Sheet having a layer with different light modulating materials

[0002] 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, U.S. patent application Ser. No. 09/146,656 filed Sep. 3, 1998, entitled “Reflective Sheet Display With Laser Patternable Coating”, by Stephenson et al, and U.S. patent application Ser. No. 09/337,312 filed Jun. 21, 1999 concurrently herewith, entitled “Multi-Layer Neutral Density Sheet With Memory Properties” by Stanley W. Stephenson, the disclosures of which are incorporated herein.

[0003] The present invention relates to sheets that can be used as displays that selectively transmit or reflect light.

[0004] 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.

[0005] 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 provided with means to individually address each page. 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.

[0006] 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.

[0007] 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 sheet 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.

[0008] 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.

[0009] U.S. Pat. No. 5,847,798 discloses a continuous tone, neutral density display that can be driven between a reflective and light absorbing state. The display uses 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.

[0010] U.S Pat. No. 4,576,891 discloses encapsulated dye-forming capsules that generate color images in response to pressure. Separate capsules responsive to red, green and blue wavelengths of light and forming colors responsive to those respective wavelengths are selectively hardened and form color images when crushed. Capsules are formed separately and mixed to create a single pressure sensitive layer. The sheets are light written and the dye forming process is irreversible.

[0011] Currently, privacy windows are created using the scattering properties of conventional nematic liquid crystals. Such materials require continuous electrical drive to remain transparent.

[0012] Accordingly, it is an object of the present invention to provide a neutral density display apparatus permitting continuous tone, neutral density images that are stable in a zero-field condition.

[0013] A further object of the invention to provide a neutral density privacy window that is stable in either a tight blocking or light transmitting field in a zero state condition.

[0014] Another object is to provide a sheet that can be re-written using electronic means.

[0015] Yet another object is to improve optical properties of a sheet by using a single field to operate on a plurality of liquid crystal formulations.

[0016] These objects are achieved in a light modulating, electrically responsive sheet comprising:

[0017] (a) a transparent substrate;

[0018] (b) a transparent, electrically conductive layer formed over the transparent substrate; and

[0019] (c) a light modulating layer including a plurality of electrically responsive light modulating materials dispersed in the light modulating layer and having a plurality of optical states.

[0020] An advantage of the present invention is the use of reflective characteristic of multiple cholesteric materials to provide light reflection across the visible spectrum.

[0021] Sheets made in accordance with the present invention can be used to provide a re-writable image sheet. The present invention uses a plurality of cholesteric liquid crystal materials in a layer which 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.

[0022] FIG. 1A is a sectional view of a sheet having a layer with different liquid crystal materials in accordance with the present invention;

[0023] FIG. 1B is a sectional view of the sheet in FIG. 1A having a conductive layer;

[0024] FIG. 1C is a sectional view of the sheet of FIG. 1B having a scribed pattern in the conductive layer;

[0025] FIG. 2A is a view of the optical characteristics of a chiral nematic material in a planar state reflecting light;

[0026] FIG. 2B is a view of the optical characteristics of a chiral nematic material in a focal-conic light transmitting state;

[0027] FIG. 3 is a sectional view of a domain containing chiral nematic liquid crystal material;

[0028] FIG. 4A is a diagram of blending a three component neutral density dispersion;

[0029] FIG. 4B is a diagram of blending a two component neutral density dispersion;

[0030] FIG. 4C is a diagram of blending a two component dispersion of materials having opposite polar reflectivity;

[0031] FIG. 5 is a sectional view of a sheet having three types of dispersed chiral nematic liquid crystals to produce a neutral density sheet;

[0032] FIG. 6 is the spectral reflection of the sheet in FIG. 5;

[0033] FIG. 7 is a sectional view of a sheet having two types of dispersed chiral nematic liquid crystals to produce a neutral density sheet;

[0034] FIG. 8 is the spectral reflection of the sheet in FIG. 7;

[0035] FIG. 9 is a sectional view of a sheet having two different polarities of dispersed chiral nematic liquid crystals to increase reflectivity; and

[0036] FIG. 10 is the spectral reflection of the sheet in FIG. 7.

[0037] 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, the transparent, electrically conductive layer 32 is sputtered onto the substrate 30 to a resistance of less than 250 ohms per square.

[0038] A light modulating layer 11 is deposited over transparent, electrically conductive layer 32. The liquid crystal materials in light modulating layer 11 are preferably chiral doped nematic liquid crystals, also known as cholesteric liquid crystals, dispersed in a polymeric binder. These types of liquid crystal molecules can align in a reflective planar structure and the chirality of the molecules set to reflect a wavelength of visible light.

[0039] 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.

[0040] 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. 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 layer 11 maintains a given optical state indefinitely. The states are more fully discussed in U.S. Pat. No. 5,437,811.

[0041] Chiral nematic materials are superior to un-doped 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 formulation layer liquid crystal material operates on light having only one color and polarity. Because different polymerically dispersed materials are incorporated in a single layer with a common electrical drive, optical properties can be improved over a single formulation. Having two formulations of opposing polarity will improve the reflectivity of a layer having a cholesteric material of only one polarity.

[0042] In FIG. 4A, red reflecting cholesteric 34, green reflecting cholesteric 36 and blue reflecting cholesteric 38 are separately dispersed in polymeric binders such as a UV curable polymer, an epoxy, polyvinyl alcohol (PVA) or in this invention de-ionized 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 is required to reduce the ionic content of gelatin to operable levels. Ions in the binder can migrate in the presence of an electrical field applied on chemically red reflecting cholesteric 34, green reflecting cholesteric 36 and blue reflecting cholesteric 38.

[0043] In FIG. 1A, three separate formulations are then blended to form light modulating layer 11, which has a neutral reflection density. Because each formulation is dispersed separately and then blended, interaction between formulations is prevented. FIG. 4B is an alternative blend, incorporating a blue reflecting cholesteric 38 and a yellow cholesteric material 39, blended to form a neutral density light modulating layer 11. In FIG. 4C, a right handed cholesteric 60 and a left handed cholesteric 62 having a common peak reflectance are blended to improve the reflectivity light modulating layer 11.

[0044] FIG. 3 is a cross section through a domain 25 containing a cholesteric material. Domain 25 is spherical, and cholesteric material anchors 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.

[0045] In FIG. 1B, a second conductive layer 40 is formed over light modulating layer 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 said metals can be used to darken second conductive layer 40. The metal material can be excited by energy from resistance heating, cathodic are, 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.

[0046] 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-Alunminum 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.

[0047] The transparent, electrically conductive layer 32 provides a continuous electrode for light modulating layer 11. An electrical field across conductive areas 42 and transparent, electrically conductive layer 32 operate on all formulations of light modulating layer 11 to permit selective reflection or transmission through light modulating layer 11.

[0048] Turning to FIG. 5, incident light 16 passes through transparent substrate 30 and transparent, electrically conductive layer 32. In FIG. 5, 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 a domain having 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, continues to pass through light modulating layer 11. When incident light 16 passes through subsequent domains, further portions of the visible spectrum are reflected. The reflectivity of the domains is adjusted by concentration to create a neutral density reflection. The multi-formulation light modulating layer 11 shares an applied 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 19.

[0049] FIG. 6 is a plot of the reflectivity of sheet 10 as a function of wavelength. Each of the three reflective formulation 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 58. Combined, the three layers form a neutral density.

[0050] 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.

[0051] FIG. 7 is an alternative embodiment of the current invention. Light modulating layer uses two polymer stabilized cholesteric formulations instead of three formulations. 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.

[0052] In FIG. 7, 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 light 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 56 and sheet 10 appears to be black.

[0053] FIG. 8 is a plot of the reflectivity of sheet 10 as a function of wavelength. Each of the two reflective materials 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 is set so portions of the green and red colors of incident light 16 are reflected. Combined, the two layers form a neutral density.

[0054] FIG. 9 is a partial view of an alternative structure for sheet 10. Left handed cholesteric 62 and right handed cholesteric 60 have been dispersed and blended to form a material having isolated domains having either right handed or left handed polar reflectivity. Combining the two cholesteric materials directly would cause the two materials to interfere with each other. Dispersing each cholesteric material into a binder material and mixing the two dispersions permits formation of light modulating layer 11 which has domains of materials having separate reflective polarities which do not interfere with each other. Light modulating layer 11 is applied as a single layer over transparent, electrically conductive layer 32. The domains of liquid crystal in light modulating layer 11 are smaller than the layer thickness so that multiple domains overlap. Incident light 16 passes through light modulating layer 11 and can pass through domains of right handed cholesteric 60 which reflects right handed reflected light 64. Same incident light 16 can also strike a domain consisting of left handed cholesteric 62, which reflects left handed reflected light 66. The combination of right handed cholesteric 60 and left handed reflective cholesteric 62 in a layer improves the reflective efficiency of sheet 10.

[0055] FIG. 10 is a chromatic distribution of visible light striking sheet 10 having a light modulating layer 11 with domains of both right handed cholesteric 60 and left handed cholesteric 62. The two materials have been formulated to have peak reflection at 550 nanometers. A single layer of right handed cholesteric 60 will have a peak reflectance shown by the curve of right handed light 64. Addition of left handed domains 62 increases light reflection to the right handed light 64 by the curve shown as left handed light 66.

[0056] 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.

[0057] 10 sheet

[0058] 11 light modulating layer

[0059] 12 planar liquid crystals

[0060] 14 focal-conic liquid crystals

[0061] 16 incident light

[0062] 18 reflected light

[0063] 19 absorbed light

[0064] 20 light absorber

[0065] 25 domain

[0066] 30 substrate

[0067] 32 transparent, electrically conductive layer

[0068] 34 red reflecting cholesteric

[0069] 36 green reflecting cholesteric

[0070] 38 blue reflecting cholesteric

[0071] 39 yellow reflecting cholesteric

[0072] 40 second conductive layer

[0073] 42 conductive areas

[0074] 44 non-conductive area

[0075] 52 red reflected light

[0076] 54 green reflected light

[0077] 56 blue reflected light

[0078] 58 yellow reflected light

[0079] 60 right handed cholesteric

[0080] 62 left handed cholesteric

[0081] 64 right handed reflective light

[0082] 66 left handed reflective light