Liquid crystal cells – elements and systems – Particular structure – Detector of liquid crystal temperature
Patent
1995-05-01
1999-01-12
Sikes, William L.
Liquid crystal cells, elements and systems
Particular structure
Detector of liquid crystal temperature
349184, 349199, G02F 1133, G02F 113, C09K 1902
Patent
active
058596801
DESCRIPTION:
BRIEF SUMMARY
Switching and display devices containing ferroelectric liquid-crystal mixtures are disclosed, for example, in EP-B 0 032 362 (=U.S. Pat. No. 4,367,924). Liquid-crystal light valves are devices which modify their optical transmission properties, for example due to electrical switching, in such a way that light which is incident (and possibly reflected again) is intensity- or phase-modulated. A number of such light valves can be combined to form liquid-crystal displays (LCDs). Examples are the known watch and calculator displays or liquid-crystal displays in the OA (office automation) and TV sectors. However, these also include light shutters, as employed in photocopiers, printers, etc. So-called "spatial light modulators" are also within the area of application of liquid-crystal light valves (see Liquid Crystal Device Handbook, Nikkan Kogyo Shimbun, Tokyo, 1989; ISBN 4-526-02590-9C 3054 and the papers cited therein).
Electro-optical switching and display devices are constructed in such a way that the FLC layer is included on both sides by layers which are usually, in this sequence starting from the FLC layer, at least one alignment layer, electrodes and a limiting plate (for example made of glass). In addition, they contain a polarizer if they are operated in "guest-host" mode or in reflective mode, or two polarizers if the transmissive birefringence mode is used. The switching and display elements may, if desired, contain further auxiliary layers, such as, for example, diffusion barrier or insulation layers.
Together with a distance between the limiting plates which is chosen to be sufficiently small, such alignment layers bring the FLC molecules of the FLC mixture into a configuration in which the molecules lie with their long axes parallel to one another and the smectic planes arranged perpendicular or inclined to the alignment layer. In this arrangement, the molecules are known to have two equivalent alignments, between which they can be switched by applying an electrical field in a pulsed manner, ie. FLC displays are capable of bistable switching. The response times are inversely proportional to the spontaneous polarization of the FLC mixture and are in the range of microseconds.
The major advantage of FLC displays over the LC displays which are still the ones usually encountered in industrial practice, is regarded as being the achievable multiplex ratio, ie. the maximum number of lines which can be addressed in the time-sequential process ("multiplex process"), which is virtually unlimited in FLC displays, in contrast to conventional LC displays.
In ferroelectric liquid crystals (FLCs), practical use in displays (N. A. Clark, S. T. Lagerwall, Appl. Phys. Lett. 36 (1980) 899); K. Kondo et al. Jpn. J. Appl. Phys. 27 (1988) 464; HOE 89/F 157) requires a uniform and above all invariable alignment of the smectic layers. The layer perpendiculars are approximately parallel to the display plane; their mean direction should remain the same over the display area. The polarizers used can thereby be aligned relative to the liquid crystal so that maximum contrast and/or maximum brightness can be achieved over the entire display.
During the switching process, the molecules are switched, without a change in the layer perpendiculars relative thereto, into states which change the preferential direction of the anisotropic refractive index in the display plane. The physical ideal here is an angle change of 45.degree. when the birefringence effect is used and 90.degree. when guest-host mode is used.
In the alignment process during production of the display, the liquid crystal is usually heated and then recooled to room temperature. During the cooling, in particular at the transition from the smectic A phase to the smectic C phase, the smectic layer thickness generally drops and results in the so-called chevron structure (definition: T. P. Rieker et al., Phys. Rev. Lett. 59, 2658), which in turn significantly reduces the angle change of the preferential direction of the anisotropic refractive index during switching in the
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Escher Claus
Fujiwara Shozo
Illian Gerhard
Jungbauer Dietmar
Murakami Mikio
Hoechst Aktiengesellschaft
Nguyen Tiep H.
Sikes William L.
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