Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1999-05-12
2002-04-09
Wu, Xiao (Department: 2674)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S096000
Reexamination Certificate
active
06369789
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to reducing the effects of ionic memory in ferroelectric liquid crystal (FLC) materials.
BACKGROUND OF THE INVENTION
Liquid crystal devices incorporating a ferroelectric smectic liquid crystal material (FLCDS) are particularly suitable for use in displays and shutters in which their fast switching times and memory characteristics are of advantage. Because of the bistable nature of the material, it is not necessary to continue to apply an electric field to the material in order to maintain the material in a given switched state. A conventional FLCD comprises a layer of ferroelectric smectic liquid crystal material between two parallel glass substrates, electrode structures being typically provided on the inside facing surfaces of the glass substrates in the form of row and column electrodes (on opposite sides of the liquid crystal material) which cross one another to form a matrix array. The intersections of the electrodes define an array of pixels within the material. As is well known, pulses are applied to the row and column electrodes in order to produce electric fields which switch the molecules within the material between two polar states having different molecular orientations. As a result of the different light transmitting properties of the two molecular orientations, when the material is disposed between two polarizers having polarizing axes which are arranged transversely to one another, a display element or pixel at the intersection of two electrodes will appear dark or light depending on the state to which the molecules of the pixel have previously been switched by the electric field due to the voltage difference between the pulses applied to the relevant row electrode on one side of the layer and the relevant column electrode track on the other side of the layer.
Some prior art documents will now be discussed. P Maltese. J. Dijon, T. Leroux, D. Sarrasin, Ferroelectrics (1988), vol. 85, p26s discusses the use of blanking periods containing multiple up and down voltage pulses to provide an improved erasure of previous states. It mentions the problem of using blanking pulses with large V·t products of the same polarity. The opposite polarity pulses substantially cancel out any ionic build-up, however both polarities cause full switching of the FLC and the total blanking period is impracticably long. It does not discuss any modification of either the switching pulse itself or the time periods following it.
British Patent No. 2 262 831 seeks to speed up the addressing time of FLCDS whilst improving their contrast ratio. It describes extending the strobe (row or scan) voltage beyond the select period of the row to which it is applied, so as to temporally overlap with the select period of the strobe voltage applied to subsequent row or rows. Regards these extended strobe waveforms it states that “Each strobe pulse may be immediately followed by a pulse of opposite sign”, no reason for this is given, although in Table 15 a result is given that shows for two field operation of Malvern 2.1 a larger operating window is obtained than with Malvern 2.0.
M. Nitta, N. Ozaki, H. Suenaga, K. Nakaya and S. Kobayashi ‘Electrooptic Characteristics of a Charge-Transfer Complex-Doped Surface-Stabilized Ferroelectric Liquid Crystal Device’ Japanese Journal of Applied Physics (1988) vol. 27, no. 4 pL477-L478 describes an improvement of bistability and response speed obtained by doping an FLC material with a charge-transfer complex. The FLC material showed a minimum in response time for both the doped and undoped cases. In the case of the doped material the device exhibited only reverse switching to a single polarity pulse which was faster than forward switching in the undoped material. There is no discussion on the effect this could have on analogue or digital grayscale.
K. Nakaya, B. Y. Zhang, M. Yoshida, I. Isa, S. Shindoh and S. Kobayashi ‘Electrooptic Bistability of a Ferroelectric Liquid Crystal Device Prepared Using Charge-Transfer Complex-Doped Polyimide-Orientation Films’ Japanese Journal of Applied Physics (1989) vol. 28, pL116-L118 describes an enhancement of bistability and response speed obtained by doping the polyimide alignment layer in an FLC device with charge transfer complex. The FLC material used is the same as in (iii) and therefore shows a minimum in response time. The improvement is attributed to neutralization of accumulated surface charge from the spontaneous polarization. There is no discussion on the effect this could have on analogue or digital grayscale.
K. Nakaya, B. Y. Zhang, M. Yoshida, I. Isa, S. Shindoh and S. Kobayashi ‘Electrooptic Bistability of a Perroelectric Liquid Crystal Device Prepared Using Charge-Transfer Complex-Doped Polyimide-Orientation Films’ Japanese Journal of Applied Physics (1989), vol. 28 pL116-L118 describes an enhancement of bistability and response speed obtained by doping the polyimide alignment layer in an FLC device with charge transfer complex. The FLC material used is the same as in (iii) and therefore shows a minimum in response time. The improvement is attributed to neutralization of accumulated surface charge from the spontaneous polarization. There is no discussion on the effect this could have on analogue or digital grayscale.
J. R. Hughes and F. C. Saunders ‘Inversion of contrast in ferroelectric smectic C liquid crystal displays’ Liquid Crystals (1988), vol. 3 p1401-1410 describes two switching regimes when using two slot bipolar multiplex waveforms. At lower voltages and longer slot times the switching occurs due to the trailing pulse while at higher voltages and shorter slot times the switching occurs due to the leading pulse. The strobe waveform during the line address period is made up of equal and opposite pulses and no pulses are extended beyond this period.
P. W. H. Surguy, et al. Ferroelectrics (1991), vol. 122, p63 describes the JOERS/Alvey addressing scheme for FLC displays with &tgr;-V min characteristics.
Japanese Patent Publication No. 215616/1992 describes a method to prevent reverse switching by applying an opposite polarity voltage pulse after the first applied pulse.
It is well documented that the performance of ferroelectric liquid crystal devices is significantly affected by ionic contamination: Firstly, ionic response to the internal dipole field causes memory of previous states. This is a particular problem when stable analogue gray levels, which utilize the partial switching behavior of the FLCD, are sought. Ionic memory can also cause problems when temporal dither is used to achieve gray levels because the transmission level in each subframe will depend on the previous subframes even if only the two bistable states are addressed.
Secondly, ionic response to an external applied field destabilizes the switched state and can cause reverse switching after field removal (Ionic Field Latching Effect). If this behavior occurs following a strobe pulse then the operating drive window is cut off by the switching resultant failing to switch at longer pulse widths. If this behavior occurs following a blanking pulse then the operating window is reduced because it causes the non-switching resultant to switch at a lower threshold.
The solution to the first of these problems might be to compensate for previous state dependence by appropriate modification of the data. This requires a real-time computation of the data required to achieve a particular gray level as well as a means of storing the switch history of each pixel. If the dependence on previous states accumulates over several frame times this could require multiple frame stores and becomes impractical to both evaluate and implement. Another solution to this problem is to use longer duration and larger amplitude blanking pulses to reset the ionic field to a uniform state (see prior art (i)). Both of these solutions are limited by the response time of the ions to both external fields and the internal field from the spontaneous polarization. In order to make practical either of these solutions or to resolve the prob
Bonnett Paul
Kabe Masaaki
Towler Michael John
Ulrich Diana Cynthia
Renner Otto Boisselle & Sklar
Sharp Kabushiki Kaisha
Wu Xiao
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