Liquid crystal cells – elements and systems – Liquid crystal optical element
Reexamination Certificate
1998-06-23
2001-07-10
Parker, Kenneth (Department: 2871)
Liquid crystal cells, elements and systems
Liquid crystal optical element
C349S001000, C349S086000, C283S072000, C283S090000, C283S901000
Reexamination Certificate
active
06259506
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to document and currency authentication and, in particular, to security articles used in documents and currency.
BACKGROUND OF THE INVENTION
In a publication entitled “Preparation and characterization of polymer dispersed liquid crystal films using photosensitive polymers”, Die Angewandte Mackromolekulare Chemie 231 (1995) 109-121 (Nr. 4051), J.-H. Liu, M.-Z. Chen and Y.-F. Wang report on the preparation of polymer dispersed liquid crystal (PDLC) films using a photosensitive technique.
As is reported by these authors, a light scattering state in an a.c. electric field off state depends on optical heterogeneities such as a spatial distortion of nematic directions and/or mismatching in refractive indices of the compounds. The light scattering and light switching properties of the composite films are decisively influenced by the phase separation structure of the composite films. The size of a liquid crystal domain (channel) can be controlled on the basis of the solvent evaporation rate or the curing rate during the separation of the composite film.
The authors employed commercial liquid crystals ZLI-2444, 2452, 2459, and BDH-E7 without further treatment. For the polymer matrix the authors used hydrophilic monomers of 2-hydroxythyl methacrylate (HEMA) and acrylic acid (Aa), hydrophobic monomers of methyl methacrylate (MMA) and styrene (St) that were purified by distillation in vacuo under nitrogen. 2-Methoxy-2phenylacetophenome (benzoin methyl ether, BME) was used without purification. The hydrophilic monomers were found to be suitable for the PDLC system. UV radiation was employed to photopolymerize the mixture, and it was found that sufficient UV irradiation was needed to complete the phase separation of the liquid crystals in the polymer matrix.
It is also known in the art to include isotropic dyes in an epoxy resin and liquid crystal when making optical shutters. The dye concentration can be adjusted to maximize the per cent transmission through the film between the on and off states. High contrast, colored displays have been made using an isotropic dye containing PDLC films and complementary colored backgrounds. Reference in this regard can be made to a publication entitled “Polymer Dispersed Liquid Crystals Incorporating Isotropic Dyes”, SPIE Proceedings, 1080 (1989), J. L. West et al., and to a publication entitled “Characterization of polymer dispersed liquid-crystal shutters by ultraviolet/visible and Infrared absorption spectroscopy”, J. Appl. Phys. 70(7), Oct. 1, 1991, pgs. 3785-3790, J. L. West et al. In U.S. Pat. No. 5,448,582, issued Sep. 5, 1995, entitled “Optical Sources Having a Strongly Scattering Gain Medium Providing Laser-Like Action”, the inventor disclosed a multi-phase gain medium including an emission phase (such as dye molecules) and a scattering phase (such as TiO
2
). A third, matrix phase may also be provided in some embodiments. Suitable materials for the matrix phase include solvents and polymers. The material is shown to provide a laser-like spectral linewidth collapse above a certain pump pulse energy.
FIGS. 9
a
and
9
b
illustrate a display system embodiment having, in one embodiment, a liquid crystal display (LCD) array that is positioned adjacent to a surface of a pixel plane comprised of pixels or sub-pixels of the inventive gain medium. The LCD array is arranged so as to be selectively energized for passing the emission from the pixels through to an observer. The pixels operate so as to be substantially non-saturable and to output electromagnetic radiation within a narrow band of wavelengths.
It is well known in the art to use security articles, for example, security threads in paper to hinder a non-authorized production of the paper or to authenticate already manufactured paper and/or a document or currency printed on the paper. Reference in this regard can be had to the following U.S. patents: U.S. Pat. No. 5,486,022, “Security Threads Having At Least Two Security Detection Features and Security Papers Employing Same”, by T. T. Crane; U.S. Pat. No. 4,534,398, “Security Paper”, by T. T. Crane; and U.S. Pat. No. 4,437,935, “Method and Apparatus for Providing Security Features in Paper”, by F. G. Crane, Jr.
OBJECTS AND ADVANTAGES OF THE INVENTION:
It is a first object and advantage of this invention to provide an improved security article for inclusion within documents and currency.
It is another object and advantage of this invention to provide an improved machine-readable security article for inclusion within documents and currency.
It is a further object and advantage of this invention to provide an improved security thread having a polymer dispersed liquid crystal material layer that is responsive to an applied electric field for varying a visual characteristic of the security thread.
It is a still further object and advantage of this invention to provide a security thread that includes a polymer dispersed liquid crystal material layer that includes an ink, such as a polymer-based ink, a solvent-based ink, or a water-based ink, wherein the security thread is responsive to an applied electric field for varying a visual characteristic of the security thread.
It is another object and advantage of this invention to provide an improved security article having micro-encapsulated liquid crystal material that is responsive to an applied electric field for varying a visual characteristic of the security article.
It is still another object and advantage of this invention to provide a security article that includes micro-encapsulated liquid crystal material having at least one orientable dye, such as a dichroic or an isotropic dye, wherein the security article is capable of laser-like action when excited by a source, and wherein the laser-like action can be switched on and off by switching an electric field on or off.
It is still another object and advantage of this invention to provide a security article that is capable of laser-like action when excited by an essentially solid-state optical source, and wherein the laser-like action can be switched on and off by switching an electric field on or off.
Further objects and advantages of this invention will become more apparent from a consideration of the drawings and ensuing description.
SUMMARY OF THE INVENTION
The foregoing and other problems are overcome and the objects and advantages of the invention are realized by the embodiments of this invention.
In a first aspect this invention provides a security article, i.e. a security thread, comprising an electrically actuated optical switch comprised of a liquid crystal (LC) material. In one embodiment, the LC material is disposed in a layer of polymer dispersed liquid crystal material. In another embodiment, the security thread comprises a plurality of micro-capsules which include the LC material and an orientable dye.
In a second aspect this invention provides a document or currency that comprises a paper matrix that includes at least one security article, the at least one security article being comprised of at least one layer of electrically actuated liquid crystal material. The electrically actuated liquid crystal material being responsive to an applied electric field for varying a visual characteristic of the security article. In one embodiment a security thread comprises a plurality of micro-capsules that contain the electrically actuated LC material.
In a third aspect this invention provides a device for verifying an authenticity of a document or currency of a type that includes at least one security thread. The device includes first and second electrodes and an excitation source coupled to the electrodes. The first and second electrodes are spaced apart for accommodating therebetween the document or currency, and at least one of the electrodes is transparent. The at least one security thread includes at least one layer of polymer dispersed liquid crystal material having a visual characteristic that is switched between two states as a function of a presence or absence of an electric field b
Ohlandt Greeley Ruggiero & Perle LLP
Parker Kenneth
Spectra Science Corporation
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