A multicolor liquid crystal display

Abstract: A multicolor display (300) which comprises a high con­ trast liquid crystal device (320), light source (314) and a luminance-balanced color mask (318). The liquid crystal device is the transmissive, twisted nematic type having high extinction ratio polarizers on the input and output sides and functions as a light shutter for the display. The liquid crystal material has a dichroic dye admixed therein to prevent build-­ up of the contrast limiting orthogonal mode electric vector of polarized light. The color mask contains different colored patches with gray surrounds, and the colored patches are luminance balanced in order that the different colored data displays are of substantially equal brightness. During the off-­ state there is no light bleed through so that the display pres­ ents a uniformly dark state or dead front.

Description: The present invention relates to multicolor liquid crystal displays and, more particularly, to a multicolor liquid crystal display utilizing a transmissive, high-contrast, twisted-nematic liquid crystal cell which functions as a light shutter, a selected light source, for example a rich-in red light source, and a luminance-balanced color mask. Multicolor displays using a twisted nematic (TN) liquid crystal cell and polarizers have been typically realized through two known schemes. In one scheme, the liquid crystal layer serves only to twist linearly polarized light, as in the usual TN liquid crystal cell. Using polarized white light, a transmitted color is then obtained as a result of the selective absorption produced by two external, crossed colored pleochroic polarizing filters. In a second scheme, colors are transmitted by selective interference obtained whith white light, using an externally positioned optical retardation plate. Refer to Scheffer, T. J., (1973), "New MultiColor Liquid Crystal Display That Used a TN Electro-optic Cell." J. Applied Physics, 44(11), pp. 4799-4803. The first scheme is preferable because its construction is simpler. Figure 2 of the accompanying drawings shows the operating principle of such a two-color liquid crystal cell 210 mentioned above as the first scheme. This cell consists of a neutral (panchromatic) polarizer 212, a 90° twist TN liquid crystal cell 214 and two dichroic analyzers 218 and 218. In this arrangement, it is assumed that the polarizing axes of the neutral polarizer 212 and the dichroic analyzer 218 are positioned vertically and that the polarizing axis of the dichroic analyzer 216 is positioned horizontally. It is also assumed that the polarizer 212 absorbs all the incident light, except that light with electric vector in the vertical direction. Finally, it is assumed that the analyzers 216 and 218 allow light with electric vector in the direction of their polarizing axis to pass through irrespective of color and absorb incident light at all other polarization directions, except for the light of one color component, which is allowed to pass through regardless of the direction in which it vibrates. In the case ot Fig. 2, it is assumed that the analyzer 216 allows the green color of all polarization directions to pass through and that the analyzer 218 allows the red color component to pass through. The liquid crystal molecules in the TN cell 214 are aligned on the inside surface of the cell parallel to the polarizing axes of the corresponding adjacent polarizers when the cell is in the off-state. In operation, incident white light from source 200 is polarized vertically by the polarizer 212 and then passes through the liquid crystal layer, the polarization direction essentially following the alignment of the molecules. The white light thus leaves the cell horizontally polarized, so that all of its color components can pass through the analyzer 218. When the light strikes the analyzer 218, only the red component can pass through because the polarizing axis of the analyzer 218 is perpendicular to that of the analyzer 218 and an observer perceives red light. On the other hand, when a voltage in excess of the threshold voltage is applied accross the electrode of the liquid crystal cell 214, the molecules are oriented perpendicular to both electrodes. As a result, incident polarized light passes through the cell with its polarization direction unchanged. In this case, only the green component of the light will pass througth the analyzer 216. This light will also pass through the analyzer 218 without being absorbed and the observer will perceive a green color. Analyzers of the type described can be purchased commercially in the form of two dichroic polarizers of arbitrary color with their polarizing axes oriented at 90° with respect to each other. Refer to Shanks, I, A., (1974), "Electro-Optical Color Effects by TN Liquid Crystal," Electron. Letters 10(7), pp. 90-91. Many of the performance characteristics of TN two-color display cells, such as response time, operating voltage, and power consumption, are the same as those for conventional TN display cells, since both types of display have the same basic construction except that the two cells have additional polarizers. Therefore, a two-color matrix display can be made with this scheme in which the applied voltage controls the color (instead of brightness) of each pixel of the matrix. Although good color separation can be obtained, no more than two colors can be displayed with this scheme. A serious shortcoming of such displays is their low brightness, especially when a reflecting type of display is used, since a large amount of light is absorbed by the polarizer and the analyzers. It is also well known to use a typical transmissive, twisted-nematic, liquid crystal display device as a light shutter and insert a colored filter between a light source and the input side of the TN liquid crystal display. However, these prior art devices permit bleed-through during the off-state condition, so that the underlying color filters or patches can be seen by the human eye even though unactivated. Existing prior multicolor displays are adequate for some uses; however, they are unduly unattractive for uses in the office equipment environment or in auto instrument panels. The present invention overcomes this bleed-through problem by incorporating a high contrast. TN liquid crystal display device, a selected light source and luminance-balanced color mask, as more fully described below. Since the high contrast, TN liquid crystal display is an important part of the present invention, and is an improvement over the prior art in its own right, a discussion of the conventional TN liquid crystal display devices is presented below to enable the understanding of the improvement of the high contrast liquid crystal display over the art. A typical prior art transmissive-type, twisted nematic liquid crystal device 10 is shown in the accompanying drawings, in Figure 1A in the off state and in Figure 1B in the energized state. A polarizer 11 and an analyzer 12 are attached to the outside surfaces of the glass plates 13, 13a. The polarizing axes of these are made parallel, coinciding with the direction of alignment of the liquid crystal molecules 14 at the top plate 13a. Because of the relatively gradual twist of the liquid crystal molecules, polarized light entering the liquid crystal material gradually twists its direction of polarization as it passes therethrough, emerging with its polarization rotated by 90 degrees. Since this light is blocked by the output analyzer 12, the device appears nearly opaque when viewed from the output side. Generally, the prior art disclosures state that the twisted nematic devices possess waveguide-like properties. That is, a first polarizer plane polarizes white light which, because of a waveguide-like action, is rotated by the twist angle of the liquid crystal molecules. While this is approximately true, the statement is not exact. As will be explained later, approximately 2% of the light exists in an orthogonal polarized component which easily passes through the analyzer resulting in "bleed-through." The multicolor liquid crystal display of the present invention prevents this orthogonal light component from exiting the liquid crystal material, as will be fully described later. The elimination of the passage of the orthogonal component of light increases the white light optical contrast above levels previously thought possible and substantially reduces the bleed-through problem encountered with a conventional TN liquid crystal display. Generally, liquid crystal materials used in twisted-nematic devices have a positive dielectric anisotropy, so that, when a voltage is applied, the direction of the molecules in the center of the layer tends to orient itself parallel to the applied field, as shown in Figure 1B. Above some threshold voltage (e.g., 1 to 3 volts) the molecules become oriented with the director everywhere parallel to the applied field, except for those bound at the immediate surfaces of the glass. Since, in this condition, the liquid crystal layer no longer twists the direction of polarization of the light, it passes through the analyzer unobstructed. The fact that at intermediate voltages partial light transmission occurs is of importance in devices where gray scale is required. If the applied voltage is cut off, intermolecular forces causes the liquid crystal molecules to return to the twisted configuration of Figure 1A, blocking most of the light transmission. Essentially, no de current is required to produce a change in optical state because of the high resistivity (>10 10 ohm cm) of the liquid crystal materials. The power for switching such a device on, therefore, is low, usually less than 1µw/cm 2 ; It is important to realize that conventional displays i.e. those using low extinction ratio polarizers, are not limited in contrast by fundamental properties of the liquid crystal itself. Rather, contrast is polarizer limited if the polarizers are of the low extinction ratio type. Manufacturers of these displays, mostly of the reflective type, choose polarizing materials which are optically efficient to achieve the brightest display possible when used in ambient illumination. Film polarizers have the property that those that give the highest extinction ratio also tend to be the most lossy. By extinction ratio it is meant the intensity of the light passed by two parallel polarizers divided by the value for the crossed case. Therefore, high contrast and high brightness are not simultaneously obtained. In fact, a trade-off must be made between contrast and brightness. Tables 1(a) and 1(b) below illustrates this point. In Table 1(a) a list of polarizers as well as measured extinction ratios are shown. The Polaroid Corporation conveniently identifies their products with a number which is a measure of the white light transmission of a single polarizer to randomly polarized white light. For example, an HN55 transmits 55% of unpolarized white light, etc. In Table 1(b), the on-axis white light photopic contrast ratio of a 10 µm thick LCD cell filled with Merck liquid crystal #1132, a biphenyl mixture, is shown with polarizers which have a wide range of extinction ratios. Polarizer Extinction Ratio Polaroid HN-55 3.59 Polaroid HN-48 3.80 Polaroid HN-42 143 Polaroid HN-32 ∼8,000 Polarizer Extinction Ratio Contrast Ratio Polaroid HN-48 3.80 3.69 Sanritz LC-82-13 15.6 11.0 Polaroid HN-32 ∼8,000 46.7 Conventional twisted nematics (TN) materials only change the state of light polarization. Polarizers are required to transform the light polarization changes to light intensity differences which are detectable by the human eye. Using HN48's, a contrast in transmission of 3.69 was obtained which is within 3% of the maximum of 3.8. If Sanritz Limited LC-82-13's are used, a contrast ratio of 11.0 is obtained, 30% lower than the maximum of 15.6. If very high extinction ratio plarizers such as Polaroid HN32's are used, the contrast increases substantially, a value of 46.7 is obtained. However, the contrast ratio is now orders of magnitude below the extinction ratio of 8,000 which is possible with polarizers alone. Clearly, the first two cases give results which are polarizer limited while the final one is not. This result indicate that a typical TN cell passes about 2% of white light which is not because of any deficiency in the polarizers themselves, as mentioned above. Thus, to operate in the regime where contrast and density are not polarizer limited, high extinction ratio polarizers must be used. Further, increases in contrast and density require changes in the liquid crystal cell itself. Such changes, as described below, form part of the present invention. One would conclude if complete waveguide action existed that plane-polarized white light would exit the last layer of liquid crystal molecules. In fact, the light leaving the liquid crystal material is weakly elliptically polarized. A more complete description of this phenomenon follows. In addition to the main plane polarized mode which follows the twist of the cell, a second mode of propagation with its electric vector at right angles (orthogonal) to the first mode builds up. This mode is free to be passed by the final polarizer (analyzer). Since about 2% of the light is in this mode, contrast is limited to 50:1. Gooch and Terry have described in the following reference, a mathematical treatment of the light propagation which includes the orthogonal mode. C. H. Gooch and H. A. Terry, "Optical Characteristics of Twisted Nematic Liquid Crystal Films," Electronic Letters, 10,1, 1974. By using Gooch and Terry's formulation, it can readily be shown that liquid crystal layer thicknesses in excess of 10µm greatly reduce the light transmission. Twisted nematic LCDs are typically operated in the so-called Mauguin limit; that is, the wavelength of light (X) is much less than twice the birefringence (Δn) times the liquid crystal layer thickness d: λ<<2Δηd A typical value for 2Δηd (Δη = 0.14, d = 10 µm) is about 3µm. Such a liquid crystal cell would operate in what may be referred to as the near Mauguin limit. If the liquid crystal layer thickness and/or birefringence is increased, the optical contrast is increased. If the layer thickness were to be increased so that 2Δηd is much greater than 3µm. which may be deemed the far Mauguin limit, to say a value of 10µm, the photopic contrast ratio becomes several hundred to one. However, there is a price to pay for this improvement in contrast; viz, the viewing angle and turn-off time are compromised. Since the turn-off time is proportional to the thickness squared, a long turn-off time would be encountered. This increased turn-off time is unacceptable for uses in multicolor liquid crystal displays utilized in, for example, office automation equipment and automotive instrument panels. A technique for reducing the orthogonal light component without using thick cells which result in increase turn-off times, involves the addition of a dichroic dye to the liquid crystal material. The inclusion of a dichroic dye to prevent the build-up of the orthogonal component pertains to the subject of the present invention. The incorporation of dichroic dyes in liquid crystals was first described in the following reference: G. H. Heilmeier and L. A. Zanoni, Appl. Phys. Lett. 13, 91, 1968. Heilmeier coined the term "Guest-Host Effect" which is now widely used. The liquid crystal, present in high concentration is the "host", while the dichroic dye, present in small concentration, is the "Guest." The liquid crystal essentially governs the electrical properties, while the dye is there for its advantageous optical properties. Simply stated, when the dichroic dye has its long, absorbing axes parallel with the electric vector of incident light, maximum absorption takes place. When the absorbing axes and light electric vector are at right angles, minimum absorption takes place. Figure 3 illustrates two typical states for a "Heilmeier-type" device. Note no molecular twist exists in this configuration. Further note that a single polarizer on one side of the device only is used. The first public disclosure of a dichroic dye in a twisted configuration was made in the following reference: D. L. White and G. N. Taylor, J. Appl. Phys. 46, 4718, 1974. In this device, there are a number of turns of the molecular helix, allowing incident randomly polarized white light to be absorbed to a high degree. This allows the device to be operated with no polarizers. In liquid crystal materials, thermally-induced temporal fluctuations in the molecular axis exist. Thus, the plane of polarization of incident light is not always parallel to the liquid crystal and/or dichroic dye molecular axis. Consequently, contrast ratio is reduced at a given temperature. There exists liquid crystal materials which fluctuate by different amounts. Those with minimal fluctuation have high order parameters and are the most desirable. The addition of dichroic dyes to liquid crystal material is well known. However, dyes are generally used for features such a broad viewing angles or elimination of one of both polarizers. Since less than about 10 weight percent of dye can be dissolved in state-of-the-art liquid crystal materials (order parameter ≅ 0.75), the prior art leaches that the contrast ratio of the images that is obtained is limited to about 10:1. The high contrast LCD which forms a part of the present invention overcomes this limitation. US-A-4,288,147 discloses a host liquid crystal admixed with various guest dichroic dyes. In this patent, the electro-optic composition has a host liquid crystal which may include conventional and well-known materials such as nematic and cholestric liquid crystal, while the guest dichroic dye is characterized as a linear, non-condensed aromatic compound which includes a quinonoid ring as the chromophore or color-generating group. In positive dichroic dyes, such as discussed in this patent, light which impinges on the dye molecule (generally having a long, rod-like structure) perpendicular to its long axes is absorbed more strongly than light which impinges on the molecule parallel to its long axes. This patent discloses that the advantageous properties of dyes of this type are attributable to their long structure which results in a high order parameter and thus improved contrast and to the quinonoid ring as the color-generating group. These dyes impart color when incident light impinges on the dye molecules perpendicular to their long axes. The liquid crystal solution is colorless when the incident light impinges parallel to the dye long axes. US-A-4,211,473 discloses a reflective-type liquid crystal display device having front and rear polarizers and a reflector. An enhanced contrast is achieved for this type of device by the use of pleochroic dye in the liquid crystal display material, a rear polarizer, pleochroic and fluorescent materials are incorporated in balanced proportion and have absorption and emission spectra, respectively, that are complementary. Light incident on the display is converted so that the light reaching the eye from the bright state regions is neutral grey in contrast to the light from the dark state regions which is strongly attenuated and may be strongly colored. US-A-2,213,382 discloses an apparatus for enhancing the quality of color reproduction. The apparatus contains a filter in the projection system which consists of vertical strips of various colors with uncolored strips therebetween. The uncolored strips may consist of a gray filter with a characteristic absorption of light equal to the absorption of light by the colored strips. US-A-4,342,046 discloses a contact screen for making color separation halftone blocks for use in a picture reproducing machine consisting of a plurality of colored halftone screens having different screen angles which are arranged on a base film in positions corresponding to color separation reproduction pictures to be reproduced and with transparent blank spaces formed between the adjacent halftone screens through which register lines can be recorded. Vortrag Nr. 23 der 13. Freiburger Arbeitstagung Flüssigkristalle, 23.03 - 25.03.1983 discloses a colored LCD however this prior art device does not comprise a luminance-balanced color mask having colored patches and a gray surround area, the colored patches being luminance-balanced with each other and the gray surround area in order that the colored patches and gray surround area are of substantially equal luminous transmission. The present invention may be summarized as a multicolored display according to claim 1 and a method of providing a multicolored display according to claim 8. The liquid crystal device is the transmissive, twisted-nematic type having high extinction ratio polarizers on the input and output sides and it functions as a light shutter for the display. The liquid crystal material has a dichroic dye admixed therein to prevent buildup of the contrast-limiting orthogonal mode electric vector of polarized light. The color mask contains different colored patches with gray surrounds, and the colored patches are luminance balanced with each other and the gray surrounds in order that the different colored data displays are of substantially equal brighthness. During the off-state, there is no light bleed-through and the combined luminance balanced color mask and high optical contrast and density of the TN liquid crystal device enables the multicolored display to present a uniformly dark state or dead front. The invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1A is a cross-sectional side view of a schematical representation of conventional transmissive-type, twisted nematic liquid crystal device in the off-state condition. Figure 1B is a cross-sectional side view of a schematical representation of a conventional transmissive-type, twisted nematic liquid crystal device in the energized or on-state condition. Figure 2 is an exploded schematic representation of a conventional two-color liquid crystal display showing the display in both the off-state and on-state condition. Figure 3 is a cross-sectional side view of a schematical representation of a typical prior art guest-host effect, transmissive-type liquid crystal display device depicting the off-state and on-state condition. Figure 4 is a schematic representation of a high-contrast liquid crystal device used in the present invention. Figure 5 is a graphical plot of contrast ratio as a function of liquid crystal cell thickness comparing the contrast ratio of a liquid crystal device incorporating a dichroic dye with (a) a single polarizer on the input side and (b) the two polarizer arrangement of Figure 4. Figure 6 is a schematic of a xerographic copier with which the present invention has utility. Figure 7 is a schematic of the electronics used both to control and monitor xerographic functions inside the copier of Figure 6. Figure 8, is an exploded schematical representation of the multicolor display of the present invention, incorporating the liquid crystal display device of Figure 4. Figure 9 is a graphical plot of Percent Optical Contrast versus Brightness in foot-Lamberts. Figure 10 is the Standard Eye Sensitivity Curve plotted as spectral luminous efficiency versus wavelength in nanometers. Figure 11 is a graphical plot of wavelength of light in nanometers versus watts per 10 nanometers for a fluorescent light (Cool White Deluxe F40T12). Figure 12 is a schematical representation of color patch brightness enhancement and brightness reduction. Figure 1A and 1B show a typical prior art transmissive-type, twisted nematic liquid crystal device 10 in the off-state and the on or energized state, respectively. The device comprises parallel, upper transparent glass substrate 13a and lower transparent glass substrate 13, each having transparent electrodes 16 on the inner confronting surfaces of the glass substrates. A thin layer of nematic liquid cyrstal material is sandwiched between the transparent electrodes. A polarizer 11 and an analyzer 12 are attached to the outer surfaces of the glass substrates. The polarizing axes of the polarizer and analyzer are made parallel and coinciding with the direction of the alignment of the liquid crystal molecules 14 at the upper glass substrate 13a. The liquid crystal molecules gradually twist so that the molecules at the lower substrate have been rotated 90 degrees and are perpendicular to those adjacent the upper glass substrate. The polarized light from source 18 is substantially blocked when viewed from the output side 20 because of the polarizing direction of analyzer 12. When the electrodes 16 have an alternating voltage from source 21 that is above a certain threshold applied across them, the liquid crystal molecules become oriented in a direction substantially parallel to the applied electric field, except for those molecules bound at the surfaces of the glass substrates. In this energized condition, the liquid crystal layer no longer twists the direction of polarization of the light and, therefore, it passes through the analyzer 12. Figure 3 is an early, prior art, transmissive-type liquid crystal device 40 having dichroic dye admixed with the nematic liquid crystal material. The dye molecules 42 align themselves with the liquid crystal molecules 44 in a typical guest-host arrangement. The device 40 comprises parallel glass substrates 46 with transparent electrodes 48 placed on the inner, confronting substrate surfaces 50, which substrates sandwich a thin layer of nematic liquid crystal material with a small concentration of dichroic dye. A polarizer 52 was attached to the input surface 56 of the device 40. The polarizing axis of the polarizer is parallel and coincides with the direction of the nematic liquid crystal molecules 44. When an alternating voltage is not applied to the transparent electrodes (off-state condition), the dichroic dye molecules absorb much of the polarized light passing therethrough, thus eliminating the need for a second polarizer/analyzer to block the light when some light bleed-through is acceptable. The device of Figure 3 is generally referred to as a "Heilmeier-type" device. When the electrodes 48 have an alternating voltage from source 49 that is above a certain threshold applied across them, the host liquid crystal molecules 44 and guest dye molecules 42 become oriented in a direction parallel to the applied electric field. In this energized condition, the liquid crystal layer with dye molecules therein passes the polarized light therethrough. The high contrast, TN liquid crystal device 70 used in the present invention is shown in Figure 4, wherein a transmission, guest-host type, twisted nematic liquid crystal device 70 is depicted in which a dichroic dye, such as anthraquinone or azo dye, is admixed with any of the widely used liquid cyrstal materials. Figure 4 shows the device 70 in both the off-state and on-state condition. The device 70 comprises parallel, transparent, upper and lower glass substrates 71 and 72, respectively. Transparent electrodes 73 are placed on the inner, confronting surfaces 71a and 72a of the glass substrates. An alignment layer 78 covers the electrodes 73. A thin layer of twisted nematic liquid crystal material, with a relatively small concentration of dichroic dye admixed therein, is sandwiched between the glass substrate surfaces having the electrodes. Polaroid HN32 polarizers 74a, 74b marketed by the Polaroid Corporation are attached to each of the glass substrate surfaces opposite the surfaces having the transparent electrodes. The polarizing axes of both polarizers are made parallel and coincident with the direction of the alignment of the liquid crystal molecules 75 at the surface 73a of the transparent electrode 73 on the lower glass substrate surface 72a. Alternatively, the polarizers may be rotated in tandem by 90 degrees and subtantially the same results achieved. The liquid crystal molecules gradually twist in a manner well known in the art so that the molecules at the upper glass substrate surface have been rotated 90 degrees and are perpendicular to those adjacent the lower glass substrate surface. The axes of the dichroic dye molecules 76 are parallel with axes of the liquid crystal molecules in the typical guest-host arrangement. Light from source 77 enters the liquid crystal layer through the polarizer 74a on the lower glass substrate 72. In the off-state condition, the polarized light from light source 77 is blocked when viewed from the output side (viz., viewed from the polarizer 74b side) because of the polarizing direction of polarizer 74b after the polarized light has been twisted 90 degrees by the twisted nematic liquid crystal layer. A second component or the orthogonal mode of the polarized light builds up as the light impinges upon layer after layer of liquid crystal molecules. For a 10 µm liquid crystal cell with a liquid crystal birefringence of about 0.15, approximately 2% of the light would ordinarily exist in this mode because its electric vector is orthogonal to the main component. The light in the orthogonal mode would be almost completely passed by the polarizer 74b, but the dichroic dye, specifically added to prevent the build-up of the orthogonal propagating mode, prevents its passing during the off-state condition. In the 'on' or energized state, an alternating voltage from source 79 is applied to the electrodes 73 of the appropriate magnitude (e.g., 1 or 3 volts) to achieve an electric field across the liquid crystal material in a manner well known in the art. The host liquid crystal molecules align themselves with the direction of the electric field and the guest dichroic dye molecules maintain themselves substantially parallel to the host molecules. Thus, the polarized light is transmitted through the device 70. It is not necessary that a 90 degree twist cell be used to realize the benefits of the described invention. Other twist angles can be used. The twist of the liquid crystal molecules depends both upon the surface treatment and the amount of chiral dopant, which can be a cholesteric liquid crystal. Applications for a high contrast, TN liquid crystal device as used in the present invention are numerous, but offer an especial appeal for multicolor displays used in automotive instrument panels and office automation equipment, such as copier instrument panels. One of the reasons is that the prevention of light bleed-through enables the liquid crystal display to present a better dead front panel. By 'dead front' panel, it is meant the various messages available for activation by the LCD electrodes cannot be detected in the 'off' state. This dead front or very dark 'off' state, as well as increased contrast, is realizable with this invention and both are very beneficial for improving the 'off' state appearance of multicolor liquid crystal display devices. A plot showing contrast enhancement provided by the present invention is shown in Figure 5. The lower curve was taken with a single high extinction ratio polarizer on the input side of the device of Figure 4. Thus, contrast was because of absorption differences with and without the dichroic dye rotated by an electric field. The role the liquid crystal material plays in the one polarizer configuration is to orient the molecules. The upper curve was obtained using the two polarizers configuration shown in Figure 4. The contrast in the single polarizer device is given approximately by the expression: contrast ratio (CR) = e 3Sα i d Here S is the order parameter of the dye/liquid crystal mixture, α i is the isotropic absorption coefficient, and d is the liquid crystal layer thickness. No closed form expression for the relationship yet exists for the upper curve which is the subject of the present invention. The liquid crystal/dye mixture used to obtain this experimental data was obtained from Hoffmann-LaRoche Inc. and the mixture has the product designation ROSA-605. Contrary to the prior art teaching, valuable gains in contrast is achieved without a large amount of dichroic dye. These data were taken with a diluted dye/liquid crystal mixture. The liquid crystal ROTN-605, also a product from Hoffmann-LaRoche, was used for dilution purposes. The data were taken using a tungsten source at 2800 Kelvin color temperature. A Pritchard 1980B photometer was used to measure the photopic contrast ratio. Using Polaroid HN32 polarizers, an off-state density greater than 3 was measured using the device shown in Figure 4. It is well known that a multicolor liquid crystal display can be created by placing a mask with colored films in a patch-wise fashion therein closely adjacent a conventional transmissive, TN liquid crystal display and by directing light through the colored films and into the input side of the liquid crystal device. In using such an arrangement different colored messages and/or symbols can be created on a dark background. The background of a typical multicolor TN liquid crystal device is not opaque in the off-state condition because some light leaks making the color patches and/or electrode information configuration within the liquid crystal apparent to the viewer. In the present invention, the high contrast liquid crystal device described earlier is used as a light shutter in conjunction with a multicolor mask having a plurality of color patches in a gray surround 317. The high contrast of the liquid crystal device used prevents the detection of color by an observer because the normal bleed-through prevalent in conventional devices that is attributable to the orthogonal mode of the electric vector of polarized light has been eliminated as discussed above. Furthermore, the brightness (or more accurately the luminance) of the light coming through the patches has been matched to that coming through the surround 317. With this combination, visualization of the patches as well as any messages or data is wholly eliminated and a completely dark output or "dead front" is presented to an observer, making such a display more appealing for use in such equipment as automotive instrument panels and office copier panels. The threshold for detection of a contrast or density gradient has been determined to be approximately 0.5% optical contrast. Refer to studies on the visibility threshold by: P. W. Cobb and F. K. Moss, "The Four Variable of Visual Threshold", J. Franklin Inst., 205, 831 (1928) and J. P. Connor and R. E. Ganoung, "An Experimental Determination of Visual Thresholds at Low Values of Illumination," J. Opt. Soc. Am. 25,287(1935) Cobb and Moss studied high lumination level situations, while Connor and Ganoung studied the low regime. The visibility threshold is somewhat dependent on brightness and Figure 9 shows the observed dependence. It should be noted that the results depend on other variables also, such as object size. The data presented to develop the Figure 9 curve were for objects of angular size of 10 minutes of arc. The portion of the curve of interest are for typical illumination levels which are in the 1 to 100 foot-Lamberts regime. The high contrast/high density liquid crystal display used in the present invention in conjunction with a highly balanced color mask underlay drops the contrast to about the threshold value. The intensity of the light is also reduced to a value outside the dynamic range of the eye, which is about 3 orders of magnitude. The balanced color underlay is sufficiently reduced in luminance by the high contrast/high density liquid crystal device to render it invisible even when the illuminator behind it is quite bright. The illuminator used in this invention has a surface luminance of approximately 1000 foot-Lamberts. Figure 8 shows an exploded view of the multicolor display 300 of the present invention. The illuminator 310 comprises a reflector 312, fluorescent light source 314 (preferably, Cool White Deluxe F40T12 for reasons discussed below), and a light diffuser 316. The color-balanced mask 318 is positioned between the illuminator 310 and input side of the high contrast TN liquid crystal device which is more fully described with reference to Figure 4. The transparent electrodes 73 of Figures 4 and 8 that are on the upper and lower glass substrates 71 and 72, respectively, comprise a plurality of closely spaced electrodes. The electrodes on one of the substrates are perpendicular to the electrodes on the other so as to form an addressable matrix in a manner well known in the art. Alternately, the electrodes can be of arbitrary shape. Activation of selected ones of the electrodes on the substrates enables the passage of light through the liquid crystal material in selected regions. Therefore, by appropriately applying a potential difference to selected electrodes, messages or graphical information may be displayed. Furthermore, large area blocks of the liquid crystal cell could be activated and the information displayed from color underlays. The mask 318 contains multicolor patches 319 in a gray surround 317. High extinction ratio polarizers 324 and 326 are attached to the input and output sides of the liquid crystal display 320 as discussed before with reference to Figure 4. Overlays 330 such as one shown with a machine configuration may be placed on the output side of the liquid crystal device 320 and/or included in an underlay (not shown). This invention uses the high contrast and high density liquid crystal device and a balanced color mask to reduce the contrast below the gray scale threshold for discrimination. It also allows operation at a sufficiently low luminance level that an observer's ability to detect differences based upon color discrimination is lost. Hence, a completely "dead front" appearance is realized. Balancing of the luminance of the color patches 319 in the mask 318 does not have to be done exactly to provide a non-discernible density difference which, with the high contrast liquid crystal device, prevents light bleed-through and, thus, a completely "dead front" panel to an observer. Even approximately matching of the luminance of the color patches gives a greatly improved performance. One set of color patches tested had the following percent luminance transmission: yellow 84% blue 34% red 21% green 39% When a gray surround 317 having a luminance transmission of 50% was used as the mask 318 to hold the color patches 319 above, a substantial reduction in the visibility of the patches was realized. Accordingly, a part of the present invention is the matching of the color patches 319 and the surround 317 more precisely to achieve perception by an observer of a panel having uniform density with no light passing through. It is important to realize that the necessary requirement is to balance the luminances of the color patches 319 to the surround 317, not the luminance transmission. The reasons for this distinction is that the invention can be used with a variety of light sources which typically differ in their spectral output. For example, incandescent light sources are very rich in red light, white fluorescent light sources are typically deficient in red light. Fluorescent sources have advantages such as longer life and smaller amounts of heat generated, however, which makes them attractive for displays. Because the fluorescent sources have a number of intensity spikes at particular wavelengths, these spikes must be taken into consideration in the display design. Figure 11 shows the spectral profile of a fluorescent phosphor light source 314 (Cool White Deluxe F40T12) used in the present invention, because the white phosphor of this light has a higher output in the red portion of the spectrum than other fluorescent sources and a more uniform overall spectral content. The rectangular shaded spikes represent additional light emitted at particular wavelengths because of the gas discharge process. The balanced color mask used in this invention can be fabricated using a variety of techniques; for example, color photographic film may be used. In principle, all that is necessary is to adjust the film exposure to give the required light transmission properties. Other possible approaches are silk screen printing of colored inks as well as conventional printing technologies practiced in the graphic arts. The preferred technique to provide a luminance balanced color mask is by a photographic procedure with light-sensitive material whereby the various color patches are created on a material marketed by Agfa-Gevaert called Copyproof by contact exposure. The color patches created on Copyproof were found to produce brighter blue and red patches; i.e. more colored light is transmitted through them. Before describing the results obtained using Copyproof, it is useful to define a few terms. The human visual system has its maximum sensitivity in the green portion of the spectrum (550 nanometers). The eyes' sensitivity drops off on either side of this point. Simply stated, our eye is less sensitive to red and blue than it is to green light. Figure 10 shows the eye sensitivity as a function of wavelength. Photometric units, such as lumens and foot Lamberts, have this weighting function built in. A useful concept for selecting color patches is their luminous transmittance, which is defined as the fraction of white light transmitted by a sample weighted by the eye's spectral sensitivity function. This term applies to any white light source. Since different white light sources of light vary somewhat in their output spectral profile, the luminous transmittance depends on the light source as well. As discussed above, fluorescent lighting for this invention was selected, the preferred one being Cool White Deluxe F40T12 (Figure 11). The luminance transmittance of the Agfa-Gevaert Copyproof material is given below in Table 2(a) for various colors. Note that they vary from 17 to 75%. Using a halftoning technique described later, the lumination transmission is balanced within a few percent as shown in Table 2(b). SOURCE WEIGHTED LUMINOUS TRANSMISSION OF AGFA-GEVAERT MATERIAL Red 17% Blue 21% Green 43% Yellow 75% SOURCE WEIGHTED LUMINOUS TRANSMISSION OF AGFA-GEVAERT MATERIAL AFTER BALANCING Red 29% Blue 29% Green 30% Yellow 33% White 31% As mentioned above, halftoning of all of the color patches are necessary to obtain the perceived uniform density of the multicolor display panel of this invention by an observer; that is, a "dead front" panel. Halftones are accomplished by adding small black dots in the colors more sensitive to the eye and small white holes to emit more light in those colors less sensitive to the eye. The number of dots per unit length is fixed in this invention by the use of 150 dot/inch screens to create the halftoning photographically. Opaque black dots are used to dim the bright green, yellow and white patches. Typically, the black dots occupy 30 to 50% of the area. The brightnesses of the red and blue patches are increased by opening small transparent holes to increase the brightness of the relatively dim blue and red regions. Since the holes are clear to all wavelengths, color saturation is decreased by this approach. Therefore, these holes cannot be too large and for best results should be in the range of 10 to 20% open area; i.e.; the transparent holes comprises no more than 10% to 20% of the total colored patch area. Figure 12 shows the halftoning concept used in the present invention. Figure 12 shows surround 317 with a color patch 319 therein. When the color of the patch is red or blue, the brightness must be enhanced and small transparent holes 332 are created in the patch. When the color of the patch is yellow, green or white, the brightness must be reduced and small opaque black dots 334 are placed therein. Brightness matching by halftoning techniques is an attractive approach because it is highly reproducible and because it is digital in nature rather than analog, thus giving all the benefits of a two-state system. By two-state system it is meant the holes are fully transparent and the dots are essentially opaque without varying the degrees of light transmittance therethrough. The present invention relates to multicolor liquid crystal display which presents a uniformly dark state or dead front during the off-state and has particular utility, as mentioned above, for use in automotive instrument panels and office equipment such as xerographic printers and copiers. Figure 6 shows an example of the use of a multicolor liquid crystal display (LCD) 82 in a copier 80. The detail description of the multicolor LCD 82 is more fully described with reference to Figures 4 and 8. The multicolor display 82 cooperates with alphanumeric display 84 to prompt the user efficiently regarding copier status, inform him of certain faults as they occur, and refer the user to flip charts 86 in the event that instructions to be given require more detail than can be conveniently displayed on the multicolor LCD 82 and alphanumeric display 84. The copier 80 includes a housing 88 which provides an attractive appearance and covers typical copier components and subassemblies. The particular copier 80 schematically illustrated includes a platen and platen cover 90. The platen cover 90 is hinged to allow the user to raise and lower the cover 90 and insert original documents onto the platen (not shown) for copying. The copier 80 also includes a sorter 92 which provides collated copy sets of multiple document originals. A control panel 94 allows a user to select copy size, copy contrast, number of copies to be made, and the manner in which the copies are to be made (e.g., duplex or simplex copies). The panel 96 having the multicolor liquid crystal display 82 also includes a button 95 for starting the copying operation. As the user approaches the copier 80 both the LCD 82 and alphanumeric display 84 are blank and can display nothing until the user activates the "Power On" switch 95 to energize the power supply inside the copier 80. Once the power has been turned on, a "Standby" message will appear on the alphanumeric display 84 indicating the copier is not yet ready for use. Once the copier 80 is ready for making xerographic copies, the alphanumeric display 84 shows a "Ready To Make Copies" message telling the user that the copier 80 is ready for operation. The functioning of the components comprising the copier 80 is controlled and monitored by an electronics subsystem 110 (Figure 7) comprising a number of programmable controllers which communicate with a master central processor 112. An interface 114 between the panel 96 and main processor 112 apprises the processor 112 of inputs entered by the user regarding number of copies, etc. The main processor 112 responds to user inputs by executing its operating system stored in a main memory unit 116. The algorithm in main memory 116 causes the master central processor 112 to communicate along a communications bus 118 to a number of remote electronics units 120-125 used to monitor and control the copier. The specific units 120-125 vary with copier architecture so the Figure 7 schematic is representative of one of many possible electric subsystems schematics. Each unit 120-125 has its own microprocessor with accompanying memory (both RAM and ROM) and support circuitry. The multicolor LCD 82 and alphanumeric displays 84 are electrically coupled to a display console remote unit 125. The display console remote unit 125 receives status information, fault information, or program control information from the main processor 112 and then displays an appropriate message on the alphanumeric display 84, and if appropriate, energizes one of a plurality of liquid crystal segments on the multicolor LCD display 82 or addresses the electrode matrix within the liquid crystal cell to create information. The multicolor liquid crystal display 82 is mounted directly above the alphanumeric display 84 and located to the side of the flip chart 86. The multicolor alphanumeric display 84 comprises a vacuum fluorescent tube capable of generating messages helpful to the copier user. The multicolor liquid crystal display 82 positioned directly above the alphanumeric display includes various liquid crystal segments as well as an addressable electrode matrix to aid the copier user in both interacting with the copier and correcting faults should they occur during operation. The particulars regarding the liquid crystal cell operation within the liquid crystal display device 70 have been discussed with reference to Figure 4. Figure 8 shows an exploded schematic representation of one configuration of a multicolor LCD 300 that may be used in a copier environment such as that of Figure 6. In Figure 8, fluorescent lights 314 direct light with the aid of reflector 312 and diffuser 316 through a color mask 318 behind the liquid crystal device 320. When the liquid crystal electrode elements are in an unenergized state, no light from the illuminator 310 reaches the user. When the electrode elements in the multicolor liquid crystal display 82 (or 320), however, are energized, they allow light to pass through the energized segment of the liquid crystal to the user. Thus, when an appropriate ones of the liquid crystal electrodes are energized by signals from the display console remote unit 125, it is rendered visible because of light passing through that segment. Recapitulating, the present invention relates to a multicolor display which comprises a high contrast liquid crystal device, light source and a luminance-balanced color mask which during the off-state presents a uniformly dark state or dead front because there is no light bleeding through the liquid cystal devices and the luminance-balanced color mask presents a uniform optical density below the threshold for visual detection by an observer. The liquid crystal device is the transmissive, twisted nematic type having high extinction ratio polarizers on the input and output sides and functions as a light shutter for the display. The liquid crystal material has a dichroic dye admixed therein to prevent buildup of the contrast-limiting orthogonal mode electric vector of polarized light. The color mask contains different colored patches with gray surrounds, the colored patches being luminance-balanced. Therefore, the different colored data displays are of substantially equal brightness. The specific use of the present invention in a copier environment discussed above was only for purposes of illustrating a particular use and was not intended as an only use. To the contrary, the present invention is important for many different environments.

Claims: A multicolor liquid crystal display device (300) selectively capable or being in the "on" or "off" state and providing a uniformly-dark output panel (330) when in the 'off' state, comprising: a liquid crystal cell (320) including two transparent, parallel, substrates of insulation material each with a confronting surface bearing at least one transparent electrode (73), a layer of twisted nematic liquid crystal material between the substrates, the cell being capable of transmitting light in the vicinity of the electrodes upon the application of an electric field across the layer when a potential difference of appropriate magnitude is applied between the electrodes; a light source (314) for directing light towards one of the substrates; a high extinction ratio polarizer (324,326) located on each substrate surface remote from the confronting surfaces, the polarizers being of the type which enables the attainment of a contrast ratio of greater than 100 from the multicolor display, and having polarising axes which are parallel with or orthogonal to the director of the liquid crystal molecules adjacent the surface nearer to the light source; a relatively-small amount of dichroic dye mixed with the liquid crystal material to form a guest-host mixture therewith for the purpose of absorbing, and thus preventing the buildup and subsequent passage of, the orthogonal vector component of polarized light created by imperfect rotation of the polarized light by the orientation of the twisted nematic liquid crystal molecules when no electric field is present; means for applying a voltage across the electrodes (73) to change the orientation of the molecules of the mixture, and to enable the passage of light when desired, and a luminance-balanced color mask (317) having colored patches and a gray surround area, the colored patches being luminance-balanced with each other and the gray surround area in order that the colored patches and gray surround area are of substantially equal luminous transmission located between the light source and the polariser on the input substrate of the liquid crystal cell, so that the twisted nematic liquid crystal cell with the dichroic dye added, and the two polarisers in combination with the luminance-balanced color mask, cause the output panel to appear uniformly dark to an observer when the multicolor liquid crystal display device is in its 'off' state. The multicolor liquid crystal display device as claimed in claim 1, wherein the cell has a plurality of electrodes on each substrate surface, and in which each electrode is capable of being individually addressed by the voltage-applying means to create information readable by an observer. The multicolor liquid crystal display device of claim 2, wherein the plurality of electrodes on each substrate surface comprise a series of closely-spaced electrodes, the electrodes on each of the substrate surfaces being mutually perpendicular so that the application of a potential across selected electrodes permits matrix addressing. The multicolor liquid crystal display device of any preceding claim, wherein the liquid crystal/dye mixture is Hoffmann-LaRoche ROSA-605 or equivalent, and the thickness of the layer of the mixture is 10 µm. The multicolor liquid crystal display device of any preceding claim, wherein the light source is a fluorescent light having a uniform overall spectrum with a high red light content. The multicolor liquid crystal display device of any preceding claim, wherein the mask has on it color patches which are luminance balanced by dimming the colors to which the eye of an observer is more sensitive with a matrix of relatively-small opaque dots, and/or by brightening the colors to which the eye of an observer is less sensitive, by a matrix of relatively-small transparent holes. The multicolor liquid crystal display device of claim 6, wherein the opaque dots and transparent holes are photographically produced in the color patches by the use of six cots/mm screens, the transparent holes comprising no more than 10-20% of the total color patch area, and the opaque dots comprising no more than 30-50% of the total color patch area. A method of providing a multicolor liquid crystal display device selectively capable of being in the 'on' or 'off' state and providing a uniformly-dark output panel when in the 'off state, comprising the steps of: providing a liquid crystal cell (320) having a pair of parallel, transparent substrates (71,72) with transparent electrodes (73) on their confronting surfaces, and a twisted nematic liquid crystal material contained between the substrates; providing a light source (314) for directing light to one ot the substrates, this substrate (72) being the input substrate: attaching high extinction ratio polarizers on each of the substrate surfaces remote from the confronting surfaces, the polarizers being of the type which enables the attainment of a contrast ratio of greater than 100 from the multicolor display, the polarising axes of both polarisers being either parallel with or orthogonal to the director of the liquid crystal molecules adjacent the surface of the input substrate; mixing a relatively small amount of dichroic dye with the liquid crystal material to form a guest-host mixture of liquid crystal and dye molecules for the purpose of absorbing, and thus preventing the build-up and subsequent passage of, the orthogonal vector component of polarised light created by imperfect rotation of the polarised light by the orientation of the twisted nematic liquid crystal molecules; providing means for applying an alternating voltage across the electrodes (73) to change the orientation of the molecules of the mixture, and to enacle the passage of light when desired, and inserting a luminance-balanced color mask (317) having colored patches and a gray surround area, the colored patches being luminance-balanced with each other and the gray surround area in order that the colored patches and gray surround area are of substantially equal luminous transmission, the luminance-balanced mask being between the light source (314) and the polarizer (324) on the input substrate of the liquid crystal cell, so that the twisted nematic liquid crystal cell with the dichroic dye added and the two polarizers in combination with the luminance-balanced color mask cause the output panel to appear uniformly dark to an observer when the multicolor liquid crystal cisplay device is in its 'off' state. A xerographic reprographic machine with a multicolor liquid crystal display device as claimed in any of claims 1 to 7.

Inventors: DIR GARY A, DIR, GARY A.
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1984
Main classification: G02F1
IPC codes: G02F1