Compositions – Liquid crystal compositions
Patent
1990-08-27
1994-05-03
Stoll, Robert L.
Compositions
Liquid crystal compositions
25229966, 359 93, 359106, 359 73, 359 94, 359 96, 428 1, C09K 1952, C09K 1912, G02F 113
Patent
active
053085353
DESCRIPTION:
BRIEF SUMMARY
SUMMARY OF THE INVENTION
The invention relates to an electro-optical system containing at least one addressable liquid-crystal layer and a device for compensating for the optical path difference d.multidot..DELTA.n of the addressable liquid-crystal layers.
Compensating for the optical path difference d.multidot..DELTA.n of optically uniaxial media is based on a well-known physical principle which is implemented, for example, in the Babinet-Soleil compensator. A second optically uniaxial medium which has the same optical path difference d.multidot..DELTA.n is added to the optically uniaxial medium, the optical axes of the two media being, however, perpendicular to each other. Linearly polarized light whose plane of polarization is inclined at 45.degree. to the optical axis of the first medium is split up into an ordinary and an extraordinary ray. Since the optical axes of the two media are perpendicular to each other, the ordinary ray of the first medium passes through the second medium as an extraordinary ray and vice versa. The optical path difference in the first medium is d.multidot.(n.sub.e -n.sub.o) and in the second medium, on the other hand, d.multidot.(n.sub.o -n.sub.e), with the result that the total path difference is zero and the system composed of the two optically uniaxial media does not exhibit birefringence.
This principle, which is also known as delay compensation, was first proposed in 1981 for TN-displays (U.S. Pat. No. 4,443,065). In that case, a second TN display was used as compensating medium to eliminate the undesirable interference colors. It is also possible to compensate for the interference colors of STN displays by adding a second STN display having an opposite helical direction of rotation but otherwise identical properties (K. Katoh, Y. Endo, M. Akatsuka, M. Ohgawara and K. Sawada, Jap. J. Appl. Phys. 26 (1987) L 1784; SID Digest Vol. 20, 1989, papers 22.3, 22.4, 22.5 and 22.6). In the case of ECB displays, a combination of two ECB cells is used, the liquid-crystal molecules in the two cells having the same pre-tilt and having the same angle with respect to the homeotropic direction in the switched state, but the direction of rotation of said angle being opposite. Furthermore, the use of a planarly oriented, optically negative discotic liquid-crystal layer has been proposed (DE 39 11 620) for ECB displays.
The use of a second, not necessarily addressable liquid-crystal layer as compensating medium in electro-optical systems is advantageous, on the one hand, since the liquid-crystal layer used for displaying information or for compensation has an identical temperature dependence of the birefringence, and this makes compensation possible in a wide temperature range. In addition, both liquid-crystal layers exhibit a substantially identical dispersion, i.e., a substantially identical wavelength dependence of the birefringence. On the other hand, however, the use of two or more liquid-crystal layers results in increased production costs, a reduced output in production and a markedly higher display weight.
Because of these disadvantages, organic compensating films based on polycarbonate, polyvinyl alcohol or polyethylene terephthalate have been developed. Compared with two-layer or multilayer displays, however, electro-optical systems provided with such compensating films have a reduced contrast, a higher dependence of the contrast on viewing angle and a markedly smaller working temperature range. Owing to the difference in dispersion between compensating film and LCD, color displays require more than one compensating film. Furthermore, the production of organic compensating films is fairly complicated and requires a precise two-dimensional stretching of the organic polymeric material to achieve the optically uniaxial characteristic.
There is consequently still an appreciable need for electro-optical systems which contain a device for compensating for the optical path difference d.multidot..DELTA.n, in which said device does not have the disadvantages of the compensating media hith
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Matthias Ballauff, "Flussig-kristalline Polymere," Chemie in unserer Zeit, Jan. 22, 1988, Nr. 2, pp. 63-68.
Dorsch Dieter
Rieger Bernhard
Scheuble Bernhard
Harris C.
Merck Patent Gesellschaft mit beschrankter Haftung
Stoll Robert L.
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