Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2000-02-23
2002-03-19
Ton, Toan (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S118000, C349S121000
Reexamination Certificate
active
06359671
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a liquid crystal display having at least two retardation film layers.
Liquid crystal materials are useful for electronic displays because the 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 is typically required for illuminating displays. Liquid crystal displays (LCDs) are now commonly used in such applications as digital watches, calculators, portable computers, desktop computers, avionic cockpit displays, car navigation systems, and many other types of electronic devices which utilize the liquid crystal display advantages of long-life and operation with low voltage/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 in order to 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 array of pixels which are connected by an X-Y sequential addressing scheme between two conventional sets of perpendicular conductor lines (i.e. row and column lines). More advanced addressing schemes typically use arrays of thin film transistors, diodes, MIMS, etc. which act as switches to control the drive voltage at the individual pixels.
Contrast ratio is one of the most important attributes determining the quality of both normally white (NW) and normally black (NB) liquid crystal displays. The contrast ratio in a NW display is determined in low ambient conditions by dividing the “off state” light transmission (high intensity white light) by the “on state” or darkened intensity. For example, if the “off state” transmission is 200 fL at a particular viewing angle and the “on state” transmission is 5 fL at the same viewing angle, then the display's contrast ratio at that particular viewing angle is 40 or 40:1 for the particular driving voltage utilized.
Accordingly, in normally white (NW) LCDs, the primary factor adversely limiting the contrast ratio is the amount of light which leaks through the display in the darkened or “on state”. In normally black (NB) liquid displays, the primary factor limiting the contrast is the amount of light which leaks through the display in the darkened or “off state”. The higher and more uniform the contrast ratio of a display over a wide range of viewing angles, the better the LCD.
The contrast ratio problems are compounded in bright environments such as sunlight and other high intensity ambient conditions where there is a considerable amount of reflected and scattered ambient light adjacent the display. The lesser the amount of ambient light reflected from the display panel, the better the viewing characteristics of the display. Therefore, it is desirable to have a LCD reflect as little ambient light as possible. The amount of ambient light reflected by a display panel is typically measured via conventional specular and diffused reflection tests. In color liquid crystal displays, light leakage causes severe color shifts for both saturated and gray scale colors. These limitations are particularly important for avionic applications, where the copilot's viewing of the pilot's displays is important.
The legibility of the image generated by both normally black (NB) and normally white (NW) liquid crystal display devices depends on viewing angle, especially in matrix addressed devices with a large number of scanning electrodes. Absent a retardation film, the contrast ratio of a typically NW (and sometimes NB) liquid crystal display is usually at a maximum only within a narrow viewing or observing envelope centered about normal (0 degrees horizontal viewing angle, 0 degrees vertical viewing angle) and drops off as the angle of view increases.
It would be a significant improvement to provide a liquid crystal display capable of presenting a uniform high quality, high contrast ratio image over a wide field of view with little or no ambient light reflection.
Normally black (NB) twisted nematic displays typically have better contrast ratio contour curves or characteristics than do their counterpart NW displays in that the NB displayed image can be better seen at larger viewing angles. However, NB displays are much harder to manufacture than NW displays due to their high dependence on the cell gap “d” of the liquid crystal material, as well as on the temperature of the liquid crystal material itself. Accordingly, a long felt need in the art has been the ability to construct a NW display with high contrast ratios over a large range of viewing angles, rather than having to resort to the more difficult to manufacture NB display to achieve these characteristics.
What is generally needed in normally white displays is an optical compensating or retarding element(s), i.e. retardation film, which introduces a phase delay that restores the original polarization state of the light, thus allowing the light to be blocked by the output polarizer in the on state. Optical compensating elements or retarders are known in the art. It is known that the polyimides and copolyimides can be used as negative birefringent retarding elements in normally white liquid crystal displays and are said to be custom tailorable to the desired negative birefringent values without the use of stretching. The polyimide retardation films may be uniaxial but with an optical axis oriented in the Z direction, i.e. perpendicular to the plane defined by the film.
The type and orientation of optical compensation or retardation normally used depends in part upon the type of display, normally black or normally white.
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. In the un-energized OFF state (no voltage above the threshold voltage Vth is applied across the liquid crystal material), normally incident light from the backlight is first polarized by the rear 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 twisting effect. The twist angle is set, for example, to be about 90 degrees so that the light is blocked or absorbed by the front or output polarizer when the pixel is in the OFF state. 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 optical effect 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 twist effect such that the light polarization state is unchanged by propagation through the liquid crystal layer so that light can pass through the output polarizer. Patterns can be written in a normally black display by selectively applying a variable voltage to the portions of the display which are to appear illuminated.
Turning again to normally white (NW) LCD cells, in a normally white liquid crystal display configuration, a twisted nematic cell preferably having a twist angle of about 80 degrees to 100 degrees (most preferably about 90 degrees), is placed between p
Chernoff, Vilhauer, McClung and Stenze
Planar Systems Inc.
Ton Toan
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