Abstract:
A display comprising: a) a substrate; b) first transparent conductors; d) second conductors patterned to form segments; e) inter-segment material disposed between said second conductors, electrically conductive and having optical properties similar to said second conductors; f) an imaging layer comprising a light-modulating material disposed between said first and second conductors, electrically switched between two field-stable optical states and having a third as-coated optical state; and g) applying an electrical field to said inter-segment material and said first conductors to write said light-modulating material from the as-coated optical state to a different optical state more closely matching one of the two field-stable optical states.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to segmented cholesteric (chiral nematic) liquid crystal displays.  
       BACKGROUND OF THE INVENTION  
       [0002]     Currently, information is displayed using assembled sheets of paper carrying permanent inks or displayed on electronically modulated surfaces such as cathode ray displays or liquid crystal displays. Printed information cannot be changed. Electrically updated displays are often heavy and expensive. Other sheet materials can carry magnetically written areas, for example, to carry ticketing or financial information. Such magnetically written data, however, is not visible.  
         [0003]     Media systems exist that maintain electronically changeable data without power. Such system can be electrophoretic (Eink), Gyricon, or polymer dispersed cholesteric materials. An example of such electronically updateable displays can be found in U.S. Pat. No. 3,600,060 issued Aug. 17, 1971 to Churchill et al., which patent shows a device having a coated, then dried emulsion of cholesteric liquid crystals in aqueous gelatin to form a field-responsive, bistable display. U.S. Pat. No. 3,816,786 also to Churchill et al. discloses a layer of encapsulated cholesteric liquid crystal responsive to an electric field. The electrodes in the patent can be transparent or non-transparent and formed of various metals or graphite. It is disclosed that one electrode must be light absorbing, and it is suggested that the light absorbing electrode be prepared from paints contains conductive material such as carbon.  
         [0004]     Fabrication of flexible, electronically written display sheets is disclosed in U.S. Pat. No. 4,435,047 issued Mar. 6, 1984 to Fergason. A substrate supports a first conductive electrode, one or more layers of encapsulated liquid crystals, and a second electrode of electrically conductive ink. The conductive inks form a background for absorbing light, so that the information-bearing display areas appear dark in contrast to background non-display areas. Electrical potential applied to opposing conductive areas operates on the liquid crystal material to expose display areas. Because the liquid crystal material is nematic liquid crystal, the display ceases to present an image when de-energized, that is, in the absence of a field. The patent discloses the use of dyes in either the polymer encapsulant or liquid crystal to absorb incident light. The patent further discloses the use of a chiral dopant. The dopant improves the response time of the nematic liquid crystal, but does not cause the nematic host to operate in a bistable light-reflective state.  
         [0005]     U.S. Pat. No. 5,251,048 to Doane et al. discloses a light-modulating cell having a polymer-dispersed chiral-nematic liquid crystal. The chiral-nematic liquid crystal has the property of being electrically driven between a planar state, reflecting a specific visible wavelength of light, and a focal-conic state, transmitting forward scattering light. Chiral-nematic liquid crystals, also known as cholesteric liquid crystals, potentially in some circumstances have the capacity of maintaining one of multiple given states in the absence of an electric field. Black paint can be applied to the outer surface of a rear substrate to provide a light-absorbing layer forming a non-changing background outside of a changeable display area defined by the intersection of segment lines and scanning lines.  
         [0006]     U.S. Pat. No. 5,636,044 to Yuan et al. discloses a seven-segment display, using cholesteric liquid-crystal material, which display has two substrates. The substrates are rigid glass with patterned transparent electrodes on each of two facing surfaces. A continuum of cholesteric liquid crystal fills the gap between the two electrode sets. The first substrate is divided into segmented and non-segmented areas which are defined by gaps in transparent, electrically conductive Indium-Tin-Oxide (ITO) disposed on the substrate. The second substrate is divided into common electrodes in an ITO coating corresponding to segmented and non-segmented areas on the first substrate. The device can change the state of the segmented areas as well as non-segmented areas, permitting the display of a positive or negative image. Both electrodes are transparent electrodes, requiring an additional light-absorbing layer on the back of one substrate. Inter-segment material, or gaps in the electrode materials, requires electrode contacts to each segment area to write; requiring separate electrical connection to each segment area. It would be useful to have a structure that could provide simple electrode connect to each segment area, in a matrix fashion, without requiring point connection to each segment area.  
         [0007]     U.S. Pat. No. 6,236,442 to Stephenson et al. discloses a display sheet with a metallic conductive layer over a cholesteric layer. A process is disclosed for vacuum depositing a continuous metallic layer and laser patterning the metallic layer to form segment electrodes. Moralized areas remain between electrically driven areas. However, those areas between etched segments remain in an as-coated state. A circuit board with contacts is pressed against each segment electrode. The circuit board provides electrical drive to segment electrodes.  
         [0008]     U.S. Pat. No. 6,394,870 to Petruchik et al discloses an opaque conductive material with non-conductive areas to delineate images stored in polymer-dispersed cholesteric liquid crystal. The patent discloses printing the opaque conductive material, and providing contact to each image-bearing area. No conductive material is provided between segments. In one embodiment, a dielectric area with openings to each opaque conductive area is printed over the opaque conductive material. Traces are then printed over the dielectric layer to eliminate a circuit board with contacts. Areas between etched segments remain in an as-coated state.  
         [0009]     There is a need for a display using polymer-dispersed cholesteric liquid crystals having background material with optical properties matching optical properties of the written segments.  
       SUMMARY OF THE INVENTION  
       [0010]     The need is met according to the present invention by a display having a display driver and a display area capable of displaying a plurality of characters, each character having a character region and a background region, wherein each character region comprises a plurality of segments, said display comprising: 
        a) a substrate;     b) transparent first conductors, wherein said character region corresponds to at least one of the first conductors,     c) second conductors patterned to have electrically separate areas corresponding to the segments of the character region;     d) at least one imaging layer comprising a light modulating material disposed between said first and second conductors, which material has the property of having a first and second field-switchable stable optical state which states correspond, respectively, to a first and second contrasting optically visible state, and which material has the further property, when coated on a substrate and before application of a electromagnetic field, of exhibiting an initial state or optical appearance (as-coated or as-fabricated prior to exposure to a field to change its optical state) that is nearer to the first optically visible state (for example, a reflective state in a liquid-crystal display) and, after being subjected to a field (such as capable of switching from the second visible state to the first visible state) of exhibiting a field-induced optical appearance (for example, the reflective state corresponding to a planar orientation in a liquid-crystal material) and     e) an inter-segment background element, corresponding to the background region, comprising one or more sections, said background element and said second conductors being substantially level with each other (e.g., in the same layer], and which background element is disposed outside the peripheral boundary of the patterned areas of said second conductors, which background element comprises a material that is electrically conductive such that during manufacture it is capable of electrically writing the background region into one of the optically visible states (which background element may be patterned to have electrically separate areas);     f) third conductors connected to second conductors in more than one character region;     wherein the display is arranged such that, during use, the background element cannot be used to electrically write the background region into one of the optically visible states employing a display driver and/or using third conductors.        
 
         [0018]     In one embodiment, a polymer-dispersed cholesteric liquid-crystal display comprising: 
        a) a substrate;     b) first transparent conductors;     c) second conductors having preselected optical properties patterned to form segments;     d) inter-segment material disposed between and/or around said second conductors, electrically conductive and having optical properties preselected to substantially match those of said second conductors, but which inter-segment material is electrically isolated from the second conductors (e.g. separated by a narrow gap);     e) a coated polymer-dispersed cholesteric liquid crystal layer disposed between said first and second conductors, electrically switchable between two field-stable optical states and also having a third as-coated optical state that is substantially closer to one of the two field-stable states than the other;     wherein the display is adapted, during its intended display use, for applying an electrical field to said first conductors, employing a display driver, in order to write said polymer-dispersed material from either one of the two-field stable optical states to the other of the two field-stable states, thereby displaying an image (for example, a picture, symbol, digit, letter, or other information-bearing or ornamental data intended for human perception); and     wherein the display is not adapted, during its intended display use, for applying an electrical field to said inter-segment material employing the display driver connected during display use.        
 
         [0026]     In one embodiment, an electric field has been applied to said inter-segment material during manufacture of the display in order to write a corresponding polymer-dispersed liquid crystal material from its as-coated optical state to one of the two field-stable optical states corresponding to optical state of the background region of the display during use.  
         [0027]     The present invention is also directed to a method of making a display, one embodiment of which comprises the steps of: 
        a) providing a substrate;     b) forming a first patterned conductor layer on the substrate having electrically separate areas defining character regions;     c) depositing a layer of light modulating material over the first patterned conductor layer;     d) forming a second patterned conductor layer over the layer of light modulating material and having electrically separate areas defining the segments of the characters and the background;     e) forming inter-segment background conductors at substantially the same horizontal level as the second patterned conductor layer over the layer of light modulating material and having electrically separate areas defining the background;     f) optionally depositing a dielectric layer over the second patterned conductor layer, the dielectric layer defining holes over each segment; and     f) forming a third patterned conductor layer defining a plurality of conductors connected to the areas defining the character segments in the second patterned conductor; at least one of the conductors being connected to a segment in more than one character, whereby the display may be addressed in a matrix fashion by electrically addressing the first and second patterned conductors;     g) employing the inter-segment background conductors to electrically change the as-coated or heated light-modulating material controlled by said conductors to one of two contrasting optical states;     connecting the display to a driver capable of addressing the display in a matrix fashion by electrically addressing, via electrical contact with the conductors in the third patterned conductor layer, the first and second patterned conductor layers, but wherein the inter-segment background conductors defining background is not electrically addressable by the driver via electrical contact with the conductors in the third patterned conductor layer.        
 
         [0037]     A voltage can be applied to the background element before application of a dielectric layer or a spaced circuit board, and voltage can be applied to the background element alone or to the entire display. As indicated above, a preferred embodiment involves electrically writing the background of the display, or manufacturing intermediate thereof, in a reflective or planar optical mode once before sale or commercial use of the display. However, another option, prior to step (g) of the method, the display may be heated to convert the light-modulating material controlled by the background conductors to a dark optical state. In such as option, one embodiment involves the manufacture of a display in which a liquid-crystal material is in the focal-conic or darker optical state in the background region of the region.  
       ADVANTAGES  
       [0038]     The invention has the advantage that inter-segment areas in a display provide an improved optical match with at least of the optical modes of switched segments. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0039]      FIG. 1  is a perspective of one embodiment of a display in accordance with the present invention, in which a polymer-dispersed liquid crystal-material is used;  
         [0040]      FIG. 2  is a schematic sectional view of a chiral nematic material, respectively, in a planar and focal-conic state responding to incident light;  
         [0041]      FIG. 3  is a schematic sectional view of an as-coated chiral nematic material responding to incident light;  
         [0042]      FIG. 4  is a plot of the spectra for a polymer-dispersed liquid-crystal material in the planar, focal-conic, and near-planar states;  
         [0043]      FIG. 5  is a rear view of a sheet in accordance with the present invention having a patterned first conductor;  
         [0044]      FIG. 6  is a rear view of a sheet in accordance with the present invention having a polymer-dispersed cholesteric liquid-crystal layer and a dark layer;  
         [0045]      FIG. 7  is a rear view of a sheet in accordance with the present invention having exposed first conductors;  
         [0046]      FIG. 8A  is a rear view of a display in accordance with prior art having second conductors;  
         [0047]      FIG. 8B  is a rear view of a display in accordance with the present invention having second conductors and conductive inter-segment material between second conductors;  
         [0048]      FIG. 9  is a sectional side view of a display in accordance with the present invention having an electrical field applied to inter-segment material;  
         [0049]      FIG. 10  is a rear view of a display in accordance with the present invention having an electric field applied to inter-segment material;  
         [0050]      FIG. 11  is a section view of a display in accordance with the present invention attached to a circuit board;  
         [0051]      FIG. 12  is a rear view of a display in accordance with the present invention having an applied dielectric area;  
         [0052]      FIG. 13  is a rear view of a display in accordance with the present invention having row contacts; and  
         [0053]      FIG. 14  is a front view of a display in accordance with the present invention connected to electric drive means.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0054]      FIG. 1  is a perspective section view of one preferred embodiment of a display in accordance with the invention, which display employs a polymer-dispersed cholesteric material. A sheet designated as display  10  is made in accordance with the present invention. Display  10  includes a display substrate  15 , which can be a thin transparent polymeric material, such as Kodak Estar® film base formed of polyester plastic that has a thickness of between 20 and 200 micrometers. In an exemplary embodiment, display substrate  15  is a 125-micrometer thick sheet of polyester film base. Other polymers, such as transparent polycarbonate, can also be used.  
         [0055]     One or more first transparent conductors  20  are formed on display substrate  15 . First transparent conductors  20  can be tin-oxide, indium-tin-oxide (ITO), or polythiophene, with ITO being the preferred material. Typically the material of first transparent conductors  20  is sputtered or coated as a layer over display substrate  15  having a resistance of less than 1000 ohms per square. First transparent conductors  20  can be formed in the conductive layer by conventional lithographic or laser etching means. Transparent first transparent conductors  20  can also be formed by printing a transparent organic conductor such as PEDT/PSS, PEDOT/PSS polymer, which materials are sold as Baytron® P by Bayer AG Electronic Chemicals. Portions of first transparent conductors  20  can be uncoated to provide exposed first conductors  22  for this embodiment.  
         [0056]     Cholesteric layer  30  overlays first transparent conductors  20 . Cholesteric layer  30  contains cholesteric liquid-crystal material, such as those disclosed in U.S. Pat. No. 5,695,682 to Doane et al., the disclosure of which is incorporated by reference. Such materials are made using highly anisotropic nematic liquid crystal mixtures and adding a chiral doping agent to provide helical twist in the planes of the liquid crystal to the point that interference patterns are created that reflect incident light. Application of electrical fields of various intensity and duration can be employed to drive a chiral-nematic (cholesteric) material into a reflective state, to near-transparent or transmissive state, or an intermediate state. These materials have the advantage of having first and second optical states that are both stable in the absence of an electrical field. The materials can maintain a given optical state indefinitely after the field is removed. Cholesteric liquid crystal materials can be formed, for example, using a two-component system such as MDA-00-1444 (undoped nematic) and MDA-00-4042 (nematic with high chiral dopant concentrations) available from E. M. Industries of Hawthorne, N.Y.  
         [0057]     In a preferred embodiment, cholesteric layer  30  is a cholesteric material dispersed in deionized photographic gelatin. The liquid crystal material is mixed at 8% cholesteric liquid crystal in a 5% gelatin aqueous solution. The mixture is dispersed to create an emulsion having 8-10 micrometer diameter domains of the liquid crystal in aqueous suspension. The domains can be formed using the limited coalescence technique described in U.S. Pat. No. 6,423,368 by Stephenson et al. The emulsion is coated over first transparent conductors  20  on a polyester display substrate  15  and dried to provide an approximately 9-micrometer thick polymer dispersed cholesteric coating. Other organic binders such as polyvinyl alcohol (PVA) or polyethylene oxide (PEO) can be used in place of the gelatin. Such emulsions are machine coatable using coating equipment of the type employed in the manufacture of photographic films. A gel sub-layer can be applied over first transparent conductors  20  prior to applying cholesteric layer  30  as disclosed in U.S. Pat. No. 6,423,368 by Stephenson et al., hereby incorporated by reference in its entirety.  
         [0058]      FIG. 2  is a schematic section view of optical states of one embodiment of a display in accordance with the present invention. The left diagram demonstrates the optical path when the cholesteric material is in a planar state. Incident light  60  strikes planar liquid crystal  72  which reflects a portion of incident light  60  as reflected light  62 . The remaining light passes through dark layer  35 .  
         [0059]     Dark layer  35  can be a complementary light-absorbing dye layer that operates on a portion of the light passing though dark layer  35 . Particular wavelengths of light are absorbed, and the remaining light strikes reflective second conductor  40 . Light is reflected from second conductor  40  and passes through dark layer  35  a second time, then passes through planar material  72  to become complementary light  64 . Complementary light  64 , operating in conjunction with cholesteric liquid crystal having peak reflectance near 575 nanometers, forms a substantially color-neutral reflective surface.  
         [0060]     On the right side of  FIG. 2 , the liquid-crystal material is in a focal-conic state  74  and transmits incident light  60 . Dark layer  35  provides complementary light  64  when the liquid crystal material is in a focal conic state. As one alternative, dark layer  35  can be a thin, black layer that absorbs across all wavelengths of light. With a black dark layer  35 , when the cholesteric material is in the focal-conic state, the image will be essentially black.  
         [0061]      FIG. 3  is a schematic sectional view of an as-coated chiral nematic material responding to incident light. Polymer-dispersed cholesteric material as-coated assumes a near-planar state (“nP”) which is less reflecting compared to the planar state obtained by an electrical field, in an area of an image or other information formed during display use. The amount of reflected light  62  is less. The cholesteric material is bright but does not match the reflectance of the cholesteric material when it is electronically written into the planar state.  
         [0062]      FIG. 4  is a plot of the spectra for a yellow polymer-dispersed cholesteric liquid-crystal material with a blue dark layer in the planar (P), focal-conic (FC) and near-planar (nP) state. When the cholesteric material is in the planar state, the display appears substantially color neutral. When the display is written into the focal-conic state, the display appears blue. In the as-coated, electrically unwritten near planar (nP) state, the display does not have the full reflectance of the planar (P) state and has an appearance different from an electrically written planar state. Displays that do not electrically activate the full surface area, such as seven-segment displays, will have characters in optical states that do not match the background state. Similarly, in the case that dark layer is black, the as-coated state in the near-planar (nP) will also differ from the reflectance of polymer-dispersed cholesteric liquid-crystal material electrically written into the planar state. It should also be recognized that although, in  FIG. 4 , the spectra for a polymer-dispersed cholesteric liquid-crystal material in the near-planar (nP) state is below the spectra of the material in the planar (P) state, the point of significance is that the two spectra do not match well and, for example, the spectra of the material in the near-planar (nP) state could be above the spectra of the material in the planar (P) in other embodiments.  
         [0063]     Thus, based on  FIG. 4 , the skilled artisan will appreciate that any regions in a display having as-coated chiral nematic material will not match well the regions in a display having chiral nematic material exhibiting optical states obtained during display use, a problem described in U.S. Pat. No. 5,636,044 to Yuan et al., hereby incorporated by reference in its entirety. The skilled artisan will also recognize that a chiral-nematic material can provide a gray scale in which case the optical states referred to in this application preferably correspond to lightest and darkest contrasting states.  
         [0064]     Returning to  FIG. 1 , dark layer  35  overlays cholesteric layer  30 . In a preferred embodiment, dark layer  35  is a complementary light-absorbing layer composed of pigments that are milled below 1 micrometer to form “nano-pigments” in a binder. Such pigments are very effective in absorbing wavelengths of light in very thin (sub-micrometer) layers. Such pigments can be selected to be electrically inert to prevent degradation interference from electrical display fields applied to display  10 . Such pigments are disclosed in copending U.S. patent application Ser. No. U.S. Ser. No.______ (Docket 84,140), hereby incorporated by reference.  
         [0065]     In the present embodiment, in  FIG. 1 , dark layer  35  is coated over cholesteric layer  30  to provide a light-absorbing layer that provides a specific contrast state to reflected light. As mentioned above, dark layer  35  can be designed to provide a specific amount of light at wavelengths not operated on by the cholesteric liquid crystal to create a more color-neutral image. The coating can be simultaneous with the deposition of cholesteric layer  30  or as a separate step. In a preferred embodiment, multi-layer coating equipment of the kind used in the photographic industry provides cholesteric layer  30  and dark layer  35  as two co-deposited layers. Dark layer  35  is significantly thinner than cholesteric layer  30  and has minimal effect on the electrical field strength required to change the state of the cholesteric liquid-crystal material.  
         [0066]     Second conductors  40  overlay dark layer  35 . Second conductors  40  have sufficient conductivity to induce an electric field across cholesteric layer  30  strong enough to change the optical state of the polymeric material. Second conductors  40 , especially for applications requiring complementary light  64 , are preferably formed of reflective metal, for example, by vacuum deposition of conductive and reflective material such as aluminum, chrome or nickel. In the case of vacuum-coated second conductors  40 , aluminum or silver provide very high reflectance and conductivity. The layer of conductive material can be patterned using well-known techniques such as photolithography, laser etching or by application through a mask.  
         [0067]     In another embodiment, second conductors  40  can be formed by screen printing a reflective and conductive formulation such as UVAG® 0010 from Allied Photochemical of Kimball, Mich. Such screen printable conductive materials comprise finely divided silver in a ultraviolet-curable resin. After printing, the material is exposed to ultraviolet radiation greater than 0.40 Joules/cm 2 , the resin will polymerize in 2 seconds to form a durable surface. Screen printing is preferred to minimize the cost of manufacturing the display. Alternatively, second conductors  40  can be formed by screen printing a thermally cured silver-bearing resin. An example of such a material is Acheson Electrodag® 461SS, a heat cured silver ink. In the case that the dark layer  35  is black, any type of conductor can be used including black carbon in a binder.  
         [0068]     The present invention is applicable to displays  10  in  FIG. 1 , which display has significant or sufficiently large areas between second conductors  40 . In accordance with this invention, areas between second conductors  40  are filled with inter-segment material  42 . Inter-segment material  42  is preferably formed with electrically conductive material that provides essentially the same optical properties as the material used in second conductors  40 , that is, so that the inter-segment material  42  and the second conductors  40  provide the same appearance in the display. In the case of a vacuum-coated second conductor  40 , a continuous metallic layer can be deposited and a laser used to delineate second conductors  40 . Vacuum-deposited material outside second conductors  40  is retained as inter-segment material  42  in the laser etching process. In the case of printed second conductors  40 , material can be printed in areas between second conductors  40  to create inter-segment material  42 . In any case, it is desirable that inter-segment material  42  be electrically isolated from second conductors  40  to prevent fields applied to second conductors  40  during display use from activating inter-segment material  42 . In the printed as-coated state, cholesteric liquid-crystal material adjacent to inter-segment material  42  is in the near planar (nP) state and appears to be a different color than the material written into either the planar or focal-conic state.  
         [0069]     Referring still to the embodiment of  FIG. 1 , a dielectric layer  50  can be provided over second conductors  40 . Dielectric layer  50  is provided with “through via”  52  that permit interconnection between second conductor  40  and conductive row contacts  54 . Dielectric layer  50  can be formed, for example, by printing or coating a polymer such as vinyl dissolved in a solvent. Row contacts  54  can be formed by screen printing the same screen-printable, electrically conductive material used to form second conductors  40 . The row contacts  54  (better seen in  FIG. 13 ) enable the connection of common segments in different characters, thereby creating functional rows of electrically addressable areas in the polymer-dispersed cholesteric liquid-crystal layer. The row contacts and exposed first conductors  22  form a set of backside display contacts that are used to electrically address the display.  
         [0070]     The use of: a flexible support for display substrate  15 ; thin first transparent conductors  20 ; machine-coated cholesteric liquid-crystal layer  30 ; and printed second conductors  40  permits the fabrication of a low-cost flexible display. Small displays according to the present invention can be used as electronically rewritable tags or labels for inexpensive, rewrite applications.  
         [0071]     A process for fabricating display  10  will now be described.  FIG. 5  is a rear view of a sheet in accordance with the one embodiment of the present invention, which sheet has a patterned first conductor. A substrate  15  is provided with a plurality of patterned first transparent conductors  20 .  FIG. 6  is a rear view of a sheet in accordance with the present invention having a polymer-dispersed cholesteric liquid-crystal layer and a dark layer  30 ,  35  (only the top layer is viewable). In a preferred embodiment, cholesteric layer  30  and dark layer  35  are co-deposited.  FIG. 7  is a rear view of a sheet in accordance with the present invention having exposed first conductors. Portions of cholesteric layer  30  and dark layer  35  can be removed, for example, using a solvent to form exposed first conductors  22 .  
         [0072]      FIG. 8A  is a rear view of a display in accordance with prior art having second conductors, which second conductors  40  have been printed over dark layer  35 . Printing portions of the same material used to create second conductors  40  over exposed first transparent conductors  20  can provide protective covering  24  over exposed first conductor  22 .  FIG. 8B  is a rear view of a display in accordance with the present invention having second conductors  40  and conductive inter-segment material  42  between second conductors  40 . Preferably, printed material is provided in substantially or essentially all display areas not covered by second conductors  40 . In the embodiment of  FIG. 8B , however, there is a relatively thin gap between inter-segment material  42  and second conductors  40  so they are conductively isolated. A pattern of second conductors  40  and inter-segment material  42  can also be formed by applying a continuous vacuum coated metal, and laser etching separated second conductors from inter-segment material  42 .  
         [0073]      FIG. 9  is a sectional side view of a display in accordance with the present invention showing diagrammatically an electrical field being applied to inter-segment material  42  and, hence (through dark layer  35 ) to the cholesteric liquid crystal material  30  between the inter-segment material  42  and the first conductor  20 . This can occur during manufacture prior to coating of a dielectric layer or may occur, afterwards. The electrical field can be applied on-line during manufacture only to the inter-segment material or to both the inter-segment material and the segment material. The electric field may be applied by contact with a voltage source or by introduction of the display in a sufficiently strong electromagnetic field. A tool can be designed by the skilled artisan that connects an electrical current to a point of contact for each electrically isolated intersegment-element in the display.  
         [0074]      FIG. 10  is a rear view of a display (manufacturing intermediate) in accordance with the present invention showing points of contact for an electric field being applied to inter-segment material in a manufacturing intermediate of a display. An electrical field is be applied to inter-segment material  42  at this step of the manufacturing process to convert cholesteric liquid-crystal material covered by inter-segment material  42  from near-planar (nP) to an electrically written planar (P) state. After conversion of cholesteric material under inter-segment material  42  to an electrically written state, display  10  can be completed.  
         [0075]      FIG. 11  is a sectional view of one embodiment of a display  10  comprising a substrate  15  and dark layer  35  in accordance with the present invention in which the display comprises a circuit board attached to the assembly of  FIG. 10 . Contacts  80  on circuit board  82  provide electrical connection to each second conductor  40  and protective covering  24  over a first conductor in accordance with prior art, as will be understood by the skilled artisan. Instead of coating a dielectric layer, air may be used as a dielectric material in combination with suitable spacing achieved by contacts  80 .  
         [0076]     Circuit board  82  of display  10  can be replaced with additional printed layers to form a matrix drive for seven segment displays.  FIG. 12  is a rear view of a display in accordance with the present invention having an applied dielectric layer  50 . Dielectric layer  50  covers both second conductors  40  and inter-segment material  42 . Through via  52  permit access to second conductors  40 . Alternatively, through via  52  can furthermore permit connection to inter-segment material  42  to permit writing of cholesteric liquid-crystal material to either the focal-conic or planar state during manufacture or prior to display use. Design of multiple printed layers to create a matrix driven seven segment display having electrically writable inter-segment material are incorporated in co-pending U.S. application U.S. Ser. No.______(docket 85,836) by the same inventors, which application is hereby incorporated by reference in its entirety.  
         [0077]     A display employing dark images on a light background provides advantageous image quality. That configuration corresponds, in the embodiment of  FIG. 1 , to planar written background material with focal-conic characters. In a preferred embodiment, cholesteric liquid-crystal material below inter-segment material  42  is, accordingly, electrically written into the planar state. However, the present invention is not limited to such a configuration.  
         [0078]      FIG. 13  is a rear view of a display (manufacturing intermediate) in accordance with the present invention having row contacts  54 . Row contacts  54  are conductive traces printed to connect common second conductors using through via  52  in dielectric layer  50 .  
         [0079]      FIG. 14  is one embodiment of a completed display in accordance with the present invention connected to an electric driver. Row driver  84  is connected by contacts  80  to row contacts  54 . Column driver  86  is connected by contacts  80  to first transparent conductors  20  though protective covers  24 . Electrical signals can be applied to row driver  84  and column driver  86  to write images onto display  10 . Because inter-segment material  42  was used to write the material into the planar state, the display appears to have a continuous bright state. Segments of display  10  are written into the darker, focal-conic state to present image data. Writing data segments back into the electrically written planar state merges the previously written area into an optically continuous background.  
       EXAMPLE  
       [0080]     In an experiment, an as-coated yellow cholesteric liquid-crystal material with a blue dark layer was printed with silver ink to form both second conductors and inter-segment material. A high voltage bipolar electrical field (e+, e−) was applied between each inter-segment area and associated one or more second conductors. Thus, the areas between segments were electrically written from a near-planar (nP) to a planar (P) state. The process was found to significantly improved the image quality of the completed displays.  
         [0081]     The printing and inter-segment electrical writing process was repeated using a green cholesteric liquid-crystal material and a carbon-black dark layer. Electrical pulses converted green cholesteric liquid-crystal material from a near-planar (nP) to a substantially more fully planar (P) state, thereby matching the planar state of written segment electrodes. It was concluded that electrically pulsing polymer-dispersed cholesteric liquid-crystal material under inter-segment material converted the optical state to a state more closely matching the planar state of electrically written segment areas. The process, therefore, provided an improvement in display quality.  
         [0082]     Another experiment was performed to determine the effect of electrically writing cholesteric liquid-crystal material in the inter-segment (background) areas into the planar and focal-conic states. When the cholesteric liquid-crystal material in the background was written into the focal-conic state, cholesteric liquid-crystal material between second conductors and inter-segment material remained in the near planar (nP) state. The resulting display accordingly had distinct bright borders around each segment between the inter-segment material and second conductor. Accordingly, the thinly narrow areas of near planar (nP) between second conductors and inter-segment material are close enough to the planar (P) state to be relatively unnoticeable when the inter-segment material is in the planar (P) state, but may be objectionable when inter-segment material is in the focal-conic (FC) state. In the as-coated state, therefore, the inter-segment material is preferably written into the planar (P) state.  
         [0083]     Nevertheless, heating a display can convert a cholesteric liquid-crystal material into a near focal-conic state, but the displays will have an objectionable dark border when the background is written into the planar (P) state. In this case, then, because material between second conductors and inter-segment material is black, it is preferable that inter-segment material be written into the focal-conic state. Thus, when manufacturing a display in which the image or other information is positively displayed in the planar state and the background is in the focal-conic state or darker state, conversion by heat or otherwise of essentially or substantially all of the display, including the as-coated cholesteric liquid-crystal layer, into the focal-conic state preferably proceeds electrically writing the cholesteric liquid-crystal material in the background into the more pure of fully focal-conic state.  
         [0084]     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.  
       PARTS LIST  
       [0085]    
       
           10  display  
           15  display substrate  
           20  first transparent conductors  
           22  exposed first conductors  
           24  protective covering  
           30  cholesteric layer  
           35  dark layer  
           40  second conductors  
           42  inter-segment material  
           50  dielectric layer  
           52  through via  
           54  row contacts  
           60  incident light  
           62  reflected light  
           64  complementary light  
           72  planar liquid crystal  
           74  focal conic liquid crystal  
           80  contacts  
           82  circuit board  
           84  row driver  
           86  column driver  
          P Planar state  
          FC focal-conic state  
          nP near planar state