GB1586123A - Light valves - Google Patents

Description

(54) LIGHT VALVES (71) We, RESEARCH FRONTIERS INCORPORATED, of 31 Cain Drive, Plainview, New York, United States of America, a corporation organised under the Laws of the State of 'New York, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to light valves; more particularly, it relates to changing the optical transmission of a liquid suspension that includes a dissolved halogen by the addition of a compound that includes in its molecular structure a large percentage of atoms of another halogen having an atomic weight less than the atomic weight of the dissolved halogen.

In the making of suspensions having halogen as an ingredient in the formation of the suspended particles, an excess amount of halogen is very often used to ensure that all or almost all of the suspended particles have the desired amount of the halogen in the molecular structure thereof and thereby have suitable properties and are properly formed. If insufficient halogen was used, some or all of the particles would be of little value and they might be improperly formed. However, the excess or residual halogen present has the following disadvantages. It goes into solution in the suspending medium to colour the suspending medium, an especially undesirable result since the desire is to have the suspending medium either clear or tinted in a predetermined colour.A typical situation is the one with iodine as the halogen where the iodine will tint the suspension an undesirable colour, usually yellow.

Thus, it would be highly desirable if this residual dissolved halogen could be removed from solution.

Also, in fluid suspensions used in light valves, especially ones where the suspended particles'are polarizing particles, when the particles are formed, they very often have a thin coating of a halogen on the surface thereof. This coating is very advantageous in that it improves the functioning of the particles in the light valve and improves the thermal stability thereof. However, the coating very often does not remain on the particles after the formation thereof. Some of it quickly goes into solution in the same manner as with the residual dissolved halogen mentioned above. It would, therefore, be desirable to return the halogen to the surface of the suspended particles. Furthermore, if the particles are formed with insufficient halogen coating, it is desirable to add an amount of halogen to the surface of the particles.

Halogen compounds have been mentioned in the prior art as being used in fluid suspensions containing polarizing, halogen-containing particles. However, the halogen compounds were ones where the amount of halogen atoms in the molecule in proportion to the total number of atoms was insufficient to yield the desired result, i.e. at least one, preferably at least two, halogen atoms per non-halogen atoms of the molecule. These prior-art compounds were not used for and would not work for the present purposes.

Also, if a compound is deleterious to the suspension and especially to the particles, the compound obviously is not useful for the purposes of the present invention.

Furthermore, certain volatile compounds containing halogen were used in the formation of light-polarizing particles, but were not used in the suspension for the particles, they were only used in forming the particles themselves. In any case, the compounds were so volatile that they were completely evaporated before the particles were placed in final suspension. They therefore, could not be used for the purposes of the present invention.

The present invention relates to a light valve which comprises a cell containing a liquid suspending medium, suspended therein, particles of one or more halogenated alkaloid acid salts (as hereinafter defined) or metal halides and dissolved therein, a polymeric stabilizer to prevent agglomeration of the particles, the liquid suspending medium comprising a liquid halogenated hydrocarbon having a ratio of halogen atoms to all other atoms therein of greater than 1:1, the halogen atoms of the particles being iodine and/or bromine and the halogen atoms of the halogenated hydrocarbon being of lower atomic weight than the halogen atoms of the particles and at least 60% of the halogen atoms of the halogenated hydrocarbon being fluorine and/or chlorine.

(By the term "alkaloid acid salt" is meant a compound of the type obtainable by reaction of an alkaloid with an acid).

More generally, the present invention is concerned with changing the optical transmission of a liquid suspension by adding a compound containing a large proportion of halogen in its molecular structure to the fluid or fluid suspension, the added halogen having an atomic weight less than the atomic weight of the original halogen in the suspension. The compound containing the halogen may be an aliphatic saturated compound m which the number of atoms halogen comprises a large proportion of the number of atoms in the molecular structure of the compound. There has to be more than one halogen atom for every other atom of the compound's molecular structure. In many cases there are at least two halogen atoms for every other atom of the compoundt s molecular structure. In many cases there are at least two halogen atoms for every other atom.

In one embodiment of the present invention all or nearly all of the suspending medium is composed of the compound.

Fluids containing halogen therein and especially fluid suspensions are used in a wide,' variety of application, in particular light valves, as will be discussed below. The suspensions generally consist of a suspending medium in which a plurality of minute particles are dispersed. The suspended particles are solid, colloidal particles which may be, for example; rod-like or plate-like, A A particular example of a fluid suspension is a suspension of herapathite particles in a suspending medium, such as isopentyl acetate.

The fluid suspension also very often contains a substance that protects the particles in suspension. This substance tends to prevent the particles from grouping together to form large clumps of particles, referred to as "agglomerates". The substance may be a long chain mdlecular copolymer having available OH and/or acidic groups. Some of these materials include: copolymers of 3,5,5 trimethylhexyl acrylate/bis-2-ethylhexyl fumarateX2- hydroxypropyl acrylate/fumaric acid and bis-2-ethylhexyl fumaratei3,5,5-trimethylhexyl acrylate/vinylidene chloride/ mesaconic acid.

Another material which may be used as the protective material is nitrocellulose. This material is well known in the prior art.

In making fluid suspensions, the particles are very often formed in a chemical reaction among certain ingredients. Usually, more of the halogen is added than is stoichiometrically required in the reaction to ensure complete formation of the particles. If an insufficient amount of the halogen is present, either all or some of the particles would be malformed. In either case, the suspension made with these particles is of little value. Thus, when the particles are placed in a suspending medium, such as a liquid, the excess of the halogen, since it cannot be removed, is also placed in the suspending medium with the particles; this excess ingredient is very often detrimental to the suspension, as mentioned above, for example, by colouring the suspension an undesirable colour.If the excess ingredient is a halogen, such as iodine, the suspension may appear to be an undesirable yellowish colour. Such a colour may be a problem, especially in clear or transparent suspensions where no colour is desired. Excess halogen is a problem also in suspensions where a particular colour is desired, and the halogen changes the desired colour to an undesired one. The'present invention envisions the use of particular substances which, when added to the final suspension, will cause the excess halogen in solution in the suspension to be rendered ineffective.Now, in accordance with the present invention, if a material which includes a halogen having an atomic weight less than the atomic weight ol the halogen in solution is added to the suspension, the lower atomic weight halogen will cause the higher atomic weight halogen to cease producing the undesirable effects.

One particular example of the above is the addition of chloroform (CHC13) to a suspension of herapathite particles in a suspending medium of isopentyl acetate. The herapathite particles' are formed by combining quinine bisulthate, iodine and hydriodic acid. The formula for the herapathite particles is 4(C20H24N202) 3H2SO4 2H1 14 6H2O. Therefore, iodine is a significant ingredient in herapathite crystals. To ensure that enough iodine is present properly to form the particles, more iodine is added than may be used stoichiometrically by the quinine bisulphate and hydriodic acid to form the herapathite particles. The excess iodine is allowed to remain with the mass of particles because any process to remove the iodine might remove some of it from the particles and impair the operability thereof. Therefore, when the mass of particles is added to a suspending medium for herapathite, such as isopentyl acetate or diisooctylphthalate, the excess iodine goes into solution.

However, by adding chloroform to this suspension, the chlorine in the chloroform having a lower atomic weight than the iodine will cause the iodine colour to disappear from the solution.

The result of this transfer may often be readily seen. For instance, the herapathite isopentyl acetate suspension is often a purplish colour in appearance. This colour is partially due to the excess iodine in solution. However, as soon as the chloroform is added to the suspension, the suspension turns a blue or bluish colour in appearance. When chloroform is used as above described in a herapathite-isopentyl acetate suspension, the amount of chloroform added will depend on a number of factors. These are: the type of suspended particles; the amount of iodine in solution; the type of suspending medium; the temperature of the system (which affects the solubility of iodine in the suspending medium); and the type of protective material, if any, added to the suspending medium to reduce agglomeration of the particles.

It will be appreciated that for the compound that is added to function properly, it must be soluble in the suspending medium; it should be a relatively poor solvent for the dissolved halogen.

Now referring once again to the addition of chloroform, when the chloroform is added to a herapathite suspension the chloroform must be added in a manner such that it will not affect any of'the desired ingredients of the suspension. One such ingredient is the protective material, nitrocellulose. If too much chloroform is added, the protective material will precipi tate but of solution, causing the particles to group together or agglomerate. At ordinary room temperatures, using nitrocellulose as the protective material, up to 50%,-by weight, of chloroform 'may be added to a herapathite-isopentyl acetate suspension. At higher tempera tures, the amount of chloroform that may be added decreases significantly.For example, only 20% or less chloroform, by weight, may be tolerated near the boiling point of chloroform, 61.29C. If more chloroform is added, the suspension may turn into a gel. The chloroform or other material added should be a non-solvent for the suspended particles, and should be one that is miscible with any other solvent used in this suspension. Also, as mentioned above, the halogen in the compound that is being added should have a lower.atomic weight than the halogen that is in solution for the present invention to be effective. Also, preferably, but not necessarily, the halogen-including substance being added should be of relatively low molecu lar weight, such as chloroform. It should also preferably be a solvent for any protective material which is used to coat the particles.Also, and most significantly, the halogen containing compound is one where, as mentioned above, there are more halogen atoms than the other atoms in the compound. For example, in CHC13 there are three chlorine atoms and only two other atoms. Chloroform is also a material that is aliphatic and saturated, i.e. there are no multiple bonds between carbon atoms.

In addition to chloroform (CHC13) other materials that have been found effective include: carbon tetrachloride (CC14); dibromotetrafluoroethane (CBrF2-CBrF2); sym tetrachlorodifluoroethane (CC12F-CC12F); and polychrlorotrifluoroethylene, the latter being a polymeric oil having a general formula (CF2CFCl)n.

Taking these compounds one at a time, carbon tetrachloride has four chlorines for the single carbon, is aliphatic and is saturated. The dibromotetrafluoroethane, has three halogens for every other atom, i.e. two fluorine atoms and one bromine atom for each carbon atom.

Sym-tetrachlorodifluoroethane also has three halogens for every other atom, i.e. two chlorines and one fluorine for each carbon. Finally, polychlorotrifluoroethylene has two halogens for each carbon. Thus, this relationship of the number of halogens to the other atoms appears to be a critical criterion to the effective functioning of the present invention.

When there is less than this ratio of halogen the benefits of the present invention are not observable.

The boiling points of the last-mentioned polymeric substances may vary depending upon the degree of polymerization. However, briefly the usual boiling points of the aforesaid compounds are as follows: TABLE 1 Name of Compound Boiling Point (0C) Chloroform 61.2 Carbon tetrachloride 76.7 Dibrombtetrafluoroethane 47.3 Sym-tetrachlorodifluoroethane 92.8 Polychlorotrifluoroethylene 220-255 These relatively high boiling points are advantageous because they indicate that these compounds have relatively low vapour pressures at the temperatures at which liquid suspension light valves are operated. High vapour pressures would tend to impair the valves by rupturing the liquid seals, or generating leaks, or bowing the faces of the valves, or otherwise deteriorating the valves.

Some of the halogen-including compounds have more than one type of halogen in the molecular structure thereof. Also, the compounds may either be solid or fluid at room temperature before being added to the fluid suspension.

Care must be taken in choosing the halogen compound, protective substance and suspend ing medium particularly when used at relatively high temperatures, not to exceed the solubility of the protective substance in the suspending medium nor to substantially adversely affect the balance of attraction between the protective substance and the suspended particles.

The suspending medium may be a mixed solvent including the halogen-including compound, or the compound and the suspending medium may be the same so that only one material need be used.

Light polarizing particles may also be formed from quinine bisulphate, iodine and any of the various iodides other than hydriodic acid. When hydriodic acid is used, heraphatite is formed. When other iodides are used, such as potassium iodide, ammonium iodide, rubidium iodide or cesium iodide, other types of light polarizing particles are formed to which the present invention is applicable.

The above-mentioned discussions with regard to suspensions are particularly applicable to fluid suspensions which are used in light valves. A light valve basically comprises a structure for controlling the transmission of light or other radiation. One form of a light valve consists of two parallel sheets of glass or other transparent material which are spaced apart a small distance, such as 20 mils or less. They may be held in this position by inert plastic spacers and the cell sealed with a sealant or adhesive material. Each of the transparent walls usually has a thin coating of a transparent electrically conductive material that is located, for example, on each of the inside facing wall surfaces so that a voltage may be applied across the gap between the two transparent wall sections.This gap is filled with a fluid suspension of the type that has been described above, such as a herapathite isopentyl acetate suspension containing particles in suspension. Herapathite particles, which are polarizing particles, may be oriented on the application of an electric field across the suspension. This field is placed across the suspension by the application of the voltage between the conductive coatings. Normally, the suspension appears darkly coloured, such as a blue colour for herapathite. However, when a field is applied across the suspension the suspended particles become aligned.The particles are tsreferably needlershaped or rod-shaped and have a length to diameter ratio of between 5 ; 1 and 20:1. When they are aligned, the long dimension of the particle usually orients itself so that it is parallel to the applied field. The particles are then perpendicular to the transparent wall sections. In this position, because the particles are so aligned that the person viewing the suspension will be looking at them parallel to the long dimension thereof, the entire suspen sion will appear transparent and light will readily pass therethrough. It is in this position that it is especially important that the suspension be transparent.If the present invention is not used, the iodine will tint the suspension a colour in this aligned ("on") position when the desire is to have a non-coloured, clear and transparent suspension. This tint, for example, may be a yellow tint which will substantially detract from the light transmission capabilities of the suspension and therefore of the entire light valve. Thus, it is especially important for the suspension to be clear in the actuated condition. Also, if it contains excess halogen above a negligible amount, as has been described above, the halogen might also turn the suspension from a blue to a purple colour in the "off" or non-aligned position; and to some other colour if particles other than herapathite are used. This is also unsatisfactory. Thus, in the "on" condition it will not be the desired colour.When the voltage is "off" and no field applied across the suspension, the particles are in a random, non-oriented position due to the forces of Brownian motion. In this condition very little light may pass through the suspension because the particles are randomly oriented and intercept or extinguish more light. When the voltage is removed from across the suspension, Brownian motion immediately takes over to cause this disalignnent from the aligned or "on" condition. The protective substance that has been mentioned is used to prevent agglomeration, that is, when the voltage is applied, the particles which have mutual attraction for each other tend to group together or agglomerate.

It was described above that in certain situations the addition of a halogen-containing compound in accordance with the present invention will not only seem to remove the halogen from solution but will produce a superior particle. This is especially true in fluid suspensions which are used in light valves. When herapathite particles are used in a light valve, the presence of halogen, especially iodine, increases the polarizing properties of the particles.

The suspension then appears darker in the "off" condition. This is a very desirable situation.

This will not affect the "on" or transparent condition. The ratio of optical densities in the dark and light conditions, which is sometimes referred to as the optical density ratio, will thus be increased; i.e. the difference between the optical densities in the open and closed conditions will be increased.

The following is one possible theory as to how and why the present invention operates on a molecular level. When the halogen-containing material used in accordance with the present invention, e.g. chloroform, is added to the suspension, the halogen of lower atomic weight, e.g. chlorine, in the material drives the original halogen (of higher atomic weight), e.g. iodine, out of the solution. The original halogen is thereby driven to the surfaces of the suspended particles, where the original halogen is adsorbed or absorbed by the particles, or coats the particles, or combines chemically with the particles.The beneficial results of this are: the original halogen no longer colours the solution because the halogen is removed from solution; the suspended particles become more effective polarizers, thereby increasing the optical density of the suspension when used in a light valve in the closed state; the original halogen, now added to the particles, retards agglomeration.

The following experiment and observation tend to support the above theory that the original halogen is driven to the suspended particles.

When chloroform is added to a solution of iodine in isopentyl acetate without the presence of any herapathite or other type of suspended particles, the colour of the solution does not significantly change (except for a slight lightening of colour due to dilution). This demons trates,that, the chloroform does not cause the iodine to disappear by means of any chemical combination with the chloroform or the isopentyl acetate. However', if herapathite or other suitable crystals are present in the isopentyl acetate, the iodine colour disappears when chloroform is.added.. is added.

As a further discussion of a possible theory, it may be noted that the iodine will not displace the chlorine in the chloroform because the iodine ion is a weaker base than the chlorine ion.

In the!halogen column of the Periodic Table, the base formed by an ion of one halogen is stronger than the base formed by the ion of a halogen of higher atomic number. Thus, the halogen of lower atomic number will displace the halogen of higher atomic number, but not vice versa.

Continuing the discussion of the aforesaid theory, when a suspension contacts the atmosphere the iodine escapes from the suspension through the air-liquid interface. When a light valve is hermetically sealed there is less chance of this happening. However, the possibility still exists. When it does happen, the suspension, generally changes in colour. If it is a herapathite suspension it goes.from a blue through a purple to a red-purple and finally to a red colour. This indicates (according to the theory under discussion) that the iodine is coming off the surfaces of the particles, going into solution in the suspending medium, and escaping to the atmosphere.The present invention, as discussed above, will cause the iodine to go back from -solution onto the .surfaces of the particles to produce the superior polarizing effects described above. Merely adding excess iodine to a solution from which the iodine has escaped into the atmosphere, or from which the iodine coating the particles has gone into solution, will not solve the problem because the added excess iodine will also go. into solution in the suspending medium and will not coat the surfaces of the particles as desired. The present invention overcomes this problem.

The following Examples illustrate the present invention: EXAMPLE 1 A suspension of herapathite in isopentyl acetate was placed in a 2" square inactivated light valve. The colour of the suspension was purple and its optical density for white light was 3.0 (corresponding to a transmission of 0.1 ito). When the light valve was activated by applying a voltage gradient of about 30 volts/ mail AC across the suspension, its optical density decreased to 0.8. Tlle light valve was.deactivated and its optical density returned to 3.0: Upon addition of 15 %, ry weight, chloroform the optical density increased to 3.2. The light valve was again activates and the open optical density was nearly 0.8 as it was before adding chloroform. The light valve was deactivated.Then, enough isopentyl acetate was added to the suspension to dilute it back to a closed optical density of 3.0. Upon activiation by the same voltage as previously used. the open optical density decreased to 0.7. Thus, the density ratio of the suspension, (i.e. the closed optical density divided by the open optical density), was increased frojm 3.0/0.8 or 3.75 to.3'.0/0.7 or 4.3 by the addition of the chloroform, and efficiency of the light valve improved. The increase in optical density of the suspension was caused by the increase in polarizing effectiveness of the particles. The polarizing effectivensss is increased because the- particles with the application of the present invention, extinguish each other in the inact Jated condition more than before the application of the present invention.

EXAMP ;E2 Example 1 was repeated, except that carbon tetrachloride is substituted for chloroform with substantially the same results.

EXAMPLE 3 Example 1 was repeated, except that dibromotetrafluoroethane is substituted for chloroform with substantially the same results.

EXAMPLE 4 Example 1 was repeated, except that sym-tetrachlorodifluoroethane is substituted for chloroform with substantially the same results.

EXAMPLE 5 Example 1 was repeated, except that polychlorotrifluoroethane was substituted for chloroform and it was necessary to use in excess of 20%of the added compound, by weight, to get substantially the same change in colour and optical density of the suspension as in Example 1.

EXAMPLE 6 A suspension of herapathite in a suspending medium comprising 80%, by weight, diisodecyladipate and 20 % chloroform was placed in a sealed test tube in an oven at 165 9F for one week. At the end of that period the suspension had substantially the identical colour, closed optical density and density ratio as it had prior to the test.

EXAMPLE 7 Example 1 is repeated, except that quinine bisulphate potassium lodide periodide is substituted for herapathite with substantially the same results.

EXAMPLE 8 Example I is repeated, except that quinine bisulphate ammonium iodide periodide. is substituted for herapathite with substantially the same results.

EXAMPLE 9 Example 1 is repeated, except that quinine bisulphate rubidium iodide periodide is substituted for herapathite with substantially the same results.

EXAMPLE 10 Example 1 is repeated, except that quinine bisulphate cesium iodide periodide is substi tuted for herapathite with substantially the same results.

EXAMPLE 11 A suspension of cupric bromide, prepared by mortar grinding cupric bromide in isopentyl acetate, was placed in the same light valve as Example 1. The suspension had a green appearance and a closed optical density of 2.2 in white light. A small drop of bromine, about 0.01 gm. was added to the suspension making it appear visually as a lighter shade of green.

The closed optical density decreased to 1.98. Upon addition of less than 4%, by weight, chloroform, the suspension turned a dark orange, and its closed optical density in white light increased substantially to 2.85. Detailed analysis at wavelengths between 400 and 700 millimicrons showed that addition of chloroform increased the closed optical density at all wavelengths, but particularly in the centre of the visible spectrum. At 400'millimicrons, the density increased from about 2.4 to 3.9; at 500 millimicrons from about 1.9 to 3.1; and at 600 millimicrons it increased from about 1.7 to 2.7.

It will be appreciated that a plasticizer or other agent may be added to the above suspensions at a stage in the preparation thereof before the particles are placed in the suspending medium so that they will remain in the liquid state.

In one embodiment of the present invention, all or nearly all of the suspending medium is comprised of one of the additive compounds mentioned above, such as chloroform. It is theorized that, when particles. such as herapathite particles, containing iodine are added to the chloroform. the halogen in the chloroform keeps the excess iodine on the surface of the particles. The reason the present invention is effective is, theoretically, because the iodine neyer substantially leaves the surfaces of the particles. Also, since the iodine is not very soluble in chloroform, this also tends to keep the iodine on the particles' surfaces.

In practice. in order to minimize voltage and power requirements, it is best that the suspending medium be comprised of less than 100 percent of the halogen-containing com pound. the remainder being a more polar liquid. such as an ester.

The'present invention may be used with light valves that are used as displays, windows, including double glazed units. windshields. mirrors and many similar devices.

In the present context and in that of the related cases mentioned below, the terms "perhalide" and "polyhalide" may be used interchangeably.

U.K. Patent No. 1586122 (Application No.21213/77) relates, inter alia. to a light valve which comprises a cell containing a liquid suspension comprising; an electrically resistive liquid suspending medium; suspended therein, a plurality of anisometne,' polarizing, halogen - containing particles; and, substantially dissolved therein, a copolymer comprising at least one monomer having a sterically unhindered functional group, which is a hydroxyl group or an acidic group. and at least one monomer having a branched group, the distance from the backbone of the copolymer to the most distant sterically unhindered functional group being less than the distance to the terminal carbon atom of the branched group.

U.K. Patent No. 1586125 (Application No. 21216/77) relates, inter alia, to a process for the preparation of a light-polarizing material which comprises; hydrogenating an unsaturated bond of a branch chain of an unsaturated alkaloid; forming an acid salt of the resulting saturated compound; and forming a polyhalide of the resulting alkaloid acid salt (as therein defined).

U.K. Patent No. 1586126 (Application No. 21217/77) relates, inter alia, to a light valve which comprises a cell containing a suspension in a liquid (as therein defined) suspending medium of particles responsive to an electrical field to change the transmission of radiation through the suspension, the liquid suspending medium comprising an ester of a phenol and a carboxylic acid, and the liquid suspending medium comprising, dissolved therein, a polymeric material for preventing agglomeration of the particles.

U.K. Patent No. 1586127 (Application No. 21218/77) relates, inter alia, to a light polarizing perhalide of an alkaloid salt having incorporated in its molecular structure at least one halide of calcium, rubidium or cesium, the perhalide being the reaction product of an alkaloid acid salt (as therein defined), elemental iodine and the halide.

U.K. Patent No. 1586124 (Application No. 21215/77) relates, inter alia, to a light valve which comprises a cell containing a suspension in a liquid suspending medium of particles responsive to an electrical field to change the transmission of radiation through the suspension, the liquid suspending medium comprising an ester of a phenol and a carboxylic acid.

WHAT WE CLAIM IS: 1. A light valve which comprises a cell containing a liquid suspending medium, suspended therein, particles of one or more halogenated alkaloid acid salts (as hereinbefore defined) or metal halides and, dissolved therein, a polymeric stabilizer to prevent agglomeration of the particles, the liquid suspending medium comprising a liquid halogenated hydrocarbon having a ratio of halogen atoms to all other atoms therein of greater than 1:1, the halogen atoms of the particles being iodine and/or bromine and the halogen atoms of the halogenated hydrocarbon being of lower atomic weight than the halogen atoms of the particles and at least 60% of the halogen atoms of the halogenated hydrocarbon being fluorine and/or chlorine.

2. A light valve as claimed in claim 1, wherein the halogenated hydrocarbon has at least two halogen atoms per non-halogen atom.

3. A light valve as claimed in claim 1 or claim 2 wherein the halogenated hydrocarbon is chloroform, carbon tetrachloride, dibromotetrafluoroethane, symtetrachlorodifluoroethane or polychlorotrifluoroethylene.

4. A light valve as claimed in any of claims 1 to 3 wherein the halogenated alkaloid acid salt islight polarizing.

5. A light valve as claimed in claim 4 wherein the alkaloid is a quinine alkaloid.

6. A light valve as claimed in claim 5 wherein the halogenated alkaloid acid salt is quinine bisulphate ammonium iodide periodide, quinine bisulphate rubidium iodide periodide, or quinine bisulphate cesium iodide periodide.

7. A light valve as claimed in any of claims 1 to 6 wherein the halogenated hydrocarbon comprises all or nearly all of the liquid suspending medium.

8. A light valve as claimed in any of claims 1 to 7 wherein the halogenated hydrocarbon is a saturated aliphatic hydrocarbon.

9. A light valve as claimed in claim 1 substantially as herein described.

10. A light valve as claimed in claim 1 substantially as herein described with reference to the Examples.

11. A process for the production of a light valve as claimed in claim 1 substantially as herein described.

12. A process for the production of a light valve as claimed in claim 1 substantially as herein described with reference to the Examples.

13. A light valve as claimed in claim 1 when produced by a process as claimed in claim 11

Claims (1)

1. or claim 12.