Electric lamp and discharge devices: systems – Plural power supplies – Plural cathode and/or anode load device
Reexamination Certificate
1999-06-07
2001-03-06
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Plural power supplies
Plural cathode and/or anode load device
C345S074100
Reexamination Certificate
active
06198225
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to optical display systems, in particular to flat panel display systems containing ferroelectric material.
2. Statement of the Problem
One broad category of flat panel display systems comprises a luminescent, or phosphor, layer that is energized to produce visible light. A phosphor is a luminescent material that converts part of the absorbed primary energy into emitted luminescent radiation. (The term “phosphor”, as used herein, includes any material that converts energy from an external excitation and, by means of the phenomenon of phosphorescence or fluorescence, converts such energy into visible light. The term “luminescent” as used herein includes “phosphor” as well as any other any other material or device which absorbs energy and thereby emits light.)
For example, in an electroluminescent (EL) display, an electric field is applied across the luminescent layer in sufficient magnitude to cause avalanche breakdown of the phosphor. The light generated by recombination of electron-hole pairs can be tuned in wavelength by the addition of various impurity ions to the phosphor. As in virtually all flat panel display (FPD) devices, the display panel is formatted in an X-Y matrix of pixels. The drive circuitry supports the application of individual voltage differences between two electrode layers at each pixel location. Unfortunately, the voltage required to trigger light emission from the luminescent layer in a thin-film EL device is as high as 200-250 V, and this requires that the driving circuits serving as switching elements should also be capable of withstanding such high voltage. The manufacture of such high-voltage devices is expensive. Furthermore, it is desirable that flat panel displays operate at the voltage level of many integrated circuit devices, that is, in the 3-10 volt range.
Flat panel field emission displays (FEDs) are also known. A field emission display typically comprises a flat vacuum cell with a matrix of microscopic field emitter cathode tips formed on the back plate of the cell, and a phosphor-coated anode at the front plate of the cell. The field emitter tips emit electrons upon application of appropriate voltages. The emitted electrons are directed to strike the luminescent layer with sufficient beam current intensity and kinetic energy to cause the luminescent layer to generate visible light.
An advantage of displays with phosphor layers is that backlighting of the display is thereby eliminated. Backlighting can be impractical because the color and intensity of the light is delivered to the display unmodified, and the system must modify it to produce an optical image. One typical way to include color in a backlighted display is to pass light through a color filter. But, the filter absorbs up to 70 percent of the incident light, resulting in inefficiency or low intensity. Similarly, methods forming an image by controlling the transmissivity of light through the panel also result in inefficiency. An advantage of FED systems, and phosphor-emission systems in general, is that the luminescent material generates the required image intensity based on the energy impinging the material without significant losses. Thus, displays with high brightness can be built. Unfortunately, FEDs typically require tens to hundreds of volts for electron emission, making it difficult to use these displays in many applications. Also, the electron field emitter tips typically need to be surrounded by a very high vacuum, at least 10
−5
Torr, and often as high as 10
−8
-10
−9
in order to prevent degradation of the tips. Such high vacuums are difficult to maintain in the small volume enclosing field emitter tips. Furthermore, FEDs cannot be fabricated in “plane-to-plane” geometry.
It is known that ferroelectric materials can emit electrons when subjected to polarization switching. Ferroelectrics have the property of spontaneous polarization along a polarization axis. The material remains neutral internally as the end of each dipole is paired with the opposite end of the next dipole along that polar axis. At any boundary with a normal component to this axis, the dipoles are unpaired and a material-dependent bound charge will exist. As a consequence of this abnormally high energy state, free screening charges collect to neutralize the surface. It is possible to eject a pulse of these charges and/or induce a field emission pulse by altering the material's internal polarization. This process is not yet fully understood. The most common view of the process is that ferroelectric emission results from the expulsion of the free screening charge from the material's surface upon a rapidly induced change of the internal polarization. Another possibility is that ferroelectric emission is actually a field emission process wherein an extremely large electric field, generated by the spontaneous bound charge, is caused to exist across a nonferroelectric layer on the surface.
One advantage of a ferroelectric emission display, in particular, is that it can be fabricated in “plane-to-plane” geometry, which is not possible for field emission displays. Significant uses would include flat panel television screens and computer display devices.
Ferroelectric electron emission used in luminescent flat panel displays is known in the art. See, in particular, U.S. Pat. No. 5,453,661, issued Sep. 26, 1995 and U.S. Pat. No. 5,508,590, issued Apr. 16, 1996, which are hereby incorporated by reference as if fully contained herein. These disclose ferroelectric-emission FPDs. Both of these patents teach using lead zirconium titanate (PZT) and lead zirconium lanthanum titanate (PZLT) as ferroelectric electron emitters.
A second broad category of flat panel display system is the liquid crystal display (LCD). A liquid crystal layer in a flat panel display is arranged so that the molecules follow a specific alignment. This alignment can be changed with an external electric field, resulting in a corresponding change in the transmissivity of the liquid crystal material to light passing through it. Since the liquid crystal molecules respond to an external applied voltage, liquid crystals can be used as an optical switch, or light valve. In a typical configuration, the liquid crystal display comprises a front glass plate and a back glass plate. The space between the plates is filled with liquid crystal polymer. Various types of liquid crystal polymer are used. The principal classifications of liquid crystal material are twisted pneumatic, guest-host (or Heilmeier), phase change guest-host and double layer guest host. The type of liquid crystal employed determines the type of optical modulation that is effected by the light valve. For example, twisted pneumatic material reorients the polarization of the light (usually by ninety degrees). Guest-host materials, so-called by the presence of a dye that aligns itself with the liquid crystal molecules, modulate light as a consequence of the property of the dye to absorb or transmit light in response to the orientation of the liquid crystal molecules. In phase-change guest-host, the molecules of the liquid crystal material are arranged into a spiral form that blocks the majority of the light in the OFF state. The application of a voltage aligns the molecules and permits the passage of light. A double-layer guest-host liquid crystal comprises two guest-host liquid crystals arranged back-to-back with a ninety degree orientation between the molecular alignment of the two cells. Liquid crystal displays may be arranged to operate in a transmissive mode, requiring backlighting, or in a reflective mode for operation under high ambient light conditions, or in a combination of the two.
Liquid crystal displays are typically used such that pixels of liquid crystal material are arranged in a matrix form. The matrix displays are classified into passive and active types in terms of the driving method. In a typical passive display, transparent electrodes are patterned on both facing glass pla
Arita Koji
Cuchiaro Joseph D.
Hayashi Shinichiro
Kano Gota
McMillan Larry D.
D Chuc Tran
Duft, Graziano & Forest, P.C.
Symetrix Corporation
Wong Don
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