Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
2000-12-05
2003-09-16
Shalwala, Bipin (Department: 2673)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S089000, C349S172000
Reexamination Certificate
active
06621476
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of driving a liquid crystal display device, and in particular to a method of driving a surface mods LCD such as a pi-cell device. It also relates to a liquid crystal display device.
2. Description of the Related Art
The pi-cell (otherwise known as an “optically compensated birefringent device” or OCB) is described in “Mol. Cryst. Liq. Cryst.”, 1984, Vol 113, p329-339, and In U.S. Pat. No. 4,635,051. The structure of a pi-cell is schematically illustrated in FIG.
1
(
a
). The device comprises transparent substrates
2
,
2
′ on which are disposed alignment layers
3
,
3
′. A layer of nematic liquid crystal
1
is disposed between the substrates
2
,
2
′.
The alignment layers
3
,
3
′ create parallel alignment of the liquid crystal molecules in the liquid crystal layer
1
at its boundaries with the alignment layers
3
,
3
′. This can be achieved by using parallel-rubbed polyamide alignment layers. The pretilt induced by the alignment layers is generally under 45° and is typically in the range 2° to 10°.
Addressing electrodes (not shown) are provided on the substrates
2
,
2
′, so that an electric field can be applied to selected areas of the liquid crystal layer.
FIG.
1
(
a
) shown the device when no electric field is applied across the liquid crystal layer. The liquid crystal is in an H-state (homogenous state, also known as a splay state), in which the liquid crystal molecules in the centre or the liquid crystal layer are substantially parallel to the substrates. The short lines in the figure represent the director of the liquid crystal molecules.
When an electric field greater than a threshold value is applied across the liquid crystal layer, the liquid crystal molecules adopt a V-state (or a bend state). In this state, the liquid crystal molecules in the centre of the liquid crystal layer are substantially perpendicular to the substrates. FIG.
1
(
b
) shows a first V-state which occurs at a low applied voltage across the liquid crystal layer, and FIG.
1
(
c
) shows a second V-state which occurs when a higher voltage is applied across the liquid crystal layer. The pi-cell is operated by switching the liquid crystal layer between the first, low voltage V-state and the second, higher voltage V-state.
As can be seen by comparing FIGS.
1
(
b
) and
1
(
c
), modulating the electric field applied across the liquid crystal layer causes the director of liquid crystal molecules close to the substrates to be reoriented, while the directors of liquid crystal molecules in the central region (in the thickness direction) of the liquid crystal layer remain substantially perpendicular to the plane of the substrates. For this reason, the pi-call is known as a surface mode device, and operates with the director in a bend state. Surface mode LCDs are disclosed in “Sov. J. Quant. Electronics”, 1973, Vol 3, page 78-79.
Surface mode devices have the advantage that they tend to exhibit a more rapid electro-optic response than most other nematic liquid crystal devices. Since liquid crystals have optically anisotropic properties, placing a pi-cell between two polarisers and varying the voltage applied across the liquid crystal layer causes a variation in the optical transmission, and this makes possible the formation of a light modulating device. For example, a pi-cell may be placed between linear polarisers whose transmission axis are crossed with one another and are at 45° to the optic axis of the liquid crystal layer. Alternatively, a pi-cell may be arranged to operate in a reflective mode using only a single polariser.
One known problem is that when the electric field across the liquid crystal layer is reduced below the threshold voltage for the transition from the H-state to the V-state, the directors of the liquid crystal molecules adopt the H-state, or splay-state, shown in FIG.
1
(
a
). The transition from this 0V splay-state to the required operating state is slow, and when a display device that incorporates a pi-cell is turned on there is a delay before the required operating state forms.
One attempt to overcome this problem, often referred to as the “nucleation problem”, is described in U.S. Pat. No. 4,566,756. This patent addresses the nucleation problem by adding a chiral material to the liquid crystal, so that the liquid crystal director adopts a 180° twist state under a condition of no applied voltage. This is shown in FIG.
2
. In contrast, the device illustrated in FIGS.
1
(
a
) to
1
(
c
) has a 0° twist angle.
When a sufficiently high voltage (typically around 3V or greater) is applied across a chiral doped pi-cell having a 180° twist angle it will exhibit an essentially identical director bend state to a non-doped, 0° twist pi-cell. In fact, a 180° twist p
1
-cell does not reach a true bend state (that is, a state where the director in the central region of the liquid crystal layer is perpendicular to the substrates) at any finite voltage. However, at high applied voltages the liquid crystal state of a 180° twist pi-cell is a good approximation to a band state. In contrast, at low voltages (typically around 3V or below), a chiral doped pi-cell and a non-doped pi-cell will differ significantly in their operating properties.
Although a chiral doped 180° twist state pi-cell of the type disclosed in U.S. Pat. No. 4,566,756 overcomes the nucleation problem, it has the disadvantage that the voltage applied over the pi-cell must remain above a certain level (typically around 3V or above) to ensure that the device operates in a surface mode. For example, if the voltage applied to such a device were switched between 0V and 10V the director of liquid crystal molecules in the centre of the liquid crystal layer would switch between 0° (that is, parallel to the substrates) and substantially 90° (that is, substantially perpendicular to the plane of the substrates). The device clearly would not then perform as a surface mode switching device, and thus would not achieve the high speed of operation expected for a surface mode device.
In order for the pi-cell disclosed in U.S. Pat. No. 4,566,756 to function as a surface mode device and thus retain the short switching time associated with a surface mode device, it is necessary for the voltage applied across the pi-cell not to fall below around 3V. This requirement means that the full range of optical response of liquid crystal is not available. In particular, bright regions of the optical response curve may not be available.
FIG. 3
is a schematic illustration of the relationship between equilibrium optical transmissivity of a chiral doped 180° twist pi-cell against the voltage applied across the cell. Curves of this sort are routinely used to determine the voltages that should be applied across a liquid crystal layer in a display device. The inserts in
FIG. 3
schematically show the director configuration of the liquid crystal molecules for various applied voltages.
When voltage D its applied across the liquid crystal layer, the display has a low transmissivity, and the director of liquid crystal molecules in the centre of the liquid crystal layer is predominantly perpendicular to the cell substrates. That is, the liquid crystal state is a close approximation to a bend state. In contrast, when a voltage close to zero (voltage A) is applied across the liquid crystal layer, it adopts a 180° twist state, and the director of the molecules in the centre of the liquid crystal layer is parallel to the plane of the substrates. At intermediate voltages, the liquid crystal undergoes a complex variation in its optical transmissivity as the molecules re-orient themselves under the action of the applied voltage.
It will be seen that the transmissivity of the pi-cell only slowly tends towards zero as the applied voltage increases. It may therefore be desirable to incorporate a fixed retarder in a pi-cell, so that zero transmissivity is obtained at a finite applied voltage.
From a consideration of just the transmissivi
Towler Michael John
Walton Harry Garth
Dharia Prabodh M.
Shalwala Bipin
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