US2667596A - Storage electrode for signal-converting devices - Google Patents

Description

Jan. 26, 1954 c. s. szEGHo ET AL STORAGE ELECTRODE FOR SIGNAL-CONVERTING DEVICES Filed Nov.' l5, 1950 Scanning LIAM O. REED CoNsTNTlN S. SzEGHo Wu. Y

'1% E a i.. mllllNlll S stem Volclge Source INVENTORS THE/? ATTORNEY l Patented Jan. 26, 1954 UNER-D 2,667,596 'STORAGE amenacen soir VERTING DEVICES Constantin S. Szegh and cago, Ill., assignors to tion, a corporation of Ill sIGNALmoN;

vviiiiam o. need, chi- The `Bauland Corporainos Application November- 15, 195o, serial No. leases s claims. (ci. 31e-34e) This invention relates to signal-converting devices, and more particularly to a new and improved storage electrode for `a directlyeviewed signal converter.

The use of signal-storage devices incorporating image-storage electrodes for developing a spacemodulated charge image of an incoming electrical signal by scanning the storage electrode with a signal-modulated cathode-ray beam is weil known in the' art. Arrangements of this type have vbeen found particularly useful for radar display purposes and the like in which it is desired to convert radar pulse-type input signals to television-type output signals for application to an image-reproducing device. Such devices as have been Vemployed in the .past are characterized by several undesirable limitations. In the first place, conventional image-storage electrodes are incapable of satisfactory half-tone reproduction, so that conventional signal-storage devices may be operated only at a full-signal condition or a 11o-signal condition. Thus, for example, radar signals from an object passing through a cloud are obscured by the signals returning from the cloud itself.

rthermore, conventional signal-storage devices are usually operated at relatively high voltages, thereby imposing severe power supply requirements.

It is an important object of lthe present invention to provide an improved signal-storage device of the directly-viewed type, particularly appl-ilcable for radar display purposes and the like, which is capable of half-tone storage and reproduction.

A further object of the invention is to provide :an improved directly-viewed signal storage device adapted to be operated at relatively low pov/er supply voltage y Still a further object of the invention is to provide a novel storage electrode Vfor a signalconverting device.

In the copending application of 'Constantin S. Szegho et al., Serial No. 99,421, filed June i6, 19-1'9, for Signal Storage Devices, and assigned to the present assignee, there is disclosed and claimed a novel signal-storagedevice of the type comprising a storage electrode and a pair of electron gun structures for respectively storing a space-modulated charge image on the storage electrode and releasing the stored information. The storage electrode comprises a conductive mesh screen and a dielectric lm distributed acrossthe intrstices of the screen in substantially coplaia-r relation therewith. The 'storage electrode structure disclosed and claimed in the copending application is capa-ble of half-torre storage and reproduction land is particularly adapted to use in scanning converters, memory tubes, and the like. The present invention is directed to a mcdication of the storage electrode which is adapted to use in a directly-viewed signal-converting device;

In accordance with the present invention, a storage electrode for 'a signal-converting device comprises a conductive grid-like structure having a plurality of interstice's. A dielectric flm is distributed on a single sur-face of the conductive structure and extends into and across the inter'- stices in substantially coplanar relation with 'the Photoemissive material is The features of the present invention which are believed to be Ynovel are set forth with particularity in the appended claims. together with further objects and advantages thereof, may best be understood, however, by reference to the following description taken in connection with the accompanying drawing, in the several gures of which like reference numer als indicate like elements, and in which:

Figure 1 is a sectional view, of a signal-'storage device including a storage electrode constructed in accordance with the present invention, together with4 a schematic representation of suitable external circuit con'- nections;

storage electrode of the line'S-S of Figure 2.

site dielectric -lm I3. The construction of which is preferably of substantially uniform thickness. Dielectric clin 'r3 extends iii-to and across the interstices of conductive mesh screen I2 in substantially coplanar relation with the screen in order to present a substantially planar surface with the mesh screen. Photoemissive material I4 is supported adjacent the exposed surface of conductive mesh screen I2, preferably being deposited directly thereon in a manner to be hereinafter described in detail.

With reference again to Figure 1, envelope I also encloses an electron gun I5 which constitutes an electrode system for projecting a recording or Writing cathode-ray beam on storage electrode II during writing intervals. A decelerating electrode I6 and an accelerating electrode I1 are disposed on opposite sides of'storage electrode I I.

Magnetic focussing coils I8 and deection coils I9 are associated with electron gun I5 and are connected to a suitable scanning system which operates to cause the writing beam to scan storage electrode II during Writing intervals. Scanning system 20 may be a conventional televisiontype sweep-signal generator; alternatively a circular-sweep system or any other desired type of scanning system may be employed.

It is preferred that the image to be stored on storage electrode II be recorded from the insulator side thereof, and that the photoemissive material I4 be deposited on the opposite, exposed. surface of the conductive mesh I2.

The cathode 22 of electron gun I5 is returned to ground through a signal source 23 and a suitable source of unidirectional operating potential 24. The control grid 25 of electron gun I5 is maintained at a xed direct potential relative to cathode 22 by means of a suitable negative biasing potential source, here shown as a battery 26; the bias of control grid 25 relative to cathode 22 may be adjusted as desired by means of a variable tap 21 on a resistor 28 shunting battery 26. A coupling condenser 55 is connected between cathode 22 and control grid 25, and a decoupling resistor 56 is connected between control grid 25 and variable tap 2. The rst accelerating electrode 29 of electron gun I5 is maintained at a constant positive unidirectional operating potential With respect to cathode 22 by connection to a tap 30 associated with a potentiometer 3I which in turn is connected in shunt with a battery 32 or other suitable constant-potential source. The second accelerating electrode 33 of electron gun I5 is connected to a second tap 34 associated with potentiometer 3I.

Decelerating electrode I6 and accelerating electrode I? are connected to respective variable taps 35 and 36 associated with potentiometer 3I. Anode cylinder 33 is preferably provided with a wide mesh screen 31 closing the output end of the cylinder, in order to insure a substantially uniform electrostatic field between anode 33 and decelerating electrode I6.

The conductive portion I2 of storage electrode II is connected to an additional variable tap 38 associated with potentiometer 3|.

In order to achieve visual reproduction of the charge image recorded on storage electrode II, a fluorescent screen 40 is positioned at the end of envelope I0 opposite electron gun I5; an electron-permeable metal backing layer 4I, preferably constituted by an aluminum film, is provided to prevent charge accumulation and to increase the brightness of the reproduced image in a manner well known in the art. Conductive lm 4I also serves effectively to prevent spurious excitation of photosensitive mosaic I4 by light originating at the fluorescent screen 4G. Light radiation from an external source 42 is directed and focussed onto photoemissive material I4 deposited on the conductive portion of the storage electrode II by means of a suitable collimating system, here schematically represented as a single lens 43. A pair of accelerating electrodes 44 and 45, which may conveniently assume the form of metal coatings on the inner wall of envelope I0, are provided between accelerating electrode Il and fluorescent screen 40. Accelerating electrodes 44 and 45 are connected to taps 46 and 41, respectively, associated with potentiometer 3I, and conductive iilm 4I may be connected to accelerating electrode 45. Bypass condensers (not shown) may be inserted between each of the several taps associated with potentiometer 3I and ground, in order to prevent signal-frequency variations in the potentials of the various electrodes.

As explained in the above-identified copending application, the storage properties of electrode II are dependent upon the secondary elec tron emission characteristics of that electrode, which in turn are primarily determined by the secondary emission properties of dielectric nlm I3. Three distinct operating conditions may be obtained for the composite storage electrode II. Within a rst range of primary electron voltages less than a first cross-over voltage Voi, the secondary electron emission ratio (the number of secondaries released for each incident primary electron) of the composite storage electrode is ess than unity and the net electron flow is to the storage electrode, i. e., negative current. For primary electron voltages within a second range greater than the iirst cross-over voltage Voi and less than a second cross-over voltage Voz, the secondary electron emission ratio is greater than unity and the net electron flow is away from storage electrode I I, i. e., positive current. Within a third range of primary electron voltages greater than the second cross-over voltage Voz, the secondary electron emission ratio is again less than unity, and negative current flows in the storage electrode II` At the two cross-over voltages Vez and Voi, the secondary electron emission ratio is exactly unity, and the current flow in storage electrode II is zero.

Similar characteristics obtain for each of the components of the storage electrode. Thus, the secondary-electron emission characteristic of the dielectric or insulating portion of the storage electrode exhibits two well defined ages V11 and V12. The cross-over voltages for the dielectric lm with those of the composite storage electrode.

The preferred mode of operation of the device of Figure l will now be described and explained in detail, bearing in mind the secondary electron emission properties of the storage electrode and its component parts.

In order to condition storageY electrode II for recording incoming signals from source 23, the potential of conductive portion I2 of storage electrode II is raised by means of tap 38 on poten-y tiometer 3l to a value between the rst and second cross-over voltages Vn and V12 of the insulator portion I3 of composite storage electrode II.

from electron gun I5 onto dielectric lm I3; because the primary electron voltage is greater than V11, the number of secondary electrons emitted is greater than the number of incident primary electrons, and a positive charge is built up on the t cross-over volt- I3 do not necessarily coincide beam is projectedv surface of storage electrode I I.

Thisaction is cu.- mulative until such4 timel as the charge potential of insulator portion I3 of storage electrode Il is substantially the same as the potential applied to decelerating electrode I 6, which is maintained at or near the potential applied to conductive mesh I 2 by suitable adjustment of tap 35 on potentiometer 3 I, and which serves to maintain a substantially uniform field for the incident pri--` mary electrons. When the charge on insulator portion I3 of storage electrode II has reached a potential substantially equal to that of decelerating electrode I6, an equilibrium condition is reached, and further scanning of storage elec-- trode II by the unmodulated electron beam has no further substantial efect on the charge accumulation.

The unidirectional operating potential applied toconductive mesh I2. is now lowered by means of variable tap 38 o-n potentiometer 3| to a value below the first cross-over potential V11 of the insulator, in order to bias the conductive mesh I2 to a lower voltage than that to which the insulator portion I3 is charged, thereby to reduce the tendency of subsequently stored negative charges to leak on through the insulator to the mesh. The lcharge on the insulator portion I3 remains at a potential well above the rst crossover voltage Vn, since secondary electrons emitted from the insulator are either collected by decelerating electrode IS or returned to insulator portion I3 thereby slightly lowering the charge potential. However, the charge on the dielectric film I3 is still maintained at a potential higher than its first cross-over potential Vn.

Writing gun I5 is now prepared to store an image on electrode II by reducing the bias on control grid 26 by means of variable tap 27 on potentiometer 28, and the unidirectional operating potential small positive value by means of D. C. voltage source 24 so that electrons from Writing gun I5 impinge on insula-tor portion I3 of storage electrode II at an effective potential below the first cross-over voltage Vn of the insulator. Signals representing the image to be stored are supplied to cathode 22 from source 23 -to effect velocity modulation of the writing beam, and deilection coils I8 are energized by scanning system 2t to cause the writing beam to scan the storage electrode. Because the writing electrons imp-inge on insulator portion I3 at an eiective voltage below the first cross-over potential V11 of the insulator, the number of secondary electrons emitted is less than the number of incident primary electrons, and the insulator assumes a negative charge distribution to form a spacemodulated charge image of the input signal from source 23.

Alternatively, the potential of cathode 22 may be adjusted to a large negative value by means of D. C. Voltage source 24 so that the writing electrons impinge on insulator portion I3 of storage electrode I I at an effective voltage greater than the second cross-over voltage V12 of the insulator; this mode of operation also establishes a negative charge distribution on dielectric film I3 constituting a space-modulated charge image of lthe input signal from source 23.

In order to reproduce a Visual image of the space-modulated charge pattern stored on dielectric lm I3, photoemissive material I4 is illuminated by light radiation from external source 42. The electrostatic field induced Iby the stored charges extends through the dielectric of cathode -22 is adjusted to a film I3 and innuences the intensity-ofthe photo-f emission from each elemental portion of the photoemissive mosaic I4 in accordance with thecharge distribution, since the field induced by. the stored charges retards the photoelectronsy and opposes the field set up by accelerating electrode I7. This eect is large owing to the sub-- stantial coplanarity of the dielectric film I3 and the conductive mesh I2. Thus, a beam of photoelectrons varying in intensity throughout its cross-section is emitted by photcemissive mosaic I4; electrodes I 7, 4d, and 45 serve to accelerate and focus the lphotoelectron beam and project it toward fluorescent screen 40. Consequently, fiuorescent screen 4t is excited in accordance with Ithe stored charge image. Due to the intensity-control action of the stored charge image on the photoemission fro-rn mosaic I t, and due to the space-modulation of the stored charge image in accordance the input signal, halttone storage and reproduction are obtained.l

The storage time obtainabe with the device of Figure 1 is determined primarily by the leali;-v

age time constant of the insulator employed as i the dielectric iilm I3. in the event that it should 'oe desired to change the stored signal before the insulator has become completely discharged through leakage, it is always erate the discharge by raising the potential of conductive mesh I 2 above the rst cross-over potential V11 and scanning the storage electrode with an unmodulated cathode-ray beam, as in Athe conditioning process outlined above.

As a further alternative, a second primary electron source, for projecting either a focussed beam which is caused to scan the storage electrode or a flooding beam, may be provided to perform the conditioning function. Moreover, amplitude modulation rather than Velocity modulation may be used on the writing beam from electron gun l5; to achieve this mode of oper ation, cathode 22 is maintained at a xed potential, and the potential of control grid 25 is varied in accordance with the incoming signal.

rlhis mode of operation is particularly advantageous when it is desired to utilize a writing beam of high voltage above the second cross-over potential but may also be used to advantage with low-voltage writing lbeams below the first 'crossover potential.

It may also be desired to store a white signal on a black background, and for this purpose it negatively is possible to bias storage electrode Il by scanning with an unmodulated electron beam below the first cross-over potential or second cross-over potential and subsequently writing with electrons -between the first and second cross-over potentials to produce a positive charge distribution on storage electrode I I which corresponds to a space-modulated charge image;

of the input signal.

With reference again to Figures 2 and 3, the storage electrode construction comprises a conductive grid-like structure I2 having a plurality of interstices. A dielectric lm a3 is distributed i2 and extends into I2, preferably by being deposited directly thereon. In accordance with a preferred embodiment of the invention, grid-like structure I2 may consist of a conductive mesh screen constructed of gold, chromium, gold-plated or chromium-plated coppossible to accel-v above the per, or other suitable material, and dielectric film I3 may be constructed of silicon dioxide, aluminum oxide, chromic oxide, a uoride of calu cium, barium or magnesium, or other suitable material. Photoemissve mosaic I preferably comprises a conventional antimony-caesium composite photoemissive surface although other types of photoemissive mosaics may be used. For example, if a composite silver oxide-caesium lm is employed as mosaic I4, improved contrast may be obtained by using infra-red light to illuminate the mosaic; moreover, photoelectrons emitted by a mosaic of this type have lower velocities and are therefore more readily controlled by the charges stored on dielectric film I3.

Merely by Way of illustration, a method for producing the storage electrode which has been successfully employed comprises forming a layer of antimony on one surface of a gold mesh screen of as fine a mesh as possible. An organic film, of nitrocellulose or the like, is disposed in contact with the same antimony-coated surface of the mesh screen, and silicon dioxide is evaporated in a low pressure of oxygen onto the mesh side of the organic lm. This process is repeats f. a number of times from different approach angles to provide uniformity of dielectric film thickness and to insure complete coverage of the mesh by the dielectric film. The nitrocellulose drawn into the interstices of the gold mesh by surface tension, so that the dielectric nlm extends into and across the interstices in substantislly coplanar relation with the mesh. After die lm evaporation, the nitrocellulose film is dissolved in amyl acetate or other suitable solvent, and the structure is immersed in a mixture alcohol and ether to remove any remaining traces of organic film. The storage electrode is then mounted Within envelope I0, and the latter i. evacuated in any convenient manner, The antimony layer on the gold mesh is caesiated to render the layer Ill photoemissive; for this purpose a tubulation (not shown) on envelope I may be provided. The techniques of forming photoemissive mosaics are well known in the art and are therefore not described in detail.

The mesh size or" screen l2 determines the resolution of the reproduced image and is preferably made as fine as practicable. A 0.0002-inch thick gold screen having four hundred openings or interstices per inch has been successfully used for structure I2, and a dielectric nlm thickness of from 1,000 to 8,000 Angstrom units has been found satisfactory. Other methods of producing the desired structure may occur to those skilled in the art, and the invention is not restricted to electrodes produced in accordance With the foregoing illustrative example.

While any of a number of insulating materials may be used in the preparation of dielectric iilm I3, it is preferred that the film be constituted of silica (silicon dioxide) to avoid the possibility of caesium deposits combining with or adhering to the insulator When the antimony layer is caeslated. The other conventionally used oxides and uorides are more diflicult to process Without encountering leakage in or across m. Thus, the present invention the insulator provides a novel storage electrode for use in a signal-'converting device of the directly-viewed type. The improved Storage electrode is eminently suitable for halftone storage and reproduction, and satisfactory operation may be attained at materially lower operating potentials than in the case of conventional prior art devices, thereby materially reducing the stringency of thepower supply requirements.

While a particular embodiment of the present invention has been shown and described,r it is apparent that various changes and modications may be made, and it is therefore contemplated in the appended claims to cover all such changes and modifications as fall Within the true spirit and scope of the invention.

We claim:

l. A storage electrode for ya signal-converting device comprising: a conductive grid-like struc; ture having a plurality of'interstices; a thin dielectric lilm of substantially uniform thickness less than that of said grid-like structure distributed on a single surface of said conductive structure and extending into and across said interstices in substantially coplanar relation with said structure; and photoemissive material supported adjacent the exposed surface of said con-- ductive structure.

2. A storage electrode for a signal-converting device comprising: a conductive grid-like structure having a plurality of interstices; a thin silica iilm of substantially uniform thickness less than that of said grid-like structure distributed on a single surface of said conductive structure and extending into and across said interstices in' Substantially coplanar relation with said structure; and photoemissive material supported adjacent the exposed surface of said conductive structure.

3. A storage electrode for a signal-converting device comprising: a conductive mesh screen having a plurality of interstices; a thin silica lm of substantially uniform thickness less than that of said mesh screen distributed on a single surface of said screen and extending into and across said interstices in substantially coplanar relation with said screen; and an antimonycaesium photoemissive layer depostied on the exposed surface of said screen.

CONSTANTIN Si. SZEGI-IO. WILLIAM O. REED.

References Cited in the rile of this patent UNITED STATES PATENTS Number Name Date 2,013,162 McCreary Sept 3, 1935 2,100,841 Farnsworth Nov. 30, 1937 2,149,977 Morton Mar. 7, 1939 2,136,393 Ring et al. Jan. 9, 1940 2,322,807 Iams June 29, 1943 2,523,132 Mason Sept. 19, 1950 2,558,647 Freeman June 26, 1951 FOREIGN PATENTS Number Country Date 491,011 Great Britain Aug. 19, 1938 607,178 Great Britain Aug. 26, 1943 716,614 Germany Jan. 24, 1942

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