GB2050777A - Electroluminescent Storage CRT Display Device and Operating Method - Google Patents

Abstract

In a direct viewing electroluminescent storage CRT display system, an electron beam is used both to directly write and selectively erase information on an a.c. thin film EL panel that constitutes the CRT's storage target. The target consists of electrodes 42, 34, dielectric layers 40, 36 and an EL layer 38 consisting of material which exhibits voltage/luminance hysteresis. Writing and erasure of information are accomplished by scanning the beam over the rear surface of the target (which has an inherent memory characteristic), in synchronization with a train of alternating polarity pulses 44 that is applied across the electrodes. Information is written on the panel when it is addressed by the (pulsed) beam during the application of either a positive or negative pulse. Erasure occurs when the panel is addressed in the interval following the application of a positive pulse to the rear electrode. <IMAGE>

Description

SPECIFICATION Electroluminescent Storage CRT Display Device and Operating Method Background and Objects of the Invention The present invention relates to improved electroluminescent storage CRT display devices and to a method of operating such devices to achieve direct electron beam writing and selective image erasure.

Cathode-ray storage tubes are widely used for their ability to provide visual representations of stored electrical signals. In general, such tubes include an electron gun for generating an electron beam, a storage target upon which the beam is focused, and means for deflecting the electron beam to "write" an image of the signals on the target.

A variety of different storage target structures are known. Very short term storage is provided by display screens that incorporate long persistence phosphors. Because the only energy available for exciting the phosphor is that imparted by the writing electron beam, stored image brightness diminishes rapidly with time. In another type of storage device - the direct viewing bistable phosphor storage tube - the target includes a patterned or thin porous layer of a dielectric bistable storage phosphor overlaying a substantially transparent conductive backplate. Images written on the dielectric phosphor layer are maintained in a luminescent state by being bombarded with low velocity electrons from one or more flood electron guns in the tube.Although such devices are capable of displaying stored images for an indefinite length of time and have very high resolution, many users consider the stored image brightness and contrast to be near the lower limit of acceptability.

One means of providing a stored CRT display with improved brightness and contrast is to utilize an electroluminescent (EL) storage target as shown, for example, in U.S. Patent Nos. 3,087,086 to Turner, 3,796,909 to Chang et al. and 3,908,148 to Lehrer et al., and by application Serial No.

922,950 filed July 10, 1 978 in the name of Gary S. Barta and assigned to the assignee of the present invention.

Turner discloses a direct-viewing CRT with a faceplate-supported screen that includes, beginning at the faceplate, a transparent conductive layer, a layer of an electroluminescent phosphor, a layer of a ferro-electric material that becomes conductive when subjected to electron bombardment, and a second conductive layer that is directly addressed by the tube's writing electron beam. To write an image into storage, a d.c. potential is applied between the conductive layers and the screen is scanned by the electron beam modulated so that the beam is "on" where a dark area is required in the image. In other words, a negative or reverse version of the desired image is written on the target, after which the d.c. potential is removed and the conductive layers are connected together to store the image.To view the stored information, the connection between the conductive layers is broken and an a.c. potential applied between them, causing the electroluminescent layer to fluoresce in the unwritten regions of the target. Erasure is accomplished by again connecting the conductive layers together and then scanning the entire target with the electron beam. The system disclosed by Turner has a number of significant drawbacks, including the need for both a.c. and d.c. target potential sources, and a slow and overly complex operational sequence.

Chang et al. describe an electroluminescent storage display tube with a target structure consisting of a layer of electroluminescent material, a fine mesh screen supported in front (on the electron gun side) of the EL layer and a transparent conductive backplate behind it. An a.c. voltage is applied between the screen and backplate, together with a suitable d.c. bias. Regions of the EL layer addressed by the tube's writing electron gun are held at a fixed potential by flood gun action, as in a bistable phosphor storage target. The a.c. voltage creates an alternating field across the fixed potential regions to produce electroluminescence in those regions. The remaining regions float with the applied voltage and remain "off". As with the Turner CRT, both d.c. and a.c. sources are required.Other disadvantages include the requirement for flood guns in the CRT, and the need to provide a fine mesh screen suspended close to the EL layer, both of which add to the complexity and cost of manufacturing the device.

Lehrer et al. characterize an electro-optical transducer and storage tube including a target that comprises a silicon diode array overlaying an a.c.- or d.c.-responsive electroluminescent layer. With a suitable potential applied across the series-connected EL layer and diode array, the diodes are initially in a reverse-biased, non-conductive state. An electron beam from a writing gun in the tube then is used to scan the target in accordance with a signal to be stored and displayed. Diode elements in regions of the target struck by the beam are swtiched to a conductive state, turning on corresponding areas of the EL layer. The display remains on until it is erased by momentarily removing the applied voltage.

Resolution of such a target is limited by the diode element density of the array. In addition, the target structures described by Lehrer et al. are relatively complex, requiring from seven to ten separate layers, of which several are patterned layers.

A significant drawback of the storage display devices proposed by all of the above-mentioned patents is the lack of a selective erase capability. Thus, to change even one element of a displayed image, the entire display must be erased and rewritten. The Barta application referred to above describes an electroluminescent storage CRT that includes a target comprising a thin-film EL panel having an ultraviolet light-emitting phosphor layer on one side. With a suitable a.c. signal applied to the EL panel, an electron writing beam is directed onto the phosphor layer. Ultraviolet radiation from areas addressed by the beam switches on corresponding areas of the electroluminescent panel, which remain in a luminous state as long as the a.c. signal is applied. By suitably reducing the potential of the image-sustaining a.c. signal, the stored image may be erased in its entirety.Selected portions of the image can be erased by addressing corresponding areas of the phosphor layer with the writing beam during the intervals between the alternating positive and negative pulses of the sustaining signal.

In general, prior art electroluminescent CRT storage and display systems are more complex in construction or operation, or both, than is desirable. It is thus a principal object of the invention to provide a simpler, lower cost version of such a system.

It is another object of the invention to provide electroluminescent CRT apparatus capable of storing and erasing information by direct electron beam writing, i.e., without using intermediate switching or energy conversion means in the EL target structure.

Still another object of the invention is to provide an improved method of operating display apparatus comprising an a.c. thin-film electroluminescent panel to provide information storage and selective erasure by direct electron beam action.

A further object is to provide improved a.c. thin-film electroluminescent CRT apparatus and a method of operating the same to permit direct electron beam writing and selective erasure at reduced beam accelerating potentials.

Additional objects, features and advantages of the present invention will become apparent as the following detailed description is read in conjunction with the accompanying drawing.

Brief Description of the Drawings Fig. 1 is a simplified sectional view of an electroluminescent cathode-ray storage tube, together with certain associated circuitry, according to the present invention; Fig. 2 is an enlarged, fragmentary cross-sectional view of the electroluminescent storage and display target incorporated in the Fig. 1 CRT; Fig. 3 shows a plot of display brightness versus applied voltage for the Fig. 2 target; Fig. 4 is a fragmentary front elevation view taken along line 3-3 of Fig. 2; and Figs. 5-7 depict certain waveforms illustrating the operation of the Fig. 1 CRT and associated circuitry.

Detailed Description of the Invention Referring now to the drawings, Fig. 1 shows a direct-viewing electroluminescent cathode-ray storage tube 10 and certain associated circuitry, in accordance with the best mode presently contemplated for practicing the invention. Storage tube 10, which is generally similar in overall configuration to prior art electroluminescent CRT's, includes an evacuated envelope 12 having a transparent faceplate 14 at one end. Supported by faceplate 14 is an a.c. thin-film electroluminescent (ACTFEL) storage panel, or target 1 6. The faceplate/storage target combination may be referred to as a viewing screen for the CRT.

Mounted at the opposite end of the storage tube within the neck of envelope 12 is a writing gun 18 for forming a beam 20 of high velocity electrons directed toward target 16. Writing gun 18 is conventional in construction and thus need not be described in detail. It may be noted, however, that the electron gun includes a cathode 22 coupled to the negative side of a high voltage d.c. power supply 24, the positive side of which is coupled to ground, and a grid 26 coupled to beam blanking circuitry 28. Power supply 24 suitably may be set at about 4 kV. The accelerator electrode of gun 18 is coupled to ground, thereby providing a 4 kV beam acceleration potential.

The electron beam from writing gun 18 is scanned across the surface of the storage target by electrical signals supplied by deflection circuitry 30 to an electromagnetic deflection yoke 32 supported on the neck of CRT 10. Alternatively, electrostatic deflection means -- e.g., horizontal and vertical deflection plates - may be used to direct the beam. It should be noted that blanking circuitry 28 and deflection circuitry 30 are of conventional design, and thus for purposes of illustration are shown only in simplified block form in Fig. 1.

Referring for the moment to Fig. 2, the viewing screen of CRT 10 comprises a transparent front electrode 34, suitably of tin oxide (SnO2) or indium tin oxide, applied to the inner surface of faceplate 14. Overlaying electrode 34 is a first insulating layer 36 of a high dielectric strength material such as yttrium oxide (Y203), silicon nitride (Si3N4) or aluminum oxide (Al203). An active thin-film layer 38 of an electroluminescent material, preferably manganese-activated zinc sulfide (ZnS:Mn) is deposited on layer 36, and is in turn covered by a second insulating layer 40 of yttrium oxide or the like. A rear electrode 42, suitably of aluminum, is formed on insulating layer 40 to complete the ACTFEL target structure.

Target 16 may be fabricated using known procedures, such as those described by Yamauchi et al.

in "Inherent Memory Effects in ZnS:Mn Thin Film EL Devices", International Electron Devices Meeting Digest, pp. 348-351, Dec. 1974. By way of specific example, a suitable target may be constituted as follows: Element Material Thickness, Ang.

Front Electrode 34 SnO2 300-500 1 st Insul. Layer 36 Y203 200S3000 Active Layer 38 ZnS:Mn 4000-7000 2nd Insul. Layer 40 Y203 2000-3000 Rear Electrode 42 Al 400-1000 As will be understood, it is necessary that the ACTFEL structure have an inherent memory characteristic -- i.e., that a plot of the device's brightness versus applied voltage shows it to have a hysteretic behavior. Fig. 3 illustrates the hysteresis demonstrated by a target provided in accordance with the example described above.The brightness and hysteresis characteristics of the target will, of course, vary depending on the physical and electrical properties of the dielectric layers and the material forming the active electroluminescent layer. For example, in panels of the type exemplified herein i.e., with Y203 insulating layers on both sides of a ZnS:Mn film-it has been found that a manganese concentration of about 5 mol percent in the active layer produces the greatest hysteresis and that maximum brightness occurs at about 1 mol percent.

Now referring to Fig. 1 along with Fig. 2, electrodes 34 and 42 of ACTFEL panel 16 are coupled to a suitable alternating polarity voltage pulse source 44. The output from source 44, which is referenced to ground, is a train 46 of spaced, alternating polarity pulses 48 having a nominal pulse width of about 10 to about 100 ys, suitably about 30 ys, and a frequency of about 500 Hz to about 500 kHz, suitably about 1 kHz. A suitable coupling (indicated by a dash-double dot line in Fig. 1) is provided between the pulse source and blanking circuitry 28 for reasons which will become apparent.

In operation of the display device according to the invention, EL panel 1 6 is placed in a ready-towrite condition by activating source 44 to apply alternating polarity pulses 48, which will be referred to as sustaining pulses, to electrodes 34 and 42. The source is preset to provide pulses having a peak amplitude Vs below the electroluminescence threshold voltage for the panel, which in this case is about 1 50 V, but high enough to sustain written images at a usable level of brightness. With a target having the hysteresis characteristic shown in Fig. 3, for example, sustain pulse voltages of about 140-1 50 volts are suitable, although values in the upper half of the range are preferred.

Information is written on target 1 6 by scanning the electron beam from gun 1 8 over the target's rear surface at the same time that a positive or negative sustain pulse 48 is applied to electrodes 34 and 42. Fig. 4 shows a portion of CRT 10's viewing screen on which alphanumeric information, including a block letter "A", has been written in such a manner using a conventional stroke-writing technique. As will be evident, large or complex images may be written on the EL panel in intermittent steps, the beam being turned off by blanking circuitry 28 during the intervals 50 (Fig. 5) between individual sustain pulses 48.Fig. 6 shows the intervals 52 during which beam 20 may be enabled, or turned on by blanking circuitry 28 to write an image on panel 1 6. Intervals 52 coincide with the durations of sustain pulses 48 in train 46.

When electrons from gun 18 bombard the rear surface of ACTFEL panel 1 6 during the writing step, polarization is induced in corresponding regions of active layer 38, producing electroluminescence in those regions. As long as the train of alternating polarity pulses 48 is applied to the target electrodes, the luminescence will be sustained, providing a stored display of the written information. As will be understood, the brightness with which the stored information is displayed will depend upon the amplitude of the sustaining pulses and the degree of polarization induced by writing beam, the latter being a function of the written beam's energy and current density.

Information is selectively erased from panel 1 6 in a manner similar to that used to write it. More particularly, luminescent regions of the thin-film active layer are selectively depolarized by scanning beam 20 over corresponding areas of the targets rear surface during the time intervals when electrodes 34 and 42 are at the same potential (ground) following the termination of a sustaining pulse 48. By so doing, luminescence in the selected regions is reduced to that of the unwritten areas of the panel. Thus, it will be seen that all or any portion of a stored display may be erased by writing over it in appropriately timed relation to the sustaining pulses in train 46.Referring by way of example to Fig. 4, the letter "A" may be selectively erased from the displayed information by scanning beam 20 in a raster pattern (indicated by dashed lines) over an area of the panel that includes the letter, or by tracing over it using a stroke-writing technique.

in accordance with a preferred aspect of the invention, electron beam 20 is operated at an energy sufficient for it to penetrate rear electrode 16, but below the level necessary for it to pass through insulating layer 40 and enter the active layer. With the exemplified active layer material (ZnS:Mn), beam penetration into the active layer during the writing step is indicated by the production of cathodo luminescence, which produces light of a color different from that produced by electroluminescence. A significant advantage of this approach is that it greatly reduces the required beam acceleration potential, and permits the use of ACTFEL storage targets in mono-accelerator CRT's of generally conventional design. CRT 10, for example, uses a conventional oscilloscope electron gun operating at a potential of about 4 kV (measured with respect to cathode 22).It is hypothesized that, at the low acceleration potentials used according to preferred practice of the invention, the writing beam penetrates the metal rear electrode of panel 1 6 and builds up a localized charge in insulating layer 40.

The charge locally reinforces the field impressed on active layer 38 by alternating pulses 48, thereby inducing polarization and electroluminescence in localized regions of the layer. As viewed graphically in Fig. 3, the direct electron beam writing method of the invention switches incremental areas of the panel from a low luminance state S1 to a high luminance state S2, which is sustained by pulse train 46. Erasure with the beam reverses the process, returning the areas to initial luminance state Si. With the preferred low acceleration potential, erasure is performed by scanning beam 20 over the target's rear surface in an interval following the termination of a positive sustaining pulse 48. Fig. 7 shows the intervals 54 during which the electron beam from gun 18 may be enabled by blanking circuitry 28 to erase an image on panel 1 6 under the control of deflection circuitry 30.

Claims (11)

Claims

1. A method of operating a cathode-ray tube of the type that includes an electroluminescent panel exhibiting voltage/luminance hysteresis, and means for producing an electron beam and directing it toward said panel, which panel consists in essence of an active layer of electroluminescent material, first and second electrodes of conductive material, each disposed opposite a different face of the active layer, and first and second dielectric layers separating and insulating said electrodes from the active layer, said method comprising the steps of: (a) with a suitable excitation voltage applied across said electrodes to place said panel in a relatively non-luminous, ready-to-write state, directing said electron beam onto selected first areas of the panel to write a desired image thereo, said first areas being thereby changed to a stable luminous state; and (b) subsequently erasing selected portions of the thus-produced luminous image by the subsequential steps of (b1) removing said excitation voltage; (b2) placing said electrodes at the same electrical potential; (b3) directing said electron beam into selected second areas of the panel that include said selected portions; and (b4) reapplying said excitation voltage to the electrodes.

2. The method of claim 1, wherein said electron beam is operated at an energy level insufficient for it to penetrate into said active layer of the panel.

3. The method of claim 2, wherein said excitation voltage comprises a pulse component of a train composed of spaced, alternating polarity voltage pulses, and wherein step (b3) is performed in the interval following the termination of a positive voltage pulse applied to one of said first and second electrodes.

4. The method of claim 1, wherein said active layer consists essentially of manganese-activated zinc sulfide.

5. The method of claim 4, wherein the concentration of manganese in said active layer is in the range of about 1 to about 5 mol percent.

6. The method of claim 4, wherein said dielectric layers are formed of yttrium oxide.

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7. A method of selectively erasing portions of an image displayed by apparatus including a cathode-ray tube of the type that includes an electroluminescent panel exhibiting voltage/luminance hysteresis and means for producing an electron beam and directing it toward said panel, said panel consisting in essence of an active layer of eiectroluminescent material, first and second electrodes of conductive material, each disposed opposite a different face of the active layer and first and second dielectric layers separating and insulating said electrodes from the active layer, said method comprising the steps of: (a) placing said first and second electrodes at the same electrical potential; (b) directing said electron beam onto selected areas of the panel that include portions of a previously written image desired to be erased; and (c) applying a suitable excitation voltage across said electrodes to produce luminescence of those portions of the previously written image laying outside said selected area.

8. The method of claim 7, wherein said active layer consists essentially of manganese-activated zinc sulfide.

9. The method of claim 8, wherein said dielectric layers are formed of yttrium oxide.

10. A method of operating a cathode-ray tube substantially as hereinbefore described with reference to the accompanying drawings.

11. A method of selectively erasing portions of an image displayed by apparatus including a cathode-ray tube substantially as hereinbefore described with reference to the accompanying drawings.