Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Electrical excitation of liquid crystal
Reexamination Certificate
1999-06-10
2002-04-23
Sikes, William L. (Department: 2164)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Electrical excitation of liquid crystal
C349S074000, C349S080000, C349S176000
Reexamination Certificate
active
06377321
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a liquid crystal display device that can reflect light in both the visible and infrared ranges of the electromagnetic spectrum, and to a method of fabricating such a device.
BACKGROUND OF THE INVENTION
Cholesteric displays are bistable in the absence of a field, the two stable textures being the reflective planar texture and the weakly scattering focal conic texture. In the planar texture, the helical axes of the cholesteric liquid crystal molecules are substantially parallel to the substrates between which the liquid crystal is disposed. In the focal conic state the helical axes of the liquid crystal molecules are generally randomly oriented. By adjusting the concentration of chiral dopants in the cholesteric material, the pitch length of the molecules and thus, the wavelength of radiation that they will reflect, can be adjusted. Cholesteric materials that reflect infrared radiation have been used for purposes of scientific study. Commercial displays are fabricated from cholesteric materials that reflect visible light.
Liquid crystal displays are useful as instrumentation in vehicles. For example, commercial airlines employ LCD instrumentation in the cockpits. Vehicles such as for military use, may use LED or LCD instrumentation. In military vehicles used to conduct stealth night operations, such as army helicopters, pilots wear night vision detectors or goggles that enable them to view objects in the air and on the ground without using visible light. The night vision goggles enable the wearer to view infrared radiation, such as the heat from the motor of an automobile. The night vision goggles may also utilize the ambient infrared light from the night sky to view objects that do not emit infrared radiation. The night vision goggles are worn spaced from the eyes of the pilot so that the LED instrumentation panels can be read when the wearer looks down, without looking through the goggles. Use of current night vision goggles limits the depth perception of the wearer. In addition, visible light may saturate the night vision goggles and render them ineffective. The goggles thus may filter out certain wavelengths of visible light.
SUMMARY OF THE INVENTION
The present invention is directed to a liquid crystal display including a single chiral nematic liquid crystal material or at least two chiral nematic liquid crystal materials, which can reflect light across a particular range of wavelengths. One aspect of the invention is directed to a display that reflects in both the visible and infrared ranges of the electromagnetic spectrum at an intensity that is observable to the human eye. The radiation in the infrared spectrum is observed using a device suitable for detecting infrared radiation, such as night vision goggles. Another aspect of the invention is the cell wall configurations of the displays. The invention may employ a single cell using one chiral nematic liquid crystal that reflects in the visible and in the infrared ranges or at least two liquid crystal cells each having a different chiral nematic liquid crystal disposed in each. When two or more cells are used, the cells may be stacked on top of one another. The chiral nematic liquid crystal composition may be tailored to have a certain peak intensity and bandwidth according to the invention. Although the preferred operation of the inventive display utilizes light reflecting from the liquid crystal, it would be appreciated by those skilled in the art in view of this disclosure that the display may be used in a transmissive mode using backlighting.
In general, the present invention is directed to a liquid crystal display device comprising cell wall structure and a chiral nematic liquid crystal material. The cell wall structure and the liquid crystal cooperate to form focal conic and twisted planar textures that are stable in the absence of a field. A device applies an electric field to the liquid crystal for transforming at least a portion of the material to at least one of the focal conic and twisted planar textures. The liquid crystal material reflects radiation having a wavelength in both the visible and the infrared ranges of the electromagnetic spectrum at intensity that is sufficient for viewing by an observer. In particular, the liquid crystal has a positive dielectric anisotropy. At least about 20% of the radiation incident on the material is preferably reflected from the material. The liquid crystal material may have an optical anisotropy of at least about 0.10.
In one embodiment of the invention, the display device employs a single liquid crystal material that is disposed in one region and yet reflects both visible and ago infrared radiation. This is accomplished by selecting the peak reflection wavelength of the radiation and by broadening the bandwidth or the range of wavelengths in which the radiation is reflected.
Another embodiment of the invention utilizes two regions, a liquid crystal material being disposed in each region. The cell wall structure forms a first region in which a first chiral nematic liquid crystal material is disposed and a second region in which a second chiral nematic liquid crystal material is disposed. The first liquid crystal reflects radiation having a wavelength in the visible range and the second liquid crystal material reflects radiation having a wavelength in the infrared range.
The particular cell wall structure that is used to form the two regions may be a stacked display employing three, four or more substrates. The liquid crystal material is disposed between opposing substrates. In one aspect using four substrates the first region is disposed between first and second substrates and the second region is disposed between third and fourth substrates. The first and second regions are arranged in series with respect to one another in the direction toward the observer. In this regard, the first liquid crystal reflecting visible light is disposed downstream of the liquid crystal reflecting infrared radiation in the direction toward the observer. In the case of the three substrate stacked display, the first region is disposed between first and second substrates and the second region is disposed between the second substrate and a third substrate.
The spacing between substrates in the single cell display ranges from about 4 to about 10 microns. The spacing between the substrates in the stacked cell a display is at least about 4 microns.
The three cell display employs a photolithography method of the present invention to form a substrate that employs patterned electrodes on both sides. This substrate can be used in any stacked display. This method of the invention includes applying radiation in the ultraviolet region of the electromagnetic spectrum through a mask. The radiation is reflected through a substrate, each opposing surface of the substrate containing a layer of photoresist material over a conductive layer disposed on the surface. The photoresist layer is exposed to the UV radiation on both sides of the substrate. Exposed photoresist material and underlying electrode material are removed from the substrate to form an electrode pattern on both surfaces of the substrate.
In particular, the ultraviolet radiation is applied at a level effective to compensate for the transmission loss of the photoresist, the electrode and the substrate. The ultraviolet radiation is applied at a level that is at least two times the level of ultraviolet radiation that is normally used to expose photoresist on a substrate.
A preferred embodiment of the present invention is directed to instrumentation of the type that is used by personnel employing a night vision detector such as goggles. The instrumentation reflects light having a wavelength in the visible region of the electromagnetic spectrum. This embodiment of the present invention has military applications, such as use in instruments in the cockpit of army helicopters. The present invention includes a liquid crystal display device comprising cell wall structure and a chiral nematic liquid cr
Huang Xiao-Yang
Khan Asad A.
Lu Minhua
Urban Ellison C.
Yuan Haiji
Chowdhury Tarifur R.
Kent Displays, Inc.
Sikes William L.
Watts Hoffman Fisher & Heinke
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