WO1997001792A1

WO1997001792A1 - Full color display of subtractive color type

Info

Publication number: WO1997001792A1
Application number: PCT/US1995/013747
Authority: WO
Inventors: William C. Heinze; Brent D. Larson; Sonia R. Dodd; John R. Trimmier
Original assignee: Honeywell Inc.
Priority date: 1995-06-26
Filing date: 1995-10-20
Publication date: 1997-01-16

Abstract

A liquid crystal display which uses subtractive color technology to provide a color image. Color polarizers adjacent to active switching elements act to selectively subtract out the primary colors of light and to transmit the desired color to the viewer. The light source used is tuned to emit light which peaks at the three primary colors instead of covering the whole visible spectrum. Dichroic liquid crystal polarizers are used with the nematic liquid crystal active switching elements in order to subtract out color. The liquid crystal configuration is tuned to compensate for birefringence characteristics. Guest-host dye is incorporated into the active switching element in order to improve contrast and color gamut.

Description

Full color display of subtractive color type

FIELD OF THE TNVENTTON

This invention relates to liquid crystal displays, or more specifically to liquid crystal displays utilizing subtractive color.

BACKGROUND OF THE INVENTION

Previous segmented or matrix display technologies used for generating full-color alphanumeric, graphic and/or television type video image have relied on additive color synthesis via high-density arrangements of small red (R), green (G), and blue (B) primary color pixels.

Color encoding has become a common feature in visual information displays. Although many types of color display systems and applications presently exist, there are many potentially useful applications of color which have not been developed due to limitations and existing color display technology. Nearly all existing color displays are additive color systems, in that full color is produced by either the spatial integration of very small primary color points (i.e. very small R,G and B pixels), the geometric combining of independently generated color fields, or the temporal integration of sequentially presented image fields of alternating primary colors.

These additive approaches to color synthesis have significant limitations. Spatial additive color synthesis requires high pixel density or resolution, since the projected angle subtended by small primary color elements (i.e. R,G and B pixels) must be encompassed within the spatial integration zones of human visual system. If primary color elements are too large, then complete color synthesis will fail to occur and color fringes or patterns will be apparent in the image. The requirement for three "populations" of spatially separated primary color elements to produce a full-color image, as in the shadow-mask cathode ray tube, results in a reduction of available image sampling resolution ofthe display device. For applications requiring full color and very high image resolution, such as systems for the display of sensor video information, spatial additive approaches to color synthesis are generally not feasible due to the resultant losses in image sampling resolution. In addition, many applications for color information displays require only low image resolution such as color-coded alphanumeric or symbolic displays. For low-resolution displays, a spatial additive color technology is generally not appropriate since relatively high pixel resolution or density is required for adequate color synthesis even though image resolution requirements are substantially lower. High pixel density usually incurs high costs, and many potentially useful applications of color in low resolution displays remain undeveloped due to the relatively high cost of spatial additive color display technology.

Combined additive color methods utilize bulky and potentially costly optical arrangements and elements to merge separate components of a full-color image. Often, a backlight source must also be split into individual color components, resulting in additional bulk and cost. Aligning and maintaining spatial relationships can be difficult, although essential to high performance. While this approach eliminates some ofthe sampling constraints ofthe spatial additive approach, the size and weight of such a system is prohibitive in many applications.

Temporal color synthesis does not rely on three "populations" of spatially separated R,G and B pixels to produce a full-color image but rather achieves color synthesis by rapid sequential alternation of primary color images. This approach to color synthesis does not degrade image resolution as does spatial color synthesis. Full color control is effectively achieved at each individual image pixel. Temporal synthesis is generally implemented by a broad-band image forming source passing light sequentially in time through color filters (typically R,G and B). The image forming source must be synchronized with three color filters such that appropriate parts ofthe image within an intended color are displayed while the respective filter or filters are in front ofthe image forming source. The most popular implementations of such "frame- sequential" color display systems are typified by the use of a cathode ray tube with a broad band phosphor, (i.e. emitting white light) as the image forming source and a rotating color wheel containing R,G and B filters as the color rendering component. More recently, the color wheel has been replaced by a non-mechanical component consisting essentially of a liquid crystal (LC) switchable optical polarizer and several layers of polarized color filter films.

The disadvantages of color display systems which use temporal color synthesis are rooted in the fact that, in such systems, the individual primary color image fields are separated in time and are only present for one-third ofthe total display viewing period. Since three color image fields must be presented in the same amount of time as a single field in a spatial additive color display or monochromatic display, frame-sequential displays require an extremely high system bandwidth in order to produce a full-color image at a refresh rate high enough to minimize image flicker. Even with high system bandwidths and full-color refresh rates equivalent to monochromatic or spatial additive color displays, frame-sequential color displays are prone to image flicker due to the luminance modulation existing between sequential color image fields. A more important limitation ofthe temporal synthesis approach to color mixture, however, is the mixture colors are often observed to smear or separate into their individual primary color image components during motion of either the display image or the observers eye. An alternative to these additive approaches is subtractive color. In subtractive each picture element in a display is made up of three stacked switching elements and color filters for each ofthe primary colors. White light is transmitted through the switching elements, where portions ofthe primary colors are filtered out until the desired color is emitted from the stack. Each switching element is individually actuable to control the color content and image makeup.

In one subtractive color scheme, three guest/host liquid crystal cells, each containing a different dichroic guest dye (typically magenta (minus G), cyan (minus R) and yellow (minus B) dyes), are stacked in registration along with associated structural components and optical components (e.g. polarizers and/or fiber optic plates). The cells include pattern electrodes (and for some applications integral sample-and-hold features such as thin film transistors (TFT) at individual pixels) when the device is configured as either a low or high resolution full-color display or a uniform electrode layer when the device is configured as a simple electronic color filter. When the device is configured as a color display, only a broad band source of illumination is required for full-color image presentation. In the simple electronic color filter configuration, the device is used in conjunction with a broad band image forming source, such as a cathode ray tube with white-emitting phosphor or a back-lit pattern illuminator with broad-band lamp. In another type of subtractive liquid crystal color display, the display is comprised of first, second and third subtractive LCD filters, each filter comprising means for independently subtracting one ofthe primary colors red, green or blue from a polychromatic light beam, without substantially affecting the other primary colors. Each ofthe subtractive LCD filters combines wavelength selective dichroic polarizers with a liquid crystal cell to provide a filter that can selectively subtract varying amounts of incident spectral radiant energy from within one of three primary energy bands. In prior art subtractive filters, it has been shown that super twisted nematic (STN) can be used as the active switching element_^where the predominant color is derived from birefringence effects in the highly twisted LC structure. In another prior art subtractive filter, twisted nematic (TN) LCD's are used in a way which does not rely upon birefringent characteristics ofthe liquid crystal cells.

The prior art subtractive color displays usually incorporate sheet type colored polarizers which are commercially available. These are usually made of thin sheets of glass or plastic which are embedded with an aligned dichroic dye in order to polarize a particular band ofthe visible spectrum. A disadvantage of this type of polarizer is its thickness. Thick polarizers increase parallax and increase the difficulty in developing designs for focusing optics.

As described above, subtractive displays may employ STN liquid crystal cells. Here, extending the passive matrix multiplexibility ofthe display is stressed. The resulting_disadvantage of STN LC is that it lacks acceptable video speed and broad gray scale capability. Additional constraints may be put on the multiplexibility or number of pixels, contrast, viewing angle and color gamut. . Subtractive color displays in the prior art, which are based upon highly doped guest host devices, yield disadvantages which commonly include limited contrast and color gamut as well as dopant related effects such as reduced voltage holding ratio and slow switching speed. Approaches using TN with colored polarizers are generally limited by the characteristics of available polarizers. While ideal polarizer properties can be easily identified, such elements are elusive, and significant performance trade-offs must be made when using practical elements. An additional shortcoming ofthe prior art in this area is the failure to consider and compensate for the optical mode-mixing and related birefringence effects which occur in nematic devices which are optimized for high switching speed. As a result, the prior art is prone to reduced performance in one or more of several potential areas, including efficiency, contrast, color gamut, gray scale response, viewing angle, switching speed and degree of interaction ofthe ideally independent subtractive layers. Further, the use of broad band light sources in the prior art limits the range of achievable color gamut performance, especially with the use of non-ideal polarizers.

Therefore, the objective of this invention is to develop a subtractive color display using twisted nematic liquid crystals which provides full color, large color gamut, high contrast, improved gray scale, video speed, fast liquid crystal response time and low voltage operation.

SUMMARY OF THE INVENTION Herein described is a liquid crystal display which uses subtractive color. The liquid crystal display is comprised of a white light source which emits incident light upon first, second and third liquid crystal cells. The liquid crystal cells are active switching elements which selectively alter the polarization state of incident light in a controllable way. A first color polarizer is located between the white light source and the first liquid crystal cell. This polarizer polarizes the first of three primary colors while allowing the other two to pass. Located between the first liquid crystal cell and the second liquid crystal cell is a second color polarizer which polarizes the first and second primary colors but allows the third to pass. Located between the second and third liquid crystal cells is a third color polarizer which polarizes the second and third primary colors while allowing the first to pass. And finally there is a fourth color polarizer on the opposite side ofthe third liquid crystal cell from the third polarizer which polarizes a third color and allows the first and second primary colors to pass. Selective activation ofthe individual active switching elements provides filtering ofthe white light emitted from the light source so that the desired color is emitted from the display. The liquid crystal cells utilized are tuned together with the polarizers to provide high throughput, high contrast and fast response time. This is achieved by constructing and operating a thin nematic LC layer with moderate static orientational deformation geometry. Thickness and polarizer orientation are tuned to compensate for birefringence and non-adiabatic optical mode-mixing effects associated with the pitch of the static deformations. The combination ofthe thinness ofthe LC layer and the moderate static deformations such as twist and bend ofthe LC director result in improved temporal as well as optical performance over prior art subtractive color systems. In order to improve the performance ofthe subtractive liquid crystal stack further, a tuned white light source is provided. This light source emits light only in three isolated bands ofthe primary colors. This has the effect of greatly improving contrast and color performance, especially in the presence of limited or non-ideal polarizers. Also, the typical sheet polarizers which are well known in the prior art are replaced by anti-parallel aligned nematic liquid crystal cells containing dichroic dyes as passive color polarizers. In this type of color polarizer, the liquid crystal layer thickness and dye concentration are easily adjustable to control the color polarizer transmissions according to the R, G and B lamp peak ratios for additional improvements in a color gamut with subtractive color. The polarizers are also advantageous in that the liquid crystal layer thickness required for adequate saturation is generally less than or equal to

15 microns.

In addition to the use of passive color polarizers and appropriate birefringence compensation in the LCD structure, preferred embodiments are also provided in which guest-host dye is incorporated into the active switching element. This improves contrast and color performance, especially in the presence of limited (and non-ideal) polarizer selection.

BRIEF DESCRIPTION OF THE DRAWINGS Figures 1 A and IB show two existing approaches for producing full-color images using the spatial additive method of color synthesis: the shadow-mask color cathode ray tube (1 A), and the active-matrix address liquid crystals display panel with

R, G and B color filter array (IB).

Figure 2 is a conceptual exploded view of a subtractive color liquid crystal display.

Figure 3 is a 3-D exploded view of a picture element within the first embodiment ofthe subtractive color liquid crystal display.

Figure 4 is a truth table for the preferred embodiment ofthe subtractive color liquid crystal display. Figure 5 displays the transmission of light through a given switching element as a function of cell thickness for various colors of light.

Figure 6 is a 3-D exploded view of a picture element within the second embodiment ofthe subtractive color liquid crystal display. Figure 7 is a graph showing the output of a 300 watt broad band lamp over the visible spectrum.

Figure 8 is a graph showing the output of a 300 watt broad band lamp tuned to red, green and blue peaks with notch filters.

Figure 9 is a sheer view showing the construction of anti-parallel dichroic LC polarizers.

Figures 10A and 10B_show the visible spectral response ofthe selected primary colors, red and green, when the subtractive color stack is illuminated with the broad band lamp.

Figures 11 A and 1 IB show the visible spectral response ofthe selected primary colors red and green, when a subtractive color stack is illuminated with the RGB lamp tuned from the broad band lamp with notched filters.

Figure 12 is a chromaticity diagram ofthe subtractive liquid crystal display with the performances of all the embodiments.

DESCRIPTION OF THE PREFERRED EMBODIMENT Figures IA and IB illustrate two commonplace embodiments of spatial additive color information displays. Referring first to Figure 1 A, the typical shadow mask cathode ray tube 100 such as is used in commercial colored television receivers and which is the predominant device for color information display, is shown. Full color is achieved with the shadow-mask color cathode ray tube by the spatial integration of luminous emissions from closely-spaced R,G and B phosphor dots 106, each of which is excited by an associated electron beam 102. The phosphor dots are positioned on the cathode ray tube face 104. Electron beams 102 are generated by a plurality of electron guns. The R,G and B phosphor dots 106 are arranged in pixel groups 105. The electron beams 102 exciting each phosphor dot of a pixel group 105 pass through an aperture associated with each pixel group 105 in the shadow mask 103. Note that the spatial integration of chromatic information is performed by the observer's eye and not the display device, thus requiring the display device to possess sufficient resolution such that the individual primary color elements are not individually resolvable by the eye of the observer. Referring next to Figure IB, another full-color display device which relies on spatial-additive color synthesis is shown. This display is generally referred to as an active-matrix addressed liquid crystal color matrix display. While the basic principles of image formation and color mixture are the same as those used in the shadow mask color cathode ray tube, the liquid crystal color matrix display 120 employs a liquid crystal material which serves as an electronically controlled light valve at each picture element individually to gate incident light through a microlayer of color filters (typically R,G and B). Backlight 130 is transmitted through polarizing material 127. The backlight is then transmitted through the glass substrate 126 upon which are positioned thin film transistors 128. Liquid crystal material 125 is contained between glass substrate 126 and common (transparent) electrode 123. Associated with each thin film transistor 128 is a filter 124. The thin film transistor 128 controls the polarization of light transmitted through the associated filter 124. Three filters (R,G and B) 124 form an image pixel. The filtered light is then transmitted through glass substrate 122 and polarizing unit 121.

Figure 2 shows conceptually the structure for a subtractive color liquid crystal display. Light source 2 emits light which is incident upon the RGB subtractive stack 8. At the conceptual level, the stack 8 is made up of three separate color filters 10, 12 and 14. Each ofthe filters selectively removes a portion ofthe visible spectrum from the light emitted by light source 2. In the configuration shown, yellow filter 10 acts to remove blue light, cyan filter 12 acts to remove red light, while magenta filter 14 acts to remove green light. Within each of these filters are individual picture elements 11 which can be turned on and off depending on the output desired. By manipulating the individual picture elements within the filters 10, 12 and 14 a color image is created. A detailed view of a picture element in a first preferred embodiment ofthe display is shown in Figure 3. The picture element 30 is comprised of three switching elements 32, 34 and 36 with color polarizers on either side. The combination of elements forms a stacked structure. Each switching element is comprised of two transparent electrodes with liquid crystal contained between. The liquid crystal is ofthe nematic type and acts to alter the polarization of incident light when no voltage is applied across the electrodes. When the full saturation voltage is present the incident light is allowed to pass virtually unchanged in its polarity. The polarizers 31, 33, 35 and 37 each polarize a particular portion ofthe visible spectrum. In the context of this embodiment, polarizer 31 between switching element 32 and light source 2 polarizes blue light while allowing red and green light to pass unchanged. Polarizer 33 between switching elements 32 and 34 polarizes blue light along the same axis as polarizer 31 in addition to polarizing red light. Polarizer 35 between switching elements 34 and 36 polarizes red light along the same axis as polarizer 33 in addition to polarizing green light. Finally, polarizer 37 polarizes green light along the same axis as polarizer 35.

An understanding ofthe operation of an individual picture element in the subtractive liquid crystal display can be had by study ofthe truth table shown in Figure 4. A truth table is an aid in tracing the polarization states of light as it passes through each subtractive element. Initially, the light which is incident upon the picture element is white containing elements of all three primary colors. As the light travels through the picture element, the different primary colors are either subtracted or allowed to pass depending on the desired color to be transmitted to the viewer. In this embodiment of the invention, the polarizer order is such that the cyan polarizer is first, the blue polarizer is second, the red polarizer is third and the yellow polarizer is last. As was described above, the cyan polarizer polarizes red light alone, the blue polarizer polarizes red light and green light, the red polarizer polarizes green light and blue light, and the yellow polarizer polarizes blue light. According to the truth table, when white light strikes the cyan polarizer, red light is polarized while green and blue pass unchanged. As described above, liquid crystal cells are either in their on or off states. The off state is shown on the right side ofthe truth table while the on state is on the left. Ifthe liquid crystal is on the light is allowed to pass and its polarization is unchanged. Ifthe liquid crystal cell is off, the plane of polarization for all three components ofthe light is rotated.

At the blue polarizer, the red light which had been rotated by the off liquid crystal cell is blocked and no red light is allowed to pass any further. The red light which passed through the "on liquid" crystal cell passes through the blue polarizer unaffected. The green light which passed through either the "on" or "off' liquid crystal cell is polarized at the blue polarizer, while the blue light passes unaffected. At the second liquid crystal cell, all components ofthe light are again rotated through the "off cell, and allowed to pass unchanged through the "on" cell. At the red polarizer, green light which had passed through the "off liquid cell is blocked at the red polarizer. Green light which went through the "on" liquid crystal cell passes through the red polarizer unchanged. Blue light which passes through either the "on" or "off liquid crystal cell is then polarized at the red polarizer.

Finally, the yellow polarizer acts to either extinguish the blue light which had been rotated through the "off liquid crystal cell or allows to pass blue light which passed through the "on" liquid crystal cell unchanged. As is seen at the top ofthe truth table, eight different combinations of light are allowed to exit the picture element.

However, this does not take into account various shading techniques and gray scale which can be created by partially turning on and off the liquid crystal cells. This truth table merely shows the basic operation of a particular picture element in the subtractive liquid crystal display. The polarizer configuration shown in the first embodiment is referred to as normally black (NB), or alternately as white-select, in the case of a subtractive color device in which all ofthe primary colors red, green and blue are controlled. Essential for operation of this mode is that in the unpowered or partially powered state, light which is passed by the first polarizer to act on a particular wavelength band must be removed by either the LC mixture or by the second polarizer to act on that wavelength band. For example, polarizer 33 polarizes red light. In the presence of a non-absorbing LC mixture, polarizer 35 must remove the remaining red light. Hence, in passing through switching element 34 the polarization of red light must be rotated by 90 degrees, since in this NB case the polarizers are parallel.

One method for rotating the polarization with LC is the use of a twisted nematic structure. Twist is one ofthe three intrinsic deformation modes for a uniaxial LC, the others being bend and splay. The 90 degree rotation can be achieved via a wide variety of twisted nematic configurations. Variables include the total twist angle ofthe LC director, or locally averaged molecular orientation, the cell gap or LC layer thickness, and the birefringence ofthe LC. These variables can be selected to optimize performance based upon the application requirements. Taking the example ofthe STN device, multiplexibility is optimized in part by selecting a highly twisted configuration. While making the LC in the gap highly nonlinear in orientation as a function of applied field, this also leads to somewhat slow response. Hence in this preferred embodiment, a lower twist angle, in the range of zero to 180 degrees is selected, with a range of zero to 90 even more preferred. Advantages of using twist include reduced voltage operation, potentially improved switching speed and the opportunity to reduce the reliance on LC birefringence.

While reduced, there is still in general a significant birefringence effect under certain conditions. Figure 5 represents the degree of optical transmission through a normally black 90 degree TN LCD with parallel neutral polarizers and a particular LC birefringence in the absence of an applied field, plotted as a function ofthe LC layer thickness or cell gap for several wavelength values. The points where the curve touches zero represent where true 90 degree rotation is achieved. Changing the birefringence of the LC essentially rescales the cell gap axis. Figure 3 makes it very clear that the contrast and resulting color gamut from a 90 degree TN device is intimately tied to the birefringence and thickness ofthe LC layer, unless one extrapolates to significantly large cell gaps where the rotation of polarization ofthe incident light occurs very slowly, or adiabatically. While this is certainly possible, there are several reasons why it is preferable to retain a small cell gap despite the need to deal with the birefringence. These reasons include faster switching, increased stability, reduced spacer dimensions and increased lateral resolution capability. For this preferred embodiment, the cell gap and LC birefringence of each layer are selected to be at or near one ofthe lower order minima of this curve for the primary color modulated by that layer, most preferably at or near the first minimum. This yields the optimum in both optical performance and response time. As stated previously, the LC geometry is not restricted to a 90 degree TN. Twist angles from 0 degrees to 90 degrees or more may be used. Curves similar to that in Figure 5 may be generated, either theoretically or experimentally, to determine the optimum first minimum configuration for each. Bend deformations may also be used to improve the performance ofthe LC structure. These may be achieved by providing differing alignment geometries on opposite substrates for the LC, in terms of varying the pre-tilt, or angle between the surface and the LC director at the surface. Examples include the pi-cell and other hybrid-aligned cells, such as a device having a small pretilt angle on one surface and a director perpendicular to the substrate on the other surface. Response time can be made very fast, depending upon the details ofthe LC alignment and physical parameters. In a thin configuration, all of these deformation-enhanced nematic configurations require careful consideration of birefringence in the proper tuning ofthe devices. Such tuning may include the use of birefringent retardation films, and may also call for situations in which the respective polarizers for a given color band are neither parallel nor perpendicular. This would occur, for example, in an optimized system with 60° twist angle on the LC rather than 90°.

As the optical characteristics of these configurations are naturally sensitive to the wavelength of light, the individual layers ofthe subtractive color display are preferentially tuned for their respective wavelengths. In the present embodiment, either the cell gap or the LC birefringence is varied between the layers to ensure that an appropriate color extinction is achieved in the NB state.

Normally black displays of this type generally provide excellent performance and relatively wide viewing angle compatibility. Even when used in projection modes, the large viewing angle extends the suitability ofthe display for wide aperture applications such as high efficiency projectors and head mounted displays having large fields of view and large exit pupils.

A second preferred embodiment is shown in Figure 6. Here a normally white (NW) or black-select display is shown. When the viewing characteristics ofthe NB displays are not required, NW may be preferred due to it's increased tolerances without sacrificing contrast ratio over the useful viewing angle. Assuming a 90° twist angle in the LC, the corresponding pair of polarizers for each color band in a NW display are oriented perpendicular to each other rather than parallel as in the first (NB) embodiment. In Figure 6, these corresponding polarizer pairs are 31 and 40, 40 and 35, and 35 and 41 respectively. In this way, the suppression of polarization effects in the field-driven nematic device will result in effectively crossed polarizers and zero output for each color band. While it is not necessary to rotate the polarization exactly throughout a known twist angle in order to achieve display output in the undriven state, providing for a 90 degree rotation does optimize the display transmittance and uniformity. In this embodiment, a comparable tuning ofthe LC birefringence and thickness is used to obtain high transmittance at or near a low order maximum, preferably the first maximum, ofthe NW display. The first maximum for a 90 degree NW twist cell is defined analogously to the first minimum in Figure 5, except that the minimum positions become maxima.

As with the first embodiment, the second embodiment can also be modified to incorporate a nematic with a twist angle anywhere in the range of 0 to 180 degrees, preferably in the range of 0 to 90 degrees to maximize the performance ofthe device.

Whereas these embodiments have been described as being either normally white or normally black, combinations of these modes may be used. For example, the device for modulating green light might be made normally black (non-transmissive to green light) and the other devices normally white (transmissive to their respective colors without applied voltage). Thus, the relative advantages of normally white or normally black can be weighed independently for each active layer. A variety of light sources exists which can be used to create the incident light upon the picture element. Most prior art light sources are broad band lamps which emit light whose spectral components span the visible spectrum. In order to improve the performance of a color display and especially a subtractive color display, it is advantageous to have a tuned light source in which light emitted is tuned to R,G and B peaks. The advantage of this is that tuned light sources provide better contrast and greatly enhanced color gamut.

Figure 7 is a graph ofthe output of a typical broad band lamp. It is seen that light of every wavelength in the visible spectrum is emitted. In order to improve performance ofthe subtractive color display, it is advantageous to emit light whose components are more like those shown in Figure 8. This particular graph is of a 300- watt broad band lamp tuned to red, green and blue peaks with notch filters. These notch filters can be incorporated as part ofthe lamp assembly. Notch filters can be used to tune broad band light sources such as a xenon arc lamp, a metal halide lamp, a Tungsten halogen lamp, or a fluorescent lamp. Another solution is a phosphor triband lamp which does not require notch filters and its emissions are only in narrow bands of red, green and blue light. Another option is individual lasers which emit the primary colors in narrow wavebands.

In order to further enhance the performance ofthe subtractive color liquid crystal display, a new color polarizer design can be employed which is superior to the commercial sheet polarizers used in the prior art. As seen in Figure 3, the color polarizers are located on both sides ofthe active switching elements. In the prior art, these color polarizers were typically constructed of sheets of glass and plastic embedded with dichroic dye. In order to properly tune these color polarizers the manufacturing procedure must be modified and once the polarizer is built it can be tuned no further. The biggest drawbacks ofthe commercial polarizers is their size and its lack of tuneability.

In the preferred embodiment ofthe invention, anti-parallel aligned nematic liquid crystals containing dichroic dyes are utilized as passive color polarizers. This type of color polarizer can be used as a replacement for a commercial sheet polarizer or in conjunction with the commercial sheet polarizers to improve performance.

Figure 9 shows the construction of a typical anti-parallel nematic liquid crystal color polarizer. The support substrates 50 are typically glass plates. They may be the transparent electrode substrates ofthe active switching elements. The substrates 50 are assembled with spacers 56, which control the distance between substrates and also act as a perimeter seal. A gap is left in the spacer 56 so that liquid crystal may pass into the space between substrates. The glass substrates 50 are oriented so that the alignment directions of both their surfaces are parallel and the distance between the substrates is uniform. Air between the substrates is evacuated with a vacuum and a pool of dyed liquid crystal 54 is placed in contact with the gap in spacer 56. Pressure is reintroduced and the package is filled with the dyed liquid crystal 54. Dichroic dye aligns with its optical axis parallel to the liquid crystal and the liquid crystal aligns parallel to the alignment layer 52 with the long axis ofthe molecules parallel to the substrate plane. In this way, the dyes are aligned with respect to the substrate the same way as in a sheet polarizer. The thickness ofthe package is controlled with spacers 56, where the thickness ofthe liquid crystal 54 controls the amount of optical extinction in the subtractive polarization band. The liquid crystal thickness is typically between 1 and 15 microns, and depends on the amount of extinction desired from the dye-saturated liquid crystal. The same variation in extinction can be affected by controlling the percentage of dichroic dye in the liquid crystal mixture. Either method is acceptable although to minimize the layer thickness, a liquid crystal mixture fully saturated with dye is used and desired extinction of color is achieved by fixing the gap between the plates 50. Using the dichroic liquid crystal polarizer it is easier to evaluate and modify the optical extinction. This type of adaptability is not available in a commercial sheet polarizer.

The liquid crystal layer thickness and dye concentration is adjusted to control the color polarizer and transmission according to the R,G and B lamp peak ratios for additional improvements and a color gamut with subtractive color.

In yet another embodiment ofthe invention, a combination of guest-host dye incoφorated into the active switching element is used with either the commercial sheet polarizers, the anti-parallel aligned nematic liquid crystal color polarizers or both. The incorporation of guest-host dye into the active switching element is well known and is described in U.S. Patent #5,032,007 (Silverstein, et al.) which is hereby incorporated by reference. By introducing dye in the form of a moderately doped twisted nematic structure, several significant benefits result. The combination ofthe color polarizers with the dyes in the active switching element works to improve the purity and saturation ofthe subtractive colors, especially when the color polarizers have non-ideal spectral properties. The alignment ofthe dichroic dye with the applied field amplifies the contrast ratio ofthe device more than would be achieved had the same dye been added in the form of a non-switched polarizer. Further, the guest-host dye acts to minimize the dependence on the birefringence and optical mode-mixing ofthe non-adiabatic polarization rotation mechanism. All of these work to extend the color gamut and move the chromaticities to more saturated positions than would be achieved through use of just the color polarizers. Detrimental effects due to high doping concentrations are avoided by minimizing the amount of dye used. Benefits are still achieved because the dye acts as a color enhancing mechanism but is not relied upon as the primary color producing mechanism. Figures 10 and 11 show graphs which illustrate the performance ofthe subtractive stack using either a tuned or broad band lamp source. Figure 10 illustrates the use of a broad band lamp source and it can be seen that in either green or red select states there is much leakage ofthe color into other areas ofthe visible spectrum. In contrast, Figure 11 shows the use ofthe tuned light source in either the red or green select states and the color transmitted through the stack is sharper and there is not as much overlap with other bands ofthe spectrum.

Performance improvements described in the above embodiments can be seen in the chromaticity diagram of Figure 10. It is seen that the various enhancements provide superior performance over standard polarizers and broadband lamp. In the chromaticity diagram line 1 is a standard polarizer and broadband lamp, line 2 is a tuned lamp with standard polarizers, line 3 is a broadband lamp with anti-parallel aligned nematic liquid crystal polarizers, line 4 is a tuned light source with anti-parallel aligned nematic liquid crystal polarizers and color sheet polarizers, and line 5 is a tuned light source, anti¬ parallel aligned nematic liquid crystal polarizers, color sheet polarizers and active switching elements with guest-host dye. In the chromaticity diagram, it is desirable to cover the largest area possible, representing a large color gamut. Each ofthe succession of curves represents an improvement over the baseline as defined by line 1 using commercial sheet polarizers. This clearly demonstrates that the methods described can yield a subtractive color display with dramatically improved color performance. The color gamuts shown in Figure 12 illustrate extraordinary improvements in color purity and color saturation due to the notch filter tuned lamp, the addition of anti parallel aligned dichroic liquid crystal polarizers, and the addition of guest host twisted nematic LCD's in a subtractive color stack. The reference color gamut signified by line 1 utilized TN LCD's with commercial sheet polarizers and an untuned broad band white light source. Color gamuts signified by lines 2 and 3 show improvements due to the utilization of a notch filter tuned lamp and the addition of tuned dichroic LC polarizers separately. The color gamut signified by line 4 illustrates the improvement due to the interactive tuning ofthe dichroic LC polarizers and notch filter tuned lamp together, and finally the color gamut signified by line 5 is the color gamut of a fully interactively tuned subtractive color stack using a notch filter tuned lamp, dichroic anti-parallel aligned liquid crystal polarizers, color sheet polarizers and guest host TN LCD's.

The foregoing is a description of a novel and nonobvious optimized full color display using subtractive color. The applicants do not intend to limit the invention through the foregoing description but instead define the invention through the claims appended hereto.

Claims

1. A liquid crystal display comprising: an illumination means; and a plurality of picture elements, each of said picture elements comprised of : first, second, and third active switching elements which selectively rotate incident light about a known twist angle; a first color polarizer between the illumination means and the first active switching element which polarizes a first of three subtractive primary colors (red, green, and blue) while allowing second and third primary colors to pass; a second color polarizer between the first active switching element and the second active switching element which polarizes the first and second primary colors while allowing the third primary color to pass; a third color polarizer between the second and third active switching elements which polarizes the second and third primary colors while allowing the first primary color to pass; and a fourth color polarizer adjacent to the third active switching element and opposite the third color polarizer which polarizes the third primary color and allows the first and second primary colors to pass.

2. The color liquid crystal display of claim 2 wherein the illumination means is tuned to emit white light tuned to red, green, and blue intensity peaks.

3. The color liquid crystal display of claim 1 wherein the active switching element is a twisted nematic liquid crystal cell of a predetermined thickness.

4. The color liquid crystal display of claim 3 wherein the color polarizers are arranged to provide a normally black display.

5. The color liquid crystal display of claim 4 wherein the thickness of each nematic liquid crystal in the active switching elements is selected to be at or near a thickness for a low-order minimum for transmission.

6. The color liquid crystal display of claim 5 wherein the thicknesses ofthe active switching element liquid crystal cells are not identical, the thickness of each ofthe cells being determined by the color of light being switched by that element.

7. The color liquid crystal display of claim 4 wherein the color polarizers are arranged to provide a normally white display.

8. The color liquid crystal display of claim 7 wherein the thickness of each nematic liquid crystal cell in the active switching elements is selected to be at or near the thicknesses for a low-order maximum for transmission.

9. The color liquid crystal display of claim 8 wherein the thicknesses ofthe active switching element liquid crystal cells are not identical, the thickness of each ofthe cells being determined by the color of light being switched by that element.

10. The color liquid crystal display of claim 4 wherein the twisted nematic liquid crystal cells have a twist angle of 90 degrees.

11. The color liquid crystal display of claim 4 wherein at least one ofthe twisted nematic liquid crystal cells operates in a normally black mode and at least one ofthe twisted nematic liquid crystal cells operates in a normally white mode.

12. The color liquid crystal display of claim 4 wherein the twisted nematic liquid crystal cell has a twist angle of less than 90 degrees.

13. The color liquid crystal display of claim 3 wherein at least one ofthe active switching elements contains guest host dye which absorbs the corresponding primary color.

14. The color liquid crystal display of claim 1 wherein the color polarizers are constructed of anti-parallel aligned nematic liquid crystal.

15. The color liquid crystal display of claim 14 wherein commercial sheet polarizers are used in combination with the anti-parallel aligned nematic liquid crystal polarizers.

16. The color liquid crystal display of claim 2 wherein the color polarizers are constructed of anti-parallel aligned nematic liquid crystal.

17. The color liquid crystal display of claim 16 wherein at least one of the active switching elements contains guest host dye which absorbs the corresponding primary color.

18. A liquid crystal display comprising: an illumination means which emits white light tuned to primary color (red, green, and blue) intensity peaks; and a plurality of picture elements comprising: a first color polarizer which polarizes a first of three subtractive primary colors while allowing second and third primary colors to pass, a second color polarizer which polarizes the first and second primary colors while allowing the third primary color to pass, a third color polarizer which polarizes the second and third primary colors while allowing the first primary color to pass, and a fourth color polarizer which polarizes the third primary color and allows the first and second primary colors to pass, where at least one ofthe color polarizers is comprised of anti-parallel aligned nematic liquid crystal; a first active switching elements which selectively rotate incident light about a known twist angle positioned between the first and second color polarizers; a second active switching elements which selectively rotate incident light about a known twist angle positioned between the second and third color polarizers; and a third active switching elements which selectively rotate incident light about a known twist angle positioned between the third and fourth color polarizers.

19. The color liquid crystal display of claim 18 wherein the illumination means is a broad band light source tuned with notch filters.

20. The color liquid crystal display of claim 19 wherein the broad band light source is a xenon arc lamp.

21. The color liquid crystal display of claim 19 wherein the broad band light source is a metal halide lamp.

22. The color liquid crystal display of claim 19 wherein the broad band light source is a tungsten halogen lamp.

23. The color liquid crystal display of claim 19 wherein the broad band light source is a fluorescent lamp.

24. The color liquid crystal display of claim 18 wherein the illumination means is a phosphor tuned tri band lamp.

25. The color liquid crystal display of claim 18 wherein the illumination means is three lasers which emit narrow bands of red, green, and blue light.

26. The color liquid crystal display of claim 18 wherein commercial sheet polarizers are used in combination with the anti-parallel aligned nematic liquid crystal in the color polarizers.

27. The color liquid crystal display of claim 18 wherein the first, second, third and fourth polarizers consist ofthe anti-parallel aligned nematic liquid crystal.

28. The color liquid crystal display of claim 27 wherein the thickness of each the first, second, third, and fourth polarizers is between 1 and 20 microns.

29. The color liquid crystal display of claim 18 wherein the active switching elements are twisted nematic liquid crystal cells.

30. The color liquid crystal display of claim 18 wherein at least one ofthe liquid crystal cells contains guest host dye which absorbs the corresponding primary color.

31. A liquid crystal display comprising: an illumination means which emits white light tuned to primary color (red, green, blue) peaks; and a plurality of picture elements, where each picture element is comprised of: a plurality of first, second, and third active switching elements comprised of twisted nematic liquid crystal where the thickness ofthe liquid crystal for each ofthe switching elements is at a minimum to compensate for optical mode-mixing due to birefringent characteristics ofthe twisted nematic liquid crystal; a first color polarizer between the illumination means and the first active switching element which polarizes a first of three subtractive primary colors (red, green, and blue) while allowing second and third primary colors to pass; a second color polarizer between the first active switching element and the second active switching element which polarizes the first and second primary colors while allowing the third primary color to pass; a third color polarizer between the second and third active switching elements which polarizes the second and third primary colors while allowing the first primary color to pass; and a fourth color polarizer adjacent to the third active switching element and opposite the third color polarizer which polarizes the third primary color and allows the first and second primary colors to pass.

32. The color liquid crystal display of claim 31 wherein at least one ofthe active switching elements contains guest host dye which absorbs the corresponding primary color.