Liquid crystal cells – elements and systems – With specified nonchemical characteristic of liquid crystal... – Within nematic phase
Reexamination Certificate
2002-01-22
2004-08-24
Chowdhury, Tarifur R. (Department: 2871)
Liquid crystal cells, elements and systems
With specified nonchemical characteristic of liquid crystal...
Within nematic phase
C349S117000, C349S119000, C349S178000
Reexamination Certificate
active
06781664
ABSTRACT:
The invention relates to an electro-optical liquid-crystal switching element comprising at least one polariser and a liquid-crystal layer which has an initial alignment in which the liquid-crystal molecules are aligned essentially parallel to the substrates and essentially parallel to one another, in which the realignment of the liquid crystals from their initial alignment essentially parallel to the substrates is caused by a corresponding electric field, which, in the case of liquid-crystal materials of negative dielectric anisotropy, is aligned essentially parallel to the substrates and, in the case of liquid-crystal materials of positive dielectric anisotropy, is aligned essentially perpendicular to the substrates, where the liquid-crystal layer has an extremely low optical retardation d-An in the range from 0.06 &mgr;m to 0.43 &mgr;m, and the liquid-crystal switching element preferably contains, in addition to the liquid-crystal layer, a further birefringent layer, preferably a &lgr;/4 layer or two &lgr;/4 layers or a &lgr;/2 layer, and 2 liquid-crystal display systems containing liquid-crystal switching elements of this type.
The present invention furthermore relates to liquid-crystal media, in particular of low birefringence, for use in the liquid-crystal display systems. These liquid-crystal display systems containing the liquid-crystal switching elements are, inter alia, display screens of television sets, computers, such as, for example, notebook computers or desktop computers, central control units and of other equipment, for example gambling machines, electro-optical displays, such as displays of watches, pocket calculators, electronic (pocket) games, portable data banks, such as PDAs (
p
ersonal
d
igital
a
ssistants) or of mobile telephones.
In particular, the liquid-crystal display systems according to the invention are highly suitable for applications with display of grey shades, such as, for example, television sets, computer monitors and multimedia equipment. Both mains-independent operation and also operation on the mains are possible here. Mains operation is often preferred.
These liquid-crystal display devices are also known as liquid-crystal displays.
The liquid-crystal switching elements typically used in the liquid-crystal display devices of this type are the known TN (
t
wisted
n
ematic) switching elements, for example in accordance with Schadt, M. and Helfrich, W. Appl. Phys. Lett. 18, pp. 127 ff (1974) and in particular in their special form with low optical retardation d·&Dgr;n in the range from 150 nm to 600 nm in accordance with DE 30 22 818, STN (
s
uper
t
wisted
n
ematic) switching elements, such as, for example, in accordance with GB 2.123.163, Waters, C. M., Brimmel, V, and Raynes, E. Pproc. 3
rd
Int. Display Research Conference, Kobe 1983, pp. 396 ff and Proc. SID 25/4, pp. 261 ff, 1984, Scheffer, T. J. and Nehring, J. Appl. Phys. Lett. 45, pp. 1021 ff, 1984 and J. Appl. Phys. 58, pp. 3022 ff, 1985, DE 34 31 871, DE 36 08 911 and EP 0 260 450, IPS (
i
n-
p
lane
s
witching) switching elements, as described, for example, in DE 40 00 451 and EP 0 588 568, and VAN (
v
ertically
a
ligned
n
ematic) switching elements, as described, for example, in Tanaka, Y. et al. Taniguchi, Y., Sasaki, T., Takeda, A., Koibe, Y., and Okamoto, K. SID 99 Digest pp. 206 ff (1999), Koma, N., Noritake, K., Kawabe, M., and Yoneda, K., International Display Workshop (IDW) '97 pp. 789 ff (1997) and Kim, K. H., Lee, K., Park, S. B., Song, J. K., Kim, S., and Suk, J. H., Asia Display 98, pp. 383 ff, (1998).
In these liquid-crystal display devices which were known hitherto and are for the most part already commercially available, the optical appearance is inadequate, at least for demanding applications. In particular the contrast, especially in the case of coloured displays, the brightness, the colour saturation and the viewing-angle dependence of these parameters are in clear need of improvement and have to be improved if the display devices are to compete with the performance features of the widespread CRTs (
c
athode
r
ay
t
ubes). Further disadvantages of the liquid-crystal display devices are often their poor spatial resolution and inadequate response times, in particular in the case of STN switching elements, but also in the case of TN switching elements or IPS (“in-plane switching”) and VAN (vertically aligned nematic”) switching elements, in the case of the latter especially if they are to be used for the reproduction of video, such as, for example, in multimedia applications on computer display screens or in the case of television sets. Particularly for this purpose, but also even for the display of rapid cursor movements, short response times, preferably of less than 32 ms, particularly preferably of less than 16 ms, are desired.
The requirements regarding the viewing-angle dependence of the contrast are highly dependent on the application of the display devices. Thus, for example, the horizontal viewing-angle range is the most important in television screens and computer monitors, whereas centrosymmetrical or at least approximately centrosymmetrical viewing-angle distributions are desired in other applications. Displays having virtually centrosymmetrical viewing-angle distributions are required, in particular, in projection displays in order to utilise the optical apertures as well as possible, but also in computer display screens having a swivel base. These display screens allow the display to be tilted through 90° in order to change from portrait mode to landscape mode while retaining the resolution of the display. Displays of this type obviously have to have similar horizontal and vertical viewing-angle ranges since these are interchanged on tilting.
In general, it should be noted that for practical acceptance of a display, it is not primarily its contrast or its maximum contrast ratio that is crucial, but instead the viewing-angle dependence of the contrast is frequently important. However, these properties should be weighted differently depending on the application.
TN switching elements having a d·&Dgr;n in the range from 0.2 &mgr;m to 0.6 &mgr;m, as described in DE 30 22 818, generally have very good colour saturation and colour depth, but an inadequate viewing angle for demanding applications, such as, for example, desktop computer monitors.
In some embodiments, such as, for example, in typical IPS display devices, the brightness of the display can be achieved to an inadequate extent or can only be achieved at great expense with backlighting. In contrast, VANs are frequently characterised by inadequate colour saturation and colour depth, and furthermore the production of VANs is complex owing to the homeotropic alignment, which is difficult to achieve, and owing to the long filling times.
EP 0 264 667 describes TN cells having twist angles (&phgr;, known as twist for short) in the range from 100 to 80° with a d·&Dgr;n in the range from 0.2 &mgr;m to 0.7 &mgr;m. Although these have both an improved viewing-angle dependence of the contrast and lower steepness of the electro-optical characteristic line compared with TN cells having a 90° twist, they have, however, significant disadvantages. Thus, inter alia, their brightness and their contrast are significantly lower than those of conventional TN switching elements. In addition, the TN switching elements in accordance with EP 0 264 667 switch relatively slowly.
Raynes, E. P., Mol. Cryst. Liq. Cryst. 4, p. 1, ff, 1986, describes the voltage dependence of the tilt angle in the centre of the liquid-crystal layer (&phgr;
M
, also known as mid-plane tilt angle or mid-plane tilt for short) as a function of the addressing voltage for cells containing a nematic liquid crystal with a tilted alignment having a tilt angle of from 0° to 270°.
DE 40 10 503 and WO 92/17831, which corresponds thereto, describe, inter alia, TN switching elements having twist angles in the range from greater than 0° to 90° which contain one or more compensation layers, where the compensation layers for compensation of the optica
Götz Achim
Heckmeier Michael
Plach Herbert
Schuler Brigitte
Chowdhury Tarifur R.
Merck Patent GmbH
Millen White Zelano & Branigan P.C.
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