Wilbert J. A. M. Hartmann

Ferroelectric liquid crystal displays for television application

Abstract The surface-stabilized ferroelectric liquid crystal (SSFLC) effect is a serious candidate for application in television displays, due to its high switching speed and viewing angle independence. For the television application, however, several severe requirements must be fulfilled, of which the demand for a large amount of gray levels is very crucial in view of the bistable character of the SSFLC effect. A review and discussion of the different approaches to realize gradation in SSFLC structures is given. The so-called dither techniques yield discrete gray levels but become complex when many levels are required. Analogue gradation methods are based on in-pixel domain switching but require an accurate control of the multidomain structures. Active-matrix addressing of the SSFLC effect enhances the reproducibility of the multidomain gray levels via the charge-control process, resulting in a fast-switching, viewing-angle-independent video display. Passive-matrix addressing (multiplexing) takes advanta...

Uniform SSFLC director pattern switching

Abstract Dynamical and polarisation-dependent transmission intensity measurements on 2.0 μm SSFLC cells clarify that the director pattern profiles are uniform from top to bottom glass plate in the fully-switched, the memory and all transient states. Switching occurs by two processes: a continuous shift on the smectic C cone and a bistable multidomain transition. This behaviour is caused by tilted smectic layers. Removal of this tilt results in a true bistability in which memory and switched states arc nearly indistinguishable and possess high contrast.

SSFLC field-induced director pattern modulation described in terms of smectic layer bending

Abstract It is proposed to modify the description of the electric-field-induced modulation of the director pattern in surface stabilized ferroelectric liquid crystal (SSFLC) structures. This modification is based on a constantly planar-oriented director pattern, combined with the boundary conditions that the smectic layer tilt is relatively fixed near the surface, but free at the chevron interface. As a consequence the interaction between an electric field and the polarization causes a bending of the smectic layers. This description is able to provide a consistent interpretation of reported data on the electro-optical behaviour of SSFLC structures.

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.

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 ...