Abstract:
A display for presenting image forming light to a viewer, includes a transparent substrate; a transparent, electrically conductive layer formed over the transparent substrate; a light modulating layer formed over a portion of the transparent, electrically conductive layer being effective in a first stable state to reflect light and in a second stable state to transmit light; and a layer formed over the light modulating layer which includes separate conductive portions. Electrical connections are provided which are selectively connected to separate conductive portions and being effective in a first condition to apply a first field across selected portions of the light modulating layer which correspond to separate conductive portions to be in the first stable state to reflect light and to apply a second field across selected separate conductive portions of the light modulating layer which correspond to separate conductive portions to be in the second stable state to transmit light.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Reference is made to commonly-assigned U.S. patent application Ser. No. 08/961,056 filed Oct. 30, 1997, entitled “Single Sheet Display Having Patternable Conductive Traces” by Stanley W. Stephenson; commonly-assigned U.S. patent application Ser. No. 08/990,891 filed Dec. 15, 1997, entitled “Method of Producing a Display Having Patternable Conductive Traces” by Stanley W. Stephenson; commonly-assigned U.S. patent application Ser. No. 08/990,853 filed Dec. 15, 1997, entitled “A Sheet Having Patternable Conductive Traces for Use in a Display” by Stanley W. Stephenson; and commonly-assigned U.S. patent application Ser. No. 09/027,321 filed Feb. 20, 1998, now issued as U.S. Pat. No. 5,912,716 entitled “Selectively Presenting Viewable and Conductive Images” by Stanley W. Stephenson, the disclosures of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to image displays which can 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 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 bistable liquid crystal system, and thin metallic conductor lines on each page. Various ways are disclosed to produce said conductor lines including photolithography, but not selective exposure and photographic development of traces from a photosensitive emulsion. One disadvantage of this structure is that individual pages are bound together and that many multi-layer conductors must pass across the pages to interconnect at the spine of the book. 
     Fabrication of flexible, electronically written display sheets are disclosed in U.S. Pat. No. 4,435,047. A first sheet has transparent 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 ceases to present an image when de-energized. Kaychem Industries form electrical flexible displays interconnection by offsetting the two sheets and contacting trace conductors from each of the two sheets. 
     The prior art typically requires multiple, separate layers to build up the display. The electrical traces and transparent conductive layers are typically formed through repeated vacuum deposition and photolithography of materials on the substrate. These processes are expensive and require long processing times on capital intensive equipment. Because most display structures are formed of glass, two sheets are used and are offset to permit connection to two separate and exposed sets of traces that are disposed on separate sheets 
     In the case of electronic display means, power must be provided to view images. Printed sheets receive ink and cannot be rewritten. In the case of magnetically written media such as magnetic areas on the back of credit cards, the information is not readable. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a display apparatus which uses a minimum number of layers for changing the transmissivity of image forming light. 
     Another object is to provide a display that can be re-written using electronic means. 
     These objects are achieved by a display for presenting image forming light to a viewer, comprising: 
     (a) a transparent substrate; 
     (b) a transparent, electrically conductive layer formed over the transparent substrate; 
     (c) a light modulating layer formed over a portion of the transparent, electrically conductive layer being effective in a first stable state to reflect light and in a second stable state to transmit light; 
     (d) a layer formed over the light modulating layer which includes separate conductive portions; and 
     (e) electrical conduction means being adapted to be selectively connected to separate conductive portions and being effective in a first condition to apply a first field across selected portions of the light modulating layer which correspond to separate conductive portions to be in the first stable state to reflect light and to apply a second field across selected separate conductive portions of the light modulating layer which correspond to separate conductive portions to be in the second stable state to transmit light. 
     Displays made in accordance with the present invention can be used to provide a rewritable image display sheet. The sheet can be formed using inexpensive, fast photographic means to expose and develop a display. A single large volume of sheet material can be coated and formed into various types of sheets and cards. 
     Advantageously, sheets which form displays can be made from simple coatings, and they receive and retain a viewable image with a simple writer and retain the image data without a power source. 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. 1 is a sectional view of a display sheet in before it has been completed in accordance with the present invention; 
     FIG. 2 is a sectional view of the sheet of FIG. 1 in a completed condition; 
     FIG. 3 a  is a partial top view of the completed display sheet of FIG. 2; 
     FIG. 3 b  is a magnified view of a portion of the display sheet of FIG. 3 a;    
     FIGS. 4 a - 4   c  show various steps in the formation of the conductive pixels of the display sheet of FIG. 2 in accordance with the present invention; 
     FIG. 5 a  is a front sectional view showing a writer writing to a processed sheet; 
     FIG. 5 b  is a side sectional view showing of the printer of FIG. 5 a;    
     FIG. 6 is a sectional view showing the optical effect of a sheet on light; and 
     FIG. 7 is a schematic view of circuitry for writing on displays in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 depicts a sectional view of an incomplete display sheet  10  used in the invention. The display sheet  10  includes a substrate  12 . Substrate  12  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  12  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  13  is formed over the substrate  12 . The transparent, electrically conductive layer  13  can be formed of tin-oxide or Indium-Tin-Oxide (ITO), with ITO being the preferred material. Typically, the transparent, electrically conductive layer  13  is sputtered onto the substrate  12  to a resistance of less than 250 ohms per square. 
     A light modulating layer  30  is formed over the transparent, electrically conductive layer  13 . Light modulating layer  30  is formed from a chiral doped nematic liquid crystal such as those disclosed in U.S. Pat. No. 5,695,682. A chiral doped nematic liquid crystal material is supported in a binder of hardened gelatin. The nematic liquid crystal has a chiral dopant that reflects light in a first. homeotropically aligned state. For an example of a imager which uses liquid crystals see U.S. Pat. No. 4,603,945. 
     The liquid crystal molecules start in a pitched formation across the light modulating layer  30 , and the twist (or chirality) of the molecules is set to reflect a wavelength of visible light. A first, low voltage electric field can disrupt the orderly pitch of the material and the material switches to a focal-conic texture that is a hazy and light diffusing. If the field strength increased, the material becomes optically clear, In this transparent state, incident light passes through light modulating layer  30  and onto a light absorbing layer, which creates “black”. See, for example, the &#39;682 patent cited above. If the voltage is switched off in this state, the material snaps to the original, light reflecting condition. If the voltage is removed at a slower rate, the display will return to a light transmitting, black state. The transition to the light transmitting state is progressive, and varying the time that the voltage is removed 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 level  30  maintains a given optical state indefinitely. 
     For another approach, for creating gray levels in Hashimoto et al, “Reflective Color Display Using Cholesteric Liquid Crystals”, SID 98 Digest, Article 31.1, 1998, pp. 897-900. A first, high voltage pulse clears all pixels, and a second, lower voltage pulse puts the liquid crystal molecules in the focal-conic scattering mode. The pulse time of a third, intermediate voltage pulse returns the liquid crystal material to different degrees of reflectivity based on the time of the third voltage pulse. 
     The light modulating layer  30  preferably includes liquid crystal material from a polymeric a binder such as a UV curable polymer, an epoxy, and in this invention de-ionized gelatin or polyvinyl alcohol (PVA). The binder content can be between 0.5% and 20.0% of the material in the modulating material and permits such materials to have a “memory” for either a reflective or transitive state. 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. Additionally, ions in the binder can migrate in the presence of an electrical field, chemically damaging the light modulating layer  30   
     The layer thickness, the structure of the polymer network within the liquid crystal material designed to optimize the reflection and transmission of light through light modulating layer  30 . Other bi-stable materials can also-be used for light modulating layer  30 , such as electro-chromic or micro-spherical particles. The light modulating layer  30  is effective in two conditions, which will be described in more detail below. Light modulating layer  30  will have low strength at low polymer concentrations, and photosensitive layer  14  can serve as a protective, stabilizing cover over a weak light modulating layer  30 . 
     A barrier layer  20  is coated over light modulating layer  30 . Barrier layer  20  protects light modulating layer  30  from processing chemicals used on display sheet  10 . Barrier layer  20  can be a layer of de-ionized gelatin or PVA that has been polymerized to resist ionic diffusion into light modulating layer  30 . A photosensitive layer  14  is coated over barrier layer  20 . The photosensitive layer  14  must form metal deposits of conductivity sufficient to carry a field to operate on the light modulating layer  30 , and is preferably an emulsion of silver halide grains. Alternatively, other photosensitive materials can be used, such as gold or copper salts. In the case of silver halide emulsions, high concentrations of crystalline silver halide in a binder, such as gelatin or PVA, are used to improve conductivity over conventional imaging emulsions. Conductive additives such as fine Indium-Tin-Oxide or fine silver with particle sizes between 0.5 and 2 microns can be added to the emulsion to improve the electrical conductivity of photographically produced metallic silver. 
     FIG. 2 is a sectional view through the display sheet  10  after processing. The photosensitive layer  14  has been exposed and processed to create conductive areas  16  and non-conductive areas  18 , as shown in FIG.  2 . Conductive areas  16  should have sheet resistance equal to or greater than the sheet resistance of the transparent, electrically conductive layer  13 . Sheet resistivity of less than 200 ohms per square have been formed and will operate on light modulation layers  30 . When silver halide grains in gelatin are used for the photosensitive layer  14 , conductive areas  16  are metallic silver formed from exposed silver halide grains in the unprocessed display sheet  10 . Conductive areas  16  appear black, having an optical density of greater than 2.0 D. The light absorbing characteristic of conductive areas  16  provide the “black” level for the display. Unexposed silver halide in non-conductive areas  18  has been removed by conventional photographic development processes to define the extent of conductive areas  16 . Non-conductive areas  18  are typically gaps in developed silver approximately 5-50 microns wide that electrically isolate electrically conductive areas  16 . Non-conductive areas  18  should be fine enough that photosensitive layer  14  appears to be uniformly black. 
     The transparent, electrically conductive layer  13  provides a continuous electrode across light modulating layer  30 . An electrical field across conductive areas  16  and transparent, electrically conductive layer  13  will operate on light modulating layer  30  to selectively permit either reflection or absorption of light in conductive areas  16 . 
     FIG. 3 a  is a partial top view of the completed sheet. Conductive areas  16  and non-conductive areas  18  cover the majority of the sheet, and power areas  35  have been formed on two sides of display sheet  10 . Power areas  35  are areas on display sheet  10  with all coatings removed with the exception of transparent, electrically conductive layer  13 . Layers above the transparent electrically conductive layer  13  are removed to form power areas  35 . Such removal can be accomplished by chemical etching. Power areas  35  are areas that permit electrical connection to transparent, electrically conductive layer  13 . 
     FIG. 3 b  is a magnified rear view of a portion of the surface of display sheet  10 . Conductive areas  16  are small pads of conductive silver that define pixel elements on display sheet  10 . Non-conductive area  18  define a fine silver-free mesh that limits each conductive area  16 . Preferably, nonconductive areas  18  can be 25 micron across, and non-conductive area  18  can be 10 microns apart. Typical display resolutions require 150 dots per inch (75 micron pitch) for readability. The size of the pixels permits 4 to 9 conductive areas  16  per a 300 dpi pixel. Nonconductive areas  18  are required to limit an electrical field operating between transparent, electrically conductive layer  13  and conductive areas  16 . When the light modulating layer  30  is employed in a display sheet  10  which is effective in only two states, in the first state light modulating layer  30  transmits light, which is absorbed by conducting areas  16 , and in the second state the light modulating layer  30  reflects light over conductive areas  16 . Defined areas of light absorption and light reflectance create “black” and “white” areas respectively, permitting the recording of text or image data. 
     FIGS. 4 a - 4   c  are schematic representations of various steps in showing how conductive areas  16  are formed in the photosensitive layer  14 . Unexposed silver halide  42  is the light sensitive material of the photosensitive layer  14 . In FIG. 4 a,  photo-mask  40  selectively blocks a source of light that strikes and exposes exposed silver halide  44  while unexposed silver halide  42  remains inactivated. 
     In FIG. 4 b,  display sheet  10  has been photographically developed to convert exposed silver halide  44  to metallic silver  46 . Barrier layer  20  prevents developing chemicals from contaminating light modulating layer  30 . Metallic silver  46  forms conductive areas  16  in display sheet  10 . In FIG. 4 c,  a conventional photographic fixing step has removed the unexposed silver halide  42 . Removal of unexposed silver halide  42  forms non-conductive areas  18  in display sheet  10 . Additionally, the conductive areas  16  of silver halide can be chemically plated with harder materials such as nickel to provide further abrasive strength and improve conductivity in conductive areas  16 . 
     FIG. 5 a  is a front sectional view of a writer  66  used to write information on display sheet  10 . FIG  5   b  is a side sectional view of writer  66 . A pressure roller  80  is used to advance display sheet  10  (in arrow direction) through the writer  66 . Power rollers  65  disposed to the sides of display sheet  10  contact power areas  35  to form an electrical connection to transparent, electrically conductive layer  13 . A write head  67  supports a series of contact pads  70  which have a 300 dots per inch (dpi) pitch (82.5 micron) with 10 micron gaps between each contact pad  70 . Contact pads  70  can be copper traces with a nickel overcoat. Each contact pad  70  contacts a plurality of conductive areas  16 . Nonconductive areas  18  define a set of conductive areas  16  that record a pixel of image information. 
     Display sheet  10  is advanced under power roller  65  and sequential elements of image data are written to display sheet  10 . A first electrical potential is applied across light modulating layer  30  to reset all pixels. A second electrical potential is then selectively applied to write gray levels onto display sheet  10 . In the case of light modulating layer  30  being a polymer stabilized chiral nematic material, light modulating layer  30  will be transparent after the high voltage reset. 
     If the applied voltage is removed rapidly, the pixel returns to a reflective state. If the applied voltage is removed slowly, light modulating layer  30  will relax into a transparent state. Display sheet  10  is sequentially advanced to each line of pixels at approximately 3 milliseconds for each line of pixels. When the light modulating layer  30  includes an electrophoretic material, write head  67  applies fields of different polarities and in response thereto particles move to one of two states. 
     FIG. 6 is a sectional view showing the optical effect of a display sheet  10  on light. In FIG. 6 the center of light modulating layer  30  over conductive areas  16  has been written into a transmissive, black state. Absorbed light  94  strikes the black silver material in conductive area  16  and is not reflected from display sheet  10 . Conductive area  16  traps absorbed light  94 , causing the pixel area to appear black in a normally white sheet. On the sides of sheet  10 , light modulating layer  30  has been written into the reflective state and reflected light  90  forms a “white” pixel. 
     FIG. 7 shows schematic circuitry for writing to the display sheet  10 . Digital image data  100  is applied to a writer controller  102  and is stored in memory (not shown). These digital image data  100  are converted to electrical signals that are applied to drivers  104  which provide voltages to contact pads  70 . Writer controller  102  controls power supply  106  to provide various voltage levels to power roller  65  that are required to initialize and write to display sheet  10 . Display sheet  10  is advanced to a first line of pixels. A first low voltage is applied to the row of pixels on display sheet  10  which is then raised to drive all pixels to the clear state. The field for each individual pixel is dropped at different rates, corresponding to the degree of reflection required for each pixel that corresponds a given gray level of light reflectance. Display sheet  10  is then advanced a distance corresponding to the next row of pixels. The process is repeated until display sheet  10  contains a representation of digital image data  100 . 
     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 
       12  substrate 
       13  transparent, electrically conductive layer 
       14  photosensitive layer 
       16  conductive areas 
       18  non-conductive areas 
       20  barrier layer 
       30  light modulating layer 
       35  power areas 
       40  photo mask 
       42  unexposed silver halide 
       44  exposed silver halide 
       46  metallic silver 
       60  reflected light 
       64  absorbed light 
       65  power roller 
       66  writer 
       67  write head 
       70  contact pad 
       80  pressure roller 
       90  reflected light 
       94  absorbed light 
       100  image data 
       102  writer controller 
       104  driver 
       106  power supply