US2598401A - Electron discharge device suitable for use as television transmitting tubes - Google Patents

Description

y 1 1952 H. G. LUBSZYNSKI ET AL 2,598,401

ELECTRON DISCHARGE DEVICE SUITABLE FOR USE AS TELEVISION TRANSMITTING TUBES Filed Feb. 16, 1949 //2 Var/Tar H. g. 1. 1/532 m/s/o, n45 MRK 5y Am I!- CASS/MA /v Patented May .27, 1952 2,598,401 ELECTRON DISCHARGE DEVICE SUITABLE gglEIsE AS TELEVISION TRANSMITTING Hans Gerhard Lubszynski, Northwood, James Dwyer McGee, Ealing, London, Walter. Ernest Turk, Hayes, and Harry Cassman, Harlington, England, assignors to Electric .& Musical Industries Limited, Hayes, England, a company of Great Britain Application February 16, 1949, Serial No. 84,596

In Great Britain February 18, 1948 This invention relates to electron discharge pick-up tubes suitable for use in television.

A known form of pick-up tube includes a target electrode comprising a photo-electric mosaic 5 Claims. (Cl. 315

found that the above-mentioneddisadvantages can be reduced if instead of using insulating material for the target electrode material is employed which is slightly conducting so as to enable formed on one side of an insulating surface the 5 leakage of charge to occur. opposite side of which is in contact with a signal According therefore to the present invention electrode from which picture signals can be oba pick-up tube is provided comprising a target tained after projecting an optical image on to electrode in contact withasignal electrode, means said photoelectric mosaic and after scanning said for scanning said target electrode with a lowmosaic with an electron beam. Such a type of 10 velocity electron beam in such a manner that the tube can be constructed so that the target elecelements of the target electrode can be stabilised trode is scanned by a low-velocity scanning-beam substantially to the potential of the cathode of whereby the elements of the mosaic are stabilised said tube and wherein said target electrode has at an equilibrium potential corresponding subsuch a conductivity that leakage of chargefrom stantially to that of the cathode of the tube. In the elements of said-target electrode can occur such a form of tube the beam is usually scanned through said target electrode to said signal elecin the presence of a longitudinal magnetic field trode during a frame scanning Period. By em-' so that the scanning beam impinges on the target ploying a target electrode of this form it is poselectrodesubstantially normally throughout the sibl to ensure that the elements of the target scanning cycle. 7 V 7. 2O electrode are not permitted to increase in poten- In Operating Such a tube, if any element of the tial to an undesirably high extent and to ensure target is brightly illuminated it may increase in that any charge accumulated by an element of Potential to Su a extent d g a frame scan said-target electrode which is not neutralised period that the secondary electron emission ratio h n scanned leaks away appreciably before of the element when scanned is greater than being scanned again. The extent to which the un y and in such a 'case it is found that the tube leakage of charge is permitted to occur durin becomes unstable and it is not possible to stabilise one frame scan will naturally depend on the cirh a ri h ly u at e e t to cathode cumstances under which; the tube is being emp t tial.v urt rm f a m ai element is ployed,. and whilst it is preferable to arrange for Weekly illuminated, su h an element does not s the target electrode to have such a conductivity appreciably in potential between successive frame t t any charge ulated on an element of Scans W th e r t that y a small prop r the target electrode which is not neutralised when of the electrons from the scanning beam impinge an ed completely leaks away before being on said element and it may require several suescanned again nevertheless, Substantial cessive scans to restore such an element to cathm nt over, the known type of tubes which ode potential. The result of this is that instead employ an insulating target electrode can be obof a a u Signal being devebped mV tained if the conductivity of the target electrode an eleme t Wh Sca ed a Smaller S l is such that the charge leaks away in two or three VEIODBd d e element y r qu re o be frame scan periods. The conductivity of the tar canned several tim s before m ximum si n l is get electrode will of course also be chosen having developed- If an image of a moving object s regard to the duration of a frame scan and when reconstituted from signals developed in this manemploying a tube in accordance with the invenner'the moving object in the reconstituted image ticn in a television transmitting system in which will then produce a low intensity smear as it is one complete frame of a picture is scanned in moving and the object will exhibit an increased 5 405 lines at 25 frames per second, the specific illumination as soon as its motion ceases. resistance of t target l tr d may b th 'The object of the present invention isto proorder of 3.10 10 ohm centimetre. v on proved pick-up tube with a vi w to In order that the said invention may be clearly el minat or reducing either or both of these understood and readily carried into effect, it will disadvantages. 1 now be more fully described with reference to the Normally the target electrode in the pick-up accompanying drawing which illustrates diatubes referred to above comprises a sheet ofingrammatically a pick-up tube in accordance with si'ilating material, such as glass or mica, and when the invention and with circuit connections employing such an insulator there is practically therefor. no leakage of charge there-through. We ave 56 As shown in the drawing, the pick-up tube 1 5. The side of the target'electrode having -the mosaic elements 5 is arranged tobescann'edby a low-velocity scanning beam which is generated by a suitably disposed electron: gun. The-electron gun comprises a cathode 6, a catho'deshi'eld'l, an.

apertured anode 8 and a furtherelectrodea'con sisting of a metallic wall ooatirig. --The cathodetmay be maintained at earth'po'tential, the'shield I at a negative potential thereto. the anode 8 at a positive potential and the electrodes usually-tat a slightly lower positive potential than the anode 8. Near to the target electrode 2 is a decelerating electrode which is maintained at a lesspositive' potential than the electrode 9. The electrodes 6 to [care conventionally shown as deriving their potentials from a potentiometer it connected across a source of potential 12. The electron beam from the gunisaccelerated by the anode 8 and decelerated by the electrodes 9 and land is scanned over the surface-of the target-electrode 2' at line and frame'fr'equencies by scanningcoils indicated alt-l3. The electron beam -is-f0oussed by-and is arrangedto -scan-the surface of the target electrod'e in the presenceo-f a longitudinal magnetic field set up by -a solenoid M and the arrangemen-t is such that-the-beam'is caused to impinge' on the target electrode-substantially normally throughout the wholesca'nning cycle in known manner. An optical image for-transmission is projected by an o'pticalsystem l through the signal electrode- 4- through the sheet 3 on to the photoel'ect'ricmosaic elements 5. Photoelectrons are thereby caused to be emitted from the elements '5 causing positive charges to be accumulatedaccording to the intensity oi. elementary areas of theopticalimage. The target electrade is then scanned at line and frame fre- 'quericies with the low-velocity scanning beam emanating from the cathode 6, in known manner. Scanning of the target-electrode with the scanhing beam restores-the elements-of the target electrode to a potential corresponding substantially to that of the-cathode 6, restoration of said charges serving to set upacross a signal resistance 16 4 connected tothe signal electrode '4 :picture signals which are fed via condenser I! to an amplifier l8.- The signal 'resistance I6 is connected to -earth or to-a-source of positive potential as hereinafter referred to.

The type et tube shown in Figure l is known arid normally the sheet 3 'is made of-in'sulating material such as glass or mica having a specific resistance of the orde'r of 10 -10 ohm'centi- In accordance, however, with "the present invention the sheet 3 is'arran'ged to b'e'slighttre. Such a material may be a known form of socalledconductingtglass. It will be appreciated that theelemei itsof thetarget electrode form a multiplicity of minut condensers and due to the conductivity of the sheet-3 'each of these condensers iseffe'ctively' shunted by aresistance. By

arranging the conductivity of the sheet. 3 to be such that the time constants of the condensers and resistances are equal to or less than atrame scan period, or a half or a third of a frame scan period, the disadvantagesreferred to above can be substantially reduced since leakage of charge can occur to the signal electrode.

The use of a slightly conducting material gives rise to loss in sensitivity of the tube since the charge accumulated on the mosaic elements is not sohigh as the charge which would be accumulated if a true insulator were employed. Some degree of sensitivity can, however, be regained asc'ompared with the true insulator for the case of moving. objects at low illuminations, where the true irrsulator produces weak, blurred signals due to insuihcient'restoring power of the scanning beannby maintaining the signal electrode associated with the mosaic elements at a few volts positive with respect to the cathode of the tube by connecting the signal resistance IE to a source of positive potential. Under those conditions, and assuming that the mosaic elements are not illuminated, they will increase-in potential due to leakage from the signal electrode through the sheet 3 to the positive potential of the signal electrode during a frame scan-period and will-be restoredto a potential substantially that of the cathode on being scanned. If, however, the mosaic elements are i-lluminatedthey will rise to a potential-Sonia what higher than that of the signal electrode and since all the elements of the mosaic, whether illuminated or not, will be at slightly positive potential when scanned, a large proportion of the electrons 'ofthe scanning beam will beable to impinge on the elements so that the signal output will be increased for any-given increase of illumination.

In the types of tube shownin the drawing," i-t is known that electrons from the scanningbeam which are not utilised for 'restoring the elements of the targetelectrode to an equilibrium potential are returned from the target electrode and can beemployed to generate picture signals, said electrons, if desired, being multiplied in an "electron multiplier. The present invention can be-applied to tubes in which the returned electrons are employed to generate picture signals and in tubes employed in this manner it will stillof course be necessary to employ the electrode! in contact with 'the layer 3 although this electrode willnotr then-serve the function of a signal electrade. The signal electrode l in'tubes employed in this manner may-be connected directly to' earthor to a source of positive potential. It will'be understood therefore that in the following claims the term signal electrode is intended toinclude an electrode whether employed asra signal electrode or not.

What we claim is:

1. A pick-up tube having-a cathode,- a charge storage target electrode comprising a sheet of partially conducting material, a mosaic of:pho"- toelectric conducting elements on one side-ofsaid sheet and a; signal electrode in contact with the opposite-side of said sheet, and means forscannin'g said mosaic elements with a low velocity electron beam, said sheet having a specific-resistance not greater than 10 ohm-centimeter to conduct charge from substantially .all otsaid mosaic elements to said signal electrode.

2'. A circuitarrangement including a pick-up tube having a cathode, a charge storage target electrode comprising a-sheet of partially conducting material, a mosaic of photoelectric conducting elements on one side ofsaid sheet-and asigrial-electrode in contact with the oppositeside of said sheet means ior maintaining said cathode predetermined potential.

at a predetermined potential, and means for successively scanning each mosaic element with a low velocity electron beam to stabilize said elements substantially to said potential, the time constants of the condensers and resistances effectively provided by said partially conducting sheet having values equal to at least one-third of the interval between successive scannings of the same element.

3. A circuit arrangement including a pick-up tube having a cathode, a charge storage target electrode comprisinga sheet of partially conducting material, a mosaic of photoelectric conducting elements on one side of said sheet and a signal electrode in contact with the opposite side of said sheet, means for maintaining said cathode at a predetermined potential, and means for successively scanning said mosaic elements with a low velocity electron beam to stabilize said elements substantially to said potential, said sheet having a specific resistance not greater than ohm centimeter to conduct charge from substantially all of said mosaic elements to said si nal electrode.

4. A circuit arrangement according to claim 2 including means for maintaining said signal electrode at a positive potential with respect to said 5. A circuit arrangement according to claim 3 including means for maintaining said signal electrode at a positive potential with respect to said predetermined potential.

HANS GERHARD LUBSZYNSKI. JAMES DWYER McGEE. WALTER ERNEST 'I'URK. HARRY CASSMAN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,150,980 Lubszynski et a1. Mar. 21, 1939 2,175,691 Iams Oct. 10, 1939 2,234,292 Uhlmann Mar. 11, 1941 2,404,046 Flory July 16, 1946 2,404,098 Schade July 16, 1946 2,407,485 Essig Sept. 10, 1946 2,460,093 Law Jan. 25, 1949 2,470,875 Snyder, Jr May 24, 1949 2,503,949 Jensen et al Apr. 11, 1950 2,506,741 Rose May 9,1950 2,506,742 Rose May 9, 1950 2,518,434 Lubszynski Aug. 8, 1950 2,520,244 Iams Aug. 29, 1950 2,544,753 Graham Mar. 13, 1951 2,544,754 Townes Mar. 13, 1951 2,544,755 Johnson et al Mar. 13, 1951 2,555,091 Lubszynski May 29, 1951

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