Color liquid crystal display employing dual cells driven with an EXCLUSIVE OR relationship

A liquid crystal display device and method for generating the three primary colors and combinations thereof including black and white is disclosed using, at most, two adjacent discrete subpixels which together form a full color pixel. At least one dual STN cell system is included in each device, such controlled by EXCLUSIVE OR (XOR) logic.

FIELD OF THE INVENTION 
This invention relates to liquid crystal display pixel arrangements, and 
more specifically to STN cell arrangements for generating the three 
primary colors in different combinations and black and white using 
compensation technology. 
BACKGROUND OF THE INVENTION 
For use in small portable televisions or laptop computer displays, both 
monochrome and color liquid crystal displays (LCDs) have become popular. 
LCD panels have many significant advantages over light emitting diode 
(LED) displays and cathode ray tubes (CRTs). For example, because LCD 
panels have very low power dissipation, they are useful for 
battery-operated equipment. Moveover, because of LCDs' high ambient light 
levels, they are in utilized in equipment for use outdoors. Also, for 
displays that require custom shapes and symbols or displays with many 
digits or characters, LCD's high pixel density is preferred over the low 
pixel density of LEDs. Because LCDs are compact in size, energy efficient, 
and have the potential 1or high performance, LCDs are considered very 
promising. 
A significant limitation of twisted nematic LCD displays is the relatively 
long switching time and resultant limitation on driver multiplexing due to 
the physical nature of the LCD phenomenon. Therefore, although acceptable 
for displays with modest information content and interactivity, available 
twisted nematic LCDs are not yet competitive with CRTs requiring high 
information content and interactivity. To overcome the multiplexing 
problems, a transistor driver can be provided for each pixel, however, 
this solution is accompanied by a substantial increase in display 
complexity and cost. 
Color LCDs are available, however, because they suffer even more than their 
monochrome counterparts from complexity and slow switching time, the 
technology is not competitive with color CRTs except where low power and 
thin-form factor is critical. 
Super twisted nematic (STN) LCDs have become popular in many applications 
because their multiplexibility is superior to other types of liquid 
crystals. STN cells exhibit bi-stable behavior when they switch rapidly 
from a deselect state to a select state and back again as the excitation 
root mean square (RMS) voltage crosses a switch threshold. The select and 
non-select regions can be made quite close to one another and permit cells 
to be multiplexed at high rates. The use of STN cells with their superior 
multiplexibility avoids panel complexity because each cell does not need 
its own transistor driver. 
While a benefit of STN cells is their high multiplexibility, their 
detriment is that they are optically anisotropic crystals having two 
indices of refraction and therefore they exhibit undesirable birefringence 
effects. Their double refractance makes them unsuitable, particularly for 
black and white LCD panel applications because the STN cell unavoidably 
produces colors. An STN cell polarizes light of different wavelengths 
differently during their passage through the cell, and as a result an STN 
cell cannot be operated in black or white with a high contrast ratio. 
Moreover, the colors produced by birefringence are regarded as too limited 
in range and too inferior in quality to be suitable for use in color 
displays. 
The birefringence operating mode of STN cells, however, has been exploited 
by different arrangements. For example, since the degree of the 
birefringence is a function of the voltage applied to the liquid crystal 
material, by switching the applied voltage to different values, different 
colors can be produced by a properly configuring of the STN cells. For 
example, by utilizing this inherent birefringence property, an arrangement 
is shown in U.S. Pat. No. 4,917,465 where STN cells are stacked to form a 
plurality of tri-pixels tuned to generate different subtractive primary 
colors (i.e. yellow, cyan, and magenta). 
In a modification of the tri-subpixel stacked LCD configuration, U.S. Pat. 
No. 4,966,441 shows a bi-subpixel system which allows the second panel to 
have twice as many pixels as the first. In the bi-subpixel arrangements, 
the colors are generated by both birefringent color and color filters. In 
principle, all eight basic colors can be generated with these approaches, 
however, because black and white transmission is poor, they both suffer 
from low contrast ratios. In an optimized configuration of that shown in 
U.S. Pat. No. 4,966,441, only about 25% of the light transmits through the 
cell at the select state and when the cell is in the non-select state, and 
is therefore turned off, about 5% of the light still passes through the 
cell, therefore reducing its transmissivity characteristics. 
Another common approach to achieve color display with STN panel uses 
adjacent tri-pixels covered with mosaic color filters. Tri-subpixels are 
usually placed in a "parallel" or side-by-side way as compared to the 
stacked configuration where the tri-subpixels are placed in "series" or 
coaxially. In order to compensate for the color dependence of the 
birefringent effect, a compensation panel or a retardation film is needed 
in the tri-pixel color mosaic arrangement so that it can display black and 
white color. When the double layer supertwisted display (DSTN) 
configuration is used, one of the cells is passive and usually has no 
electrodes. LCD with the mosaic color filter tri-pixel configuration 
usually has a relatively low light transmissivity as compared to the 
stacked cell configuration. 
SUMMARY OF THE INVENTION 
In order to overcome the problem of the prior art as described above, a 
method and device to improve the contrast ratio of liquid crystal display 
in the stacked configuration with the incorporation of compensation 
technology is disclosed herein. The conventional stacked STN LCD 
technology uses birefringence color. However, the principal of 
birefringence color, which changes the transmissivity of light of one 
primary color by switching the state of a particular LCD panel without 
changing the transmissivity of light of the other two primary colors, 
conflicts with the principal of compensation technology which eliminates 
the light wavelength dependence of the transmissivity in both the select 
and the non-select states. 
In the present invention, each pixel generates the three primary colors 
which are each controlled by separate optical assemblies. In optically 
subtractive and additive combinations, the three secondary colors plus 
black and white are generated with a high contrast ratio, a high 
resolution and high transmissivity using highly multiplexible STN cells. 
The present invention uses STN cells for their high multiplexibility but 
avoids their birefringence effects. In each optical assembly (OA) 
arrangement, light of a primary color is linearly polarized before it 
passes through a first STN cell where it is elliptically polarized, and 
then by passing the light through another STN cell or a retardation film, 
the light is restored to its linear polarization. In doing so, the present 
invention provides mirror symmetry for the polarization state of light 
originally linearly polarized. Because light can be made linearly 
polarized upon exiting the compensating STN cell or retardation film, it 
can be completely blocked by an exit polarizer. By nearly complete 
blockage, a high contrast ratio is achieved. Unlike the prior art of the 
DSTN configuration when one of the LCD cells is used only passively, in 
many embodiments of the present invention, both cells in a compensating 
pair are used actively in the operation that generates all eight basic 
colors. 
In accordance with the present invention, two, three or four LED cells form 
an ordered arrangement. In various embodiments, compensation is provided 
by as many as three dual STN cell systems. In other embodiments, fewer 
dual STN cell systems are arranged, with single STN cells capable of being 
coupled with retardation films. In all embodiments, exit polarizers block 
restored linearly polarized light so that a high contrast ratio is 
achieved. 
As indicated above, the STN cells can be in either a non-select state or 
select state. These states can be equated with the binary numbers, zero 
and one and therefore color can be defined in terms of binary logic, that 
is, any particular color can be identified as a specific binary 
combination (herein after also referred to as B/C) of select and 
non-select states. For example, in the case of an overall arrangement 
containing four cells, when all of the cells are in non-selected states, 
the binary number equivalent of their combined states is zero (0000) and a 
color, such as black is associated with the binary number zero (of course, 
depending on the arrangement). On the other hand, when a first cell is in 
the non-select state and the other three cells are in a selected state, 
the binary number equivalent of their combined states is seven (0111), and 
a different color, such as cyan is associated with that number (again, 
depending on the arrangement). 
Here, the logic that governs a particular color by using a pair of 
compensating cells is the "EXCLUSIVE OR" (XOR). Light of this particular 
color passes through the invented display device if and only if the two 
liquid crystal display panels in this pair are in different states; with 
one panel in the select state while the other one is in the non-select 
state. 
The present invention uses STN cells for their high multiplexibility but 
avoids their bireference effects. In embodiments disclosed here, one or 
more pairs of compensating supertwisted nematic liquid crystal display 
panels are stacked to generate colored display with high contrast ratio. 
In a compensating pair, the two liquid crystal display panels are 
similarly constructed but with their liquid crystal molecules twisted in 
the opposite sense. The linearly polarized light passing through the first 
STN layer becomes elliptically polarized with an ellipticity and principal 
axis direction strongly wavelength dependent. However, whatever the first 
STN layer does to the light, the second, compensating layer undoes and 
restores the linear polarized state so that the light can be completely 
absorbed by an exit polarizer. By being able to nearly completely block 
the transmission of light, a high contrast ratio is achieved.

DETAILED DESCRIPTION OF THE INVENTION 
The complete or nearly complete block of light of all wavelengths in the 
"black" state is required to achieve a high contrast ratio. The display of 
black requires that light of all wavelengths be linearly cross-polarized 
with the exit polarizer to effect complete light blockage. The display of 
white requires that all wavelengths of light has equal passage through the 
exit polarizer. Because of the way the present invention controls 
polarization of light before it passes through the exit polarizer, nearly 
complete blockage for black can be effected. 
Turning to the first embodiment shown in FIG. 1, a single picture pixel 
arrangement 9, which is a full-color pixel, including two adjacent 
subpixel systems 10 and 20 is shown. In this arrangement, 1 dual 
compensating STN cell system is used for a first primary color. The other 
two primary colors are controlled by single STN cells coupled with 
retardation films. Subpixel system 10, on the right, includes a first 
optical assembly 1 for controlling a first primary color. The first 
optical assembly 1 includes a first STN LCD display cell 13 and a second 
STN LCD cell 14. Adjacent to subpixel 10 is subpixel 20 which includes two 
linearly aligned optical assemblies, 2 and 3 for controlling a second and 
third primary color, respectively. The second optical assembly 2 includes 
a third cell STN 23 and a retardation film 23'. The third optical assembly 
3 includes a fourth cell 24 and a retardation film 24'. 
In the embodiment shown in FIGS. 1-4, the cells 13 and 23, and 14 and 24, 
are individual cell pixels which are adjacent to one another on a common 
cell panel. In the manufacturing process, the cells are individual units 
or, alternatively, are cell pixels on a common cell panel, depending upon 
the materials used. Such configuration is shown in FIGS. 1-4. In either 
case, cells 13 and 23, and 14 and 24, operate electrically independently 
of one another. Hereinafter, cell pixels 13 and 23 of panel 4 and cell 
pixels 14 and 24 of cell panel 5 will be referred to as cells 13 and 23 
and cells 14 and 24, although both types will be considered. In other 
words, cells 13 and 23 will be considered as part of panel 4, and 14 and 
24 will be considered as part of panel 5 in this discussion. 
The four cells are specifically arranged so that when they are their 
respective in zero/non-select states or in the one/select states and 
certain polarizers and/or color filters are positioned in series 
therewith, the four cells generate eight colors, including black and 
white, according to the following binary order: 
TABLE 1 
______________________________________ 
OA 1 OA 2 OA 1 OA 3 Binary 
Cell 13 
Cell 23 Cell 14 Cell 24 
Number Color 
______________________________________ 
non- non- non- non- 0000 = 0 
blue 
select select select select 
non- non- non- selected 
0001 = 1 
cyan 
select select select 
non- non- select non- 0010 = 2 
magenta 
select select select 
non- non- select select 0011 = 3 
white 
select select 
non- select non- non- 0100 = 4 
black 
select select select 
non- select non- select 0101 = 5 
green 
select select 
non- select select non- 0110 = 6 
red 
select select 
non- select select select 0111 = 7 
yellow 
select 
______________________________________ 
Eight numbers, 0-7, are represented by the binary representation of the 
possible states of cells 13, 14, 23 and 24, except that in this 
arrangement, cell 13 always remains in the zero/off/non-select state. In 
other embodiments described below, all four cells operate both in the on 
and off states. 
Cells 13 and 14 of optical assembly 1 have the same configuration except 
that cell 13 has its liquid crystal molecules twisted in the opposite 
direction as does cell 14. When both cells are in the same state, the 
second compensates the first. For example, when both cells are in the 
non-select state, the linearly polarized light beam passes cell 13 and 
becomes elliptically polarized with its principal axis direction strongly 
wavelength dependent, but as the light passes cell 14, the light is 
restored to being linearly polarized. Therefore, because the exit 
polarizer 17 is appropriately positioned, light is blocked by exit 
polarizer 17. However, when the two cells are in opposite states, the 
light is not blocked. (See Table 1, optical assembly 1, cells 13 and 14, 
for controlling red.) 
In the example shown in FIG. 1, cells 23 and 24 of optical assemblies 2 and 
3 are compensated by retardation films 23' and 24' respectively. The two 
retardation sheets have identical retardation values but the second 
retardation film 24' is rotated by 90 degrees with respect to the first 
retardation film 23' so that the slow axis of the first film 23' is 
parallel to the fast axis of the second film 24'. 
As indicated above, the three primary colors controlled by the three 
individual optical assemblies are each chosen by the polarizers and/or 
filters which are strategically positioned as entrance elements to the 
optical assembly and as exit elements. The polarizers and/or filters can 
easily be arranged so that different primary colors are controlled by 
different optical assemblies than those described here. The particular 
arrangement of polarizers and/or filters described with regard to FIG. 1 
is provided as an example. 
On subpixel system 10, the entrance polarizer 15 is a green polarizer which 
polarizes blue and red light. The second polarizer 16 is a red polarizer 
which polarizes blue and green light. The exit polarizer 17 is a blue 
polarizer which polarizes green and red light. The entrance polarizer 15 
and the exit polarizer 17 are positioned in a "cross" orientation to one 
and other. On subpixel system 10, between polarizer 15 and cell 13, is red 
filter 18 which blocks blue and green light. On subpixel system 20, cyan 
(blue and green) filter 28 blocks red light. These polarizers and/or 
filters are selected so that the first optical assembly 1 controls red, 
the second optical assembly 2 controls blue and the third optical assembly 
3 controls green. As mentioned above, by additive and subtractive optics, 
the three secondary colors and black and white are generated by the 
combination of red, blue and green. 
The following description outlines how the primary colors, red for optical 
assembly 1, blue for optical assembly 2 and green for optical assembly 3 
are generated according to the first embodiment of the present invention 
shown in FIG. 1. As described above, red color filter 18 allows the red 
polarized light to pass through the first STN cell 13. The red light 
therefore becomes elliptically polarized. The red light then passes 
through red polarizer 15 without any change. Red polarizer 15 operates in 
conjunction with subpixel system 20, but it only polarizes green and blue 
light. Moreover, the two retardation films 23' and 24' can be considered 
optically isotropic for on-axis light and therefore can be ignored for 
operation in subpixel system 10. Thus, the STN cell 13 elliptically 
polarized red light which then passes through the second STN cell 14. 
As shown in Table 1, red light cannot be transmitted through the full color 
pixel 9, if cells 13 and 14 are in the same states since LCD panels 4 and 
5 form a compensating pair. However, when the cell 14 is in the select 
state while pixel 13 is kept in the non-select state (cell 13 may be kept 
always in the non-select state), the compensation effect does not apply 
and the red light which passes through cell 14 will not be restored to its 
original polarization state. It will therefore not be totally absorbed by 
the exit polarizer 17 and will become visible to an observer. As a result, 
red light can transmit through the full color display pixel 9 only if cell 
13 in panel 4 and cell 14 in panel 5 are in different states, and a high 
contrast in red light can be achieved. 
Blue light is controlled by optical assembly 2 on subpixel system 20 and is 
transmitting when cell 23 is in the off state (see Table 1). Cyan (blue 
and green) filter 28 blocks red light and allows transmission of blue and 
green light. Blue light which has been polarized by green entrance 
polarizer 15, will pass through panel 4 and become elliptically polarized. 
The light then passes retardation film 23' which provides reasonable 
compensation for the panel 4 for blue light in the non-select state. The 
red polarizer 16, which polarizes blue and green light and acts as the 
exit polarizer for optical assembly 2, is arranged to permit the 
transmission of polarized blue light when cell 23 in panel 4 is in the 
off/non-select state and block it when the cell 23 is in the on/select 
state. 
Finally, green light is controlled by third optical assembly 3 on subpixel 
system 20 and is polarized by red polarizer 16 (which also acts as the 
exit polarizer for optical assembly 2). The green light is unaffected by 
cell 23 and the first retardation film 23' because the green light is 
unpolarized until it passes red polarizer 16. The green polarized light 
first passes the retardation film 24' and then STN cell 24. Because there 
is mirror symmetry about the center of the optical assembly 2 - optical 
assembly 3 system with respect to the polarized state of blue and green 
light, the green polarized light will pass through the retardation film 
24' and then the STN cell 24 just like a polarized beam is reflected by a 
mirror located by the red polarizer 16, back through retardation film 23' 
and cell 24 in panel 4. Transmitting of green light is therefore 
controlled by the optical assembly 3 in a way exactly like the control of 
blue light's transmission by the optical assembly 2. 
Polarizers 15 and 17 are only used to create and detect the polarization 
state of the light. Since polarizers 15 and 17 are positioned in a "cross" 
orientation, the logic of the two STN cells, 23 and 24, respectively 
responsible for the passage of blue and green light, is opposite. That is, 
if the select state for green light provides maximum transmission, the 
select state for the blue light provides minimum transmission. Note that 
STN cell 24 and polarizer 17 do not affect the display of blue light, and 
that it is irrelevant what effect cell 24 has on the blue light. Blue exit 
polarizer 17 only polarizes red and green, and therefore it does not block 
polarized blue light. It simply passes through the polarizer 17 and is 
viewed by an observer of the display. 
As noted above, the particular arrangement of polarizers and/or filters 
described with regard to FIG. 1 has been provided as an example. The 
polarizers and/or filters can easily be arranged so that different primary 
colors are controlled by different optical assemblies than those shown in 
FIG. 1. For example, a blue polarizer can replace polarizer 15, a green 
polarizer can replace polarizer 16, and a red polarizer can replace 
polarizer 17. In accordance with the present invention, the choice as to 
which configuration to use depends on the availability of color filters 
and polarizers as well as the tune of the display picture that best fits 
the particular need. The function of select and non-select states of the 
two colors displayed by optical assemblies 23 and 24 can be interchanged 
as well. Three versions with interchanged polarizers is summarized in 
Table 2. 
TABLE 2 
______________________________________ 
VERSION 
ELEMENTS 1st 2nd 3rd 
______________________________________ 
1st polarizer 
blue green red 
2nd polarizer 
red or green 
blue or red 
green or blue 
3rd polarizer 
green or red 
red or blue 
blue or green 
color filters 
red cyan blue yellow 
green magenta 
______________________________________ 
Also, in the embodiment shown in FIG. 1, the color filter 18/28 can be 
positioned differently. For example, FIG. 2 shows the color filter 18/28 
positioned between panel 4 and polarizer 16. In this example, the 
retardation films are not shown. The configuration shown in FIG. 2 may be 
desired because when the color filter 18/28 is so positioned, parallax is 
minimized. A parallax effect is mainly caused by the separation of two 
stacked cells and the color filters. However, if a backlighting system 
with a sufficiently high degree of collimation is employed, parallax is 
less of a problem and therefore the location of the color filter is not 
important. 
An arrangement including additional filters is shown in FIG. 3. In addition 
to adjacent filters 18 and 28, adjacent filters 19 and 29 are positioned 
between panel 5 and polarizer 17. As indicated above, red filter 18 allows 
the red polarized light to pass and cyan filter 28 allows blue and green 
light to pass. In this arrangement, adjacent filters 19 and 29 are 
linearly aligned with adjacent filters 18 and 28 and are of the same type. 
Accordingly, in the event of insufficient backlighting collimation or 
scattering inside the assembly, color leakage can be avoided. Moreover, 
color saturation can be improved. Also, this arrangement provides a less 
expensive manufacturing process than others disclosed herein. The color 
filter dye used for manufacturing color LCDs is expensive and its 
processing is complicated since in the traditional configuration the color 
filter is deposited between the glass sheet and the transparent electrode 
and must withstand a temperature as high as 250 degrees C. during other 
manufacturing processes. By placing the color filter outside the liquid 
crystal cell, conventional dyes can be used to manufacture color filter 
layers and the cost of color filters is decreased significantly. Such 
color filters can be created either by printing dyes on to the surface of 
a liquid crystal cell or by depositing the dye on to a separated thin 
plastic sheet. 
In the arrangement shown in FIGS. 1-3, for controlling one primary color, 
optical assembly 1 has dual cells for compensation. However, for the other 
two optical assemblies 2 and 3, for compensation, STN cells are paired 
with retardation films. In this arrangement, there may also be undesirable 
retardation effects caused by color polarizers. For example, while a red 
polarizer polarizes blue and green light, it has a retardation effect on 
red light if the red polarizer is made of stretched dyed plastic film. 
In the above discussion, ideal color polarizers which do not affect the 
polarization state of light are assumed. However, in reality, a color 
polarizer may affect, in a retarding manner, the polarization state of 
colored light which is not specifically polarized by the color polarizer. 
The color polarizer may also act as a retardation plate to colors which it 
does not specifically polarize and are meant to simply pass through 
unchanged. This is because polarizers made by stretching a plastic film 
dyed with a particular color will have its molecules in the plastic film 
aligned in the stretching direction and make the film anisotropic. Light, 
which is not polarized by this color filter may nevertheless have its 
polarization state changed by the polarizer since light polarized along 
and perpendicular to the aligned direction of the molecule will experience 
a different optical path when propagating the film. For this reason, it is 
sometimes necessary to use an additional retardation film, positioned 
properly, to compensate the retardation effect of some color polarizers. 
A different configuration is shown in FIG. 4, which enhances the attribute 
of the present invention, adding better compensation by using a second, 
dual compensating STN system and by using gray polarizers in place of 
color polarizers. FIG. 4 shows the use of two gray polarizers and a yellow 
polarizer which are chosen to reduce the use of retardation film, and 
therefore enhance compensation while including two dual STN systems. In 
some situations, it is preferable to use the configuration of FIG. 4 over 
that of FIGS. 1-3. 
Turning to FIG. 4, subpixel system 10 uses the dual STN cell system of 
optical assembly 1 to control red in the manner described above with 
reference to FIG. 1. However, in this embodiment, on subpixel system 20, 
green is also controlled in a symmetrical manner by a dual STN cell 
system. By addition of gray entrance polarizer 30 and exit polarizer 32, 
which polarize light of all colors, and the addition of yellow polarizer 
31 which cooperates with retardation films 23' and 24', compensation 
mirror symmetry for optical assembly 2 is provided for yellow light. 
Therefore, for controlling green, a dual STN cell system includes cell 23 
compensated by cell 24. Blue is controlled by optical assembly 3 including 
just cell 24 and retardation film 24' as it was in FIG. 1. The arrangement 
shown in FIG. 4 provides the logic between color and switching state as 
follows: 
TABLE 3 
______________________________________ 
OA 2 
OA 1 OA 2 OA 1 and 3 Binary Number 
Cell 13 
Cell 23 Cell 14 Cell 24 
and Color 
______________________________________ 
non- non- non- non- = zero or black 
select select select select 
non- non- non- select = one or cyan 
select select select 
non- non- select non- = two or red 
select select select 
non- non- select select = three or white 
select select 
non- select non- non- = four or green 
select select select 
non- select non- select = five or blue 
select select 
non- select select non- = six or yellow 
select select 
non- select select select = seven or magenta 
select 
______________________________________ 
Eight numbers, 0-7, are represented by the binary representation of the 
possible states of cells 13, 14, 23 and 24, except that here as in Table 
1, cell 13 remains in the zero/off/non-select state. Note that with regard 
to cells 14, 23 and 24, to generate red and blue, the logic has an 
opposite configuration. 
In this arrangement, the entrance 30 and exit polarizers 32 are grey 
polarizers which polarize all colors. By suitably arranging the yellow 
polarizer 31 and the retardation films 23' and 24', blue light is 
controlled by optical assembly 2 and the transmission of blue can be 
blocked when cells 23 and 24 are in the off/non-select state, thus 
improving the contrast ratio. 
The yellow polarizer 31 is sandwiched between LCD panels 4 and 5. While it 
polarizes only blue light, it however has a retardation effect on light of 
other colors (green and red). Retardation film 23' and 24' are on either 
side of yellow polarizer 35. On the one hand, retardation films 23' and 
24' compensate the retardation effect of the yellow polarizer 35 so that 
red and green light can pass without much change in their polarization 
state. On the other hand, the retardation film, together with the yellow 
polarizer, break the symmetry of optical assemblies 2 and 3 for blue light 
so that the transmission of blue light can be controlled only by one of 
the optical assemblies, optical assembly 3 in the current arrangement, 
with properly chosen retardation films. 
Other embodiments of the present invention include linear configurations 
such as those shown in FIGS. 5-10. In each of these examples, the optical 
assemblies, each for controlling a primary color, are linearly aligned. 
Therefore, the surface area of the display occupied by one pixel, is 
equivalent to only one optical assembly. Accordingly, the resolution of 
the display can be maximized for a given screen area. 
FIG. 5 shows a first linearly aligned embodiment wherein three cells are 
used. The first cell 33 and the third cell 35 have their liquid crystal 
molecules twisted in identical directions and are also identically 
constructed. The second cell 34 is constructed similarly to cell 33 and 
35, but has its liquid crystal molecules twisted in the opposite 
direction. The three cells are arranged and ordered in such a way that: 
cells 33 and 34 form a compensating pair while cells 34 and 35 form 
another compensating pair. In this arrangement, cells 33 and 34 form the 
first optical assembly which follows the logic of "XOR" for its operation. 
Similarly, the third optical assembly which is composed of cells 34 and 35 
also follows the logic of XOR for its operation. The optical assembly 2 
contains only one cell, 34. With the polarizers and the liquid crystal 
display cells arranged as shown in FIG. 5, all eight colors, including 
black and white, can be generated according to the following binary order: 
The three cells are ordered so that when they are in the on/select state or 
in the off/non-select state and certain color polarizers are appropriately 
positioned, the three cells generate eight colors, including black and 
white according to the following binary order: 
TABLE 4 
______________________________________ 
Optical Assembly 1 
Optical Assembly 2 
Optical Assembly 3 Binary Number 
Cell 33 Cell 34 Cell 35 and Color 
______________________________________ 
non-select 
non-select non-select 
zero = black 
non-select 
non-select select one = green 
non-select 
select non-select 
two = white 
non-select 
select select three = magenta 
select non-select non-select 
four = red 
select non-select select five = yellow 
select select non-select 
six = cyan 
select select select seven = blue 
______________________________________ 
According to this arrangement, the transmission of red light is controlled 
by switching cell 33 and the transmission of green light is controlled by 
switching cell 35. The transmission of blue light is principally 
controlled by switching cell 34. Optical assembly 1 include cells 33 and 
34, optical assembly 2 includes cell 34 and a retardation film 43' and 
optical assembly 3 includes cells 34 and 35. 
The high contrast for red and green is secured by the mirror symmetrical 
construction of optical assemblies 1 and 3 when they are operated in a 
state to block light from transmitting. Optimization of the optical 
assembly 2, which contains only one cell (cell 34), can be achieved by 
using compensators. Suitable retardation films 42' and 43' are needed to 
be placed next to polarizers 42 and 43 to compensate the retardation 
effects of the color polarizers if the polarizers, 42 and 43, are made of 
dyed stretched film. with this approach, a display of high contrast can be 
constructed without color filters. 
Light from the backlighting, first passes through a cyan entrance polarizer 
41 which polarizes red light. Green polarizer 43, which polarizes red and 
blue acts, as an exit polarizer for optical assembly 1. Polarizer 41 and 
polarizer 43 are cross-aligned. Therefore, when cell 33 and cell 34 are in 
the same states, the linear polarization of the red light is restored and 
is blocked by green polarizer 43. When cell 33 and cell 34 are in 
different states, polarizer 43 permits the red light to pass. 
Similarly, red polarizer 42 which polarizes blue and green, is the entrance 
polarizer for optical assembly 3 which controls the transmission of green 
light. If both cell 34 and cell 35 are in the same state, then the 
linearly polarized green light is blocked by magenta exit polarizer 44, 
which polarizes green light and is cross aligned with polarizer 42. When 
cell 34 is in the non-select state and cell 35 is in the select state, 
magenta polarizer 44 permits the green light to pass. Therefore, green 
light is transmitted. 
Red polarizer 42 which polarizes blue and green, is the entrance polarizer 
for optical assembly 2 which controls the transmission of blue light. By 
switching all three cells to the select state, the transmission of pure 
blue can be obtained because red and green can be blocked owing to their 
mirror symmetry. 
Another linear arrangement of the present invention is shown in FIG. 6, 
such provided without the use of color filters. Optical assemblies 1, 2, 
and 3 are aligned and include four STN cells, 36, 37, 38 and 39 which are 
shared by neighboring optical assemblies so that three dual STN cell 
systems are formed. In other words, in this arrangement, each optical 
assembly overlaps with another neighboring optical assembly because they 
share an STN cell. Optical assembly 1 for controlling a first primary 
color includes cells 36 and 37 in a dual STN cell system. Optical assembly 
2 for controlling a second primary color includes cells 38 and 39. Optical 
assembly 3 for controlling a third primary color includes cell 37 in a 
shared arrangement with optical assembly 1 and includes cell 38 in a 
shared arrangement with optical assembly 2. 
Cells 36 and 38 have their liquid crystal molecules twisted in identical 
directions while cells 37 and 39 have their liquid crystal molecules 
twisted in the opposite direction of cells 36 and 38. By appropriately 
positioning and cross-orienting color polarizers 45, 46, 47, 48 and 49, 
the cell pairs of optical assemblies 1, 2 and 3 compensate one another. 
For example, FIG. 6A depicts the cartesian orientation for polarizers in 
FIG. 6 where the arrow indicates that particular polarizer's orientation 
direction. Retardation films 46', 47', 48' compensate polarizers 46, 47, 
48 as discussed with reference to FIG. 4. Retardation film 49' operates to 
keep the mirror symmetry of optical assembly 2. In this example, polarizer 
45 is a yellow polarizer (polarizing blue light), 46 is a magenta 
polarizer (polarizing green light), 47 is a green polarizer (polarizing 
red and blue light), 48 is magenta polarizer (polarizing green light) and 
49 is a cyan polarizer (polarizing red light). 
Turning to Table 5, the four cells operate with binary symmetry to generate 
each primary and secondary color and black and white. Note in Table 5 (and 
Table 6 below) that there are two possible switching combinations for each 
of the eight basic colors. The arrangement shown in FIG. 5 provides the 
logic between color and switching state as follows: 
TABLE 5 
______________________________________ 
Optical Assembly 1 
Optical Assembly 2 
Binary Number 
Optical Assembly 3 or Color 
Cell 36 
Cell 37 Cell 38 Cell 39 
Color 
______________________________________ 
non- non- non- non- = zero or black 
selected 
selected selected selected 
non- non- non- select = one or red 
selected 
selected selected 
non- non- select non- = two or yellow 
selected 
selected selected 
non- non- select select = three or green 
selected 
selected 
non- select non- non- = four or cyan 
selected selected selected 
non- select non- select = five or white 
selected selected 
non- select select non- = six or magenta 
selected selected 
non- select select select = seven or blue 
selected 
select non- non- non- = eight or blue 
selected selected selected 
select non- non- select = nine or magenta 
selected selected 
select non- select non- = ten or white 
selected selected 
select non- select select = eleven or cyan 
selected 
select select non- non- = twelve or green 
selected selected 
select select non- select = thirteen or yellow 
selected 
select select select non- = fourteen or red 
selected 
select select select select = fifteen or black 
______________________________________ 
As indicated above, each optical assembly has an entrance polarizer and an 
exit polarizer and in some cases they are within the confines of a 
neighboring optical assembly. For example, magenta polarizer 46, which is 
the entrance polarizer optical assembly 3, is present within the confines 
of optical assembly 1. However, polarizer 46 does not affect the blue 
light because a magenta polarizer only polarizes green light. Therefore, 
while cell 37 is shared, magenta polarizer 46 does not impact optical 
assembly 1. 
In the example shown in FIG. 6, optical assembly 1 controls blue, optical 
assembly 2 controls red and optical assembly 3 controls green. Each dual 
STN compensation system includes an entrance polarizer and an exit 
polarizer appropriately chosen so that each optical assembly controls a 
primary color. For example, yellow entrance polarizer 45 polarizes blue 
light, which acted upon by the dual STN compensation system. If cell 45 
and cell 46 are in different states, the linearity of the blue polarizes 
light has not been restored and therefore green exit polarizer 47 which 
polarizes blue and red light allows transmission of the non-linearly 
polarized blue light. If cells 45 and 46 are in the same state, exit 
polarizer 47 blocks transmission of blue light. In optical assembly 2, red 
light is blocked when cells 38 and 39 are in the same state, and in 
optical assembly 3, green light is blocked when cells 37 and 38 are in the 
same state. Each primary color is transmitted when the dual STN systems 
have cells in different states. 
In this embodiment, black is generated when all four cells are in the same 
state. Light of one color will pass through the assembly when the mirror 
symmetry of one optical assembly is broken. For example, if cell 36 is 
switched to a select state and all the other cells are in the non-select 
state, cells 36 and 37 cannot compensate each other and blue light can 
pass through the display assembly. Furthermore, if the symmetry of two 
optical assemblies is broken, two colors will pass through the assembly. 
For example, cyan light is transmitted when optical assemblies 1 and 3 
have broken symmetries and yellow light is transmitted when optical 
assemblies 2 and 3 have broken symmetries. Finally, white is generated 
when the symmetries of all three optical assemblies are broken. In each 
optical assembly in this embodiment, transmitting of light of all three 
primary colors follows the logic of XOR. 
Another linear arrangement of the present invention is shown in FIG. 7, 
such provided without the use of color filters and with the use of fewer 
components than the arrangement shown in FIG. 6. In this arrangement, a 
gray polarizer 37 acts as exit polarizer for optical assembly 1 and as 
entrance polarizer for optical assemblies 2 and 3, allowing them to 
function independently. Green light is controlled by cells 36 and 37 and 
magenta entrance polarizer 51, red light is controlled by cells 38 and 39 
and cyan exit polarizer 54 and blue light is controlled by cell 38 in 
combination with retarder 38' and exit yellow polarizer 53. In optical 
assembly 3, retardation films 38' and 53' are properly chosen so that the 
polarization state of red light will not be affected by the yellow 
polarizer 53 and retardation film 53' combination. As a result, the 
transmission of red light in optical assembly 3 is controlled, with a high 
contrast ratio, by cells 38 and 39. Retardation film 38' as well as the 
orientation of the yellow polarizer 53 (which polarizes blue light) are 
properly chosen to ensure control of the transmission of blue light by 
cell 38 with a high contrast ratio. Suggested polarizers' orientation is 
provided by the arrows on each polarizer and is comparable to the 
orientation guide of FIG. 7A. 
Turning to Table 6, the four cells operate to generate each primary and 
secondary color and black and white. Note that there are two possible 
switching combinations for each of the eight basic colors. The arrangement 
shown in FIG. 6 provides the logic between color and switching state as 
follows: 
TABLE 6 
______________________________________ 
Optical Binary 
Assembly Number 
Optical Assembly 1 
2 or 
Optical Assembly 3 
Color 
Cell 1 Cell 2 Cell 3 Cell 4 Color 
______________________________________ 
non- non- non- non- zero = black 
selected 
selected selected selected 
non- non- non- selected 
one = red 
selected 
selected selected 
non- non- selected non- two = magenta 
selected 
selected selected 
non- non- selected selected 
three = blue 
selected 
selected 
non- selected non- non- four = green 
selected selected selected 
non- selected non- selected 
five = yellow 
selected selected 
non- selected selected selected 
six = white 
selected 
non- selected selected selected 
seven = cyan 
selected 
selected 
non- non- non- eight = green 
selected selected selected 
selected 
non- non- selected 
nine = yellow 
selected selected 
selected 
non- selected non- ten = white 
selected selected 
selected 
non- selected selected 
eleven = cyan 
selected 
selected 
selected non- non- twelve = black 
selected selected 
selected 
selected non- selected 
thirteen = red 
selected 
selected 
selected selected non- fourteen = magenta 
selected 
selected 
selected selected selected 
fifteen = blue 
______________________________________ 
Yet another linear arrangement of the present invention is shown in FIG. 8, 
such provided without the use of color filters and with the use of even 
fewer components than the arrangement shown in FIG. 7. A Polarizer 
Orientation Guide is shown in FIG. 8A. Here, three STN cells are used as 
opposed to four cells so that optical assemblies 1 and 3 include single 
STN cell and retardation film systems and optical assembly 2 includes a 
dual STN cell system. 
In this arrangement, the orientation of magenta polarizer 51 does not need 
to be in a "cross" or "parallel" direction with respect to the gray 
polarizer 52. Just like in the case of a monochromatic STN panel, one or 
two retardation films (not shown) may be placed between cell 36 and its 
neighboring polarizers to improve contrast. Retardation film 37' 
compensates cell 37 and retarder 53' acts to compensate polarizer 53. The 
logic between color and switching state of individual cells is summarized 
in Table 7. The logic for the operating of the cyan assembly is again the 
XOR. 
TABLE 7 
______________________________________ 
Optical Assembly 2 
Optical Optical Binary Number 
Assembly 1 Assembly 3 or Color 
Cell 1 Cell 2 Cell 3 Color 
______________________________________ 
non-selected 
non-selected 
non-selected 
zero = black 
non-selected 
non-selected 
selected one = red 
non-selected 
selected non-selected 
two = magenta 
non-selected 
selected selected three = blue 
selected non-selected 
non-selected 
four = green 
selected non-selected 
selected five = yellow 
selected selected non-selected 
six = white 
selected selected selected seven = cyan 
______________________________________ 
Modifications of the embodiment shown in FIG. 8 can be arranged by 
interchanging the position sequence of three polarizers to give all eight 
colors. 
The embodiment shown in FIG. 9 is similar to that shown in FIG. 8., except 
that the logic of the arrangement shown in FIG. 9 is described by Table 8. 
Cyan polarizer 54 (which polarizes one color, red)is replaced by a green 
polarizer (which polarizes blue and red). Moreover, the arrangement 
includes that colored polarizers 51 and 54 are complementary to one 
another, and polarizer 53 polarizes one of the two colors polarized by 
polarizer 54. A polarizer orientation guide is shown in FIG. 9A. Such an 
arrangement also provides all eight colors. 
Finally, in FIG. 1 0, a final example of an arrangement incorporating the 
present invention is shown. In this arrangement, transmission of blue 
light is controlled by cells 36 and 37, following the logic of XOR, while 
the transmission of green light is controlled by cells 37 and 38, also 
following the logic of XOR. Red light, on the other hand, is controlled by 
a combination of cells 36, 37 and 38. When cells 36 and 37 are in the same 
state, red light is polarized by the first grey polarizer 51, however, it 
is not affected by polarizers 52 and 53. The red light will keep its 
linear polarization state unchanged as it passes cells 36 and 37, since 
these two cells compensate each other. A properly constructed cell 38 can 
therefore minimize red light from transmitting through the assembly from 
the first grey polarizer 51 to the second grey polarizer 54 when cells 36, 
37 and 38 are all in the non-select state. Since blue light and green 
light are also blocked with cells 36, 37 and 38, all in the non-select 
state, black will be displayed in this situation. When cells 36, 37 and 38 
are all in the select state, the situation, however, is not quite the 
same. Under this condition, as before, red light will keep its linear 
polarization unchanged as it passes cells 36 and 37. However, since the 
cell 38 is now in the select state, red light will pass through the 
assembly. The situation with cells 36 and 38 in the same state while cell 
37 is in different state, is somewhat more complicated. However, the 
combined state of (0,1,0) is not equivalent to the combined state of 
(1,0,1). It is therefore possible to properly design the cell so that one 
of the state can be defined as what while the other one can be defined as 
cyan. This configuration can therefore give all the eight colors. 
Here, two gray polarizers 51 and 54 and a magenta polarizer 52 (polarizes 
green), a yellow polarizer 53 (polarizes blue) and a cyan polarizer 55 
(polarizes red) are used. A polarizer orientation guide is shown in FIG. 
10A and suggested polarization directions are shown on the polarizers in 
FIG. 10. The cells and the polarizers are arranged so that the pairs of 
cells identified within the confines of optical assemblies 1 and 2 
compensate each other. Optical assembly 1 and optical assembly 2 overlap 
one another, and optical assembly 3 overlaps both optical assemblies 1 and 
2. 
In this arrangement an optional optical assembly 3' is included because the 
arrangement without optical assembly 3' tends to allow red to transmit 
even when it is desired that red be blocked. Therefore, optional optical 
assembly 3' is added to ensure the blockage of red. However, as indicated, 
optical assembly 3' is optional. It is possible to chose cells and the 
orientations of polarizers to avoid the use of optical assembly 3'. 
Optical assembly 3' is shown to illustrate a method for correcting certain 
color leakages. 
The logic between color and switching state of the embodiment shown in FIG. 
10 for the case with optical assembly 3' added is shown in Table 8. The 
logic between color and switching state where optional optical assembly 3' 
is not added is shown in Table 9. In this embodiment, the operation of the 
first optical assembly as well as the second optical assembly follows the 
logic of XOR. 
TABLE 8 
______________________________________ 
Optical Assembly 3 Binary 
Optical Assembly 2 Number or 
Optical Assembly 1 Color 
Cell 36 
Cell 37 Cell 38 Cell 39 
Color 
______________________________________ 
non- non- non- non- zero = black 
selected 
selected selected selected 
non- non- non- selected 
one = black 
selected 
selected selected 
non- non- selected non- two = green 
selected 
selected selected 
non- non- selected selected 
three = yellow 
selected 
selected 
non- selected non- non- four = cyan 
selected selected selected 
non- selected non- selected 
five = white 
selected selected 
non- selected selected selected 
six = blue 
selected 
non- selected selected selected 
seven = magenta 
selected 
selected 
non- non- non- eight = blue 
selected selected selected 
selected 
non- non- selected 
nine = magenta 
selected selected 
selected 
non- selected non- ten = cyan 
selected selected 
selected 
non- selected selected 
eleven = cyan 
selected 
selected 
selected non- non- twelve = green 
selected selected 
selected 
selected non- selected 
thirteen = yellow 
selected 
selected 
selected selected non- fourteen = black 
selected 
selected 
selected selected selected 
fifteen = red 
______________________________________ 
TABLE 9 
______________________________________ 
Optical Assembly 3 Binary 
Optical Assembly 2 
Number or 
Optical Assembly 1 Color 
Cell 1 Cell 2 Cell 3 Color 
______________________________________ 
non-selected 
non-selected 
non-selected 
zero = black 
non-selected 
non-selected 
selected one = yellow 
non-selected 
selected non-selected 
two = white (cyan) 
non-selected 
selected selected three = blue 
selected non-selected 
non-selected 
four = magenta 
selected non-selected 
selected five = cyan (white) 
selected selected non-selected 
six = green 
selected selected selected seven = red 
______________________________________ 
In each of the embodiments shown, each of the assemblies can be modified by 
rearranging, reorienting and substituting the shown color polarizers for 
others. Some of the retardation films can be replaced by STN cells for a 
higher contrast ratio. 
A display subassembly according to the present invention can be 
incorporated into a number of direct view display systems, such as color 
graphics display for computers and color monitor of television sets. In 
direct view displays, a backlighting system that can provide the display 
assembly light with a substantially high degree of collimation is required 
to avoid parallax effects. On the viewing side of the display, it is 
desirable to have a diffuser screen to achieve a wide viewing angle. 
An example direct view embodiment is shown in FIG. 11. A backlighting 
assembly 62 which provides a sufficiently high degree of collimated 
illumination is placed behind the display assembly 60. A diffuser plate 63 
is placed on the exit side of the display assembly so that a real image of 
the picture displayed by the LCD assembly can be formed. The diffuser 
plate can be made of ground glass of matt plastic plates or a commercially 
available diffusion material (i.e. Rolux Film manufactured by the Rosco of 
Port Chester, N.Y.). To reduce the reflection of ambient light by the 
diffuser, the matt side of the diffuser should be facing the display panel 
and the outside flat surface may be coated with a particular 
anti-reflection layer. In addition, a color polarizer (blue in the 
example) may be placed over the diffuser and aligned in such a way that 
the transmission of ambient light to the diffuser can be minimized. If the 
diffuser does not cause depolarization, it is also possible to place the 
diffuser between the third polarizer and the second STN cell. In this 
case, the orientation of individual elements in the invented display 
system should be correspondingly arranged so as to ensure their maximum 
passage through the polarizer which is used to maximize the reduction of 
environment light reflection. The diffuser may also be a plate with many 
microlenses. Exiting collimated light will then be dispersed by the 
microlenses on the diffuser plate 63, thereby permitting the color image 
to be viewed from a wide range of angles. 
A typical projection embodiment of the invention is shown in FIG. 12, in 
which a display subassembly 60 is positioned on the transparent projection 
surface 71 of a conventional overhead projector 62. such a projector 
usually includes an illumination source 73 and a Fresnel lens sheet 74 
under the projector surface to produce light beams that pass through a 
transparency and converge onto a projection lens assembly 75. When display 
assembly 60 is used in such an embodiment, it is desirable to provide a 
diverging Fresnel sheet lens to collimate the converging light from the 
projection surface 72 prior to illumination of the display subassembly. 
The light exiting the subassembly is then focused by a converging Fresnel 
sheet lens 76 onto the projection lens assembly 77. The converging Fresnel 
sheet lens is used to recover the original trace of light beams in the 
absence of the display subassembly. 
Projection technology may also be used to provide a self-contained display 
in which an image is projected onto the rear of a viewing screen. Both a 
color television set and a color monitor for a computer may be realized in 
this fashion. One of such arrangement is shown in FIG. 13. In this 
embodiment, a collimation reflector 81 is used to create a high intensity 
collimated lighting. The resulting image is projected by a Fresnel sheet 
lens 76 and a projection lens either onto a translucent medium 85 if the 
image is viewed from the opposite direction by a user or onto a reflection 
medium 85 if the image is viewed from the same side by a user. 
There are a wide variety of embodiments to which the principals of our 
invention may be applied and the invented display assembly can be 
employed. For example, the sequence of the panels can be interchanged 
without affecting the properties of the system very much. As another 
example, additional black/white liquid crystal panels can be added to the 
system to achieve a better contrast ratio and generate additional gray 
scales. Moreover, the STN cells used in the invented display assembly can 
be replaced by twisted nematic, ferroelectric, and any other types of 
liquid crystal display panels which are based on changing light 
polarization state for achieving colored display. For this reason, the 
illustrated embodiment should be considered illustrative only and not as 
limiting the scope of the invention. 
In accordance with the present invention, each embodiment includes three or 
four cells form an ordered arrangement. In various embodiments, 
compensation is provided by as many as three dual STN cell systems. In 
other embodiments, fewer dual STN cell systems are arranged, with single 
STN cells being coupled with retardation films. In all embodiments, exit 
polarizers block restored linearly polarized light so that a high contrast 
ratio is achieved.