Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1999-09-01
2001-11-20
Malinowski, Walter J. (Department: 2164)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S118000, C349S119000
Reexamination Certificate
active
06320634
ABSTRACT:
TABLE OF CONTENTS
1. REFERENCES
2. BACKGROUND OF THE INVENTION
2.1 LCD Technology Overview
2.2 Normally White Twisted Nematic LCDs
2.3 Normally White Twisted Nematic LCD Characteristics
2.3(a) C-Plate Compensation
2.3(b) Gray Scale Stability
2.3(c) O-Plate Gray Scale Compensation
2.3(d) O-Plate Technology
2.4 Summary
3. SUMMARY OF THE INVENTION
4. BRIEF DESCRIPTION OF DRAWINGS
5. DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS
5.1 Introduction
5.2 Nematic Embodiment
5.3 Smectic-C Substrate Aligned Embodiment
5.4 Smectic-C Electric Field Alignment Embodiment
5.5 Possible Variations
6. BIBLIOGRAPHY
7. CLAIMS
8. ABSTRACT
BACKGROUND OF THE INVENTION
This invention is concerned with the design of liquid crystal displays (LCDs) and, more particularly, with techniques for maximizing the field of view of such displays by maintaining a high contrast ratio and minimal variance in relative gray levels over a wide range of viewing angles. These goals are achieved through the fabrication and manufacture of LCDs using O-plate compensator technology.
LCD TECHNOLOGY OVERVIEW
Liquid crystals are useful for electronic displays because polarized light traveling through a liquid crystal layer is affected by the layer's birefringence, which can be changed by the application of a voltage across the layer. By using this effect, the transmission or reflection of light from an external source, including ambient light, can be controlled with much less power than is required for the luminescent materials used in other types of displays. As a result, liquid crystal displays are now commonly used in a wide variety of applications, such as, for example, digital watches, calculators, portable computers, and many other types of electronic equipment. These applications highlight some of the advantages of LCD technology including very long operational life in combination with very low weight and low, power consumption.
The information content in many liquid crystal displays is presented in the form of multiple rows of numerals or characters, which are generated by segmented electrodes deposited in a pattern on the display. The electrode segments are connected by individual leads to electronic driving circuitry. By applying a voltage to the appropriate combination of segments, the electronic driving circuitry controls the light transmitted through the segments.
Graphic and television displays may be achieved by employing a matrix of pixels in the display which are connected by an X-Y sequential addressing scheme between two sets of perpendicular conductors. More advanced addressing schemes, applied predominantly to twisted nematic liquid crystal displays, use arrays of thin film transistors to control driving voltages at the individual pixels.
Contrast and stability of relative gray scale intensities are important attributes in determining the quality of a liquid crystal display. The primary factor limiting the contrast achievable in a liquid crystal display is the amount of light which leaks through the display in the dark state. In addition, the contrast ratio of the liquid crystal device also depends on the viewing angle. The contrast ratio in a typical liquid crystal display is a maximum only within a narrow viewing angle centered near normal incidence and drops off as the angle of view is increased. This loss of contrast ratio is caused by light leaking through the black state pixel elements at large viewing angles. In color liquid crystal displays, such leakage also causes severe color shifts for both saturated and gray scale colors.
The viewing zone of acceptable gray scale stability in a typical prior art twisted nematic liquid crystal display is severely limited because, in addition to color shifts caused by dark state leakage, the optical anisotropy of the liquid crystal molecules results in large variations in gray level transmission, i.e., a shift in the brightness-voltage curve, as a function of viewing angle. The variation is often severe enough that, at extreme vertical angles, some of the gray levels reverse their transmission levels. These limitations are particularly important for applications requiring a very high quality display, such as in avionics, where viewing of cockpit displays from both pilot and copilot seating positions is important. Such high information content displays require that the relative gray level transmission be as invariant as possible with respect to viewing angle. It would be a significant improvement in the art to provide a liquid crystal display capable of presenting a high quality, high contrast image over a wide field of view.
FIGS. 1A and 1B
show a conventional normal white, twisted nematic liquid crystal display
100
including a polarizer
105
, an analyzer
110
with a polarization axis perpendicular to that of the polarizer
105
, a light source
130
, and a viewer
135
.
In the normally white configuration of
FIGS. 1A and 1B
, a “nonselect” area
115
(no applied voltage) appears light, while a “select” area
120
(those which are energized by an applied voltage) appear dark. In the select area
120
the liquid crystal molecules tend to tilt and rotate toward alignment with the applied electric field. If this alignment perfect, all the liquid crystal molecules in the cell would be oriented with their long axes normal to the cell's major surface. This configuration is known as homeotropic alignment.
Because the liquid crystals used for twisted nematic displays exhibit positive birefringence, this arrangement, known as the homeotropic configuration, would exhibit the optical symmetry of a positively birefringent C-plate. As is well known in the art, a C-plate is a uniaxial birefringent plate with its extraordinary axis (i.e., its optic or c-axis) perpendicular to the surface of the plate (parallel to the direction of normally incident light). In the select state the liquid crystal in a normally white display would thus appear isotropic to normally incident light, which would be blocked by the crossed polarizers.
One reason for the loss of contrast with increased viewing angle which occurs in a normally white display is that a homeotropic liquid crystal layer will not appear isotropic to off-normal light. Light propagating through the later at off-normal angles appears in two modes due to the birefringence of the layer; a phase delay is introduced between those modes and increases with the incident angle of the light. This phase dependence on incidence angle introduces an ellipticity to the polarization state which is incompletely extinguished by the second polarizer, giving rise to light leakage. To correct for this effect, an optical compensating element must also have C-plate symmetry, but with negative birefringence (n
e
<n
o
). Such a compensator will introduce a phase delay opposite in sign to the phase delay caused by the liquid crystal layer, thereby restoring the original polarization state and allowing light passing through energized areas of the layer to be blocked more completely by the output polarizer. C-plate compensation, however, does not impact the variation of gray scale with viewing angle, which is addressed by the present invention.
FIG. 2
depicts the coordinate system which is used to describe the orientation of both liquid crystal and birefringent compensator optic axes. Light propagates toward the viewer
200
in the positive z direction
205
which, together with the x-axis
210
and the y-axis
215
, form a right-handed coordinate system. Backlighting is provided, as indicated by the arrows
220
, from the negative z direction. The polar tilt angle &THgr;
225
is defined as the angle between the liquid crystal's molecular optic axis ĉ
230
and the x-y plane, measured from the x-y plane. The azimuthal or twist angle &PHgr;
235
is measured from the x-axis to the projection
240
of the optic axis into the x-y plane.
Normally White Twisted Nematic LCDs
FIG. 3
is a cross sectional schematic view of a prior art twisted nematic, transmissive type normally white liquid crystal display. The display includes a pola
Chung Young J.
Li Zili
Rosenblatt Charles
Ryang Hong-Son
Warren, Jr. Leslie F.
Foley & Lardner
Malinowski Walter J.
Rockwell International Corporation
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