Gray scale in zenithal bistable LCDs: The route to ultra‐low‐power color displays

Abstract

Abstract— Zenithal bistable devices (ZBD™) exhibit rugged image storage, excellent optical performance, fast latching, and infinite multiplexibility. The excellent performance characteristics are achieved using a surface‐relief grating to align the nernatic liquid crystal. Such gratings offer a high degree of design flexibility, at manufacturing costs equivalent to today's STN displays. In the present work, the grating shape is varied within each pixel to produce several separate latching thresholds, thereby introducing error‐free analog gray scales. Passive‐matrix waveforms, suitable for use with commercial STN drivers, are reported and shown to discriminate between at least seven error‐free grays in a test cell. The combination of analog and digital grays was considered. Careful selection of the digital dither weighting factors allows for a large total number of grays to be achieved, while reducing the effect of errors if all of the analog levels are not achieved.

Zenithal bistable device: Comparison of modeling and experiment

“…The Zenithal Bistable Device (ZBD) technology [1,2,3,4] introduces a patterned or profiled surface structure into an otherwise standard liquid crystal display (LCD) device. The patterned surfaces give rise to an elastic deformation of the liquid crystal to form multi-stable alignment states with different stable pre-tilts [4,5], each with a flexo-electric polarisation.…”

“…These two states then appear white and black, respectively, when viewed between crossed polarizers. Typically, the surface is a near-sinusoidal grating with a locally homeotropic surface condition for the liquid crystal director, though more complex structures are also used to give wide temperature range [5] and greyscale [3,6].…”

Applications of Flexoelectricity

“…[43][44][45][46] The energy barrier between the topologically different states is, as in the Boyd device, 36 related to the creation and annihilation of line defects, while the selection of states is achieved using the flexoelectric coupling as in Barberi et al 41 The basic principle of the ZBD mode, Fig. 7.12, is to use a non-reconfigurable surface that can support and stabilize both a homeotropic and a planar director structure in the bulk of a nematic liquid crystal cell.…”

“…By variation of the grating shape within each pixel, separate latching thresholds can be achieved over the pixel area and hence shades of grey can be introduced. 45 In a practical display device it is fundamental that adjacent domains with different states are stable in time and over wide temperature ranges at zero field even though the two states have different energies. As mentioned above, the ZBD is not symmetrically bistable as the two states are generally different with one state energetically more stable than the other.…”

Bistable bend-splay liquid crystal display

“…The zenithal bistable display (ZBD) is interesting in that the surface anchoring condition can give rise to either a homeotropic or homogeneous alignment. 7 Another bistable display that relies on surface anchoring switching has also been demonstrated. 8 Each technology has its own merits and difficulties.…”