US 3626241 A
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United States Patent  Inventor Dinh-Tuan Ngo Colts Neck, NJ.  Appl. No. 887,994  Filed Dec. 24, 1969  Patented Dec. 7, 1971  Assignee Bell Telephone Laboratories, incorporated Murray lllill, NJ.
 GRAY SCALE GASEOUS DISPLAY 15 Claims, 11 Drawing Figs.
 U.S.Cl 315/167, 313/112, 315/169  Int. Cl 11051 37/00  Field of Search 313/1095, 112; 315/167, 169, 169TV  References Cited UNITED STATES PATENTS 3,015,747 2/1962 Rosenberg 315/169 TV SIGNAL SOURCE ADDRESS CCT.
susmmme SIGNAL souncr I CO NT ROL cmcupi e0 3,042,823 7/1962 Willard 3,114,065 12/1963 Kaplan Primary Examiner-Roy Lake Assistant Examiner-Lawrence .l. Dahl Attorneys-R. J. Guenther and Kenneth B. Hamlin PATENTED DEC 7 I971 SHEET 2 BF 3 AQQQLE! FIG. 6
SIGNAL SOURCE PATENTED DEC 7197! 3,626, 241
SHEET 3 OF 3 WRITE PULSE SUSTAINING Vb SIGNAL ISIS IS CELL ADDRESS GRAY FIG. 7C SCALE SIGNALS l I I 7m DISPLAY k FIG. 70 CELL xb CURRENT 703 P I DISPLAv FIG. 75 CELL X0 1 CURRENT GRAY SCALE GASEOUS DISPLAY BACKGROUND OF THE INVENTION This invention relates to display devices, and more particularly, to display devices upon which images are generated by the selective energization of individual display cells or elements.
Display devices are typically used for generating patterns of information or images in a two-dimensional raster for information display media, television, radar, computer input/output terminals, and the like. The principal types of display devices currently available include cathode-ray tube presentations, which suffer from well-known disadvantages related to size, cost, ruggedness and power requirements. The need for a display device which would overcome these disadvantages has been apparent for some time and considerable effort has been expended toward achieving such a display device.
Currently one of the areas of greatest promise appears to be gaseous displays of the type generating display images through the breakdown discharge of a gas, particularly such displays utilizing pulsed discharges to breakdown a gas to lightemitting plasma. Plasma displays are digitally addressable and have inherent memory, the latter eliminating the need for external memory storage and associated circuitry to regenerate the display image. However, known plasma display devices, as well as other gaseous discharge display devices, in view of their basic ON-OFF characteristic, suffer from an absence of the multilevel gray scale or variable contrast required for television and other similar display applications.
Time division multiplexing arrangements have been proposed for providing such displays with the required gray scale by varying the duty cycle; that is, by energizing the individual display cell elements for varying durations during each scanning frame. However, such known arrangements have so far proven to be too costly and complex from a manufacturing standpoint.
Another approach has been to array the display cells in spot clusters, selected cells or combinations thereof in a cluster being energized for different desired intensities or gray. scale levels. This approach has not proven entirely satisfactory due principally to various manufacturing problems and to problems related to cell addressing and image resolutions.
SUMMARY OF THE INVENTION It is accordingly an object of this inventionto provide a new and improved gaseous display arrangement having a multilevel gray scale.
More particularly, it is an object of this invention to provide a new and improved gray scale gaseous display arrangement which alleviates the disadvantages of known arrangements.
A conventional gaseous display device, such as a plasma display device, typically comprises a coordinate array of crosspoint display cells defined by row and column conductors which are spaced apart by first and second layers of dielectric material having a layer of gaseous display material disposed therebetween. According to a feature of my invention, the above and other objects are attained in a simple and economical manner in an illustrative embodiment of a gray scale gaseous display arrangement by stacking a plurality of conventional gaseous display devices separated by respective light-attenuating layers. Each successive light-attenuating layer in the stack attenuates the light reaching the viewing surface by a predetermined amount, illustratively by a factor of two. Thus, the light reaching the viewing surface from the second display device level in the stack isattenuated by a factor of two, from the third by a factor of four, from the fourth by a factor of eight, etc.
Accordingly, a 2" level gray scale is achieved in accordance with my invention with a stack ofn display devices. For example, a 64-Ievel gray scale, which is more than adequate for commercial television applications, is provided by an arrangement of six gaseous display device levels and five light attenuating layers interleaved therewith, having a combined thickness on the order of one-tenth inch or less.
An important advantage ofagray scale gaseousdisplay arrangement in accordance with my invention is that addressing may be effected via a single conventional addressing circuit employed in common for all display device levels, with each bit of an n-bit gray scale codeword determining the ON-OFF character of the respective display levels. Moreover, an arrangement according to my invention is compact and reliable, is significantly less expensive to manufacture, than known arrangements, and provides better'resolution than arrangements using cell clusters. In addition, my invention is particularly suited to color television applications by appropriate selection of display gases or by the use of appropriate filters between the display levels.
BRIEF DESCRIPTION OF THE DRAWING The above and other objects and features of the invention may be fully apprehended from the following detailed description and the accompanying drawing in which:
FIG. l-is'a diagram of an illustrative embodiment of-a gaseous display arrangement providing a multilevel gray scale in accordance with the principles of my invention;
FIG. 2 shows a portion of the display embodiment of FIG. 1 in cross section;
FIG. 3 is a graphical representation of a typical voltage-current characteristic for electric discharge across a display cell;
FIG. 4 is an alternative illustrative embodiment of a display according to my invention;
FIG. 5 is another alternative illustrative embodiment of a display according to my invention;
FIG. 6 is a block diagram of illustrative display arrangement 1 showingportions of the control circuitry therefor in greater a detail; and
FIG. 7 is a time chartuseful in describing the operation of my invention.
DETAILED DESCRIPTION In FIG. 1 of the drawing an illustrative plasma display embodiment of the invention is shown comprising a stack of conventional plasma display devices 101, l02'and 103, separated "by respective light-attenuating layers 121 and 122, for
generating mural images by the selective energization of individual ones of the cross-point display cells of each device. Illustratively in FIG. I, the three display devices 101, 102 and 103 each comprise an 11x14 coordinate array of I54 crosspoint display cells, the display cells of each device being substantiallyin registration with the corresponding display cells of the other display devices in the stack. However, it will be apparent from the description herein that the display cells may be employed individually or in comlbinationin any form of array desired for particular display applications. For example, the display cells may be arranged in'a spiral row or in concentric circles for radar display applications.
The cross-point display cells of each display device in FIG. 1 aredefmed by respective sets of rowand column conductors, such as row conductors Rll Rlll and column conductors 'C'lI-CIS of device 101, which are spaced apart by dielectric material layers,'such as layers 51 and 52. Dielectric material layers SI and 52, are in turn, spaced apart such as by spacers 55, and a substantially uniform continuous layer of gaseous display material 53 is disposed between dielectric layers 51 and 52. Suitable gaseous display materials are well known in the art and may comprise, for example, one or more of the inert gases or mixtures of these gases with other gases. Different gases, of course, have different lightgspeed and color characteristics and the particular gaseous display material chosen will depend generally upon the application of the display arrangement. Moreover, it will-be apparent that ditferent gaseous display materials may be employed in the various display device levels in the stack, or alternatively that appropriate filters may be employed between the various levels, such as for multicolor display applications.
As is well known in the art, the dielectric mat'eriaL-such as layers 51 and 52, may comprise plates of glass, plastic or other similar transparent material, each illustratively on the order of 5 mils or so in thickness and spaced apart a like distance by spacers 55. The row and column conductors may comprise transparent metal or metal oxide conducting strips, for example, vapor-deposited on the respective dielectric material layers. Illustratively, the row and column conductors may be on the order of 15 mils wide and spaced apart on the order of mils on the respective dielectric material layers.
A cross section of a portion of the display arrangement of FIG. 1 is depicted in FIG. 2 showing, by way of example, the eight cross-point display cells of device 101 defined by column conductor C14 and row conductors R12-R19. Similarly, the corresponding eight cells of device 102 defined by column conductor C24 and row conductors R22-R29, and the corresponding eight cells of device 103 defined by column conductor C34 and row conductors R32-R39, are shown in FIG. 2.
Display devices 101, 102 and 103, in the illustrative embodiment of FIG. 1, each utilize the mechanism of electrical discharge breakdown of the gaseous display material to the light-emitting plasma at selected cross-point display cells for generating images. When an electric field is applied across a cross-point display cell, such as the cell defined by row conductor R12 and column conductor C14 of device 101, of a breakdown magnitude V determined by the pressure-distance characteristic of the particular gaseous display material employed, the gas in the cross-point region 220 breaks down and provides a light-emitting discharge of low current density. A typical voltage-current characteristic for such breakdown of a gas is shown in FIG. 3. As may be appreciated from FIG. 3, when voltage of increasing magnitude is applied across the display cell, very little current flows until the breakdown voltage V is reached. At this point, the cell breaks down in a so-called Townsend discharge characterized electrically by a substantially constant low current density.
As the breakdown discharge and the resultant current flow are established initially at a cross-point display cell, charge is stored on the surfaces of dielectric material layers 51 and 52 in the immediate vicinity of the cell cross-point. The stored charge opposes the voltage drop across the display cell and quickly reaches a level where the voltage across the cell becomes too low to maintain the discharge, thereby quenching the discharge at the cell.
In operation, an alternating current sustaining signal voltage provided by source 20, which may be either sinusoidal or pulsed, is extended by control circuit 80 across each display cell of devices 101, 102 and 103 via the row and column conductors. The sustaining signal voltage extended by source 20 across each display cell is of a magnitude less than the breakdown voltage level V,,. For example, the sustaining signal voltage may be on the order of one-half the breakdown voltage level.
Addressing of a selected display cell of a particular display device in the stack is effected via address circuit 30 under control of control circuit 80 by the application of coincident signals to the particular row and column conductors defining the selected display cell. The voltage thus extended across the selected display cell by the coincident row and column signals, by itself or in conjunction with the sustaining signal voltage applied to the row and column conductors, is sufficient to effect breakdown of the gaseous display material at the selected cell. At the same time, however, the voltage extended across the other display cells connected to the addressed row and column conductors, is insufficient to effect breakdown of the gas at these other cells.
As mentioned above, according to an important aspect of my invention, the individual display device levels in the stack are separated by respective layers of light-attenuating material, such as layers 121 and 122, which may be translucent glass or plastic, for example. Each successive light-attenuating layer in the stack attenuates the light reaching the viewing surface by a predetermined amount, illustratively by a factor of two. Thus, the light reaching the viewing surface from display device 101 due to discharge at a cross-point display cell thereof is essentially unattenuated, while the light reaching the viewing surface from display device 102 due to discharge at a cell of device 102 is attenuated one-half by light-attenuating layer 121. Similarly, the light emitted by discharge at a cell of display device 103 is attenuated one-half by layer 122 and an additional one-half by layer 121, such that the light finally reaching the viewing surface from device 103 is attenuated by a factor of four.
It will be appreciated therefore, that at a selected crosspoint on the viewing surface of the embodiment of FIG. 1, the light presented to the viewer is controllable over a multilevel gray scale, having a range zero to seven, by energization of the selected cross-point in various combinations of the display device levels. For example, the light reaching the viewing surface at cross-point 5,4 in FIG; 2 due to energization of devices 101, 102 and 103 at that cross-point would be the maximum level attainable (e.g., a level of seven); while that at crosspoint 8,4 due to energization of device 103 at that cross-point would be the next-to-lowest level (e.g., a level of one); and the light at cross-point 2,4 due to energization of device 101 would be in the middle of the range (e.g., a level offour).
Addressing of the individual cross-point display cells in FIG. 1 is effected via conventional addressing or scanning techniques, such as those known to the television art, advantageously employing a single address circuit 30 in common for all the plasma display devices in the stack. As corresponding cross-point cells of devices 101, 102 and 103 are addressed in common, the-cells of the respective devices are selectively energized at the addressed cross-point in accordance with a gray scale input signal received by input circuit 50 from signal source 10. The gray scale input signal from source 10 may be in analog form as is usual in commercial television, or preferably it may be in the form of a multibit digital word with respect bits of the word determining the ON- OFF character of the individual display device cells at an addressed cross-point. If the signals from source 10 are received in analog form, control circuit must be provided with analog-to-digital encoding circuitry to place the signals in digital form for energizing the individual display cells of devices 101, 102 and 103.
Alternative display device arrangements are shown in FIGS. 4 and 5. In the embodiments of FIGS. 4 and 5, the row and column conductors, except for the outermost sets of conductors, are each shared by two adjacent display device levels. Thus, column conductors C42 are shared by display devices 401 and 402, and row conductors C43 are shared by devices 402 and 403 in FIG. 4. An arrangement of n display devices therefore would require only n+1 sets of conductors interleaved with the display device levels, alternating between row conductors and column conductors as shown in FIG. 4. This arrangement advantageously facilitates manufacture by significantly reducing the number of sets of conductors and, consequently, the number of connections which must be made to the display devices. Moreover, the embodiments of FIGS. 4 and 5 permit closer spacing between display device levels in the stack than the embodiment of FIG. 1.
It will be appreciated that light-attenuating layer 421 may be disposed at any level in the stack intermediate the gaseous display material of device 401 and the gaseous display material of device 402, and similarly that light-attenuating layer 422 may be disposed at any level in the stack intermediate the gaseous display material of devices 402 and 403. Layer 421, for example, may comprise a light-attenuating film disposed on either surface of dielectric material layer 452 of device 401, or on either surface of dielectric material layer 451 of device 402.
In the illustrative embodiment of FIG. 5, the row and column conductors CSI-C54 comprise thin conductors coated or encased in dielectric material such as glass or plastic. The sets of conductors are supported and spaced apart by suitable spacers such as spacers 555 and adjacent display device levels are separated by respective light-attenuating layers 521 and 522. Conductor CSIb-Cfid and light-attenuating layers 521 and 622 are disposed in housing 630 having a transparent viewing surface 53H, the housing being substantially filled with gaseous display material. Thus, conductors C51 and C62 in FIG. 5 comprise a first display device level corresponding to display device 401i in FIG. 4, conductors C52 and C63 comprise a second display device level corresponding to display device 462, and conductors C53 and C54 comprise a third display device level corresponding to device 4503. The advantages of the arrangement of FIG. 5 relative to facilitating manufacture are manifest. Moreover, unlike the embodiments of FIGS. l and 43, the arrangement in FKG. 3 permits the individual conductors to be removed and replaced, if required for repair or maintenance purposes.
With the above description in mind, and with reference to FIG. 6 and to FlGS. 7A, 7B and 7C, the operation of an illustrative display arrangement according to my invention will now be considered. In FIG. 6 a three-level display 666 is shown comprising plasma display device levels 6621, 662 and 603, which may be similar to the display arrangement shown in FIG. t or H6. 6, for example. For the purposes of describing the operation of the invention, only the three display cells Xa, Xb and X0 of a single cross-point of the display are depicted in FlG. 6. Assume initially that display cells Xa, Xb and Xc are OFF; i.e., that no charge appears on the adjacent dielectric material surfaces and that none of the three display cells are lighted. The sustaining signal voltage from source 620 extended through OR gates 623 to row and column conductors C6l-C64l defining cells Xa,Xb and X0 appears across each display cell, as shown in FlG. 7A. However, since the sustaining signal voltage is less than the breakdown voltage V for the particular gaseous display material employed, no significant current flow occurs through the display cells.
Each individual row and column conductor of display 666 is connected to the output of a respective OR gate 623, one input of which, as just mentioned, is connected to source 626 and the other input of which is connected to the output of a respective address gate associated with the individual row or column conductor. A single address circuit 636 is employed in common for all three plasma display levels in display 606, and is illustratively shown in FIG. 6 as providing an address signal on a respective one of leads 63l63m for each cross-point in the display. Thus each of leads 63l-63m is connected to an input of the four address gates associated with the individual row and column conductors defining the three display cells at a particular cross-point. The cross-point comprising display cells Xa, Xb and X0 is addressed, for example, by an address signal over lead 635 to one input of address gates 665, 666, 647 and 668, gates 665 648 being individually associated with conductors C6l-C6fil, respectively.
The other input of each address gate is connected to a respective one of four gray scale code leads Dl-Dd from address circuit 630. As each cross-point is addressed, a gray scale input signal from signal source 6% is registered as a three-bit word in input circuit 656, each bit determining the ON-OFF character of a respective cell at the addressed crosspoint. The gray scale codeword bits are extended, one-at-atime, to address circuit 636 which responds to each bit to provide a signal of like character on the corresponding pair of leads Dll-Dd. Thus, responsive to the first bit of a codeword from input circuit 650 address circuit 636 provides a gray scale signal of like character on code leads D1 and D2, such as indicated by pulse 733 in FlG. 7C; responsive to the second bit, address circuit 630 provides a signal on code leads D2 and D3, such as indicated by pulse 732 in FIG. 7C; and responsive to the third bit, a signal is provided on leads D3 and D41, such as indicated by pulse 733.
Assume now that it is desired to turn ON display cells Xb and Xc defined by conductors C62 and C63 and by conductors C63 and C64 respectively. This is accomplished by addressing the selected display cross-point, extending a write pulse to display cell Xb in the form of coincident signals on conductors C62 and C63, and extending a write pulse to display cell X0 in the form of coincident signals on conductors C63 and C66. The write pulses extended to display cells Xb and Mo are of sufficient potential to effect breakdown of the gaseous display material at these cells.
Accordingly, with the illustrative example assumed, the gray scale word Oil is registered in input circuit 660 and extended, one-bit-at-a-time, to address circuit 630 during the interval that the cross-point including cells Xa, Xb and Xc is addressed. No output appears on leads Dl-D4 responsive to the first bit from input circuit 650 since this bit is illustratively a binary zero. Responsive to the second bit, illustratively a binary one, address circuit 630 provides coincident signals on lead D2 and D3 to address gates 646 and 647. The signals on leads D2 and D3, in combination with the address signal on lead 635 as shown in FIG. 7B, enables gates 646 and 667 to provide a write pulse over leads C62 and C63 to display' cell Xb. This is shown as occurring at time I by way of example in FIG. 7A.
The write pulse applied to conductors C62 and C63 causes momentary breakdown of the gaseous display material at dis play cell Xb, permitting current flow thereacross to store charge on the adjacent dielectric material surfaces. The resulting current flow across the display cell during breakdown is in the form of a current pulse, shown as pulse 701 in FIG. 7C, which may illustratively have a duration on the order of 50-75 nanoseconds.
Similarly, responsive to the third bit, illustratively a binary one, address circuit 630 provides coincident signals on leads D3 and D4 to address gates 6 37 and 648 which, in combination with the address signal on lead 635, provides a write pulse over leads C63 and C64 to display cell Xc, at time !,in FIG. 7. Momentary breakdown of cell Xc occurs, permitting current flow thereacross, shown as pulse 702 in FIG. 7D, to store charge on the adjacent dielectric material surfaces of cell Xc.
The level of charge stored .on the dielectric material surfaces is determined principally by the net voltage across the display cell during breakdown. During the following negative pulse of sustaining signal voltage applied across the display cell, the stored charge adds to the sustaining signal voltage as shown in FIG. 7A. At time t the combined voltage exceeds the breakdown voltage V causing a momentary breakdown discharge at display cells X12 and Xc. The resulting negative current pulse 7'03, between conductors C62 and C63, removes the stored charge from the dielectric material surfaces adjacent cell Xb and charges the surfaces in a reverse direction. Similarly, the negative current pulse 704, between conductors C63 and C64, removes the stored charge from the dielectric material surfaces adjacent cell X0 and charges the surfaces in a reverse direction.
During the following positive pulse on. the sustaining signal voltage, therefore, the reverse charge on the dielectric material surfaces of display cells Xb and Xcadds to the sustaining signal voltage thereacross as shown in Flg. 7A, reaching a level sufiicient to break down the gas at these display cells again at the time t During succeeding half-cycles of the sustaining signal voltage, the charge stored on the dielectric material surfaces of display cells Xb and X0, in combination with the sustaining signal voltage thereacross, causes periodic breakdown of the gas at these display cells to emit light in the form of pulsed discharges at a frequency twice that of sustaining signal source 620.
Additional ones of the display cells at other cross-points in display 660 are turned ON in a similar manner by application of a write pulse to the particular row and column conductors which define the additional cells. Conversely, a selected display cell is turned OFF by applying an erase pulse to the row and column conductor defining the selected cell, such that the erase pulse substantially removes or erases the charge stored at the cell. This is effected, for example, by applying an erase pulse to the particular row and column conductors at a point between successive sustaining signal pulses, or at a point when the instantaneous magnitude of the sustaining signal voltage applied to the row and column conductors is at or near zero in the case of a continuous sinusoidal sustaining signal.
Although in the description above it is tacitly assumed that only a single display cell is addressed by a write or erase pulse during each cycle of the sustaining signal voltage, it will be apparent that more than one cell can be addressed during each sustaining signal voltage cycle by consecutively or concurrently addressing a number of cells in each cycle. Further, the entire display can be erased if desired by terminating the sustaining signal voltage for a period sufficient to permit the stored charges at the display cells to dissipate. lt is to be understood, therefore, that the above described arrangements are but illustrative of the application of the principles of my invention. Numerous other arrangements may be devised by those skilled in the art without departing from the spirit and scope of the invention.
1. In combination, a plurality of conductors, a plurality of gaseous display cells, each of said display cells being defined by a pair of said conductors separated by two layers of dielectric material having a gaseous display material therebetween, means arranging said cells in a stack, each cell comprising a respective level in said stack, and means disposed between said levels for attenuating light passing therethrough.
2. The combination in accordance with claim 1 wherein n display cells are defined by n+1 conductors, said conductors being interleaved with said cells in said stack, and wherein n-l light attenuating means are interleaved with said cells in said stack.
3. The combination in accordance with claim 2 further comprising a viewing surface, each of said light-attenuating means individually attenuating the intensity of light reaching said viewing surface therethrough by a factor of two.
4. The combination in accordance with claim l. further comprising means for selectively addressing individual ones of said display cells to initiate a discharge breakdown thereat, and means connected to each of said display cells and operative upon the initial discharge breakdown of individual ones of said cells for thereafter periodically breaking down said individual ones of said cells.
5. The combination in accordance with claim 4 wherein said addressing means comprises means for addressing selected ones of said display cells in common, and means for selectively initiating said discharge breakdown at individual ones of said common addressed cells.
6. A display device comprising a plurality of gaseous display levels, a plurality of sets of conductors interleaved with said levels and arranged so as to define a respective array of display cross-points at each of said levels, the display cross-points of each said array being substantially in registration with corresponding display cross-points of the other ones of said arrays, and individual light-attenuating layers separating adjacent ones of said display levels.
7. A display device according to claim 6 wherein each of said conductors comprises an electrically conductive member separated from said gaseous display levels by nonconductive material.
8. A display device according to claim 7 further comprising a housing, means stacking said sets of conductors in spacedapart relationship in said housing, said display levels comprising a substantially uniform gaseous display material disposed in said housing between said stacked sets of conductors.
9. A display device according to claim 6 further comprising a viewing surface, said light-attenuating layers each equally attenuating the light reaching said viewing surface by passing therethrough.
M). A display device according to claim 9 wherein each light-attenuating layer attenuates the light reaching said viewing surface by a factor of two.
iii. A display device according to claim 6 further comprising means for addressing corresponding cross-points at each of said display levels in common, and means for initiating breakdown discharge at selected ones of said common addressed cross-points.
12. A display device comprising a housing having a viewing surface, gaseous display material disposed throughout said housing, n+1 sets of conductors, each conductor comprising an electrically conductive member from said gaseous display material, means supporting said sets of conductors in spacedapart relationship within said housing so as to define a stack of n arrays of display cells, the cells of each array being substantially in registration with corresponding cells of other ones of said arrays, and means disposed between adjacent ones of said arrays for attenuating light passing therethrough by a predetermined amount.
13. A display device according to claim 12 wherein said conductors each comprise an individual electrically conductive member coated with dielectric material, and wherein said conductors are supported by said supporting means so as to be individually removable from said housing.
M. A display device according to claim )12 wherein adjacent sets of said conductors are disposed orthogonally so as to define individual coordinate arrays in said stack.
RE. A plurality of conventional two state display cells, means arranging said display cells in a stack, each cell in said stack being substantially in registration with all other cells in said stack, and individual light-attenuating means disposed between adjacent ones of said display cells.