CA1191587A - Thermally addressed liquid crystal device - Google Patents

Abstract

THERMALLY ADDRESSED LIQUID CRYSTAL DEVICE

ABSTRACT OF DISCLOSURE

A visual display device is featured which uses a new thermal electro-optic effect to provide a dark image upon a ligher background. The display is capable of being multiplexed to a large number of rows. The device comprises a liquid crystal material mixed with a pleochroic dye of high order parameter. When the material experiences a rapid transition from a higher to a lower, smectic thermal phase, two different textures or light states are developed; a transparent state and a light absorbing state.
The transparent state is developed by applying a sensitizing voltage to certain portions of the medium. The pleochroic dye absorbs the light passing through the unsensitized portions of medium to provide the dark image.
Heating electrodes are made diffusely reflective so as to provide a visual display that has high contrast and wide viewing angle.

Description

2 5 1 , 2 4 0 C A N / W B B

THERMALLY ADDRESSED LIQUID C~YSTAL DEVICE

FIELD OF THE INVENTION
The invention pertains to thermally addressable liquid crystal display devices, and more particularly to thermally addressed visual display devices which use a light absorption technique to provide a dark image upon a lighter background.

BACKGROUND OF THE INVENTION
Heretofore, the ability to Eabrica~e large scale multiplexed liquid crystal displays was very dificult.
This difficulty was primarily due to "cross-talk" efects, and the necessity to quickly refresh the slow responding liquid crystal medium. Large scale multiplexed displays notoriously have had problems with "cross-talk", i.e. t the unwanted sensitizing o partially selected display elements. This problem results from the small root mean square voltage ratio between the "on" and "off" elements achievable in a large scale multiplexed liquid crystal display.
As displays become larger, a new problem appears.
Most device effects do not have intrinsic storage. The display must therefore be repeatedly scanned to update;
this is o~ten with typical display effects done a~ 60 Hz (per frame). The result ~or large area matrices is a small 25 duty cycle for each individual row or column. Most display media only partially respond to small duty cycle voltage information and the resulting effect is only a fraction of the dc equivalent voltage. The result is low contrast or brightness. As the display matrix gets larger, the duty 30 cycle gets less and less and optical performance gets poorer and poorer. The result is a very poor (below commercial standards) optical performance as the X-Y matrix gets larger and larger.
These two problems have severely limi-ted 1~9~5~7 the ability to provide large scale multiplexed displays, and to dateJ not one has produced a device which has high contrast, wide viewing angle, is easy to fabricate, easy to operate, and which has low cost.
The pre~ent invention has developed a low cost, large scale multiplexed, visual display device that has resolved the aforementioned problems, while providiny a new liquid crys~al device having many advantages over the prior art.
While the present invention is concerned primarily with large scale, thermally addressed multiplexed devices, its new light absorbing method is easily applicable to devices which are not large scale, and which do not utilize multiplexing. The subject invention is 15 believed to have wide applica~ion in the field of thermally addressed liquid crystal displays, and is not considered as being limited to any particular device or system.

DISCUSSION OF RELATED ART
The invention features certain classes of smectic 20 liquid crystal hosts that have a nematic phase upon heating. A small percentage of pleochroic dye is added to the material~ The display is addressed in a thermal electric mode. For the "on" elements, the liquid crystal texture is absorbing due to the dye which strongly absorbs incoming light. The "off" elements and the background have homeotropic smectic A texture, where the dye exhibits minimum absorption.
The concept of pleochroic dye switching as the Guest Host effect in nematic liquid crystal, was first 30 suggested in an article to: G. H. Heilmeier, J. A.
Castellano, and L.A. Zanoni, Mol Crystals and Liquid Crystals 8, 293 (1969)o Others have suggested that the liquid crystal structure can be twist nematic, homogeneous, or 35 homeotropic. Most of these devices using pleochroic dyes mixed with the liquid crystal material have generally required external devices such as polarizers or wave plates to improve the contrast of the image.
Dyes of high order parameter in a cholesteric liqud crystal host were first suggested in an article to:
D. L. White, G. N. Taylor, J. of Appo Phys. 45 4718 (1974).
~ isplays using this liquid crystal medium have high contrast and do not require external ploarizers.
These displays have high brightness and a wide viewing angle not available with ~he field effect twist nematic lO liquid crystal displays~ Such devices use a cholesteric to nematic transition efect with liquid crystals of positive dielectric anisotropy.
In the no field (off) mode, the dye molecules follow the helical structure of the host and exhibit strong l5 light absorption. In the on condition, the dye is in a homeotropic nematic host and the absorption is minimized.
Thus, the display presents a white image against a dark (or colored) background. A white image against a dark background is, however, generally not desirable.
Recently, high order parameter and ligh~ stable dyes have become available. Devices using these dyes will provide viable displays for many applications. However, they have two major drawbacks which may restrict their application to simple displays of very low information 25 content only.
These dye displays are very difficult to multiplex. Even a few rows represent a state oE the art development. Large size matrix addressing has only been achieved by adding external non-linear elements to each 30 display elements.
For non-emissive (reflective) displays, a white image against a dark background is formed. This is esthetically undesirable, and of limited commercial utility. Techniques to reverse the îmage contrast to a 35 more pleasing dark against a light background are available, but the added complication increases the complexity and cost.

In 1978 C. Tani and T. Ueno discussed the application of pleochroic dyes to certain smectic liquid crystals in a scientific paper (~ppl. Phys. Lett. Vol. 33 No. 4, 15 Aug. 1978). The authors, however, specifically teach aqainst the use of the smetic "Al' phas~ as having utility in the pleochroic dye system: they indicate that it has application only in scattering applicaitons such as in laser addressed light valves~ They concluded that only materials having smectic H or possibly B phase structure have useful properties in combination with pleochroic dyes.
Further, they discuss the utilization of slow cooling as having utility with pleochroic dyes and that rapid cooling of the elements is ony applicable to light scattering devices.
The present invention utilizes pleochroic dyas to produce an absorbing state rather than a scattering state and uses thermal XY local heating as distinct from the laser heating as described in other art. Further it utilizes rapid cooling of the element with liquid crystals preferentially of the smectic "A" phase. The last factor is directly against the teaching oE Tani and Ueno, but has been found to be most effective in this application.
Also recently, a system has been reported in the French literature, which uses a thermally addressed smectic "A" crystal medium. Such a system is described in an article entitled: MATRIX ADDRESSED SMECTIC LIQUID CRYSTAL
DISPLAY: M~ Hareng, S. Le Berre, R. Heh]en, and J. N.
Perbet, Thomson-CSF Laboratoire Central de Rechereches.
Society of Information Display, Conf. Record of 1980 Biennial Display ResO Conf. "Post deadline paper".
Such a system does not use dyes, and employs a sca~tering light t~chnique, rather than a light absorption technique as decribed by this invention.
In addition, the system described is embodied in a very different device than detailed in this invention.
Because of the crucial difference of the light scattering as compared to light absorption, the device can only be `

l5~

viewed through a projection optical system that results in a very bulky, power intensive system.
While the prior art teaches the use of pleochroic dyes of high order parameter Eor use in liquid crystal media, it should also be noted that these dyes are used primarily to enhance the ligh~ a~fects produced by the thermal phase transition of the media. The invention by contrast, relies upon the dye to do most of the light absorption for the crystal medium; the medium acting as a vehicle for orientating the dye to develop a light absorbing stance.

BRIEF DESCRIPTION OF T~E lNv~N~ oN
The invention relates to a thermally addressed visual device which provides a dark image against a lighter background. The device comprises a liquid crystal medium including at least one liquid crystal compound mixed with at least one coloring agent, generally a pleochroic dye of high order parameter. The medium has positive dielectric anisotropy. The medium is thermally sensitive and has a transition between at least two thermal phases; the lower thermal phase is a smectic phase. The medium develops two textures in the smectic phase: a light absorbing texture and a homeotropic texture. The homeotropic texture is developed in portions of the medium by sensitizing the medium as it passes rapidly from an upper thermal phase to the lower, smectic phase~ The light absorbing texture develops in the unsensitized portions of the medium as it goes through the transition to the smectic phase.
The medium is sensitized by applying a voltage to those portions of the medium to be addressed. The addressed portions develop a substantially transparent light state, while the unaddressed portions develop a sub-stantially light absorbing state. The coloring agent or dye which is locked within the liquid crystal medium as it develops its light absorbing texture in the smectic phase will absorb most of the light passing through the medium;

5 Sl~
^ 6 _ the liquid crystal acting as a vehicle to orient the dye molecules into a light absorbing position. Electrodes are provided to sensitize the medium. They are disposed adja~
cent the medium. Heating electrodes are also provided to 5 heat the medium to an upper thermal phaseO In a multi-plexed device, these electrodes define a matrix of columns and rows disposed substantially at right angles to each other, and in different planes.
In order to obtain a pleasing direct viewable 10 display device, the row electrodes are made diffusely reflective to provide high contrast as well as wide viewing angle. The reflective electrodes provide for a double pass of light through the cell enhancing light absorption.
The liquid crystal medium will generally contain an octyl cyano biphenyl compound and will have two thermal transitions: between an isotropic and nematic phase, and hetween the nematic and a smectic phase, in particular a smectic "A" phase.
More particularly, the liquid crystal will be comprised of a mixture of cyano biphenyl compounds of the following formulas:

C8H17 ~ ~ ~ ~ -CN

ClOH21 ~ ~ ~ ~ -CN

In one type of mixture, the octyl cyano biphenyl can be represented in the mixture in a weight percentage from approximately 35 to 6S, and the deca cyano biphenyl can be represented in a weight percentage from approxi-mately 30 to 60.
In another type of mixture, there can be added ~o the above materials cyano alkyl oxy biphenyl compounds of the following formulas:

:~glS87 C8H17 ~ ~ ~ -CN

ClOH21 ~ ~ ~ ~ ~ -CN

The octyl cyano oxy biphenyls can be represented in the mixture in a weight percentage from approximately 15 5 to 30 and the deca cyano oxy biphenyl in a weight percen tage ~rom approximately 11 to 26 while the octyl cyano biphenyl is approximately in the range 20 to 35 and the deca cyano biphenyl is approximately in the range 24 to 39 by weight The pleochroic dye can be:
4~4'-N=N-Dimethylamine phenylazo~ azobenzene r and can have a weight percent range rom 0.5 to 3.0 of the total composition.
More particularly, the octyl and deca cyano 15 biphenyls may be mi~ed by weight 55.6 and 44.4 percent, respectively, and the 4(4'-N=N-Dimethylamino phenylazo) azobenzene can have 1 to 1.5 weight percent of the total composition.
Also, the octyl, deca, octyl oxy, and deca oxy 20 cyano biphenyls can be mixed by weight percentage 27.5, 31.5, 22.5, and 18.5, respectively. The dye will be mixed with the above in 0O75~ to 1.75% weight percent of the total, respectively.
In operating the device, the liquid crystal 25 medium is passed through its thermal transition from its upper thermal phase to its lower thermal, smectic phase.
The transition must be accomplished reasonably rapidly, hence rapid thermal pulses are used that heat the liquid crystal locally but do not signiicantly heat the 30 surrounding glass. Hence the natural cooling period immediately following the passage of the heat pulse is also rapid and hence the liquid crystal medium passes through the nematic phase rapidly. This greatly enhances ~he optical effect and results in greater contrast.

`..;

~1~315137 Certain portions of the medium are sensitized.
These portions define the background of the medium. These sensitized portions develop the substantially light trans-parent state when the medium passes into the smectic ther-5 mal phase. The remaining unsensitized portions oE themedium develop a light a~sorbing state~ When light (generally ambient) is passed through the medium, the unsensitized portions absorb the light to provide a dark image upon the lighter sensitized background. The 10 addressed portions of the medium may be sensitized in a chronological sequence.
It is an object of this invention to provide improved liquid crystal device, method, and liquid crystal compositions;
It is another object of the invention to provide an improved large scale, multiplexed, thermally addressed visual display;
It is a further object of this invention to pro-vide a highly contrasted dark image on a lighter background 20 or a thermally addressed liquid crystal device.
These and other objects of the invention will become more apparent and will be better understood with reference to the following detailed description considered in conjunction with the accompanying drawings.

Figure 1 is a perspective, exploded, schematic view of a visual device made in accordance with the invention;
Figure 2 is a plan schematic view of the device 30 shown in Figure 1, illustrating how an image can be formed in the liquid crystal medium by a multiplexing technique;
Figure 3 is a graphical illustration of the chronological sequencing of the row and column electric waveforms of the device depicted in Figure 1.
Figures 4a and 4b ~how a schematic view of two different light modulating textures developed in the liquid LS15'7 g crystal medium of the device of Figure 1, when the medium passes into its smectic phase; Figure 4a depicts a homeo-tropic, substantially light transparent texture, and Figure 4b illustrates a substantially light absorbing texture.

Generally speaking, this invention relates to new methods, compositions, and visual devices utilizing the thermal addressing o liquid crystal media. The visual devices of this invention feature a highly contrasted dark image on a lighter background O
Where the devices of the invention are multi-plexed, they are capable of being multiplexed up to a large number of rows.
This invention provides new displays that incor~
porate pleochroic dyes of high order parameter into a smectic A liquid crystal material that has a nematic phase upon heating. By using a thermal electric addressing tech-nique described hereinafter, this display has major advan-tages over the previously known dye switching displays based on the gues~ host effect and the cholesteric to nematic transition effect.
Smectic A phase is one of the most common liquid crystal phase~ encountered. Some materials having smectic A phase also exhibit a nematic phase upon heating. One example is ~he octyl cyano biphenyl:

C8~17 ~ ~ - ~ -CN

which has a phase transition as follows The smectic A phase can be aligned homeotropic-ally, as shown in Figure 4a, if the surface of the display is treated with materials such as lecithin. In this struc-ture, the material is transparent.
.~' 15~'7 - 1 o There are two forms of thermally addressed smectic A displays. One type uses a scanning laser beam to address the display elements. The other type is x y matrix addressed. The row electrodes are heated sequentially with electric current and the display is written by applying voltages on the columns. ~uring the writing process, only the dots associated to the row where the heating current has just been removed are affected. In other words, only the dotswhere the liquid crystal material is rapidly cooling to the smectic state respond to the column voltage pulses.
As the liquid crystal material cools rapidly through the nematic phase to the smectic phase, it can form two different textures. With a voltage applied on the 15 column, the liquid crystal material is switched to a homeo-tropic state during the nematic phase and assumes the homeotropic smectic A texture after cooling is completed~
Without the applied voltage, a light absorbing texture is developed instead~ Thus, the dots associated with a 20 rapidly cooling row electrode can be written into trans-parent state or a light absorbing state by applying or not applying voltages on the columns. The smectic material used in the inventive display device has positive dielec~
tric anisotropy.
The transition must be accomplished reasonably rapidly; hence, rapid thermal pulses are used that heat the liquid crystal locally but do not significantly heat the surrounding glass. Hence the natural cooling period imme-diately following the passage of the heat pulse is also 30 rapid and hence the liquid crystal medium passes through the nematic phase rapidly. This greatly enhances the optical effect and results in greater contrast. Table 1 below, illustrates this effect:

5~3'~

Impact of Rapid Cool Rapid Cool(l) Slow Cool(2~
Test# CR Brightness CR Brightness 1 3.1 28% 1.1 29%
2 5.2~ 34% 1.0 33~

3 6.6 29% 1.2 29%

4 8.~ 28% 1~2 29%
8.7 32~ 1.2 31%

(1) Typical rapid cool; greater than 500C per sec.
~2) Typical slow cool; less than 500C per sec.

The present invention, however, must be carefully distinguished from other similar systems wherein a scattering texture rather than a light absorbing texture is developed in the smectic material~ ~isplays developing the scattering texture are generally not suitable for direct viewing, and are often used only in projection systems~
The optical contrast developed by a scattering texture against a transparent texture is similar to ~hose obtained with the dynamic scattering effect. Under many commonly encounted illumination conditions, it will not give a pleasing legible, high contrast image.
The situation becomes quite different, however, when a pleochroic dye of high order parameter is introduced into the smectic A material. The dye becomes locked into the liquid crystal, and assumes the orientation of the liquid crystal molecules. When a randomly oriented texture is produced due to rapid cooling through the nematic phase to the smectic phase, the dye molecules in the host absorb light strongly, transforming it into a light absorbing tex-ture, either deeply colored or black. In fact, the ran-domly oriented texture may not scatter light as it usually does, if liquid crystal materials of very low birefringence are used. It will, however, strongly absorb the light due . . , 58~7 to the orientation of some of the dye molecules having their light absorbing axis partially transverse to the light pathO In the homeotroplc smectic -texture, the dye molecules have minimum absorp~ion, since they do not absorb

5 light inciden~ upon the edge of their molecular structure.
This texture, therefore, develops a clear tex~ured back-ground. This results in a high contrast display that is suitable for direct viewing. No external po:Larizers are required. The addressiny technique is substantially the 10 same as smectic displays without the dyea Now referring to Figure 1, an exploded view of a typical multiplexed, visual display device 10, is illustrated. The device comprises a liquid crystal medium 11 containing the pleochroic dye, which materlal is 15 disposed between ~wo glass substrate plates 12 and 13, respectively. The top substrate plate 12 supports a plurality of column electrodes Cl, C2, C3t etc., which make up one half of the x y matrix for addressing the liquid crystal material 11~ The column electrodes are made of 20 electrically conductive, light transmitting material such as indium tin oxide.
The bottom plate 13 supports a plurality of row electrodes rl, r~, r3, etc., which make up the remaining half of the x y matrix. The row electrodes are electric-25 ally conductive and light reflective, and can be, forexample, silver or aluminum, which can be deposited on the glass plate 13.
The liquid crystal medium 11 is generally sealed between the two substrate plates 12 and 13 with the elec~
30 trodes in contact on either side. Light (generally ambient) is passed through (arrow 18) the glass composite, as shown.
The physical operation of this display 10 can best be illustrated with a simple example of a 5 X 7 matrix 35 displaying a character "A", as shown in Figure 2. The rows of the matrix are tied together at one end to the common 16 and are sequentially heated by applying electric pulses to the other ends 17. In time æone 0, (see Figure 3) row 1 is heated such that -the liquid crystal material over the row 1 electrode r1 is in the isotropic state. In time zone 1, row 2 electrode r2 is heated. In the meantime, row 1 rapidly cools down and the dots associated with it are written by applying electric voltage on the column elec-trodes. In this example, electrodes Cl and C5 have voltage applied such that the dots rlcl and rlc5 will be in the clear state. C2, C3, C4 have no voltage applied, and the dots rlc2, rlc3, rlc4 have a colored light absorbing texture. During time zone 2, row 3 electrode r3 is heated and row 2 rapidly cools down, and the voltage on the columns assume the values corresponding to the l'on'l and lloff" pattern of dots associated to row 2. The entire wave-form for displaying a character IIAI', is shown in E'ig. 2.
The colored light absorbing texture associatedwith the "on" dots is metastable and has long relaxation time of the order of at least months. This texture can be automatically erased by heating the row during rewriting of 20 the display, or can be erased by applying a voltage on the columns substantially higher than the wri~ing voltage.
Once the colored light absorbing texture is formed, it will not be affected by the writing or sensitizing voltage.
This assures that "cross-talk" will not be a problem, and 25 makes possible a large scale matrix display~
The erase-writing process for this display is very fast. Generally, less than a 100~second writing time can be achieved. If the display is refreshed at fr times per second, the total number of rows that can be multi-30 plexed will be:
r x Tw Where Tw = the time required to write the row.
With fr = 30 hertz, which is a similar rate as aconventional CRT, and Tw = 100~sec., we have n = 333 rows.
35 Thus, the display can be multiplexed up to a rather large number of rows.

In practical display driving, the heating pulse can be applied over several time zones before the cooling and writing cycleO This lowers the voltage requirement for the heating pulses. However, the heating pulse should be short enough to avoid heat spreading to the neiyhboring rows and to minimize glass hea~ing that then prevents the rapid cooling necessary for yood optical images.
A high contrast is achieved for the colored or black image due to the light absorbing character of the dye 10 material vis-a-vis the transparent background~
The contrast is further improved by the reflec-tive nature of the heating electrodes, which provide a double light pass back through (arrow 15~ the medium ll, wherein the unaddressed dye molecules in the light 15 absorbing state (image) can absorb more light as compared to the addressed transparent background.
The medium ll is depicted in the sensitiæed (addressed) homeotropic phase in Figure 4a, and is shown in the unaddressed light absorbing phase in Figure 4b. 1ight (arrow 20) entering the homeotropic material of Figure 4a, passes between the liquid crystal molecules 21. The dye molecules 22 are not light absorbing in this phase r because they are locked in the crystal to confront the light rays upon their edge, as shown.
However, in the light absorbing phasev the dye molecules 22 are locked in the crystal molecules 21 in a randomly angled pattern, as shown in Figure 4b. In this phase t the dye molecules 22 will strongly absorb the impinging light rays 20 to produce an intensely colored or 30 dark imageO
The crystal liquid medium ll can be comprised of at least one sctyl cvano biphenyl compound.
More particularly, the liquid crystal will be comprised of a mixture of cyano biphenyl compounds of the following formulas:

C8H17 ~ - ~ -CN
C10~21 ~ ~ ~ ~ -CN
CllH23 ~ ~ -CN

In one type of mixture, the octyl cyano biphenyl can be represented in the mixture in a w~ight percentage from approximately 35 to 55, and the decyl cyano biphenyl can be represented in a weight percentage from approximately 30 to 60.
In another type of mixture, the undecyl cyano biphenyl can be added to the above, and can have an approximate weight percent range from 15 to 35.
The pleochroic dye can be:
4(4'-N=N-Dimethylamino phenylazo~ azobenzene, and can have a weight percent range from 0.5 to 3~0 of the total composition.
More particularly, the octyl and decyl cyano biphenyls may be mixed by weight 55O6 and 44.4 percent, respectively, and the 4(4'-N=N-Dimethylamino phenyla~o) azobenzene can have 1 to 1,5 weight percent of the total composition, Also the octyl decyl, and undecyl cyano biphenyls can be mixed by weight percentage: 44~4; 31.3; and 24.5, respectively~ The dye will be mixed with the above in 1 to 1,5 weight percent of the total composition.
Other compositions of the liquid crystal medium can be:

C10H21 ~ ~ ~ ~ - CN 4 - Cyano - 4~ - n -decyl biphenyl 45% by weight C8H17 ~ ~ ~ ~ - CN 4 - Cyano - 4~ ~
octyl biphenyl 55%; or l~lS~';'' - 1.6 -C8H17 ~ ~ - CN 4 - Cyano - 4~ ~
octyl biphenyl 27~5% by wt.

r~ , ClOH21 ~ ~ ~ ~ - CN 4 - Cyano - 4 decyl biphenyl 31.5%

C8H17 ~ ~ ~ - CN 4 - Cyano - 4' -octoxy biphenyl 22,5~

ClOH21 ~ ~ - CN 4 - Cyano - 4' -decyloxy biphenyl 18.5%; or C2H5 - ~ -CN

4' - Ethyl - trans, trans - bicyclohexyl - 4 -carbonitrile.

To the above liquid crystal compositions, any one of the following coloring agents may be added:

N ~ - N = N - ~ - N = N - ~ - N = N ~ ~ N 1% by weight; or c f 2 wt.; or .

151S'~

a dye mixture 1.4% by weight of:

C2H5 0,CH2 C2H~
30% wt. ~ N - ~ - N = N - ~ - N = N - ~ - N = N - ~ -N
C~l2 H

54% C4Hg - ~ - CH = N - ~ - N = N - ~ - N = CH - ~ - OC~Hg O NH - ~ ~ C5 16% C5~110 - ~ - NH

Having thus described the invention, what is desired to be protected by Letters Patent is presented by the following appended claims:

Claims (34)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In a thermally addressed visual display device having a liquid crys-tal medium including at least one liquid crystal compound mixed with at least one coloring agent, the method of displaying a dark image against a lighter background, comprising the steps of: a) passing said medium through a rapid thermal transition from an upper thermal phase to a lower thermal, smectic phase; b) sensitizing certain portions of said medium defining a background to develop a substantially light transparent state in said background when said medium passes rapidly into said lower thermal, smectic phase, the remaining unsensitized portions of said medium developing a light absorbing state; c) pas-sing light through said medium, and d) absorbing light in said unsensitized portions of said medium to provide a dark image upon said lighter sensitized background.

2. The method of claim 1, wherein said dark image is observable from the same side as the light passing through said medium, and comprising the further step of: e) diffusely reflecting the light passing through said medium back through said medium.

3. The method of claim 1, wherein said medium has dielectric anisotropy and further wherein the sensitized portions of said medium are sensitized by the further step of: e) applying a voltage to said certain portions of said medium in order to sensitize said certain portions into developing said light transparent state.

4. The method of claim 3, wherein said voltage is applied in a given chronological sequence to said certain portions of said medium.

5. The method of claim 1, wherein said sensitized portions of said medium are sensitized in chronological sequence.

6. The method of claim 1, further comprising the step of: e) passing said medium through said transition from said lower thermal, smectic phase to said upper thermal phase prior to step (a).

7. The method of claim 6, wherein said transition step (e) is accom-plished heating said medium.

8. The method of claim 1, wherein said transition step (a) is accom-plished by rapidly cooling said medium.

9. The method of claim 1, wherein said medium passes through rapidly two thermal transitions between an upper isotropic phase, an intermediate nematic phase, and a lower smectic phase.

10. The method of claim 1, wherein said light passing through said medium in step (c) comprises ambient light.

11. In a thermally addressed visual display device having a liquid crys-tal medium including at least one liquid crystal compound mixed with at least one pleochroic dye of high order parameter and having dielectric anisotropy, said medium being thermally sensitive and having a transition between an upper and a lower thermal phase, said lower thermal phase being a smectic phase, said medium having two possible light modulating states in said lower thermal, smectic phase characterized by development of respective light absorbing and homeotropic textures in said medium, said light absorbing texture being normally substantially light absorbing and said homeotropic texture being substantially transparent, the method of displaying a dark image against a lighter background, comprising the steps of: a) rapidly passing said medium through said transition from said upper thermal phase to said lower thermal, smectic phase; b) sensi-tizing certain portions of said medium defining a background to develop said second substantially light transparent state in said background when said medium passes rapidly into its lower thermal, smectic phase, the remaining unsensitized portions of said medium developing said light absorbing state;
c) passing light through said medium, and d) absorbing light in said unsensi-tized portions of said medium to provide a dark image upon said lighter sen-sitized background.

12. The method of claim 11, wherein said dark image is observable from the same side as the light passing through said medium, and comprising the further step of (e) diffusely reflecting light passing through said medium back through said medium.

13. The method of claim 12, wherein the sensitized portions of said med-ium are sensitized by the further step of: (e) applying a voltage to said certain portions of said medium in order to sensitize said certain portions into developing said light transparent state.

14. The method of claim 13, wherein said voltage is applied in a given chronological sequence to said certain portions of said medium.

15. The method of claim 12, wherein said sensitized portions of said medium are sensitized in chronological sequence.

16. The method of claim 12, further comprising the step of: (e) passing said medium through said transition from said lower thermal, smectic phase to said upper thermal phase prior to step (a).

17. The method of claim 16, wherein said transition step (e) is accom-plished by heating said medium.

18. The method of claim 12, wherein said transition step (a) is accom-plished by rapidly cooling said medium.

19. The method of claim 12, wherein said medium passes rapidly through two thermal transitions, namely between an upper isotropic phase and an inter-mediate nematic phase, and between said intermediate nematic phase and a lower smectic phase.

20. The method of claim 12, wherein said light passing through said medium in step (c) comprises ambient light.

21. A liquid crystal composition which can be thermally addressed and which can provide a dark image upon a lighter background, comprising a mixture comprising approximately from 35 to 65 weight percent of "x" and approximately from 30 to 60 weight percent of "y", wherein "x" and "y" are represented by the formulas:

at least one pleochroic dye of high order parameter approximately from 0.5 to 3 weight percent based upon the total composition mixture.

22. The liquid crystal composition of claim 21, wherein said pleochroic dye comprises:
4(4' -N=N-Dimethylamino phenylazo) azobenzene

23, The liquid crystal composition of claim 21, further comprising approx-imately from 15 to 30 weight percent of u and 11 to 26 weight percent of u together with 20 to 35 weight percent of x and 24 to 39 weight percent of y, where u is -22-

24. A. liquid crystal composition which can be thermally addressed and which can provide a dark image upon a lighter background, comprising a mixture comprising approximately 44 percent by weight of "x" and approximately 56 percent by weight of "y", wherein "x" and "y" are represented by the formulas:

at least one pleochroic dye of high order parameter approximately from 1.0 to 1.5 percent by weight based upon the total composition mixture.

25. The liquid crystal composition of claim 24, wherein said pleochroic dye comprises:
4(4'-N=N-Dimethylamino phenylazo) azobenzene.

26, A liquid crystal composition which can be thermally addressed and which can provide a dark image upon a lighter background comprising a mixture comprising approximately 44 percent by weight of "x", approximately 27.5 per-cent by weight of "y", and approximately 31.5 percent by weight of "v", and approximately 22.5 weight percent of u and approximately 18.5 weight percent of v, wherein x, y, and v are represented by formulas:

at least one pleochroic dye of high order parameter approximately from 1.0 to 1.5 percent by weight based upon the total composition mixture.

27. The liquid crystal composition of claim 26, wherein said pleochroic dye comprises:
4(4'-N=N-Dimethylamino phenylazo) azobenzene.

28. An electro-thermally addressed visual display device utilizing a medium containing at least one liquid crystal compound mixed with at least one pleochroic dye, said medium having at least two thermal phases, one of said phases being a smectic phase and including means for heating said medium to cause a transition to another phase, thereby altering the optical properties of the device, characterized by a) said medium comprising a liquid crystal compound having a type A smectic phase and having dielectric anisotropy, and a pleochroic dye of high order parameter, the high order parameter of the dye providing in combination with the smectic A phase of the liquid crystal com-pound an electrically orderable texture in which the mixture is substantially transparent, while normally, in the absence of such electrical ordering, the mixture has a substantially light absorbing texture, and b) means for applying a sensitizing voltage to address portions of said medium when said medium is in said another phase and is rapidly cooled into said smectic A phase to develop said substantially transparent ordered texture while the remaining portions develop said substantially light absorbing texture, thus providing a dark image against a lighter background.

29, A device according to claim 28, wherein said medium has at least three thermal phases, a said smectic A phase, an intermediate nematic phase and an upper isotropic phase, further comprising means for rapidly passing the medium between said upper isotropic phase and said intermediate phase and between said intermediate phase and said lower smectic phase.

30. A device according to claim 28, characterized by said liquid crystal compound comprising an alkyl cyano biphenyl.

31. A device according to claim 28, characterized by means for rapidly heating said medium to said another phase without substantially heating the surroundings such that upon cessation of said heating pulses the medium rapidly cools to said smectic A phase, including at least one electrode adjacent said medium for heating said medium to said another phase.

32, A device according to claim 31, further characterized by said heating electrode being disposed adjacent said medium for diffusely reflecting light passing through the medium back through the medium to thus provide a dark image observable from the same side as a light source passing light toward said device.

33. A device according to claim 28 characterized by at least one elec-trode for applying a sensitizing voltage to said addressed portions of said medium.

34. A device according to claim 28, further characterized by a matrix of electrodes disposed about said medium, a number of said matrix electrodes defining rows and a number of said matrix electrodes defining columns, said row electrodes for heating said medium to cause said medium to pass through a thermal transition between said smectic A phase and said another phase, and said column electrodes for applying sequential sensitizing voltages to address portions of said medium to develop said substantially transparent texture when said medium rapidly cools and passes rapidly into said smectic phase, the remaining unaddressed portions of said medium developing said substan-tially light absorbing texture.