US2996623A

US2996623A - Electroluminescent image device

Info

Publication number: US2996623A
Application number: US652162A
Authority: US
Inventors: Koury Frederic
Original assignee: Sylvania Electric Products Inc
Current assignee: GTE Sylvania Inc
Priority date: 1957-04-11
Filing date: 1957-04-11
Publication date: 1961-08-15
Anticipated expiration: 1978-08-15

Description

Aug. 15, 1961 F. KOURY ELECTROLUMINESCENT IMAGE DEVICE Filed April 11, 1957 FREQUENCY (CPS) IN V EN TOR. FREDER/C KDURY VOL T A GE United States Patent 2,996,623 ELECTROLUMINESCENT IMAGE DEVICE Frederic Koury, Lexington, Mass, assignor, by mesne assignments, to Sylvania Electric Products Inc., Wilmington, Del., a corporation of Delaware Filed Apr. 11, 1957, Ser. No. 652,162 3 Claims. (Cl. 250-213) This invention relates to electroluminescent image-producing devices, and especially to those in which there is an array of parallel conductors on one side of an electroluminescent layer, and a similar array oriented in a different direction on the other side.

When a proper voltage is applied between a conductor of one array and a conductor of the other, the spot where one conductor crosses the other Will luminesce, and thus an image, moving or still, can be produced on the electroluminescent device by applying potential to particular conductors at particular times. Such devices are known to'the cross over point, because capacity effects between conductors tend to cause other points along each con-,

ductor to lurninesce, thus providing a sort of halation around each active point and tending to make the image unclear.

The voltage across the electroluminescent layer is, how: ever, greater at the cross-over point than at the other luminescing points, and I have discovered that electroluminescence at the additional points can be prevented or reduced by keeping the voltage at the cross-over point low enough to insure that the voltage at the additional points will be below the threshold voltage for electroluminescence.

The voltage across the layer at the cross-over point will then be so low that the luminescence at that point will be reduced, but the luminescence there can be raised by increasing the frequency of the applied voltage.

Thus by operating at a voltage-low enough to prevent luminescence at noncrossover points, and increasing the frequency at the same time, I obtain bright luminescence at the desired point, without luminescence at undesired points.

Other objects, features and advantages of the invention will be apparent from the following specification tafkfier;1 in connection with the accompanying drawing in w c FIGURE 1 is a top view in perspective of a device according to the invention;

FIGURE 2 is a graph of the brightness in foot lamberts against frequency in cycles per second, and one embodiment of the invention; and

FIGURE 3 is a graph of relative brightness against voltage for the same embodiment.

In FIGURE 1, the glass base plate 1 carries an array A of narrow parallel conductors 2 spaced apart and insulated from each other. Over these conductors and in close contact therewith is a layer of phosphor and dielectric material 3 and over said layer is another array B of narrow parallel conductors 4, spaced apart and insulated from each other, the conductors 3 being oriented in a plane parallel to the plane of the conductors 2 but said conductors 3 being at an angle with said conductors 2.

The dielectric is preferably a ceramic material such as shown in an application of Richard M. Rulon, Serial No; 365,617, filed July 2, 1953, and the phosphor used can also be a phosphor such as used in said application for example, a zinc sulfide activated with copper, lead and chlorine.

The conductors in at least one of the two arrays are preferably of a transparent conductive material such as the now well-known tin chloride or the like. Such materials and the methods of application are shown in said copending application of Richard M. Rulon. The tin chlorides may be applied to the glass plate 1 and the phosphor-dieletric layer 3 through a mask to provide the series of parallel conductors, or the coating may be applied to the entire top surfaces of the glass layer 1 and the phosphor dielectric layer 3 and then removed from those parts of said surfaces on which it is not desired. Methods of removing parts of such coatings are shown, for example, in my copending application, Serial No. 631,131, filed December 28, 1956. The word top is here used merely to denote the surfaces which appear as top surfaces in FIGURE 1, but of course they would not be the top surfaces if the device of FIGURE l were turned over. Connections to the various conductors 2, 4 can be made by methods well-known in the art.

In FIGURE 2, the electroluminescent brightness of a device such as shown in FIGURE 1 is seen to rise sharply with an increase of frequency and also to be higher for higher voltage. FIGURE 3 shows, however, that there is a threshold voltage E below which no electroluminescence occurs. I have found that this threshold voltage is substantially the same for all frequencies. Thus voltage which is considerably below the threshold value for one frequency will be below it for all frequencies.

Accordingly a voltage V can be selected which is above the threshold voltage E, and this voltage can be applied between a conductor 2a in array A and a conductor 4a in array B, by connections 5, 6 from A.C. voltage source V. The cross-over point C will then show electrolumi nescence.

Due to capacity eifects between the wires at points such as D, where either of the live conductors to which voltage is directly applied crosses over an electrically floating conductor to which no potential is directly applied, but which provides a capacitative path joining the two live conductors, a voltage will exist between the live conductor and the floating conductors, through a series of capacitors. There will generally be two or three such capacitive connections between the live conductors, and accordingly the voltage between the line conductor and a floating conductor will generally be less than half the voltage between the live conductors.

Accordingly it is possible to have the voltage between the two live conductors above the threshold voltage, and the voltage with respect to other conductors below the threshold voltage. Cross-over point C will then have voltage enough to emit light, whereas points D will not. The light emission will then be confined to the crossover point.

The light emission at point C will, however, be dim because of the need to keep the voltage at point D below the cross-over value. However, the frequency can then be increased until the desired brightness is obtained.

In that way, the brightness of the cross-over point C will be increased, but not that at the point D, because the voltage at the latter points will be below the threshold voltage. If the threshold voltage B were substantially independent of frequency, an increase in brightness of C with frequency would be accompanied by an increase in brightness of D also, and the device would be ineffective. However, I have discovered that the threshold voltage is substantially independent of frequency, and that my device is quite effective.

In one specific example, the base plate 1 was of glass two inches square and about .070 inch thick. The

conductors

2, 2, and 4, 4 were of transparent conductive film of tin chloride, as previously mentioned, and the electroluminescent layer was about 0.002 inch in thickness, and composed of a copper-activated zinc sulfide, containing lead and chlorine, as in US. Patent 2,728,730, issued December 27, 1956, to Keith H. Butler, embedded 3 in a ceramic material as shown in copending application Serial No. 365,617, filed by July 2, 1953, by Richard M. Rulon.

A sine-wave alternating voltage of about 300 volts R.M.S., at about 4000 cycles per second was applied between a conductor of array A and a conductor of array B, and point C glowed brightly, whereas points such as D did not glow at all.

If the voltage were raised to 500 volts, points D glowed also, and if the voltage were reduced to 300 volts at 60 cycles per second, the spot at C Was dim. The use of a rfrequency of several hundred cycles per second, for example 500 cycles per second, would improve the brightness.

In some cases, it may be desirable to arrange the two arrays in polar coordinate form, that is, one array can be made up of conductors arranged in a series of concentric circles, and the other in a series of radial lines emanating from a center in register with that of the circles.

A layer of dielectric material, without phosphor, can be used on either or both sides of the electroluminescent layer, if desired. The dielectric material can be, if desired, the same used in the phosphor-dielectric layer 3, and one at least of the added dielectric layers, should be of a light-transmitting material.

What I claim is:

1. An electroluminescent image display device comprising a layer of electroluminescent phosphor embedded in a dielectric material, a first array of parallel conductors on one side of said layer, a second array of parallel conductors on the other side of said layer, the conductors in said second array being at an angle to those in the first array, and means for supplying an alternating voltage to a selected conductor in said first array and a selected conductor in said second array, the full voltage of said means being above the threshold voltage for electroluminescence in said layer, in order to provide electroluminescence at the point where one of said selected conductors crosses the other, but being low enough to insure that the lower voltage is below the threshold voltage at points where a selected conductor in one array crosses a non-selected conductor in the other array, the frequency of said alternating voltage being at least several hundred cycles per second.

2. An electroluminescent image display device comprising an electroluminescent layer, a first array of parallel conductors on one side of said layer, a layer of dielectric material on the other side of said electroluminescent layer, a second array of parallel conductors on said dielectric layer, the conductors in said second array being at an angle to those in the first array, and means for Supplying an alternating voltage to a selected conductor in said first array and a selected conductor in said second array, the full voltage of said means being above the threshold voltage for electroluminescence in said layer, in order to provide electroluminescence at the point where one of said selected conductors crosses the other, but being low enough to insure that the lower voltage is below the threshold voltage at points where a selected conductor in one array crosses a non-selected conductor in the other array, the frequency of said alternating voltage being at least several hundred cycles per second.

3. The device of claim 1 in which there is a layer of dielectric material between one of said arrays of parallel conductors and the layer of electroluminescent phosphor embedded in dielectric material.

References Cited in the file of this patent UNITED STATES PATENTS 2,660,686 Putnam Nov. 24, 1953 2,698,915 Piper Ian. 4, 1955 2,837,660 Orthuber June 3, 1958 FOREIGN PATENTS 717,169 Great Britain Oct. 20, 1954 OTHER REFERENCES Bramley et 211.: Review of Scientific Instruments, June 1953, vol. 24, 471-472.