Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
2000-06-15
2003-04-15
Saras, Steven (Department: 2675)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C252S299010
Reexamination Certificate
active
06549188
ABSTRACT:
SEQUENCE LISTING
Not Applicable.
CROSS-REFERENCE TO RELATED APPLICATIONS
Not Applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable.
INCORPORATION BY REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC
Not Applicable.
BACKGROUND OF THE INVENTION
(1) Field of the Invention
Ferroelectric liquid-crystal display having a broad operating temperature range.
(2) Description of Related Art
Owing to their unusual combination of anisotropic and fluid behavior, liquid crystals have found a multiplicity of possible applications in electro-optical switching and display devices.
Besides nematic liquid-crystal phases, which have been employed for some time, recent years have also seen the use of smectic liquid-crystal phases, in particular ferroelectric liquid-crystal phases (FLCs).
The practical use of ferroelectric liquid crystals in electro-optical switching and display elements requires chiral, tilted smectic phases, such as S
C
* phases (see, for example, R. B. Meyer, L. Liebert, L. Strzelecki and P. Keiler, J. Physique 36 L-69 (1975)), which are stable over a broad temperature range.
Switching and display devices containing ferroelectric liquid-crystal mixtures (‘FLC light valves’) are disclosed, for example, in EP-B 0 032 362. LC light valves are devices which, for example owing to electrical switching, modify their optical transmission or reflection properties in such a way that transmitted or reflected light is modulated in intensity. Examples are the known watch and calculator displays or LC displays in the area of office communications and television.
BRIEF SUMMARY OF THE INVENTION
The response time t of a ferroelectric liquid crystal or of the mixture in the display is inversely proportional to the spontaneous polarization P
s
:
t~&ggr;/P
s
&ggr;=viscosity
(see, inter alia, J. W. Goodby et al., “Ferroelectric Liquid Crystals, Principles, Properties and Applications”, Ferroelectricity and Related Phenomena, Vol. 7, 1991, Gordon and Breach Science Publishers).
From EP-A-O 370 649 an addressing scheme for FLC matrixes is known in which bipolar pulses are employed in a manner that single pixels of the matrix may be switched in different dates using a reduced number of pulses. The application contains a diagram in which the relation between the operating voltage of the display and the pulse width necessary for switching the liquid crystal mixture is shown with different values for the spontaneous polarization Ps.
From EP-A-O 283 916 a FLC-device is known in which the phase sequence of the FLC-mixture a smectic A-phase is following a S
C
*-phase and in which either the elastic constant is according to a specific formula or the spontaneous polarization is higher than a specific value. For the experimental use the device is operated with pulses of 30V and 2 ms length.
From DE-A-36 30 012 a FLC-device is known in which the angles between the average directions of the molecular axes obey specific relations.
FR-A-2 731 537 discloses a chip-card containing a FLC-display.
The viscosity is temperature dependent as follows:
&ggr;(
T
)~exp
V/k
b
T
V=activation energy
k
b
=Boltzmann constant
T=temperature
This means that the response time t is highly dependent on the temperature (see, for example, E. P. Pozhidayev et al., Mol. Cryst. Liq. Cryst., 1985, Vol. 124, pp. 359-377).
In the case of fluctuating temperatures, therefore, either the operating voltage, for a constant response time, or the response time, for a constant operating voltage, must be constantly adjusted.
The quotient Q of the threshold voltage at 0° C. and at 40° C. may be regarded as a measure of the temperature dependence:
Q=V
(0° C.)/
V
(40° C.)
In the FLC displays developed hitherto, this quotient is of an order of magnitude which makes voltage adjustment necessary in practice (see, for example, the “Merck Liquid Crystals, Licristal®, Ferroelectric Smectic Mixtures, June 1991” catalog from Merck, Darmstadt, Germany).
Surprisingly, it has now been found that, however, at long response times, generally of greater than or equal to t=1 ms, and relatively high values for the spontaneous polarization P
s
, generally of greater than 5 nCcm
−2
, the temperature dependence of the threshold voltage, and thus the value Q, becomes small.
The invention therefore relates to a ferroelectric liquid-crystal display containing a ferroelectric liquid-crystal mixture, wherein the display is addressed with voltage pulses of longer than 0.9 ms.
The invention likewise relates to a display as described above having improved temperature dependence of the operating voltage.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)
Not Applicable.
DETAILED DESCRIPTION OF THE INVENTION
The invention furthermore relates to a process for the production of a ferroelectric liquid-crystal display having improved temperature dependence of the operating voltage, which comprises matching the ferroelectric liquid-crystal mixture and the electrical addressing to one another in such a way that the product of the spontaneous polarization (in nanocoulombs/square centimeter) and the width of the voltage pulses used for addressing (in milliseconds) is greater than 15 preferably 20, particularly preferably 100, in particular 500.
The invention likewise relates to a process for improving the temperature dependence of the operating voltage of an FLC display, wherein the display is addressed with voltage pulses which are longer than 0.9 ms.
The display according to the invention is particularly suitable for applications in which very fast switching is unnecessary. Such displays can be switched with the same voltage over a very broad temperature range; adjustment of the voltage as a function of the temperature is unnecessary.
In addition, the requisite operating voltages are very low and can be less than 15 V, in some cases less than 5 V, in particular even less than 3 V.
Preferred applications are therefore, for example, displays in chip cards, electronic price and display signs (shelf labels), PDAs (personal digital assistants) or pagers.
The display according to the invention is constructed in such a way that an FLC layer is enclosed on both sides by layers, which are usually, in this sequence, starting from the FLC layer, at least one alignment layer, electrodes and an outer plate (for example made of glass or plastic). In addition, they contain least one polarizer if they are operated in ‘guest-host’ or reflective mode, or two polarizers if the mode used is transmissive birefringence. The switching and display elements can, if desired, contain further auxiliary layers, such as diffusion barrier or insulation layers.
The alignment layer(s) are usually rubbed films of organic polymers or silicon oxide vapor-deposited at an angle.
In order to achieve a uniform planar alignment in the S
C
* phase over the entire display, it is advantageous if the phase sequence of the liquid-crystal mixture is, with decreasing temperature:
isotropic−nematic−smectic
A
−smectic
C
(see, for example, K. Flatischler et al., Mol. Cryst. Liq. Cryst. 131 21 (1985); T. Matsumoto et al., pp. 468-470, Proc. of the 6th Int. Display Research Conf., Japan Display, Sep. 30-Oct. 2, 1986, Tokyo, Japan; M. Murakami et al., ibid. pp. 344-347).
In order to achieve the highest possible contrast in the display, the planar alignment in the display should be uniform. This is achieved if the pitch of the helix in the S
C
* phase of the liquid crystal or of the liquid-crystal mixture is sufficiently high to prevent formation of a helix in the display. Furthermore, the helix in the N* phase should be sufficiently large that no twisted state, but instead a homogeneous nematic phase, forms in the display during the cooling process.
Switching the molecules back and forth (and thus the light or dark position in the case of a fixed polarizer setting) is carried out, as already mentioned, by applying an electric field in a pulsed manner. Because of the bistability of the FLC molecules, voltage need only be applied for a c
Dübal Hans-Rolf
Hornung Barbara
Takeichi Ayako
Anyaso Uchendu O.
Aventis Research & Technology Deutschland GmbH & Co.KG
Saras Steven
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