Light Valve With Flowing Fluid Suspension

Abstract

A light valve having a cell containing a fluid suspension of minute particles dispersed therein capable of orientation by an electric or magnetic field to change the transmission of light through the suspension, and means for applying such a field thereto, includes circulating means for producing a flow of the fluid suspension through the cell during operation thereof to reduce or avoid agglomeration of the particles. Various means are described for producing a smooth generally laminar flow of the fluid suspension in the active region of the cell. The circulating means may include means for dispensing agglomerated particles which may be produced during cell operation. A sheet for polarizing material in the path of light from the valve, with its direction of polarization perpendicular to fluid flow in the cell, markedly increases the closing speed. Two valves with fluid flow at right angles increases the closing speed without seriously decreasing the density ratio between closed and open states.

Parent Case Text

This application is a continuation of Ser. No. 25,541, 4/1/70, now abandoned.

Description

BACKGROUND OF THE INVENTION

This invention relates to light valves of the type including a cell containing a fluid suspension of minute particles capable of orientation by an electric or magnetic field to change the transmission of light through the suspension.

Light valves of this type have been known for many years. Fluid suspensions of herapathite in a suitable liquid have commonly been preferred, although other types of particles have been suggested. In general, the shape of the particles should be such that in one orientation they intercept more light than in another orientation. Particles which are needle-shaped, rod-shaped, lath-shaped, or in the form of thin flakes, have been suggested. The particles may variously be light absorbing or light reflecting, polarizing, birefringent metallic or non-metallic, etc. In addition to herapathite, many other materials have been suggested such as graphite, mica, garnet red, aluminum, periodides of alkaloid sulphate salts, etc. Preferably dichroic, birefringent or polarizing crystals are employed.

Very finely divided or minute particles are employed, and are suspended in a liquid in which the particles are not soluble, and which is of suitable viscosity. In order to help stabilize the suspension when in the non-actuated state, a protective colloid should preferably be used to prevent agglomeration or settling.

A fluid suspension which has been used with success uses generally needle-shaped particles of herapathite, isopentyl acetate as the liquid suspending medium, and nitrocellulose as a protective colloid. Plasticizing agents such as dibutyl phthalate have also been used in the suspension to increase the viscosity.

Both electric and magnetic fields have been suggested for aligning the particles, although electric fields are more common. To apply an electric field, conductive area electrodes are provided on a pair of oppositely disposed walls of the cell, and an electric potential applied thereto. The electrodes may be thin transparent conductive coatings on the inner sides of the front and rear walls of the cell, thereby forming an ohmic type cell wherein the electrodes are in contact with the fluid suspension. It has also been suggested to cover the electrodes with a thin layer of transparent material such as glass in order to protect the electrodes. Such thin layers of glass form dielectric layers between the electrodes and the fluid suspension, and the cells may be termed capacitive cells. Direct, alternating and pulsed voltages have been applied to the electrodes in order to align the particles in the fluid suspension. When the voltage is removed, the particles return to a disoriented random condition due to Brownian movement.

Commonly the front and rear walls of the cell are transparent, for example, panels of glass or plastic. With no applied field, and random orientation of the particles, the cell has a low transmission to light and accordingly is in its closed condition. When a field is applied, the particles become aligned and the cell is in its open or light transmitting condition. Instead of making the rear wall transparent, it may be made reflective. In such case light is absorbed when the cell is unenergized and is reflected when the cell is energized. These principal actions may be modified by employing light reflecting rather than light absorbing particles.

In such cells a serious problem is the agglomeration of the particles. While protective colloids are helpful in reducing or avoiding agglomeration in the stored or inactive condition, when the cell is in use the tendency to agglomerate increases. Depending on the particular suspension employed, and the voltage and frequency used, agglomeration may become noticeable in a matter of seconds, minutes or hours of use. Once agglomeration has occurred, it tends to remain more or less permanently even though the exciting voltage is removed.

Such agglomeration considerably impairs the usefulness of the light valve since it creates inhomogeneities in the suspension and hence changes the light transmission from point to point. Also it reduces the ratio of optical density between the closed state and the open state. Further, the density in the closed state may decrease.

The possible settling of particles out of the fluid suspension has previously been recognized. In this connection it has been suggested to furnish a constantly renewed supply of particles to the suspension, or to stir up and redistribute settled particles, or to cause a slight current or flow of particles within the container so as to assure a constant suspension. While such expedients may help to avoid settling, agglomeration may still occur during use. Also, the turbulence involved may appreciably affect the performance of the cell.

In copending application, Ser. No. 25,542, filed Apr. 1, 1970 by Matthew Forlini for "Light Valves with High Frequency Excitation," the use of frequencies higher than heretofore proposed is described, in order to avoid agglomeration of the particles in the suspension. While effective, the use of high frequencies involves considerable power consumption in the high frequency power source, and the power source may be quite expensive.

The present invention is designed to prevent agglomeration, while allowing the use of a wide range of frequencies including power line frequencies, hence avoiding the need for expensive power supplies. Other advantages are also obtained, as will be described hereinafter.

SUMMARY OF THE INVENTION

In accordance with the invention, a circulating means is employed to cause the fluid suspension to flow through the cell during operation thereof. To this end the cell is provided with inlet and outlet connections, and a pump is connected thereto, thereby forming conduit means connected to the cell at spaced points thereof and means for producing a circulation of the fluid suspension through the cell and conduit means.

Within the cell, means are employed to insure that the suspension will flow through the active cell region in a smooth, non-turbulent, generally laminar manner. Any turbulence, or markedly non-uniform flow has deleterious effects, as will be described hereinafter. Several ways of providing the desired smooth flow are described in the specific embodiments.

Advantageously a rate of flow is selected so that, with the particular fluid suspension and operating conditions employed, the fluid suspension passes through the cell before appreciable agglomeration takes place. However, to prevent an increase in the size of agglomerates due to repeated passages through the cell, dispersive forces may be applied to the suspension while passing through the circulating system so as to disperse any agglomerates that may have formed.

In addition to preventing agglomeration, the fluid flow is found to reduce the time required for the cell to change its light transmission upon removal of the applied field. Usually the cell becomes more opaque upon removal of the applied field, and the fluid flow thus reduces the closing time.

With fluid flow, it has been found that the cell suspension partially polarizes the light passing therethrough when the energizing field is removed. Thus, in accordance with another feature of the invention, a sheet of light-polarizing material is placed in the path of light through the cell, with the direction of polarization perpendicular to the direction of flow in the cell, thereby markedly increasing the closing speed of the cell when the applied field is removed.

In accordance with a further feature of the invention, two flow cells are placed in the path of the light to be controlled, and arranged with their flow directions perpendicular to each other. This arrangement also increases the closing speed, and permits obtaining a greater ratio of densities between closed and open conditions than that obtained with a single flow cell and polarizing sheet.

Further features of the invention will be explained in connection with the description of specific embodiments given hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a general view of a known type of light valve with which the invention may be used;

FIGS. 2 and 3 are cross-sectional views showing ohmic and capacitive type cells known in the art;

FIGS. 4a and 4b illustrates the closed and open states of the cell suspension, respectively;

FIG. 5 shows a recirculating flow cell in accordance with the invention;

FIG. 6 is a cross-section along the line 6--6 of FIG. 5;

FIG. 7 is a cross-section similar to FIG. 6, showing an alternative construction of the cell;

FIGS. 8 and 9 are face and cross-sectional views of another embodiment of the cell;

FIG. 10 illustrates the use of a sheet of polarizing material in conjunction with a flow cell in accordance with the invention;

FIG. 11 shows two cross flow cells;

FIG. 12 is a cross-section of another embodiment of the invention taken along the line 12--12 of FIG. 13; and

FIG. 13 is a cross-section taken along the line 13--13 of FIG. 12.

Referring to FIGS. 1 and 2, a light valve generally indicated as 10 is formed of two sheets of glass 11 and 12 having transparent conductive coatings 13 and 14 on the inner surfaces thereof. The conductive coatings form area electrodes for the application of energizing voltage to the cell. The glass plates are separated by a spacer 15 sealed to the glass plates around the edges thereof to provide a chamber 16 therebetween in which the fluid suspension of minute particles is placed. Once the fluid suspension has been introduced, the cell is sealed. The conductive coatings 13 and 14 are connected to a suitable power supply 17. Inasmuch as the fluid suspension in chamber 16 is in contact with conductive coatings 13 and 14, this may be termed an ohmic type cell.

FIG. 3 is similar to FIG. 2 and corresponding parts are similarly designated. However, in FIG. 3 thin transparent coatings 18 and 19, for example glass, are placed over the area electrodes 13 and 14 so that the conductive coatings are protected from the fluid suspension. Since layers 18 and 19 are of dielectric material, the electrodes are, in effect, capacitively coupled to the fluid suspension in chamber 16.

FIG. 4a shows the closed or opaque condition of the cell 10. Here tiny acicular particles 21 are illustrated in random orientation. A beam of light impinging on cell 10, indicated by arrows 22, is substantially cut off.

FIG.4b shows the open or light transmitting condition of the cell 10. Here, due to the application of an electric field, the particles 21 are aligned with their major axes perpendicular to the wall faces. In this condition, the particles intercept much less light than in the random condition shown in FIG. 4a. Consequently a considerable portion of the beam of light 22 passes through the cell, as indicated by the arrows 23.

Referring now to FIGS. 5 and 6, a flow cell in accordance with the invention is illustrated. The cell itself is generally designated as 31 and may be considered to be divided into three sections, namely an active section A and input and output sections B and C. The front and rear walls 32, 33 of the cell are of glass and, in the active region A, are coated on their inside surfaces with conductive coatings 34, 35 forming area electrodes. If desired, the area electrodes may be protected by thin coatings of glass, as described for FIG. 3. In this active region the parallel wall sections with electrodes 34, 35 are spaced apart a distance which is small compared to the lateral dimensions of the sections, to confine the fluid suspension therebetween to a thin layer designated 36. The herapathite suspension previously described is advantageously employed. In this and subsequent drawings, the dotted representation of the suspension previously used has been omitted, to avoid confusion. The parallel area electrodes 34, 35 are connected to a power supply 37.

Instead of sealing the fluid suspension within the cell, in accordance with the invention inlet and outlet tubes 38 and 39 are connected to end sections 41 and 42 of the cell. A pump 43 continuously circulates the fluid suspension through the cell during operation. An orbital pump has been used with success, but other types may be employed as desired. Advantageously a reservoir 44 is provided in the circulating system. The reservoir provides a heat interchanger to cool the suspension in applications where intense beams of light are used, allows air or gas bubbles to escape, provides for differential thermal expansion in various parts of the system, and allows a large volume of suspension to be used so that operation over a long period of time without substantial deterioration is facilitated.

It has been found to be important to produce a smooth generally laminar flow of the fluid suspension in the active region of the cell, that is, in the thin layer 36 between the electrodes 34 and 35. If turbulent flow exists in this region, there will be variations in light transmission with the cell energized. Accordingly, as illustrated in FIG. 6, the spacing between the front and rear walls of the cell in the inlet and outlet regions 41, 42 is substantially greater than the spacing of the wall sections in the active region 36. Consequently, with suitable rates of flow, turbulence in the region where tube 38 enters chamber 41 is dissipated before the fluid suspension reaches the adjacent side of the action region at 43. Since it is preferred that the rate of flow be substantially constant across the entire width of the active region of the cell, the spacing of the front and the rear walls in the outlet region 42 is likewise substantially increased. Some variation in velocity of flow across the active region 36 may be tolerated in practice, so long as the flow is reasonably smooth.

It will be understood that the spacing of the wall sections in region 36 may be quite small, say of the order of 20 or 30 mils, and that the spacing of the walls in sections 41 and 42 may be many times this spacing, in order to obtain the desired smooth flow of the suspension in region 36. In the drawings, it is impractical to illustrate the relative spacings accurately.

FIG. 7 illustrates another way to produce smooth, generally laminar flow in the active region 36 of the cell. Here again, the separation of conductive coatings 34, 35 is greatly exaggerated as compared to their lateral dimensions. With the inlet and outlet tubes 38, 39 spaced a substantial distance from the active region 36, and with a sufficient distance from each tube to the active region, the initial turbulence may be reduced to acceptable values so that the flow through the active region 36 is smooth and generally laminar. The distance from the tubes to the inlet and outlet sides 44 and 45 of the active region may be much greater than that shown, in order to obtain the desired smooth flow. Also, the tubes 38, 39 may be placed on opposite walls 32, 33 of the cell, if desired.

Referring to FIGS. 8 and 9, another construction for insuring smooth, generally laminar flow in the active region 36 is illustrated. Here barrier walls 51 and 52 are positioned between the inlet and outlet tubes 38, 39 and the respective sides 44, 45 of the active region 36. The barrier walls extend to the spacers 53 on either side of the cell. The height of the barrier walls is somewhat less than the separation of the cell walls so as to provide thin passages 54 and 55 through which the fluid suspension flows. Considering the inlet region, as the fluid suspension enters the wall through inlet 38, it must pass over the barrier wall 51, through the thin passage 54, before reaching the active area 36. Consequently, turbulence is removed. In the outlet region, the fluid suspension must pass over barrier wall 52, through the thin passage 55, before reaching the outlet 39, so that the generally laminar flow in the upper portion of region 36 is not substantially impaired. If desired, the cell may be extended above and below barrier walls 51 and 52 so as to provide a greater volume in the spaces 56 and 57 to eliminate turbulence and promote smooth flow in active region 36.

In the embodiments of FIGS. 5-9, the inlet and outlet sections have been made similar, which is believed to enhance the smooth generally laminar flow desired in the active region 36. However, they may be made dissimilar if desired, and various combinations of inlet and outlet constructions may be employed.

In operating closed-type cells such as illustrated in FIGS. 1-3, the opening time of the cell is quite short, since the particles become rapidly aligned upon applying the electric field. However, the closing time is much longer. With a flowing fluid suspension, it has been found that the closing time is decreased. This is believed to be due to the hastening of the disorientation of the particles from their aligned condition by the fluid flow when the applied voltage is removed. Also the scatter of light from the cell is reduced by the flow.

It has also been found possible to greatly increase the closing speed by placing a sheet of polarizing material in the path of light through the cell, either in front of or behind the cell, with the direction of polarization perpendicular to the direction of flow of the fluid suspension. This is illustrated in FIG. 10, wherein the direction of fluid flow is indicated by arrow 61 and the direction of polarization of polarizing sheet 62 is shown by the double-headed arrow 63. As herein used, the direction of polarization may be considered to be that of the magnetic vector of the light radiation, so that the orientation of polarizing sheet 62 is such as to cut off the polarized portion of the light passing through the cell.

When the energizing voltage is removed, it is believed that the laminar flow of the fluid suspension tends to align the particles more or less in the direction of flow, so that the light passing through the cell as it is closing tends to become partially polarized. In any event, it is found that by placing a sheet of polarizing material in the path of light through the cell, with proper orientation, closing speeds can be obtained which are several times faster than the closing speeds observed without the use of the polarizing sheet.

The use of a sheet of polarizing material decreases the amount of light passing by the polarizing sheet when the cell is in its open condition. Consequently the ratio of light transmission from open to closed states may be decreased. This may not be disadvantageous in certain applications, or may be more than offset by the faster closing.

FIG. 11 shows an arrangement whereby rapid closing may be obtained without seriously decreasing the light transmission ratio. In FIG. 11 two flow cells 31 and 31' are placed one in front of the other, with the directions of fluid flow therein at right angles. Thus, the fluid suspension enters cell 31 via inlet tube 38 and travels in a vertical direction to outlet tube 39. Outlet tube 39 is connected through tube 65 to inlet tube 38' of cell 31', and travels in a horizontal direction to outlet tube 39'. The two cells are energized from a common source 37. In the open condition of the cells, the light transmission through the cells will be less than that for a single cell, since light must pass successively through the two cells. However, the same is true when the cells are closed. Consequently, the ratio of transmission between open and closed conditions is approximately the same as that for a single cell. However, during the closing of the cells after the energizing voltage is removed, the polarizing effect of one cell will be at right angles to that of the other, thereby increasing the speed of closing.

In FIGS. 10 and 11 the directions of polarization are mutually perpendicular, so as to obtain the maximum effect. If desired, angles other than 90.degree. may be employed. It will also be understood that the direction of light from cell to polarizing sheet in FIG. 10, or between the cells in FIG. 11, may be changed by mirrors, prisms, etc. and the orientation of the components suitably changed so that the effective angles are those described herein.

Referring to FIGS. 12 and 13, another embodiment of a flow cell is shown using barrier walls as in FIGS. 8 and 9, but providing an overall more compact structure. Here barrier walls 71 and 72 are employed which are tapered from top to bottom. As shown, the spacing of the top of each barrier wall from the front wall 32 is uniform, although it also could be tapered if desired. Bearing in mind that the tops of the barrier walls are very close to the front wall 32, perhaps only a few mils separation, the greater width of the barrier walls at the top of the cell, as viewed in FIG. 12, provides a greater resistance to fluid flow thereacross than at the bottom of the cell. Consequently, fluid suspension entering at inlet 38 travels downward and also sidewise over the top of barrier wall 71. The change in resistance to flow of the wedge-shaped wall tends to equalize flow in the horizontal direction in the space 36 between electrodes 34 and 35. Wedges 71 and 72 may terminate slightly short of the bottom edge of the cell so as to leave open channels in regions 73 of full cell thickness which aid flow of the suspension out of the lower corners.

In FIG. 12 the stippled area 35 designates the lower area electrode. As will be observed, the electrode does not extend fully to the upper and lower edges of the cell. The velocity of flow along upper and lower edges of the cell may be somewhat lower than that in other regions, and hence these regions of the cell are excluded from the active region. If desired, the corners of the cell may be rounded to further improve the flow. As will be clear from FIGS. 5-9, 12 and 13, the specific circulating arrangements shown include conduit means connected to the respective cell at spaced points thereof and means for producing a circulation of the fluid suspension through the cell and conduit means.

As has been stated before, the time required for agglomeration to become noticeable varies with the particular suspension employed and the voltage and frequency of the power source. While in some cases agglomeration may become noticeable in a matter of seconds or minutes, in other cases several hours may elapse before it becomes serious. Such agglomeration creates inhomogeneities in the suspension which may seriously affect the usefulness of the light valve. Also, the density ratio between closed and open states may be seriously reduced, and the valve may be less opaque in its closed state.

It is desirable to select a rate of flow such that a given volume of the suspension flows through the cell before agglomeration becomes visible. Even so, some agglomeration may be initiated, and become more serious as the cell is continued in operation. Accordingly, it is desirable to eliminate any agglomeration that has taken place within the cell, before that portion of the suspension again reaches the cell. Thus, it is desirable to apply dispersive forces to the suspension during the recirculation outside the active region of the cell to break up any groups of particles that may exist. Such dispersive forces can be produced by friction between the agglomerated particles and the walls of the tubes or vessels through which the suspension flows, by the use of narrow orifices or obstructions, or by rapidly agitating the suspension. Frequently the liquid pump may be relied upon to produce sufficient turbulence and liquid shear to break up the agglomerates, but if necessary additional expedients may be employed, as discussed above. It may be noted that the rate of flow through the cell is somewhat reduced when the electric field is applied, and this may be taken into account in selecting the overall rate of flow.

Although the light valves described are commonly used with visible light sources, with suitable suspensions it may be possible to control the passage of similar types of electromagnetic radiation such as infrared and ultraviolet. Also, instead of using continuous area electrodes within the active region of the cells, the electrodes may be formed in patterns so as to exhibit a desired display. Further, instead of allowing light to pass through the cell from front to rear, the rear surface may be made reflective so as to provide a mirror of variable reflectivity. It will be understood that the term "light valve" applies to these various types of applications.

Claims

We claim:

1. A light valve including a cell for containing a fluid suspension of minute particles dispersed therein capable of having their orientation changed by an electric or magnetic field to change the transmission of light through the suspension, said cell having front and rear wall sections spaced apart a distance which is small compared to the lateral dimensions of the sections, and means for applying an electric or magnetic field to the suspension between said wall sections to change the light transmission thereof, in which the improvement comprises circulating means connected to said cell at spaced points thereof for producing a flow of said fluid suspension through said cell during operation thereof, and means for producing a smooth generally laminar flow of said fluid suspension between said wall sections.

2. A light valve including a cell containing a fluid suspension of minute particles dispersed therein capable of having their orientation changed by an electric or magnetic field to change the transmission of light through the suspension, said cell having front and rear wall sections spaced apart a distance which is small compared to the lateral dimensions of the sections, and means for applying an electric or magnetic field to the suspension between said wall sections to change the light transmission thereof, in which the improvement comprises circulating means including conduit means connected to said cell at spaced points thereof and means for producing a flow of said fluid suspension through said cell and conduit means, and means for producing a smooth generally laminar flow of said fluid suspension between said wall sections.

3. A light valve in accordance with claim 2 in which said conduit means is connected to said cell to supply said fluid suspension to the cell on one side of the region between said wall sections and withdraw the fluid suspension from the cell on the opposite side of said region.

4. A light valve in accordance with claim 2 in which said circulating means includes means for dispersing agglomerated particles of said fluid suspension.

5. A light valve in accordance with claim 2 including a sheet of polarizing material positioned generally perpendicular to the direction of light passing through said cell, the direction of polarization of said sheet being effectively at a substantial angle to the direction of fluid flow in said cell.

6. A light valve in accordance with claim 5 in which said direction of polarization is effectively approximately perpendicular to the direction of fluid flow in said cell.

7. A pair of light valves in accordance with claim 2 positioned so that light passes through said valves in succession in a direction generally perpendicular to the direction of fluid flow therein, said cells being oriented so that the direction of fluid flow in one valve effectively makes a substantial angle with the direction of fluid flow in the other valve.

8. A pair of light valves in accordance with claim 7 in which the directions of fluid flow in said valves are effectively approximately mutually perpendicular.

9. A light valve including a cell containing a fluid suspension of minute particles dispersed therein capable of having their orientation changed by an electric field to change the transmission of light through the suspension, said cell having front and rear wall sections spaced apart a distance which is small compared to the lateral dimensions of the sections to confine the fluid suspension therebetween to a layer, and area electrodes of opposite sides of said layer for producing an electric field through the layer to change the light transmission thereof, in which the improvement comprises circulating means including an inlet and an outlet connected to said cell at spaced points for producing a flow of said fluid suspension through said cell during operation thereof, said cell being designed and adapted to produce a smooth generally laminar flow of said fluid suspension in the region between said electrodes.

10. A light valve in accordance with claim 9 in which said circulating means includes means for dispersing agglomerated particles of said fluid suspension.

11. A light valve in accordance with claim 9 in which said inlet and outlet are positioned and adapted to supply said fluid suspension on one side of the region between said electrodes and withdraw the fluid suspension from the opposite side of said region.

12. A light valve in accordance with claim 11 in which said inlet and outlet are positioned on respectively opposite sides of the region between said electrodes, at least one of said inlet and outlet being spaced a substantial distance from said region between the electrodes to aid in producing said smooth generally laminar flow in said region.

13. A light valve in accordance with claim 11 in which said cell has inlet and outlet sections between said opposite sides of the region between said electrodes and the respective inlet and outlet, the spacing between the front and rear walls of the cell at at least one of said inlet and outlet sections being substantially greater than the spacing of the wall sections at the region between said electrodes.

14. A light valve in accordance with claim 13 in which the spacing between the front and rear walls at both of said inlet and outlet sections is substantially greater than the spacing at the region between said electrodes.

15. A light valve in accordance with claim 11 including a barrier wall positioned within said cell between said region between the electrodes and said inlet and extending along the respective side of said region for at least a major portion of the length thereof, the height of said barrier wall being less than the separation of the front and rear walls at the location thereof to thereby allow a flow of the fluid suspension thereover.

16. A light valve in accordance with claim 15 including a second barrier wall positioned within said cell between said region between the electrodes and said outlet and extending along the respective side of said region for at least a major portion of the length thereof, the height of said second barrier wall being less than the separation of the front and rear walls at the location thereof to thereby allow a flow of the fluid suspension thereover.

17. A light valve in accordance with claim 15 in which said inlet is adjacent one end of said respective side of the region between the electrodes, and the width of said barrier wall tapers from wider to narrower dimensions from the inlet end toward the other end of said respective side.

18. A light valve in accordance with claim 16 in which said inlet and said outlet are adjacent corresponding ends of the respective sides of said region between the electrodes, and the width of said barrier walls taper from wider to narrower dimensions from respective inlet and outlet ends toward the other ends of the respective sides.

19. The method of preventing minute suspended particles in a fluid suspension from significantly agglomerating in a light valve cell comprising

producing a smooth generally laminar flow of the fluid suspension to prevent significant agglomerating of the particles and

flowing said laminar flow through the light valve cell.

20. The method of claim 19 wherein the fluid suspension is caused to flow through the light valve by means of a circulating system.

21. The method of claim 19 wherein the smooth generally laminar flow is produced by means of a barrier.

22. A light valve including a cell adapted to contain a fluid suspension of minute particles dispersed therein capable of having their orientation changed by an electric or magnetic field to change the transmission of light through the suspension, said cell having front and rear wall sections, means for applying an electric or magnetic field to the suspension between said wall sections to change the light transmission thereof and means for producing a smooth generally laminar flow of the fluid suspension between said wall sections.

23. The light valve of claim 22 including a circulating means for producing a flow of fluid suspension through said cell.

24. The light valve of claim 23 wherein said means for producing a smooth generally laminar flow includes a barrier means.

25. The light valve of claim 24 wherein the circulating means is connected to said cell to supply fluid suspension to the cell on side of a region between said wall sections and withdraw the fluid suspension from the cell on the opposite side of said region.