Stock material or miscellaneous articles – Liquid crystal optical display having layer of specified... – With viewing layer of specified composition
Reexamination Certificate
2001-10-09
2004-05-18
Wu, Shean C. (Department: 1756)
Stock material or miscellaneous articles
Liquid crystal optical display having layer of specified...
With viewing layer of specified composition
C252S299100, C252S299500, C252S299660
Reexamination Certificate
active
06737126
ABSTRACT:
DESCRIPTION
The present invention is related to a liquid crystal mixture, a liquid crystal cell for a color liquid crystal display LCD and the use of a dye with a dipole for a liquid crystal mixture.
Liquid crystals and liquid crystal mixtures can be used for a wide area of applications. One of the most popular applications for liquid crystal is the liquid crystal display LCD especially for notebook computers but increasingly also for monitors of desktop computers of stationary systems.
One of the most important advantages of the liquid crystal displays is, in comparison to other monitors, the small size of the displays as well as the flicker free picture they produce that is very convenient for the user and is especially protective for the eyes of the users, especially in case of long term working before the display.
A liquid crystal display comprises an array of liquid crystal cells. Different liquid crystal cells have been developed in the recent years, the most important liquid display cells are the TN cell (twisted nematic cell), the STN cell (super twisted nematic cell), PDLC cells (polymer dispersed liquid crystal cells) etc. Liquid crystal cells normally use nematic liquid crystals, however, also smectic liquid crystals or cholesteric liquid crystals may be utilized.
All the above mentioned liquid crystal materials generally have common characteristics. They provide a rod-like molecular structure, a rigidness of the long axis and dipoles and/or easily polarisable substituents, therefore providing permanent or induced dipoles.
The distinguishing characteristic of the liquid crystalline state is the tendency of the molecule to point along in the same direction, called the director. The tendency of the liquid crystal molecules to point along the director leads to a condition known as anisotropy. This term means that the properties of the material, especially the light transmission properties, depend on the direction in which they are measured. This anisotropic nature of liquid crystal is responsible for the unique optical properties.
In liquid crystal cells and liquid crystal displays the orientation of the molecules can be controlled by applying an electric or a magnetic field to the liquid crystal material or mixture or the liquid crystal cell. The liquid crystal molecules, having a permanent dipole or an induced dipole, tend to orient themselves along the direction of the field.
By applying an electric or a magnetic field to the liquid crystal mixture or the liquid crystal cell, the molecules can be switched generally between two stages or orientations, an “on-state”, where the liquid crystal cell is transparent in a preferred direction, and an “off-state”, where the liquid crystal cell is not transparent in this preferred direction.
Depending on the liquid crystal cell, the transparency is influenced by different effects: with a TN cell (twisted nematic cell) the polarization of the light is influenced within the liquid crystal material, whereas on both sides of the liquid crystal materials polarizing filters are provided. Depending on the applied field, the polarization is influenced or not influenced within the liquid crystal material, therefore light can transmit both polarizing filters in the above mentioned “on-state”, or cannot transmit through the polarizing filters in the “off-state”.
In a PDLC cell (polymer dispersed liquid crystal cell), small liquid crystal droplets are dispersed uniformly in a transparent polymer matrix. The diameters of the liquid crystal micro-droplets are comparable to the wavelength of visible light. Since these diameters are small compared with the film thickness, a light ray emitted in a first direction (being the direction the transmitting will be essential in regarding the function of the liquid crystal cell or display, therefore also referred to as the functional or the preferred direction) will be scattered many times before emerging from the film, provided that the mismatch of refractive index between liquid crystal and polymer is sufficiently large in the “off-state”. The film will appear milky white in the absence of an applied field because of the above mentioned scattering. On the other hand, in the “on-state”, when the director of the individual droplets is aligned with a field, the refractive index of the liquid crystal within the droplets becomes sufficiently close to the index of the polymer matrix material and therefore the liquid crystal material becomes transparent in the above mentioned preferred direction. When the field is removed, the liquid crystal domains recover their initial orientations because of surface forces and because the cavities are not ideal spheres. The degree of “off-state” scattering will depend on the amount of refractive index mismatch, size and the number density of the liquid crystal droplets.
In order to enable display of color information, dichroic dyes are incorporated in the liquid crystal materials.
In general, liquid crystals are excellent solvents for especially organic molecules. Therefore, when a small amount of a dye with geometric anisotropy is mixed in a liquid crystal, the dye molecules couple with the anisotropic intermolecular interaction field of the liquid crystal; they tend to arrange in such a way that their long molecule axes align along the liquid crystal director. When a field, preferably an electric field is applied, the orientation of the dye molecules can be switched along the liquid crystal orientation. This phenomenon is known as “guest-host interaction”, the respective liquid crystal cell is therefore called guest-host effect cell.
The quality of liquid crystal cell or liquid crystal display is provided on the one hand by a good contrast and a good color, therefore by good optical properties, on the other hand by the response time of the liquid crystal cells, when switching the cell from e.g. the off-state to the on-state or vice versa.
A rise time is usually defined as the time needed for the material or the cell to reach 90% of the on-state transmission when a field is applied. Similar, a decay time is defined as the time needed for the material in the on-state to reach 10% of the on-state transmission, when the field is set to zero. Fast response times are very important, especially for moved pictures but also for standard computer applications with changing pictures.
It is therefore an object of the present invention to provide a liquid crystal mixture and a liquid crystal cell for liquid crystal displays capable of providing color information with very high response times, both short rise times and short decay times.
This object is solved by a liquid crystal mixture according to claim
1
, a liquid crystal cell for a color liquid crystal display (color LCD) according to claim
25
and the use of a dye with a permanent dipole for a liquid crystal mixture. Claims
2
to
24
,
26
to
32
and
33
to
45
are related to preferred embodiments of the liquid crystal material, the respective cell and to the use of a dye with a dipole.
According to the invention, the liquid crystal mixture that is especially used for liquid crystal cells for a color liquid crystal display comprises a liquid crystal material and a dye, wherein said dye has a dipole. The dye preferably has a permanent dipole, however, an induced dipole will be sufficient for some applications. By the addition of the dye having a dipole, the rise time can be remarkably shortened. This is due to the additional dipole of the dye, directly influenced by the applied field so that the overall “orientating force”, realized by the interaction of the applied field and the overall dipole moment of the liquid crystal mixture could be increased. On the other hand, the decay time is hardly influenced, especially not increased by the addition of the dye with a permanent dipole.
A decrease of the response time, especially the rise time, is important for a good optical quality, furthermore, the lower limit of the operating temperature of the display can be decreased, because the switching time will in generally increa
Bloor David
Cross Graham
Love Gordon
Masutani Akira
Yasuda Akio
Frommer William S.
Frommer & Lawrence & Haug LLP
Megerditchian Samuel H.
Sony International (Europe) GmbH
Wu Shean C.
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