Abstract:
A multi-layer, neutral-density sheet with memory properties, includes a transparent substrate and a transparent, electrically conductive layer formed over the transparent substrate. The multi-layer sheet further includes a plurality of light modulating layers formed over the transparent electrically conductive layer, each light modulating layer being formed of polymer with a dispersion of cholesteric liquid crystal material having memory properties and selected so that in combination with cholesteric liquid crystal in other layers are controllable between a first, light reflecting neutral-density state and the second transparent state.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Reference is made to commonly-assigned U.S. patent application Ser. No. 09/045,016 filed Mar. 20, 1998, entitled “Display Having Viewable and Conductive Images” by Stanley W. Stephenson now U.S. Pat. No. 6,267,697, U.S. patent application Ser. No. 09/146,656 filed Sep. 3, 1998, entitled “Reflective Sheet Display With Laser Pattemable Coating”, by Stephenson et al now U.S. Pat. No. 6,236,442, and U.S. patent application Ser. No. 09/336,931 filed concurrently herewith, entitled “A Sheet Having a Layer with Different Light Modulating Materials” by Stanley W. Stephenson now U.S. Pat. No. 6,359,673, the disclosures of which are incorporated herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to imaging sheets which can be used in displays that selectively transmit or reflect light. 
     BACKGROUND OF THE INVENTION 
     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. Other sheet materials can carry magnetically writable areas to carry ticketing or financial information, however magnetically written data is not visible. 
     A structure is disclosed in PCT/WO 97/04398, entitled “Electronic Book With Multiple Display Pages” which is a thorough recitation of the art of thin, electronically written display technologies. Disclosed is the assembling of multiple display sheets that are bound into a “book”, each sheet is arranged to be individually addressed. The patent recites prior art in forming thin, electronically written pages, including flexible sheets, image modulating material formed from a bi-stable liquid crystal system, and thin metallic conductor lines on each page. 
     Fabrication of flexible, electronically written display sheets are disclosed in U.S. Pat. No. 4,435,047. A first sheet has transparent indium-tin-oxide (ITO) conductive areas and a second sheet has electrically conductive inks printed on display areas. The sheets can be glass, but in practice have been formed of Mylar polyester. A dispersion of liquid crystal material in a binder is coated on the first sheet, and the second sheet is bonded to the liquid crystal material. Electrical potential applied to opposing conductive areas operate on the liquid crystal material to expose display areas. The display uses nematic liquid crystal materials which ceases to present an image when de-energized. 
     U.S. Pat. No. 5,223,959 discloses a plurality of polymer dispersed liquid crystal material, each having a different dye material of red, green or blue dye material. Differing electrical signals to common electrodes operate on each of the materials to control the state of each type of dyed liquid crystal material. The patent requires the use of conventional nematic liquid crystals with a dye to absorb light. The droplets are chemically treated to be stable in either a clear or a light absorbing state. The invention also requires materials having different response times to electrical signals. The device must be continually driven so that the human eye perceives complementary colors. This arrangement has the disadvantage of requiring continuous, high speed electrical drive because the materials do not maintain their state. The material must be driven to achieve a neutral color density. 
     U.S. Pat. No. 5,437,811 discloses a light modulating cell having a polymerically 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 light scattering focal-conic state. The structure has the capacity of maintaining one of the given states in the absence of an electric field. 
     U.S. Pat. No. 5,847,798 discloses a continuous tone, neutral density display which can be driven between a reflective and light absorbing state. The display use a single cholesteric material to provide a constant reflectance across the visible light spectrum to provide a neutral color. The material is reflective in the non-visible spectrum and appears clear in the visible spectrum. The display can also be driven in to a stable light scattering state that is uniformly scattering in the visible spectrum. A neutral “white” state occurs, but the provides a very low reflective intensity. 
     Currently, privacy windows are created using the scattering properties of conventional nematic liquid crystals. Such materials require continuous electrical drive to remain transparent. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a neutral density sheet with memory properties. 
     It is another object of the present invention to provide a neutral density sheet which permits continuous tone, neutral density images that are stable in a zero-field condition. 
     A still further object is to provide a sheet usable as a display that can be electronically written repeatedly. 
     These objects are achieved in a multi-layer, neutral-density sheet with memory properties, comprising 
     (a) a transparent substrate; 
     (b) a transparent, electrically conductive layer formed over the transparent substrate; and 
     (c) a plurality of light modulating layers formed over the transparent electrically conductive layer, each light modulating layer being formed of polymer with a dispersion of cholesteric liquid crystal material having memory properties and selected so that in combination with cholesteric liquid crystal in other layers are controllable between a first, light reflecting neutral-density state and the second transparent state. 
     The present invention provides a neutral density privacy sheet that is stable in either a light blocking or light transmitting field in a zero state condition. The reflective characteristic of cholesteric materials provided in multiple layers generates a light reflection across the visible spectrum. 
     Sheets made in accordance with the present invention can be used to provide a re-writable image display sheet. The present invention uses a plurality of layers of cholesteric liquid crystal materials that are effective in at least two states, a reflective state and a transmissive state. This invention permits the use of light modulating, electrically responsive sheets with improved reflective efficiency. The sheet can be formed using inexpensive, efficient photographic layer methods. A single large volume of sheet material can be coated and formed into various types of sheets and cards. Displays in the form of sheets in accordance with the present invention are inexpensive, simple and fabricated using low-cost processes. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a sectional view of a sheet having multi-layers of liquid crystal in accordance with the present invention; 
     FIG. 1B is a sectional view of the sheet in FIG. 1A having a conductive layer; 
     FIG. 1C is a sectional view of the sheet of FIG. 1B having a scribed pattern in the conductive layer; 
     FIG. 2A is a view of the optical characteristics of a chiral nematic material in a planar state reflecting light; 
     FIG. 2B is a view of the optical characteristics of a chiral nematic material in a focal-conic light diffusing state; 
     FIG. 3 is a sectional view of a domain containing chiral nematic liquid crystal material; 
     FIG. 4 is a sectional view of a sheet having three layers of dispersed chiral nematic liquid crystals; 
     FIG. 5 is the spectral reflection of the sheet in FIG. 4; 
     FIG. 6 is a sectional view of a sheet having two layers of dispersed chiral nematic liquid crystals; and 
     FIG. 7 is the spectral reflection of the sheet in FIG.  6 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1A is a sectional view of a sheet  10  used in the invention. The sheet  10  includes a substrate  30 . Substrate  30  can be made of a transparent polymeric material, such as Kodak Estar film base formed of polyester plastic, and have a thickness of between 20 and 200 microns. For example, substrate  30  can be a 80 micron thick sheet of polyester. Other polymers, such as transparent polycarbonate, can also be used. An optically transparent, electrically conductive layer  32  is formed over the substrate  30 . The transparent, electrically conductive layer  32  can be formed of tin-oxide or Indium-Tin-Oxide (ITO), with ITO being the preferred material. Typically, transparent, electrically conductive layer  32  is sputtered onto the substrate  30  to a resistance of less than 250 ohms per square. 
     Light modulating layers  11  are deposited over transparent, electrically conductive layer  32 . The liquid crystal materials are chiral doped nematic liquid crystal, also known as cholesteric liquid crystals, dispersed in a polymeric binder. These types of liquid crystal molecules can align in a planar structure and the chirality of the molecules set to reflect a given wavelength of visible light. 
     FIG. 2A, and FIG. 2B show states of cholesteric liquid crystals. In FIG. 2A, a high voltage field has been applied and quickly switched to zero potential, which causes the liquid crystal molecules to become planar liquid crystals  12 . Incident light  16  can consist of red, green and blue fractions of light. The pitch of the molecules can be adjusted to create a Bragg diffraction of reflected light  18  comprised of light of a given color and polarity. In this example, the chirality of planar liquid crystals  12  is adjusted to reflect green light. 
     In FIG. 2B, application of a lower voltage field has caused molecules of the chiral nematic material to break into tilted cells that are known as the focal conic liquid crystals  14 . The lower voltage field can progressively drive the molecules of the cholesteric material towards a transparent state. A light absorber  20  can be positioned on the side opposing the incident light  16 . In the fully evolved focal-conic state, incident light  16  becomes absorbed light  19 . Progressive evolution of the focal-conic state causes a viewer to perceive green light that transitions to black as the cholesteric material changes from a planar to a fully evolved focal-conic state. The transition to the light transmitting state is progressive, and varying the low voltage time permits a variable level of reflection. These variable levels can be mapped out to corresponding gray levels, and when the field is removed, light modulating layers  11  maintains a given optical state indefinitely. The states are more fully discussed in U.S. Pat. No. 5,437,811. 
     Chiral nematic materials are superior to undoped nematic crystals having incorporated dichroic dyes because chiral doped nematic materials maintain a given state between reflective to transparent states when the electrical drive field is removed. However, in a single layer configuration they operate on light having only one color. Returning to FIG. 1A, a plurality of polymer dispersed cholesteric materials are coated as red reflecting cholesteric  34 , green reflecting cholesteric  36  and blue reflecting cholesteric  38  over transparent, electrically conductive layer  32 . The concentration and layer thicknesses are adjusted to create a neutral density “white” reflective layer in the planar state. Because each layer is polymerically dispersed, multiple layers having separable chromatic reflectivity can be incorporated into a multilayer layer sharing a common drive field. 
     Red reflecting cholesteric  34 , green reflecting cholesteric  36  and blue reflecting cholesteric  38  are dispersed in a polymeric binders such as a UV curable polymer, an epoxy, polyvinyl alcohol (PVA) or in this invention deionized photographic gelatin. The binder content can be between 5% and 30%. Compounds such as gelatin and PVA are machine coatable on equipment associated with photographic films. It is important that the binder have a low ionic content. The presence of ions in such a binder hinders the development of an electrical field across the dispersed liquid crystal material. De-ionized photographic gelatin reduces the ionic content of gelatin to operable levels. Ions in the binder can migrate in the presence of an electrical field on red reflecting cholesteric  34 , green reflecting cholesteric  36  and blue reflecting cholesteric  38 . 
     FIG. 3 is a cross section through a domain  25  containing a cholesteric material. Domain  25  is spherical, and cholesteric materials anchor on the surface of the domain. Because the surface of domain is spherical, incident light  16  from any angle of observation is reflected. The result is that these polymer dispersed (cholesteric) liquid crystals (PDLC) have good off-axis reflectivity. 
     In FIG. 1B, a second conductive layer  40  is formed over light modulating layers  11 . Other commonly used materials and processes can be used to provide a vacuum deposited coat to second conductive layer  40 . In a vacuum environment, materials such as Aluminum, Tin, Silver, Platinum, carbon, Tungsten, Molybdenum, Tin or Indium can be used. Oxides of the metals can be used to darken second conductive layer  40 . The metal material can be excited by energy from resistance heating, cathodic arc, electron beam, sputtering or magnetron excitation. Use of Tin-Oxide or Indium-Tin Oxide in the layer permits layers of second conductive layer  40  that are transparent. An ITO layer can be sputtered over light modulating layers  11  to less than 250 ohms per square of resistance and over 80% light transmission. 
     FIG. 1C is a sectional view through the sheet  10  after laser processing. The laser removes portions of second conductive layer  40  to create non-conducting areas  44 . Remaining portions of second conductive layer  40  form conductive areas  42  which can appear black, having an optical density of greater than 2.0 D or be nearly transparent. Nominally conductive metal in non-conductive areas  44  has been removed using a Ytterium-Aluminum Garnet (YAG) laser to define the extent of conductive areas  42 . Non-conductive areas  44  are typically gaps approximately 2-5 microns wide that separate conductive areas  42 . The YAG laser can generate patterns in second conductive layer  40  for both opaque and transmissive layers of materials. Alternatively, a light sensitive, metal forming layer can be used to create conductive areas  42  and non-conductive areas  44 . A material such as silver halide can be light patterned and developed with nucleated silver ions to create a light absorbing, electrically conductive layer. 
     The transparent, electrically conductive layer  32  provides a continuous electrode for light modulating layers  11 . An electrical field across conductive areas  42  and transparent, electrically conductive layer  32  operate on all of light modulating layers  11  to permit selective reflection or transmission of light through sheet  10 . 
     Turning to FIG. 4, incident light  16  passes through transparent substrate  30  and transparent, electrically conductive layer  32 . In the diagram, red reflecting cholesteric  34 , green reflecting cholesteric  36  and blue reflecting cholesteric  38  have been written into the reflective, planar state. When incident light  16  passes through red reflecting cholesteric  34 , red light is reflected from red reflecting cholesteric  34  as red reflected light  52 . Incident light  16 , minus red reflected light  52 , then passes subsequent layers. When incident light  16  passes through subsequent layers, further portions of the visible spectrum are reflected. The reflectivity of the layers is adjusted to create a neutral density reflection which appears as white or grey. The multi-layer structure is simultaneously written when a field is applied between conductive areas  42  and transparent, electrically conductive layer  32 . If the materials are fully driven into the focal-conic state, all wavelengths of light can pass through sheet  10 . If conductive areas  42  are absorptive, then incident light  16  becomes absorbed light  19  and the sheet appears black. 
     FIG. 5 is a plot of the reflectivity of sheet  10  in the planar state as a function of wavelength. Each of the three reflective layers acts on a component of visible light to create a neutral density. Red reflecting cholesteric  34  creates the peak of red reflected light  52 . Green reflecting cholesteric  36  creates the peak of green reflected light  54 . Blue reflecting cholesteric  38  creates the peak of blue reflected light  56 . Combined, the three layers form a neutral density. 
     If second conductive layer  40  is formed of a transparent material, sheet  10  acts as a neutral filter to block light in the planar state and appears transparent in the fully evolved focal-conic state. An un-patterned sheet  10  can serve as a privacy window that can selectively block or transmit light. Such a sheet provides a neutral density privacy screen that can be momentarily written and maintain state in the absence of an electrical field. 
     FIG. 6 is an alternative embodiment of the current invention. Light modulating layers  11  is two layers of polymer stabilized cholesteric material instead of three layers. Cholesteric materials have a range of reflection, and in this embodiment, two materials are used, blue reflecting cholesteric  38  and yellow reflecting cholesteric  39 . Blue reflecting cholesteric  38  is a nematic liquid crystal with a concentration of chiral dopant to create a greenish blue at about 490 nanometers. 
     In FIG. 6, blue reflecting cholesteric  38  and yellow reflecting cholesteric  39  have been written into the reflective, planar state. When incident light  16  passes through blue reflecting cholesteric  38 , blue light is reflected from blue reflecting cholesteric  38  as blue reflected light  56 . The shift in peak wavelength towards green causes some green light to be reflected. Incident light  16 , minus blue reflected light  56 , then passes subsequent layers. When incident light  16  passes through yellow reflecting cholesteric  39 , much of both the red and green components of incident light  16  are reflected from yellow reflecting cholesteric  39  as yellow reflected light  58 . The reflectivity of the layers is adjusted to create a neutral density reflection, appearing white or gray. The multi-layer structure shares a common field between conductive areas  42  and transparent, electrically conductive layer  32 . When the materials are fully driven into the focal-conic state, all wavelengths of light can pass through sheet  10 . If conductive areas  42  are absorptive, then incident light  16  becomes absorbed light  39  and sheet  10  appears to be black. 
     FIG. 7 is a plot of the reflectivity of sheet  10  as a function of wavelength. Each of the two reflective layers acts on a component of visible light to create a neutral density. Blue reflecting cholesteric  38  creates the peak of blue reflected light  56  with some additional reflection of green light due to the shift in peak reflectance. Yellow reflecting cholesteric  39  creates the peak of yellow reflected light  58 . The peak reflection of yellow reflecting cholesteric  39  and portions of both the green and red colors of incident light  16  are reflected. Combined, the two layers form a neutral density. 
     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  sheet 
       11  light modulating layers 
       12  planar liquid crystals 
       14  focal-conic liquid crystals 
       16  incident light 
       18  reflected light 
       19  absorbed light 
       20  light absorber 
       25  domain 
       30  substrate 
       32  transparent, electrically conductive layer 
       34  red reflecting cholesteric 
       36  green reflecting cholesteric 
       38  blue reflecting cholesteric 
       39  yellow reflecting cholesteric 
       40  second conductive layer 
       42  conductive areas 
       44  non-conductive area 
       52  red reflected light 
       54  green reflected light 
       56  blue reflected light 
       58  yellow reflected light