Abstract:
A display system includes a display arranged to receive an image wise pattern of light to form an image, including, a pair of conductors, at least one conductor being transparent, a layer of cholesteric liquid crystal material disposed between the conductors, the liquid crystal material having multiple stable optical states at zero electrical field, and a light absorber for forming an image wise thermal pattern in the liquid crystal sufficient to change the optical state of the cholesteric liquid crystal in response to an image wise pattern of light; a display writer, including, a light source for producing a flash of light of sufficient intensity to generate sufficient heat in the light absorber to change the optical state of the cholesteric liquid crystal, a mask located between the light source and the display for defining the image wise pattern of light, a display drive connectable to the conductors for generating an electric field between the conductors for changing the optical state of the cholesteric liquid crystal, and a controller connected to the light source and the display drive for controlling the intensity of the electrical field and actuating the light source to create an image on the display.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a display system having a polymer dispersed cholesteric liquid crystal that change optical states in response to heat, light and electrical field. 
     BACKGROUND OF THE INVENTION 
     Cholesteric liquid crystals have the property of maintaining several different optical states in the absence of electrical field. Additionally, cholesteric liquid crystals can change optical states in response to applied electrical and/or thermal fields. Those properties make them useful in the development of field-stable, re-writable displays. 
     U.S. Pat. No. 3,401,262 issued Sep. 10, 1968 to Fergason et al. discloses a cathode ray tube to apply light to a screen. The screen has a photoconductive layer that is excited by an electrical field applied by fine leads across the photoconductive layer. The screen has a layer of a temperature sensitive cholesteric material that changes reflective wavelength with slight changes in temperature, and changes hue in heated areas. Light from the cathode ray tube strikes the photoconductor layer, creating heat which can be used to selectively change the color of the sheet of cholesteric material. The system uses a complex cathode ray tube and a photoconductor layer and ceases to present an image in the absence of an electrical field. 
     U.S. Pat. No. 3,578,844 issued May 18, 1971 to Churchill discloses a sheet of gelatin encapsulated cholesteric material without a photosensitive layer. The sheet is put into a first reflective state by heating. Portions of the sheet are written into a black (clear) state by the application of DC fields. The sheet is heated to reset the display. The encapsulated material in the sheet retained written information without fade at ambient conditions for eight weeks. 
     U.S. Pat. No. 3,789,225 issued Jan. 29, 1974 to Leder discloses a glassy cholesteric liquid crystal between glass plates. Glassy liquid crystal materials are solidified liquid crystals in an orderly state at ambient temperatures. They are not responsive to electrical fields in the glassy state. The apparatus writes the sheet to an initial state by heating the material above the isotropic (liquid) transition point. As the material is cooled, a high-intensity xenon flash lamp is used to disturb the material so that flash disturbed areas solidify into a state different than areas not receiving flash energy. The imaging system requires that the materials be raised to a high temperature, and cooled at a fast rate in the presence of selective high-intensity flash light. No electrical fields are applied to the media. 
     Conventional, non-glassy liquid crystals have the property of being electrically driven between a planar state reflecting a specific visible wavelength of light and a light scattering focal-conic state at ambient temperatures. Chiral nematic liquid crystals, also known as cholesteric liquid crystals have the capacity of maintaining one of multiple given states in the absence of an electric field. U.S. Pat. No. 5,437,811 issued Aug. 1, 1995 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 driven between a planar state reflecting a specific visible wavelength of light and a weakly light scattering focal-conic state. Chiral nematic liquid crystals, also known as cholesteric liquid crystals, have the capacity of maintaining one of multiple given states in the absence of an electric field. The Doane et al. patent discloses the use of only electrical fields to change the optical state of cholesteric liquid crystals. The technology writes image data line sequentially. Sequentially writing data lines is slow compared to writing all pixels at once and requires electrical drivers on each column and row line. 
     Yamamoto et al. in A Novel Photoaddressable Electronic Paper Utilizing Cholesteric LC Microcapsules and Organic Photoconductor, SID 2001 DIGEST, pp. 362-365, create an electronic paper having a photoconductive layer and a polymer encapsulated cholesteric liquid crystal that is field responsive at ambient temperatures. A high electrical field is applied across both layers, and the photoconductive layer provides a bias voltage in the presence of light. The high and low field states across the material write cholesteric material into different optical states. 
     Prior art light sensitive sheets have required expensive and complex photosensitive layers for operation. Electrical drive systems must write data sequentially, requiring complex electronic drives. Glassy liquid crystals change state with the application of large amounts of heat and no electrical field. There is a need therefore for a light written sheet that could have image data written simultaneously without a photosensitive layer at low temperatures. 
     SUMMARY OF THE INVENTION 
     The need is met according to the present invention by providing a display system that includes a display arranged to receive an image wise pattern of light to form an image, including a pair of conductors, at least one conductor being transparent; a layer of cholesteric liquid crystal material disposed between the conductors, the cholesteric liquid crystal material having multiple stable optical states at zero electrical field; and a light absorber for forming an image wise thermal pattern in the liquid crystal sufficient to change the optical state of the cholesteric liquid crystal in response to an image wise pattern of light; a display writer, including, a light source for producing a flash of light of sufficient intensity to generate sufficient heat in the light absorber to change the optical state of the liquid crystal, a mask located between the light source and the display for defining the image wise pattern of light, a display drive connectable to the conductors for generating an electric field between the conductors for changing the optical state of the liquid crystal, and a controller connected to the light source and the display drive for controlling the intensity of the electrical field and actuating the light source to create an image on the display. 
     ADVANTAGES 
     The present invention has the advantage that it provides a simple sheet structure that can be written and re-written using a xenon flash lamp and an electrical field. A single writing process can be used to write the sheet without regard to prior written information. The writing process is fast, and improves image quality over sheets written only electrically. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partial cross sectional view of a display in accordance with the present invention; 
         FIG. 2  is a schematic side view of a prior art chiral nematic material in a planar and focal-conic state responding to incident light; 
         FIG. 3  is schematic side view of an experimental setup used to simulate a display system in accordance with the present invention; 
         FIG. 4  is a plot of the response of the display of  FIG. 1 , originally in the planar state, at constant flash lamp energy and various voltages; 
         FIG. 5  is a plot of the contrast ratio of the data of  FIG. 4 ; 
         FIG. 6  is a plot of the response of the display of  FIG. 1 , originally in the focal-conic state, at constant flash lamp energy and various voltages; 
         FIG. 7  is a plot of the contrast ratio of the data of  FIG. 6 ; 
         FIG. 8  is an electrical schematic diagram for a display writer in accordance with the present invention; 
         FIG. 9  is a side view of the display of  FIG. 1  attached to an object; 
         FIG. 10  is a side schematic view of a display writer in accordance with the present invention; 
         FIG. 11  is a side view of the display writer connected to a display to write the display in accordance with the present invention; and 
         FIGS. 12A and B  are front views of a display according to the present invention in prewritten state and written states, respectively. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to  FIG. 1 , a display  10  according to the present invention includes a display substrate  15 , such as a thin transparent polymeric material, for example, Kodak Estar film base formed of polyester plastic that has a thickness of between 20 and 200 (preferably 125 microns). Other polymers, such as transparent polycarbonate, can also be used. 
     A first transparent conductor  20  is formed on display substrate  15 . First transparent conductor  20  can be tin-oxide, indium-tin-oxide (ITO), or polythiophene, with ITO being the preferred material. Typically the material of first transparent conductor  20  is sputtered or coated as a layer over display substrate  15  having a resistance of less than 1000 ohms per square. 
     In a preferred embodiment, a first conductor cover  22  is printed over first transparent conductor  20 . First conductor cover  22  can be screen printed conductive ink such as Electrodag 423SS screen printable electrical conductive material from Acheson Corporation. Such screen printable conductive materials comprise finely divided graphite particles in a thermoplastic resin. First conductor cover  22  protects first transparent conductor  20  from abrasion. 
     Light modulating layer  30  overlays a first portion of first transparent conductor  20 . A portion of light modulating layer  30  is removed to create exposed first conductor  20 ′ to permit electrical contact. Light modulating layer  30  contains cholesteric liquid crystal material, such as those disclosed in U.S. Pat. No. 5,695,682 issued Dec. 9, 1997 to Doane et al., the disclosure of which is incorporated by reference. Application of electrical fields of various intensity and duration can be employed to drive a chiral nematic material (cholesteric) into a reflective state, to a substantially transparent 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 Merck BL112, BL118 or BL126, available from E.M. Industries of Hawthorne, N.Y. 
     In a preferred embodiment, light modulating layer  30  is E.M. Industries&#39; cholesteric material BL-118 dispersed in deionized photographic gelatin. The liquid crystal material is mixed at 8% concentration in a 5% gelatin aqueous solution. The liquid crystal material is dispersed to create an emulsion having 8-10 micron 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 issued Jul. 23, 2002 to Stephenson et al. The emulsion is coated on a polyester display substrate over the first transparent conductor(s) and dried to provide an approximately 9-micron 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 the first transparent conductor  20  prior to applying light modulating layer  30  as disclosed copending U.S. Ser. No. 09/915,441 filed Jul. 26, 2001 by Stephenson et al. 
       FIG. 2  is a schematic side sectional view of a chiral nematic material in a planar and focal-conic state responding to incident light. In the figure on the left, after a high voltage field has been applied and quickly switched to zero potential, the liquid crystal molecules become planar liquid crystal  72 , which reflect portions of incident light  60  as reflected light  62 . In the figure on the right side of  FIG. 2 , upon application of a lower voltage field, the molecules of the chiral nematic material break into weakly forward scattering cells known as focal-conic liquid crystal  74 . Increasing the time duration of a low-voltage pulse progressively drives the molecules that were originally reflective planar liquid crystal  72  towards a fully evolved and light scattering focal-conic liquid crystal  74 . 
     A light absorber  35  is positioned on the side opposing the incident light  60 . Light absorber  35  can be a thin layer of light absorbing, sub-micron carbon in a gel binder as disclosed copending U.S. Ser. No. 10/036,149 filed Dec. 26, 2001 by Stephenson. As fully evolved focal-conic liquid crystal  74 , the cholesteric liquid crystal is forward light scattering and incident light  60  is absorbed by light absorber  35  to create a black image. Progressive evolution towards the focal-conic state causes a viewer to perceive reflected light  62  that transitions to black as the cholesteric material changes from reflective planar liquid crystal  72  to a fully evolved light scattering focal-conic liquid crystal  74 . When the field is removed, light modulating layer  30  maintains a given optical state indefinitely. The states are more fully discussed in U.S. Pat. No. 5,437,811, referenced above. 
     Returning to  FIG. 1 , light absorber  35  is disposed between second conductor  40  and light modulating layer  30  to improve contrast. A second conductor  40  overlays light modulating layer  30 . Second conductor  40  has sufficient conductivity to provide an electric field between the first transparent conductor  20  and second conductor  40  strong enough to change the optical state of the cholesteric material in light modulating layer  30 . Second conductor  40  can be formed, for example, by the well known technique of vacuum deposition for forming a layer of conductive material such as aluminum, tin, silver, platinum, carbon, tungsten, molybdenum, tin or indium or combinations thereof. The layer of conductive material can be patterned using well known techniques of photolithography, laser etching or by application through a mask. 
     In a preferred embodiment, second conductor  40  is formed by screen printing a conductive ink such as Electrodag 423SS screen printable electrical conductive material from Acheson Corporation. Such screen printable conductive materials comprise finely divided graphite particles in a thermoplastic resin. Screen printing is preferred to minimize the cost of manufacturing the display. 
     The use of a flexible support for display substrate  15 ; first transparent conductor  20 ; machine coated light absorber  35  and light modulating layer  30 ; and printed second conductor  40  and first conductor cover  22  permits the fabrication of a low cost flexible display. Small displays according to the present invention can be used as electronically rewritable tags for inexpensive, limited rewrite applications. 
     Referring to  FIG. 3 , a display  10  in accordance with the preferred embodiment was positioned so that a conventional xenon flash  52  exposed portions of display  10  through mask  54 . In the experiment, flash  52  was a Vivitar model 285HV professional flash lamp and mask  54  was a sheet of Dupont Mylar transparency with an electro-photographic printed image. The output of flash  52  could be adjusted to imprint an image on display  10  if the cholesteric material was initially either planar liquid crystal  72  or focal-conic liquid crystal  74 . 
     A set of electrodes was applied to first conductor cover  22  and second conductor  40 . An electrical field was applied across electrodes  50 , and flash imprinted images on display  10  were erased. Display  10  could be imprinted and erased multiple times without damage to display  10 . Display  10  was positioned so that the black second conductor  40  faced flash  52  and mask  54 . Flash  52  could be adjusted so that images were imprinted through second conductor  40 . Images made in the reversed manner could be electrically erased using a field across electrodes  50 . From these experiments, it was concluded that the printing process occurs due to thermal energy being applied to second conductor  40  through either side of display  10 . A heat pulse of correct power and duration provides a thermal flux effect that can write cholesteric material into either the planar or focal-conic state. These experiments are the first demonstration of the use of masked high-intensity light to thermally print and reprint images on polymer dispersed cholesteric liquid crystals. The method and materials permit multiple erasure and writing cycles. Furthermore, the structure of display  10  is flexible and low-cost. 
     Experiments were performed to determine the response of display  10  to the application of electrical fields during the thermal heat pulses from flash  52 . In a first experiment, display  10  was electrically written into the planar state using a high voltage pulse. The output of flash  52  was set so initially planar liquid crystal was selectively written into the focal-conic state in unmasked areas. The display was repeatedly reset to the planar state, and written using a series of voltages. 
       FIG. 4  shows a plot of the response of the display of  FIG. 1 , originally in the planar state, at constant flash lamp energy and various voltages. For each test, the material was initialized into the planar using a high voltage pulse. Then a test voltage was applied during a flash event. The resulting curve is equivalent to the response curve found in the Doane et al. patent for materials tested without flash. The curve for the masked material (filled boxes) is the same response of materials in the absence of the thermal pulse provided by flash  52 . The clear curve is the same display in the presence of the heat pulse from the flash combined with an electrical field. At zero applied voltage and without a mask, planar material is written into the focal conic-state and has a reflectance of about 7 percent. Applying a low voltage, such as 10 volts, improves the clarity of focal-conic state from 7 percent to about 2 percent reflectance. 
       FIG. 5  shows a plot of the contrast ratio of the data of FIG.  4 . The combination of the flash and electrical field creates two high contrast states which are improved from a standard 4:1 contrast ratio to a 14:1 contrast ratio. The two optimal states are optically reversed states. One optimum contrast ratio, 14:1, occurs at 20 volts. Material initially in the planar state is unaffected by the low-voltage pulse without the presence of the flash energy; the presence of energy from flash  52  writes the material into the focal-conic state. A second high-contrast state, 13:1 contrast ratio occurs at 60 volts applied field. Without the flash energy, initially planar material is written into the focal-conic state; the presence of the flash energy writes the material into the planar state. 
       FIG. 6  is a plot of the response of the display of  FIG. 1 , originally in the focal-conic state, at constant flash lamp energy and various voltages.  FIG. 7  is a plot of the contrast ratio of the data of FIG.  6 . Application of a low field again reduces the reflection of focal-conic material from 7 percent to 2 percent. Only one pair of bistable states is possible in this system, again at 60 volts, which has a contrast ratio of about 13. Material initially in the focal-conic state remains in the focal-conic state at a medium level voltage. Energy from flash  52  causes initially focal-conic material to be driven into the planar state. Apparently, heat from flash  52  reduces the voltage required to drive cholesteric material into the planar state. 
     It was observed that at 60 volts of applied field, the final state of the material was defined by mask  54  if the material was initially in either the planar or focal-conic state. The phenomenon eliminates the need to initially write the material into an initial state before flash-writing an image. The single writing process, without an initialization step, provides a fast, parallel method of writing display  10 . 
     The multiple states of the system suggest a variety of drive schemes, shown in Table 1. A first scheme (1) initializes display  10  into the planar state using either a 100 volt pulse ( 1 A) or a 60 volt pulse with flash ( 1 B). A low, 20 volt, pulse in conjunction with the flash permits selective writing using mask  54 . In a second scheme (2), display  10  is written into the focal conic state either without flash ( 2 A) or with a flash ( 2 B). In scheme 2, 60 volts is applied to display  10 , and areas receiving flash light are written into the planar state, and areas that are masked remain in the focal-conic state. In a third scheme (3) is essentially identical to scheme  2 B, but eliminates initialization to a focal-conic state. A 60 volt pulse in the presence of flash energy provides image-wise writing of display  10  using mask  54  without initialization. The use of an initialization step may be useful in applications that require display  10  to be erased in a first operation, and a later re-writing of display  10 . 
     
       
         
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Initial 
                 Initial 
                 Initial 
                 Write 
                   
                   
               
               
                 SCHEME 
                 Flash 
                 voltage 
                 state 
                 Voltage 
                 Masked 
                 Clear 
               
               
                   
               
             
             
               
                 1A 
                 No 
                 100 
                 Planar 
                 20 
                 Planar 
                 F-C 
               
               
                 1B 
                 Yes 
                  60 
                 Planar 
                 20 
                 Planar 
                 F-C 
               
               
                 2A 
                 No 
                  60 
                 F-C 
                 60 
                 F-C 
                 Planar 
               
               
                 2B 
                 Yes 
                  20 
                 F-C 
                 60 
                 F-C 
                 Planar 
               
               
                 3 
                 — 
                 — 
                 XXXX 
                 60 
                 F-C 
                 Planar 
               
               
                   
               
             
          
         
       
     
       FIG. 8  is an electrical schematic for a display writer made in accordance with the present invention. A power supply  91  provides power to a flash capacitor  92  and to the display drive  93 . A masking display  94  is disposed to selectively mask the output of flash  52 . Masking display  94  can be a simple twisted-nematic (TN) or super-twisted-nematic (STN) display of conventional design. Controller  95  supplies information to masking display  94 . Controller  95  applies writing voltage to electrodes  50  through display drive  93 , connected to display  10 . A trigger circuit  96  triggers flash  52  in conjunction with the application of a bipolar electrical field from display drive  93 . The flash energy is masked by masking display  94  to apply an image-wise light pattern from flash lamp  52  in conjunction with an applied field to write an image on display  10 . 
     In an experiment, a dot-matrix super-twisted-nematic (STN) display, part number TM 13164 BCHG-1 from Tianma Corporation in Taiwan was placed over a display  10  which was built in accordance to the preferred embodiment. A Vivitar flash, already described, was adjusted so that light absorbing portions of the STN display masked the flash and transparent portions of the STN display passed flash light. The masking effect was sufficient to write areas of display  10  into the focal-conic or planar states depending on the optical state of the masking display  94 . The flash unit was discharged through the STN display repeatedly with no observable harm to the structure of the STN display or display  10 . The experiment shows that it is possible to use simple, low-cost STN displays as masking display  94 . 
       FIG. 9  is a side view of the display of  FIG. 1  attached to an object  80 . Object  80  can be a tray that supports display  10  or an article to which display  10  is attached, such as a card. Display  10  is attached to object  80  using contacts  82 . Contacts  82  can be an electrically conductive adhesive having two areas, a first area in contact with first transparent conductor  20  and a second area in contact with second conductor  40 . In  FIG. 9 , the left portion is connected to first transparent conductor  20  and the right portion is connected to second conductor  40 . Contacts  82  are connection areas for electrodes  50  to connect to display  10  as well as an adhesive to secure display  10  to object  80 . 
       FIG. 10  is a side schematic view of a separable writer in accordance with the present invention. In the preferred embodiment, writer  90  is connected to display  10  for a writing event. Writer  90  contains the electrical components of FIG.  8 . Writer  90  has a housing (not shown) which positions flash  52 , masking display  94  and electrodes  50  in positions to write a display  10 . Details (not shown) of conventional design in object  80  and the housing of writer  90  align writer  90  to display  10 . 
       FIG. 11  is a side section view of the writer connected to the display to write display  10  in accordance with the present invention. Writer  90  is connected to display  10  when electrodes  50  are pressed against contacts  82 . Sensors (not shown) of conventional design can be connected to controller  95  to signal that writer  90  is connected to display  10 . Sensors can also be provided to signal controller  95  to write an image to display  10 . The writing method, previously disclosed, can be used to write an image on display  10 . Writer  90  can be detached from display  10 , and used to write other displays  10 . Object  80  has attached information on display  10  that has been updated. 
       FIG. 12A  shows a display  10  according to the present invention in a prewritten state. When display  10  is manufactured according to the present invention, the material is uniformly in the planar state. Alternatively, display  10  may have been written and can have areas having cholesteric material in either a focal-conic or planar state, or any intermediate state. Alternatively, previous information may have been erased by writer  90  in a previous writing event. Returning to  FIG. 11 , writer  90  is brought into a writable position with respect to display  10 . In that position, electrodes  50  contact areas of contacts  82  which are connected to first transparent conductor  20  and second conductor  40 . Using method  3 , no initialization writing is performed. A masking display  94  in writer  90  receives image data from controller  92 . Controller  92  applies a field through electrodes  50  to display  10  and discharges flash  52 . Radiation from flash  52  selectively heats areas of display  10  in the presence of an electric field to write an image on display  10 . Areas of display  10  receiving light are written into the planar state and areas masked from light from flash  52  are written into the focal conic state regardless of initial state of the cholesteric liquid crystal.  FIG. 12B  is a display according to the present invention in a written state. Brighter, lighter areas have been written into the planar state and the darker areas have been written into the focal-conic state. 
     The method and apparatus of this invention provides low-cost, simple and rapidly written labels with high information content. The displays are inexpensive, having a few simple, un-patterned, mass produced layers. The writing apparatus itself is simple, requiring a low-cost flash system, an inexpensive masking display and a 2 wire electrical exciter for the display  10 . Separating the drive from the display permits many inexpensive labels to be written by a single, simple display writer  90 . 
     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 
     
         
           10  display 
           15  display substrate 
           20  first transparent conductor 
           20 ′ exposed first conductor 
           22  first conductor cover 
           30  light modulating layer 
           35  light absorber 
           40  second conductor 
           50  electrodes 
           52  flash 
           54  mask 
           60  incident light 
           62  reflected light 
           72  planar liquid crystal 
           74  focal-conic liquid crystal 
           80  object 
           82  contacts 
           90  writer 
           91  power supply 
           92  flash capacitor 
           93  display drive 
           94  masking display 
           95  controller 
           96  trigger circuit