Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1999-09-29
2003-03-25
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S123000, C349S136000, C349S183000
Reexamination Certificate
active
06538712
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates in general to the field of liquid crystal display design and, more particularly, to the fabrication of organic O-plate compensator elements. Specifically, the invention describes an O-plate compensator incorporating polyimide materials having bulky side-chain groups.
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 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 (LCDs) 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 LCI)s 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 LCDs, use arrays of thin film transistors to control driving voltages at individual pixels.
Contrast and stability of relative gray scale intensities are important attributes in determining the quality of a LCD. The primary factor limiting the contrast achievable in a LCD is the amount of light which leaks through the display in the dark state. In addition, the contrast ratio of the LCD also depends on the user's viewing angle. The contrast ratio in a typical LCD 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 LCDs, such leakage can also cause 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 LCD 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 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 normally white, twisted nematic LCD
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
. (The polarizer
105
and the analyzer
110
both polarize electromagnetic fields. Typically, however, the term ‘polarizer’ refers to a polarizer element that is closest to the source of light while the term ‘analyzer’ refers to a polarizer element that is closest to the viewer of the LCD.) 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 were 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 compensator 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 liquid crystal layer 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 angle of incidence 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, in general, 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 arrows
220
. The polar tilt angle &thgr;
225
, also referred to as a pretilt angle, is defined as the angle between the liquid crystal's molecular optic axis ĉ
230
and the x-y plane, as 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 onto 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 polarizer layer
300
and an analyzer layer
305
, between which is positioned a liquid crystal layer
310
, consisting of a liquid crystal material in the nematic phase.
It is convenient in describing the orientation of various compensation elements of the display to refer to a normal axis perpendicular to the display, which is depicted by a dashed line
370
. In the case of a normally white LCD, the polarizer
300
(with a polarization direction in the plane of the drawing
315
) and the analyzer
305
(with a polarization direction
320
perpendicular to the plane of the drawing) are oriented with their polarization directions at 90° to one an
Winker Bruce K.
Zhuang Zhiming
Duong Tai
Rockwell Science Center LLC
Sikes William L.
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