Full spectrum reflective choleterics display employing...

Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only

Reexamination Certificate

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C349S175000, C349S176000

Reexamination Certificate

active

06344887

ABSTRACT:

BACKGROUND OF THE INVENTION
Cholesteric liquid crystal displays are characterized by the fact that the pictures stay on the display even if the driving voltage is disconnected. The bistability and multistability also ensure a completely flicker-free static display and have the possibility of infinite multiplexing to create giant displays and/or ultra-high resolution displays. In cholesteric liquid crystals, the molecules are oriented in helices with a periodicity characteristic of material. In the planar state, the axis of this helix is perpendicular to the display plane. Light with a wavelength matching the pitch of the helix is reflected and the display appears bright. If an AC-voltage is applied, the structure of the liquid crystals changes from planar to focal conic texture. The focal-conic state is predominately characterized by its highly diffused light scattering appearance caused by a distribution of small, birefringence domains, at the boundary between those domains the refractive index is abrupt changed. This texture has no single optic axis. The focal-conic texture is typically milky-white (i.e., white light scattering). Both planar texture and focal-conic texture can coexist in the same panel or entity. This is a very important property for display applications, whereby the gray scale can be realized.
Current cholesterics displays are utilizing “Bragg reflection”, one of the intrinsic properties of cholesterics. In Bragg reflection only a portion of the incident light with the same handedness of circular polarization and also within the specific wave band can reflect back to the viewer, which generates a monochrome color display. The remaining spectrum of the incoming light, however, including the 50% opposite handedness circular polarized and out-off Bragg reflection wave band, will pass through the display and be absorbed by the black coating material on the back surface of the display to assure the contrast ratio. The overall light utilization efficiency is rather low and it is not qualified in some applications, such as billboard at normal ambient lighting condition. The Bragg type reflection gives an impression that monochrome display is one of distinctive properties of the ChLCD.
In many applications the human eyes are friendlier with full color spectrum, i.e., white color information written on the dark background. With the development of the flat panel display more and more displays with neutral color is come into being, such as black-and-white STN display and AMTFT display, etc. Unfortunately, both of these approaches involve major disadvantages and limitations. The AMTFT displays are not true zero field image storage systems, as they require constant power input for image refreshing. The STN displays do not possess inherent gray scale capability as a result of the extreme steepness of the electro-optical response curve of the display. To realize a gray scale the resolution has to be reduced by using, for example, four pixels instead of one per area. Anywhere from one to four pixels are activated at a particular time to provide the gray scale effect. The AMTFT devices use semiconductors to provide memory effects and involve use of expensive, ultra high resistance liquid crystal materials to minimize RC losses. Additionally, these displays are both difficult and costly to produce and they are, at present, limited to relatively small size displays. The cholesteric display has many advantages over the STN and AMTFT display with its zero field memory effect, hemispheric viewing angle, gray scale capability and other optical performances but it needs obviously to come up with black-and-white solution in order to keep its superiority.
U.S. Pat. No. 5,796,454 introduces a black-and-white back-lit ChLC display. It includes controllable ChLC structure, a first circular polarizer laminating to a first substrate of cell has the same circular polarity as the liquid crystals, a second circular polarizer laminating to a second substrate of the cell has a circular polarity opposite to the liquid crystals, and a light source. The black-and-white back-lit display is preferably illuminated by a light source that produces natural “white” light. Thus, when the display is illuminated by incident light, the circular polarizer transmits the 50% component of the incident light that is right-circularly polarized. When the ChLC is in an “on” state, the light reflected by the ChLC is that portion of the incident light having wavelengths within the intrinsic spectral bandwidth, and the same handedness; The light that is transmitted through the ChLC is the complement of the intrinsic color of ChLC. The transmitted light has right-circular polarization, however, is thus blocked by left-circular polarizer. Therefore, the observer will perceive that region of the display to be substantially black. When the display is in an “off” state, the light transmitted through the polarizer is optically scattered by the ChLC. The portion of the incident light that is forward scattered is emitted from the controllable ChLC structure as depolarized light. The left-circularly polarized portion of the forward-scattered light is transmitted through the left-circular polarizer, thus, is perceived by an observer. The black-and-white display, in “454” patent, is generated by back-lit component and the ambient light is nothing but “noise”.
It is well known that ChLCD can be used perfectly for daytime information display for its bright reflection to the front environmental lighting. Nowadays available cholesterics display is generally working in reflection mode and the black painting absorbs incoming light both opposite the handedness and out-off the selective reflection wave band. It is the black coating material that completely rolls out the possibilities of adopting the back-lit approach.
In some applications it is needed to work after dark. Take a cell phone for example, a customer need make phone call any time including nighttime and dark environment (travelling in the car). This makes ChLCD an artificial lighting system necessary. It is convenient to use front light arrangement for some types of displays such as gas pump and billboards. In the case of hand-held type of display however, the compact and ergonomic designing requires a back-lit structure.
SUMMARY OF THE INVENTION
It is the primary intention for this invention to realize the full spectrum of reflection (white color in optical “on” state) while maintaining the cholesterics display's superiority such as high ambient environment contrast ratio, hemispheric viewing angle, zero-field long time memory and so on.
It is the other intention for this invention to create the dark state for the optical “off” state so as to achieve black-and-white display, which is the foundation of the achromatic display and the full color display.
It is the other intention of this invention to render the display dual working function, i.e., during the day or bright ambient light the display works as front-lit mode and during the dark environment, back-it mode. Compared with the prior art ChLC displays the skill of the art endows the display with more user friendly and better viewing quality.
The invention is based upon the principles described as following:
Optical “On” State
First, the cholesteric material in planar texture reflects the light component with the same handedness as ChLC and a narrow bandwidth determined by its helical pitch and the optical birefringence (prior art Bragg reflection). Secondly the remaining light component out of the selective bandwidth passes the cell again, is reflected by a metal material without changing the polarization state, i.e., handedness (there is no half wave phase loss) and this component reentry the ChLC cell from back side without attenuation. The two components, one reflected by Bragg reflection with a center wavelength &lgr;
0
, and the other reflected by the metal surface, are compensatory each other and will meet together and emanate toward the viewer as full gamut of visible light. When the ChLCD is tuned in invi

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