Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1998-04-28
2003-05-20
Kim, Robert H. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S118000
Reexamination Certificate
active
06567143
ABSTRACT:
This invention relates to a normally white (NW) liquid crystal display (LCD) including tilted and negative retardation films or retarders. More particularly, this invention relates to a NW twisted nematic (TN) LCD including a pair of tilted retarders arranged in a manner so as to provide improved contrast at vertical viewing angles and reduce gray level inversion.
RELATED APPLICATIONS
This application is related to commonly owned U.S. Pat. Nos. 5,576,861; 5,594,568; and 5,570,214; and U.S. Ser. Nos. 08/559,275, filed Nov. 15, 1995; U.S. Ser. No. 08/711,797, filed Sep. 10, 1996, U.S. Ser. No. 08/768,502, filed Dec. 18, 1996, U.S. Ser. No. 08/869,973, filed Jun. 5, 1997, and U.S. Ser. No. 8/876,043, filed Jun. 13, 1997, the entire disclosures of which are all hereby incorporated herein by reference. Each of these commonly owned applications and/or patents relates to a liquid crystal display with specific retarder values, contrast ratios, retarder positions or orientations, and/or retarder types.
BACKGROUND OF THE INVENTION
Informational data in liquid crystal displays (LCDs) is presented in the form of a matrix array of rows and columns of numerals or characters (e.g. pixels) which are generated by a number of segmented pixel electrodes arranged in a matrix pattern. The segments are connected by individual leads to driving electronics which apply a voltage to the appropriate combination of segments and adjacent liquid crystal (LC) material in order to display the desired data and/or information by controlling the light transmitted through the liquid crystal (LC) material.
Contrast ratio (CR) is one of the most important attributes considered in determining the quality of both normally white (NW) and normally black (NB) LCDS. The contrast ratio (CR) in a normally white display is determined in low ambient conditions by dividing the “off-state” light transmission (high intensity white light) by the “on-state” or darkened transmitted 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 voltages utilized.
Accordingly, in normally white LCDs, a significant factor adversely limiting contrast ratio is the amount of light which leaks through the display in the darkened or “on-state.” In a similar manner, in normally black displays, a significant factor limiting the contrast ratio achievable 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 particular display over a wide range of viewing angles, the better the LCD in most applications.
Normally black (NB) twisted nematic displays typically have better contrast ratio contour curves or characteristicsthan do their counterpart NW displays (i.e. the NB image can often be seen better at large or wide viewing angles). However, NB displays are optically different than NW displays and are much more difficult to manufacture due to their high dependence on the cell gap or thickness “d” of the liquid crystal layer as well as on the temperature of the liquid crystal (LC) material itself. Accordingly, a long-felt need in the art has been the ability to construct a normally white (NW) display with high contrast ratios over a large range of viewing angles, rather than having to resort to the more difficult and expensive to manufacture NB displays in order to achieve these characteristics.
What is often needed in NW LCDs is an optical compensating or retarding element(s), i.e. retardation film(s), which introduces a phase delay that restores the original polarization state of the light, thus allowing the light to be substantially blocked by the output polarizer (analyzer) in the “on-state.” Optical compensating elements or retarders are known in the art and are disclosed, for example, in U.S. Pat. Nos. 5,184,236; 5,189,538; 5,406,396; 4,889,412; 5,344,916; 5,196,953;: 5,138,474; and 5,071,997.
The disclosures of U.S. Pat. Nos. 5,570,214 and 5,576,861 (incorporated herein by reference) in their respective “Background” sections illustrate and discuss contrast ratio, and driving voltage versus intensity (fL) graphs of prior art NW displays which are less than desirable. Prior art NW LCD viewing characteristics are problematic in that, for example, their contrast ratios are limited horizontally and/or vertically (and are often non-symmetric), and their gray level performance lacks consistency.
Gray level performance, and the corresponding amount of inversion, are also important in determining the quality of an LCD. Conventional active matrix liquid crystal displays (AMLCDs) typically utilize anywhere from about 8 to 64 different driving voltages. These different driving voltages are generally referred to as “gray level” voltages. The intensity of light transmitted through the pixel(s) or display depends upon the driving voltage utilized. Accordingly, conventional gray level voltages are used to generate dissimilar shades of color so as to create different colors and images when, for example, the shades are mixed with one another.
Preferably, the higher the driving voltage in a normally white display, the lower the intensity (fL) of light transmitted therethrough. The opposite is true in NB displays. Thus, by utilizing multiple gray level driving voltages, one can manipulate either a NW or NB LCD to emit desired intensities and shades of light/color. A gray level voltage V
ON
is generally known as any driving voltage greater than V
th
(threshold voltage) up to about 3.0 to 6.5 volts, although gray level voltages may be as low as 2.0 in certain applications.
U.S. Pat. Nos. 5,576,861 and 5,570,214 discuss, in their respective “Background” sections, prior art NW LCDs with inversion problems (e.g. inversion humps). As discussed therein, inversion humps are generally undesirable.
U.S. Pat. No. 5,583,679 discloses an LCD including an optical compensating sheet that includes a discotic structure and negative birefringence, with the discotic structure unit having an inclined plane. Unfortunately, the contrast ratios and inversion characteristics resulting from displays of the '679 patent have been found by the instant inventors to be less than desirable. Certain embodiments of the instant invention described herein exhibit surprisingly improved results with respect to contrast ratio and/or inversion as compared to the '679 patent.
In the prior art, some have been able to separately and independently reduce inversion or improve contrast in given viewing zones, but typically if contrast is improved upon, then inversion characteristics suffer. To date, those in the art have been unable to improve both contrast and inversion in the same viewing area of a display as taught below in accordance with the instant claimed invention.
The examples set forth in U.S. Application Ser. No. 08,876,043, incorporated herein be reference, have excellent viewing characteristics. However, it has been surprisingly found by the instant inventors that contrast ratios can be improved in the vertical viewing region(s) by the inventions set forth herein.
It is apparent from the above that there exists a need in the art for a normally white TN liquid crystal display wherein the viewing zone of the display has high contrast ratios and/or little or no inversion over a wide range of viewing angles, and wherein contrast ratios may be improved in vertical region(s). Furthermore, there exists a need in the art for improved contrast and reduced inversion in the same viewing zone (e.g. in the upper vertical viewing zone principally utilized by pilots of aircraft in avionic applications).
The term “rear” when used herein but only as it is used to describe substrates, polarizers, electrodes, buffing films or zones, and orientation films means that the described element is on the backlight side of the liquid crystal material, or in other words, on the side of the LC material opposite
VanderPloeg John A.
Xu Gang
Guardian Industries Corp.
Kim Robert H.
Michael Best & Friedrich LLC
Nguyen Dung
Saret Larry L.
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