Abstract:
A display device comprises a substrate, cathode means for emitting electrons, a permanent magnet and one or more supports between the substrate and the magnet. A two dimensional array of channels extends between opposite poles of the magnet, the magnet generating, in each channel, a magnetic field for forming electrons from the cathode means into an electron beam. A screen receives an electron beam from each channel, the screen having a phosphor coating facing the side of the magnet remote from the cathode, the phosphor coating comprising a plurality of pixels each corresponding to a different channel. Grid electrode means is disposed between the cathode means and the magnet for controlling flow of electrons from the cathode means into each channel, the grid electrode means having a plurality of apertures, each aperture corresponding to one of the channels. The apertures are of varying cross-section in the vicinity of the supports such that localized variations in the emission of electrons by the cathode means caused by the one or more supports is compensated. The display also has one or more spacers between the screen and the magnet and anode means disposed on the surface of the magnet remote from the cathode for accelerating electrons through the channels. The anode means is of varying shape in the vicinity of the spacers such that localized variations in the electron beam shape and position caused by the one or more spacers is compensated.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a magnetic matrix display device and more particularly to spacers, supports, grid and anode electrodes for use in such a display. 
     BACKGROUND OF INVENTION 
     A magnetic matrix display of the present invention is particularly although not exclusively useful in flat panel display applications such as television receivers and visual display units for computers, especially although not exclusively portable computers, personal organizers, communications equipment, and the like. 
     Conventional flat panel displays, such as liquid crystal display panels and field emission displays, are complicated to manufacture because they each involve a relatively high level of semiconductor fabrication, delicate materials, and high tolerances. 
     GB Patent Application 2304981 discloses a magnetic matrix display having a cathode for emitting electrons, a permanent magnet with a two dimensional array of channels extending between opposite poles of the magnet, the direction of magnetisation being from the surface facing the cathode to the opposing surface. The magnet generates, in each channel, a magnetic field for forming electrons from the cathode means into an electron beam. The display also has a screen for receiving an electron beam from each channel. The screen has a phosphor coating facing the side of the magnet remote from the cathode, the phosphor coating comprising a plurality of pixels each corresponding to a different channel. 
     The magnetic matrix display uses a thick glass for the screen and backplate in order to ensure that a self supporting structure can be obtained when the glass envelope is evacuated. The thickness of glass required to provide this self supporting structure effectively limits this type of design to screen diagonals up to about 24 inches (610 mm). 
     To allow screen sizes with a larger than 24 inch (610 mm) diagonal, or to allow the thickness of the glass and hence the weight of the display to be reduced, thinner glass must be used. This requires the use of front spacers and back supports to withstand the atmospheric pressure on the outside of the glass envelope due to the vacuum within the glass envelope. Such spacers and supports can be designed, but the permittivity of an insulating support modifies the position of the remote cathode and changes the electron density in the vicinity of the support. This effects grid cutoff and the emission of electrons. The permittivity of an insulating spacer modifies the electrostatic field patterns in the vicinity of the spacer or of the support and hence will change the shape and/or direction of the electron beam. 
     Thus a visible pattern will appear on the screen at the locations where the spacers or the supports are located. If one component (spacer or support) is used between every pixel of the screen, then such visible patterning will not cause a problem, since it will be consistent across the whole area of the screen. In a practical design, such spacers and supports are positioned at intervals of about 10 mm and so the patterning can be discerned in the screen image generated by the display. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, there is now provided a display device comprising: a substrate; cathode means for emitting electrons, a permanent magnet; one or more supports between said substrate and said magnet; a two dimensional array of channels extending between opposite poles of the magnet; the magnet generating, in each channel, a magnetic field for forming electrons from the cathode means into an electron beam; a screen for receiving an electron beam from each channel, the screen having a phosphor coating facing the side of the magnet remote from the cathode, the phosphor coating comprising a plurality of pixels each corresponding to a different channel; grid electrode means disposed between the cathode means and the magnet for controlling flow of electrons from the cathode means into each channel, said grid electrode means having a plurality of apertures, each aperture corresponding to one of said channels, said apertures being of varying cross-section in the vicinity of the supports such that localized variations in the emission of electrons by the cathode means caused by said one or more supports is compensated. 
     The variation in cross-section of the apertures allows any shift in remote cathode position and change in electron density in the vicinity of the support to be compensated. This compensation may be by means of variation in the diameter of the apertures, or by variation in the shape of the apertures. Typically the apertures located nearest to the spacers are non-circular in shape, and preferably elliptical. 
     In a preferred embodiment, said cathode means for emitting electrons comprises an extraction grid; and said one or more supports between said substrate and said magnet are positioned so as to support the extraction grid. This allows the spacer to perform a second function, thereby eliminating the need for a separate support for the extraction grid. Also, in a preferred embodiment, said cathode means for emitting electrons comprises thermionic cathode filaments; and said one or more supports between said substrate and said magnet are positioned so as to support the thermionic cathode filaments. This allows the spacer to perform a third function, thereby eliminating the need for a separate support for the thermionic cathode filaments. 
     In a further preferred embodiment, the display further comprises one or more spacers between said screen and said magnet; and anode means disposed on the surface of the magnet remote from the cathode for accelerating electrons through the channels, said anode means being of varying shape in the vicinity of the spacers such that localized variations in the electron beam shape and position caused by said one or more spacers is compensated. 
     The varying shape of the anode means in the vicinity of the spacer allows any variation of the electrostatic field patterns in the vicinity of the spacer and hence change in the shape of direction of the electron beam to be compensated. 
     Preferably, said one or more supports between said substrate and said magnet and said one or more spacers between said screen and said magnet have a low conductivity such that charge accumulation is prevented. In a preferred embodiment, said one or more supports between said substrate and said magnet and said one or more spacers between said screen and said magnet are glass ceramic. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which: 
     FIG. 1 is a simplified cross-sectional view of an example of a prior art Magnetic Matrix Display device; 
     FIG. 2 is a cutaway plan view of the example of FIG. 1; 
     FIG. 3 is a simplified cross-sectional view of a prior art practical Magnetic Matrix Display device; 
     FIG. 4 is a graph of display weight versus glass thickness for varying prior art viewable diagonal sizes of screen; 
     FIG. 5 is a simplified cross-sectional view of a practical Magnetic Matrix Display device according to the present invention; 
     FIG. 6 is a view of spacers according to the prior art; 
     FIG. 7 is a view of spacers according to the present invention; 
     FIG. 8 is a view of a spacer located on the magnet surface; 
     FIG. 9 is a view of back supports according to the present invention; 
     FIG. 10 shows a prior art thermionic remote virtual cathode used in a Magnetic Matrix Display; 
     FIG. 11 shows the effect of a back support on cathode position and electron density; and 
     FIG. 12 shows the varying grid holes according to the present invention in the vicinity of the back support. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, an example of a magnetic matrix display device  10  comprises a plane cathode  20  facing a plane anode  30 . A phosphor coating  150  is disposed on the side of the anode  30  remote from the cathode. A permanent magnet  140  is disposed between the anode  30  and the cathode  20 . The magnet  140  is perforated by a two dimensional matrix of channels  160 . A grid assembly is disposed between the magnet  140  and the cathode  20 . The grid assembly comprises first and second electrically isolated arrays of parallel conductors hereinafter referred to as first grids  71  and second grids  72  respectively. The first grids  71  are arranged orthogonally to the second grids  72  to form a lattice pattern. Apertures are formed in the first grids  71  and the second grids  72 . The apertures are located at each intersection of a first grid  71  and a second grid  72 . Each aperture is aligned with a different channel  160 . The phosphor coating comprises a plurality of pixels each corresponding to a different channel. In a colour magnetic matrix display, each of the corresponding phosphor pixels may be a group of phosphor elements, each group corresponding to a different channel and each group typically comprising a Red, a Green and a Blue phosphor element. Deflection anodes  302 ,  304  are arranged as a pair of combs between the magnet  140  and the anode  30  to sequentially address electron beams emerging from the channels to different ones of the phosphor elements. 
     Referring to FIG. 2, column drive circuitry  170  is connected to the first grids  71 . Row drive circuitry  180  is connected to the second grids  72 . This has the advantage that for a conventional display having a four to three aspect ratio, with more columns than rows, the number of more complex expensive analog drivers is reduced at the cost of having more simple, cheap digital switches. Referring back to FIG. 1, in operation, the anode  30  is held at a higher potential than the cathode  20 . Electrons emitted from the cathode  20  are thus accelerated towards the anode  30 . As electrons enter each of the channels  160  in the magnet  140  they are collimated into a dense beam by the magnetic field therein. In operation, admittance of electrons to the channels is selectively controlled via the grid assembly. Each channel  160  is addressable by appropriate voltage signals applied by the row drive circuitry  180  and the column drive circuitry  170  to the corresponding first grid  71  and second grid  72 . Electrons are thus selectively admitted or blocked from entering each channel  160 , passing through the magnet  140  and reaching the corresponding region of the phosphor coating  150  to generate a pixel of a displayed image on the screen. The pixels of the displayed image are scanned in a refresh pattern. To produce the refresh pattern, a column of pixels is energized by applying an appropriate voltage, via the row drive circuitry  180  to the corresponding second grid  72  with the voltage on the first grids  71  set via the column drive circuitry  170  so that no beam current flows. The voltages on the remaining first grids  72  are set by the column drive circuitry  170  so that no beam current flows for any operating voltage on the second grids  71 . The voltages on the second grids  72  are then modulated by row drive circuitry  180  as a function of input video data corresponding to the energized column of pixels. The process is then repeated for the next successive column. The row and column functions are transposed relative to that conventionally used in LCDs, that is the rows are driven by an analog voltage and the columns are switched between two analog levels, however such transposition is not an essential feature of a magnetic matrix display. 
     FIG. 3 shows an exploded view of a prior art construction of a magnetic matrix display device. The magnet  140 , first  71  and second  72  grids and deflection anodes  302 , 304  are shown at the center together with the driver circuits  170 , 180  for the first  71  and second  72  grids. The magnet  140  is of the order of 2 mm thick. A metallized grid stand off is attached to the lower face of the magnet. The stand off comprises an insulating layer  314  having a thin metallized coating  316 . 
     On the side of the magnet having the metallized grid stand off is a substrate glass  310  on which the display device is mounted. The substrate glass  310  has exhaust holes  312  for evacuating the completed glass envelope. The substrate glass  310  is typically 20 mm thick so that the substrate is self supporting when the display structure is evacuated. The substrate glass  310  is separated from the magnet  140 , grids  71 , 72  and deflection anodes  302 , 304  by lower glass side supports  318  formed with the substrate glass  310 . These lower side supports  318  are of the order of 5.5 mm high and provide a separation between the glass substrate  310  and the grid stand off in which a cathode  20  is located. The cathode consists of cathode filaments  24  and an extraction grid  22 . 
     On the side of the magnet away from the metallized grid stand off is the screen glass  308 , on which the phosphor coating  150  and the aluminium backing forming the anode  30  is located. This screen glass is also typically 20 mm thick so that the screen glass is self supporting when the display structure is evacuated. The screen glass  308  is separated from the magnet  140 , grids  71 , 72  and deflection anodes  302 , 304  by upper glass side supports  306 . These upper side supports  306  are of the order of 5 mm high. Between the upper side supports  306  and the screen glass is a glass frit seal  318 . A connection  312  to the anode  30  passes through the glass envelope at the glass frit seal  318  to allow a connection to be made. An anti-reflective coated implosion protection film  309  is present on the front surface of the screen glass. 
     FIG. 4 shows a graph of the magnetic matrix display weight versus minimum thickness of glass used for the substrate and screen glass for varying sizes of display screen. Line  402  corresponds to a 14″ (356 mm) viewable diagonal magnetic matrix display. Lines  404  and  406  correspond to 16″ (406 mm) and 20″ (508 mm) viewable diagonal magnetic matrix displays respectively. As an example, for a 15″ (381 mm) display, glass having a thickness of between 12 and 14 mm is needed. Such a display weighs between 7 and 8 Kg. For a 21″ (533 mm) display, the required thickness increases to around 20 mm. Such a display weighs about 14 Kg. For a display of this size, with this thickness of glass, the weight is significant. Additionally, the thickness of glass usable reaches an upper limit because optical distortions occur which give an image a concave or “dished” appearance. In practice, about 24″ viewable diagonal is the limit for a flat screen display device with an unsupported glass faceplate. 
     FIG. 5 is a simplified cross-sectional view of a practical Magnetic Matrix Display device according to the present invention. Compared with FIG. 3, the screen glass  308  is reduced in thickness from 20 mm to 2 mm. Any reduction in thickness from the typical 20 mm of the prior art may be used, but reducing the glass thickness to 2 mm utilizes, to best advantage, the present invention. The substrate glass is similarly reduced in thickness to 2 mm. This may also be any thickness, but 2 mm is chosen to utilize, to best advantage, the present invention. The spacing between the magnet and the aluminium backing to the phosphors is reduced from 5 mm to 2 mm. Again, this dimension may be varied without departing from the present invention. 
     Conical spacers  502  are used to separate the magnet  140  assembly from the faceplate glass  308  and to withstand the atmospheric pressure on the outside of the glass envelope due to the vacuum within the glass envelope. Similarly back supports  504  are used to separate the magnet  140  assembly from the substrate glass  310  and to withstand the atmospheric pressure on the outside of the glass envelope due to the vacuum within the glass envelope. The use of the conical spacers  502  and the back supports  504  allows the use of thinner glass for the screen glass  308  and for the substrate glass  310 . The front spacers and back supports shown in FIG. 5 are not to scale and are illustrative of location between the magnet and the glass faceplate and substrate. The front spacers are typically positioned about 10 mm apart in a preferred embodiment. The back spacers are also typically positioned about 10 mm apart in a preferred embodiment. The size of the front spacers and back supports is shown in FIGS. 7 and 9 respectively. The location of the spacers relative to the channels in the magnet is shown in FIG.  8 . 
     FIG. 6 shows front screen spacers used in prior art Field Emission Displays (FEDs). At  602  is shown a FED spacer made by Micron having a length of about 0.7 mm and a diameter of about 0.03 mm. At  604  is shown a FED spacer made by Candescent having a length of about 0.5 mm and a cross-section of about 0.1 mm by 0.2 mm. Also shown in FIG. 6 for comparison are two standard size SMT elements. At  606  is shown a  201  sized SMT which has a height of 20 thou (0.5 mm) and a cross section of 10 thou (0.25 mm) by 10 thou (0.25 mm). At  608  is shown a  402  sized SMT which has a height of 40 thou (1 mm) and a cross section of 20 thou (0.5 mm) by 20 thou (0.5 mm). 
     FIG. 7 shows spacers according to the present invention. At  702  is shown a conical spacer having a length of about 1 mm and a maximum diameter of 0.244 mm. At  704  is shown an alternative design of spacer in the shape of a star. This spacer has a length of about 1 mm and a maximum diameter of 0.72 mm. At  706  is shown a conical spacer having a length of about 2 mm and a maximum diameter of 0.244 mm. At  708  is shown an alternative design of spacer in the shape of a star. This spacer has a length of about 2 mm and a maximum diameter of 0.72 mm. At  720  is shown, to scale, the phosphor/black matrix pattern on the screen. The tip of the spacer must fit onto the black matrix area, which in  720  is shown as 50 μm wide. 
     The spacers according to the present invention of FIG. 7 are relatively large compared with those of the prior art of FIG.  6 . They may be relatively easily manufactured out of ceramic or glass. Placement of the spacers in the display is a technology which is well known in the art for use with spacers for FEDs. 
     FIG. 8 shows the positioning of the spacers when viewing the magnet  140  surface. The individual channels  160  of the magnet can be seen, together with the deflection anodes  302 , 304 . In FIG. 8, both a conical spacer such as  702  or  706  from FIG. 7 and a star-shaped spacer such as  704  or  708  from FIG. 7 are shown. In a practical display, in general, both types of spacer would not be used in the same display. Additionally, the pitch between the spacers would be greater than that shown. The channels  160  shown are on a 300 μm pitch and are 180 μm in diameter. The tips of the spacers can be seen located on the surface of the magnet  140  between the channels  160 . 
     FIG. 9 shows back supports according to the present invention. At  902  is shown a star shaped back support having a length of about 5.5 mm, a maximum diameter of 2 mm and a minimum diameter of 0.244 mm. Also shown for comparison are the  201  sized SMT  606  and the  402  sized SMT  608  of FIG.  6 . At  904  is shown a star shaped back support having a length of about 5.5 mm, a maximum diameter of 2 mm and a minimum diameter of 0.72 mm. At the point of minimum diameter, the spacer is still star shaped, rather than being circular. 
     The effect of the back support on the electrostatic field will now be explained. FIG. 10 shows a prior art remote cathode used in a magnetic matrix display. The cathode may be of a thermionic or of a cold cathode type. The example of FIG. 10 shows a thermionic cathode in cross section. Electrons are extracted by a grid ( 22  in FIG. 5, not shown in FIG. 10) above an array of filaments  20 . The extracted electrons cycle between a position close to the magnet  140  and the rear glass substrate. The result is a plane of low velocity electrons close to the control grids on the magnet. The distance of this plane of electrons from the control grids and the density of the electrons in the plane are parameters which affect the cut off voltage set on the first grids  71 , and the electron beam pixel current set by the second grids  72  voltage. 
     The rear supports added between the magnet and the rear glass substrate have two effects: 
     1. The physical obstruction of the support will cause some electrons to be lost; 
     2. The permittivity will change the local electrostatic field potential and cause a local variation in electron position and density. The typical spacer is made of a glass ceramic material and has a permittivity of 5. 
     The exact disturbance which occurs is difficult to predict, but the disturbance itself is repeatable for any given structure. FIG. 11 shows a typical result, including the shift in remote cathode position and the change in electron density. 
     In the applicants co-pending GB Patent Application 9711744.4 (Attorney Docket Reference UK9-97-001) is disclosed that the cut off voltage and the gain of each pixel can be controlled by the first grid  71  and second grid  72  aperture diameter. These aperture diameters can be different from the aperture diameter in the magnet. Typically, the second grid  72  aperture is slightly larger in diameter than the magnet  140  aperture and the first grid  71  aperture is slightly larger than the second grid  72  aperture. The sizes of each or both of these aperture diameters may be increased or decreased in the regions surrounding a support by small amounts in order to equalise the cut off and gains to an appropriate value. Since the disturbance effect of the rear support decreases as we move further away from the support, then the aperture diameters would gradually revert back to the nominal diameters. So the present invention provides for modification of the first grid  71  and second grid  72  aperture diameters in a progressive manner around a support. The support can also be used to hold the virtual cathode extraction grid  22 . 
     In a variation of the present invention the first grid  71  and second grid  72  aperture diameters are not circular, but vary around the support. For example, the holes near to the support may be elliptical or “egg” shaped. FIG. 12 shows how the grid hole shape varies in a preferred embodiment of the present invention. The precise details of the hole shape, size and pattern for any particular application have to be determined by simulation and experimentation. 
     The effect of the front support on the electrostatic field will now be explained. In the magnet  140  to screen  150  area an electron exits from a pixel well or channel and passes through an electron lens formed by the EHT voltage on the final anode  30 , the deflection anode  302 , 304  voltage and the permittivity of the magnetic material, before hitting the phosphors on the screen. A spacer placed between the magnet  140  and the screen  150  must be physically designed so as not to obstruct the electron beam. However, the permittivity of the spacer component, typically 5, will cause a change to the beam shape and position due to the effect on the electrostatic fields. In addition, although the resistance of the spacer will be high, to avoid charge build up, there must be some finite level of conductivity. The resistance of such a slightly conductive spacer will vary with cross sectional area, further complicating the field pattern. 
     The shape and position of the electron beam can be controlled by the precise design of the deflection anodes around the pixels. The shape of the deflection anodes  302 , 304  may be modified in the region of the spacer in a progressive manner in such a way as to counter the effects of the material permittivity.