Liquid crystal cells – elements and systems – With specified nonchemical characteristic of liquid crystal... – Within nematic phase
Reexamination Certificate
2001-09-13
2004-08-31
Ngo, Huyen (Department: 2871)
Liquid crystal cells, elements and systems
With specified nonchemical characteristic of liquid crystal...
Within nematic phase
C349S184000, C349S185000
Reexamination Certificate
active
06784968
ABSTRACT:
This invention relates to the addressing of bistable nematic liquid crystal devices.
One known bistable nematic liquid crystal device is described in WO-97/14990, PCT/GB96/02463, GB98/02806.1, EP96932739.4 and has been described a zenithal bistable device (ZBD™). This device comprises a thin layer of a nematic or long pitch cholesteric liquid crystal material contained between cell walls. Optically transparent row and column electrode structures arranged in an x,y matrix of addressable pixel allow an electric field to be applied across the layer at each pixel causing a switching of the material. One or both cell walls are surface treated to permit nematic liquid crystal molecules to adopt either of two pretilt angles in the same azimuthal plane at each surface. Opposite surfaces may have pretilt in differing azimuthal planes. The two states are observed as a dark (e.g. black) and a bright (e.g. light grey) state. The cell can be electrically switched between these two states to allow information display which can persist after the removal of power; i.e. the liquid crystal material is latched into either of the two allowed states and remain in the one latched state until electrically switched to the other latched state.
Another bistable nematic device is described in WO99/34251, PCT/GB98/03787. This uses grating structures to provide bistable alignment similar to WO-97/14990 but uses a negative dielectric anisotropy material.
The terms switching and latching need some explanation: in monostable nematic devices, the effect of a suitable applied electric field is to move the liquid crystal molecules (more correctly the director) from one alignment condition to another, i.e. from a zero applied voltage OFF state to an applied voltage ON state. In a bistable device, the application of a voltage may cause some movement of the liquid crystal molecules without sufficient movement to cause them to permanently move into a different (one of two) state. In the present application, the term switch and latch are used to mean the molecules are caused to move from one bistable state to the other bistable state; where they remain until switched or latched back to the first state.
The term same azimuthal plane is explained as follows; let the walls of a cell lie in the x,y plane, which means the normal to the cell walls is the z-axis. Two pretilt angles in the same azimuthal plane means two different molecular positions in the same x,z plane.
Another bistable nematic liquid crystal device is described in GB-2,286,467. This uses a grating alignment surface to give two stable states in two different azimuthal planes.
Most presently available liquid crystal devices are monostable and are addressed using rms. addressing methods. For example twisted nematic and phase change type of liquid crystal devices are switched to an ON state by application of a suitable voltage, and allowed to switch to an OFF state when the applied voltage falls below a lower voltage level. In these devices the liquid crystal material responds to the rms. value of the electric field. Various well-known addressing schemes are used; all use ac rms. voltage values. This is convenient because liquid crystal material deteriorat if the applied voltage is dc.
EP 569,029 describes a long pitch cholesteric liquid crystal display having two metastable switched states. The material is first switched to a Frederick's transition, then switched with other voltages to either of the two metastable states. Each state lasts for about 10 seconds after voltage is removed; i.e. the display has (temporary) bistability providing the display is continually addressed.
Another type of device is the ferroelectric liquid crystal display (FELCD) which can be made into bistable device with the use of smectic liquid crystal materials and suitable cell wall surface alignment treatment. Such a device is a surface stabilised ferroelectric liquid crystal device (SSFELCDs) as described by: -L J Yu, H Lee, C S Bak and M M Labes, Phys Rev Lett 36, 7, 388 (1976); R B Meyer, Mol Cryst Liq Cryst. 40, 33 (1977); N A Clark and S T Lagerwall, Appl Phys Lett, 36, 11, 899 (1980). Then device switch upon receipt of a suitable unipolar (dc) pulse of suitable voltage amplitude and time. For example a positive pulse switches to an ON state, and a negative pulse switches to an OFF state. A disadvantage of this in that the material will degenerate under dc. voltages. Therefore the many known addressing schemes must ensure a net zero value dc. For example by periodically inverting all voltages.
Known addressing schemes for bistable smectic devices include those described in EP-0,542,804 PCT/GB91/01263, EP-0,308,203, EP-0,197,742, Surgey et al ferroelectric 1991, vol. 122 pp63-79 etc. Some use mono pulse strobe pulses, others bipolar strobe pulses in combination with bipolar data pulses.
Bistable nematic devices, as mentioned above, switch between or latch into their two bistable states upon receipt of suitable unipolar (dc) pulses. This may allow use of existing addressing schemes previously used for ferro electric bistable devices. However, the switching characteristics of bistable nematic devices are different from that of ferro electric bistable devices.
The present invention addresses the problem of switching bistable nematic liquid crystal devices by providing new addressing schemes, which take account of the different switching characteristics of bistable nematic devices.
According to this invention a method of addressing a bistable nematic device formed by two cell walls enclosing a layer of nematic or long pitch cholesteric liquid crystal material with electrode structures carried by the walls to form a series of row electrodes on one wall and a series of column electrodes on the other wall to form a matrix of intersecting regions or pixels with a wall surface treatment on at least one wall providing a molecular alignment permitting the molecules at or adjacent the wall to align into two different stable states upon application of appropriate unipolar voltage pulses, the method comprising the steps of:
applying a row waveform to each row in a sequence whilst simultaneously applying one of two data waveforms to each column electrode whereby each pixel can be independently switched between two bistable states;
the row waveform having a period of at least two time slots, at least two unipolar pulses for switching the device to a first state, and at least two unipolar pulses for switching the device to a second state,
both data waveforms having a period of at least two time slots with a unipolar pulse in each time slot, with at least one data waveform shaped to combine with the row waveform to cause a switching to one latched state
whereby each pixel can be addressed to latch into either stable state to collectively provide a desired display, with a substantially net zero dc voltage applied to the device.
Preferably the alignment treatment on a cell wall is arranged to give two different switching characteristics; namely lower voltage/time values for switching from one latched state to the other latched state. This may be arranged by variation of the height of grooves in a grating structure, and/or variation of the period of the grating, and/or selection of a surfactant on the grating, and/or selection of material elastic constants. The surfactant may be lecithin or a chrome complex surfactant.
The addressing of the device may be in two field times, one for switching to one stable state, and the other for switching into the second stable state. The field times may be identical or different in length. The device may be addressed by selectively switching pixels to one state in one field time and selectively switching pixels to the other state in the second field time. Alternatively, some or all of the pixels may be blanked into one state, then selectively switched to the other state. The blanking can be done at the same time to all pixels, a row at a time (e.g. one or more rows ahead of selective addressing), or the blanking and selective addressing may be combined as
Bryan-Brown Guy P
Graham Alistair
Hughes Jonathan R
Jones John C
Ngo Huyen
ZBD Displays Limited
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