WO2001041176A2 - Virtual cathode having a segmented backing electrode - Google Patents

Abstract

A display device comprises a face plate assembly (14), a segmented cathode subsystem having a segmented backing electrode (19) and an emitter array (20), an accelerator (52), and first and second grid elements (42,44). The face plate assembly has an electrically positive receptor (16) that causes an image to be displayed as a result of electron impingement thereon. Active segments of the segmented backing electrode and the emitter array establishing a uniform space-charge cloud of free electrons functioning as a proximity virtual cathode. The segmented backing electrode has segments and a controller to activate the segments individually. The accelerator located a first distance from the electron emitter array creating at least one virtual cathode remotely located a second distance from the accelerator. The first and second grid elements are located between the face plate assembly and the at least one remotely located virtual cathode to select electrons from the remotely located virtual cathode to impinge the electrically positive receptor.

Description

SEGMENTED VIRTUAL CATHODE

FIELD OF THE INVENTION

The present invention relates generally to flat electron control devices and more particularly to a specifically designed flat virtual cathode device which differs significantly from the prior art.

BACKGROUND OF THE INVENTION

Figure 1 illustrates a typical prior art approach to flat cathode ray visual display devices. Referring to Figure 1, the flat cathode ray visual display device is a prior art high vacuum device that is generally indicated by reference numeral 10. High vacuum device 10 includes a face plate assembly 12 having a face plate 14 and an electrically positive phosphorescent coated and aluminized back face 16 (also referred to as a screen or anode) which, as a result of impingement of electrons thereon, provides a visual image as viewed from the front face of face plate 14.

Spaced rearward of the screen and in front of back plate 18 and backing electrode 19 are a series of thermionically heated wire electron emitters 20 disposed in a plane parallel with both the back face 16 and back plate 18. Each of the wires of electron emitters 20 is responsible for producing its own supply of free electrons in a cloud of free electrons or virtual cathode around and along the length of itself, as generally indicated by the individual clouds or virtual cathodes 22. These free electrons are acted upon by a grid stack 24 having addressing electrodes, a buffer electrode, focusing electrodes, and in some cases a deflecting mechanism, all of which will be discussed immediately below, so as to cause the electrons acted upon to impinge on specific areas of the back face 16 of the face plate assembly 12 in order to produce a desired image at the front face of face plate 14. For purposes of description, the planes containing the emitters, screen, grid stack and back plate will be defined by the x and y axes and the axis perpendicular thereof will be the z axis.

Still referring to Figure 1, the grid stack 24 of electrodes includes an electrically isolated buffer electrode 25, one or more apertures address plates 26 and one or more focusing electrodes, two of which are exemplified as electrodes 28 and 30. As an example of the address plate 26 the latter may include a dielectric substrate 32 having a front face 36 and a back face 38 and closely spaced apertures 40 extending in the z direction between these faces in an array of rows and columns.

The particular address plate illustrated also includes a first set of parallel strip address electrodes 42 disposed on the back face of substrate 32 and a second set of parallel strip electrodes 44 normal to electrodes 32 on front face 36. For purposes of discussion, the address electrodes 42 will be referred to as the first address electrodes and the electrode strips 44 will be referred to as the second address electrodes, as these are the closest and second closest address electrodes to the supply of electrons. It should be noted that while electrodes 42 are the first address electrodes, the buffer electrode 25 is actually the first electrode in the stack. The components making up overall display device 10 as described so far are conventional components and hence will not be discussed in any further detail.

Also it is to be understood that not all of the components making up the device 10 have been illustrated. For example, the overall device 10 includes a housing or envelope that may or may not integrally incorporate face plate 12 and back plate 18 but which nevertheless defines an evacuated interior containing the phosphorescent-coated electrically positive screen 16, backing electrode 19, thermionic emitter array 20 and the grid stack 24 as described above. The device 10 also includes gas absorption devices such as getters to maintain high vacuum, a suitable mechanism for energizing the thermionic emitter array 20 in order to produce their respective clouds of virtual cathodes 22 for providing a controlled positive unidirectional field and a mechanism (not s lown) for voltage biasing the various other electrodes including placing a bias on backing electrode 19 with respect to the virtual cathode voltage, in order to act on free electrons produced by the emitters in an attempt to cause those electrons acted upon to move in a relatively uniform current density with increasing z axis velocity toward the buffer electrode. Throughout this process, the buffer electrode 25 is maintained at a positive voltage relative to the electron emitter voltage, thereby taking a positive role in drawing electrons to it. At the same time, mechanisms (not shown) are provided for addressing (by appropriately voltage biasing) selected sectors of the first and second control electrodes at any given time in order to draw electrons through specific apertures 40 and in the direction of back face 16. Once those electrons pass through the selected apertures, the remaining electrodes 28 and 30 (and any others if they are provided) function to focus or deflect or otherwise further direct the electrons passing there through onto back face 16.

It is to be understood that device 10 has been provided as a generalized example of some categories of the prior art and is not intended to incorporate all of the features of prior art devices or represent a specific device. For example, other prior art devices may utilize a different arrangement of addressing and focusing electrodes and /or may provide different types of individual electron emitter systems. However, in each of the prior art applications of the type generally illustrated in Figure 1 (of which applicant is aware), a spatially non-uniform supply of free electrons is produced and acted upon directly by the buffer, addressing and focusing electrodes (and possibly deflecting electrodes) in order to produce the desired image. In the case of device 10, the clouds 22 of free electrons surrounding electron emitter 20 provide such a supply which is acted upon directly by the grid stack 24.

Flat display devices exemplified by device 10 have been found to produce visual displays that tend to vary uncontrollably in brightness from a spatial standpoint. There are two basic causes for this "washboarding" effect. First, there are density variations in the free electrons produced by and relative to the emitter wires. More specifically, the number of free electrons approaching the grid stack 24 immediately behind and available to one sector of the address plate might differ from the amount behind and available to another sector. Therefore, even if two different apertures are addressed for the same amount of time with the intent of causing the same number of electrons to pass there through in order to provide equally illuminated pixels on the back face 16, different amounts might in fact pass through the apertures and therefore result in pixels having entirely different illumination intensities. The second washboarding effect is a result of the wide-angle approach of some of the electrons being caused to move into a given aperture being addressed. These "wide angle" electrons tend to pass through the particular aperture off axis, thereby making focusing variable.

Ideally, one way to eliminate the washboarding effect described is to provide device 10 with electron emitter wires 20 directly behind and in close proximity and precisely spaced with respect to each and every aperture 40 so that each of these apertures could draw from similar reservoirs of electrons.

In that way, if any two or more apertures are addressed for the same amount of time, they would under ideal conditions draw the same number of electrons and therefore illuminate the screen with the same degree of intensity. However, it should be apparent that from a practical standpoint in these systems there are far too many apertures in the address plate to provide an equal number of electron emitters, nor could the cathodes and the apertures of the address plate be precisely aligned.

Another drawback of devices exemplified by device 10 resides in its use of buffer electrode 25. As stated above, this electrode is maintained at a relatively high positive voltage relative to the electron emitter voltage. As a result, the buffer electrode 25 acts as a constant current drain as does the backing electrode 19 if the latter is maintained at a positive voltage.

Exemplary device 10 is one approach to flat visual display devices. Another approach is illustrated in U.S. Patent Nos. 4,227,117; 4,451,846; and 4,158,210. These patents describe devices that use a series of focusing, deflecting and accelerating electrodes working in unison to produce an array of individual scanning electron beams on a cooperating electrically positive screen. While devices of this type do not generally have washboarding problems, they are subject to cathode emission variations and problems associated with deflection distortion and borderline registration. In still another prior art approach, electrons are produced by means of a plasma- generated cloud by means of an address stack in front of the cloud and directed onto an electrically positive screen. A problem with this technique is that the light output on the screen is limited (weak). There are also other known disadvantages to this approach.

Another category of flat display devices utilizes single, multiple or ribbon beams directed initially essentially parallel to the plane of the display and are then caused to change directions essentially in the Z direction to address appropriate areas of the display target either directly or by way of a selecting and /or focusing grid structure. Examples are the Aiken and Gabor devices, U.S. Patent Nos. 2,928,014 and 2,795,729, respectively, using single guns, the RCA multibeam channel guide system as exemplified by U.S. Patent Nos. 4,103,204 and 4,103,205 and the Siemens A.G. controlled slalom ribbon device (U.S. Patent No. 4,437,044). The major drawback of these systems resides in their construction and /or electrical and electron optical control complexities. The Siemens approach issued in U.S. Patent No. 4,435,672 by Heynisch utilizes a cathode region permeated by very low velocity electrons described as having velocities of 1 to 2 volts and described variously as "electron reservoir," "electron cloud," "cloud of low velocity electrons," "electron storage space" and "electron gas." Some of the problem areas of these systems involve:

1. The ability to maintain density uniformity, since even minor magnetic fields will disturb the uniformity of the space charge cloud, such as those occasioned by the earth's magnetic field or those generated by currents in the circuitry; 2. The lack of adequate electron density due to the relatively large volume required for the overall cathode space; and

3. There is no reasonably fixed cathode distance which can act as a virtual cathode for the purpose of controlling the subsequent focusing action required to obtain small, well defined spots at the screen.

An example of a visual display device that attempts to overcome some of these problems is described in U.S. Patent No. 4,719,388. This display device creates a uniform space-charge cloud of electrons as a virtual cathode. An apertured address plate acts upon the electrons within the cloud so as to cause the electrons acted upon to impinge on specific areas of an electrically positive back face plate. However, the display device described is limited, to some extent, by the problems described above. SUMMARY OF THE INVENTION

A display device and method for displaying have been disclosed. In one embodiment, the display device comprises a face plate assembly, a segmented cathode subsystem having a segmented backing electrode and an emitter array, an accelerator, and first and second grid elements. The face plate assembly has an electrically positive receptor that causes an image to be displayed as a result of electron impingement thereon. Active segments of the segmented backing electrode and the emitter array establish a uniform space-charge cloud of free electrons functioning as a proximity virtual cathode. The segmented backing electrode has segments and a controller to activate the segments individually. The accelerator located a first distance from the electron emitter array creates at least one virtual cathode remotely located a second distance from the accelerator. The first and second grid elements are located between the face plate assembly and the at least one remotely located virtual cathode to select electrons from the remotely located virtual cathode to impinge the electrically positive receptor. BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood more fully from the detailed description given below and from the accompanying drawings of various embodiments of the invention, which, however, should not be taken to limit the invention to the specific embodiments, but are for explanation and understanding only.

Figure 1 is a diagrammatic illustration, in side elevation, of a flat display device designed in accordance with the prior art;

Figure 2 is a partially broken away exploded perspective view of one embodiment of a flat visual display device;

Figure 3 is a diagrammatic illustration, in side elevation, of the device of Figure 2;

Figure 4 diagrammatically illustrates operational aspects of the device of Figures 2 and 3;

Figure 5 is a diagrammatic illustration, in side elevation, of a flat visual display device designed in accordance with a second embodiment;

Figure 6 shows a basic unidirectional planar electrode diode system in terms of voltages and spacing;

Figure 7 diagrammatically illustrates an approach in terms of electrical and spatial parameters to attain an oscillatory remote virtual cathode system;

Figure 8 diagrammatically illustrates systems more general than that of Figure 7 and used to attain oscillatory diode systems for remote virtual cathode applications; and Figure 9 diagrammatically illustrates a system which reduces the energy requirements in the establishment of the virtual cathode system particularly in large area and high luminance devices.

Figure 10 diagrammatically illustrates a layered break away top view of Figure 9 showing partial views of various electrodes and the relationship between the active remote virtual cathode and the delineation of the active first grid element in one embodiment.

DETAILED DESCRIPTION

A flat electron control device is described. In the following description, numerous details are set forth. It will be apparent, however, to one skilled in the art, that the present invention may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form, rather than in detail, in order to avoid obscuring the present invention.

Overview

In one embodiment, a flat visual display device is disclosed herein and includes a flat face plate having a front face and opposite back plate and electrically positive receptor on the latter which as a result of impingement of the electrons thereon, provides a visual image through the front face of the face plate. The device establishes a uniformly dense planar space charge cloud of free electrons, which is also a remote virtual cathode, within a planar band parallel with and rearward of the back face of the display faceplate. The device includes an apertured address plate disposed in spaced-apart confronting relationship with the back face of the faceplate between the latter and in very close proximity of the uniform planar virtual cathode to act on the electrons within the virtual cathode in a controlled way to cause the electrons acted upon to impinge on specific areas of the electrically positive receptor of the display faceplate in order to produce a desired image through the transparent face plate's front face.

A flat visual display device is described herein that includes a planar receptor, for example a flat or slightly curved display screen, which may be identical to the one forming part of device 10 of Figure 1. To the extent necessary, U.S. Patent No. 4,719,388, entitled "Flat Electron Control Device Utilizing a Uniform Space-Charge Cloud of Free Electrons as a Virtual Cathode," issued January 12, 1988, is incorporated herein by reference.

In one embodiment, the planar receptor comprises a face plate assembly having a front face and a coated electrically positive back face and a mechanism on the latter which, as a result of impingement of electrons thereon, provides a corresponding visual image as viewed from the face plate's front face. The planar receptor need not be a visual display screen. It could be, for example, an end plane of individual electronic leads to activate other devices such as, for example, a liquid crystal display. However, for purposes of discussion, the receptor will be described as a display screen and the overall device will be referred to as flat visual display device.

The display device also includes a grid stack which may be identical to stack 24 forming part of device 10 in Figure 1 or an arrangement which only includes the apertured address plate. The grid stack 24 may incorporate address electrodes and a supply of free electrons for use by the address electrodes. In one embodiment, the electrodes forming part of the stack 24 or any other electrodes do not draw any appreciable power from the free electrons during operation of the device.

In addition and in accordance with the present invention, the flat visual display device disclosed herein utilizes an electron emitter mechanism for establishing a uniformly dense space-charge cloud of free electrons within a planar band parallel with and just rearward of the back side of the first address grid so that each and every aperture in the address plate sees and acts upon a reasonable uniform or nearly equal supply of electrons during operation of the device. The addressed apertures of the grid stack 24 as shown in Figure 1 pass a similar number of electrons for a given increment of time, thereby effectively eliminating or substantially reducing the non-uniformity or washboarding effect described above.

In one embodiment, the above noted dense planar space charge cloud forms a virtual cathode, i.e., the density of the cloud may be such that the electrical field within the cloud at some plane (e.g., within the band referred to above) drops to slightly below emitter potential. The virtual cathode potential is very slightly, less than 1 (one) volt, below the emitter potential due to the thermal energy of the electrons emitted by the emitter. In devices using free electrons, space-charge limited emission may be used. In one embodiment, this device only uses space charge limited emission. This means that more electrons are thermally ejected into the virtual cathode than are subsequently used by the device. The excess electrons in the virtual cathode fall back into the emitter due to the increasing thermal injection of electrons as well as any decreases in electrons used by the device.

In one embodiment, the uniformly dense space-charge cloud of free electrons or "virtual cathode" is established by use of a backing electrode and an accelerator electrode in combination with the previously described first address electrode of the device's grid stack, all three acting on electrons supplied by suitable electron emitter mechanism such as emitter array 20 in Figure 1. These three components cooperate with one another in order to cause free electrons emitted by the emitter to oscillate back and forth in a pendulum-like fashion between two planar bands, one essentially coplanar with and incorporating the emitter plane and the other one behind and adjacent to the first address electrode.

Thus, at least one embodiment of a flat high vacuum visual display device has been described which is not subject to the non-uniformity or washboarding effects discussed above nor excessively sensitive to magnetic fields. Also, at least one embodiment of a flat visual display device is energy efficient in operation. Detailed Embodiments

Turning now to the drawings, wherein like components are designated by like reference numerals throughout the various figures, attention is immediately directed to Figures 2 and 3, as Figure 1 has been discussed previously.

Figure 2 illustrates a flat visual display device which is generally indicated by the reference numeral 46. This device 46 may include the same face plate assembly 12 (or other such planar receptor), back plate 18, electron emitter array 20, and apertured address plate 26, as described previously with respect to device 10 illustrated in Figure 1. While face plate 14 is shown flat, it can be made slightly curved (defining a relatively large radius) for manufacturing purposes as can all of the otherwise flat components making up the overall device, although it is not basically necessary that the remaining components be curved. For purposes herein the term flat is intended to include those slight curvatures.

The apertured address plate 26 is located directly behind and in parallel relationship with the back face 16 of face plate assembly 12. In one embodiment, back face 16 comprises a phosphorescent coated and aluminized back face. In alternative embodiments, the coating on back face 16 may be made of copper, gold or other well-known vacuum-compatible material.

The first addressing electrodes 42 extend into the depth of apertures of address plate 26 and in one direction on the back face 38 of the address plate's substrate 32 and second addressing electrodes 44 extend into apertures of address plate 26 and in normal directions on the opposite side of the address plate 26. The apertures 40 in the address plate are illustrated in both Figures 2 and 3.

Note that device 46 does not necessarily include or at least does not have to include (although it may include) additional focusing, deflecting and /or addressing electrodes between the address plate 26 and back face of screen 16 such as focusing electrodes 28 and 30 and other such electrodes which may make up the grid stack 24 in device 10 of Figure 1.

The wire-like electron emitter array 20 in device 46 runs perpendicular to addressing electrodes 42 and also to these electrodes in device 10. The wires of electron emitter array 20 could run in either direction.

In one embodiment, display device 46 has an outermost envelope which, while not shown in its entirety, includes face plate 14 and back plate 18 and defines an evacuated chamber containing the screen 16 of the display face plate 14, wire-like electron emitter array 20 and address plate 26 as well as other components to be discussed hereinafter.

In addition to the components thus far described, in one embodiment, overall flat visual display device 46 includes a plate like backing electrode 19 located behind the wires of electron emitter array 20 in a plane adjacent to and parallel with (and possibly supported by) backing plate 18 and a grid- shaped accelerator electrode 52 disposed within a plane parallel with and between address plate 26 and the wires of electron emitter array 20. The way in which these two additional components operate in device 46 will be described hereinafter.

For the moment it suffices to say that these two additional components in combination with those described previously establish a planar uniform dense virtual cathode 56 of free electrons in a planar band (e.g., a flat layer having some thickness) essentially coincident with the plane of electron emitter array 20 denoted as the proximity virtual cathode due to its incorporation of the emitter wires. A second virtual cathode denoted as remote virtual cathode 54 is disposed in parallel relationship with and immediately behind the address electrodes 42 or the entrance electrode (25 of Figure 5) if used. The components including the backing electrode 19, the emitter array 20, the accelerator 52, and the space up to but not including the first address electrodes 42 (or the entrance electrode 25 if used) shall be denoted as the cathode sector 60 (which will also include the quiescent virtual cathode). As will be shown, the remote virtual cathode 54 is essential to the operation of the device 46 while the proximity virtual cathode 56 is a result of the way in which the virtual cathodes are established. Therefore, some of the discussion henceforth will be directed primarily to the remote virtual cathode 54 although it will be understood that the entire cathode sector 60 is of major importance to the design of the device as detailed subsequently.

From the way in which remote virtual cathode 54 is established, as will be described, it will be apparent that this reservoir of free electrons has essentially zero forward and rearward z-axis velocities (e.g., in the direction normal to the plane of address plate 26) and the individual electrons have a random Maxwellian cross beam velocity distribution (parallel to the plane of the address plate 26). Stated another way, each and every point or sub-area within space-charge cloud 54 at a given planar distance from the address electrodes 42 includes essentially the same density of free electrons displaying the same essentially zero velocity conditions as each and every other point or sub-area. In that way, the "virtual cathode sectors" which are identical to one another are established at each and every of the apertures 40 immediately behind addressing electrodes 42 (or the entrance electrode 25 if used). As electrons are drawn from these virtual cathode sectors by the apertures during the addressing mode of the device 46, the voids they leave are immediately filled so as to preserve the uniformity of the overall virtual cathode, provided the number of electrons emitted by the emitter array 20 is well in excess of the current which is drawn by the grid stack 24 and accelerator electrode 52 as will be discussed. This is because the virtual cathode 54 is made to be sufficiently dense, in the manner to be described hereinafter, as compared to the number of free electrons drawn to the addressed aperture, so that addressing the cloud by the aperture has a reduced, and perhaps minimal, effect on the cloud's field. When electrons are drawn from the virtual cathode 54, the tendency of the virtual cathode 54 to maintain equilibrium causes a redistribution in which electrons in the immediate surrounds move in to fill the void. This assures that each of the apertures 40 has a continuous supply of electrons to draw from and that each supply is the same as the other.

Having described space-charge cloud of remote virtual cathode 54 and before describing how this cloud is established, attention is directed to the way it is utilized in combination with address plate 26 for directing controlled beams of electrons from the cloud through selected apertures 40 and on to back face 16 in order to produce a desired visual image on the latter. To this end, certain nomenclature should be noted. Specifically, those apertures which are energized or addressed are ones which are caused to direct electrons from remote virtual cathode 54 towards back face 16. On the other hand, those apertures which are not energized or addressed are maintained electronically closed to the passage of electrons.

Whether any specific aperture is addressed or not depends upon the voltages on the particular first address electrodes 42 and second address electrodes 44 which orthogonally cross that aperture. In the case where no apertures are being addressed, that is, during the quiescent mode, the first address electrodes 42 are maintained (biased) at a voltage at most equal or slightly negative with respect to the most negative field in the remote virtual cathode 54 while the second address electrodes 44 are also maintained at a quiescent potential of the first address electrodes 42 or any negative cutoff potential. Thus, in the case where no apertures are being addressed, none of the electrons from remote virtual cathode 54 are attracted by the address plate 26 and thus there is no current passage to the screen 16 due to electron passage through the specific aperture being addressed. If a row conductor in the first address electrodes 42 is only addressed, then only the row in question attracts electrons from the virtual cathode 54 at relatively low potential. This is to be distinguished from device 10 and the prior art where there is continuous current drain in the grid stack 24 by a buffer electrode where the buffer electrode is unipotential and is maintained at a positive voltage with respect to its electron emitter array 20. If a negative unipc tential entrance electrode 25 electrically insulated from the first address electrodes 42 is used in the grid stack 24, each of the first address electrodes 42 does not necessarily have to be zero or negative relative to the unipotential entrance electrode 25, but it must be such that in combination with the entrance electrode 25 no current will flow into the grid stack 24.

The precise "cutoff" voltages on each set of address grids (of first address electrodes 42 or second address electrodes 44) is adjusted so that no current due to field penetration flows to the back face of screen 16 as a result of the turn-on pulse voltage of the other electrodes or due to capacitive coupling. If an entrance electrode 25 is used aft of the first address electrodes

42, as will be described with respect to Figure 5, then the combination field established with the latter provides the "cut off" function similar to the first address electrode without the presence of an entrance electrode 25. It is also a purpose of the entrance electrode 25 to prevent collection of electrons by the current conductor delineations of the grid of first address electrodes 42.

The electrical conductors of both address electrodes 42 and 44 operate the control grids of apertures 40 to a considerable depth so as to create electron optical lens systems with height to diameter ratios of the apertures that may be in the order of two to three. In one embodiment, the electron optical lens systems are cylindrical. In an alternative embodiment, the electron optical lens systems comprise tapered or conical electron optical lenses. In one embodiment, the purpose of using an entrance electrode is to:

1) Prevent loss of electron current from the remote virtual cathode

54 by the conduction strips of the first address electrodes 42. In one embodiment, the entrance electrode 25 shields these areas by its continuous unipotential surface, typically fixed at a cut off potential.

Thus, the entrance electrode 25 provides isolation of the remote virtual cathode 54 from the first address electrodes 42 to shield conductors and prevent electron absorption. 2) Prevent voltage variations across the grid stack 24 at the first address electrodes 42 due to current and associated conductor resistances.

3) Prevent lens distortions at the entrance lenses to the first address electrodes 42 during operation resulting from field distortion caused by the conduction strips of first address electrodes 42.

4) Prevent electron flow into the surfaces of the first address electrodes 42 by avoiding electron collection at the conduction strips and also at the first grid aperture electrodes during active operation due to the now convergent configuration of the entrance lenses.

5) Provide field shaping control upon the entrance lenses to aid in focussing.

6) Aid in control of the location of the remote virtual cathode 54. Thus, the entrance electrode 25 may be controlled in order to perform these functions in a manner not performed in the prior art.

Similarly, an exit electrode, located between the address electrodes and the face plate, may be controlled to provide the following functionality:

1) isolation to prevent cross talk between adjacent apertures in grid stack 24, and

2) field shaping to aid in focusing.

3) Prevent lens distortions at the exit lenses to the second address electrodes 44 during operation resulting from field distortion caused by the conduction strips of second address electrodes 44.

In order to energize or address a particular aperture, its specific first and second address electrodes are energized to voltage levels positive with respect to the potential of the remote virtual cathode 54. For purposes herein, it is to be understood that the potential of the remote virtual cathode 54 is not necessarily at the same potential as the proximity virtual cathode. However, the potential of the remote virtual cathode 54 is the reference potential for all elements forward of the remote virtual cathode. If emitters of electron emitter array 20 are directly heated structures, then there is a non- addressing mode or period in order to heat up to maintain electron emissions and thereby maintain the virtual cathodes during visual operation of the device. During the heating mode which generally occupies only a very short time compared to the visual operating time, a short heating pulse is applied to the ends of the emitter wires which causes the emitter potentials to vary along the wire lengths and thus also the potentials of the associated virtual cathodes, which is not acceptable. For proper visual operation, the overall potential at the virtual cathodes remains essentially unipotential during visual operation requiring that visual operation be interrupted during the heating cycles. If the emitters of emitter array 20 are indirectly heated, then there would be no need for a non-addressing mode. This would, however, require a very complex system involving insulated heater elements inside tubular emitters replacing the wire emitter, which is not practical.

The use of a segmented virtual cathode subsystem (using the segmented backing electrode and emitter array combination), described below, substantially reduces energy consumption of the cathode subsystem.

Line Sequential Operation

The device described herein can be operated in a "line sequential operation" which includes operation of one line at a time or several spaced apart lines at a time. It is of course also possible to interchange lines and columns in this arrangement. For simplicity, one embodiment of the method to be discussed herein will be the "one line at a time" line sequential operation. It should however be understood that the basic principle will apply to all systems noted. In this method of operation, pulse width (or length of time) rather than amplitude modulation is used to obtain gray scales.

When operating the display device in line sequential operation, a full line of first address (control grid) electrodes 42 is activated for a period of time and during such period of time all necessary second address (control grid) electrodes 44 are activated for units of time corresponding to the desired intensity of the required image. After such period of time, another full line of first address electrodes 42 is activated for a period of time and during such period of time all necessary second address electrodes 44 are activated for units of time corresponding to the desired intensity of the required image. In one embodiment, the activation of first address electrodes 42 progresses sequentially until all first address electrodes 42 have been activated. This permits a long writing time per pixel as compared to the scanning beam of a standard CRT. The required current per pixel is thereby considerably reduced and /or the luminance can be increased.

By having the first address electrodes 42 on for an extended period of time, all (or a large portion) of the second address electrodes 44 may be on over a length of time or a fraction thereof, with each of the second address electrodes 44 being independently manipulated. By varying the amount of time any particular pixel is on, the intensity of that particular pixel may be controlled in order to achieve a gray scale.

Using the line sequential operation, many apertures in grid stack 24 may be operated simultaneously. Thus, while increasing the number of apertures being operated, the line sequential operation also reduces the frequency of switching required, thereby increasing the time any one pixel is on.

Establishing a Remote Virtual Cathode

Having described the remote virtual cathode 54 and the way in which address plate 26 is operated, attention is now directed to Figure 4 which illustrates how remote virtual cathode 54 is established.

It is first assumed that only systems similar to the one in Figure 1 exist incorporating the entrance electrode 25, the emitter array 20, the virtual cathodes 22 and the backing electrode 19. Initially, the entrance electrode 25, the emitter array 20, and the backing electrode 19 are all at emitter potential. The emitter array 20 (if eating is pulsed and only the non-pulsed conditions are observed) then thermally ejected electrons will cause virtual cathodes 22 to surround the emitter wires. These electrons will form slightly negative potentials that actually constitute the virtual cathode. Some of these electrons because of their negative potential eventually fall back to the emitters. Their volumetric density is a function of the emitter temperature.

If a positive potential is applied to the entrance electrode 25 (e.g., 40 volts) we have a diode structure and electrons are drawn from the virtual cathodes

22 to the entrance electrode 25. Additional electrons are injected into the virtual cathodes 22 as current is drawn by the buffer (anode). This current will be limited by the volumetric density distribution p in the inter-electrode space and self-adjusted until the slope of the potential curve at the virtual cathode is zero. This condition is one aspect of "space charge limited operation." If the temperature at the emitter is too low and/or the voltage gradient too high the space charge will disappear and temperature limited operation will prevail, a condition not useful for vacuum displays.

The system as described above will cause electrons to be accelerated and be totally absorbed in a non-uniform "washboard like" distribution at the buffer. The approach of the electrons is also not angularly uniform. If electrons in a mesh type buffer as in Figure 1 are further accelerated through apertured address plate 26 and toward the back face 16 the beam spots will be distorted in both luminous distribution and focus.

The operation in Figure 4 differs dramatically from that in Figure 1. Although the electron emission theory is the same, from thereon the similarity ends. On the initial instance of turning on the display, electrons start from cylindrical virtual cathodes. These are then altered into a planar proximity virtual cathode as noted hereinafter. The initial electrons drawn from the cylindrical virtual cathodes are accelerated toward the high transmission accelerator 52 (for example at 95% transmission) where they attain maximum velocity and minimum density. Under the foregoing assumption, 5% of the electrons will be captured and the remainder will travel toward the grid stack 24. Since the operation is quiescent, no electrons will enter the grid stack 24. Because the electrons are decelerated they will attain zero velocity before again being accelerated toward the accelerator 52.

At the accelerator 52, 5% will be absorbed and 95% of the amount returning will move toward the emitter plane again being decelerated after passing the mesh of accelerator 52. The voltage at the backing electrode 19 is adjusted so that the field intensity in the plane of the emitters in emitter array 20 is essentially at proximity virtual cathode potential. Under these conditions, electrons will return to any point at the reference potential plane and will be re-accelerated toward the accelerator. At 95% accelerator transmission, this procedure will statistically be repeated for about 10 (ten) times for each electron emitted from the proximity virtual cathode. As electrons are absorbed by the accelerator 52, they will be replaced from the proximity virtual cathode and ultimately from the emitters.

The oscillations and replacements are continuous and non- synchronous.

The thickness of the virtual cathode regions is due to slight differences in the thermal energy between the statistical distribution of electrons and actual individual electrons ejected from the emitters and is very small.

The density uniformity of the virtual cathode regions is due to the mutual electron repulsion in the x-y planes caused by the extremely high volumetric density in the virtual cathodes. This effect is also operative when electrons are drawn from the remote virtual cathode in the nonquiescent mode by single or dual lines or columns of the first electrodes of the grid stack 24. It should also be noted that the number of electrons drawn from the remote virtual cathode is very much smaller than the number available at the remote virtual cathode if the system is operated under space charge limited conditions.

The details of operation of the remote virtual cathode system and its operation will now be discussed. Numerical examples will be used which will serve only to explain technical details that are of major importance in the operation of the system.

Essentially, the system is predicated on planar space charge current limited diode operation as explained by the use of Figures 6 through 8 and the Child-Langmuir equations given below.

where J is the current density in amperes (A) per unit area (e.g., A/unit area), a² is a constant given as 2.335 -lO^"6 ,

V is the anode voltage in volts at the anode or accelerator, and z is the distance from the virtual cathode to the anode or accelerator. In one embodiment, the two derived equations for V and z are: V = 5.682 • 10³ J^{2 3} z ^4/3 volts (2)

1.528 - 1Q- V %

-unit length (3)

The emitter in Figure 6 is assumed to be planar and continuous. Those in Figures 7 and 8 are spaced apart planar wire arrays. In the basic planar diode system of Figure 6, the anode is a planar conductive surface. In the case of the abutted oscillatory systems Figures 7 and 8, the anodes are assumed to be a 95% transmission mesh denoted as the accelerator. In one embodiment, the stipulated voltage at the anode is at 40 volts relative to the virtual cathode in Figure 6 and the proximity virtual cathodes in Figures 7 and 8. In Figures 7 and 8, aft of the emitter plane and parallel to it is a backing electrode (not shown) whose function is to maintain a near zero reference potential between the wires of the emitter array. The diode dimensions in diagrams Figures 6, 7, and 8 are referenced from the most negative point of the space charge cathodes for both z and V. The planar dimensions of components such as the backing electrode, the emitter plane, anode or accelerator, and subsequent tube components are all planar and assumed to be infinite to avoid discussions about edge or boundary effects.

The basic planar diode system as illustrated in Figure 6 shows the z-V relationship of Equation (1) above. The derived value of the current density J is 57 mA/cm² and is of course constant for all intermediate values of z. At the virtual cathode, the slope (dV/dz) is zero. The curve is not a straight line function due to the variations of the volumetric space charge density p which is maximum at the virtual cathode and decreases toward the anode. Increasing the end values of V or decreasing z will increase the current density J. Other examples of parametric relationships from the Child- Lagmuir equation are:

Volumetric charge density p ∞ z ^{~2 3}

Velocity v x z ^2/3

Voltage V x z ^4/3

Field Gradient E ∞ z ^1/3

In the above arrangement, the current density J is unidirectional which will not be the case in oscillatory systems.

In the abutted near-mirror-image oscillatory diode system, conditions are shown in Figure 7. Using the previously assumed input values, the operation of the system will now be demonstrated and evaluated.

Referring to Figure 7, the sector between proximity virtual cathode and the accelerator will be denoted as side 1 and that of the sector between the accelerator and the remote virtual cathode as side 2. Since the voltage V and the z dimension are specified, the value of the current density on side 1 is fixed even with electron absorption by the accelerator or when current is drawn by the address electrodes during active operation. This is due to the fact that the value of J remains constant since z and V are fixed by equation (1). It also needs to be noted that the electron current density J is now bidirectional and since the Child-Langmuir equation depends on current density distribution, the two components of J are additive. Thus, the value of J is the same for the unidirectional case of the basic planar diode system and for the oscillatory system. The value of J does not differ and remains at

57 mA/ cm² regardless as to how many oscillations are involved. In this situation, there is no current density or space charge density positive gain due to oscillation after the display device has been turned on.

Since the current density on side 1 of Figure 7 is constant and a 5% current absorption exists when the current passes through the accelerator, then the value of J on side 2 is 5% lower or 95% of the value of side 1. This must be taken into account in evaluating the Child-Langmuir equation on side 2. If the voltage V on side 2 is the same as on side 1, then the value of z must differ from that of side 1 because there is no emitter to replace electrons on side 2. For this reason, the two sides can only be near mirror images.

Assuming that the accelerator on side 2 is the only existing electrical field, then the distance z to the remote virtual cathode can be calculated directly from equation (3) provided the lower value of J is used and also the voltage V is the same on both sides.

To use the remote virtual cathode as the source of electrons, the first address electrode voltage should be operated a very small and precise distance from the remote virtual cathode at somewhat below reference potential of the remote virtual cathode in order to control the geometry of the first lens of the subsequent electron gun optics for purposes of focusing and intensity control of the resulting electron beams and their cutoffs.

If an entrance electrode is used, the situation is similar except that the voltage of the entrance electrode is constant and held at slightly below space charge potential. In one embodiment, the entrance electrode extends over the entire grid system (selection array) other than the apertures .

The above noted procedure may be difficult to obtain because of precise spacing requirements in the z dimensions.

Other methods are shown in Figure 8. In Figure 8, the design and assembly precision is of considerably less importance. For the sake of comparison, the system shown in Figure 7 has also been incorporated in this diagram. The major difference between the abutted duo diode system of Figure 7 is the use of variable voltages on side 2. The values of z and V on side 1 in Figure 8 have been held constant for the three examples illustrated.

The values in case 2 on side 2 were held as given in Figure 7. The positive values of the voltages in cases 1 and 3 need to be and have been matched with the positive value of the accelerator voltage (+40V) on side 1. The voltages on side 2 have been chosen as +30 V in case 1 and +50 V in case 3.

The corresponding values of J and z on side 2 have been calculated on the basis of 95% of J on side 1 and the stipulated values of V for each case on side 2 are noted in the diagrams. It is of course equally appropriate to calculate V by selecting the values of z. In practice, z can be the initial design value and the actual value of V can be determined by the cutoff conditions at the first address electrode or the entrance electrode. A high degree of precision is therefore no longer required. This method can also be used in correcting mechanical errors in the device of Figure 7, however the equality of the reference potentials on sides 1 and 2 will no longer apply.

Corrective action to change the distance the remotely located virtual cathode is from the accelerator for variable voltage methods can be accomplished by using the remote virtual cathode as a new reference potential for all remaining elements forward of the remote virtual cathode 54, or by using the remote virtual cathode potential as the reference potential for all elements in the cathode system 60.

Returning again to Figure 8, side 2 has a large range of remote virtual cathode potentials and design values for z available for any existing values of voltage V and the terminal value of z on side 1. The terminal value of V on side 1 will always be at the same point as the terminal value voltages for all possible diode systems on side 2. The values of the integrated bidirectional current density J for all possible systems on side 2 will be common but less than that of side 1 by the reduced transmission through the accelerator toward side 2. The reference voltages on side 2 will be the difference in the diode voltage of side 1 and that of side 2. Thus for case 1, this will be 40V - 30V =

+10V and for case 3 it will be 40V - 50V = -10V.

The voltage at the first address electrode under quiescent conditions or that of the entrance electrode should be negative to these values by about 1 volt or less to provide for current cutoff.

For fixed conditions on side 1 and any fixed value of z on side 2, there is but one diode voltage value possible on side 2 that solves the equation.

In the case of side 2 in Figure 8, the ratio of V^3/ /z² remains constant such that for any value of V there is a corresponding value of z.

Active Sectors

In one embodiment, the power consumption by the cathode system 26 as described herein consists of two components: 1) the power required to maintain emitter temperature levels adequate to keep the necessary electron emission for sustained space charge limited thermionic operation, and 2) the power required to operate the remote virtual cathode 54 consisting mainly of the accelerator voltage and current. Thermal emitter power losses are subject mainly to the size of the device and the electron emission requirements to satisfy luminous intensity requirements of the system and cannot readily be reduced. However, the power used at the remote virtual cathode 54 can be reduced considerably by the arrangement noted previously in Figure 9 and Figure 10 as described below.

Instead of extending the remote virtual cathode 54, which is planar, to encompass the entire area of the grid stack 24, the remote virtual cathode 54 can be reduced to smaller active sectors which correspond to the active lines of the first address electrodes 42, such as, for example, active electrode 38, and then keeping the active sectors synchronized with the active lines of the first address electrodes 42.

Note that the entrance electrode 25 in Figure 9 has not been included in Figure 10. The details of this arrangement will now be described with reference to Figure 9 and Figure 10. As described below, the backing electrode 19 is altered by segments 61 and 62 disposed perpendicular to the electrodes of the emitter array 20. If "line sequential operation" is used as described above, then the segmentations should be in the "x" direction if the emitters are disposed in the "y" direction. To prevent electron current flow in non- active areas the associated backing electrode sectors 61 will be operated at a negative potential relative to the emitter array 20 in such a manner that a sufficiently high negative potential 63 can be established forward of the emitters in the inactive region 64. This will cut off electron current from reaching the accelerator electrode 52, thereby reducing the total accelerator current. The accelerator voltage is, however, not altered. To accomplish this, the backing electrode segments 61 and 62 should be located as closely as reasonable to the emitter electrodes of emitter array 20. The emitter wires of emitter array 20 are still surrounded by cylindrical virtual cathodes but the ejected electrons fall back to the emitter wires of emitter array 20.

At the active sectors 62 the system functions as described above. In one embodiment, these sector groups or strips are sufficiently wide and generally centered on the active first grid electrodes to avoid reduction of space charge density in the central sector and avoid adverse boundary conditions from affecting the active first grid electrode 54. There may be more than one active sector group. As shown in both Figure 9 and Figure 10, in one embodiment, the addressing of the backing electrode strip 61 and 62 is timed and generally centered with the position of the active electrode of the first address electrodes 42.

The operation of the electrodes in the display device described herein throughout are controlled using a controller that controls which electrodes have potentials applied to them and the amount of the potential that is applied.

Whereas many alterations and modifications of the present invention will no doubt become apparent to a person of ordinary skill in the art after having read the foregoing description, it is to be understood that any particular embodiment shown and described by way of illustration is in no way intended to be considered limiting. Therefore, references to details of various embodiments are not intended to limit the scope of the claims which in themselves recite only those features regarded as essential to the invention.

Thus, a display device has been described.

Claims

CLAIMSI claim:

1. A display device comprising: a face plate assembly having an electrically positive receptor on the face place that causes an image to be displayed as a result of electron impingement thereon; a segmented cathode subsystem having an emitter array and a plurality of backing electrode segments to establish a uniform space-charge cloud of free electrons functioning as a proximity virtual cathode; an accelerator located a first distance from the plurality of electron emitter electrodes creating at least one virtual cathode remotely located from the accelerator; and first and second grid elements located between the face plate assembly and the at least one remotely located virtual cathode to select electrons within a space-charged cloud of the remotely located virtual cathode to impinge the electrically positive receptor.

2. The display device defined in Claim 1 wherein the plurality of backing electrode segments are controlled in singular sectors.

3. The display device defined in Claim 1 wherein the plurality of backing electrode segments are controlled in multiple sectors.

4. The display device defined in Claim 1 wherein the segmented cathode subsystem comprises a plurality of electron emitter electrodes operating in combination with plurality of backing electrode segments to establish the uniform space-charge cloud of free electrons functioning as a plurality of proximity virtual cathodes between the plurality of backing electrode segments and the accelerator, the plurality of electron emitter electrodes being located between the face plate assembly and the backing electrode.

5. The display device defined in Claim 4 wherein the plurality of backing electrode segments substantially reduces electron current flow to the areas of the accelerator.

6. The display device defined in Claim 5 wherein the plurality of backing electrode segments are disposed substantially perpendicular to the plurality of emitter electrodes.

7. The display device defined in Claim 4 further comprising a controller to prevent electron current flow by operating non-active segments of the plurality of backing electrode segments at a negative potential relative to the plurality of proximity virtual cathodes in such a manner as to establish a sufficiently high negative field potential forward of emitter electrodes of the plurality of emitter electrodes substantially perpendicular to the non- active segments of the plurality of backing electrode segments.

8. The display device defined in Claim 4 wherein the plurality of backing electrode segments are disposed in a first plane perpendicular to a second plane containing the plurality of emitter electrodes.

9. The display device defined in Claim 1 further comprising a controller to activate each of the plurality of backing electrode segments individually in response to application of a potential.

10. The display device defined in Claim 9 wherein the controller activates only a number of segments of the plurality of backing electrode segments and the first grid elements coinciding with activated backing electrode segments.

11. The display device defined in Claim 1 further comprising a controller to cause different voltages to be applied simultaneously to different segments of the plurality of backing electrode segments.

12. A method for controlling a display device comprising: establishing a uniform space-charge cloud of free electrons functioning as a proximity virtual cathode; activating portions of a segmented backing electrode and emitters in an emitter array to create at least one remotely located virtual cathode at a first location using an accelerator positioned a first distance from the proximity virtual cathode, the first location being a second distance from the accelerator; and activating first and second grid elements coinciding with activated portions of the segmented proximity virtual cathode to select electrons from at least one remotely located virtual cathode to impinge an electrically positive receptor, including activating at least a portion of one or more of the first grid elements for a first period of time and activating a portion of the second grid elements during the first period of time.

13. The method defined in Claim 12 further comprising preventing electron current flow forward of non-activated portions of the segmented backing electrode.

14. The method defined in Claim 13 wherein preventing electron current flow forward of non-activated portions of the segmented proximity virtual cathode comprising operating non-activated segments of the segmented backing electrode at a negative potential relative to the proximity virtual cathode in such a manner as to establish a sufficiently high negative field potential forward of emitters in the emitter array.

15. The method defined in Claim 12 further comprising activating only a number of segments of the segmented backing electrode and the first grid elements coinciding with the activated segments of the segmented backing electrode to select electrons.

16. An apparatus for controlling a display device comprising: means for establishing a uniform space-charge cloud of free electrons functioning as a proximity virtual cathode; means for activating a number of segments of a segmented virtual cathode to create at least one remotely located virtual cathode at a first location using an accelerator positioned a first distance from the proximity virtual cathode, the first location being a second distance from the accelerator; and means for activating first and second grid elements coinciding with activated portions of the segmented emitter to select electrons from the at least one remotely located virtual cathode to impinge an electrically positive receptor, including activating at least a portion of one or more of the first grid elements for a first period of time and activating a portion of the second grid elements during the first period of time.

17. The apparatus defined in Claim 16 further comprising means for preventing electron current flow forward of non-activated portions of the segmented backing electrode.

18. The apparatus defined in Claim 17 wherein the means for preventing electron current flow forward of non-activated segments of a segmented backing electrode comprising means for operating non-activated segments of the segmented backing electrode at a negative potential relative to the proximity virtual cathode in such a manner as to establish a sufficiently high negative field potential forward of the segmented backing electrode.

19. The apparatus defined in Claim 16 further comprising means for activating only a number of segments of the segmented virtual cathodes and the first grid elements coinciding with the activated portion of the segmented backing electrodes to select electrons.

20. A display device comprising: a face plate assembly having an electrically positive receptor that causes an image to be displayed as a result of electron impingement thereon; an electron emitter array and a backing electrode to establish a uniform space-charge cloud of free electrons functioning as a proximity virtual cathode; an accelerator located a first distance from the electron emitter array creating at least one virtual cathode remotely located a second distance from the accelerator, the first and second distances being different; and first and second grid elements located between the face plate assembly and the at least one remotely located virtual cathode to select and focus electrons from the remotely located virtual cathode to impinge the electrically positive receptor.

21. A display device comprising: a face plate assembly having an electrically positive receptor that causes an image to be displayed as a result of electron impingement thereon; an electron emitter array and a backing electrode to establish a uniform space-charge cloud of free electrons functioning as a proximity virtual cathode; an accelerator located a first distance from the electron emitter array creating at least one virtual cathode remotely located from the accelerator; first and second grid elements located between the face plate assembly and the remotely located virtual cathode to select electrons from the at least one remotely located virtual cathode to impinge the electrically positive receptor; and a controller coupled to activate first and second grid elements, wherein the controller activates at least a portion of one or more of the first grid elements for a first period of time and activates a portion of the second grid elements during the first period of time.