US2795729A - Cathode ray tube - Google Patents

Description

June 11, 1957 GABOR 2,795,729

CATHODE RAY .TUBE

Filed Nov. 29, 1955 11 Sheets-Sheet 1 PIE 3 BYWWMVJ/ZQW/ ATTORNEYS June 11, 1%57 GABOR 2,795,729

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CATHODE RAY TUBE Filed Nov. 29, 1955 11 Sheets-Sheet 5 INVEN-TOR BYW, {Age/m. v-m

ATTORNEYS June 11, 1957 D. GABOR CATHODE RAY TUBE 11 Sheets-Sheet 6 Filed Nov. 29, 1955 INVENTOR ATTORNEYS June 11, D. GABOR CATHODE RAY TUBE Filed Nov. 29. 1955 11 Sheets-Sheet 7 1 19.24. Frg.25

INVENTOR e 7 {M091 PM AT ORNEYS June 11, 1957 D. GABOR CATHODE RAY 'TUBE ll Sheets-Sheet 9 Filed Nov. 29, 1955 omdl INVENTOR BY came/b071,,

- ATTORNEYS June 11, 1957 n. GABOR CATHODE RAY TUBE 11 Sheets-Sheet 10 Filed Nov. 29, 1955 Q INVENTOR 19W m BY W, W m

ATTORNEYS June 11, 1957 D. GABOR Y 2,795,729

CATHODE RAY TUBE Filed Nov. 29. 1955 V 11 Sheets-Sheet l1 ATTORNEYS cArrronn RAY TUBE Dennis Gabor, London, England, assignor to National Research Development Corporation, London, England, a British corporation Application November 29, 1955, Serial No. 549,712

58 tfllainnsa ((31. 315--13) The present invention relates to cathode ray tubes and more particularly to a new form of tube primarily suitable for television. The tube according to the invention has particular advantages for the production of television in color.

This is a continuation-impart of application Serial No. 309,677, filed September 15, 1952, now abandoned.

In the known types of cathode ray tube it is usual to employ an electron gun positioned at a distance from a fluorescent screen so as to direct an electron beam towards the screen along an axis substantially normal to the plane of the screen, like a straight pointer. The point of impact of the electron beam is moved over the screen by deflecting the beam in two directions parallel to the fluorescent screen and symmetrically about a mean axis so that such tubes normally assume a substantially conical shape, the screen forming the base of the cone. It follows that the larger the screen to be employed, the deeper the tube must be in the axial direction of its conical shape in order that the beam may not be required to be deflected over too wide a solid angle.

The present invention has for its primary object to provide a cathode ray tube of relatively small depth in the direction normal to the screen.

A further object of the invention is to provide a cathode ray tube of novel form which is especially Well adapted for the production of television pictures color.

Another object is to provide a new type of cathode ray tube having a plane screen.

Yet another object of the invention is to provide a cathode ray tube which will be compact and not greatly exceed in any of its dimensions the dimension of the picture screen.

Further objects will appear as the description proceeds.

In the cathode ray tube of the present invention the picture producing electron beam is first deflected in one direction only, parallel to the screen surface, and then proceeds in a direction which is also substantially parallel to the screen, at a relatively small distance from its surface. When the beam reaches a certain zone, it is strongly deflected towards the screen by a locally applied electric field, so that it forms a curved pointer, meeting the screen at the desired point. According to the invention this localized electric field extends over a substantially linear zone which always intercepts the beam, irrespective of its first mentioned deflection, and is shaped in such a way that the beam is focused by said field at the same time as it is deflected thereby. In the application of the invention to television, the linear zone of the electric field is in the direction of the line scan, while the frame scanning is achieved by moving said field parallel to itself, in the direction of the frame scan.

It follows from this functional principle of the invention that, as the beam moves substantially parallel to the screen and close to it, the new tube can be made very flat. No throw at right angles to the screen is required, as in all conventional cathode ray and television tubes.

A further advantage is that the deflecting and focusing electric field acts like a cylindrical electron lens of short focal length, which makes it possible to achieve a convergence angle of the electron trajectories at the screen many times larger than in conventional tubes. This, by the known laws of electron optics, enhances the specific brightness of the spot, because a larger current can be concentrated into a smaller screen area with a given guri design and prescribed screen potential. p H Another advantage of the new design is that thelarg'e' final convergence angle makes it eminently suitable for color television. Color tubes are known in which the color is varied by changing the incidence angle of one or several electron beams by means of a grid or sieve disposed at a certain distance from a fluorescent screen with locally varying color response, in such relative position that a ray passing in a certain direction through an aperture of said grid or sieve falls on a spot of the screen which has a definite color response. The Wide convergence angle in the new tube allows dispensing with a special foraminated organ, spaced and positioned in accurate relation to the screen, and makes it possible to produce a definite color response by suitable shaping of the screen surface, as will be described later in more detail.

The functional principle of the invention may be translated into structural terms in the following manner. Within a glass envelope there is provided a substantially plane fluorescent picture screen adapted to be maintained at a maximum positive potential of the order 515 kv. Anarray of linear conductors is arranged in a plane parallel to the screen, the conductors being substantially parallel to one another and to the direction of the line scan, insulated from one another and preferably associated with a common capacitive backing plate. This array of conductors may be called the scanning array. An electron gun is positioned in the envelope and arranged so as to project an electron beam into the space between the scanning array and the screen. If the array is charged up to a maximum positive potential, equal to the potential of the fluorescent screen, there is no electric field between said array and the screen, and the cathode ray beam will pass through between them without being deflected. If, however, a zone of the array is discharged, so that in the zone successive conductors assume a graduated range of potentials extending from the maximum positive potential down to a potential in the neighborhood of that of the cathode of the electron gun, the electrons will be repelled by said zone and thrown towards the fluorescent screen. If the conductors of the array are discharged, one by one, progressively across the array in the' direction towards the electron gun, the zone will travel across the array so that the beam will be thrown on the screen after a progressively diminishing length of travel, and this effect may be used to provide the frame scan for a television picture presentation employing the tube. The discharge should not be abrupt but gradual; that is to say, the conductors should not be fully discharged one by one, but the discharging process is caused to be spread over a plurality of conductors constituting the deflecting zone so that an electric field wave of a certain desired shape travels across the array, the wave shape being such that it focuses the beam on the screen at the same time as it deflects the beam towards" the screen. This wave can be made to run at a constant speed, i. e., the speed required for the frame scan.

Ina preferred-form of the invention a second, similar array of substantially parallel conductors is disposed at or in close proximity to the surface of the fluorescent screen so that the electron beam passes between thetwo arrays: The conductors of this second-mentioned array are electrically connected one by one or'in' small groups with the conductors of the first-mentioned array, the interconnection being so effected that the conductors of one array are staggered in relation to the conductors of the other array to which they are connected. Corresponding conductors therefore do not face one another across the gap at right angles to the path of the electron beam but are displaced relatively, so that the conductor on the screen side is beyond its electrically connected fellow on the other side,looking in thedirection of the electron beam. Such a double array may be made as a single array folded upon itself, and has the advantage, compared with the simple scanning array previously described, that it enables more precise focusing of the end portion of the beam and focusing of a wider beam to be achieved.

The discharging of the conductors to form the electric field wave may be achieved by a special organ which will be called .the scanning valve, contained in the vacuum space .of the tube itself. -The. scanning valve, to be described below, comprises a further array of substantially parallel conductors, which may be made continuous with the conductors of the other array or arrays referred to above. The scanning valve conductors are arranged in a special pattern on an approximately U-shaped cylindrical surface and form the outer electrode of the valve. Associated with-this further array are a cathode, at least one electrode acting as a grid, and at least two electroncollecting electrodes. All these electrodes extend over the whole length of the valve.

At the far end, looking in the same direction as that in which the picture producing electron beam travels, the scanning array is terminated or bounded by a plate-like electrode permanently at or near the potential of the electron gun cathode, and atthe near end by a second platelike electrode permanently at maximum positive potential. Thescanning valve array is similarly terminated or bounded by electrodes which may be continuations of those of the scanning array. e

The scanning valve effects both the frame scanning operation and the flyback. In the scanning phase the electrons emitted by the scanning valve cathode discharge the array in such a way that a potential wave of the desired shape runs down its length, at constant speed. The speed is determined by the capacity of the array to the backing plate, and by the electron current, which in turn is determined by the design of the valve, and can be controlled by the grid potential. In the fiyback phase the whole array may be raised to the maximum positive potential by releasing secondary electrons from its conductors, the secondary electrons being collected by one or more of the electron-collecting electrodes.

Apart from the novel method of effecting-the frame scan, the operation of the new tube is similar tothat of conventional television tubes, the line scan being effected by magnetic or electrostatic deflection means operating on the picture producing electron beam in known manner.

The invention will be better understood from the following description given with reference to the accompanying drawings in which:

Figures 1, 2 and 3 are a front view, a'partly sectioned elevation and a partly sectioned plan view, respectively, all somewhat diagrammatic in character, of one form of cathode ray tube according to the invention, designed for the presentation of television pictures.

Figure 4 is an enlarged fragmentary vertical section of one form of screen and scanning array according to the invention. Y

Figure 5 is a diagram, showing the distribution of the potential over the arrayconductors of Figure 4, at one instant. V

Figure 6 is a perspective view of a part of the scanning valve of the tube illustrated in Figures 1-3, and Figure 7 a longitudinal section thereof looking into the grid electrode.

Figures 8-10 are transverse sections showing the electron trajectories in three phases of operation 'of the scanning valve of Figures 6 and 7.

the folded array of Figures 12-15.

Figure 18 is a diagram of the equipotential lines and of the electron trajectories in the field of a folded array such as that shown in Figure 17, and illustrates the principle of color control.

Figure 19 is a developed view of the folded conductor array and associated plate-like electrodes employed in the embodiment of Figures 12-17. v

Figure 20 is an enlarged cross section of one type of color screen according to the invention, and Figures 21-23 illustrate three phases in the manufacture thereof.

Figures 24 and 25 are similar sections of two other types of color screen, while Figures 26 and 27 illustrate two stages in the manufacture of a thirdtype.

Figures 28-30 illustrate two stages in the manufacture of one type of scanning array embodying the invention.

Figures 31-34 are three sections and one face view of a preferred type of color screen. a

Figure 35 is a series of waveform diagrams illustratin the operation of the scanning valve of Figures 6-10, while Figure 36 is a diagram of a circuit which may be used for operating-said valve.

Figure 37 is a developed view of the conductor array and associated plate-like electrodes of the modified form of tube illustrated in Figure 38.

Figure 38 is a horizontal cross section of the scanning a valve, array, screen, and associated electrodes of a modified form of tube embodying the invention.

Figure 39 is a fragmentary perspective view of the electron gun of the modified scanning valve of Figure 38.

Figures 40 and 41 are enlarged cross sections of the modified scanning valve of Figure 38 illustrating two phases of the operation of this organ.

Figure 42 is a schematic view of the modified scanning valve of Figures 38-41, partly opened out and showing the electron trajectories in the scanning zone.

In Figure 1 the vacuum envelope 1, which has approximately the shape of a hand mirror, consists of glass on the side from which the screen 2 is viewed, while the other parts may bemade of glass or metal. The fluorescent screen 2 is in the form of a phosphor coating on a sheet of suitable material'such as glass, or glass cloth suitably tensioned, and may be backed with the now widely used metallic layer. Facing the screen, and at a relatively small distance from it, is arranged the scanning or rear array 3, details of which are shown in Figures 4 and 11. The array is backed, at least in part, by the metal plate 4, which may be used as support for the rear array but is insulated therefrom and constitutes the common capacitive backing of the array. The scanning array is folded round the backing plate 4, and into a loop 6 which forms the outer electrode of the scanning valve, whose grid 7 and collecting electrode 8 are also shown, and which will be more fully described later.

The electron beam E is formed by the electron gun 9, which may beof conventional design, and is not shown in detail. In the drawings the electron beam E is shown as if it were astigmatic; focused at the point P in the plane of Figure 1, while in the plane of Figure .2 it is shown collimated by the gun, i. e. focusedat infinity. In this plane the beam is brought to a focusonly by the local deflecting field. This representation has been chosen chiefly for simplicity and clarity. Though an astigmatic gun may be advantageous, 'it is not essential for the invention. It will be shown later that the focus of the beam in the plane of Figure 2, i'. e. at right angles to the screen, is not at all critical.

The line scan, that is to say the deflection parallel to the fluorescent screen, in the plane of Figure 1, is effected by magnetic deflection coils 10, 10.

Two pairs of small deflector electrodes 11 and 12 are provided which operate in opposition to one another so as to displace the beam parallel to itself. For a tube operating only in black and white these electrodes are given fixed potentials adjusted to direct the electron beam exactly into the narrow gap between the screen 2 and the array 3, thus correcting for small inaccuracies in the mounting of the electron gun relative to said gap. In color tubes they have the additional function of color control, to be explained later.

As the beam has to proceed for an appreciable distance in the relatively narrow space between screen 2 and array 3, it is essential to eliminate stray magnetic field components at right angles to the plane of Figure 2. Magnetic screening can be effected, as usual, by a screen outside the tube, or preferably by making the backing plate 4 of material of high magnetic permeability.

Figure 4 is a cross section of a portion of the fluorescent screen 2 and of the scanning array 3 on a larger scale. Screen 2 with its fluorescent coating 17 is covered by a thin layer 18 of aluminum or the like, and is maintained at the maximum positive potential of the tube, hereinafter designated Vm, as well known in the art. The scanning array 3 of this embodiment is formed by a row of metal wires of round cross section, partly embedded in an insulating plate 19, which in turn is backed by the metal plate or layer 4, forming the common capacitive backing of the array. The deflection of the end portion of the electron beam E towards the screen 2 at different vertical levels to provide the frame scan is achieved, as above described, by setting up a potential distribution between the wires of the electrode array 3 in a small vertical zone, and causing this zone to move progressively across the array at the desired frame scanning speed. The effect is as of a wave of discharge running down the array.

Figure 5 is a diagram of the potential distribution in the region of this wave, potential being plotted horizontally against vertical distance 2 (transverse to the array wires). Strictly this is a stepped function, but if the array wires are sufficiently fine and closely packed it is admissible to consider V as a smooth function of z at all but extremely small distances from the plane of the array. Though in principle the wave could run in either direction, it is preferable to make it run against the beam, with a speed 11. The reasons for this preference are that, first, the beam has to traverse only a completely field-free region, not affected by the charge-residues of those parts of the picture which have been already drawn, and, secend, it is more convenient to operate the scanning valve in such a way that it discharges the array during the scan and re-charges it to a positive datum level during the flyback. Assuming then that the electrons travel upwards in Figure 4, the wave V travels downwards with a speed 11, which is the speed of the frame scan. Below the wavezone the potential is the positive maximum Vm; above it the potential is at or near the potential of the electron gun cathode, which has been denoted by O in Figure 5.

The charge and discharge of the conductors of the rear or scanning array 3 is achieved by means of the organ which has been referred to above as the scanning valve. This organ includes a further array of conductors formed as a continuation of the scanning array, bent round into a loop 6 as shown in Figure 3. Associated with this array are a cathode, a grid electrode, a. bucket the arrangement of which is best shown in Figures 6, 7,.

8, 9 and 10.

Figure 6 is a perspective view of a portion of the array of conductors constituting the outer electrode 6 of the scanning valve, together with parts of the electrodes associated therewith. For simplicity the conductors are shown without any insulating backing or fibres or other supporting means. The conductors 3 are arranged on a cylindrical surface, with a base-line approximately in the form of a letter U, with a constriction at the opening. At the near side of the loop as seen in Figure 6 the conductors, which come from the scanning array, run horizontally to the vertical line I, where they turn upwards. At the line I they turn horizontal again, and run level to the line K, where they again turn upwards. After running horizontally between L and M, the conductors dip until at the terminating line N they reach a level below that at which they started. On the near side the scanning valve array is terminated or bounded at the top by an electrode 20, permanently at or near the potential of the cathode of electron gun 9, which also extends over the conductors of the scanning array. A similar electrode (not shown) may be provided at the bottom of the array on the far side of the structure as seen in Figure 6, and permanently maintained at the maximum positive voltage Vm. The array of conductors and electrode 20 are also shown in Figure 11 developed into a fiat pattern which will produce the configuration of Figure 6 when bent into the cross-sectional form shown in Figures 8-10.

In lieu of using metal wires partly embedded in an insulating plate as represented in Figure 4, the conductor array may be formed by printing conducting lines on a flexible insulator, such as mica, which can be bent into the configuration of Figure 6, or by cementing insulating rods to a fabric of the character illustrated in Figure 28 and hereinafter described, displacing the rods longitudinally until the desired shape is obtained, and then destroying the unwanted fibres of the fabric.

Figure 7 is avertical section through the scanning valve, at about the lines J, K, showing some of the as-.

sociated electrode structure. The electrode 7, referred to as the grid, is a long box-like electrode with a slotted partition 25. Behind the partition 25 is the long hotcathode 23, with heater 24. Full horizontal sections through the valve are shown in Figures 8-10. These figures show, in addition to the grid 7 and bucket electrode 8, already shown in Figure 3, the rods 26, 26 whichv serve as collectors for secondary electrons liberated in the flyback phase of operation of the valve, and schematically illustrate the following explanation of the operation of the valve. t

The operation of the scanning valve is required to do two things. The first is to charge the conductors 3 of the scanning array to the maximum H. T. voltage, Vm. This charging operation takes place during the interval between television frames in a television picture presentation system and will, therefore, be called the flyback phase. The second is to produce the wave of potential variation referred to above, which must be made to sweep across the array in the direction transverse to the conductors, so as to provide a deflecting field for the electron beam serving to turn the end of the beam over towards the screen at a progressively varying level across the screen so that successive line scans are executed one below the other to produce the normal picture raster. This will be called the scanning phase.

To start with, consider the conductors of the scanning valve array (and therefore, of course, the conductors of the scanning array) to be fully charged to Vm. Consider the cathode 23 to be heated to operative temperature by its heater 24 and to be held at or near the potential of the picture producing gun cathode, the grid 7 to be held at a lowvoltage just sufficient to cut off any electron flow from the cathode, and the bucket" electrode 8 to be held at a positive potential sufliciently high tocollect electrons from the cathode 23 when they are released by the grid 7. The collector electrodes 26, which are in the form of rods extending the full length of the scanning valve, are employed during the flyback phase for the purpose of collecting secondary electrons emitted from the conductors; during the scanning phase, they may be at or about the potential of the cathode of th scanning valve.

If now the potential of the grid 7 is raised a beam of electrons will be released from cathode 23 in the form of a ribbon or sheet electron beam which will pass straight through between the two sides of the scanning valve array (all at Vm) and will be collected by the bucket electrode 8, as illustrated in Figure 8. This ribbon beam of electrons extends over the full length of the scanning valve. However, at the top end of the array, on the near side as seen in Figure 6, the electrode lies opposite the top conductors of the array on the far side between i L and M, and is maintained permanently at or about the potential of the cathode of the picture producing electron gun 9. There will, therefore, exist at the top of the scanning valve a transverse electric field between the electrode 20 and the top conductors of the array which are at Vm. This transverse field will serve to deflect the top portion of the electron stream towards the far side of the array as seen in Figure 6, as illustrated in Figure 9, providing a current flow operating to discharge these top conductors, lowering their potential towards cathode potential. Since, however, by virtue of the vertical stagger introduced into the conductors between the two sides of the scanning valve array, the parts of the top conductors on the near side are opposite portions of lower conductors on the far side, a transverse field such as previously existed between electrode 20 and the top conductors will now exist between the top conductors on the near side and the opposite conductors on the far side, which field will serve to deflect a lower portion of the electron stream towards the. lower conductors on the far side, thus discharging them. Clearly, this effect is progressive, and the electron flow from cathode 23 towards bucket 8 will be deflected towards the conductors of the array at progressively lower levels down the array.

If the electron flow to the conductors is suflicient they will, of course, be discharged to cathode potential so that both' sides of the array will assume cathode potential, though it is not absolutely necessary that complete discharge should be achieved. The electron stream at portions of the array where this condition is reached will tend to be cut off, but any residual electron flow will find itself in a region free from transverse field and will therefore flow to the bucket electrode 8.

At any given time during a frame scan, therefore, the electron discharge from cathode 23 can be divided into.

three zones, namely, the upper zone where the electron stream is cut off, or passes between at least partially discharged conductors to the bucket electrode 8 (conditions as in Figure 8), a scanning zone in which the electron fiow from cathode 23 is deflected to the conductors of the scanning valve array (conditions as in Figure 9), and a lower zone in which the electron stream passes between fully charged conductors straight through to the bucket electrode 8 (conditions as in Figure 8).

As described above, the scanning zone moves progressively downwards, and clearly the speed at which it does so will depend upon the intensity of the electron stream and the capacity to earth to the scanning array which will determine the rate at which the potential of the array is reduced for any given level of electron flow. It will be appreciated that the potential of any given conductor in the scanning zone vwill depend upon the time for which electrons have been flowing to it, so that the conductors near- I est the lower undischarged zone will be nearly at Vm ajcertain state; in other words, there is instability. is also confirmed by a more accurate mathematical analand there will be a gradation of potential over the scanning zone, the potential of successive conductors falling off in the direction towards the discharged upper zone. This vertical potential gradient gives rise to the transverse gradient between the conductors on opposite sides of the scanning valve array by virtue of the stagger of the conductors above described. The conditions affecting the wave shape of the potential gradient in the scanning zone will be discussed below.

When the discharge wave reaches the bottom of the scanning valve array so that the whole array is discharged, the flyback phase is initiated. For the flyback phase (illustrated in Figure 10), the voltages on the cathode 23 and grid 7, 25 are lowered to about 600 v. and the voltage on the bucket electrode 8 is dropped a little more to, say, -650 v. so that it will be negative to the cathode 23 and will no longer collect electrons. The effect is that the electrons emitted from the cathode structure are no longer collimated as shown in Figure 8, but become. over-focused as indicated in Figure 10, so that they diverge and are directed towards the array of conductors on both sides of the valve. Moreover, by virtue of the increased voltage between the cathode and the array, the velocity of the electrons is raised to such an extent that secondary electron emission takes place from the conductors at a secondary emission ratio greater than 1. For this purpose the conductors may be coated with any suitable material which increases the secondary electron yield, such as a phosphor or magnesium oxide smoke. The secondary electrons emitted are collected by the rods 26, as shown in Figure 10, so that the array tends to charge itself and stabilize its potential to that of the collectors 26. The potentials of electrodes 23, 7, 25 and 26 are now raised together while maintaining between them the potential differences which provide the conditions just described. The potential of the array thus follows the potential of collectors 26. When collectors 26 reach Vm, therefore, the charging of the array is complete, and the potentials of the cathode, grid and collector electrodes can all be restored to those required for the scanning phase which then starts immediately, as above explained.

Operation of the scanning valve is required to be synchronised with television signals supplied from a television receiver. These signals arrive at the end of the frame scan, before the flyback, and may be employed to lockthe oscillations of a multivibrator controlling the generation of the waveforms applied to the scanning valve electrodes to the incoming synchronising signals. This will be more clearly understood later, when a circuit for controlling the operation of the scanning valve is described.

The above description deals with the mechanism of the scanning valve and explains how the scanning wave is propagated down the array and the array subsequently re-charged. It is now necessary to consider how the vertical potential gradient may be caused to correspond at least approximately to the desired wave shape discussed above with reference to Figure 5.

As has been explained, the transverse potential gradient is set up by the vertical stagger between the parts of the conductors on either side of the scanning valve array. The part of the electron current collected by the conductors of the array is therefore roughly proportional to the vertical potential gradient. The condition for the propagation of an undistorted wave along a line uniformly loaded with capacity is that the discharging current must be proportional to the longitudinal voltage gradient. Thus, if according to the above elementary explanation the total electron beam current were con stant, and a fraction proportional to the gradient were collected, any wave shape could be propagated, which indicates that there is no tendency of the wave to assume This ysis. But this analysis also shows that stability can be obtained if two conditions are satisfied. One is that the total beam current must increase with more than the first power of the potential at the entrance of the grid 7, or that the fraction of the ribbon beam current collected by the array increases more than linearly with the deflecting gradient. This condition is automatically satisfied, because in the case of a space-charge limited strip cathode the current increases initially with the /2 power of said potential. The second condition of stability is that the electrons must be collected at a potential somewhat below the entrance potential, i. e. below the potentials at the points I, K in Figure 6. This makes it necessary to dip the conductors at N below the level at K. -A certain addition must be made to this dip, because the electron tra jectories will not lie in a horizontal cross section, but will bedeflected downwards by the vertical potential gradient. With a suitable value of dip it can be shown that not only will the wave shape be stable, but it will approach very closely the optimum form obtained from electron-optical considerations. V

A further small addition to the dip of the conductors between K and N may be necessary in order to counteract the effects of secondary emission by the conductors. This has the effect that the current appears to be collected at a higher potential because the secondary electrons will be drawn towards the next conductor, but the effect is small, as the secondary yield of most pure metals falls to small values between 1 and 2 kev. electron energy (the second crossover point), while the favored operating voltages Vm are of the order 5-15 kv.

Instead of a slit-shaped aperture in the partition 25, a series of round apertures may be used with advantage, as this increases the mechanical stability of the grid electrode, and also because the emission in this type of electron gun increases at least initially with the 7/2 power of the impressed electric field.

It has been stated above that the speed of the frame scan is determined by the capacity of the array to earth and by the scanning valve current. More exactly the duration of the frame scan is the time in which said capacity, charged up to the potential Vm, is discharged by the electron current collected by the array. The capacity of the array to earth is mainly its capacity to the backing plate 4. It is preferable to make this capacity so large that the current required to discharge it is a multiple, say 10-20 times or more, of the mean electron beam current which produces the television picture, at least in the type of tube illustrated in Figures 1219 wherein the array extends over the screen. The reason for this is that, in this type of tube, the picture beam current also assists in discharging the array since the screen, the scanning array and the scanning valve array are all connected together, the effect of the picture beam being variable with picture brightness. picture beam current is made less than about 5% of the scanning valve current the variable stretching of the picture in the frame scan direction becomes unnoticeable. The reason is that, though a change in the frame height of the order of 5% would be noticeable if it appeared suddenly and by itself, such a large change, corresponding to the doubling of the mean brightness of the picture or to total blackout, must be necessarily connected with a sudden change of the scene or of a large part of it. However, two consecutive interlaced frames very rarely differ in more than a small fraction of their average brightness.

Even these small effects can be eliminated by the following method. The current of the scanning valve is made more than sufiicient to achieve the required speed of frame scan, so that it need flow only during a fraction of the time of a frame sweep. A negative bias is then applied to the valve grid 7, say once during every line scan, thus arresting the frame scan for a suitable period. This period can be made a function of the If, however, the l picture beam current, so that the scanning speed becomes independent of the picture brightness. This may be done by integrating,'by means well known in the art, the beam current over a line period, and blacking out the scanning valve for a time proportional to the integrated sum, for instance by adding a voltage proportional to the integrated sum as a negative bias to a negative sawtooth voltage having the period of the line scan. It is preferable to perform this arresting operation during the line flyback.

In the type of tube as illustrated in Figures 1-4, in which the screen is an equipotential screen, not connected with the scanning array, but directly supplied from a high voltage source, there is no need for the discharge current in the scanning valve to exceed the beam current. Consequently, the array capacity can be kept much smaller than in a tube of the other type, and the total power constunption of the scanning valve can be much reduced.

In the embodiment of the invention shown in Figures 12-19, the vacuum envelope 1 has the shape of a flat box, approximately square, little larger than the television picture. This great saving in space is made possible by the introduction of an electron lens or mirror of novel construction, shown in detail in Figure 16. In the form illustrated, the lens comprises two cylindrical electrodes 13, 13 placed one on each side of a third electrode 14, also cylindrical, located in the symmetry plane SS. All three of these electrodes are connected to a positive potential, preferably the highest positive potential in the tube Vm. Associated with electrodes 13, 13 and 14 is a hollow cylindrical electrode or trough 15, which is positioned symmetrically beneath the other electrodes and is kept at or near the potential of the cathode of the picture beam electron gun 9. These electrodes are so designed that the electric field between them brings a parallel electron beam to a focus in the symmetry plane, and the emerging beam is again collimated.

The system is best regarded, therefore, as an electron lens system with a curved optical axis.

Figure 16 shows the equipotential lines, numbered in terms of V/Vm, where Vm is the potential applied to electrodes 13, 13 and 14, and the potential of electrode 15 is assumed to be zero, i. e. the potential of the gun cathode. The figure shows also three electron trajectories. It is evident by inspection that if the incoming, collimated beam is focused in the symmetry plane, the outgoing beam will be again collimated and parallel to the original direction. A more accurate analysis reveals that in order to obtain this result it is not necessary to focus a wide beam exactly in one point of the symmetry plane. It is found that it is sufiicient if the focus of that thin bundle of trajectories which meets the symmetry plane at right angles is focused in the said plane. The central trajectory of this thin bundle is the optical axis. It is found that trajectories which enter the lens at some larger distance from the optical axis will meet the symmetry plane a little above its intersection with the optical axis, and at an incidence angle, relative to the normal, which. may be positive or negative. In first approximation the small height difference will be a quadratic function of the incidence angle. This, however, means that in this region a ray which meets the symmetry plane at a height h, at incidence angle +a, will continue at the other side as the ray which has the incidence angle -11 and will also emerge parallel to the symmetry plane. In other words, the present system, by its symmetry properties, has no second order error and therefore retains its property illustrated in Figure 16 for quite wide beams.

Another valuable property is that a beam which is not collimated will emerge from the system with the same angle of convergence or divergence as that with which it entered. This follows from the factthat the system as shown in Figure 16 is what is known in optics as a telescopic system. Finally, thanks to the near optimum (see Figures 12-15) at some angle a relative to the plane of the paper, will behave as if they had only the energy Vm cos or in that plane. They will be overfocuscd and will emerge as somewhat diverging bundle. This effect can be completely avoided if a voltage Vm sin a is ap plied to the electrode 15, so that the voltage drop in the lens is Vm(1--Sil'l a)=Vm cos a. This correcting voltage may be obtained, by means well known in themselves, by squaring the voltage applied between the line deflectors 16, 16. 1

An alternative method of correction is to depart slightly from exact cylindricality of' the electrode system, preferably by bending the central electrode 14 somewhat, so that its distance from 15 is a little larger at the ends than in the middle of the mirror system.

In this embodiment of the invention the picture beam electron gun 9, the line scanning, and, if provided, the trimming and color controlling electrode systems are arranged at the back of the scanning array 3 and the backing plate 4, being so positioned as to direct the beam downwardly into the lens 13, 14, 15. I

The line scan in this case is effected by a pair of. electrostatic deflecting plates 16, 16. It is known that electrostatic deflection is rather unsuitable for Wide scanning angles, but the present design enables the scanning angle to be reduced toa low value; l9 in the example illustrated. As shown in Figures 12 and 14, the effect of the electron lens 13, 14, 15, viewed in the, plane of these drawings, is the same as specular reflection on a line M-M, which is well outside the physical boundaries of the lens. Thus the deflectors 16 must be so adjusted as to scan the mirror image of the fluorescent screen relative to said line M-M, as illustrated in Figure 14. As in the design of Figures l3, trapezium correction must be applied to the scan, and a corresponding correction applied to the focus, at least in the plane of'Figu'res l2 and 14. Such techniques have been employed in the operation of previously known cathode ray tubes in which the screen is inclined relative to the tube axis. On the other hand, no keystone correction need be applied in the present case because the line in which the electron beam meet the screen is substantially predetermined by the deflecting zone, set up by the deflection array, which is straight. A minor correction for line curvature will be discussed later.

Before and behind the deflection zone of electrostatic deflecting plates 16 there are arranged two pairs of deflecting electrodes 11 and 12 which operate in opposition, so that the beam is displaced parallel ,to itself, in order to effect the color control, as will be explained in detail in connection with Figure 18.

The frame scan in the tube of Figures 12-19 is achieved by means of the scanning valve 6, 7, 8, etc., just as in the previously described embodiment 'of Figures 1 11.

This type of tube is eminently suitable for manufacture, because the fluorescent screen, the scanning array, the backing plate, the gun and deflector system and the electron lens or mirror may be fixed relative to one another by suitable beams, struts and spacers, well known in the art of electron tube manufacture, so as to form 'one composite structure. The envelope 1 is preferably made of two dish-shaped halves, divided in the symmetry plane, S-S as shown in Figures 13 and 15, and the composite structure above referred to is sealed between them, preferably supported by studs which project between'the two half-dishes, in premoulded grooves, not shown in the drawings. The electrode leads are also preferably sealed in at the joint between the two halves. Thus great accuracy of manufacture can be achieved, because the internal structure can be completely assembled in jigs,

and the relative positions of the parts are not affected by the subsequent glass sealing operations. The most critical adjustments are those which affect the parallelism of the beam and the fluorescent screen. These, however, can be made after the tube is completed. in which the beam is scanned for the line scan happens to be tilted around a horizontal axis, so that it runs at an angle instead of parallel to the screen, the bias of the trimming deflectors 11 or 12 may be slightly altered to correct it. If this plane is twisted round a vertical axis, e. g. by imperfect positioning of the deflectors 16, the twist can be compensated by a weak vertical magnetic field, produced by a few turns of a current-carrying wire wound round the tube, say at the level of the electrodes 13. If the electron optics of the arrangement shown in Figure 4 are examined, it is found that unless the distance between the screen and the array is made very small and the gradient of V' very large, the electrons will hit the screen at rather small glancing angles and the angular range in which focusing can be achieved is small. This is acceptable for black-and-white television, but in color television steeper incidence and a wider convergence angle are desirable. This is achieved in the present em bodiment by means of a double or folded array, already mentioned above, and shown in detail in Figure 17. In this example the conductors 3 of the array are in the form of conducting strips printed in conducting paint or otherwise suitably formed on an insulating foil support 19 which is preferably made thin enough to be flexible, so that it can be folded over in a loop 5, as shown in Figures 12, 14 and 15, to form the front and rear arrays, and is made in one piece with a further portion 6 adapted to form the scanning valve, array. The complete foil and conductor array supported thereby are shown in the flat inFigure-l9 and have to' be folded round the lines A--A and A'- in order to form the folded scanning array, and round the lines J] and K--K to form the scanning valve array. The part of the foil to the left of the line AA forms the fluorescent screen. The luminescent powder 17 (Figure 17) can be applied between the conductors, but it makes little difference if the conductors themselves are also covered by it.

Although the scanning array could be made rather coarse. without adversely affecting its beam deflecting function, it is essential that, in a double or folded array, the array on the screen side be made'so fine that it cannot be resolved by the eye from a normal viewing distance. In principle it is possible to connect the conductors at the two sides of the gap in a folded array in groups, rather than one-to-one, which would permit the use of a coarse array on one side and a fine array on the other. :lowever, it is preferable to avoid such group connections and to use a continuous array of relatively fine conductors on both sides.

. It is an essential part of the invention that the opposing conductors of the folded scanning array be staggered, as shown, by a distance s (Figure 17), the part of the conductor in the front array (on the screen side) being beyond the part in the rear array (on the backing plate side), when viewed intbe direction of the picture producing electron beam. This stagger is best characterised by the angle ,8, asillustratedin Figure 17.

As in the previous embodiment the rear array and part of the scanning valve array are terminated at the top, as shown in Figure 19, by a conducting surface or plate elec trode 26, which is kept permanently at or near the potential of the cathode ofelectrou gun S The part of this electrode in the scanning valve envelope serves for starting the discharge wave, as above described. Correspondingly, at the base of the scanning array and on the screen If the plane' side, the array is terminated by a conducting surface or plate electrode 20' of similar shape, kept permanently at maximum positive potential "m. This electrode 20 serves to collect the picture beam current when the frame scan has reached the end of its run down, if the beam is not immediately blacked out.

The operational principle of the folded array is illustrated in Figure 18 which shows the electric field and the electron trajectories in a plane corresponding to that of Figure 17. The electric field is fully determined by the vertical potential distribution V(z) at one of the sides of the folded array, and by the angle of stagger ,3. At the other side the vertical potential distribution V(z) is repeated, but shifted by a distance s=d tan 5, where d is the width of the gap between the two sides of the array. The field must be such that it focuses into a small spot as wide an electron beam as possible. It has been found that the best fields can be obtained when the angle is within the limits 45 to 55. The example illustrated in Figure 18 corresponds to 6:50. In addition the function V(z) must also be suitably determined. A favorable field is illustrated in Figure 18 in terms of its equipotential lines which are numbered in fractions of the maximum positive voltage Vm as V/ Vm=0, 0.1 0.9, 1. The maximum gradient dV dz must be approximately equal to Vm/d. Moreover, in order to obtain the best result, the Wave must not be symmetrical with respect to the mean voltage Vm/ 2, but must tail off more gently at its low-potential end. In other words, the inflection point must not be at Vm/Z, but above it. If these conditions are satisfied the exact shape of the V(z) curve is not very critical. In particular it does not matter much whether V descends exactly to zero or only to a level somewhat above the gun potential, because the picture beam does not penetrate into the top region of the deflecting field.

As illustrated in Figure 18, with a suitable field it is possible to focus a beam somewhat wider than (2/ 2 into a zone of only d/ 20 width, with incidence angles ranging from 5 to 50 to the normal. The focus is at about V/Vm=0.7, which means that the electrons arrive at the screen with 70% of the energy which they would have obtained at Vm, the maximum voltage available in the tube. The ratio V/Vm can be called the voltage efficiency. By reducing the voltage efficiency somewhat, even better focus and more nearly normal incidence can be obtained. With B=45 the spot can be reduced to d/50, with incidence angles from 5% to 45 at the cost of a drop in voltage efficiency from 70% to 58%. On the other hand with ,8=53 the voltage efiiciency can be raised to 80%, at the cost of a slightly less perfect focus. The efficiency off the central line of the screen is even better, because the electrons are deflected somewhat below the zone as shown in Figure 18, and reaches practically 100% at the edges.

A voltage efiiciency less than 100% means that at a given voltage Vm and given electron current the screen brightness is somewhat less than in conventional tubes. But in the new tube it is possible to increase the electron current far more than is necessary to outweigh this loss. As is well known, the limiting feature in cathode ray tubes is the maximum current density in the spot, which in turn is given by the current density at the gun cathode, by the screen potential, and by the convergence angle at the screen. With a given type of cathode the current density is proportional to the produce of the screen potential of the two convergence angles in the beam, at right angles to one another. In the new tube the convergence angle in the plane at right angles to the plane of Figure 18 can be considered as the same as in conventional tubes, but in the plane of the drawing it can be made at least times larger at its ends where it is focused and bent over by the scanning wave. This means thatfor a given Vm the beam current can be increased in such a proportion as to obtain a screen brightness several times that in ordinary tubes, even if the voltage efficiency is less than 50%. The beam current can be increased without introducing difliculties in respect of cathode loading by making the cathode aperture non-circular, e. g. by making it in the form of a slot which is narrow in the direction parallel to the screen and long in the direction at right angles thereto. The beam then becomes a ribbon beam which is converged onto the screen into a round spot by the astigmatic focusing effect produced by the scanning array. Thus the cross-sectional area of the beam is increased and hence the current in the beam may be increased without increasing the cathode loading.

This advantage is due to the deflecting field acting as a short-focus electron lens, with a focal length of the order d. An upper limit to d is set by the consideration that the spot width, which is d/ 20 in the example illustrated, must be less than a line width. With 500 lines definition this means that d should be preferably less than of the height of the picture. A lower limit is set by the thick ness of the electron beam, and by the manufacturing accuracy. Q

It can now be explained why it is not necessary to collimate the beam, i. e. make it parallel, in the plane at right angles to .the screen. As shown, the zonal deflecting field acts like a lens with a focal length of perhaps of the screen height. On the other hand the gun lens, even if it focusesvat the near end of the screen, has the effect of a lens of a focal length at least twice the screen height, compared with the adjustment when it focuses at infinity. Thus by the laws of lens optics, a focusing. of the gun even at minimum distance will shift the final focus only by about 1 or 2% of the shorter focal length, of the order a, i. e. by about d/ 50, which is absolutely negligible. Thus it is not necessary to collimate the beam in theplane at right angles to the screen although it is shown so in all the drawings for simplicity of explanation. Indeed it'is even advantageous to apply some convergence to the beam in this plane so that it is focused at or near the spot, as this enables the best utilization to be made of the gap width d.

Figure 18 applies to the case of a beam which is launched at right angles to the array, i. e. one with zero deflection in the line direction. If an electron is deflected in the line direction by an angle cc, its energy in the plane of the drawing will be only Vm 608 a; hence it will be deflected a little before the electron launched at (1:0. The effect is that though these sideways deflected beams will still be well focused, they will meet the screen a little earlier, that is to say, a little lower. Taking the example of Figures 12-15, in which the maximum value of a is 19, the smallest value of (205 a is 0.895, and it is found that the focus for these maximally deflected electrons is about d/ 10 below that shown in Figure 18. As the line width is about 11/20, this means that in the two corners at the bottom of Figure 12 the scanning line curves down by about two line widths. At the top the effect is less than one line. These are negligible distortions, but if desired they can be compensated by curving the conductors in the folded array, or in the scanning array only, upwards by the same amount. I

Figure 18 illustrates also how the angle of incidence of the beam on the screen may be controlled so that the tube may be employed for color television according to the invention. The trajectories are grouped so as to indicate three thinner beams, labelled R, G and B, which may stand e. g. for red, green and blue, separated by gaps; At the screen their angles to the screen normal are about 45, 25 and 10", respectively. Thus it is possible to vary the incidence angle within rather wide limits by displacing the beam sideways. As explained above, it is not necessary to displace the whole beam parallel to' itself; it is sufiicient to displace the point at which it meets the local deflection zone. The-displace- 15 meat from one group to the next in Figure 18 is d/S which in the previous example corresponds to line widths. Thus no excessive accuracy is required. The displacement'of the beam for this purpose is effected by means of the two pairs of deflector plates 11 and 12 of Figures 13 and 14, placed one before and one beyond the line deflector plates 16 inthe direction of the picture beam. It is evident that only one ofthe two deflector pairs 11, 12 need be used for the color control, though it is preferred to have two in order to compensate errors of manufacture, as above described, and in order to operate always with the same color changing signal, irre spective of the position of the scanning line.

Instead of employing a single beam and displacing it sideways for the purpose of color control, there are other possibilities. For example, three separate electron guns could be provided each modulated for its appropriate color and each providing a beam in the appropriate location. Alternatively, a common cathode could be employed having three apertures providing one beam for each color, the individual beams being appropriately modulated by separate grid electrodes for the three colors.

The translation of incidence angle into color will now be explained in connection with Figures 20-27, which show various types of screen which may be used for this purpose.

In the screen shown in Figure 20, asymmetrical grooves 27 are produced in a glass plate 2 which constitutes the screen. The ridges between the grooves are topped by the metallic conductors 3 constituting the front scanning array. The inside of each groove is coated with strips 28, 29 and 30 of diiferent fluorescent powders, corresponding e. g. in turn to red, green and blue. Alternatively these strips could be of three different pigments, acting as color filters, coated with whitefluorescent powder. These powders of pigments may be applied by spraying, at the same angles as the incidence angles of the electron beam in operation. The shape of the grooves shown in Figure 20 favors accurate spraying, because their air resistance is great, and a stagnant air cushion is formed, but spraying in vacuo or at reduced pressure is preferable. Pigment stripscan also be formed photographically, as known in the art. The tops of the ridges may be metallicly coated in any suitable manner, e. g. by means of an elastic roller with a conducting paint, and thickened by galvanoplastic deposition or metal spraying.

Figures 2l-23 illustrate three stages in the manufacture of the grooves by etching. Strips 31, 31 of a suitable resist are first applied, by printing or by a photomechanical method, to those portions of the glass screen plate which ultimately become the tops of the ridges between grooves. Figure 21 shows the first stage of etching, in which a symmetrical, approximately circular-cylindrical groove is formed. In order to produce the required asymmetry, more resist 32 is then sprayed on the righthand side of each groove, as shown in Figure 22, and the resist is melted down a little at both sides, to avoid undercutting. The etch now starts asymmetrically, as shown in Figure 23, and will continue asymmetrically until the shape 27 of Figure 20 is reached. This design has the disadvantage that the color strips 28, 29, 30 face the screen at dilferent angles, thus producing a color effect dependent on the direction of viewing. This may be reduced by making the surface layer of the screen 2 of diffusing opal glass.

Figure 24 is an improved version of the above-described type of screen, in which the color strips 28, 29, 30 all face the screen surface in parallelism therewith. This is achieved by making the surface 33 of the screen plate of a more easily soluble glass than the base 2. The etching will now continue up to a certain point as shown in Figures 21-23, but will stop in the plane of the relatively insoluble base 2.

Figure 25 represents a further improvement, wherein 16 a the base surface is fitted with ridges 34 which fix the shape of the etched grooves more accurately, and reduce the danger of undercutting the projections.

Figures 26 and 27 show a further variety of screen, whose manufacture starts with the same type of base as shown in Figure 25. The aperturing projections, however, are made of metal, in two steps. After coating the ridges 34 with a conducting paint, e. g. by means of an elastic roller, projections 35 are formed by galvanoplastic deposition in an electrolyte of small throwing power. As shown in Figure 27, these projections are subsequently sprayed with metal, in an inclined direction, forming the projections 36.

It has been stated above that the front and rear scanning arrays and the scanning valve array may be produced all in one, as shown in Figure 19, in the form of an insulating foil supporting the conducting strips constituting the arrays. There are several alternative methods of producing the arrays, and some examples which involve the production of a composite fabric of metallic fibres and insulating fibres will now be described.

Figures. 28-30 illustrate one methodwhich may be employed. The starting point in the making of this array is 'a fabric whose warp is metal while its weft consists at least partly of destructible insulating fibres, or vice versa. Figure 28 illustrates an example of such a fabric. In this fabric the weft is of metal tape 3, while the warp consists partly of glass tape or fibres 21 and partly of fibres 22 of a material, such as cotton, nitrocellulose, rayon or the like, which is destructible by means which leave the metal and the glass fibres intact; i. e., cotton by burning, nitrocellulose by explosive disintegration, and rayon, sodium algina'te or the like by dissolving in suitable solvents. The destructible fibres serve only for accurate fixing of the relative positions of the non-destructible warp and weft fibres, 'and are preferably destroyed after the array has been arranged in its final position. The remaining fabric, after destruction of the unwanted fibres, is shown in section in Figure 29 and in face view in Figure 30. Only enough glass fibres 21 are left for supporting the metal tapes 3 safely, without covering much of their surface. This fabric can be freely supported in a frame, or it can be cemented onto an insulating surface, for instance, by means of enamel glass or glass solder.

If the array is ultimately backed by an insulator, it is possible to destroy the non-metallic warp altogether. In this case it is preferable to use two sorts of Warp fibres, which are destructible by different agencies. As an example, part of the fibres may consist of cellulose acetate, and part of cotton. The fabric is fixed in its ultimate shape in a frame, and the cellulose acetate fibres are dissolved in a solvent which does not affect the cotton. Now the frame is pressed against the insulating backing, coated with a suitable cement such as sodium silicate. Finally the cotton is destroyed by some oxidizing agency, leaving only the metal tapes adhering to the insulating base.

It is an advantage of this type of array that the fabric can easily be twisted and deformed into the forms required by the invention. For instance, if it is intended to produce the structure shown in Figure 17, the glass plate 2 and the insulator plate 19 are laid down, displaced relative to one another by a distance s, with a suitable gap between them. The fabric is now laid straight across the two plates, and cemented to them. When the plates are brought into line and face to face, the conductors are brought into the shape shown in Figure 17. It may be advantageous to use a sufiicient number of non-destructible, glass fibres in the loop 5 (Figures 12, 14 and 15) to provide adequate support for the conductors at this point, while in those parts of the fabric which are ultimately in contact with solid insulating plates all the transverse fibres can be made of destructible material.

The required configuration of the array as a Whole can 17 be realized by taking a fabric, such as has just been described, and cementing insulating rods to it at the lines corresponding to I, I, K, L, M and N of Figure 19. These rods are then displaced longitudinally, so that the desired shape is obtained.

A further method of forming the arrays of conductors for the scanning arrays and the scanning valve consists offirst laying down on a suitable insulating support, such as mica, glass cloth, or silicone varnished glass cloth, by one of the known printing techniques, the required pattern of conductors, for example, the pattern shown in Figure 19. The sheet so printed is then folded into the required shape in which it is held on suitable supports. Alternatively,

such a printed sheet may be produced on a support of 1 material capable of being dissolved away or otherwise removed. Supporting fibres, of glass yarn for example, extending transversely to the conductors are then attached to this sheet in such a way as to hold the conductors in their relative positions when the supporting material is removed. For example, the printed sheet may be crimped onto glass yarn fibres at suitable small intervals. A machine for performing this operation is described in co-pending application Serial No. 480,407, filed January 7, 1955. The supporting material may then be dissolved away, leaving a composite fabric consisting of the metal conductors crimped onto the glass yarn fibres. terial is then mounted in the desired folded configuration on suitable supports.

The use of a fabric made by one of the above methods for the scanning arrays provides a further possibility for the production of a color screen for the tube. The color screens described above with reference to Figures 2027- have the disadvantage that their conductors 3 have to be connected electrically, either singly or in groups, with the corresponding conductors of the rear scanning array.

34 is free from this disadvantage. In this screen the conductors 3 of the front array, which are mounted on the screen, are continuous with those in the rear scanning array and in the scanning valve array. A fabric produced as above described with reference to Figures cemented to the ridges by means of a suitable adhesive, I

such as sodium silicate. The unwanted warp fibres are now destroyed, preferably while the fabric is still under pressure, so that the metal tapes 3 are made to adhere to ridges 37. This may be done e. g. by pressing through blotting paper, soaked with a solvent. Finally the remaining insoluble warp fibres may be also eliminated, by one of the means previously discussed, though a small number can remain without harm. Figures 31 and 32 are two sections at right angles to one another of the resulting screen, while Figure 34 is a top view of the same. Alternatively, all warp fibres can be made destructible in that part of the fabric which covers the screen, so that finally only the metal tapes are left after the first operation. Figure 33 is a section of such a screen, containing only equally spaced metal tapes This ma grosses '35 The type of color screen illustrated in Figures 3l-' ing the preparation ofthe screen, that part of the fabric which is to form the scanning array and the scanning valve array can be rolled up in a tight roll. Conversely, the screen may be prepared last, if preferred.

The tube according to the invention, with the color screens as described, is suitable for operation with any of the known systems of color television transmission. In a point-sequential system the frame scan is operated in the same way as in black-and-white television. This applies also to simultaneous systems, with the difference that the single electron gun, as illustrated in the previous example, must be replaced by three guns, one for each color. In line-sequential systems the operation is somewhat different, because here the line must be arrested while it is scanned in succession in three differe'nt colors. Thus the frame scan is best operated once in every three line flybacks.

Tubes embodying the present invention can be operated as television picture tubes to a large extent from conventional circuits commonly employed in television receivers. Thus the line scan, which is controlled by the deflector plates 16 in the embodiment of Figures 12-19, may be produced by a simple voltage sawtooth waveform, although some departure from the simple sawtooth may be introduced in known manner to correct trapezium distortion as has been mentioned above. The picture beam intensity modulation can be effected by grid or cathode modulation applied to the picture beam electron gun in the conventional manner.

For the frame scan, however, the part of the tube described above and called the scanning valve must be fed with waveforms not normally employed in conventional television picture tube operation. The operation of the scanning valve has already been described in detail, and it will be remembered that during the scanning phase, the scanning array is progressively discharged by the ribbon electron beam, a part of which is deflected towards the scanning valve portion of the array at progressively lower levels. The operation is entirely automatic, and the voltages applied to the various scanning valve electrodes during this phase may be constant. For the fiyback, however, it is necessary to recharge the scanning array, and this is effected by causing the electron beam in the scanning valve to set up secondary emission from the conductors constituting the scanning valve array so as to charge the array positively. The first stage in the flyback phase, therefore, consists in setting up between the electrodes of the scanning valve the relative voltage differences required to produce secondary emission from the conductors at a ratio of secondary to primary electrons greater than unity, i. e. 1:1. These conditions involve:

1. Lowering the potential of cathode 23 and grid 7 with respect to the potential of the collector electrodes 26 so as to increase the velocity of the electrons leaving the cathode and defocus the electron stream.

2. Lowering the potential on the bucket electrode 8 so that it will not collect electrons.

Having set up these conditions, the array conductors will stabilize themselves at a potential slightly below that of the collector electrodes 26. This potential is initially well below the maximum H. T. potential Vm to which the array must ultimately be charged. It is now necessary, therefore, to raise the potentials of the cathode, grid, and the collector electrodes of the scanning valve up to Vm while maintaining between them the relative potentials initially set up so that secondary emission from the array will be maintained. When this raising of potential has reached the point where the electrodes 26 are at Vm, the voltage on the array will have been raised to substantially the same voltage, that is nearly to Vm.

When the charging of the array has been completed in this way, the potential on the collector electrodes 26 can be dropped to zero, as can the potential on the cathodeand grid. At the same time, the potential on the bucket electrode 8 can be raised to the voltage at which it operates during the frame scan so that it is ready to collect the electron stream over those parts of the scanning valve where the array is fully charged. The next scanning phase can then start immediately.

The waveforms which these operations represent are shown in Figure 35 of the drawings, which also shows

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