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 (.DELTA.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 previous displays.  Liquid crystal displays (LCDs) are now commonly used in such applications as digital watches, calculators, portable computers, avionic cockpit displays, and man.sub. Y 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 an 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 liquid crystal displays, the primary factor limiting the contrast 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 display over a wide range of viewing angles, the better the LCD. 
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 an 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. 
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 address devices with large numbers of scanning electrodes.  Absent a retardation film, the contrast ratio of a typical NW (and sometimes NB) liquid crystal display is usually at a maximum only within a narrow viewing or observing envelope centered about normal (0.degree.  horizontal viewing angle, 0.degree.  vertical viewing angle) and drops off as the angle of view increases. 
It would be a significant improvement in the art 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 seen better 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 and are disclosed, for example, in U.S. Pat.  Nos.  5,184,236; 5,196,953; 5,138,474; and 5,071,997, the disclosures of which are hereby incorporated herein by reference.  It is known that the polyimides and copolyimides disclosed by aforesaid U.S. Pat.  No.  5,071,997 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 of U.S. Pat.  No.  5,071,997 are uniaxial but with an optical axis oriented in the Z direction, i.e. perpendicular to the plane defined by the film. 
FIG.  1 is a contrast ratio curve graph for a prior art normally white twisted nematic light valve.  The light valve for which the contrast ratio curves are illustrated in FIG.  1 included a rear linear polarizer having a transmission axis defining a first direction, a front or light-exit linear polarizer having a transmission axis defining a second direction wherein the first and second directions were substantially perpendicular to one another, a liquid crystal material having a cell gap "d" of about 5.86 .mu.m, a rear buffing zone (i.e. orientation film) oriented in the second direction, and a front buffing zone orientated in the first direction.  The LC material was Model No.  ZLI-4718 obtained from Merck.  The temperature was about 34.4.degree.  C. when the graph illustrated by FIG.  1 was plotted.  This light valve did not include a retarder.  The above-listed parameters with respect to FIG.  1 are also applicable to FIGS.  2 and 3. 
The contrast ratio graph of FIG.  1 was plotted utilizing a 6.8 V driving voltage, i.e. V.sub.on, a 0.2 volt "off state" V.sub.off voltage, and by backlighting the display with white light.  As can be seen in FIG.  1, at least about 10:1 contrast ratios extended along the 0.degree.  vertical viewing axis only to angles of about -40.degree.  horizontal and +38.degree.  horizontal.  Likewise, at least about 30:1 contrast ratios extended along the 0.degree.  vertical viewing axis only to horizontal angles of about .+-.29.degree.. This graph is illustrative of the common problems associated with typical normally white liquid crystal displays in that their contrast ratios at large horizontal and vertical viewing angles are limited. 
FIG.  2 is a contrast ratio curve plot of the same normally white light valve described above with respect to FIG.  1.  However, the FIG.  2 plot was formulated utilizing a V.sub.on of about 5.0 volts and a V.sub.off of about 0.2 volts.  Again, the temperature was about 34.4.degree.  C. and white light was used.  As can be seen by comparing the graphs of FIGS.  1 and 2, as the "on state" voltage applied to the liquid crystal material decreased, as in FIG.  2, the contrast ratio curves expanded horizontally and contracted vertically. 
The 10:1 contrast ratio area of FIG.  2 along the 0.degree.  vertical viewing axis extended a total of about 85.degree. ( from about -45.degree.  to +40.degree.  horizontal) as opposed to only about 78.degree.  in FIG.  1.  Also, the 30:1 contrast ratio area of FIG.  2 along the 0.degree.  vertical viewing axis extended horizontally about 67.degree.  as opposed to only about 58.degree.  in FIG.  1, the 30:1 ratio being, of course, represented by the contour line disposed between the 10:1 and 50:1 contour lines.  With respect to vertical viewing angles, the contrast ratio areas of 10:1 and 30:1 in FIG.  2 did not extend along the 0.degree.  horizontal viewing axis to the negative vertical extent that they did in FIG.  1.  In sum, the normally white light valve of FIGS.  1-3 had less than desirable contrast ratios at large viewing angles, these contrast ratios expanding horizontally and contracting vertically as the "on state" or driving voltage across the liquid crystal material decreased. 
FIG.  3 is a driving voltage versus intensity (fL) plot of the prior art light valve described above with respect to FIGS.  1-2, this plot illustrating the gray level behavior of the prior art light valve.  The various curves represent horizontal viewing angles from about -60.degree.  to +60.degree.  along the 0.degree.  vertical viewing axis. 
Gray level performance and the corresponding amount of inversion are important in determining the quality of an LCD.  Conventional liquid crystal displays typically utilize anywhere from about eight to sixty-four different driving voltages.  These different driving voltages are generally referred to as "gray level" voltages.  The intensity of light transmitted through the pixel or display depends upon the driving voltage.  Accordingly, gray level voltages are used to generate dissimilar shades of color so as to create different colors when, for example, these shades are mixed with one another. 
Preferably, the higher the driving voltage in a NW display, the lower the intensity (fL) of light transmitted therethrough.  Likewise then, the lower the driving voltage, the higher the intensity of light reaching the viewer.  The opposite is true in normally black displays.  Thus, by utilizing multiple gray level driving voltages, one can manipulate either a NW or NB liquid crystal display to emit desired intensities and shades of light.  A gray level V.sub.on is generally known as any driving voltage greater than V.sub.th (threshold voltage) up to about 5.0-6.5 V.
Gray level intensity in LCDs is dependent upon the displays' driving voltage.  It is desirable in NW displays to have an intensity versus driving voltage curve wherein the intensity of light emitted from the display or pixel continually and monotonically decreases as the driving voltage increases.  In other words, it is desirable to have gray level performance in a NW pixel such that the intensity (fL) at 6.0 volts is less than that at 5.0 volts, which is in turn less than that at 4.0 volts, which is less than that at 3.0 volts, which is in turn less than that at 2.0 volts, etc.  Such good gray level curves across wide ranges of viewing angles allow the intensity of light reaching the viewer via the pixel or display to be easily and consistently controlled. 
Turning again to FIG.  3, the intensity versus driving voltage curves illustrated therein of the prior art light valve of FIGS.  1-2 having no retardation film(s) are undesirable because of the inversion humps present in the areas of the curves having driving voltages greater than about 3.2 volts.  The intensity aspect of the curves monotonically decreases as the driving voltage increases in the range of from about 1.6-3.0 volts, but at a driving voltage of about 3.2 volts, the intensities at a plurality of viewing angles begin to rise as the voltage increases from about 3.2 volts to 6.8 volts.  These rises in intensity as the voltage increases are known as "inversion humps."  The inversion humps of FIG.  3 include only rise portions.  However, such inversion humps often include both rise and fall portions as will be appreciated by those of ordinary skill in the art, thus enabling the "inversion humps" to actually look like humps. 
A theoretically perfect driving voltage versus intensity curve with respect to a NW display would have a decreased intensity (fL) for each increase in gray level driving voltage at all viewing angles.  In contrast to this, the inversion humps of FIG.  3 represent increases in intensity of radiation emitted from the light valve for each corresponding increase in gray level driving voltage above about 3.2 volts.  Accordingly, it would satisfy a long felt need in the art if such a liquid crystal display could be provided with no or little inversion. 
U.S. Pat.  No.  5,184,236 discloses a NW display including a pair of retardation films provided on one side of the LC layer, these retardation films having retardation values of about 300-400 nm.  The viewing characteristics of the LCDs of this patent could be improved upon with respect to contrast ratio, inversion, uniformity of viewing zone, and flexibility of the position of the viewing envelope by utilizing retarders of different values. 
The parent of this application, i.e. Ser.  No.  08/167,652, provides an NW display with a pair of retardation films having retardation values of about 80-200 nm, one film being disposed on each side of the LC layer.  While the different embodiments of Ser.  No.  08/167,652 provides excellent results with respect to all viewing characteristics, the disclosure of this application provides similar results via different optical structure. 
FIG.  4 illustrates the angular relationships between the horizontal and vertical viewing axes and angles described herein relative to a liquid crystal display and conventional LCD angles .phi.  and .THETA.. The +X, +Y, and +Z axes shown in FIG.  6 are also defined in other figures herein.  Furthermore, the "horizontal viewing angles" (or X.sub. ANG) and "vertical viewing angles" (or Y.sub. ANG) illustrated and described herein may be transformed to conventional LCD angles: azimuthal angle .phi.;  and polar angle .THETA.,  by the following equations: EQU TAN (X.sub. ANG)=COS (.phi.).multidot. TAN (.THETA.)  EQU SIN (Y.sub. ANG)=SIN (.THETA.).multidot. SIN (.phi.)  EQU or EQU COS (.THETA.)=COS (Y.sub. ANG).multidot. COS (X.sub. ANG) EQU TAN (.phi.)=TAN (Y.sub. ANG).div. SIN (X.sub. ANG)
The term "rear" when used herein but only as it is used to describe substrates, polarizers, electrodes, buffing zones and orientation films means that the described element is on the incident light or backlight side of the liquid crystal material, or in other words, on the side of the liquid crystal material opposite the viewer. 
The term "front" when used herein but only as it is used to describe substrates, polarizers, electrodes, buffing zones and orientation films means that the described element is located on the viewer side of the liquid crystal material. 
The LCDs and light valves herein included liquid crystal material with a birefringence (.DELTA.N) of 0.084 at room temperature, Model No.  ZLI-4718 obtained from Merck. 
The term "retardation value" as used herein means "d.multidot..DELTA.N" of the retardation film or plate, wherein "d" is the film thickness and ".DELTA.N" is the film birefringence (either positive or negative). 
The term "interior" when used herein to describe a surface or side of an element (or an element itself), means the side or surface closest to the liquid crystal material. 
The term "light valve" as used herein means a liquid crystal display including a rear linear polarizer, a rear transparent substrate, a rear continuous pixel electrode, a rear orientation film, an LC layer, a front orientation film, a front continuous pixel electrode, a front substrate, and a front polarizer (without the presence of color filters and driving active matrix circuitry such as TFTs).  Such a light valve may also include a pair of retardation films disposed on either side of the LC layer as described with respect to each Example herein.  In other words, a "light valve" may be referred to as one giant pixel. 
It is apparent from the above that there exists a need in the art for a normally white liquid crystal display wherein the viewing zone of the display includes high contrast ratios over a large range of vertical and horizontal viewing angles with little or no inversion and/or ambient reflection from the display panel. 
This invention will now be described with respect to certain embodiments thereof, accompanied by certain illustrations, wherein: