Liquid crystal device

Details Liquid crystal device Liquid crystal device

2002-01-11

2003-07-29

Kim, Robert H. (Department: 2871)

Liquid crystal cells, elements and systems

Particular structure

Having significant detail of cell structure only

C349S177000, C349S179000, C349S033000

Reexamination Certificate

active

06600537

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal device of the pi-cell type. Such a device is suitable for use, for example, in transmissive and reflective flat panel displays, head-mounted displays, field-sequential colour displays, projection systems and three-dimensional image display systems.
2. Description of the Related Art
P. D. Berezin, L. M. Blinov, I. N. Kompanets and V. V. Nikitin ‘Electro-optic Switching in Oriented Liquid-Crystal Films’ July-August 1973 Sov. J. Quant. Electron Vol 3 pp 78-79 disclose a liquid crystal device of nematic type which is capable of achieving fast response times. The device comprises a non-twisted cell of low surface tilt, but it is not clear whether parallel or anti-parallel surface alignment directions are provided. Optical modulation is achieved mainly by re-orientation of the liquid crystal molecules near the surface regions whereas the orientation in the bulk of the material remains substantially homeotropic.
P. J. Bos and K. R. Koehler/Beran ‘The pi-Cell: A Fast Liquid Crystal Optical Switching Device’ 1984 Mol. Cryst. Liq. Cryst. Vol 113 pp 329-339 provide the first known disclosure of a pi-cell which, as is well known, comprises first and second alignment layers arranged to induce parallel low pre-tilt alignment in a nematic liquid crystal material disposed therebetween. The pi-cell is an example of a surface mode device in which the optical modulation is obtained mainly by reorientation of liquid crystal molecules near the surface regions, as described above. The pi-cell disclosed in this paper is substantially symmetrical in that the pre-tilt angles induced by the alignment layers are of substantially equal magnitude.
FIG. 1
of the accompanying drawings illustrates the various states of a conventional pi-cell. In the absence of an applied electric field across the liquid crystal layer, the cell is in a splay state, referred to conventionally and hereinafter as the H-state. The H
S
state for zero applied field and with symmetrical pre-tilt angles is illustrated at
1
with the liquid crystal directors being indicated by the lines such as
2
.
As the voltage across the layer (and hence the applied electric field) is increased, the H-state becomes asymmetrical for relatively small voltages as illustrated by the H
A
-state at
3
.
The H-states of a pi-cell do not have desirable optical properties for use in optical devices such as flat panel displays. Above a certain voltage, however, the pi-cell exhibits an alternative state known as a bend state (conventionally known and referred to hereinafter as the V-state) as shown at
4
in
FIG. 1
which has more useful optical properties. In the V-state, the liquid crystal molecules have relatively low tilts in the surface regions but have a homeotropic alignment in the bulk of the material with the director substantially perpendicular to the cell surfaces. Once the V-state has been established, optical modulation is performed mainly by reorientation of the liquid crystal molecules in the surface regions with the molecules in the bulk of the layer being substantially unaffected by the variations in applied voltage within the operating range of the device.
For pi-cells of the type representative of the prior art, with a typical pretilt of for example 5°, there is a “oritical” or threshold voltage U
V/H
above which the energy of the V-state is lower than the energy of the H-state. The liquid crystal in such a pi-cell will therefore prefer to align in the V-state above U
V/H
. The transition from the undesirable low voltage H-state to the desired V-state is, however, non trivial and a so-called “nucleation” process must occur which involves the creation and movement of defects in the liquid crystal. The process of nucleating a pi-cell from an H-state to a V-state is typically rather slow, taking some seconds In typical devices.
Besides the V-state and the H-state, a pi-cell may also exhibit a twist state (conventionally know and referred to hereinafter as the T-state) as shown at
5
in
FIG. 1
in which the director performs a (±)180° twist between the alignment layers. For a typical pi-cell representative of the prior art with, for example, a typical pretilt of 5°, if the liquid crystal is in the V-state and the voltage U is lowered, there is a threshold voltage U
V/T
below which the T-state becomes of lower energy than the V-state. Below this threshold voltage, the liquid crystal therefore undergoes a transition from the V-state to the T-state. This transition does not involve nucleation and may proceed fairly rapidly (in typically 10's or 100's of milliseconds). The T-state has less desirable optical properties (such as viewing angle performance and contrast ratio) than the V-state. As the voltage U is lowered towards zero volts, the H-state will reform. However, as with the H/V transition, the H/T transition involves the nucleation of defects and is typically rather slow (of the order of seconds). Thus the T-state may exist at low voltages for some seconds before it Is replaced by the H-state.
FIG. 1
summarises the behaviour of a conventional pi-cell as the applied voltage U is first increased to a maximum U>>U
V/H
and then reduced towards zero. A conventional pi-cell, which is representative of the prior art shows three main types of liquid crystal orientation: H-states, a V-state and T-states. At zero volts, the energy of the H-state is lowest, the energy of the V-state is highest and the energy of the T-state is intermediate between the energies of these other states.
The paper by Bos et al describes two modes of operating the pi-cell. In both modes, one state of the pi-cell is achieved at a relatively high voltage where the V-State is stable (its energy is lower than the energies of the H-state and the T-state.) In this operating state, the pi-cell provides a minimum of optical retardation.
The first mode applies to relatively thin cells in which the liquid crystal material is allowed to relax from the relatively high voltage V-state to a zero volt state, at which the pi-cell provides a half wave of retardation. The zero volt state is a substantially co-planar state and is achieved dynamically for in excess of 20 milliseconds (although this state may be achieved for substantially less time at higher temperatures). This state is unstable and, if it is allowed to prevail for too long, the T-state begins to form and this can then lead to nucleation of the H-state, after which re-nucleation of the V-state has to be performed in order for the pi-cell to function again. This voltage addressing scheme therefore applies a voltage U<U
V/T
to a pi-cell which, at zero volts, has a lowest energy H-state, a highest energy V-state and an intermediate energy T-state and makes use of the dynamic V-state which survices in excess of 20 msec before the T-state forms (although, again, at higher temperatures this may be substantially less).
For thicker cells the second mode of operation is used in which the half wave retardation condition is reached before the onset of any significant relaxation to the T-state. A small voltage is maintained across the liquid crystal layer to hold the cell at the half wave retardation condition.
H. Nakamua ‘Dynamic Bend Mode in a Pi-Cell’ Dec. 1-3 1999 SID Proceedings of the 6th International Display Workshop, pp 37-40 discloses a technique of “Under-Driving” a pi-cell and refers to this as a “Dynamic Bend Mode”. This driving mode is equivalent to the first driving mode described by Bos et al with the dynamic V-state having a lifetime which increases with increasing bias voltage. There is also disclosed the use of a relatively high voltage blanking pulse during each frame in order to avoid the need to reform the V-state.
U.S. Pat. No. 4,566,758 discloses a pi-cell in which the liquid crystal material Is doped with a chiral dopant such that the ratio of the thickness of the liquid crystal layer to the chiral pitch is greater than 0.25. This type of device remains In a T-state throughou

Affiliated with

Also associated with

Rating

Liquid crystal device does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Liquid crystal device, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Liquid crystal device will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3082974