US 3670323 A
An image-display panel is composed of horizontal rows and vertical columns of illumination-control cells, each of which includes anisometric particles of suspended magnetic material that normally obstruct light but become oriented to pass light in response to the application of a magnetic field. Individually associated with different cells are a plurality of magnetic storage elements that effect control of the application of the fields to the cells. A like plurality of magnetic-field-producing devices are individually associated with respective cells and storage elements. In response to vertical synchronizing signals, conditioning pulses are selectively applied to respective rows of the field-producing devices in order to create in the corresponding storage elements respective field components of magnitudes insufficient to orient the particles in the associated cells for light control. In response to horizontal synchronizing signals, control pulses are selectively applied to respective columns of the field-producing devices in order to create, in the corresponding storage elements, respective field components of magnitudes sufficient, together with the respective field components created in response to the conditioning pulses, to establish in the respective cells resultant fields sufficient to orient the affected particles for light control. At the same time, the amplitude of the conditioning or control pulses is modulated by video signals so that the total magnitude of each of the resultant fields is proportional to the instantaneous video level. The storage elements thereafter serve to maintain orientation of the particles until, finally, in response to synchronizing signals, the storage elements are periodically de-activated in time-correspondence with successive intervals of the video information. In one extension of the basic disclosure, a manually movable magnet is employed to write additional information into the display.
Claims available in
Description (OCR text may contain errors)
JJUWLUIG @amis-J2 XR  .Enne i3, 1972  IMAGE-DISPLAY DEVICES CGMPRHSIN G PARTICLE LIGHT MODULATORS WllTH STORAGE  Inventors: Alan Sobel; `lloseph Marldn, both of Evanston, Ill.
 Assignee: Zenith Radio Corporation, Chicago, Ill.
 Filed: v Dec. 14, 1970  Appl. No.: 97,867
 U.s.c|. ..34o/324R,34o/174.1M350/16o, 35o/267 [si] intel. .oozf 1/30  Field of Search .....340/324 R, 174 R, 174 EA, 174.1 M; 350/160, 267, 269
Primary Examiner-John W. Caldwell Assistant Examiner-Marshall M. Curtis Attorney-John J. Pederson [5 7] ABSTRACT An image-display panel is composed of horizontal rows and vertical columns of illumination-control cells, each of which includes anisometric particles of suspended magnetic material that nonnally obstruct light but become oriented to pass light in response to the application of a magnetic field. Individually associated with different cells are a plurality of magnetic storage elements that effect control of the application of the fields to the cells. A like plurality of magnetic-field-producing devices are individually associated with respective cells and storage elements. In response to vertical synchronizing signals, conditioning pulses are selectively applied to respective rows of the field-producing devices in order to create in the corresponding storage elements respective field components of magnitudes insufficient to orient the particles in the associated cells for light control. In response to horizontal synchronizing signals, control pulses are selectively applied to respective columns of the field-producing devices in order to create, in the corresponding storage elements, respective field components of magnitudes sufficient, together with the respective field components created in response to the conditioning pulses, to establish in the respective cells resultant fields sufficient to orient the affected particles for light control. At the same time, the amplitude of the conditioning or control pulses is modulated by video signals so that the total magnitude of each of the resultant fields is proportional to the instantaneous video level. The storage elements thereafter serve to maintain Aorientation of the particles until, finally, in response to synchronizing signals, the storage elements are periodically deactivated in time-correspondence with successive intervals of the video information. In one extension of the basic disclosure, a manually movable magnet is employed to write additional information into the display.
21 Claims, l5 Drawing Figures P'ATNTDJUH 1 s 11112 3, 670. 323
sum 2 0F 2 afge@ 11G. 13
Rece' r 85 we 83 f f Element SynChrOnlZF SeqUenCeF F58 #sa V58 57 Q) w E 2 Dlsploy LU Pone! lnvenors Alon Sobel qoseph/ Morkin IMAGE-DISPLAY DEVICES COMPRISING PARTICLE LIGHT MODULATQRS WITH STORAGE The present invention relates to image-display panels. More particularly, it pertains to display panels in which information signals are stored for periods of time at selected locations in order to create a display ofthe information.
For decades, workers in the art have been seeking image display devices sufficiently thin that they might, for example, be hung on a wall like mural. One approach has been that of using modified electron trajectories in a cathode-ray tube so that the evacuated envelope may have substantially reduced depth. Other approaches have sought to make use of generally solid-state light generation or light control. Thus, solid-state diodes, electroluminescent cells, liquid crystals and mechanical shutters have been distributed over display matrices and selectively activated in order to create the display matrices and selectively activated in order to create the display of an image. Some of these prior devices have found a significant degree of success, particularly for the display of simple stationary images. However, they leave much to be desired in brightness, contrast, speed of response, or a combination of these, in connection with the display of television or similar pictures involving high resolution and fairly rapid motion.
One prior light-control element that has attracted some interest for its possible use in connection with image reproduction utilizes finely divided conductive particles suspended in a liquid or plastic medium. In one such device, a suspension of particles was employed in combination with a cathode-ray tube in order to obtain projection of a magnified version ofthe image produced upon the screen of the cathode-ray tube. Various other attempts have been made to employ suspensions of such particles that are subjected to electric or magnetic orientation for image display. Significant success has not been achieved, however, with known system and device techniques and concepts.
It is, accordingly, a general object of the present invention to provide a new and improved image-display panel.
It is another object of the present invention to provide an image-display panel which overcomes limitations found to exist in prior, and even somewhat similar, display panels.
A specific object of the present invention is to provide a new and improved image-display panel that is capable of being fabricated to permit image reproduction in accordance with conventional television standards.
A further object of the present invention is to provide an image-display panel that affords additional variety and flexi- `bility of use.
In one embodiment, an image-display panel constructed in accordance with the present invention includes a plurality of illumination-control cells distributed over the panel in a matrix defining horizontal rows and vertical columns of the cells. Each of the cells includes particles of magnetic material suspended in a medium and normally opaque or non-reflecting to light but responsive to a magnetic field for being oriented to transmit or reflect light from an associated source. Individually associated with different cells are a plurality of magnetic storage elements that effect control of the fields applied to the cells. In turn, individually associated with respective different storage elements are a plurality of magnetic field-producing devices. The system in which the panel is employed includes means for supplying video signals composed of intervals of picture information together with row-and column-selection signals.
Responsive to the row signals are means for applying, selectively to rows of the field-producing devices, conditioning pulses that effectively create in the storage elements field components of a magnitude insufficient to orient the particles for light control. Responsive to the column signals are means for applying, selectively to columns of the field-producing devices, control pulses effectively creating in the storage elements corresponding components of a magnitude suliicient to establish respective resultant fields suicient to orient the particles for light control, only when present together with the respective field components created in response to the conditioning pulses. Means responsive to thevideo signals effect modulation of the conditioning or control pulses, or both, so that the total magnitude of each of the resultant fields varies with the level of the corresponding portions of the video signals. Finally, in response to the selection signals, the effects of the storage elements are periodically de-activated in timecorrespondence with different intervals of the video information.
The features of the invention which are believed to be novel are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings, in the several figures of which like reference numerals identify like elements, and in which:
FIG. 1 is a side-elevational view of an illumination-control cell arranged for one mode of operation;
FIG. 2 is a side-elevational view of an illumination-controlled cell arranged for another mode of operation;
FIG. 3 is a perspective view of an illumination-control cell activated in in a particular manner;
FIG. 4 is a perspective view of an illumination-control cell activated in the same manner as in FIG. 3 and also controlled in another manner;
FIG. 5 is a perspective view of an illumination-control cell activated and controlled in a manner;
FIG. 6 is another perspective view of an illumination-con.` trol cell with still different activation and control arrangements;
FIG. 7 is a front-elevational cross-sectional view of one specific embodiment of an illumination-control cell;
FIG. 8 is a perspective view of an element included in the illumination-control cell of FIG. 7;
FIG. 9 is a partially schematic cross-sectional front-eleva tional view of a portion of a portion of a display panel embodying a plurality of control cells individually like that or FIG. 7;
FIG. 9a is a cross-sectional view, to an enlarged scale, taken along the line 9a-9a in FIG. 9;
FIG. 9b is a fragmentary back view, partly cut away, of the panelof FIG. 9;
FIG. 10 is a perspective view of a modified display panel combination;
FIG. 11 is a cross-sectional view of an alternative embodiment of a portion of a display panel;
FIG. 12 is a cross-sectional view of a still different embodiment of a portion of a display panel; and
FIG. 13 is a block diagram of a system for sequentially addressing the different illumination-control cells in a display panel.
FIG. 1 illustrates a single illumination-control cell 20a through which incident light represented by arrow 2l may be permitted to emerge as represented by arrow 22. Cell 20a is in the form of a light-transparent container 23 filled with a liquid or a semi-liquid-plastic 24 in which are suspended a substantial quantity of minute, generally elongated or anisometric platelets or needles of magnetic material. Under the influence of Brownian and thermal motion within liquid 24, the suspended elongated particles assume random orientation and a substantial portion of incident light 21 is scattered, absorbed or reflected backwardly generally in the direction from which it came. On the other hand, when cell 20a is subjected to the influence of a magnetic field, the particles orient themselves in line with the magnetic flux as a result of which a sizable portion of incoming light 21 is transmitted through cell 20a and emerges as outgoing light 22. Accordingly, by selectively applying a magnetic field through cell 20a, the cell can be made to function as a light valve.
Light valves composed of suspended particles subjected to the selective action of an impressed electric or magnetic field are well-known, as evidenced by U.S. Letters No. l,963,496, issued on June 19, 1934, to Edward H. Land. Such suspensions of magnetic particles have also found use in techniques for revealing the details of domain patterns as described at pages 1252-l,262 of PROC. PHYS. SOC., Vol. 79, 1962, in an article by J. R. Garrood entitled Methods of Improving the sensitivity of the Bitter Technique. That article describes in detail techniques for preparing the particle suspensions in order to improve their sensitivity. Another article pertaining to the preparation of a suspension of platelike crystals is New Method for Making Magnetic Fields Visible by Lawrence Suchow, published in the JOURNAL OF APPLIED PHYSICS, Vol. 29, No. 2, pp. 223-224, February, 1958. Of interest with respect to the preparation of mutually contiguous pluralities of cells a, in the form of individual capsules of suspended magnetic materials with each capsule capable of acting as a light shutter, is an article by Gloria Sirine entitled Microencapsu1ation, in the STANFORD RESEARCH IN- STITUTE JOURNAL, No. l5, pp 2-6, June, 1967.
Cell 20a appropriately may be called a particle light modulator. In FIG. 1, it is operated in the transmission mode. In FIG. 2, similar cell 20b operates in a reflection mode. Cell 20b contains a suspension 26 of particles that absorb light when randomly dis-oriented or when oriented by a magnetic field so that the major axes of the individual particles are perpendicular to the direction of travel of light through the cell. Adjacent to and parallel with one major surface of cell 20b is a reflector layer 27. Upon the application of a magnetic field to cell 20b with its flux lines disposed so that the major axes of the particles become more or less perpendicular to reflector 27, a substantial portion of the light incoming along a path 28 is transmitted through cell 20b and returned by reflector 27 back through the cell along an emerging path 29. On the other hand, upon removal of the field or its re-orientation so that the particles are aligned in a direction generally parallel to reflector 27, cell 20b acts as an absorber of incoming light 28 and the light emerging along path 29 is at least substantially reduced. In the description of the various different embodiments, modifications and alternatives that follow, all of which pertain to the use of one or more such illumination-control cells, it is to be understood that the individual cells may be operated in either the transmission mode or the reflection mode.
Cell 20b in FIG. 2 may also be so constituted as to function in an inverse type of reflection-mode operation. In that case, the particles of suspension 26 are of a composition and surface finish (e.g., shiny aluminum) which reflects or scatters the incident light, while layer 27 is formed of a material such as carbon black that absorbs the incident light. Consequently, substantial light is reflected along path 29 when the particles are randomly dis-oriented or when they are oriented with their major axes in a plane transverse to the light path, while the incident light is substantially absorbed in layer 27 whenever a field is applied to orient the particles so that their major axes are generally perpendicular to layer 27. Whatever the form of either cell, it is known that the choice of specific materials and shapes for best operation usually will differ as between the transmission and reflection modes. With operation of a matrix of such cells in either reflection mode, it becomes possible to produce an image display by utilizing only ambient light.
In operation of FIG. 1, the angle over which the incident light is absorbable (which determines the effective viewing angle of the display) may be increased, if desired, by forming the wall of container 23 on the output side of the cell so that it constitutes a light-scattering surface. Similarly, a greater viewing angle is obtained in the FIG. 2 version by making reflecting layer 27 diffusely reflecting or employing the inverse type operation in which particles 26 are formed to scatter the incoming light when appropriately oriented.
When it is desired in either of FIGS. 1 or 2 that the lightcontrol effect be maintained for a period of time subsequent to removal of the magnetic field so that the cells may be said to exhibit persistence, it is customary to increase the viscosity of the suspension so that the inherent thermal or Brownian motion requires a longer time interval in which to dis-orient the particles. However, increased viscosity tends to correspondingly increase the response time. In addition, such motions are affected in velocity by changes in temperature, as a result of which it becomes difficult to control accurately the quantity or brightness of emerging light unless additional means are provided to control the temperature.
Each of FIGS. 3-6 includes a magnetic storage element 32 that enables the achievement of persistence without encountering this difculty. Moreover, the hysteresis characteristics of storage element 32 permit control ofthe field to the cell.
ln FIG. 3, element 32 is in the form of a rectangular plate of ferromagnetic material disposed in a plane oriented perpendicularly to the major plane defined by control cell 20. Conduction through plate 32 of current in the illustrated vertical direction as indicated by the arrow I creates a magnetic field having lines of flux 33 which pass through cell 20 in a direction perpendicular to its major plane. Of course, the strength of the field is a function not only of the current level but also of the magnetic characteristic of the material. Thus, the latter introduces a response non-linearity that, as will be seen, is utilized herein to provide a threshold control or selection efect. Once the ferromagnetic material of element 32 is magnetized, it retains its magnetism and continues to orient the magnetic particles in cell 20 after the energizing current representing a control signal, element 32 exhibits the property of remanence so that its stores the control signal and continues to maintain the desired orientation of the particles in cell 20 until subsequently de-magnetized. Such demagnetization may be effected by subjecting element 32 to an alternating current that initially has a peak-to-peak amplitude larger than that of the original undirectional magnetizing current and which amplitude is then gradually decreased to zero.
In addition to the magnetic field utilized to orient the particles suspended within cell 20, the simultaneous application of an electric field through cell 20 may either aid or oppose the orienting effect of the magnetic field. FIG. 4 is the same as FIG. 3 except to represent the additional application of an alternating or direct electric field in the direction indicated by arrow E which is the same as the direction of flux line 33 through cell 20. When the electric field E is alternating, the elongated particles, whether in the form of platelets, needless or ellipsoids, tend to align their major axes parallel to the direction of the electric field. At high frequencies, the particles do not follow the reversal of field polarity within each cycle but tend to be merely oriented parallel to the direction of the field vector regardless of its size. At lower frequencies, the particles may tend to clump together or exhibit electrophoretic migration.
The described alignment with respect to the electric field vector takes place whether the particles are electrically insulating or conducting. Thus, in FIG. 4, the application of the desired electric field may serve to aid the particle orientation established by the magnetic field. Accordingly, in one mode of operation the electric field is applied at an intensity less than that required to achieve sufficient particle orientation to pass a significant quantity of light while yet serving as a conditioning bias as the result of which less magnetizing current I is required to produce a sufficient flux level to complete the desired degree of orientation of the particles. Although both the magnetic and electric fields must be terminated in order to permit complete random re-orientation of the particles, there are certain applications in with which either the electric or the magnetic field may remain as a continuing bias with the other field being employed to effect the desired control.
For creation of the electric field, it is contemplated to space a pair of electrodes individually behind and in front of cell 20 and to form one or both of those electrodes, as required, of a transparent conductive material such as tin oxide. While such electrodes have been omitted in FIG. 4 for purposes of clarity, they will be illustrated in several subsequent figures.
In FIG. 5, illumination-control cell 20 is disposed so that its major plane is parallel to that of storage element 32. Upon conduction of current I in element 32 in a direction parallel to the plane of cell 20, magnetic flux created by element 32 creates flux lines 35 that travel in cell 20 parallel to the plane of the cell, and the magnetic particles within the cell are oriented with their major axes parallel to flux lines 35. Spaced from and parallel to storage element 32 is a rectangular, flat electrode 36. Upon the application of an alternating or direct electric potential between electrode 36 and storage 32, the resulting electric field E traverses cell in a direction perpendicular to the magnetic field. When storage element 32 is insufficiently conductive to carry the current I or to serve properly as one of the electric-field-developing electrodes, it may, of course, be itself coated with a conductive layer to serve that function. While transparent ferromagnetic materials exist that might be employed for element 32, they are comparatively expensive. Accordingly, it is usually preferred to operate cell 20 in FIG. 5 in a reflection mode so that only electrodes 36 need be transparent.
In FIG. 5, the orientation effects of the two fields are opposing. As a result, either one of the fields may be used to activate the desired light-modulation effect of cell 20 and the other subsequently employed to de-activate the cell. The use of one field to orient the particles and a different field to disorient them, or re-orient them to a position in which their effect on the light is similar to that when they are dis-oriented, permits the change of orientation to be achieved in a more precisely controlled and rapid manner than when relying solely upon Brownian or other thermal motion to dis-orient the particles.
FIG. 6 resembles FIGS. 3 and 4 in that cell 20 is oriented relative to storage element 32 so that flux lines 33 traverse that cell in a direction perpendicular to its major plane. In this case, an electrode 37 spaced from and parallel to element 32 is employed together with the latter to create an electric field E oriented in a direction lying in the major plane of cell 20 and, hence, perpendicular to flux lines 33. As in FIG. 5, the crossed electric and magnetic fields have opposing effects, so that one may be used to activate the cell and the other to deactivate it. The particular arrangement ofthe different parts of FIG. 6 is advantageous in that neither storage elements 32 nor electrodes 37 needs to be transparent. This allows the use of electrode materials exhibiting a higher conductivity than usually is available from transparent conductors.
The intention with respect to FIG. 3-6 has been merely to illustrate several of a number of different mutual orientations of the principal components that are possible in order to create either aiding or opposing eects from the applied magnetic and electric fields. It will become evident that the various display assemblies to be described may be modified as desired to incorporate any of these different arrangements. Moreover, other geometries than those illustrated in FIGS. 3-6 may be employed. For example, a conductor formed in a meander pattern in the plane of the display cell may be utilized for the purpose of generating a field perpendicular to that plane. Additionally, it is known that increased efficiency of reflection, and other optional effects, may be achieved by forming the particles to exhibit optical resonance at the frequencies of the light which may be employed. Such alternatives may be used herein when desired.
While FIGS. 3-6 are basically schematic in nature, FIG. 7 illustrates the actual structural details of an illustrative practical construction. An illumination-control cell 20 is disposed centrally within a pair of encircling electrical conductors 40 and 41. Situated adjacent to conductors 40 and 4l is a generally concentric ferromagnetic storage element 42. As shown, conductor 40 is a loop formed to conduct a current I, around cell 20 from an input lead 43 to an output lead 44. Similarly, conductor 4l is formed to conduct a current I2 around cell 20 from an input lead 45 to an output lead 46. Storage element 42 is in a position so that the magnetic field components created by both conductors 40 and 41 traverse storage element 42 and subject it to magnetization. Element 42, in turn, is positioned so that the magnetic field it creates, even in the absence of current in conductors 40 or 41, develops flux lines passing through the magnetic particles suspended within cell 20 in a direction perpendicular to the plane defined by a face of that cell. Each of conductors 40 and 4l and element 42 preferably have a sufcient depth, in the direction normal to the plane of the drawing, to be at least co-extensive with the corresponding depth of cell 20. An opening 47 in conductor 41 is provided for clearance of leads 43 and 44.
The development of the magnetic flux lines by the conductors is perhaps illustrated more clearly in FIG. 8 wherein conductor 40 and its leads 43 and 44 are shown apart from the other elements which make up the display units. When current is conducted into lead 43 and out of lead 44, magnetic lines of flux 48 encircle conductor 40. All flux lines travel in the same direction in traversing the area enclosed by the conductor. By selecting element 42 (FIG. 7) to be of a ferromagnetic material exhibiting substantial remanence and by supplying sufficient current to establish a substantial level of remanent magnetization in the ferromagnetic material, some flux lines 48 continue to exist even after termination of the original ener` gizing current.
While only a single current conductor is necessary to enable activation of a corresponding single control cell, both conductors 40 and 4l are included in FIG. 7 so as to enable activation of cell 20 only in response to the simultaneous appearance of current in both conductors. The current normally supplied to either conductor is insufficient to create a magnetic field component in turn sufficient to produce effective remanent magnetization of ferromagnetic storage element 42 or to change its existing remanent magnetization. However, the resultant magnetic field created by the sum of the two individual field components respectively created by the corresponding currents in conductors 40 and 41, is sufficient to retentively magnetize storage element 42 and orient the particles in cell 20. To this end, the ferromagnetic material of element 42 is selected to exhibit a relationship between its remanent magnetization and the current-developed applied field that has a reasonably pronounced knee or non-linearity. The current level supplied by either one of conductors 40 and 4l is insufficient to raise the level of magnetization above the knee at which point element 42 at least tends to become saturated and exhibits substantial remanence.
In operation, then, the absence or termination of flux from storage element 42 permits the particles within cell 20 to assume random orientations as a result of the thermal agitation of the liquid suspension within the cell. However, in the presence of the magnetic field the particles tend to align themselves with the magnetic flux. With a source of light disposed behind cell 20 as seen in FIG. 7, there is a significant increase in the quantity of light viewed from the front side of the cell when the magnetic field is applied. Consequently, the overall device of FIG. 7. constitutes a light modulator.
FIG. 9 represents a portion of an image display panel having a plurality of illumination-control cells 20 distributed over the panel in a matrix defining horizontal rows 50 and vertical columns 5l. Each of cells 20 are constructed as described in connection with FIGS. l and 7 so as to include particles of magnetic material suspended in a medium and normally obstructive to light but responsive to a magnetic field for being oriented to pass light through the cell. In such a transmissionmode embodiment, a source of light illuminating all of the cells is disposed behind the panel out of sight, and the arrangement of the current-carrying conductors and the storage elements is such as to permit activation of cells 20 to pass light through selected cells in the presence of selecting magnetic fields. Alternatively, of course, the construction may be modified for reflection-mode operation as explained in connection with FIG. 2. Each control cell 20 is partially encircled by a conductor 40 and a conductor 41, and a storage element 42 is provided for each cell 20. All of the encircling conductors 40 in each row 11 are connected in series, as are all of conductors 41 in each vertical column 5 1 If' desired, the conductors in any one row or column may alternately loop in opposite directions around successive cells 20, but is is preferred that they always loop in the same direction as illustrated.
Thus, all of the different pairs of conductors 40 and 41 constitute a plurality of magnetic-field-producing devices individually associated with respective different cells 20 and storage elements 42, and the latter likewise are individually associated with respective different cells 20. The different conductors, storage elements and control cells are supported in a preformed panel of a suitable electrical insulator 52, such as an epoxy resin, which is opaque and which does not interfere with the magnetic flux.
Because ferromagnetic materials exhibiting the desired storage and non-linearity characteristics typically are poor electrical conductors, discrete storage elements 42 preferably are provided, as illustrated. lf desired, each individual storage element 42 may be in the form of a coating encircling either or both of conductors 40 and 4l. However, when the storage element is in itself sufficiently conductive to the magnetizing current as was assumed in the discussion of FIGS. 3-6, either of conductors 40 and 41 may be formed of the storage-element material so as to function both as the storage element and as the current-carrying conductor. Accordingly, it is intended in the appended claims to contemplate the marriage of the storage and conduction or field-producing functions into a single physical element even though specified separately.
As revealed in FIG. 9a, each of cells 20 is sandwiched between a pair of auxiliary conductive electrodes disposed respectively on the back and front sides of the panel. Thus, each column 1 of cells in FIG. 9 is aligned with a vertically oriented auxiliary electrode 53 disposed on the back side of the panel. On the other hand, each of cells 20 in the uppermost row 0 in FIG. 9 is aligned with a horizontally oriented auxiliary electrode 54 disposed across the front side of the panel. Supported similarly on the front side of the panel is an auxiliary electrode 55 aligned with the middle row and an auxiliary electrode 56 aligned with the bottom row of cells 20. By applying a potential between selected auxiliary electrodes, an electric field, aiding the magnetic field as in FIG. 4, may be selectively impressed upon the individual cells 20. With individual auxiliary electrodes for each row and for each column, the electric field thus may be selectively applied to any given one or more of cells at a particular time so as to bias the selected cell or cells to a point enabling a desired level of particle orientation upon the concurrent development of a magnetic field by means of conductors 40 and 4l. Alternatively, individual selection and application of the potentials between different ones of the front and rear auxiliary electrodes may be employed as a further modulation of the degree of particle orientation within selected different cells so as to effect a control of contrast as between any one cell and another. Of course, for the model illustrated in FIG.l 9, if light is transmitted through cells 20, auxiliary electrodes 53-56 are all transparent.
In a modified version, auxiliary electrode 53 may cover the entire back side of the panel so as to be in common at one end of all cells 20 in the panel. In this manner, energization of any given one of the horizontal auxiliary electrodes S4, 55 or 56 in combination with electrode 53 enables the application of a preconditioning bias a row at a time in correspondence with line-by-line addressing techniques.
ln a further modification of FIG. 9, auxiliary electrode strips within the body of the panel are disposed parallel to and on each side of every row, or alternatively every column, of cells. Upon application of a potential between the strips of any adjacent pair and across the spanned cells, an electric field thus is created across the spanned cells, in the manner discussed in connection with FIG. 6, that opposes the particle-orienting action of conductors 40 and 41. Accordingly, the electric fields created by such electrodes may, for example, be employed to disorient the particles subsequent to their orientation as a result of the selectively applied magnetic fields.
However, the panel of FIG. 9 is particularly suited for the employment of magnetic control alone, for which case it may be assumed that auxiliary electrodes 53-56 are omitted. The coil conductors 40 of all cells in each horizontal row are interconnected by row-addressing electrodes 57, while coil conductors 41 of all cells in each vertical column are interconnected by column-addressing electrodes 58. As shown, the coils are connected in series by the addressing electrodes, but parallel or series-parallel connections may be employed if desired. The assembly of cells 20 with their associated magnetizing coils 40 and 41 and storage elements 42, supported in insulating chassis board 52, is enclosed by an encapsulating material 59 which may be of an appropriate transparent plastic or epoxy resin. ln practice, chassis board 52 with the assembled coils 40, 4l and storage elements 42 inserted therein may be molded into the front face 59A of the encapsulant while it is still in a plastic condition. After the encapsulating material 59A has hardened, the assembly may be turned to orient the storage elements 42 vertically. Storage elements 42, which may be composed of a molded ferrite material, may then be filled with the magnetic particle suspension, after which the back layer 59B of encapsulating material may be applied from above to complete the assembly.
Preferably, for optimum contrast in the image reproduced by the display panel, elements 60 of opaque material are provided on the back side of the display panel assembly to prevent the transmission of light through portions of the display panel other than through the illumination control cells 20. This may conveniently be done by coating the back surface of the assembly with a photosensitive resist such as polyvinyl alcohol or the like, actuating all of the illumination control cells 20 to an opaque or non-transmitting condition, and exposing the resist through the panel, so that the photo-resist is exposed only in those areas which transmit undesired light. The opaque light-blocking material, such as a suspension of colloidal graphite or the like, is then applied over the exposed photo-resist, after which the resist is developed to leave the opaque elements 60 only in those areas where needed.
With this construction and manner of operation, selection of a given one of cells 20 to transmit light is accomplished by applying electric current in the row and column that intersect at that cell. While all other cells in the same row will have current simultaneously supplied to the respective conductors 40 of those cells, the row current alone is insufficient to effect magnetization of any cells or their associated storage elements to a level significantly affecting particle orientation. Similarly, the current supplied to conductor 41 ofthe selected cell is also fed through the conductors 41 of the other cells in the same column as that of the selected cell; again, that current is of a value insufficient in itself to significantly affect particle orientation. However, the selected cell receives both row and column current. Each current generates a respective field component in the selected storage element, and the resultant field is sufficient to substantially orient the particles in the associated cell to make it highly transmissive to the light. As the same time, the combined or resultant field magnetizes its associated storage element 42 to a level past the knee of its hysteresis curve at which it is highly remanent. Accordingly, the particle orientation in the selected cell is retained subsequent to termination of either or both of the addressing currents. It may be noted that either one of the row and column" currents may be thought of as producing a "conditioning component and the other as producing a control component of the resultant particle-orienting magnetic field.
In order to subsequently erase the retained magnetization, a diminishing alternating current is applied to the addressing electrodes as previously described. In a line-by-line scanning system, such magnetization may be conveniently accomplished by sequentially addressing successive lines with the demagnetizing alternating current. Once the ferromagnetic storage material of any cell is demagnetized, the particles in that cell again resume a random orientation as a result of which the light transmission through the cell is sharply reduced.
It will thus be seen that storage element 42 permits the display panel to have the ability to be commanded to assume a certain state at any of a plurality of image element positions and to remain in that state until commanded to effect a change. For display devices that contain many picture elements, the property of storage of a command signal is of substantial significance. When the infomation necessary to activate a display element arrives in a short time interval, intense output levels are needed if the display is required to produce all of its output during the information-arrival time interval. For a conventional television display, for example, one picture-element addressing time interval is approximately oneeighth of a microsecond. Even when substantial persistence is exhibited, the average output during the persistence period must be large in order for the output, averaged over a frame time (the time interval between repetitions of the input data), to be sufficiently intense; however, the persistence time interval can only be a small fraction of the frame time in the ordinary case if smearing of moving images is to be avoided. With the storage technique disclosed, the average output can approach equality with the peak output. The display panel similarly is advantageous for non-repetitive output data such as in an alphanumeric computer readout device.
When displaying half-tone pictures, it is necessary for the purpose of obtaining proper contrast that the light-output in tensity exhibit varying levels in accordance with differences in shades of gray. That is, there must be intermediate levels of intensity between the conditions of off and entirely on. In the panel of FIG. 9 variation of the intensity ofthe transmitted light is secured by changing the level of magnetization of storage element 42. This, in turn, may be achieved by modulating either the row current or the column current, or both, with video signal information. The aligning torque produced by the magnetic field from the storage element is opposed by the Brownian movement of the particles in the suspending liquid, so that the average degree of alignment of the individual particles is a function of the strength of the aligning magnetic field. This relationship holds until all particles are too strongly aligned to be affected by Brownian motion.
When particles 24 or 26 are paramagnetic or weakly ferromagnetic, the moments induced on the particles are direct functions of the magnetizing field over a substantial range of field strengths. When, however, the particles are strongly ferromagnetic and exhibit a low saturation level, they experience a strong aligning torque once they are magnetized by only a weak field. For particles of the latter low-saturation characteristic, an appropriate gray scale can still be obtained by relying upon the non-uniform spatial distribution of the magnetic field; that is, the field is, of course, strongest near theferromagnetic material. As the field strength is increased, a larger volume of the particles within the control cell become magnetized to saturation; correspondingly, an increased field results in greater light transmission. Of course, a limit ultimately is reached at which all of the particles in a given cell are magnetized to saturation, in which case the amount of light transmission is at a maximum.
In general, however, it is preferred to use particles that are antiferromagnetic (e.g., gamma Fe203), ferrimagnetic (e.g., Fe3O.,) or at most weakly ferromagnetic (e.g., Ni) to combat clumping tendencies and control range limitations encountered when low-saturation (i.e., soft ferromagnetic) particles are employed.
A primary criterion for storage elements 42 is that they impose a net external moment on the suspended particles in the absence of a magnetizing eld and also that they exhibit the degree of non-linearity required to enable selection in the environment of the matrix. It should be noted that, in specifically designating the storage elements as being of ferromagnetic material, attention has been directed to that class of materials which presently appear to be most suitable. Nevertheless, other magnetic storage materials are indeed possible. For example, certain ferrites are insulators and are either antiferv romagnetic or incompletely compensated antiferromagnetic,
a property usually referred to as ferrimagnetic. In many applications, such antiferromagnets or ferrimagnets may also find utility in the panels disclosed herein.
If encountered, clumping arid migration problems may be obviated by arranging the addressing system so that oppositely poled magnetic fields are produced during successive frames, by reversing the direction of current flow from one frame to the next.
Further to the physical construction of the panel of FIG. 9, it may be observed that cells 20 constitute a plurality of small pockets of the particle-suspending liquid associated together in a larger unit. As such, cells 20 may be in the form of separately formed capsules or, in effect, bubbles" distributed over a large sheet of material. A plurality of the individual cells may be fabricated by utilizing known microencapsulation techniques. In some cases, it may even be advantageous to subdivide the particle suspension into smaller pockets in order to reduce settling and clumping difficulties.
In order to enable color reproduction, particles 24 or 26 may be selected to be of different individual sizes that resonate sharply at respective different particular wavelengths. It has been previously suggested to employ needle-like particles in associated clusters of cells, with the lengths of the particles differing between the dierent cells of each cluster so as to provide separate control of the light of different colors by separately orienting needles resonant to specific wavelengths for color selection. The resonant needles thus tuned to specific colors may be used in any of the scattering, absorbing or reflecting modes to produce areas of particular colors that are separately modulated in intensity by respective illumination-control cells 20. On the other hand, when the particles-do not exhibit wavelength discrimination, the panel may be illuminated with different colors in different locations in order to produce a color image display. For example, each cross point in the panel, as between vertical columns and horizontal rows 5 0, actually may be composed of a triad of cells, having the individually different cells in each trial assigned to the respective different primary colors. As a still further alternative, color reproduction may be obtained by utilizing filter overlays arranged in triads, stripes or other arrangements.
As an interesting adjunct to the use of the panel of FIG. 9 for displaying incoming signal information, it is contemplated to alter the magnetic fields stored in different ones of elements 42 by means of a manually controlled external magnetic stylus. For this purpose, FIG. 10 depicts an overall outline of a completed version of a panel 49 like that of F IG. 9, combined with an externally positioned permanent magnet 61 movable in any desired direction over the face of panel 49. Magnet 61 produces a field of sufficient strength to change the strength of magnetization of the different storage elements 42 encountered by the field of the magnet as the latter is moved over the surface of panel 49. Accordingly, images may be written into the display at will. When the display panel is utilized, for example, to produce an information-containing image fed from an outside source, a local operator or monitor may write in changes in the display as he desires. This is capable of use for the purpose of up-dating the displayed information or in association with a lecture in which the person handling magnet 6l desires to change the display in association with his presentation.
A variation of the immediate foregoing is to utilize magnet 61, or possibly a coordinated plurality of such magnets, for the purpose of writing onto the panel a decorative pattern when the device is not being used to display pictures. One thus may provide esthetic improvement in a large display for use in the home that would be unattractive when not in use of presenting only a blank area. It will be observed that use of a reflection mode of operation permits the display of such a decorative pattern without requiring the expenditure of any power to maintain the pattern once it is impressed upon the panel. That is, ambient light is relied upon to effect display of the attractive pattern.
More sophisticated esthetic-display arrangements also are contemplated. For example, it will be evident that simple geometric patterns may be developed by feeding information to the panel from fundamental logic circuit arrangements. More involved decorative patterns, involving complicated shadings, may be generated either by a conventional video signal generator responsive to recorded information or from a camera tube responsive to film strips. Similarly, the panel may be energized by a read-only memory coupled to an appropriate reader. In all of these alternative uses for producing a decorative effect, it is to be noted that, once the pattern has been placed upon the display panel, no further signal is required in order to maintain it.
A somewhat analogous use of panel 49 is in the selective masking of an object behind it. When all cells of the panel are addressed so as to transmit light, an image formed on a display device behind the panel then may be viewed through the panel. On the other hand, a portion or all of the cells may be rendered opaque in order to obstruct the view of the actual display device. Moreover, it is contemplated, when the purpose is to mask whatever device is to the rear of the panel, to write in a decorative pattern that not only is attractive but which assists in the masking function. However, it is not necessary to have both vertical and horizontal selection in the case when it is desired that all cells in the panel exhibit either one masking condition or the other. In a similar adaptation, the panel may be fed from one source of infomation while another image display behind it is fed from another; thus, the panel may be used as a correlator.
As another adjunct, the information read into the panel may be read out electrically in a manner analogous to known computer memory techniques. Reading pulses are modified by the level of storage existing in any storage element so as to yield a modulation representing the stored information. Such techniques are available to permit the reading while yet preserving the storage.
The particular physical construction of FIG. 9 is merely illustrative. As already indicated, and as emphasized by the several different magnetic-field and electric-field orientations discussed in connection with FIGS. 3-6, numerous alternatives and modifications may be made. Exemplifying a substantial difference in mechanical approach is display panel 62 of FIG. 1l wherein an illumination-control cell 63 again is composed of a liquid or similar medium in which are suspended a plurality of anisometric particles that orient themselves in a given direction in the presence of a magnetic field. Spaced on opposing walls of cell 63 are a pair of electrodes 64 and 65 which, when a potential is applied therebetween, create an electric field across the cell 63. A particle-orienting magnetic field is developed by currents passing through a conductor coil 66 wound around the bight of a U-shaped storage magnet 67. Disposed at and between the free ends of magnet 67 are a series of highly permeable strips 68 spaced along the external surface of electrode 65.
Upon the flow of current in conductor 66, a primary flux path 69 exists through magnet 67 and across its` open ends. Segments 68 reduce the total reluctance between the open ends of the magnet while at the same time permitting the development within cell 63 of a plurality of fringe fields 70. The material of magnet 67 has the characteristics previously described for storage elements 42 in FIGS. 7 and 9. That is, it exhibits substantial retentivity so as to hold its level of magnetization even after the cessation of current flow in conductors 66. On the other hand, segments 68 exhibit very low remanence so that, upon subsequent demagnetization of storage magnet 67, no fringe fields 70 remain within cell 63. So far as illustrated in FIG. l1, the arrangement enables but one degree of magnetic control. Of course, matrix selection may be incorporated simply by including two different sets of conductors wound upon storage element 67.
ln operation, the flow of current in conductors 66 creates fringe fields 70 which serve to align the particles in cell 63. The simultaneous application of a potential across electrodes 64 and 65 creates an electric field the direction of which is perpendicular to that of' the magnetic fields as in the fundamental configurations of FIGS. 5 and 6. Consequently, the
electric field may be employed to counteract the effect of the magnetic field in order either to modify the degree of particle orientation or to dis-orient the particles entirely even though the magnetic field persists.
In one adaptation of FIG. l l, electrode 64 is transparent so that cell 63 is appropriate for operation in the reflection mode with light entering and returning through electrode 64. On the other hand, the structure of FIG. 11 may also be used in a mode wherein light is transmitted through cell 63 in a direction perpendicular to the plane of the drawing. Moreover, each of the dilerent fringe fields 70 may represent a separate picture element position along a single row or column in a display panel. In that case, a separate magnetizing structure oriented in the coordinate direction is disposed adjacent to electrode 65 in order to obtain selection in the orthogonal direction.
In any case, FIG. 11 illustrates that it is not a necessary prerequisite for a distinct control cell to be provided at each picture element position. Instead, different fields may be applied to respectively different portions of an expanded-area single control cell in order to cause it to function as if it were a plurality of separate cells. On the other hand, the assembly of FIG. 1 1 may represent the structure of a cell corresponding to a single picture element. In that case, segments 68 merely are exemplary and may in actuality be composed of a random array of either dots or stripes of magnetic material.
A still different structural assembly is depicted in FIG. 12 wherein a cell 72 again includes a suspension of magnetically orientable particles. A storage element 73 is formed into a succession of U-shaped ferromagnetic members 74, around the bight of each of which are coiled current-carrying conductors 75. Closing the tips or free ends of all of members 74 is a plate 76 of magnetic material disposed alongside one major surface of cell 72. Each of magnetic members 74 is of a magnetic material the level of magnetization of which can be changed in correspondence with the amplitude of the current in conductors 75 and which maintains its magnetization level upon termination of such current. On the other hand, plate 76 is of a moderately high-permeability material that exhibits low remanence. In this instance, plate 76 is also electrically conductive and cooperates with a transparent conductive electrode 77 disposed on the opposite major surface of cell 72 for the purpose of impressing an electrical field across the width of cell 72 in the manner already described. For creating multiple rows or columns, a plurality of plates 76 are formed in successive strips.
In operation, magnets 74 are energized to levels such that they create a plurality of individual flux paths 78 each of which includes a fringe eld 79 that extends into cell 72 in a manner similar to that of fringe fields 70 in FIG. l1. Consequently, the manner of implementation of cell 72 into a panel display is essentially the same as that already discussed with regard to FIG. 1 1. In the case of FIG. l2, however, it is to be noted that the magnetic fields tend to cancel in the regions between each additional pair of members 74. Consequently, when each member 74 corresponds to a picture element position, cross-talk between adjacent picture elements is minimized by flux cancellation. While on first reaction, the structure of FIG. 12, as well as that of FIG. 1 1, may seem to be of undue mechanical complexity, it is to be observed that the U-shaped storage structures and plates 76 are capable of being molded out of a homogeneous magnetic material such as a ferrite. Alternatively, magnetic particles may be dispersed throughout a non-magnetic, e.g., plastic material. Moreover, conductors 66 or 7S may be molded directly into the material.
One alternative applicable to the structure of FIG. ll is to modify the configurau'on of cell 63 and electrode 65 so that segments 68 are located physically within the cell in a manner such that the suspended particles are disposed between adjacent ones of the segments. Consequently, a much greater portion of the existing flux is utilized and efficiency is correspondingly enhanced. Similarly in FIG. 12, the suspended particles may be situated in apertures formed in plate 76 individually in each of the respective different ones of flux paths 78.
Whatever the particle magnetic structure employed in association with each illumination-control cell, the utilization of a plurality of such cells in an image display panel is of primary interest. Numerous circuit techniques have been devised for the purpose of selectively addressing a matrix, such as that of FIG. 9, with row and column selection signals as well as with video signals composed of picture infonnation. FIG. 13 depicts in summary form the nature of a suitable and well-understood addressing approach for reproducing conventional television pictures. Accordingly, a video receiver 82 selects synchronizing signals from a received composite program signal and feeds them to a synchronizer 83. Vertical synchronizing pulses separated by synchronizer 83 are in tum fed to a line sequencer 84 that feeds magnetizing current successively to the leads 57 of respective different horizontal rows 0 of cells 20. Similarly, horizontal synchronizing pulses separated by synchronizer 83 are fed to an element sequencer 85 that delivers current to the leads 58 corresponding to the respective different ones of vertical columns 5 1.
Sequencers 84 and 85 may take any of a number of known forms. One conventional approach is to include in each a shift register that is stepped from each one output to the next by a series of gating pulses in turn initiated by a timing clock that is periodically synchronized with the signals from synchronizer 83. Unit 84 in this case also includes an erasing-signal generator. As already discussed, erasure of the stored magnetic energy is readily achieved by applying an alternating current wave that diminishes to zero. In FIG. 13, such as an erasing signal is applied to each of row-addressing leads 57 in succession at a time preceding the writing into that row of the video information. That is, the erasing signal is applied row by row at least a row ahead of the sequential interval that corresponds to the delivery of video information to that row. Accordingly, storage elements 42 are periodically de-activated in time correspondence with successive different intervals of` the video information.
The picture information or video signal from receiver 82 also is applied to sequencer 85 where as each succeeding column-addressing lead 58 is selected, the video signal is fed as a modulation upon the selecting signal to the selected column. Alternatively, the video signal may be applied through sequencer 84 successively to each of row-addressing leads 57 as selected. ln any event, the video signals modulate the amplitude of the conditioning or control pulses, or both, applied to rows i() and columns 5 1 so that the total magnitude of the resultant magnetic field at each selected cell is proportional to the instantaneous video signal level.
While several di`erent mechanical arrangements have been specifically discussed, and the earlier examples of FIGS. 3-6 clearly point the way to a wide variety of adaptations and modifications, in common with all of the different disclosed approaches is the utilization of an element to store the instantaneously developed resultant magnetic field to which the associated suspended particles are subjected. In this manner, persistence is obtained while at the same time enabling the use of low-viscosity suspensions that accommodate rapid response. Moreover, the magnetic storage elements themselves are selected so as to be capable of achieving a desired storage level from within a range of levels in order to permit the attainment of image gray scale. Although the drawings are greatly enlarged in order to clarify the illustration, the illumination-control cells and their magnetizing structures are capable of being fabricated in individually minute-sized units corresponding to adaptation in any of a large number of possible applications ranging from alphanumeric displays to television-display or similar high-resolution requirements.
As compared with at last most prior display panels that employ electric fields as the primary control force, much lower voltages are involved in connection with the operation of the display panels herein disclosed. Of course, this is advantageous both in reducing switching problems and in insulation requirements. In addition, it is not necessary that the entire control power be supplied by the external columnand row-sequencing circuitry. Instead, arrays of fixed electrodes may be included for the purpose of conducting the control power to each cell and the sequencing signals employed solely to operate or gate a switch associated with each respective cell and actuatable to connect the associated field-producing devices between the power-carrying electrodes.
It is to be noted that, for clarity of comprehension, the language used herein has been appropriate to orthogonally related arrays. Of course, it is intended to apply as well to other relationships such as those employed in various radar displays, including B, C and PPI presentations.
While particular embodiments of the present invention have been shown and described, it is apparent that changes and modifications may be made therein without departing from the invention in its broader aspects. The aim of the appended claims, therefore, is to cover all such changes and modifications as fall within the true spirit and scope of the invention.
l. An image-display system for controlled light transmission or light reflection comprising:
a plurality of illumination-control cells distributed in horizontal rows and vertical columns and each including particles of magnetic material suspended in a medium and responsive to a magnetic field for being changed in orientation from a first condition to a second condition, one of said conditions constituting a light-obstructing condition and the other a light-translating condition;
a corresponding plurality of magnetic storage elements respectively associated with said cells in sufficient proximity therewith to impress a controlling magnetic field thereon;
means including a corresponding plurality of magnetic fieldproducing devices individually magnetically coupled to said storage elements and responsive to an applied current in excess of a predetermined magnitude to establish a remenent magnetic field therein;
a source of picture signals and of rowand column-selection signals;
means responsive to said row-selection signals for effecting selective application to individual rows of said fieldproducing devices of a set of conditioning pulses to establish in each storage element of the selected row a magnetizing current of a magnitude less than said predetermined magnitude;
means responsive to said column-selection signals for effecting selective application to individual columns of said field-producing devices of a set of control pulses to establish in each storage element of the selected column a magnetizing current of a magnitude less than said predetermined magnitude to create a magnetic field component sufficient, when superimposed on a field component created by a conditioning pulse, to establish a resultant field sufficient to orient the particles of an illumination-control cell at the cross-point of the selected row and the selected column for controlled light translation;
means responsive to said picture signals for effectively modulating the amplitude of at least one of said sets of pulses so that the total magnitude of each of said resultant fields varies with picture content;
and means responsive to at least some of said selection signals for selectively restoring said storage elements to a reference condition.
2. An image-display system as defined in claim l, in which said storage elements exhibits substantial non-linearity in response to said field components.
3. An image-display system as defined in claim l, in which said restoring means comprises means for subjecting said cells to an electric field that opposes the effect of said resultant magnetic field.
4. An image-display system as defined in claim 3, in which said electric field acts upon said particles effectively at right angles to said resultant field.
5. An image-display system as defined in claim 1, in which said field-producing means include electrical conductors at least partially encircling said cells.
6. An image-display system as defined in claim 5, in which said storage elements are of a magnetic material and are disposed adjacent to respective ones of said conductors.
7. An image-display system as defined in claim 1, which further comprises means for subjecting said cells to an electric field that aids the effect of said resultant field.
8. An image-display system as defined in claim 1, in which said field-producing means includes a magnetic structure for enhancing the magnetic field to which each of said cells is subjected.
9. An image-display system as defined in claim 5, in which at least two separate electrical conductors at least partially encircle each of said cells.
10. An image-display system as defined in claim 9, in which separate conductors are similarly oriented with respect to each of said cells to provide substantial flux cancellation in the regions between adjacent cells.
1 1. An image-display system according to claim l, in which said magnetic storage elements also serve as containers for the magnetic particle suspension.
12. An image-display system according to claim 1, in which at least some of said magnetic field producing devices comprise the magnetic storage element associated with the corresponding illumination-control cells.
13. An image-display device comprising:
a plurality of illumination-control cells arranged in rows and columns and each comprising a suspension of anisometiic particles and means for impressing a particle-orienting field on said suspension to vary the light-translating capacity of the cell in response to variations in the strength ofthe applied field;
and a corresponding plurality of storage elements, one associated with each of said cells and each disposed in the particle-orienting field impressed on the associated cell.
14. An image-display device according to claim 13, in which said particles are light-absorbing and each of said cells is provided with a reflective backing element to provide for directreflection-mode operation of said image-display device.
l5. An image-display device according to claim 13, in which said particles are light-reflective and each of said cells is provided with a light-absorbing backing element to provide for inverse-reflection-mode operation of said image-display device.
16. An image-display device according to claim 13, in which said particles are optically resonant at a predetermined visible light frequency.
l7. An image-display device according to claim 13, in which each of said illumination-control cells is surrounded by lightabsorbing material.
18. In a two-dimensional display apparatus for displaying continuous tone images, matrix of` row and column addressable light control devices for modulating the intensity of input light in accordance with an electrical input signal, each device comprising:
a light control cell including a container enclosing an optically transparent fluid within which is suspended a large number of randomly distributed light intensity affecting particles which are urged toward a state of random orientation by Brownian motion, said container having transparent window means for admitting said input light into said container;
field-generating means responsive to said input signal for establishing across said light control cell a field to which said particles are responsive for ordering the orientation of said particles without substantially affecting the random distribution thereof so as to cause the intensity-affecting characteristic of said particles to have a value corresponding to the level of said input signal as applied to said field-generating means; and storage means associated with said cell for maintaining said field within said cell for a period of time substantially greater than a time interval during which said input signal is applied to said field-generating means such that said value of said intensity-affecting characteristics of said particles is also maintained for a period of time substantially greater than said time interval.
19. In two-dimensional visual display apparatus displaying continuous tone images, a matrix of row and column addressable elemental light control devices for modulating the intensity of input light in accordance with an electrical input signal, each of which devices includes a cell enclosing light-transmissive fluid within which is suspended a large number of randomly distributed magnetically sensitive particles and fieldgenerating means responsive to said input signal for applying across said cell a magnetic field for altering the orientation of said particles without substantially affecting the random distribution thereof to thereby alter the intensity of said input light, the improvement characterized by said device having associated with said fluid cell passive magnetizable storage means independent of said field-generating means which is located in and magnetizable by said magnetic field for causing said magnetic eld to be maintained on said particles for a period of time substantially greater than a time interval during which said input signal is applied to said field-generating means.
20. The device defined by claim 19 wherein said storage means is a hollow cylindrical member surrounding said fluid body and oriented perpendicular to said input light.
21. The device defined by claim 20 wherein said member serves to at least partially contain said fluid in said cell.