Antonius G. H. Verhulst

A wide-viewing-angle video display based on the deformed-helix ferroelectric liquid-crystal effect and a diode active matrix

— A wide-viewing-angle video display has been realized using a deformed-helix ferroelectric liquid crystal (DHF-LC) in combination with a diode active matrix. The DHF-LC is operated in the symmetric mode (ac driving), preventing the occurrence of image sticking. The application of a reset pulse before the select pulse results in a much better step response of the display and in lower drive voltages. These effects have also been calculated by making use of the electronic equivalent circuit of DHF-LC. The display has a contrast ratio of 25 and a transmission 55% that of a twisted-nematic cell.

The possibilities of ferroelectric liquid crystals for video displays

— Different approaches were used to examine the possibility of using surface-stabilized ferroelectric liquid-crystals (SSFLCs) in video displays. The advantages of such displays lie in the viewing-angle independence and the high switching speed. The large number of gray levels needed for video can, in principle, be obtained by digital or analog techniques. From simulation experiments on a high-resolution CRT, it is estimated that for video, for the case of equidistant luminance levels, 128 levels for each primary colour are needed. Discrete gradation by means of bi-level subpixels (spatial dither) is simulated for projection and direct-view displays. Efficient subpixelation (minimum number of extra connections) results in perceptual artefacts when the three primary colours (RGB) are projected onto each other, as in a projection system. In the case of a direct-view display with mosaic colour filter, the layout of this mosaic turns out to be more important than subpixelation. Analog gradation methods are based on in-pixel domain switching effect. They require accurate control of the multidomain structures. Active-matrix addressing for the SSFLC strongly enhances the reproducibility of the multidomain gray levels via the charge-control process. This is demonstrated on test displays. Passive-matrix addressing takes advantage of the bistability and switching speed of the SSFLC effect. One approach to control the multidomain gray levels in passive-matrix addressing is the “texture method,” which is based on a distribution of the threshold voltage for switching within each pixel. The mechanism, the addressing scheme, the obtainable contrast in the different textures, the applicability of various FLC mixtures, and the resulting temperature sensitivity are discussed. The performance of test displays, operating at video rate, is evaluated. Several issues, such as temperature sensitivity, texture stability, and hysteresis in the transmission-voltage curve have to be further improved.

Pretilt angle measurements on smectic A cells with chevron and tilted bookshelf layer structures

Abstract We have measured the pretilt angle induced by rubbed polymer films in a smectic A and in a nematic liquid crystalline medium using an optical phase retardation method. The pretilt angle was found to depend on the liquid crystalline phase (smectic A versus nematic) and on the smectic layer structure (chevron versus tilted-bookshelf). The occurrence of the different smectic layer structures was verified by X-ray diffraction measurements. The effect of the applied rubbing energy on the pretilt angle obtained is measured.

Switching on stray electric fields in ferroelectric liquid crystal cells

Abstract In liquid crystal dot-matrix displays light may leak through the display area between the pixels. To obtain sufficient contrast this non-pixel area has to be made non-transmissive. For ferroelectric liquid crystal (FLC) displays this may be done by switching the material in the gaps between the picture elements to a non-transmissive state by the stray electric fields that occur during application of voltages to the pixel electrodes. This is experimentally studied for test cells with an electrically modified smectic layer structure. The gap region considered is an asymmetric environment of the FLC material, as the transparent conductive coating has been removed on one substrate, whereas on the other substrate a conductor covers the glass. The FLC molecules in the non-pixel area prefer to direct their dipoles towards the covered substrate. To switch the FLC material with the stray electric fields, it is a prerequisite to outweigh this preference. We made spatially resolved observations for various ...