US2520240A - Cathode-ray tube - Google Patents

Description

Aug. 29, 1950 L. E. FLORY CATHODE-RAY TUBE Filed Sept. 5, 1946 (Ittomeg Patented Aug. 29, 1950 CATHODE-RAY TUBE Leslie E. Flory, Princeton, N. J assignor to Radio Corporation of America, a corporation of Delaware Application September 5, 1946, Serial No. 695,015

- 3 Claims. (c1.315-11 This invention relates to a cathode ray pickup tube of the type disclosed in the patent issued jointly to George A. Morton and myself, No. 2,345,282, on March 28, 1944. tube, known as the Orthicon, a beam of electrons froman electron gun is magnetically deflected to scan a mosaic which is essentially at cathode potential so that in darkness the beam electrons do not strike the mosaic but are returned to the gun end of the tube. If the entire tubeis in an axial magnetic field the beam will return approximately along the same path'over which it approached the mosaic. When areas of the mosaic are caused to become positive, as for instance by electron emission in response to a light image, the positive areas collect electrons from the beam and return the mosaic to the original condition of equilibrium. These electrons are subtracted from the returning beam resulting in a modulation of the returning beam current. This returning beam may be collected at the gun end in an electron multiplier or other collector and thus constitute the television signal. 7 r

In tubes of this type of usual construction the returning beam is shifted so that it can enter an electron multiplier placedalongside the gun.

Since it is necessary to accelerate the electron beam to several hundred volts in order to maintain the beam in good focus, it is also necessary to decelerate it in Y the region of the mosaic.

Therefore, in previous tubes an electric fleld' exists in the region of the mosaic, the lines of force of which are curved. As the electron beam enters the decelerating electric field it encounters the condition where the electric field and the magnetic field are at a relatively great angle with respect to each other. Under these conditions the beam deflected from its original course by an amount dependent upon the component of the electric fieldat right angles to the magnetic field and in a direction mutually perpendicular to both. When the beam is reversed in direction near the mosaic and re-accelerated, it is again deflected in the same direction, adding to the deflection taking place during deceleration. The electron beam, therefore, does not return over the same path by which it approached the mosaic. Furthermore, the angle between the magnetic field and the electric field becomes greater as the distance from the tube axis is increased. Thus as the beam is deflected to scan the mosaic, the deviation of the return beam from the original path varies in accordance with the deflection, resulting in the return beam scanning the multiplier dynode and making it difli- In this type of 2 cult to properly utilize the beam with a multiplier.

It is an object of this invention to reduce the scanning pattern Of the'return beam so it may readily enter the opening of an electron multiplier for all deflected positions of the beam at the mosaic.

Another object of the invention is to provide means for producing an electric decelerating field in front of the mosaic with lines of force parallel to the lines of the electromagnetic field.

Another object of the invention is to produce a decelerating field having parallelism of the lines of the electric and magnetic fields by means of a mesh of very fine wires.

Other objects will appear in the following specification, reference being had to the drawing in which: i

Fig.1 illustrates the curved lines of the electric fleld which produces the undesired scanning pattern on the multiplier dynode;

Fig. 2 is a section showing the application of my invention to television pickup tubes of the Orthicon type; r

Fig. 3 illustrates the efiect of the screen or mesh of my invention in'making the electric lines of force of the decelerating field substantially parallel to the electromagnetic lines in the region of greatest decelerating field.

Referring to Figure 2 of the invention, the tube comprises an evacuated envelope I of glass or other suitable material containing the gun G at one end and the target T at the other. gun G consists of the usual indirectly heated cathode 2, grid 3 and first anode 4. The grid and anode'have the usual small apertures for forming the outgoing or primary cathode ray beam B. Inside the'envelope l is placed an accelerating second anode 5 which may be plated on the inside wall or mounted separately thereof. This anode is usually called the wall coating regardless of its manner of mounting.

Outside the tube envelope I is located the electromagnetic deflection unit 6 which has two coil producingfields at right angles to each other and to the axis of the tube. These coils are not separately shown. One coil is supplied with a suitable varying voltage for line scansion and the other with one for frame scansion. These sources of deflection voltage are well known in the art.

Outside the deflection unit 6 is located the electromagnetic focusing coil 1 energized by direct current to produce a suitably strong magnetic focusing field axially of the tube and per- The pendicular to the target. Between the wall coating 5 on the inside of the envelope I and the target T is placed the decelerating anode or lens 8. To reduce the scanning of the multiplier dynode 9 by the return beam, I employ a mesh of very fine wires it in front of the mosaic II and between it and the decelerating lens. It is operated at a positiv voltage. with. respectto the mosaim on the targetT, preferably at about the voltage- Of the anode 8 which may be 50 volts when the wall coating anode is from 100 to 200 volts. target as usually constructed comprises a. thin transparent sheet of mica [2 in which is plated The:-

on the side thereof facing said gun and a transa thin transparent film I3 of. metals'suclras platinum, for example (Figs. 2 amid-3 Thesmosaie tt is formed on the side of the mica opposite the side containing the metal film andwiacingi. the. gun. It may consist of a layer of discrete particles of material capable of emitting photo electrons when energized by. light imaged thereon as= i's welli understood. inthe. art.

To direct the return beam B1 away from. the anode 4 and into: the first. dynode. ofi'themultip1ier'9 one stagezonly'being shownzforsimplification, a pair oflifzter plates; [4,15 are. employed. These are placed betweenthe gun andthe-target suitably near the former. These areshown in said; patent and; this: action. onzthe. beams- B and Briswell. known;

Thezmetahfilm of. thetargetmay beconnected to the power supply-unit conventionally shownvat. V so: as; to. have substantially the potential: of thecathodez of: the gun. This source; may also supply proper potentials; tothe other electrodes of the tube.

When: the-screen: ID isinotemployed the-electric field; li'nes; E are: out i of parallelism. with the electromagnetic lines M which are parallel touthe axis: of? the; tube: after passing: beyond. the deflectioncoiisz. Whemthezfinemeshscreen lil isused in front of the mosaietheelectric. field. lines E are substantially; parallel: to. the: electromagnetic lines M throughoutzthegreater partof: the deceleration field.

The deceleration and. re-acceleration. of the beam. electron takes: place in a. region where thefieldilines are approximately parallel to the tube axisand; thereiore, the undesired deflection ismuch reduced.

While certain: specific embodiments have been illustrated'and described; it. will beunderstood.

that: various changesand modifications: may. be. made therein withoutdeparting from: thes-pirit: and scope of the-invention.

Whatli claim as new-is 1. Av television. pick-up tube: systeim. compria ing an evacuated envelope containinga. cathode ray'bea-m gun having cathode,-, g-r-id and" anode electrodes; atargetspaced fromzthegun, afinemesh'screen facing. said: gun adjacent said target and. a second anodeelectrode: for producing. a

field for accelerating electrons,-.meansfor pro.- ducing" amagnetidfocusingfieldbetween the gun andthe: targetand a voltage source. connected:

to: produce. potentials at said: electrodes,. said parent-electrcdeomthe other side thereof, a fine m'eshiscreen facingsaid gun adjacent said photocathode and a second anode electrode for producing afield. for accelerating electrons between said target and said" gun, means for producing a magnetic focusing field between the gun and the target and a voltage source connected to produce potentials. at said. electrodes and said.- screen; the petentiala of said. transparent electrode and said cathode: being substantially the same and the voltage between. saidz'screen' and said transparent electrode producing. throughout the space between. the screen. and the target. a. decelerating field. having electrostatic lines parallel to. the magnet-iclines-of saidfocusing field.

31A television pick-up tube system,. comprising, a. tube containing. a cathode ray beam gfin, having, cathode, grid. and anode electrodes, a

target, a: second: anode electrode. between. the. gun

potentials atlsaid electrodes, said. target and said screemthe potentialsapplied tosaid cathode and. said target being substantially the same, the. potential. of saidscreen being intermediate those of. said. decelerating. anode. and said target to. provide. a decelerating. and accelerating field be-- tween the. screenv andetarget. parallel: with the focusing. field: at all elemental. areas of the target raster. wherebythe=return:beam leaves. the targetand. is accelerated towards: said gun. along sub,

stantially the. same paths as the. outgoing beam andliiter. anodes.- for changing. the path of the electrons andwdirecting them to. said multiplier dyno.de.

FLOR-Y.

REFERENCES. CITED" The following; references are of record in. the

file of this patent:- 1

UNITED" Sl ATES P'ATEN'IS Number Name. Date I 2,288,402. Iams June 30, 19542 2,412,086. Hallmark Dec. 3; 1946'

Images