US2875380A - Display systems - Google Patents

Description

Feb. 24-, 1959 P, 3, TOULON 2,875,380

DISPLAY SYSTEMS Filed April 11, 1958 2 Sheets-Sheet l INVENTOR Pierre M. G. Toulpn ATfORNEY 2,875,380 DISPLAY SYSTEMS Pierre M. G. Toulon, Turtle Creek, Pa., assignor to Westnghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application April 11, 1958, Serial No. 727,916

10 Claims. Cl. 315-169) This invention relates to display systems, and more particularly, to storage type display devices of thin panellike structure. The most common display tube known in the art is the conventional cathode ray tube. In the cathode ray tube, it is necessary for an electron beam to supply the energy to produce the light output from a phosphor screen as well as distribute the video information over the entire display area. In a scanned display, this requires that the electron beam excite a particular point on the phosphor screen once every scan period and the decay time of the phosphor and the persistance of the eye must combine to produce the impression of a continuous light output from the screen; If a high averagebrightness is desired from the display, it is found that the design requirements for a cathode ray tube to provide sufficient electron beam power becomes prohibitive. The present cathode ray tube not onlylacks in adequate brightness but also exhibits a certain amount of flicker and objectional line structure in the image as far as the viewer is concerned. The present type cathode ray tube is also the primary limitation in reduction of the depth of television receivers.

It is accordingly an object of this invention to provide an improved display device of thin panel-like structure.

It is another object to provide an improved display device which substantially removes flicker in the viewed image. i

It is another object to provide an improved display device in which storage is incorporated to provide a continuity of light output over the entire for a desired time interval.

It is another object to provide an improved display device which provides high brightness in comparison with presently known display devices.

' It is another object to provide an improved display device utilizing' electroluminescent phosphormaterials in which voltage or charge sensitive capacitors are utilized to control the amount of light from the electroluminescent display and in which the voltage or charge sensitive capacitors are responsive to control signals to control the amount and time of light emission.

These and other objects are exemplified by this invention as will be apparent from the following description taken in accordance with the accompanying drawings throughout which like reference characters indicate like parts, and in which:

Figure 1 illustrates a display screen embodying the principles of my invention;

Fig. 2 is an enlarged perspective view of a portion of the screen structure illustrated in Fig. 1;

Fig. 3 illustrates a circuit equivalent of the screen structure illustrated in Figs. 1 and 2;

Fig. 4 is a curve illustrating properties of a ferroelectric element; and l Fig. 5 is a curve illustrating propertiesof a nonlinear type capacitor.

Referring in detail to Figs. 1 and 2 there is shown display screen United States Pmfi 1O 2,875,380 Patented Feb. 24, 1959 an embodiment of this invention constructed in accordance with my invention. The structure shown in Fig. 2 illustrates two complete light producing elements with associated control elements. The number of light elements and associated control elements utilized in a display screen will depend on the'conditions to be met. It should be appreciated that hundreds of light elements could be assembled into one screen to meet certain display problems that may be presented.

In the specific embodiment shown, the portion of the display screen is comprised of a light transmissive support layer 12 of a suitable material such as glass. On one surface of the glass support layer 12 opposite with respect to the viewer is deposited a thin electrically conductive coating 14 of light transmissive material for providing one electrode hereinafter referred to as the front electrode of the light producing element. Any suitable electrically conductive material that is light transmissive may be utilized such as stannic oxide. A layer 16 of suitable phosphor material which exhibits the property of electroluminescence is deposited on the front electrode 14. In the specific embodiment shown the front electrode 14 and the phosphor layer 16 are in the form of continuous layers extending across the entire screen and are common to all of the lightproducing cells. It

. is obvious that if so desired, each of the cells in the screen could be provided with separate and distinct layers. Examples of suitable phosphors which exhibit the property of electroluminesence are zinc sulfide copper and manganese activated or zinc sulfide copper activated to mention a few of these Well known phosphors. The phosphor material may be dispersed Within a suitable plastic dielectric material or an inorganic material such as glass.

On the exposed surface of the phosphor layer 16, there i is deposited a plurality of electrically conductive electrodes 18 which are insulated from each other.

These electrodes 18, hereinafter referred to as back electrodes of the light producing elements, may be placed on the phosphor layer 16 by evaporating a suitable material such as aluminum through a suitable mesh structure as is well known in the art. The size and the shape of the electrodes 18 and the number will depend on the type of display device desired. In the embodiment shown, the electrodes 18 are small in area and spaced from each other a suflicient distance so that applied current to one of the electrodes 18 will excite only that elemental area of the display screen. The light producing structure just described which consists of a phosphor layer 16 with two electrically conductive electrode contacts 14 and 18 provides a light producing element which will emit lightin response to field excitation applied by voltage to the electrodes 14 and 18. This type of light producing device is discussed more fully in an article entitled Electrolurninescence and Related Topics by G.

'Destriau and H. F. Ivey in the December 1955 issue phosphor materials between two spaced electrodes and applying an alternating or time varying potential between these electrodes. The resulting electric field which is created acrossthe electrodes excites the phosphor material to luminescence, and the phosphor materials which display this electroluminescence are thus termed fieldresponsive. Such phosphor materials are normally ad- 3 mixed with adielectric material or a separate layer of dielectric material which is included between the electrodes in order to prevent any arcing thereacross, which might short out the electroluminescent cell A separate dielectric material is only desirable and not mandatory for the cells may be operated under some conditions without any dielectric wherethe appliedelectric field is as high as a'hundred kilovolts per centimeter. Norrnally the spaced electrodes are parallel but they need not be as where graded field intensities are desired.

The phosphor material layer 16 maybe deposited on the front electrode 14 by any suitable method. For example, a finely divided phosphor material such as zinc sulfide activated by copper may be admixed with a solvent such as butyl acetate and with a polyvinyl chloride lacquer. The proportions of the constituents are not critical and may vary within wide limits, but as a specilic example, thrce parts by weight of phosphor may be mixed with 50 parts by Weight of thinner and 35 parts by weight of polyvinyl chloride lacquer The forcgoing admixture may be sprayed in a plurality of coatings, for example four, according to the desired thickness, drying between each of the coatings. Other dielectrics and solvents may be substituted .for the foregoing specific examples as is well known. It is also possible to utilize a sintcred type phosphor layer. I

The control portion of the display screen comprises a separate control structure for each of the back electrode members 18 provided on the phosphor layer 16 to control the light output from each light element. In the specific embodiment shown, the control element structure is comprised of a U-shaped member 20 of electrically conductive material having its central portionZZ in electrical contact with the back electrode member 18 and with the leg portions 23 and 25 projecting outwardly from the electrode member 18. A layer 24 of a nonlinear dielectric material is provided on the end of one of the leg members 23 and a similar layer 26 of material is provided on the other leg 25. An electrical conductive bus bar 28 is provided for each of the leg portions 25 and is in physical contact with the opposite surface of the layer 26 to form a capacitor in the leg portion 23.. The capacitor consists of the conductive busbar 28, the leg member 25 which form the elec trodes with the nonlinear dielectric material layer 26 sandwiched between. An electrical conductive bus bar 39' is provided for each of the leg portions 23 and is in physical contact with the opposite surface of the layer 24 to form a capacitor in the leg portion 25. The capacitor consists of the conductive bus bar 30, the leg member 23 which form the electrodes with nonlinear dielectric material layer 24 sandwiched between. The bus bars 28 and iii) are normally parallel and extend across the screen structure contacting each control element in the row in a similar manner. The design of the structure is primarily for mechanical reasons and the electrical equivalent of a single screen element is two separate nonlinear capacitors having a common connection to a light producing element provided by the U-shaped conducting member 26 in contact with electrode member 18 and having their remaining terminals connected to two bus bars.

The nonlinear dielectric material in layers 24 and 26 may be of any suitable material such as ferroelectric dielectric material selected from the group which includes, for example, barium titanate, barium-strontium titanate, barium stannate, sodium columbate, sodium tantalate, potassium columbate and potassium tantalate. An ideal or lossless nonlinear dielectric material may be defined as a material in which the functional relationship between the electric displacement in a given principal direction and the electric field in the same direction while single valued is not that of a straight line in Cartesian coordinates. One particular class of materials a smaso' 4 that has been found satisfactory in this application are fer foelcctric dielectric materials. The preparation of titanate ceramics is fully disclosed in an article entitled Preparation of Reproducible Barium Titanate by R. M. Callahan and I. F. Murray, page 131 of the May 1954 issue of the Bulletin of the American Ceramic Society.

One method of preparing the control structure for the entire screen is to provide a thin flat dielectric sheet approximately 10 mils in thickness. The sheet is coated on both sides with a suitable paint such as one which contains finely divided silver, frit and possibly a flux. After the paint has air-dried, the coated dielectric member may be baked in an oven for 15 to 30 minutes at a temperature of. about 700 C. Both sides of the ceramic sheet are then tinned with a suitable solder, such as one containing 36% lead, 62% tin and 2% silver. Two sheets of electrically conductive material such as brass are also tinned on one side with a solder similar to that usedon the dielectric sheet. The ceramic sheet is then positioned between the two brass sheets with the tinned side of the brass sheets adjacent the dielectric sheet The sandwich of the two brass sheets with the ceramic sheet is heated to a temperature of about 230 and then cooled. The resulting lamination may then be machined to form a structure illustrated in Fig. 2. In the specific'device shown in Fig. 2, two perpendicular cuts would be sumcient to fabricate the entire control structure. Theresulting control structure may be attachedto the electrodes 18 of the light producing structure of the screen by an electrically conductive varnish or cement applied between the U-shap'edportions zit of the control structure and the electrodes 18. The entire screen structure may be imbedded in a suitable light transmissive plastic material 31 as illustrated in Fig.1. I H

An electrical conductive lead is provided from each of theelements 2t] of the control structure illustrated as leads 32. and 34 and are connected to fixed contacts of asuitable switching means 4%) illustrated as a mechanical switch. An electronic switch may be used. Other;

control elements of screen could be connected to the other fixed contacts. The movable contact of the switch 40 is connected through a bias source 42 to a source 45 o'f 'video signals. The conductive bus bars 28 and 30 are also brought out of the screen structure as well as an electrically conductive lead 44 from the frontelectrode 14 andconnected to ground. The conductive bus bar 28 is connected to a light power source 48 ,for providing a time-varying voltage. The opposite terminal ofthe power source 48 is connected to the positive terminal of abias source 50 illustrated as a battery. The negative terminal of the bias battery 50 is connected to ground. The conductive lead 44 from. the front plate electrode is also connected to ground. The other conductiyebus bar 30 is connected to a light power source 46 which also provides a varying voltage and the other terminal of the power source 46 isconnected to ground.

The operation of the device can be best described by reference to Fig. 3. The equivalent circuit of each light producing element and its associated control element is illustrated in Fig. 3. That portion of the circuit which is containedwithin the dotted lines represents one elemerit of a multi elementary screen. The remainder of the circuit may be and is usually common to all the elements of the screen with of course the control voltage being selectively impressed by means of the switching means dtl. The circuit shown may be considered to be comprised of an upper branch, at lower branch and a center branch. The upper branch of the circuit includes, beginning at the junction 6t) which is connected to ground, the bias source 50, the light power source 48, the nonlinear dielectric capacitor 62 (formed by the members and 25 in Fig. and ends at junction 64. These cn'cult elements are connected in series arrangement by suitable conducting members of which conductor 28" corresponds to the bus28 of Figs. land 2. The junction 64 is the member 22 of the control element of the screen in Figs. 1 and 2.

. i In a like manner, the lower branch of the circuit comprises in series arrangement beginning at junction 60, the light power source 46, a nonlinear dielectric capacitor 66. (formed by the members 30, 24 and 23 in Fig. 2) and ends at junction 64.

Extending from the junction 60 to the junction 64 between the two nonlinear dielectric capacitors 62 and 66 is the center branch of the circuit containing an electroluminescent element or cell 68 (formed by members 14, 16 and 18 in Fig. 2).

The complete circuit has the nature of a bridge circuit in. which the upper and lower branches determine what current, if any, passes through the center arm whichcontains the electroluminescent light producing element 68. At the ungrounded junction 64. of the upper and lower branches and the center branch is the connection to the control voltage which is here represented .by a battery 70 having its negative terminal connected to ground. In order to explain the operation of the circuit it will be first assumed that the control voltage from the source 70 is zero potential. Thenin view of the design or the adjustment of the members of the circuit system, the periodic voltage applied at the junction 64 will be substantially the same as the voltage at junction point 60, and zero or substantially zero light power potential is applied across the electroluminescent cell 68. Thus, the circuit system is in balance. This initial balance may be accomplished in several ways, for example, the value of the capacitance of the nonlinear dielectric capacitors 62 and 66 may be selected to be the same. If the nonlinear dielectric capacitors 62 and 66 are of the same value of capacitance, the bias source 50 will cause the dielectric capacitor 62 to drop in capacitance to a point where it is in effect smaller than the capacitance of the nonlinear dielectric capacitor 66 in the lower branch. In order to compensate for this changein the value of capacitance of the capacitor 62 in the upper branch, the light power voltage source 48 in the upper branch is made capable of generating a greater voltage than the light power source 46 in the lower branch and the difference between the potential of the light power sources 46 and 48 are so proportioned that there is very little unbalance between the upper and lower subsidiary circuits described hereinbefore and the voltage across the electroluminescent cell or capacitor 68 is negligible.

If it is now assumed that a control potential or control signal is delivered to the junction 64 from the source 70 then this control potential will appear directly on the two capacitors 62 and 66. The efifect on the capacitor 62 in the upper branch will be to subtract from the bias impressed on the capacitor from the bias source 50 in the upper branch, and thus will tend to increase the capacitance of the capacitor 62. The efliect of the control potential on the other capacitor 66 in the lower branch is to decrease the value of the capacitance and therefore the efiect on the two capacitors 62 and 66 are in an opposite direction and therefore they tend to aid in unbalancing the circuit. This results in a portion of the light power potential supplied from the light power sources 46 and 48 being applied across the electroluminescent cell 68 rendering it luminous and in an amount corresponding to the control bias applied to the circuit from source 70.

By varying the control signal delivered by the source 70, Which may be the video source as illustrated in Fig. l, the luminosity of the electroluminescent cell 68 may be varied as desired. Therefore, when a plurality of light power producing elements are provided and illuminated in accordance with predetermined signals any predetermined image may be presented on the display screen. As the control signal contact isremoved from one of the elements, the electroluminescent cell will emit light until the control charge gradually leaks ofi-or the charge is modified by the next signal contact tothe individual element. As illustrated in Fig. 1, it may be suitable to use in addition to the signal, a constant voltage or bias source 42 which is continuously on only during the time that the signal pulse is being applied to the individual element. The purpose of this bias is to assure that the control element operates over the optimum portion of its characteristic.

As a representative example, an electroluminescent cell 68 may have an electrode contact of an area of square inch and a capacitance of about micromicrofarads. The dielectric capacitors 62 and 66 would have an apparent capacity of about 400 micromicrofarads with no direct current voltage and volts R. M. S. at a frequency of 6000 cycles. The area of the dielectric layer in the capacitors 62 and 66 would be about 320 square mils and the layer 12 milsin thickness. The sources 46 and 48 would be of the phase shown in Fig. 3 with the source 46 of about volts R. M. S. at a frequency of 6000 cycles. The source 43 would be of a voltage of 300 volts R. M. S. at a frequency of 6000 cycles. The direct current bias source 50 would be of about 300volts. The direct current control bias range could be from zero volt to 300 volts.

Other modes of operation of the circuit shown in Fig. 3 work equally as well as that described above. For example, the two light power sources may have equal outputs while the nonlinear dielectric capacitors have difierent unbiased capacitances for example the capacitor in upperbranch may be greater than one in lowerbranch. If no control signal is applied, the bridge may 'be' balanced since the larger capacitance (and smaller impedance) or the upper capacitor may be so modified by means of the bias source 50 as to compensate the unbalancing elfect of the relative size of that capacitor. Application of a control signal will destroy the balance as in the previously described mode of operation.

Under some circumstances it may be desirable to have a light output when no control signal is applied. This may be readily eifected by having equal capacitors and equal light power sources. The initial unbalance will be due to the applied bias. Application of control signal will tend to drive the circuit toward a balanced state and reduce light output.

The use of a bias means in the circuit of Fig. 3 facilitates the attaining of good control of the light output of the electroluminescent element. However, the use of a bias means is not essential to the circuits operation. If the capacitance of the capacitor in the upper branch is less than the capacitance of the capacitor in the lower branch and the output of the light power source in the upper branch is greater than the output of light power source in the lower branch an initial balance may be attained with the bias means out of the circuit and no control signal applied. Application of control signal will unbalance the bridge.

In Fig. 4 a plot of capacitance C of a ferroelectric capacitor with respect to control voltage or bias V impressed across the capacitor is illustrated. The video information supplied from a conventional television receiver is a wave going both negative and positive and in the absence of a bias the same change of capacitance might be obtained in both negative and positive direction. To remedy this, a constant bias potential is either subtracted or added to the video yielding a control potential of either entirely negative or positive values.

A linear or normal capacitor of mica or air dielectric has a charge versus voltage curve theoretically in the form of a straight line. In the case of a non-linear capacitor, the charge Q versus voltage V curve might be similar to that shown in Fig. 5. If it is assumed that an alternating potential light power supply is applied to the capacitor, then the potential applied to the capacitor in each cycle would move the element from point A to a point B on the curve and return. If it is now assumed for purposes of explanation that in addition to the previously mentioned alternating potential applied to the capacitor a constant control signal of magnitude D is applied, then the alternating potential causes the excursion from A to B on the curve during each cycle. Since effective capacitance may be defined as AQ/AV, it is apparent that the capacitance is then smaller than in the first case where a constant potential was not applied, since AV remains the same but AG is diminished. Since the capacitance is smaller the reactance which the capacitor presents to the external circuit is larger as a result of applying a control signal on this type of capacitor known as a nonlinear capacitor.

While I have shown my invention in only one form, it will be obvious to those skilled in the art that it is not so limited but is susceptible of various other changes and modifications without departing from the spirit and scope thereof.

I claim as my invention:

1. In a solid state display element in combination, a plurality of circuit systems connected in cooperative rela tionship with a common conductor between them, one circuit system comprising a nonlinear dielectric capacitor, an alternating current light power potential source and a direct current power source connected in series circuit relationship, the other circuit system comprising a 'non linear dielectric capacitor and an alternating current source of light power potential connected in series circuit relationship, an electroluminescent cell connected in the common conductonthe members of the circuit system being so proportioned that they are in substantial balance with substantially no light power potential applied to the electroluminescent cell and mean-s for delivering signals to the circuit system to effect an unbalance and impress a light power potential across said electroluminescent 'cell to render it luminous.

2. In a solid state display element in combination, an electroluminescent cell and nonlinear dielectric capacitors connected in circuit relationship, the electroluminescent cell being responsive to an alternating current light power potential, said nonlinear dielectirc capacitors being responsive to a direct current voltage, means for impressing an alternating current light power potential across the electroluminescent cell, signal means connected in circuit relationship with said electroluminescent cell and non linear dielectric capacitors to control the alternating current light power potential impressed across the electroluminescent cell and the direct current voltage impressed across the nonlinear dielectric capacitors to regulate the luminosity of the electroluminescent cell.

3. In a solid state display screen in combination, two nonlinear dielectric elements connected in series circuit relationship, two alternating current light power potential sources connected in series circuit relationship, a direct current source connected in series circuit relationship between the two alternating light power sources and means connecting the light power potential sources and a direct current power source across the nonlinear dielectric elements connected in series circuit relationship to form a substantially balanced circuit system, an electroluminescent cell connected between a point disposed between Cid said two nonlinear dielectric elements and a point between said direct current power source and one of said alternating current light power potential sources to provide'a plurality of loop circuits in substantial balance and means for delivering signals to the loop circuit at a point between the nonlinear dielectric elements to effect the impressing of the light power potential across the electroluminescent cell in accordance with the signal delivered.

4. In a display device in combination, two separate nonlinear dielectric material members, an alternating current light power source, a direct current power source 3 and another alternating current light power source all connected in series circuit relationship providing a loop circuit, an electroluminescent material member connected across the loop circuit from a point between said nonlinear dielectric material members to a point between said direct current power source and one of said alternating current light power potential sources providing loop circuits having a common conductor, the loop circuits being in substantial balance whereby substantially no light power potential is impressed across the electroluminescent material member and means for impressing a' signal on said loop circuits to effect an unbalance and the impressing of a light power potential across the electroluminescent material member, the light power potential impressed on the electroluminescent material member having a predetermined relationship to the signal delivered to the plurality of circuits when in substantial balance whereby the luminosity of the electroluminescent material member varies with the signal.

5. A solid display element for producing light inaccordance with a controlling electrical signal, comprising a light producing element of a field responsive phosphor, a control element means associated with said light producing element, said control element exhibiting a varying capacitance in accordance with said control signal applied thereto, means for applying electric fields across said light producing element and said control element, means for applying said control signal to said control element whereby the magnitude of the electric field applied across said light producing element is modified in accordance with the magnitude of said control signal applied to said control element.

6. A display device comprising, a plurality of independently controllable light producing elements, said light producing elements responsive to a time variation of an electric field, control elements electrically connected to said light producing elements, a source of time varying potential connected to said light producing elements and said control elements to impress a field across said light producing elements, said control elements responsive to variations of electrical signals applied thereto to cause corresponding variation in the impedance of said control device whereby the magnitude of the electric field applied across said light producing element is modified in accordance with the magnitude of control signals applied thereto and means for impressing electrical signals on said control element corresponding to light values desired of said light producing element.

7. An elementfor producing light in accordance with a controlling signal, comprising a light producing element of a field responsive phosphor, control element means associated with said light producing element, said control element means comprising a bridge type circuit arrangement for controlling the light output of said light producing element comprising a first circuit comprised of a first time varying power source and a nonlinear dielectric capacitor serially connected across said light producing element, a second circuit comprised of a second time varying power source, a direct current source and a second nonlinear dielectric capacitor serially connected across said light producing element, the components of said first and second circuits selected to provide substantial balance in the bridge circuit, means for applying said control signal to said first and second circuits to modify said balance and impress light power potential across said light producing element in accordance with said control signal.

8. In a solid state display device, an electroluminescent element having a first electrode and a second electrode, means for controlling power applied across said first and second electrodes comprising a first circuit branch, a

second circuit branch and a control signal source, said first circuit branch comprising a nonlinear dielectric capacitor, a light power source, and a bias means connected in series circuit relationship with said first and second electrodes of said electroluminescent element, said second circuit branch comprising a nonlinear dielectric capacitor and a light power source connected in series circuit relationship with said first and second electrodes of said electroluminescent element in a manner such that the nonlinear dielectric capacitor of said second circuit branch and the nonlinear dielectric capacitor of said first circuit branch have a common connection with said first electrode of said electroluminescent element and with the output of said control signal source.

9. In a display device, an electroluminescent element having a first electrode and a second electrode, means for controlling power applied across said first and second electrodes comprising a first nonlinear dielectric capacitor, a second nonlinear dielectric capacitor, a first light power source, a second light power source, a bias means, and a control signal source connected in bridge circuit arrangement in a manner such that the amount of the outputs of said first and second light power sources impressed across said first and second electrodes is modified in response to signals from said control signal source applied to said first and second nonlinear dielectric capacitors.

10. An element for producing light in accordance with a controlling signal-comprising a light producing element of a field responsive phosphor, a first circuit means comprising a nonlinear capacitor and a first source for supplying a time varying electric field across said light producing element, a second circuit means comprising a nonlinear capacitor and a second source for supplying a time varying electric field across said light producing element, said first and second time circuit means connected to impress opposing time varying electric fields across said light producing element, means for impressing a control voltage to modify the capacity of said capacitors in an opposite sense to modify the electric field across said light producing element.

No references cited.

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