Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
2000-04-13
2003-11-04
Wu, Xiao (Department: 2674)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S043000
Reexamination Certificate
active
06642914
ABSTRACT:
TECHNICAL FIELD
The invention relates generally to liquid crystal displays, and, more particularly, to a liquid crystal display having improved isocontrast performance and a method for producing same.
BACKGROUND OF THE INVENTION
Liquid crystal displays (LCDs) have been in use for quite some time and are useful for displaying information. Although LCDs are attractive for use in displays for portable devices in which power consumption is a concern, LCDs are also commonly used in other display applications, such as test and monitoring equipment. Advantageously, LCDs typically consume a relatively small amount of power.
An LCD can be fabricated either as a reflective display, in which light passes through the display and is reflected back through the display for viewing, or as a transmissive display, in which a light source is located behind the display and a viewer directly views the display.
FIG. 1A
is a cross-sectional view illustrating an example of a conventional reflective LCD
10
. Briefly described, an LCD is fabricated by locating a liquid crystal (LC) material
11
between glass substrate elements
16
and
17
, to which transparent indium tin oxide (ITO) electrode lines
12
and
14
have been applied. The ITO lines
12
and
14
are applied to the glass substrate elements
16
and
17
in a manner such that the ITO lines form a matrix of intersecting lines. The ITO lines
12
and
14
are typically oriented orthogonal to each other such that a picture element (pixel) is formed in the liquid crystal material at the intersection of each ITO line
12
and
14
.
A polarizer
18
is applied to a surface of glass substrate element
16
opposite that to which the ITO line
12
is applied. Similarly, analyzer
19
is applied to a surface of glass substrate element
17
opposite to which the ITO line
14
is applied. A diffuser
21
is applied over the analyzer and a reflector
22
is applied over the diffuser. In the case of a transmissive display, a light source is located in place of the reflector
22
. The polarizer
18
is a thin film applied to the glass to serve as a filter that polarizes the impinging light so that the entering light beam is polarized in one direction. The analyzer is a thin film that aids in the polarization process. The diffuser
21
is also a thin film that diffuses, or smears, the light beam so that annoying birefringence does not occur when the display is viewed. Birefringence can be explained by understanding the refraction of a plane wave of light at the boundary between an isotropic medium (such as air) and an anisciropic medium (such as a crystal). The wavefronts of the incident wave and the refracted wave should be matched at the boundary. Because the anisotropic medium supports two modes of distinctly different phase velocities, for each incident wave there are two refracted waves with two different directions and different polarizations. This effect is known as birefringence.
Upon the application of an electrical potential between the ITO line
12
and the ITO line
14
, an electric field is established between the ITO lines
12
and
14
and passes through the LC material. In accordance with known principles, the molecules in the LC material, in response to the electric field, become mobile and. depending upon the type of LC material, will rotate, twist, or otherwise change state, thereby preventing light, by the cross polarizing of the traveling light wave. from passing through the display and appearing dark to a viewer. The display may be normally white or black. Upon the application of the electric field, the LC material will change state. In other words, if the material is “black” it will become “white” and if the material is “white” it will become “black.” Importantly, the LC material changes state in response to the electric field applied by the ITO lines
12
and
14
.
FIG. 1B
is a plan view schematically illustrating a conventional pixel
15
formed at the intersection
23
(referred to as a “pixel junction”) of ITO lines
12
and
14
of the LCD
10
of FIG.
1
A. Some of the elements have been omitted for clarity. Pixel
15
includes the LC material
11
located at the intersection of, and disposed between ITO lines
12
and
14
.
FIG. 1C
is a cross-sectional view of the conventional pixel
15
of FIG.
1
B. In response to the electric, or e field
25
, created in the region of the pixel junction
23
between ITO line
12
and ITO line
14
, the molecules that make up LC material
11
will change state, or rotate, thereby becoming visible to a viewer (
24
of FIG.
1
A).
FIG. 1D
is a cross-sectional view of the conventional reflective LCD
10
of
FIG. 1A
illustrating the difference between an “addressed” pixel and a “non-addressed” pixel. In
FIG. 1D
, the LC material
11
is illustrated as comprising individual molecules, an example of which is indicated by reference numeral
13
. The voltage source “Vs”
27
corresponding to pixel
33
indicates that the LC material
11
within pixel
33
is selected or addressed. When addressed, the orientation of the individual molecules
13
within the LC material
11
sandwiched between alignment layer
28
and alignment layer
29
change state, or twist, and appear to “straighten out”. Alignment layers
28
and
29
are each thin films which have been physically rubbed in specific directions so as to assist the LC molecules
13
adjacent to these layers to pre-rotate in favorable directions. For example, if it is desirable for an LCD to have a preferred viewing angle, these rubbed layers enhance that angular view. The aligned molecules
13
(associated with pixel
33
) allow the light from light source
24
to pass through the LC material
11
with a specific polarization. The light from light source
24
can be reflected back to the viewer
26
through glass substrate element
16
and polarizer
18
.
The molecules
13
within pixel
35
. associated with voltage source “Vna”
26
, have not been addressed. The random molecular orientation of these molecules
13
suppresses the light from light source
24
and prevents the light from passing through the LC material
11
associated with pixel
35
. Hence, pixel
35
is non-addressed and would appear dark to a viewer
26
.
FIG. 1E
is a graphical representation
31
of the isocontrast curves of pixel
15
of
FIGS. 1B and 1C
. When LCDs are viewed at angles normal to, or nearly normal to, the surface of the LCD display, the rotated liquid crystal material is easy to discern. However, when viewed at off angles, the polarizing effect of the twisted liquid crystal material on the traveling light wave quickly becomes indiscernible. This is caused by the crystalline nature of the liquid crystal material. This condition is illustrated in
FIG. 1E
, which is a graphical representation of a contrast curve (referred to as an isocontrast curve) for a conventional pixel
15
. A contrast curve which has the same contrast ratio (light returning from the addressed pixel/light returning from a non-addressed pixel) at every point on its curve is called an isocontrast curve. As shown in
FIG. 1E
, the liquid crystal material in the region of pixel
15
clearly has better contrast at some angles that at other angles. For example, isocontrast line
34
shows that the pixel has a higher contrast when viewed at approximately 180 or 360 degrees, than it does when viewed at 90 or 270 degrees. For example, arrow
37
indicates a viewing angle in which a viewer would see limited contrast.
Therefore there is a need in the industry for a liquid crystal display in which the contrast of the liquid material may be controlled and maximized depending on the viewing angle desired.
SUMMARY OF THE INVENTION
The invention is a liquid crystal display having improved and controllable isocontrast and a method for producing same.
In architecture, the invention can be conceptualized as a liquid crystal display, comprising a liquid crystal material disposed between a pair of transparent plates. The display includes a first electrical conductor and a second electrical conductor a
Hewlett--Packard Development Company, L.P.
Wu Xiao
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