Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Utility Patent
1997-06-24
2001-01-02
Dudek, James A. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S119000
Utility Patent
active
06169590
ABSTRACT:
This invention relates to the design of a liquid crystal display having at least one retardation film therein. More particularly, this invention relates to the design of a polychromatic or multicolored liquid crystal display and techniques for eliminating color leakages and maximizing the field of view of such displays.
BACKGROUND OF THE INVENTION
Liquid crystal materials are useful for electronic displays because light traveling through a layer of liquid crystal (LC) material is affected by the anisotropic or birefringent value (&Dgr;N) of the material, which in turn can be controlled by the application of a voltage across the liquid crystal material. Liquid crystal displays are desirable because the transmission or reflection of light from an external source, including ambient light and backlighting schemes, can be controlled with much less power than was required for the illuminance materials used in other previous displays. Liquid crystal displays are now commonly used in such applications as digital watches, calculators, portable computers, avionic cockpit displays, and many other types of electronic devices which utilize the liquid crystal display advantages of long life and operation with low voltage and power consumption.
The information in many liquid crystal displays is presented in the form of a matrix array of rows and columns of numerals or characters, which are generated by a number of segmented electrodes arranged in such a matrix pattern. The segments are connected by individual leads to driving electronics, which apply a voltage to the appropriate combination of segments to thereby display the desired data and information by controlling the light transmitted through the liquid crystal material. Graphic information in, for example, avionic cockpit applications or television displays may be achieved by a matrix of pixels which are connected by an X-Y sequential addressing scheme between two sets of perpendicular conductor lines (i.e. row and column lines). More advanced addressing schemes use arrays of thin film transistors or diodes which act as switches to control the drive voltage at the individual pixels. These schemes are applied predominantly to twisted nematic liquid crystal displays, but are also finding use in high performance versions of super twisted liquid crystal displays.
Contrast is one of the most important attributes determining the quality of both normally white (NW) and normally black (NB) liquid crystal displays. In normally black (NB) LCDs, the primary factor limiting the contrast achievable in these liquid crystal displays is the amount of light which leaks through the display in the darkened or OFF state. In the NW mode, the primary factor limiting the contrast is the amount of light which leaks through the display in the darkened ON state. This problem is compounded in a bright environment, such as sunlight, where there is a considerable amount of reflected and scattered ambient light. In color liquid crystal displays, light leakage causes severe color shifts for both saturated and gray scale colors. These limitations are particularly important for avionics applications, where copilot viewing of the pilot's displays is important.
In addition, the legibility of the image generated by both normally black (NB) and normally white (NW) liquid crystal devices depends on the viewing angle, especially in a matrix addressed device with a large number of scanning electrodes. Absent a retardation film, the contrast ratio in a typical NB or NW liquid crystal display is usually at a maximum only within a narrow viewing (or observing) angle centered about normal incidence (0° horizontal viewing angle and 0° vertical viewing angle) and drops off as the angle of view is increased.
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.
Several types of liquid crystal pixels or cells are in widespread use in flat panel displays. Active matrix addressing allows such displays to present a full color image with high resolution. When viewed directly at a normal or on-axis viewing angle N (0° vertical viewing angle and 0° horizontal viewing angle), a liquid crystal display provides a generally high quality output especially when the cell gap “d” is matched to the first transmission minimum, but the image degrades and exhibits poor contrast at increased viewing angles. This occurs because liquid crystal cells operate by virtue of the anisotropic or birefringent effect exhibited by a liquid crystal layer which includes a large number of anisotropic liquid crystal molecules. Such a material will be positively uniaxially birefringent (i.e., the extraordinary refractive index is larger than the ordinary refractive index) with an extraordinary refractive index associated with the alignment of the long molecular axes. The phase retardation effect such a liquid crystal material has on light passing through it inherently varies or increases with the inclination angle of light, leading to a lower quality image at larger viewing angles (see, e.g. Penz, Viewing Characteristics of the Twisted Nematic Display, Proceeding of the S.I.D., Vol. 19, p. 43 (1978); Grinberg, et al., Transmission Characteristics of a Twisted Nematic Liquid Crystal Layer, Journal of the Optical Society of America, Vol. 66, p. 1003 (1976)). By introducing an optical compensating element (or retarder) into the liquid crystal pixel or cell, however, it is possible to correct for the unwanted angular effects and thereby maintain higher contrast at both normal and larger viewing angles than otherwise possible.
The type and orientation of optical compensation or retardation required depends upon the type of display, normally black or normally white, which is used.
In a normally black (NB) twisted nematic display, the twisted nematic liquid crystal material is placed between polarizers whose transmission axes are parallel to one another. Such NB displays may be either front X-buffed or rear X-buffed. The first and second LC buffing zones are preferably perpendicular to one another thereby necessitating one of the buffs being perpendicular relative to the polarizer axes. If the first buff zone is perpendicular to the first polarizer transmission axis then the display is rear “X-buffed.” Otherwise, it is front “X-buffed.”
In the unenergized or OFF state (no voltage below the threshold voltage V
th
is applied across the liquid crystal material), normally incident light from the backlight is first polarized by the first polarizer and in passing through the pixel or cell has its polarization direction rotated by the twist angle of the liquid crystal material dictated by the buffing zones. This effect is known as the waveguiding or twisting effect. The twist angle is set, for example, to be about 90° so that the light is blocked or absorbed by the second or output polarizer. When a voltage is applied via electrodes across the normally black pixel, the liquid crystal molecules are forced to more nearly align with the electric field, eliminating the twisted nematic symmetry of the LC material. In this orientation, the optical molecular axes of the liquid crystal layer molecules are perpendicular to the cell walls. The liquid crystal layer then appears isotropic to normally incident light, eliminating the waveguiding effect such that the light polarization state is unchanged by propagation through the liquid crystal layer so that light can pass through the second or output polarizer. Patterns can be written in the NB display by selectively applying a variable voltage to the portions of the display which are to appear illuminated.
When viewed in the OFF state at both normal N and other viewing angles, however, the dark unenergized areas of a normally black display will appear colored because of angle dependent retardation effects for light passing through the liquid crystal layer at such angles. Contrast can be restored by using a compensating or retarding element which has an optical symmetry sim
Abileah Adiel
Brinkley Patrick F.
Xu Gang
Dudek James A.
Laff, Whitesel & Saret, Ltd.
OIS Optical Imaging Systems, Inc.
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