/572,778, entitled “APPLICATIONS OF A TRANSMISSIVE TWISTING BALL DISPLAY”; U.S. patent application Ser. No. 08/572,819, entitled “CANTED ELECTRIC FIELDS FOR ADDRESSING A TWISTING BALL DISPLAY”; U.S. patent application Ser. No. 08/572,927, entitled “HIGHLIGHT COLOR TWISTING BALL DISPLAY” U.S. patent application Ser. No. 08/572,912, entitled “PSEUDO-FOUR COLOR TWISTING BALL DISPLAY”; U.S. patent application Ser. No. 08/572,820, entitled “ADDITIVE COLOR TRANSMISSIVE TWISTING BALL DISPLAY”; U.S. patent application Ser. No. 08/572,780, entitled “SUBTRACTIVE COLOR TWISTING BALL DISPLAY”; U.S. patent application Ser. No. 08/572,775, entitled “MULTITHRESHOLD ADDRESSING OF A TWISTING BALL DISPLAY”; U.S. patent application Ser. No. 08/572,777, entitled “FABRICATION OF A TWISTING BALL DISPLAY HAVING TWO OR MORE DIFFERENT KINDS OF BALLS”; and U.S. patent application Ser. No. 08/573,922, entitled “ADDITIVE COLOR TRISTATE LIGHT VALVE TWISTING BALL DISPLAY.” All filed concurrently on Dec. 15, 1995, and two divisional applications from U.S. patent application Ser. No. 08/572,779, entitled “POLYCHROMAL SEGMENTED BALLS FOR A TWISTING BALL DISPLAY”, “POLYCHROMAL SEGMENTED BALLS FOR A TWISTING BALL DISPLAY” now U.S. Pat. No. A,AAA,AAA, U.S. patent application Ser. No. 08/BBB,BBB, entitled “APPARATUS FOR FABRICATING POLYCHROMAL SEGMENTED BALLS FOR A TWISTING BALL DISPLAY” filed on Jul. 10 th , 1997. 
     RELATED PATENT APPLICATIONS 
     The following copending, coassigned U.S. Patent Applications are related to this case: U.S. patent application Ser. No. 08/713,935, entitled “MONOLAYER GYRICON DISPLAY”; U.S. patent application Ser. No. 08/713,936, entitled “HIGH REFLECTANCE GYRICON DISPLAY”; U.S. patent application Ser. No. 08/716,675, entitled “GYRICON DISPLAY WITH INTERSTITIALLY PACKED PARTICLE ARRAYS”; and U.S. patent application Ser. No. 08/713,325, entitled “GYRICON DISPLAY WITH NO ELASTOMER SUBSTRATE.” 
    
    
     BACKGROUND OF THE INVENTION 
     The invention pertains to visual displays and more particularly to twisting-ball displays, such as gyricon displays and the like. 
     Gyricon displays, also known by other names such as electrical twisting-ball displays or rotary ball displays, were first developed over twenty years ago. See U.S. Pat. Nos. 4,126,854 and 4,143,103, incorporated by reference hereinabove. 
     An exemplary gyricon display  10  is shown in side view in FIG. 1A (PRIOR ART). Bichromal balls  1  are disposed in an elastomer substrate  2  that is swelled by a dielectric fluid creating cavities  3  in which the balls  1  are free to rotate. The balls  1  are electrically dipolar in the presence of the fluid and so are subject to rotation upon application of an electric field, as by matrix-addressable electrodes  4   a ,  4   b . The electrode  4   a  closest to upper surface  5  is preferably transparent. An observer at I sees an image formed by the black and white pattern of the balls  1  as rotated to expose their black or white faces (hemispheres) to the upper surface  5  of substrate  2 . 
     A single one of bichromal balls  1 , with black and white hemispheres  1   a  and  1   b , is shown in FIG. 1B (PRIOR ART). 
     Gyricon displays have numerous advantages over conventional electrically addressable visual displays, such as LCD and CRT displays. In particular, they are suitable for viewing in ambient light, retain an image indefinitely in the absence of an applied electric field, and can be made lightweight, flexible, foldable, and with many other familiar and useful characteristics of ordinary writing paper. Thus, at least in principle, they are suitable both for display applications and for so-called electric paper or interactive paper applications, in which they serve as an electrically addressable, reuseable (and thus environmentally friendly) substitute for ordinary paper. For further advantages of the gyricon, see U.S. Pat. No. 5,389,945, incorporated by reference hereinabove. 
     Known gyricon displays employ spherical particles (e.g., bichromal balls) as their fundamental display elements. There are good reasons for using spherical particles. In particular: 
     Spherical bichromal balls can be readily manufactured by a number of techniques. See the &#39;098 and &#39;594 patents, incorporated by reference hereinabove, in this regard. 
     Spheres are symmetrical in three dimensions. This means that fabrication of a gyricon display sheet from spherical particles is straightforward. It is only necessary to disperse the balls throughout an elastomer substrate, which is then swelled with dielectric fluid to form spherical cavities around the balls. The spherical balls can be placed anywhere within the substrate, and at any orientation with respect to each other and with respect to the substrate surface. There is no need to align the balls with one another or with the substrate surface. Once in place, a ball is free to rotate about any axis within its cavity. 
     “In the ‘white’ state, the gyricon display reflects almost entirely from the topmost layer of bichromal balls and, more particularly, from the white hemispherical upper surfaces of the topmost layer of balls. In a preferred embodiment, the inventive display is constructed with a single close-packed monolayer of bichromal balls.” 
     Ideally, a close-packing arrangement would entirely cover the plane with the monolayer of gyricon elements. However,. Inasmuch as a planar array of spheres cannot fully cover the plane, but must necessarily contain interstices, the best that can be achieved with a single population of uniform-diameter spherical elements is about 90.7 percent areal coverage, which is obtained with a hexagonal packing geometry. A second population of smaller balls can be added to fill in the gaps somewhat, but this complicates display fabrication and results in a tradeoff between light losses due to unfilled interstices and light losses due to absorption by the black hemispheres of the smaller interstitial balls. 
     Therefore, it would be desirable to provide a close-packed monolayer gyricon display in which areal coverage surpasses 90.7 percent or approaches 100 percent, without any need for interstitial particles. This can be done by using cylindrical rather than spherical elements. For example, a rectangular planar monolayer array of cylinders can be constructed that entirely or almost entirely covers the plane. With the white faces of the cylinders exposed to an observer, little if any light can get through the layer. 
     SUMMARY OF THE INVENTION 
     The invention provides a gyricon display having cylindrical, rather than spherical, rotating elements. The elements can be bichromal or polychromal cylinders, preferably aligned parallel to one another and packed close together in a monolayer. The close-packed monolayer configuration provides excellent brightness characteristics and relative ease of manufacture as compared with certain other high-brightness gyricon displays. The cylinders can be fabricated by techniques that will be disclosed. The substrate containing the cylinders can be fabricated with the swelled-elastomer techniques known from spherical-particle gyricon displays, with a simple agitation process step being used to align the cylinders within the sheeting material. 
     Further, the invention is well-suited to providing a gyricon display having superior reflectance characteristics comparing favorably with those of white paper. A gyricon display is made with a close-packed monolayer of cylinders, wherein cylinders are placed, preferably in a rectangular packing arrangement, so that the surfaces of adjacent cylinders are as close to one another as possible. The light reflected from the inventive gyricon display is reflected substantially entirely from the monolayer of cylinders, so that lower layers are not needed. The areal coverage fraction obtainable with cylinders is greater than that obtainable with a single monolayer of uniform-diameter spheres. 
     In one aspect, the invention provides a material comprising a substrate and a plurality of nonspheroidal (e.g., substantially cylindrical) optically anisotropic particles disposed in the substrate. A rotatable disposition of each particle is achievable while the particle is thus disposed in the substrate; for example, the particles can already be rotatable in the substrate, or can be rendered rotatable in the substrate by a nondestructive operation performed on the substrate. In particular, the substrate can be made up of an elastomer that is expanded by application of a fluid thereto so as to render the particles rotatable therein. A particle, when in its rotatable disposition, is not attached to the substrate. A display apparatus can be constructed from a piece of the material together with means (such as an electrode assembly) for facilitating a rotation of at least one particle rotatably disposed in the substrate of the piece of material. 
     In another aspect, the invention provides a material comprising a substrate having a surface and a plurality of nonspheroidal optically anisotropic particles disposed in the substrate substantially in a single layer. The particles (e.g., cylinders) are of a substantially uniform size characterized by a linear dimension d (e.g., diameter). Each particle has a center point, and each pair of nearest neighboring particles in the layer is characterized by an average distance D therebetween, the distance D being measured between particle center points. A rotatable disposition of each particle is achievable while the particle is thus disposed in the substrate. A particle, when in its rotatable disposition, is not attached to the substrate. Particles are sufficiently closely packed with respect to one another in the layer such that the ratio of the union of the projected areas of the particles to the area of the substrate surface exceeds the areal coverage fraction that would be obtained from a comparably situated layer of spheres of diameter d disposed in a hexagonal packing arrangement with an average distance D therebetween as measured between sphere centers. If the ratio D/d is made as close to 1.0 as practicable, the ratio of the union of the projected areas of the particles to the area of the substrate surface can be made to exceed the maximum theoretically possible areal coverage fraction for a maximally close-packed hexagonal packing geometry of a layer of spheres of diameter d, which is approximately equal to 90.7 percent. 
     The invention will be better understood with reference to the following description and accompanying drawings, in which like reference numerals denote like elements. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is an exemplary gyricon display of the PRIOR ART, incorporating bichromal balls; 
     FIG. 1B illustrates a spherical bichromal ball of the PRIOR ART. 
     FIG. 2 illustrates a bichromal cylinder, showing in particular the diameter and height thereof. 
     FIG. 3 illustrates bichromal cylinders in cavities in an elastomer substrate. 
     FIG. 4 illustrates bichromal cylinders arrayed in an ideal close-packed monolayer. 
     FIGS. 5A-5B are, respectively, side and top views of a gyricon display of the present invention in an embodiment wherein bichromal cylinders of unit (1:1) aspect ratio are arrayed in a monolayer configuration. 
     FIG. 6 is a side view of a gyricon display of the present invention in an alternative embodiment wherein the bichromal cylinders are arrayed in a multilayer configuration, with relatively large cavity size. 
     FIGS. 7-8 illustrate top views of gyricon displays of the present invention in alternative embodiments in which the cylinders are, respectively, staggered in their alignment or randomly oriented. 
     FIG. 9 illustrates a top views of gyricon display of the present invention in an alternative embodiment in which the cylinder aspect ratio is greater than 1:1. 
     FIG. 10 illustrates a side view of a spinning-disk mechanism for fabrication of bichromal balls in the PRIOR ART. 
     FIG. 11 illustrates a top view of a spinning-disk mechanism for fabrication of bichromal cylinders of the invention. 
     FIG. 12 illustrates an alternative embodiment of the gyricon display of the invention wherein there is no elastomer or other cavity-containing substrate to retain the monolayer of cylinders in place. 
     FIG. 13A illustrates a polychromal sphere with three display states. 
     FIG. 13B illustrates a polychromal cylinder with three display states. 
     FIG. 13C illustrates an alternative embodiment of a polychromal cylinder with three display states. 
     FIG. 14A illustrates a polychromal sphere for use in a pseudo four color gyricon. 
     FIG. 14B illustrates a polychromal cylinder for use in a pseudo four color gyricon. 
     FIG. 15A illustrates a sphere for use in a full color gyricon or as a light valve. 
     FIG. 15B illustrates a cylinder for use in a full color gyricon device or as a light valve. 
     FIG. 16A illustrates an alternative sphere for usein a gyricon device as a light valve. 
     FIG. 16B illustrates an alternative cylinder for use in a gyricon device as a light valve. 
     FIG. 17A illustrates a multiple-disk assembly for fabricating multichromal gyricon balls. 
     FIG. 17B illustrates a portion of the multiple-disk assembly shown in FIG.  17 A. 
     FIG. 17C illustrates a side view of multichromal gyricon ball made using the disk assembly shown in FIGS. 17A-B. 
     FIG. 17D illustrates a top view of multichromal gyricon ball made using the disk assembly shown in FIGS. 17A-B 
     FIG. 18 illustrates a top view of a spinning-disk mechanism for fabrication of polychromal cylinders of the invention. 
    
    
     DETAILED DESCRIPTION 
     In a preferred embodiment of the invention, bichromal cylinders are arranged in a close-packed planar monolayer, as close to one another as possible, so as to cover the plane of the monolayer. The advantages of a close-packed monolayer display are discussed at length in copending, coassigned U.S. patent application Ser. No. 08/713,935, entitled “Monolayer Gyricon Displays”; suffice it to say here that close-packed monolayer displays exhibit superior reflectance and brightness characteristics as compared with conventional gyricon displays, and that the more of the monolayer plane that is covered by the gyricon elements, the better the reflectance and the brighter the display. 
     To quote briefly from Ser. No. 08/713,935: “in the ‘white’ state, the inventive display reflects entirely from the topmost layer of bichromal balls and, more particularly, from the white hemispherical upper surfaces of the topmost layer of balls. In a preferred embodiment, the inventive display is constructed with a single close-packed monolayer of bichromal balls.” 
     Ideally, a close-packing arrangement according to Ser. No. 08/713,935 would entirely cover the plane with the monolayer of gyricon elements. However, the displays disclosed in Ser. No. 08/713,935 are all based on spherical balls of the prior art. In as much as a planar array of spheres cannot fully cover the plane, but must necessarily contain interstices, the best that can be achieved with a single population of uniform-diameter spherical elements is about 90.7 percent areal coverage, which is obtained with a hexagonal packing geometry. A second population of smaller balls can be added to fill in the gaps somewhat, but this complicates display fabrication and results in a tradeoff between light losses due to unfilled interstices and light losses due to absorption by the black hemispheres of the smaller interstitial balls. 
     The present invention provides a close-packed monolayer gyricon display in which areal coverage can approach 100 percent, without any need for interstitial particles. It does so by using cylindrical rather than spherical bichromal elements. For example, a rectangular planar monolayer array of cylinders can be constructed that entirely or almost entirely covers the plane. With the white faces of the cylinders exposed to an observer, little if any light can get through the layer. 
     FIG. 2 illustrates a bichromal cylinder  20  suitable for use as a rotating element of the inventive gyricon display. Cylinder  20  has white face  21  and black face  22 . Cylinder  20  is of height (or length) h and has diameter d. The aspect ratio of cylinder  20  is defined herein as the ratio h/d. In the presence of a dielectric fluid, cylinder  20  is electrically dipolar, with the dipole moment preferably oriented perpendicular to the plane separating the white and black portions of the cylinder and passing perpendicularly through the longitudinal axis of the cylinder. 
     FIG. 3 illustrates how bichromal cylinders can be arranged in an elastomer substrate for use in the inventive display. A portion of a gyricon display  30  is shown. In display  30 , bichromal cylinders  31  are disposed in an elastomer substrate  32  that is swelled by a dielectric fluid (not shown) creating cavities  33  in which the cylinders  31  are free to rotate about their respective longitudinal axes. Cavities  33  are preferably not much larger in diameter than cylinders  31 , so that cylinders  31  are constrained from rotating about their medial axes. Cylinders  31  are electrically dipolar in the presence of the dielectric fluid, and so are subject to rotation upon application of an electric field. As shown, cylinders  31  can be rotated so as to expose either their white or black faces to an observer at I. 
     FIG. 4 illustrates bichromal cylinders arrayed in a close-packed monolayer. A portion of a gyricon display  40  includes rows of bichromal cylinders  41   a  and  41   b  of uniform diameter. Cylinders  41   a ,  41   b  are disposed in a monolayer between the upper and lower surfaces  44   a ,  44   b  of display  40 . Preferably there is exactly one cylinder between any given point on upper surface  44   a  and the corresponding point directly beneath it on lower surface  44   b.    
     The white faces of cylinders  41   a ,  41   b  are shown turned towards transparent viewing surface  44   a . In this configuration, light from a light source L incident on upper surface  44   a  is scattered by the white faces of cylinders  41   a ,  41   b  and is reflected so as to be visible to an observer at I. Thus display  40  appears white to the observer. 
     As shown, the cylinders are aligned end-to-end within the monolayer, the circular ends of cylinders  41   a  being aligned with the circular ends of cylinders  41   b  so that the longitudinal axis of each cylinder  41   a  is colinear with the longitudinal axis of its respective neigboring cylinder  41   b . Further as shown, the cylinders are aligned side-to-side within the monolayer, so that the circumferences of neighboring cylinders  41   a  touch each other, and the circumferences of neighboring cylinders  41   b  likewise touch each other. Thus aligned end-to-end and side-to-side, the cylinders form a rectangular array, whose structure is observable from above (as by an observer at I) through surface  44   a.    
     Preferably, there are no gaps between adjacent cylinders in the rectangular array. That is, the cylinders  41   a ,  41   b  touch each other end-to-end and side-to-side, or come as close as possible to touching each other as is consistent with proper cylinder rotation. Accordingly, there is preferably little or no opportunity for incident light from source L to be scattered from the white faces of the cylinders down to the black faces, where it would be absorbed. Likewise, there is little or no opportunity for incident light to pass between adjacent cylinders, where it would be absorbed in or below lower surface  44   b.    
     FIGS. 3-4 depict their respective gyricon displays in simplified form, with details not pertinent to the discussion omitted for clarity. FIGS. 5A and 5B provide, respectively, more detailed side and top views of a gyricon display  50  of the invention in a specific embodiment. 
     In display  50 , bichromal cylinders  51  of unit (that is, 1:1) aspect ratio are arrayed in a monolayer array having a rectangular packing geometry. Preferably, bichromal cylinders  51  are placed as close to one another as possible in the monolayer. Cylinders  51  are situated in elastomer substrate  52 , which is swelled by a dielectric fluid (not shown) creating cavities  53  in which the cylinders  51  are free to rotate. The cavities  53  are made as small as possible with respect to cylinders  51 , so that the cylinders nearly fill the cavities. Also, cavities  53  are placed as close to one another as possible, so that the cavity walls are as thin as possible. Preferably, cylinders  51  are of uniform diameter and situated at a uniform distance from upper surface  55 . It will be appreciated that the arrangement of cylinders  51  and cavities  53  in display  50  minimizes both the center-to-center spacing and the surface-to-surface spacing between neighboring bichromal cylinders. 
     Cylinders  51  are electrically dipolar in the presence of the dielectric fluid and so are subject to rotation upon application of an electric field, as by matrix-addressable electrodes  54   a ,  54   b . The electrode  54   a  closest to upper surface  55  is preferably transparent. An observer at I sees an image formed by the black and white pattern of the cylinders  51  as rotated to expose their black or white faces to the upper surface  55  of substrate  52 . For example, the observer sees the white faces of cylinders such as cylinder  51   a  and the black faces of cylinders such as cylinder  51   b.    
     The side view FIG. 5A reveals the monolayer construction of display  50 . The top view of FIG. 5B illustrates the rectangular packing geometry of cylinders  51  in the monolayer. The cylinders  51  appear as squares visible through transparent upper surface  55 . The centers of cylinders  51  form a square pattern, as shown by exemplary square S. 
     The projected areas of cylinders  51  in the plane of surface  65  preferably cover as much of the total area of the plane of surface  55  as possible. To this end, cavities  53  preferably are made as small as possible, ideally no larger than the cylinders themselves (or as close to this ideal as is consistent with proper cylinder rotation). The greater the ratio between the sum of the projected areas of the cylinders in the plane of viewing surface  55  and the total area of viewing surface  55 , the greater the display reflectance and the brighter the display. It will be appreciated that, whereas the maximum areal coverage theoretically possible with spherical bichromal balls (of a single uniform diameter, without interstitial smaller balls) is about 90.7 percent, the maximum for bichromal cylinders is 100 percent. Thus a gyricon display made from a close-packed monolayer of cylinders according to the invention can be made brighter than a gyricon display made from a close-packed monolayer of spherical balls. 
     FIG. 6 shows a side view of a gyricon display  60  of the invention in an alternative embodiment. In display  60 , bichromal cylinders  61  are in a top layer  67  and additional lower layers (here represented by second layer  68 ). Elastomer substrate  62  is swelled by a dielectric fluid (not shown) creating cavities  63  in which the cylinders  61  are free to rotate. Cylinders  61  are electrically dipolar in the presence of the dielectric fluid and so are subject to rotation upon application of an electric field, as by matrix-addressable electrodes  64   a ,  64   b . The electrode  64   a  closest to upper surface  65  is preferably transparent. An observer at I sees an image formed by the black and white pattern of the cylinders  61  as rotated to expose their black or white faces to the upper surface  65  of substrate  62 . 
     To improve the brightness of display  60  so that it is comparable to the brightness of display  50  (of FIGS.  5 A- 5 B), the top layer  67  can be made close-packed, with packing geometry and reflectance characteristics similar to those of the close-packed monolayer of cylinders  51  in display  50 . In this case, cavities  63  are made as small as possible with respect to cylinders  61 , and particularly with respect to cylinders in top layer  67 , so that these cylinders nearly fill the cavities. Also, cavities  63  are placed as close to one another as possible, so that the cavity walls are as thin as possible. Preferably, cylinders in top layer  67  are of uniform diameter and are situated at a uniform distance from upper surface  65 . It will be appreciated that if top layer  67  is close-packed, almost all the light reflected from display  60  so as to be observable to an observer at I is reflected from the white faces of cylinders in top layer  67 . At least for top layer  67 , the arrangement of cylinders  61  and cavities  63  in display  60  minimizes both the center-to-center spacing and the surface-to-surface spacing between neighboring bichromal cylinders. Cylinders in the lower layers (such as layer  68 ) can also be close-packed in order to reduce overall display thickness. 
     In general, a monolayer display, such as display  50  of FIGS. 5A-5B, is preferable to a thicker display, such as display  60  of FIG.  6 . This is because a thinner display can operate with a lower drive voltage, which affords concomittant advantages such as reduced power consumption, improved user safety, and the possibility of less expensive drive electronics. Further, a thinner display can offer better resolution than a thicker one, due to reduced fringing fields between adjacent black and white pixels. A thicker display offers fringing fields a greater volume in which to develop, and bichromal cylinders caught in the fringing fields are partially but not fully rotated so that they present a mix of black and white to the observer. Consequently, the display appears gray in the fringing field regions. The thin display has minimal fringing fields, and so provides a sharp demarcation between adjacent black and white pixels. (A more detailed discussion of fringing fields in thick and thin gyricon displays, and the effects of these fields on display resolution, is given in Ser. No. 08/713,935 with reference to FIG.  14  and the accompanying text therein.) 
     Although it is preferred to align the cylinders end-to-end and side-to-side within the monolayer (or top layer) of the display, so as to form a rectangular array, in alternative embodiments other arrangements of cylinders within the layer can be used. Some examples are seen in FIGS. 7-8. 
     FIG. 7 illustrates a top view of gyricon display  70  of the present invention in an alternative embodiment in which neighboring rows a, b of cylinders  71  are staggered with respect to one another. That is, the cylinders in rows a are aligned end-to-end with each other, as are the cylinders in alternate rows b, but the cylinders in rows a are not aligned side-to-side with those in rows b. The arrangement of FIG. 7 covers the plane as completely as the arrangement of FIG. 5B; however, the arrangement of FIG. 5B can be preferable, because this arrangement produces a well-defined rectangular array of pixels for pixels as small as a single cylinder. 
     FIG. 8 illustrates a top view of gyricon display  80  of the present invention in an alternative embodiments in which cylinders  81  are in random orientations with respect to one another. That is, the longitudinal axes of cylinders  81  are not parallel to one another. This arrangement of cylinders covers the plane less completely than the arrangements shown in FIG.  5 B and FIG. 7, and so is less preferable from the standpoint of maximizing display reflectance. 
     FIG. 9 illustrates a top views of gyricon display  90  of the present invention in an alternative embodiment in which the aspect ratio of the cylinders  91  is greater than 1:1. This alternative embodiment covers the plane comparably with the arrangements of FIG.  5 B and FIG.  7 . It can be useful, for example, in situations where different display resolutions are desired in the x- and y-dimensions (e.g., a display having a resolution of 1200 by 300 dots per inch). 
     Up to this point, the discussion of gyricon displays utilizing cylinders instead of spheres has focussed on applications originally utilizing bichromal spheres and how to achieve an enhancement in brightness by using bichromal cylinders. However, gyricon displays utilizing polychromal segmented balls are also known. These displays are fully discussed in U.S. patent application Ser. No. 08/572,779, entitled “POLYCHROMAL SEGMENTED BALLS FOR A TWISTING BALL DISPLAY”; U.S. patent application Ser. No. 08/572,778, entitled “APPLICATIONS OF A TRANSMISSIVE TWISTING BALL DISPLAY”; U.S. patent application Ser. No. 08/572,819, entitled “CANTED ELECTRIC FIELDS FOR ADDRESSING A TWISTING BALL DISPLAY”; U.S. patent application Ser. No. 08/572,927, entitled “HIGHLIGHT COLOR TWISTING BALL DISPLAY”; U.S. patent application Ser. No. 08/572,912, entitled “PSEUDO-FOUR COLOR TWISTING BALL DISPLAY”; U.S. patent application Ser. No. 08/572,820, entitled “ADDITIVE COLOR TRANSMISSIVE TWISTING BALL DISPLAY”; U.S. patent application Ser. No. 08/572,780, entitled “SUBTRACTIVE COLOR TWISTING BALL DISPLAY”; U.S. patent application Ser. No. 08/572,775, entitled “MULTITHRESHOLD ADDRESSING OF A TWISTING BALL DISPLAY”; U.S. patent application Ser. No. 08/572,777, entitled “FABRICATION OF A TWISTING BALL DISPLAY HAVING TWO OR MORE DIFFERENT KINDS OF BALLS”; and U.S. patent application Ser. No. 08/573,922, entitled “ADDITIVE COLOR TRISTATE LIGHT VALVE TWISTING BALL DISPLAY.” All filed concurrently on Dec. 15 th , 1995 as well as two divisional applications from U.S. patent application Ser. No. 08/572,779, entitled “POLYCHROMAL SEGMENTED BALLS FOR A TWISTING BALL DISPLAY”, “POLYCHROMAL SEGMENTED BALLS FOR A TWISTING BALL DISPLAY” now U.S. Pat. No. 5,717,514, U.S. patent application Ser. No. 08/890,830, entitled “APPARATUS FOR FABRICATING POLYCHROMAL SEGMENTED BALLS FOR A TWISTING BALL DISPLAY” filed on Jul. 10 th , 1997. These applications have been incorporated by reference above. 
     A corresponding desirable increased display quality can be achieved for these embodiments of gyricon displays as well if the polychromal balls were replaced by polychromal cylinders. 
     For example, a highlight color gyricon display is described which uses a polychromal ball  200  as shown in FIG.  13 A. The polychromal ball  200  has 5 portions. Two end segments  202 ,  204  are made of a clear material, while the remaining segments  206 ,  208 ,  210  are made from opaque material. The broad central segment  208  may be made white while slice  206  is colored black and slice  210  is chosen to be any other desired color, for instance red as a highlight color. The polychromal ball  200  may be rotated to show either black, from segment  206 , white from segment  208  or the highlight color from segment  210 . 
     A highlight color display using cylinders can be assembled using the techniques described above and using a plurality of cylinders as shown in FIG.  13 B. FIG. 13B shows a cylinder  212  with three portions, two cylinder segments  214 ,  218  and a central cylinder slice  216 . A cylinder segment is defined as that portion of the cylinder enclosed when the cylinder surface subtended by a plane. A cylinder slice is defined as that portion of a cylinder enclosed when a cylinder is cut by two substantially parallel planes. If cylinder segment  214  is made black, cylinder slice  216  is made white, and cylinder segment  218  is made to be any other color, for example red as a highlight color, then the resulting gyricon display will operate in exactly the same manner as one made from the sphere shown in FIG. 13A except that it will have a corresponding increase in display quality due to better areal coverage obtainable by cylinders over spheres. 
     The resulting product would be configured in any of FIGS. 5 through 9 or FIG. 12 With the substitution of cylinder  212  for cylinder elements  51 ,  61 ,  71 ,  81 ,  91  or  1201  shown therein. The resulting sheet can be used in any application that previously used a gyricon sheet constructed using the polychromal ball shown in FIG.  13 A. 
     An alternative highlight color display using cylinders can be assembled using the techniques described above and using a plurality of cylinders as shown in FIG.  13 C. The cylinder  220  (FIG. 13C) should provide an increase in display quality over the cylinder  212  (FIG. 13B) when used in a gyricon system, and is therefore the preferred cylinder for use in this type of gyricon system. FIG. 13C shows a cylinder  220  with five portions, 2 cylinder segments  222 ,  230  and three cylinder slices  224 ,  226 ,  228 . If both cylinder segments  222  and  230  are made clear, cylinder slice  226  is made white, cylinder slice  224  is made black and cylinder slice  228  is made to be any other color, for example red as a highlight color, then the resulting gyricon display will operate in exactly the same manner as one made from the sphere shown in FIG. 13A except that it will have a corresponding increase in display quality due to better areal coverage obtainable by cylinders over spheres. 
     The resulting product would be configured in any of FIGS. 5 through 9 or FIG. 12 with the substitution of cylinder  220  for cylinder elements  51 ,  61 ,  71 ,  81 ,  91  or  1201  shown therein. The resulting sheet can be used in any application that previously used a gyricon sheet constructed using the polychromal ball shown in FIG.  13 A. 
     An overlay transparency gyricon display is also described which uses a polychromal ball  200  as shown in FIG.  13 A. Again the polychromal ball  200  has 5 segments however, both two end segments  202 ,  204  and the central segment  208  are made of a clear material, while the remaining segments  206 ,  210  are made from opaque material. Segments  206  and  210  may be chosen to be any desired color, for instance one segment may be red as a highlight color and the other black to provide an underline color, or one segment may be red as a highlight color and the other may be yellow as a second highlight color. The polychromal ball  200  may be rotated to be either transparent from central segment  208 , or show either of the two colors from segment  206  or segment  210 . 
     An overlay transparency display using cylinders can be assembled using the techniques described above and using a plurality of cylinders as shown in FIG.  13 B. FIG. 13B shows a cylinder  212  with three portions, two cylinder segments  214 ,  218  and a central cylinder slice  216 . If cylinder segment  214  is made one opaque color, cylinder slice  216  is made clear, and cylinder segment  218  is made to be any other color, for example red as a highlight color, then the resulting gyricon display will operate in exactly the same manner as one made from the sphere shown in FIG. 13A except that it will have a corresponding increase in display quality due to better areal coverage obtainable by cylinders over spheres. 
     The resulting product would be configured in any of FIGS. 5 through 9 or FIG. 12 With the substitution of cylinder  212  for cylinder elements  51 ,  61 ,  71 ,  81 ,  91  or  1201  shown therein. The resulting sheet can be used in any application that previously used a gyricon sheet constructed using the polychromal ball shown in FIG.  13 A. 
     An alternative overlay transparency gyricon using cylinders can be assembled using the techniques described above and using a plurality of cylinders as shown in FIG.  13 C. The cylinder  220  (FIG. 13C) should provide an increase in display quality over the cylinder  212  (FIG. 13B) when used in a gyricon system, and is therefore the preferred cylinder for use in this type of gyricon system. FIG. 13C shows a cylinder  220  with five portions, 2 cylinder segments  222 ,  230  and three cylinder slices  224 ,  226 ,  228 . If both cylinder segments  222  and  230  and cylinder slice  226  are made clear, cylinder slice  224  is made any one color and cylinder slice  228  is made to be any other color, for example red as a highlight color, then the resulting gyricon display will operate in exactly the same manner as one made from the sphere shown in FIG. 13A except that it will have a corresponding increase in display quality due to better areal coverage obtainable by cylinders over spheres. 
     The resulting product would be configured in any of FIGS. 5 through 9 or FIG. 12 With the substitution of cylinder  220  for cylinder elements  51 ,  61 ,  71 ,  81 ,  91  or  1201  shown therein. The resulting sheet can be used in any application that previously used a gyricon sheet constructed using the polychromal ball shown in FIG.  13 A. 
     A pseudo-four color gyricon is described which uses a polychromal ball  222  as shown in FIG.  14 A. The polychromal ball  222  has 7 segments  224 ,  226 ,  228 ,  230 ,  232 ,  234 ,  236 . Both two end segments  224 ,  236  and the central segment  230  are made of a clear material, while the remaining segments  226 ,  228 ,  232 ,  234  are made from opaque material. Segments  226 ,  228 ,  232 ,  234  may be chosen to be any combination of desired colors, for instance segment  226  may be red, segment  228  may be green while segment  232  is yellow and segment  234  is blue. The polychromal ball  222  may be rotated to be either transparent from central segment  230 , or to show either of the two colors from segment  226  or segment  234 . Additionally, while using a canted field electrode configuration the polychromal ball  222  may be rotated to a position intermediate between its transparent state and opaque states to partially show two colors, either a portion of segment  226  with a portion of segment  232  or a portion of segment  234  with a portion of segment  228 . Finally, a background color may be chosen, such as white, which is visible when the polychromal ball is rotated to show transparent segment  230 . 
     A pseudo-four color gyricon using cylinders can be assembled using the techniques described above and using a plurality of cylinders as shown in FIG.  14 B. FIG. 14B shows a cylinder  238  with seven portions, two cylinder segments  240 ,  252  and five cylinder slices  242 ,  244 ,  236 ,  248 ,  250 . If both cylinder segments  240 ,  252 , and the central cylinder slice  246  are made of clear material, and the remaining cylinder slices  242 ,  244 ,  248 ,  250  are made from a selection of opaque colors, then the resulting gyricon display will operate in exactly the same manner as one made from the sphere shown in FIG. 14A except that it will have a corresponding increase in display quality due to better areal coverage obtainable by cylinders over spheres. 
     The resulting product would be configured in any of FIGS. 5 through 9 or FIG. 12 with the substitution of cylinder  238  for cylinder elements  51 ,  61 ,  71 ,  81 ,  91  or  1201  shown therein. The resulting sheet can be used in any application that previously used a gyricon sheet constructed using the polychromal ball  222  shown in FIG.  14 A. 
     An additive full color RGB gyricon has been described which uses a polychromal ball  254  as shown in FIG.  15 A. The polychromal ball  254  has 3 segments  256 ,  258 ,  260 . Both of the two end segments  256 ,  260  are made of a clear material, while the remaining thin central segment  258  is made from either clear or opaque colored material. Segment  258  will be either red, blue or green. The polychromal ball  254  may be rotated to be substantially transparent, showing only the thin edge of central segment  258 , or rotated to show the fully saturated opaque color of segment  258 , or rotated at intermediate values, using a canted field electrode configuration, to show a partially saturated color of segment  258 . A pixel of the additive full color RGB gyricon is made up of at least one polychromal ball  254  having a central segment  258  in each of the three colors red, blue, and green. That is the minimal number of polychromal balls  254  needed to make one pixel is three, wherein one ball has a red central segment, one ball has a green central segment and one ball has a blue central segment, although in practice one pixel will contain more than three balls. 
     An additive full color RGB gyricon using cylinders can be assembled using the techniques described above and using a plurality of cylinders as shown in FIG.  15 B. FIG. 15B shows a cylinder  262  with three portions, two cylinder segments  264 ,  268  and one cylinder slice  266 . If both cylinder segments  264 ,  268  are made of clear material, and the remaining cylinder slice  266  is made from either a clear or opaque color, then the resulting gyricon display will operate in exactly the same manner as one made from the sphere shown in FIG. 15A except that it will have a corresponding increase in display quality due to better areal coverage obtainable by cylinders over spheres. 
     The resulting product would be configured in any of FIGS. 5 through 9 or FIG. 12 with the substitution of cylinder  262  for cylinder elements  51 ,  61 ,  71 ,  81 ,  91  or  1201  shown therein. The resulting sheet can be used in any application that previously used a gyricon sheet constructed using the polychromal ball  262  shown in FIG.  15 A. 
     A multi-layer subtractive CMY or CMYK color gyricon has been described which also uses a polychromal ball  254  as shown in FIG.  15 A. Again, both of the two end segments  256 ,  260  are made of a clear material, but the remaining thin central segment  258  is made from clear colored material. Segment  258  will be either cyan, magenta, yellow or black. The polychromal ball  254  may be rotated to be substantially transparent, showing only the thin edge of central segment  258 , or rotated to show the fully saturated color of segment  258 , or rotated at intermediate values, using a canted field electrode configuration, to show a partially saturated color of segment  258 . A pixel of the subtractive full color CMY gyricon is made up of at least one polychromal ball  254  having a central segment  258  in each of the three colors cyan, yellow, and magenta. A pixel of the subtractive full color CMYK gyricon is made up of at least one polychromal ball  254  having a central segment  258  in each of the three colors cyan, yellow, and magenta plus black. However, unlike the previously described RGB gyricon the polychromal balls of a single color reside in separate layers superposed on each other. That is, one layer will contain polychromal balls  254  wherein segment  258  is a transparent magenta color, another layer will contain polychromal balls  254  wherein segment  258  is a transparent cyan color, a the third layer will contain polychromal balls  254  wherein segment  258  is a transparent yellow color, and possibly in a fourth layer there will be polychromal balls  254  wherein segment  258  is black. The transparent segments  258  act as color filters. The three layers may be contained within one sheet or each layer may reside in its own sheet, as is known in the art for polychromal spheres. Each layer may be rotated independently of the other layers, that is, it is possible to rotate only the polychromal balls  254  which have the same color segment  258  without affecting the polychromal balls  254  which have different color segments  258 . Independent rotation of layers may be accomplished, by either locating each layer independently of the others with a dedicated addressing electrode scheme or by using for each layer elements which have different rotation thresholds and locating all the elements in one layer and using a single addressing electrode scheme. 
     A subtractive full color CMY or CMYK gyricon using cylinders can be assembled using the techniques described above and using a plurality of cylinders as shown in FIG.  15 B. FIG. 15B shows a cylinder  262  with three portions, two cylinder segments  264 ,  268  and one cylinder slice  266 . If both cylinder segments  264 ,  268  are made of clear material, and the remaining cylinder slice  266  is made from a clear color, then the resulting gyricon display will operate in exactly the same manner as one made from the sphere shown in FIG. 15A except that it will have a corresponding increase in display quality due to better areal coverage obtainable by cylinders over spheres. 
     The resulting product could be configured such that each layer appears as in any of FIGS. 5 through 9 or FIG. 12 with the substitution of cylinder  262  for cylinder elements  51 ,  61 ,  71 ,  81 ,  91  or  1201  shown therein. The resulting sheet can be used in any application that previously used a gyricon sheet constructed using the polychromal ball  262  shown in FIG.  15 A. 
     Additive full color RGB gyricons have been described which use polychromal balls as a light valve. 
     In a first approach, a polychromal ball  254 , as shown in FIG. 15A, is used. Both of the two end segments  256 ,  260  are made of a clear material, while the remaining central segment  258  is made from opaque colored material. The polychromal ball  254  may be rotated to be substantially transparent, showing only the thin edge of central segment  258 , or rotated to be completely opaque showing all of segment  258 , or rotated at intermediate values, using a canted field electrode configuration, to be partially opaque showing a portion of segment  258 . Each polychromal ball  254  is used as a valve to either reveal, obscure, or partially obscure a colored dot situated behind the polychromal ball  254  depending on the orientation of the polychromal ball  254 . In a minimum set, the colored dots will be of least three colors (red, blue and green), and a pixel will contain at least one dot of each color and its associated polychromal ball  254  to act as a light valve. 
     An additive full color RGB gyricon using cylinders as a light valve can be assembled using the techniques described above and using a plurality of cylinders as shown in FIG.  15 B. FIG. 15B shows a cylinder  262  with three portions, two cylinder segments  264 ,  268  and one cylinder slice  266 . If both cylinder segments  264 ,  268  are made of clear material, and the remaining cylinder slice  266  is made from opaque material, then the resulting gyricon display will operate in exactly the same manner as one made from the sphere shown in FIG. 15A except that it will have a corresponding increase in display quality due to better areal coverage obtainable by cylinders over spheres. Due to the better areal coverage obtainable by cylinders, the colored dots to be obscured by the light valve may be replaced with a shape described by the projection of a cylinder rather than a circle (which is the shape projected by a sphere). That shape depends on the shape of the specific cylinders used and may be either a square or a rectangle. 
     The resulting product would be configured in any of FIGS. 5 through 9 or FIG. 12 with the substitution of cylinder  262  for cylinder elements  51 ,  61 ,  71 ,  81 ,  91  or  1201  shown therein. The resulting sheet can be used in any application that previously used a gyricon sheet constructed using the polychromal ball  262  shown in FIG.  15 A. 
     In a second approach, a polychromal ball  270 , as shown in FIG. 16A, is used. Both of the two end segments  272 ,  278  are made of a clear material, while the two central segments  274 ,  276  are made from opaque colored material. One central segment  274  is colored black, while the other central segment  276  is colored white. The polychromal ball  270  may be rotated to be substantially transparent, showing only the thin edge of both central segments  274 ,  276 , to be white showing all of segment  274 , to be black showing all of segment  276  or rotated at intermediate values, using a canted field electrode configuration, to be partially opaque showing a portion of either segment  274 ,  276 . Each polychromal ball  270  is used as a valve to either reveal, obscure, or partially obscure a colored dot situated behind the polychromal ball  270  depending on the orientation of the polychromal ball  270 . In a minimum set, the colored dots will be of least three colors (red, blue and green), and a pixel will contain at least one dot of each color and its associated polychromal ball  270  to act as a light valve. 
     An additive full color RGB gyricon using cylinders as a light valve can be assembled using the techniques described above and using a plurality of cylinders as shown in FIG.  16 B. FIG. 16B shows a cylinder  280  with three portions, two cylinder segments  282 ,  288  and two cylinder slices  284 ,  286 . If both cylinder segments  280 ,  288  are made of clear material, and the two cylinder slices  284 ,  286  are made from opaque black and white material respectively, then the resulting gyricon display will operate in exactly the same manner as one made from the sphere shown in FIG. 16A except that it will have a corresponding increase in display quality due to better areal coverage obtainable by cylinders over spheres. Due to the better areal coverage obtainable by cylinders, the colored dots to be obscured by the light valve may be replaced with a shape described by the projection of a cylinder rather than a circle (which is the shape projected by a sphere). That shape depends on the shape of the specific cylinders used and may be either a square or a rectangle. 
     The resulting product would be configured in any of FIGS. 5 through 9 or FIG. 12 with the substitution of cylinder  280  for cylinder elements  51 ,  61 ,  71 ,  81 ,  91  or  1201  shown therein. The resulting sheet can be used in any application that previously used a gyricon sheet constructed using the polychromal ball  270  shown in FIG.  15 A. 
     Cylinder Fabrication Techniques 
     FIG. 10 (PRIOR ART) illustrates a side view of a spinning-disk mechanism  100  for fabrication of bichromal spherical balls. Mechanism  100  is equivalent to the “spinning disc configuration  50 ” disclosed in the &#39;098 patent incorporated by reference hereinabove; see FIG. 4 therein and the accompanying description at col. 4, line 25 to col. 5, line 7. 
     In the prior art, the spinning disk mechanism was used in conjunction with low-viscosity hardenable liquids. Low viscosity was considered necessary to ensure the formation of good-quality bichromal spheres; if viscosity was too high, the ligaments streaming off the disk would freeze in place instead of fragmenting into balls as desired. For example, as stated in the &#39;098 patent (col. 5, line 64—col. 6 line 2), “the black and white pigmented liquids are delivered . . . in a heated, molten state . . . so that they flow freely and do not harden prematurely, i.e., long enough to prevent the ligaments from freezing.” 
     According to the invention, the spinning disk mechansm is used in conjunction with high-viscosity hardenable liquids. These liquids do, indeed, “freeze”(harden) in place, the very result not desired in the prior art. However, according to the invention the frozen ligaments that are considered undesirable for making bichromal spheres can be used to make bichromal cylinders. FIG. 11 illustrates this. A spinning disk  110 , shown here in a top view, is used according to the technique of the &#39;098 patent to form bichromal ligaments, but with high-viscosity hardenable white and black liquids being used in place of the low-viscosity liquids of the prior art. The resulting ligaments  115  harden into fine bichromal filaments (roughly analogous to the way in which molten sugar hardens into filaments when spun in a cotton-candy machine). The filaments can be combed or otherwise aligned and then cut into even lengths, as with a tungsten carbide knife or a laser, to produce the desired bichromal cylinders. End-to-end and side-to-side alignment of the cut cylinders can be achieved by precise alignment of the filament ends on the working surface where the cutting takes place; for example, if the cylinders are to have aspect ratio 1:1 and diameter 100 microns, then the filament ends can be aligned with one another to within a tolerance on the order of 5 to 10 microns. 
     In the same manner that a modification of the method used to produce bichromal spheres can be used to produce bichromal cylinders, just so can a modification of the method used to produce polychromal spheres be used to produce polychromal cylinders. A modification of the spinning-disk technique can be used to fabricate multichromal balls. The modification uses a spinning multiple-disk assembly instead of a single spinning disk. An example is illustrated in FIG.  17 A. Assembly  1700  has three disks  1710 ,  1711 ,  1712  that rotate uniformly about shaft  1715 . The concave or “dish-shaped” outer disks  1710 ,  1712  curve or slope toward the flat inner disk  1711  at their respective peripheries. Other geometries are possible, and the exact geometry for a particular embodiment can be determined, for example, by hydrodynamic modeling, as will be appreciated by those of skill in the art. 
     The three-disk assembly of FIG. 17A can be used to produce multichromal balls and cylinders having certain useful properties, as will be discussed below. It will be appreciated, however, that other assemblies having different numbers of disks can also be used in the present invention, with the number and configuration of the disks varying according to the kind of ball that is to be produced. 
     If differently pigmented low viscosity hardenable plastic liquids are introduced to each side of each of the three disks  1710 ,  1711 ,  1712  in FIG. 17A, flow patterns of pigmented liquids at the edge of the disks can be obtained that result in multichromal ligaments that break up to form multichromal balls. FIG. 17B illustrates a close-up cross-sectional view of an example of the flow of pigmented plastic liquids at the edge of the three-disk assembly of FIG.  17 A. First and second liquids  1721 ,  1722  flow over opposite sides of disk  1710 , whose downward-sloping edge can be seen in the figure. Third and fourth liquids  1723 ,  1724  flow over opposite sides of disk  1711 , and fifth and sixth liquids  1725 ,  1726  flow over opposite sides of disk  1712 . The combined flows give rise to ligament  1730 , which breaks up into multilayer balls such as the ball  1740  illustrated in FIG. 17C (side view) and FIG. 17D (top view). 
     Ball  1740  has six segments corresponding to the six streams of plastic liquid used to make it. Segments  1741  and  1742  join at planar interface  1743 ; segments  1744  and  1745 , at planar interface  1746 ; and segments  1747  and  1748 , at planar interface  1749 . If different pigments are used in the various plastic liquids  1721 ,  1722 ,  1723 ,  1724 ,  1725 ,  1726 , then ball  1740  will be multichromal. In general, a three-disk assembly like the one shown in FIG.17A can produce gyricon balls having six segments of up to six different colors. 
     More generally, a multi-disk assembly with N disks can be used to produce gyricon balls having up to 2N segments in arbitrary color combinations. Black, white, or other color pigments or dyes can be used, alone or in combination, so that segments can be made in virtually any desired color or shade. Segments can be made clear by using unpigmented, undyed plastic liquid. Different segments can be made to have different widths by adjusting the flow rates of the various plastic liquids used to make the segments, with faster flow rates corresponding to wider segments and slower rates to narrower. Two or more adjacent segments can be made the same color so that they effectively merge to form a single broader segment. 
     As discussed earlier and shown in FIG. 11, when the spinning disk mechansm is used in conjunction with high-viscosity hardenable liquids these liquids do, indeed, “freeze” (harden) in place to create ligaments that can be used to make polychromal cylinders. FIG. 18 illustrates this for the case of a multiple disk system. When a spinning disk assembly  180 , shown here in a top view, is used according to the technique of the &#39;098 patent to form bichromal ligaments, but with high-viscosity hardenable liquids being used in place of the low-viscosity liquids of the prior art the resulting ligaments  185  harden into fine bichromal filaments (roughly analogous to the way in which molten sugar hardens into filaments when spun in a cotton-candy machine). The filaments can be combed or otherwise aligned and then cut into even lengths, as with a tungsten carbide knife or a laser, to produce the desired bichromal cylinders. End-to-end and side-to-side alignment of the cut cylinders can be achieved by precise alignment of the filament ends on the working surface where the cutting takes place; for example, if the cylinders are to have aspect ratio 1:1 and diameter 100 microns, then the filament ends can be aligned with one another to within a tolerance on the order of 5 to 10 microns. 
     By way of example, any given gyricon cylinder segment can be: black; white; clear (that is, essentially transparent and without chroma, like water or ordinary window glass); a transparent color (e.g., transparent red, blue, or green, as for certain additive color applications; transparent cyan, magenta, or yellow, as for certain subtractive color applications); an opaque color of any hue, saturation, and luminance; any shade of gray, whether opaque or translucent; and so forth. Any given gyricon cylinder segment can also have other optical properties polarization, birefringence, phase retardation, light absorption, light scattering, and light reflection. For ease of reference, “achromatic colors” will be used herein below to refer to colors essentially lacking in chroma, that is, to black, white, gray, and clear, and “chromatic colors” will be used hereinbelow to refer to other colors, including red, orange, yellow, green, blue, indigo, violet, cyan, magenta, pink, brown, beige, etc. 
     Alternative techniques can also be used to produce the bichromal cylinders. For example, injection molding can be used, albeit perhaps with some inconvenience. As another example, the bichromal jet technique disclosed in the &#39;594 patent can be used, again substituting high-viscosity hardenable liquids for the usual low-viscosity liquids. 
     No-Cavities Cylinder Display 
     In a gyricon display made with swelled elastomer, each bichromal cylinder is situated in a cavity. To achieve the closest possible packing of bichromal cylinders in such a display, the cavities are preferably made as small and as close together as possible. 
     To achieve still higher packing density, a gyricon display can be constructed without elastomer and without cavities. In such a display, the bichromal cylinders are placed directly in the dielectric fluid. The cylinders and the dielectric fluid are then sandwiched between two retaining members (e.g., between the addressing electrodes). There is no elastomer substrate. In this case, the packing geometry can closely approach, or even achieve, the ideal close-packed monolayer geometry shown in FIG.  4 . 
     FIG. 12 illustrates a side view of a no-cavities gyricon display. In display  1200 , a monolayer of bichromal cylinders  1201  of uniform diameter is situated in dielectric fluid  1209  between matrix-addressable electrodes  1204   a ,  1204   b . Preferably cylinders  1201  of unit aspect ratio are arranged in a rectangular array, aligned end-to-end and side-to-side within the monolayer and packed as close together as is possible consistent with proper cylinder rotation. Cylinders  1201  are electrically dipolar in the presence of dielectric fluid  1209  and so are subject to rotation upon application of an electric field, as by electrodes  1204   a ,  1204   b . The electrode  1204   a  closest to upper surface  1205  is preferably transparent. An observer at I sees an image formed by the black and white pattern of the cylinders  1201  as rotated to expose their black or white faces to the upper surface  1205  of display  1200 . 
     Electrodes  1204   a ,  1204   b  serve both to address cylinders  1201  and to retain cylinders  1201  and fluid  1209  in place. Preferably the spacing between electrodes  1204   a ,  1204   b  is as close to the diameter of cylinders  1201  as is possible consistent with proper cylinder rotation. Cylinders  1201  and fluid  1209  can be sealed in display  1200 , for example by seals at either end of the display (not shown). The close packing of cylinders  1201  in the monolayer, together with the close spacing of the electrodes  1204   a ,  1204   b , ensures that cylinders  1201  do not settle, migrate, or otherwise escape from their respective positions in the monolayer. 
     It should be pointed out that the no cavities cylinder display is not limited to the bichromal cylinders  1201  shown in FIG. 12, but in fact any of the cylinders described herein may be used to construct the no cavities cylinder display. 
     CONCLUSION 
     A new gyricon display based on cylindrical elements instead of spherical elements has been described. This new display makes possible a close-packed monolayer providing nearly  100  percent areal coverage. Such a display provides superior reflectance and brightness, and requires no interstitial particles. 
     The foregoing specific embodiments represent just some of the possibilities for practicing the present invention. Many other embodiments are possible within the spirit of the invention. For example: 
     The electrical anisotropy of a gyricon cylinder need not be based on zeta potential. It is sufficient that there is an electrical dipole moment associated with the cylinder, the dipole moment being oriented with respect to the long axis of the cylinder in such a way as to facilitate a useful rotation of the cylinder in the presence of an applied external electric field. (Typically, the dipole moment is oriented along a medial axis of the cylinder.) Further, it should be noted that a gyricon cylinder can have an electrical monopole moment in addition to its electrical dipole moment, as for example when the dipole moment arises from a separation of two positive charges of different magnitudes, the resulting charge distribution being equivalent to a positive electrical monopole superposed with an electrical dipole. 
     The optical anisotropy of a gyricon cylinder need not be based on black and white. For example, bichromal cylinders having hemispheres of two different colors, e.g. red and blue, can be used. As another example, cylinders that are black in one hemisphere and mirrored in the other might be used for some applications. In general, various optical properties can vary as different aspects of a gyricon cylinder are presented to an observer, including (but not limited to) light scattering and light reflection in one or more regions of the spectrum. Thus the gyricon cylinders can be used to modulate light in a wide variety of ways. 
     The incident light that encounters a gyricon display need not be restricted to visible light. Given suitable materials for the gyricon cylinders, the incident “light” can be, for example, infrared light or ultraviolet light, and such light can be modulated by the gyricon display. 
     On several occasions the foregoing description refers to a planar monolayer of bichromal cylinders. However, persons of skill in the art will appreciate that a gyricon display (or a sheet of bichromal cylinders for use in such a display) made of a flexible material can be temporarily or permanently deformed (for example, flexed, folded, or rolled) so as not to be strictly planar overall. In such cases, the plane of a monolayer can be defined, for example, in a locally planar neighborhood that includes the gyricon cylinder or cylinders of interest. Also, it will further be apprecated that in practice the monolayer can vary somewhat from what has been described, for example, due to manufacturing tolerances or slight imperfections of particular gyricon sheets. 
     Accordingly, the scope of the invention is not limited to the foregoing specification, but instead is given by the appended claims together with their full range of equivalents.