A revolution in the information display technology began in the early 1970s with the invention of the liquid crystal display (LCD). Because the LCD is a flat-panel display of light weight and low power which provides a visual read out that conforms to the small size, weight and battery demands of a handheld electronic device, this display technology enabled a new broad class of handheld and other portable products. Commercially, the LCD first appeared in volume as a digital readout on wrist watches, then on instruments and, later, enabled the laptop computer, personal data assistant and many other digital devices. Today LCD technology is even replacing cathode ray tubes in televisions and PCs.
Nearly every commercial LCD display manufactured and sold today is on glass substrates. Glass offers many features suitable for the manufacture of LCDs. It can be processed at high temperatures, it is rigid and suitably rugged for batch processing methods used in high volume manufacturing, its surface can be made very smooth and uniform over large areas and it has desirable optical properties such as high transparency. There are many applications, however, where glass is far from being the ideal substrate material. Glass substrates cannot be made very flexible and are not very rugged, being unsuitable for web manufacturing and subject to easy breakage. As a result there is a large worldwide effort to develop displays on more flexible and rugged substrates that cannot only conform to three-dimensional configurations but which can also be repeatedly flexed. A display is desired that has the flexibility of a thin plastic sheet, paper or fabric, so that it can be draped, rolled up or folded like paper or cloth. This would not only make the display more portable and easier to carry, it would expand its potential applications well beyond those of the typical flat panel information displays known today: A display worn on the sleeve; the back of a bicyclists coat that shows changing direction signals; textile that changes its color or design are but a few examples.
While the ability of an electrically addressable liquid crystal display to be flexible and deform like cloth or paper would be advantageous for any LCD technology, it is especially advantageous in applications suited to cholesteric liquid crystal displays. Cholesteric displays can be made highly reflective such that they can be seen in bright daylight or a dimly lit room without the aid of a heavy and power consuming backlight. Since cholesteric liquid crystals can be made to be bistable they require power only when being addressed, further adding to the power savings associated with such displays. Cholesteric liquid crystalline materials are unique in their optical and electro-optical features. Of principal significance, they can be tailored to Bragg reflect light at a pre-selected wavelength and bandwidth. This feature comes about because these materials posses a helical structure in which the liquid crystal (LC) director twists around a helical axis. The distance over which the director rotates 360° is referred to as the pitch and is denoted by P. The reflection band of a cholesteric liquid crystal is centered at the wavelength, λo=0.5(ne+no)P and has the bandwidth, Δλ=(ne−no)P which is usually about 100 nm where ne and no are the extra-ordinary and ordinary refractive indices of the LC, respectively. The reflected light is circularly polarized with the same handedness as the helical structure of the LC. If the incident light is not polarized, it will be decomposed into two circularly polarized components with opposite handedness and one of the components reflected. The cholesteric material can be electrically switched to either one of two stable textures, planar or focal conic, or to a homeotropically aligned state if a suitably high electric field is maintained. In the planar texture the helical axis is oriented perpendicular to the substrate to Bragg reflect light in a selected wavelength band whereas in the focal conic texture it is oriented, on the average, parallel to the substrate so that the material is transparent to all wavelengths except for weak light scattering, negligible on an adjacent dark background. These bistable structures can be electronically switched between each other at rapid rates on the order of milliseconds. Gray scale is also available in that only a portion of a pixel can be switched to the reflective state thereby controlling the reflective intensity.
The bistable cholesteric reflective display technology was introduced in the early 1990's as a low power, sunlight readable technology intended primarily for use on handheld devices. Such portable devices demand long battery lifetimes requiring the display to consume very little power. Cholesteric displays are ideal for this application as the bistability feature avoids the need for refreshing power and high reflectivity avoids the need for power-consuming backlights. These combined features can extend battery life times from hours to months over displays that do not have these features. Reflective displays are also easily read in very bright sunlight where backlit displays are ineffective. Because of the high reflective brightness of a cholesteric display and its exceptional contrast, a cholesteric display can be easily read in a dimly lit room. The wide view angle offered by a cholesteric display allows several persons to see the display image at the same time from different positions. In the case of cholesteric materials possessing positive dielectric anisotropy, modes of operation other than a bistable mode are possible by applying a field to untwist the cholesteric material into a transparent, homeotropic texture. Quick removal of the field transforms the material into the reflective planar texture. The more fundamental aspects of such modern cholesteric displays are disclosed in, for example, U.S. Pat. Nos. 5,437,811 and 5,453,863, incorporated herein by reference.
Bistable cholesteric liquid crystal displays have several important electronic drive features that other bistable reflective technologies do not. Of extreme importance for addressing a matrix display of many pixels is the characteristic of a voltage threshold. A threshold voltage is essential for multiplexing a row/column matrix without the need of an expensive active matrix (transistor at each pixel). Bistability with a voltage threshold allows very high-resolution displays to be produced with low-cost passive matrix technology.
In addition to bistable cholesteric displays with liquid crystalline materials having a positive dielectric anisotropy, it is possible to fabricate a cholesteric display with liquid crystalline materials having a negative dielectric anisotropy as, for example, described in the U.S. Pat. No. 3,680,950 to Haas et al., or U.S. Pat. No. 5,200,845 to Crooker et al., incorporated herein by reference. These “negative materials” like the “positive” materials are chiral nematic liquid crystals that are prepared from nematic materials that have been twisted into a helical molecular arrangement by the addition of chiral compound or collection of chiral compounds. The negative and positive materials are prepared from nematic liquid crystals with either a negative or positive dielectric anisotropy respectively.
Negative type cholesteric displays can operate in a bistable mode where the material is switched into the stable planar (e.g., color reflective) texture with an AC pulse or into the stable focal conic (e.g., transparent) texture with a DC pulse as described by U.S. Pat. No. 3,680,950. There are other modes of operation such as has been disclosed by Crooker where a droplet dispersion of negative cholesteric materials is switched into the planar, color reflective texture with an applied electric field, but relaxes back into a transparent texture when the field is removed.
Some cholesteric materials possess a dielectric anisotropy that can be negative under an applied electric field of one frequency but positive at another frequency. This feature can be used to drive a bistable display using a dual frequency drive scheme as described in U.S. Pat. No. 6,320,563, incorporated herein by reference.
Another important feature of cholesteric materials is that the layers reflecting red, green, and blue (RGB) colors as well as IR night vision can be stacked (layered) on top of each other without optically interfering with each other. This makes maximum use of the display surface for reflection and hence brightness. This feature is not held by traditional displays where the display is broken into pixels of different colors and only one third of the incident light is reflected. Using all available light is important for observing a reflective display in a dimly lit room without a backlight. Gray scale capability allows stacked RGB, high-resolution displays with full-color capability where as many as 4096 colors have been demonstrated. Because a cholesteric display cell does not require polarizers, low cost birefringent plastic substrates such a PET can be used. Other features, such as wide viewing-angles and wide operating temperature ranges as well as fast response times make the cholesteric bistable reflective technology, the technology of choice for many low power applications.
Cholesteric liquid crystals are particularly well suited for flexible substrates. Such cholesteric displays have been reported by Minolta Co. Ltd. and by Kent Displays, Inc. involving two plastic substrates filled with cholesteric liquid crystal materials (Society for Information Display Proceedings, 1998, pp 897-900 and 51-54, respectively). While the substrates themselves are flexible, the assembled displays are much less flexible because of the lamination of two substrates together. Minolta has developed procedures for manufacturing flexible displays with two substrates as seen in U.S. Pat. No. 6,459,467.
Greater flexibility can be achieved if only one substrate is used and the display materials are coated or printed on the substrate. Cholesteric liquid crystals are made suitable for standard coating and printing techniques by forming them into polymer droplet dispersions. As droplet dispersions, the materials are made insensitive to pressure and shear such that an image on a bistable cholesteric display is not readily erased by flexing the substrate. Recently, Stephenson et al., at Kodak fabricated flexible bistable reflective displays with polymer dispersions of cholesteric liquid crystals on a single transparent plastic substrate using photographic methods (U.S. Published Application No. US 2003/0202136 A1 and U.S. Pat. No. 6,788,362 B2). This process involves a sequence of depositions on transparent polyester plastic whereby the end product is a display where the images are viewed through the substrate. Such a process requires substrate materials that are transparent such as a clear plastic sheet.
In view of the foregoing, it is desirable to provide a reflective display that does not require a transparent substrate, making available a broader range of substrate materials such as fabrics made of fibers that can be deformed such as by bending, rolling, draping or folding. These added features offer many advantages and open up many new display applications. Use of flexible and drapable substrates can bring to the market place new displays that have the physical deformability of fabric so that they can be an integral part of clothing and have the feel and appearance of cloth because they can be draped and folded.
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