US2982875A - Cathode ray tubes - Google Patents

Description

May 2, 1961 D. GABOR CATHODE RAY TUBES 5 Sheets-Sheet 1 Filed Dec. 29. 1959 Inventor DENNIS GABOR Attorneys y 2, 1961 D. GABOR 2,982,875

' CATHODE RAY TUBES v F'iled Dec. 29, 1959 5 s t 5 g Inventor DENNIS GABOR Y/W Wu Attornays May 2, 1961 D. GABOR 2,982,875

- I CATHODE RAY TUBES F'iled Dec. 29, 1959 5 Sheets-Sheet a I ab 1 a 8 Inventor DENNIS GABOR By W mm W Abt rneys CA'IHODE RAY TUBES .Dennis Gabor, London, England, assignor to National Research Development Corporation, London, England, a British corporation Filed Dec. 29, 1959, Ser. No. 862,586

a I 6 Claims. (Cl. 313-78) This is a continuation-in-part of application Serial No. 592,142, filed on June 18, 1956, now Patent No. 2,926,274 issued February 23, 1960, which application was itself a continuation-in-part of application Serial No. 309,677, filed in September 1952 and now abandoned and application Serial No. 549,712, filed on November 29, 1955, now Patent No. 2,795,729 issued June 11, 1957.

The latter two applications describe and illustrate variaus forms of cathode ray tube having the feature that they can be made flat in shape, that is to say, without any appreciable depth perpendicular to the screen, the conical and tubular neck portions of conventional cathode ray tubes being dispensed with.

In one form of such a cathode ray tube, the structure is housed in an envelope the shape of which may be regarded as resembling that of a hand mirror. In a preferred arrangement, however, a more compact structure is achieved by doubling theel-ectron beam back upon itself by means of an electron 'lens adapted to receive a beam deflectable in one plane and deliver it back in an-. other plane parallel to the first but displaced therefrom.

'The application first above mentioned is concerned with electron lenses adapted to eflect such a reversal of an electron beam.

direction of. the frame scan. It follows that, as the beam moves substantially parallel to the screen and close to it, the 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.

The functional principle of the tube described in that prior application 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 5-15 kv. An array 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.

I If the conductors of the array are discharged, one by One object of the present invention is to provide a cathode ray tube which will be compact and not greatly exceed'in any of its dimensions the dimensions of the picture screen;

Another object of the invention is to provide an improved electron lens in such a cathode ray tube, which lens is adapted to effect a reversal of an electron beam such as discussed above.

A further object of the inventionis to provide in such a cathode ray tube, an electron lens of the kind set out above which serves to reverse and also oollimate an electron beam whereby a fan of rays incident upon the lens in one direction and divergent from a virtual source is reversed into a parallel array of rays, travelling in the reverse direction, in a plane parallel to that of the incident fan of rays but displaced therefrom.

l'n'thescathode'ray tubes described in my prior application SerialNo'. 5'49,7l2,'now U.S. Patent No. 2,795,729, the PlClUI'Q'PIOdIlClHg, electron beam is first deflected in one direction only, parallel to. the screen surface, and then proceeds inra 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, meetingthe screen-at the desired point. This localized electric fieldextends 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 one, progressively across the array in the direction towards the electron gun, the zone willtravel 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. f The discharging of the conductors to effect the frame scan may be achieved in any suitable manner as by a special organ, called the scanning valve, contained in the vacuum space of the tube itself. The scanning valve, which forms no part of the present invention is described and illustrated in my application Serial No. 549,712, filed November 29, 1955, now US. Patent No. 2,795,729. In one form of the device the electron beam is directed into the space between the screen and the array of conductors which perform the forward deflection from an electron gun positioned below the screen, and the whole is enclosed in an envelope which is of a shape referred to above as resembling that of a hand mirror. In the more compact arrangement of the invention which has also been referred to, however, the gun is placed behind the array of conductors and is projected downwardly, and is then turned backupon itself around the bottom edge of the array of conductors so as to enter in an upwards direction the space between the conductors and the screen. The lens has a trough-shaped repeller electrode normally maintained at low potential (e.g. the cathode potential of the electron gun), -a central or spine electrode maintained at high positive potential, and two- 7 side or flanking electrodes also maintained at high positive potential. The electron beam enters between: the spine electrode and one of the flanking electrodes, is bent round the edge of. the spine electrode back upon itself, emerging between the spine and the other flanking electrode. The lens .may be regarded as a cylindrical lens having a U-shaped optical axis.

The invention will be better understood from the fol lowing description given with reference to the accompanyrear view and a plan view, respectively, all partly sectioned and somewhat diagrammatic in character, of one form of cathode ray tube embodying the invention.

Figure 5 is an enlarged vertical section through the electron lens used in the tube illustrated in Figures 1-4.

Figure 6 is a perspective view of a reversing and collimating lens suitable for use in the present invention.

Figure 7 is an illustration of one surface of one electrode of Figure 6, and

Figure 8 illustrates the distribution of conductive areas on the inner surface of the other electrode of the lens of Figure 6 opened out into a plane.

In Figures 1-4 the vacuum envelope 1 consists of glass on the side from which the screen 2 is viewed, while the other parts may be made 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 tcnsioned, 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 array 3, details of which are given in my application Serial No. 549,712, filed November 29, 1955. 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. The scanning valve forms no part of the present invention and need not therefore be described in detail here.

The electron beam E is formed by the electron gun 9, which may be of conventional design, and is not therefore shown in detail.

The gun 9 is placed behind the scanning array 3 and positioned in the vertical plane of symmetry perpendicular to the screen and pointing vertically downwards. At the bottom of the tube the beam from gun 9 is reversed upon itself so as to pass upwards between the array 3 and screen 2, the reversal of the direction of the beam being effected by means of an electron lens formed of elements 13, 14 and 15 to be described in more detail later.

The line scan, that is to say the deflection horizontally across the fluorescent screen as shown in Figure l, is effected by electrostatic deflection plates 16, 16.

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.

It is known that electrostatic deflection is rather unsuitable for wide scanning angles, but in view of the long throw of the beam achieved by bending it back upon itself, the scanning angle can be reduced to a low value; :19 in the example illustrated.

As shown in Figures 1 and 3, 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 MM, 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 MM, as illustrated in Figure 3. Itwill be clear that trapezium correction must be applied to the scan, and a corresponding correction applied to thefocus, at least in the plane of Figures 1 and 3. 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 meets the screen is substantially predetermined by the deflecting zone, set up by the deflection array, which is straight. I

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 mirrormay be fixed relative to one another by suitable beams, strutsand spacers, well known in the art of electron tube manufacture, so as to form one com curacy of manufacture can be achieved, because the internal structure can be completely assembled in jigs, and the relative positions of the parts are not aifected 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. ,Ifthe plane in which the beam is scannedfor 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 ofthe trimming deflectors 11 or 12 may be slightly altered to correct it. If this plane is twistedround a vertical axis, e.g. by imperfect positioning of the deflectors 16, the twist can be compensated by a weak vertical magnetic field, pro

'duced by a few turns of, a current-carrying wire wound round the tube, say at the level of. the electrode 13. The

electron lens or mirror 13, 14, 15 is shown in one form in detail in Figure 5. In the form illustrated inthis figure the lens comprises twocylindrical electrodcs13, 13 hereinafter called flanking electrodes placed one on each side of a third electrode 14. which will be called .the spine,

I also cylindrical, located in the symmetry plane S-S. All

three of these electrodes are connected to a positive potential, preferably the highest positive potential in the tube, V Associatedwith electrodes 13, 13 and 14 is. a hollow cylindrical electrode or trough 15 termed the repeller electrode, 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 5 shows the equipotential lines, numbered in terms of V/V where V 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 thatif the incoming, collimated beam is focused in the symmetryplane, the outgoing beam will be again collimated and parallel to the original direction. A more accurateanalysis reveals that in order to obtain the result it is not necessary to focus a wide beam'exactly in one point of the symmetryplane. It is found that it is suflicient if the focus of that thin bundle of trajectories which meets the symmetry plane at right angles is focussed 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 3 ray which has the incidence angle 2x 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 5 for quite wide beams.

Another valuable property is that a beam which is not collirnated will, emerge from the system with the same angle of convergence or divergence as that with which it entered. This follows from the fact that the system as shown in Figure 5 is what is known in optics as a telescopic system. Finally, thanks to the near optimum properties of the electric field shown in detail in Figure 5, a slight departure from the correct shape of little consequence, and can be corrected by a small DC bias, applied to the electrode 15.

Strictly speaking, a cylindrical system will retain the properties described only for electron trajectories which are nearly parallel to the plane of the drawing in Figure 5. Electrons of an energy corresponding to V which have been deflected by the line deflector system 16, 16 (see Figures 12-15) at some angle or relative to the plane of the paper, will behave as if they had only the energy V cos a in that plane. They will be overfocused and will emerge as a somewhat diverging bundle. This effect can be completely avoided if a voltage V sin or is applied to the electrode 15, so that the voltage drop in the lens is V (lsin O6)=V 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.

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 the above-described arrangement the lens is of an electrostatic type the electrostatic electrodes being few in number and having surfaces profiled so as to set up between them the necessary electrostatic fields to achieve the required deflections of the electron beam by virtue of the separations of these electrodes.

The lens illustrated in Figs. 6-8 of the drawings adopts a different system for the design of such lens and accordmg to this aspect of the invention an electrostatic lens of the kind described, comprises an inner electrode structure and an outer electrode structure embracing said inner electrode structureso as to define between the two structures a lens field region having a U-shaped cross-section of substantially uniform dimensions throughout its length, at least one surface of at least one of said electrode structures being composed of a plurality of conductive strips adapted to be maintained each at a different potential and so contoured as to provide, when suitably energized, the desired potential distribution in the said lens field region to produce the desired lens action. v The principle which this invention adopts, therefore, is

V to simplify the spatial distribution of the electrode sur- The ray bb may be regarded as passing along the of the lens and is merely bent back upon itself so that the emergent part of the ray passes upwardly parallel to its incident part but in a plane spaced from it. The beams aa and cc emerge from the lens parallel to M1 but the initial divergence of these rays from b gives rise to a separation at the emergent aperture of the lens which is very much greater than the separation at the incident face and corresponds to a deflection angle many times that employed to scan the beam in the plane of the incident aperture which is, in the cathode ray tube above referred to, a plane parallel to the plane of the screen. This scan of the beam in this plane corresponds to the line scan required for developing a television-type raster on the screen. It follows that the angular deflection required for a television display in a flat cathode ray tube of the above kind using a reversing lens of this type will be quite small and can be achieved readily by ordinary electrostatic deflection. This elfect is produced by suitable design of the contours of the conductive elements of which the electrode surfaces are made up. By this means the same effect can be produced as is produced in thelens of application Serial No. 592,142 by bending the electrode structure to present a convex profile to the incident rays.

The potential distributions over the various surfaces of the electrodes 17 and 18 are shown in Figures 7 and 8. Figure 7 depicts the distribution of conductive'elements applied to the face of spine electrode 18 which is presented to the emergent side of the lens. On the face of this spine electrode presented to the incident beam there is a single conductive area which is, therefore, of course, of uniform potential. In the drawings the lines depicting the contours of the conductive areas, represent gaps separating these areas sufliciently to provide the necessary insulation between them. The separations between adjacent areas are, however, quite small and cannot therefore be shown as double lines on the scale of the drawing. The separation required depends, of course, upon the potential drop between successive elements. For example, the full potential drop of the lens surface may be part of the order of 10 to 20 kv. so that the individual voltage steps of which 10 are shown in the example, will be of, the order of l to 2 kv. each. At this voltage a separation of about 1 mm. between the conductive elements is sufilcient. a

The various potentials at which the individual conductive elements are maintained have been indicated on the drawing as fractions of the total accelerating voltage to which the electrons are exposed. Thus a potential of l-O corresponds to the maximum potentialjn the electron accelerating system, i.e. the final anode potential, while 0-0 indicates cathode potential.

faces and to arrange the potential distributions over the various surfaces to conform to the pattern necessary to set up the required electrostatic field between these surfaces. I

Referring to Fig. 6, the lens shown in the figure consists of two electrodes, a spine electrode 17 and a troughshaped electrode 18 which embraces the spine. The

space between these two electrodes is the field region which has the required character to perform upon incident electron beams the required reversing and collimating effect. Three typical beams aa bb and cc are shown. These three rays are incident on the incident aperture of the lens which, in the drawing, is the mout defined between the upper edge of the spine electrode and the far edge of the trough-shaped electrode. They are distributed in a fan-shaped formation as though emanating from a point source located above the incident aperture.

The distribution of conductive elements making up the interior surface of electrode 18 and their potentials are shown in Figure 8. In this figure also the required potential distribution between the various elements is achieved by interconnecting the successive strips through high resistance loops as indicated at 19 on the left hand side of the drawing. These loops thus provide. a potentiometer chain, to one end ofwhich the main HT voltage is connected and to the other the cathode potential. 'As shown, all the elements of the exterior electrode have been shown connected in this way but it may be preferred to lead out certain areas of the array for connection outside the tube, so that their voltages may be adjusted to.

- reproduced by printingtechniques as employed in so called printed circuitry, using foil bonded onto insulatin'g materials such as glass, mica or silicone impregnated glass fabric. It is convenient to prepare the electrode structures in the fla on an insulating material which is sufliciently flexible or plastic to be bent subsequently into the required configuration for the spine or for the outer electrode respectively.

I claim:

1. A cathode ray tube comprising a plane fluorescent screen, at least one electron gun so arranged as to produce an electron beam substantially parallel to said screen, an array of substantially parallel conductors insulated from one another and arranged in a plane substantially parallel to said screen and separated therefrom bya gap, a common capacitive backing plate associated with said conducw tors, a cathode additional to that of saidelectron gun so arranged as to spray portions of said conductors with electrons from' said cathode, and a substantially cylindrical electron optical system having a symmetry plane parallel to the plane of said screen comprising an approximately U-shaped electrode, two electrodes near the ends of said U-shaped electrode and one further electrode located in said symmetry plane, said electrodes being so arranged that when appropriate potentials are impressed thereon, the, electron beam produced by said gun is focused in said symmetry plane and returned, after a half-turn around the electrode in said symmetry plane, in a direction opposite to its original direction, while said beam as a whole is displaced as well as turned round, so that it passes through the gap between said screen and said array.

2.,A cathode ray tube comprising a substantially flat fluorescent screen, a plane arrayof substantially parallel linear conductors, insulated from one another and arranged in a plane substantially parallel to said screen and spaced therefrom, means for producing an electron beam passing through the space between said array and said screen in a direction transverse tothe conductors of said array, said beam producing means being so positioned as to direct said beam initially in a plane substantially parallel to, but displaced from, those of said array and said screen, means for turning said beam back upon itself and into the space between said array and said screen, means for deflecting said beam in a plane parallel to said screen, and means for so controlling the potential distribution on the conductors of said array as to deflect an end portion of said beam towards and into impact with said screen within a desired region defined by a group of adjacent conductors, said beam turning means includingan electrode of substantially U-shaped cross section, a pair of electrodes positioned one adjacent the edge of each limb of said U-shaped electrode, and an additional electrode located substantially in the plane of symmetry of said U- shaped electrode, said electrodes being so arranged that, when suitably energized, the beam of electrons entering the beam turning means will be focused in the plane of symmetry of said U-shaped electrode and will emerge 8 reversed in direction and in a plane substantially parallel to its incident plane but displaced relative thereto.

3. A cathode'ray tube as claimed in claim 2 including electrostatic deflection plates associated with said beam producing means for imposing deflections on the electron beam in a plane parallel to the fluorescent screen, and additional electrostatic deflection plates located one beyond the other in the direction ofsaid beam adapted to deflect the beam in a direction towards or away from the fluorescent screen and back into a plane substantially parallel to said screen, whereby to displace the beam in a direction normal to said screen.

4. ,A cathode ray tube comprising a substantially flat fluorescent screen, a plane array of substantially parallel linear conductors insulated from one another and arranged in a plane substantially parallel to said screen and spaced therefrom, means for producing anelectron beam passing through the space between, said array and saidscreen in a direction transverse to the conductors of bution on the conductors of said array as to deflect an end portion of said beam towards and into impact with said screen within a desired region defined by a group of adjacent conductors, said beam turning means including an inner electrode structure and an outer electrode structure embracing said inner electrode structure so as to define between thestwo structures an electrostatic field region having a U-shaped cross-section of substantially uniform dimensions throughout its length, at least one surface of at least one of said electrode structures being composed of a plurality of conductive strips adapted to be maintained each at a diflferent potential and so contoured as to provide, when suitably energised, the desired potential distribution in the said field region to produce the desired lens action.

5. A cathode ray tube as claimed in claim 4 comprising a plurality of high resistance elements series-connected to form a potentiometer chain, wherein successive ones of at least some of said plurality of conductive strips form the series connections between said elements,

6. A cathode ray tube according to claim 4 wherein each of said electrode structures is formed from plane arrays of conductive strips printed on a base of insulated material which base is moulded thereafter to a suitable shape.

References'Cited in the file of this patent UNITED STATES PATENTS

Images